A vibrissa pathway that activates the limbic system
Most sensory systems comprise parallel pathways of sensory information that encode different features of a stimulus and also take part in the control of sensor motion . The vibrissa system of rodents is no exception (reviewed in ). Ascending signals in the vibrissa system travel along two main trigeminothalamic pathways (reviewed in ): (1) a lemniscal pathway that arises from the trigeminal nucleus principalis (PrV), transits through the ventral posterior medial nucleus (VPM) of the thalamus, and projects to the primary somatosensory cortex; (2) a paralemniscal pathway that arises from the rostral part of trigeminal nucleus interpolaris (SpVIr), transits through the posterior group (Po) of the thalamus, and projects to the somatosensory cortical areas and to the vibrissa motor cortex. In contrast with PrV cells, which innervate principally VPM thalamus, SpVIr cells that project to Po thalamus also innervate a number of additional regions by means of branching axons . These include the superior colliculus, the anterior pretectal nucleus, the ventral division of zona incerta, and the dorsal lateral sector of the facial nucleus. Tract-tracing studies by means of classic tracers also reported interpolaris projections to the brainstem and spinal cord . Whether the latter projections also arise from Po-projecting cells remains an open question. While the lemniscal pathway conveys tactile information, as well as information about the relative phase of the vibrissae in the whisk cycle , the role of the paralemniscal pathway remains puzzling. It was proposed that this pathway conveys information about whisking kinematics , but later studies found that encoding of whisking along the paralemniscal pathway is relatively poor . It was also proposed that the paralemniscal pathway is specifically activated upon noxious stimulation , but it has never been shown that interpolaris cells that respond to vibrissa deflection are also activated by noxious stimuli. Thus, the general function of the paralemnical pathway remains unresolved. Here, we used virus-based tract-tracing methods and electrophysiology to document the full extent of the collateral projections of interpolaris cells that give rise to the paralemniscal pathway. We searched for new pathways of ascending information to forebrain regions, with a focus on pathways that include the Kölliker-Fuse/parabrachial complex (KF/PBc). The KF/PBc were previously described primarily in terms of their role in eliciting changes in respiration and cardiac function (reviewed in ). Yet the role of inhalation in either driving or pacing whisking  provides motivation to investigate if the KF/PBc has a broader role in orofacial motor actions. Further motivation comes from the projections of the KF nucleus to the trigeminal sensory nuclei , which contains multiple pathways for the control of motoneurons involved in whisking . Finally, we searched for feedback projections from targets along the paralemnical pathways to brainstem nuclei as a means to uncover a potential role of this pathway in rat’s behavior. 

Collateral projections of interpolaris cells that project to Po thalamus To map the collateral projections of interpolaris cells that innervate Po thalamus, we injected the retrograde virus G-pseudotyped Lenti-Cre in Po thalamus, and AAV2/8-hSyn-DIO-GFP in the vibrissa-responsive sector of subnucleus SpVIr (five juvenile rats; ). This approach revealed an unexpected diversity of axonal projections across all animals . In accordance with prior studies, terminal fields are found in Po thalamus, the ventral division of zona incerta, the anterior pretectal nucleus, the superior colliculus, the perirubral region, the dorsal lateral part of the facial nucleus, and in the other subdivisions of the sensory trigeminal complex . As new results, profuse projections are also found in the ipsilateral dorsal medullary reticular (MdD) nucleus and further caudally in the dorsal horn of the cervical cord. An ascending projection terminates ipsilaterally in the intertrigeminal region (ITr) and in the KF/PBc. Whether individual interpolaris cells project to many or all of the above-mentioned target regions remains an open issue. What is clear, however, is that interpolaris cells that project to Po thalamus broadcast sensory messages to multiple midbrain and hindbrain regions. Injection of the virus AAV1-hSyn-eGFP-WPRE in the vibrissa-responsive sector of subnucleus SpVIr (three adult rats; ) yielded similar results to those above . Yet anterograde labeling is now found in the cerebellum. This is consistent with prior studies, which reported that different populations of SpVI cells project to the thalamus and cerebellum . Of note, in sagittal sections of the brainstem, the terminal field of interpolaris axons in the KF/PBc covers an extensive crescent-shaped territory at the rostral border of the PrV and trigeminal motor nuclei . KF/PBc and MdD cells respond to vibrissa deflection Interpolaris cells that give rise to the paralemniscal pathway are known to respond to vibrissa deflection . Thus, one expects KF/PBc and MdD neurons to also respond to vibrissa deflection. We indeed found vibrissa-responsive neurons in the KF/PBc (latency: 8.0±1.2 ms; mean ± SD; 22 cells across 14 adult rats; K) and in the MdD nucleus (latency: 6.0±1.4 ms; 33 cells across three additional adult rats; ) of ketamine/xylazine anesthetized animals. The location of recorded units was assessed by single-cell juxtacellular labeling in the KF/PBc (eight cells in 8 of the 14 rats), and by iontophoretic deposit of Chicago Sky Blue in the MdD nucleus (three rats). Like interpolaris cells, the receptive field of KF/PBc and MdD cells includes multiple vibrissae. In the KF/PBc, vibrissa-responsive cells are intermingled with cells that discharge in phase with the respiratory rhythm. Yet the spontaneous activity of vibrissa-responsive cells displays no statistically significant spectral coherence with respiration (mean coherence< C > in the spectral band of breathing is |< C >|=0.16 for vibrissa-responsive cells versus |< C >|=0.72 for respiratory units . We did not investigate the parameters of vibrissa deflection that best drive KF/PBc and MdD cells. Our goal was to cross-check the tract-tracing results , which indicate that some KF/PBc and MdD cells receive input from vibrissa-responsive interpolaris neurons. Axonal projections of KF/PBc cells that receive interpolaris input s Among the targets of the paralemniscal pathway, we focused on the KF/PBc because of the well-documented hodology of its axonal projections (see review by ). As a means to visualize the axonal projections of KF/PBc cells that receive interpolaris input, we injected AAV2/1 that expresses Cre in the vibrissa-responsive sector of subnucleus SpVI to achieve anterograde transsynaptic expression of Cre , and AAV1-EF1a-DIO-hChR2-eYFP was injected in the KF/PBc (three adult rats; transsynaptically labeled cells amount to 55, 62, and 68 in each of the rats). We observed transsynaptic anterograde labeling in the lateral part of the facial nucleus, throughout the ventral lateral medulla, the MdD nucleus, the nucleus of the solitary tract, and the cervical cord in all animals . Ascending projections were found in the dorsal raphe, the Edinger-Wesphal nucleus, the periaqueductal gray, the posterior lateral hypothalamus, the paracentral and central medial thalamic nuclei, the parvicellular part of the ventral posterior thalamic nucleus, the ventral division of zona incerta, the central amygdala (CeA), and the lateral part of the bed nucleus of stria terminalis in all animals . In summary, KF/PBc cells that receive interpolaris input in turn project to most of the brainstem and forebrain regions previously identified by means of anterograde tracer injections in the KF/PBc . Yet, it is worth noting that no anterograde labeling is observed in the trigeminal sensory nuclei. This negative result is of critical importance. It indicates that KF/PBc cells that receive interpolaris input do not feedback to the relay cells that give rise to the paralemniscal pathway (see below). As PB cells that project to the CeA receive input from vibrissa-responsive SpVI neurons, one expects to find vibrissa-evoked responses in the amygdala. We addressed this issue by recording the local field potential evoked by air jet deflection of the vibrissae (three adult rats; C). These experiments were carried out under urethane instead of ketamine/xylazine anesthesia as α 2 agonists, for example, xylazine, strongly depress synaptic transmission in the parabrachial-amygdaloid pathway . Vibrissa deflection evoked a clear negative field potential in the CeA (peak latency of 12 ms as an average over all three rats and 50 trials per rat; ). This response was abolished in all animals following electrolytic lesion of the ipsilateral PB nucleus . Taken together, our anatomical  and electrophysiological  data delineate a pathway of vibrissa information that reaches the CeA and several limbic regions of the forebrain via a relay in the KF/PBc. Differential projections of KF and PB cells Prior studies in mice have shown that separate pools of KF/PBc neurons project to the forebrain and to the lower brainstem . We thus examined whether descending and ascending projections of the KF/PBc in rats arise from different or overlapping neurons. We made large injections, i.e., 100 nl, of the retrograde labels retroAAV-CAG-eGFP and retroAAV-CAG-mCherry in the CeA and the facial nucleus, respectively (two rats; ). Although a number of retrogradely labeled cells were found in the KF/PBc, none was doubly labeled. In detail, amygdala-projecting cells (326 in rat 1 and 312 in rat 2) were concentrated in lateral and medial PB nucleus, while the majority of facial projecting cells (299 in rat 1 and 140 in rat 2) were located in KF nucleus. In complementary experiments, we injected retroAAV expressing Cre in the ventral respiratory group and AAV2/8-hSyn-DIO-eGFP in the KF/PBc (two rats). Anterograde labeling was observed in the facial nucleus, the ventral respiratory group, and the MdD nucleus , but not in midbrain and forebrain regions. Taken together, these results confirm that neighboring but separate populations of KF/PBc neurons project to the forebrain and to the lower brainstem . GABAergic KF cells gate sensory transmission in subnucleus SpVI In rodents, KF projections to the brainstem arise from two neuronal populations: glutamatergic cells that project to the autonomic, respiratory, and motor regions of the medulla, and GABAergic neurons, which project principally to subnucleus SpVI . To determine whether GABAergic KF cells inhibit interpolaris cells, we made large injections, i.e., 200 nl, of the transsynaptic label anteroAAV-expressing Cre  in the KF/PBc and AAV2-Ef1a-DIO-eYFP in subnucleus SpVI (three rats; ). Unexpectedly, few interpolaris neurons were labeled (44, 48, and 53 cells, respectively, in each of three rats). Anterograde labeling was observed in the KF/PBc and in the parvicellular part of the ventral posterior thalamic nucleus, a region known to process input from the tongue and oral cavity , in all animals. No labeling was observed in the cerebellum, nor in brainstem and midbrain regions that receive input from the paralemniscal pathway . Yet, cells in the parvicellular reticular formation (PCRt) at the medial border of the trigeminal nuclei were extensively labeled, which indicates that the paucity of cell labeling in subnucleus SpVI is hardly ascribable to a technical pitfall. Taken together, these results indicate that KF projections do not innervate the SpVI interneurons or the projection cells that give rise to the paralemniscal pathway. To further assess these anatomical results, we injected AAV-hSyn-ChR2-eYFP in the KF nucleus and used an optrode to concurrently stimulate KF axons and record the activity of SpVI neurons (three adult rats; ). Interpolaris cells either discharged spontaneously or were driven by jiggling a single vibrissa with a piezoelectric stimulator. Optogenetic stimulation suppressed sensory-evoked discharges in monovibrissa-responsive cells (26 cells in the same three rats, t-test yields p<0.001; lower panel in ) but did not affect the firing of multivibrissa-responsive units (32 cells in the same three rats, p=0.84; ). Together with the lack of transsynaptic labeling of local circuit cells from the KF nucleus, these results raise the possibility that KF-induced suppression of sensory responses in local circuit interpolaris cells is mediated by presynaptic or extrasynaptic inhibition, as summarized in . 

To map the collateral projections of interpolaris cells that innervate Po thalamus, we injected the retrograde virus G-pseudotyped Lenti-Cre in Po thalamus, and AAV2/8-hSyn-DIO-GFP in the vibrissa-responsive sector of subnucleus SpVIr (five juvenile rats; ). This approach revealed an unexpected diversity of axonal projections across all animals . In accordance with prior studies, terminal fields are found in Po thalamus, the ventral division of zona incerta, the anterior pretectal nucleus, the superior colliculus, the perirubral region, the dorsal lateral part of the facial nucleus, and in the other subdivisions of the sensory trigeminal complex . As new results, profuse projections are also found in the ipsilateral dorsal medullary reticular (MdD) nucleus and further caudally in the dorsal horn of the cervical cord. An ascending projection terminates ipsilaterally in the intertrigeminal region (ITr) and in the KF/PBc. Whether individual interpolaris cells project to many or all of the above-mentioned target regions remains an open issue. What is clear, however, is that interpolaris cells that project to Po thalamus broadcast sensory messages to multiple midbrain and hindbrain regions. Injection of the virus AAV1-hSyn-eGFP-WPRE in the vibrissa-responsive sector of subnucleus SpVIr (three adult rats; ) yielded similar results to those above . Yet anterograde labeling is now found in the cerebellum. This is consistent with prior studies, which reported that different populations of SpVI cells project to the thalamus and cerebellum . Of note, in sagittal sections of the brainstem, the terminal field of interpolaris axons in the KF/PBc covers an extensive crescent-shaped territory at the rostral border of the PrV and trigeminal motor nuclei . 

Interpolaris cells that give rise to the paralemniscal pathway are known to respond to vibrissa deflection . Thus, one expects KF/PBc and MdD neurons to also respond to vibrissa deflection. We indeed found vibrissa-responsive neurons in the KF/PBc (latency: 8.0±1.2 ms; mean ± SD; 22 cells across 14 adult rats; K) and in the MdD nucleus (latency: 6.0±1.4 ms; 33 cells across three additional adult rats; ) of ketamine/xylazine anesthetized animals. The location of recorded units was assessed by single-cell juxtacellular labeling in the KF/PBc (eight cells in 8 of the 14 rats), and by iontophoretic deposit of Chicago Sky Blue in the MdD nucleus (three rats). Like interpolaris cells, the receptive field of KF/PBc and MdD cells includes multiple vibrissae. In the KF/PBc, vibrissa-responsive cells are intermingled with cells that discharge in phase with the respiratory rhythm. Yet the spontaneous activity of vibrissa-responsive cells displays no statistically significant spectral coherence with respiration (mean coherence< C > in the spectral band of breathing is |< C >|=0.16 for vibrissa-responsive cells versus |< C >|=0.72 for respiratory units . We did not investigate the parameters of vibrissa deflection that best drive KF/PBc and MdD cells. Our goal was to cross-check the tract-tracing results , which indicate that some KF/PBc and MdD cells receive input from vibrissa-responsive interpolaris neurons. 

Among the targets of the paralemniscal pathway, we focused on the KF/PBc because of the well-documented hodology of its axonal projections (see review by ). As a means to visualize the axonal projections of KF/PBc cells that receive interpolaris input, we injected AAV2/1 that expresses Cre in the vibrissa-responsive sector of subnucleus SpVI to achieve anterograde transsynaptic expression of Cre , and AAV1-EF1a-DIO-hChR2-eYFP was injected in the KF/PBc (three adult rats; transsynaptically labeled cells amount to 55, 62, and 68 in each of the rats). We observed transsynaptic anterograde labeling in the lateral part of the facial nucleus, throughout the ventral lateral medulla, the MdD nucleus, the nucleus of the solitary tract, and the cervical cord in all animals . Ascending projections were found in the dorsal raphe, the Edinger-Wesphal nucleus, the periaqueductal gray, the posterior lateral hypothalamus, the paracentral and central medial thalamic nuclei, the parvicellular part of the ventral posterior thalamic nucleus, the ventral division of zona incerta, the central amygdala (CeA), and the lateral part of the bed nucleus of stria terminalis in all animals . In summary, KF/PBc cells that receive interpolaris input in turn project to most of the brainstem and forebrain regions previously identified by means of anterograde tracer injections in the KF/PBc . Yet, it is worth noting that no anterograde labeling is observed in the trigeminal sensory nuclei. This negative result is of critical importance. It indicates that KF/PBc cells that receive interpolaris input do not feedback to the relay cells that give rise to the paralemniscal pathway (see below). As PB cells that project to the CeA receive input from vibrissa-responsive SpVI neurons, one expects to find vibrissa-evoked responses in the amygdala. We addressed this issue by recording the local field potential evoked by air jet deflection of the vibrissae (three adult rats; C). These experiments were carried out under urethane instead of ketamine/xylazine anesthesia as α 2 agonists, for example, xylazine, strongly depress synaptic transmission in the parabrachial-amygdaloid pathway . Vibrissa deflection evoked a clear negative field potential in the CeA (peak latency of 12 ms as an average over all three rats and 50 trials per rat; ). This response was abolished in all animals following electrolytic lesion of the ipsilateral PB nucleus . Taken together, our anatomical  and electrophysiological  data delineate a pathway of vibrissa information that reaches the CeA and several limbic regions of the forebrain via a relay in the KF/PBc. 

Prior studies in mice have shown that separate pools of KF/PBc neurons project to the forebrain and to the lower brainstem . We thus examined whether descending and ascending projections of the KF/PBc in rats arise from different or overlapping neurons. We made large injections, i.e., 100 nl, of the retrograde labels retroAAV-CAG-eGFP and retroAAV-CAG-mCherry in the CeA and the facial nucleus, respectively (two rats; ). Although a number of retrogradely labeled cells were found in the KF/PBc, none was doubly labeled. In detail, amygdala-projecting cells (326 in rat 1 and 312 in rat 2) were concentrated in lateral and medial PB nucleus, while the majority of facial projecting cells (299 in rat 1 and 140 in rat 2) were located in KF nucleus. In complementary experiments, we injected retroAAV expressing Cre in the ventral respiratory group and AAV2/8-hSyn-DIO-eGFP in the KF/PBc (two rats). Anterograde labeling was observed in the facial nucleus, the ventral respiratory group, and the MdD nucleus , but not in midbrain and forebrain regions. Taken together, these results confirm that neighboring but separate populations of KF/PBc neurons project to the forebrain and to the lower brainstem . 

In rodents, KF projections to the brainstem arise from two neuronal populations: glutamatergic cells that project to the autonomic, respiratory, and motor regions of the medulla, and GABAergic neurons, which project principally to subnucleus SpVI . To determine whether GABAergic KF cells inhibit interpolaris cells, we made large injections, i.e., 200 nl, of the transsynaptic label anteroAAV-expressing Cre  in the KF/PBc and AAV2-Ef1a-DIO-eYFP in subnucleus SpVI (three rats; ). Unexpectedly, few interpolaris neurons were labeled (44, 48, and 53 cells, respectively, in each of three rats). Anterograde labeling was observed in the KF/PBc and in the parvicellular part of the ventral posterior thalamic nucleus, a region known to process input from the tongue and oral cavity , in all animals. No labeling was observed in the cerebellum, nor in brainstem and midbrain regions that receive input from the paralemniscal pathway . Yet, cells in the parvicellular reticular formation (PCRt) at the medial border of the trigeminal nuclei were extensively labeled, which indicates that the paucity of cell labeling in subnucleus SpVI is hardly ascribable to a technical pitfall. Taken together, these results indicate that KF projections do not innervate the SpVI interneurons or the projection cells that give rise to the paralemniscal pathway. To further assess these anatomical results, we injected AAV-hSyn-ChR2-eYFP in the KF nucleus and used an optrode to concurrently stimulate KF axons and record the activity of SpVI neurons (three adult rats; ). Interpolaris cells either discharged spontaneously or were driven by jiggling a single vibrissa with a piezoelectric stimulator. Optogenetic stimulation suppressed sensory-evoked discharges in monovibrissa-responsive cells (26 cells in the same three rats, t-test yields p<0.001; lower panel in ) but did not affect the firing of multivibrissa-responsive units (32 cells in the same three rats, p=0.84; ). Together with the lack of transsynaptic labeling of local circuit cells from the KF nucleus, these results raise the possibility that KF-induced suppression of sensory responses in local circuit interpolaris cells is mediated by presynaptic or extrasynaptic inhibition, as summarized in . 

Our study reveals a hitherto unsuspected diversity of axonal projections of interpolaris cells that give rise to the paralemniscal pathway. We identified two new collateral projections that target the MdD nucleus and the KF/PBc ( and ). The diagram in summarizes the first-order axonal projections of interpolaris cells that give rise to the paralemniscal pathway, and the second-order projections that derive from the KF/PBc. Furthermore, we show that the KF/PBc contains two populations of vibrissa-responsive neurons. One population gives rise to an ascending pathway, which projects to limbic regions of the forebrain, and the other one projects to the autonomic, respiratory, and motor regions of the medulla ; the diagram in summarizes this circuit. Finally, the KF nucleus contains a pool of GABAergic neurons that do not receive interpolaris input, but project locally and to the trigeminal sensory nuclei ; the diagram in summarizes this circuit. Descending projection to the MdD By far the most abundant collateral projection of vibrissa-responsive interpolaris cells is to the MdD nucleus and the dorsal horn of the cervical cord. The finding of vibrissa-responsive cells in the MdD nucleus is unexpected, as most studies reported that MdD neurons best respond to noxious stimuli (reviewed in ; ). Although there is no reason to believe that vibrissa deflection per se is painful, an unexpected air puff directed toward the head of a rat elicits fear-related behavior such as a startle response , 22 kHz ultrasonic vocalization , and avoidance behavior . As the MdD nucleus sends profuse projections to the cervical cord and the facial nucleus , these results suggest that vibrissa messages to the MdD nucleus may elicit avoidance or flight behaviors. This conclusion is in line with that of the recent study, which reported that the MdD nucleus is part of a brainstem-spinal circuit to control escape responses to noxious stimuli . The brainstem medullary reticular formation comprises two main divisions: a ventral part (MdV) and a dorsal part (MdD). The MDV nucleus stands out as specifically targeting subpopulations of forelimb-innervating motor neurons. Selective ablation or silencing experiments reveal that the MdV nucleus is critically important for skilled motor behaviors . On the other hand, the MdD nucleus receives parabrachial inputs whose activation induces flight and escape behavior. Yet ‘A ctivation of PB (tac1) neurons does not produce nocifensive responses on its own but has a major effect on responses to sensory cues. It thus seems possible that peripheral inputs provide the context necessary to trigger a specific behavioral response’ . . We propose that the relay of sensory signals in the paralemniscal pathway is normally gated, and that release from gating occurs in behavioral contexts that are alarming or threatening. A recent paper  lends support for this proposal by showing that GABAergic KF/PBc cells gate the relay of aversive stimuli to limbic forebrain regions. The trigemino-parabrachial amygdaloid pathway A number of studies have reported trigeminal projections to the KF and PB nuclei ( ; ; ; ; ; see also review by ). Most of these studies focused on ascending projections from subnucleus caudalis, which convey pruriceptive and nociceptive inputs to the KF/PBc . Thus, it came as a surprise to find vibrissa-responsive neurons in KF/PBc, and further find that vibrissa signals are conveyed to the limbic regions of the forebrain via the medial and lateral PB. One study previously reported short-latency activation of amygdala neurons, at about 11 ms, upon electrical stimulation of the mystacial pad . Such a short latency is consistent with the activation of a trigemino-parabrachial-amygdaloid pathway by Aβ primary vibrissa afferents. The paralemniscal pathway and threat Although vibrissa-responsive interpolaris cells do not respond to noxious stimuli, many of the regions they innervate contain cells that process nociceptive or aversive inputs. This is the case for the MdD nucleus (reviewed in ), the KF/PBc , the superior colliculus , the zona incerta , and Po thalamus . These anatomical data suggest that the paralemniscal pathway conveys signals from threatening or alarming sensory inputs, perhaps through association with aversive contexts. In light of the evidence that interpolaris cells innervate brain regions that process nociceptive or aversive stimuli, we postulate that the KF nucleus is primarily involved in orchestrating defensive reactions to aversive and threatening stimuli. Three additional lines of evidence support this hypothesis. First, glutamatergic KF cells project to the cardio-respiratory medullary centers, the facial, hypoglossal, and ambiguous motor nuclei, the nucleus of the solitary tract, and to the preganglionic neurons of the sympathetic nervous systems (reviewed in ). Second, activation of KF cells elicits changes in respiration, heart rate, blood pressure . Third, individual KF neurons project to many of the above-mentioned targets by means of branching axons, which might form the anatomical substrate for coordinating autonomic and somatic reactions . Sensory gating in the paralemniscal pathway A prior study in the alert head-restrained rat reported that units in trigeminal nucleus principalis fire more spikes when the animal is whisking as opposed to not whisking . Yet the same study shows that spike rates are not significantly different between whisking and not whisking in subnucleus SpVI. This is an unexpected result, as primary vibrissa afferents discharge during whisking  and SpVI projection cells are typically driven by many more vibrissae than principalis neurons . The observation that whisking tends to reorganize spike times of neurons in interpolaris, rather than increase overall spike rates, supports the notion that the relay of vibrissa messages in the paralemniscal pathway is normally gated as part of fast, inhibitory feedback. The inhibition of the paralemniscal pathway by KF GABAergic neurons, together with the lack of a direct connection from KF neurons to interpolaris projections cells, suggests that proposed gating occurs by a mechanism of presynaptic or extrasynaptic inhibition . What drives the KF nucleus, consistent with the postulated involvement in orchestrating defensive reactions to aversive and threatening stimuli? The KF nucleus receives projections from different subdivisions of the periaqueductal gray, themselves downstream of inhibitory input from the amygdala . The release of the inhibition from the amygdala to the periaqueductal gray may well lead to a defensive postures. It is of interest the release of inhibition provides a release from pain , also consistent with presynaptic inhibition of the trigeminus. Conclusion Our results clearly indicate that the paralemniscal pathway broadcasts vibrissa-based sensory signals to a number of brainstem and forebrain regions that are involved in the expression of emotional reactions. They provide a different viewpoint on the role of the Kölliker-Fuse nucleus, which is currently considered to control the mechanics of respiratory and cardiac rhythms, but no more. We propose that the Kölliker-Fuse nucleus is involved in the larger role of orchestrating the expression of emotional reactions, for example, facial expressions, autonomic, and somatic reactions, of which respiration is one of many players. 

By far the most abundant collateral projection of vibrissa-responsive interpolaris cells is to the MdD nucleus and the dorsal horn of the cervical cord. The finding of vibrissa-responsive cells in the MdD nucleus is unexpected, as most studies reported that MdD neurons best respond to noxious stimuli (reviewed in ; ). Although there is no reason to believe that vibrissa deflection per se is painful, an unexpected air puff directed toward the head of a rat elicits fear-related behavior such as a startle response , 22 kHz ultrasonic vocalization , and avoidance behavior . As the MdD nucleus sends profuse projections to the cervical cord and the facial nucleus , these results suggest that vibrissa messages to the MdD nucleus may elicit avoidance or flight behaviors. This conclusion is in line with that of the recent study, which reported that the MdD nucleus is part of a brainstem-spinal circuit to control escape responses to noxious stimuli . The brainstem medullary reticular formation comprises two main divisions: a ventral part (MdV) and a dorsal part (MdD). The MDV nucleus stands out as specifically targeting subpopulations of forelimb-innervating motor neurons. Selective ablation or silencing experiments reveal that the MdV nucleus is critically important for skilled motor behaviors . On the other hand, the MdD nucleus receives parabrachial inputs whose activation induces flight and escape behavior. Yet ‘A ctivation of PB (tac1) neurons does not produce nocifensive responses on its own but has a major effect on responses to sensory cues. It thus seems possible that peripheral inputs provide the context necessary to trigger a specific behavioral response’ . . We propose that the relay of sensory signals in the paralemniscal pathway is normally gated, and that release from gating occurs in behavioral contexts that are alarming or threatening. A recent paper  lends support for this proposal by showing that GABAergic KF/PBc cells gate the relay of aversive stimuli to limbic forebrain regions. 

A number of studies have reported trigeminal projections to the KF and PB nuclei ( ; ; ; ; ; see also review by ). Most of these studies focused on ascending projections from subnucleus caudalis, which convey pruriceptive and nociceptive inputs to the KF/PBc . Thus, it came as a surprise to find vibrissa-responsive neurons in KF/PBc, and further find that vibrissa signals are conveyed to the limbic regions of the forebrain via the medial and lateral PB. One study previously reported short-latency activation of amygdala neurons, at about 11 ms, upon electrical stimulation of the mystacial pad . Such a short latency is consistent with the activation of a trigemino-parabrachial-amygdaloid pathway by Aβ primary vibrissa afferents. 

Although vibrissa-responsive interpolaris cells do not respond to noxious stimuli, many of the regions they innervate contain cells that process nociceptive or aversive inputs. This is the case for the MdD nucleus (reviewed in ), the KF/PBc , the superior colliculus , the zona incerta , and Po thalamus . These anatomical data suggest that the paralemniscal pathway conveys signals from threatening or alarming sensory inputs, perhaps through association with aversive contexts. In light of the evidence that interpolaris cells innervate brain regions that process nociceptive or aversive stimuli, we postulate that the KF nucleus is primarily involved in orchestrating defensive reactions to aversive and threatening stimuli. Three additional lines of evidence support this hypothesis. First, glutamatergic KF cells project to the cardio-respiratory medullary centers, the facial, hypoglossal, and ambiguous motor nuclei, the nucleus of the solitary tract, and to the preganglionic neurons of the sympathetic nervous systems (reviewed in ). Second, activation of KF cells elicits changes in respiration, heart rate, blood pressure . Third, individual KF neurons project to many of the above-mentioned targets by means of branching axons, which might form the anatomical substrate for coordinating autonomic and somatic reactions . 

A prior study in the alert head-restrained rat reported that units in trigeminal nucleus principalis fire more spikes when the animal is whisking as opposed to not whisking . Yet the same study shows that spike rates are not significantly different between whisking and not whisking in subnucleus SpVI. This is an unexpected result, as primary vibrissa afferents discharge during whisking  and SpVI projection cells are typically driven by many more vibrissae than principalis neurons . The observation that whisking tends to reorganize spike times of neurons in interpolaris, rather than increase overall spike rates, supports the notion that the relay of vibrissa messages in the paralemniscal pathway is normally gated as part of fast, inhibitory feedback. The inhibition of the paralemniscal pathway by KF GABAergic neurons, together with the lack of a direct connection from KF neurons to interpolaris projections cells, suggests that proposed gating occurs by a mechanism of presynaptic or extrasynaptic inhibition . What drives the KF nucleus, consistent with the postulated involvement in orchestrating defensive reactions to aversive and threatening stimuli? The KF nucleus receives projections from different subdivisions of the periaqueductal gray, themselves downstream of inhibitory input from the amygdala . The release of the inhibition from the amygdala to the periaqueductal gray may well lead to a defensive postures. It is of interest the release of inhibition provides a release from pain , also consistent with presynaptic inhibition of the trigeminus. 

Our results clearly indicate that the paralemniscal pathway broadcasts vibrissa-based sensory signals to a number of brainstem and forebrain regions that are involved in the expression of emotional reactions. They provide a different viewpoint on the role of the Kölliker-Fuse nucleus, which is currently considered to control the mechanics of respiratory and cardiac rhythms, but no more. We propose that the Kölliker-Fuse nucleus is involved in the larger role of orchestrating the expression of emotional reactions, for example, facial expressions, autonomic, and somatic reactions, of which respiration is one of many players. 

Subjects Experiments were carried out in Long Evans rats of both sexes and included five juvenile rats (P30–P35) and 36 adult rats (P80–P100; 250–350 g in mass). All experiments were carried out according to the National Institutes of Health Guidelines. All experiments were approved by the Institutional Animal Care and Use Committees at Laval University and at the University of California at San Diego. Viruses G-pseudotyped Lenti-Cre virus was designed and produced as described . AAV2/1-EF1a-DIO-hChR2-EYFP-WPRE-HGHpA, AAV2/1-hSyn-Cre WPRE, AAV2/5-hSyn-eGFP-WPRE, retroAAV-CAG-tdTomato, retroAAV-CAG-eGFP, and AAV2/8-hSyn-DIO-eGFP were obtained from Addgene. Surgery and virus injections All but three rats were anesthetized with ketamine (75 mg/kg) and xylazine (5 mg/kg), and body temperature was maintained at 37°C with a thermostatically controlled heating pad. Three rats were anesthetized with urethane (1.4 g/kg) to record the local field potential evoked in the amygdala by vibrissa deflection. All virus injections were carried out after electrophysiological identification of the target regions. When AAV injections were made in the interpolaris nucleus, we targeted the lateral most sector of the nucleus based on the well-known somatotopic representation of the vibrissae in this nucleus. This precaution proved useful to avoid infecting cells in neighboring regions. After 3–4 weeks of survival, based on the results of , animals were either perfused with saline and paraformaldehyde (4% (w/v) in PBS), or anesthetized with ketamine (75 mg/kg) and xylazine (5 mg/kg) for optogenetic stimulation. In three rats, the vibrissa-responsive sector of the KF/PBc was first located by electrophysiological recording and then lesioned by passing 0.5 mA through a stainless steel electrode with a tip (10 μm) de-insulated over a length of 500 μm. Electrophysiology Single-unit recordings were carried out in the KF/PBc and MdD with micropipettes (tip diameter, 1 µm) filled with either 0.5 M potassium acetate and 2% (w/v) neurobiotin, or 0.5 M potassium acetate and 4% (w/v) Chicago Sky Blue. To maximize the chance of cell recovery after juxtacellular labeling in the KF/PBc, we injected one cell per animal and perfused the rat thereafter. A hand-held probe was first used to identify the vibrissal receptive field of MdD and KF/PBc neurons. Then we used an air jet (1.5 s duration) to deflect the vibrissae in the rostro-caudal direction. Air jets were generated by a Picospitzer (General Valve, Brookshire, TX) connected to a pipette (tip diameter, 500 μm). The pipette was positioned at a distance of 4–5 cm of the vibrissae, in front of the animal, and the air jet was directed away from the rat’s face to avoid as much as possible stimulating other orofacial afferents. This created a cone-shaped air displacement that deflected six to seven whiskers. The delay between the command voltage and the actual motion of the vibrissae was measured by placing a piezoelectric film (Measurement Specialties, Fairfield, NJ) at the same distance from the tip of the pipette . This delay was subtracted from the recordings to build PSTHs of sensory-evoked responses (20 responses; bin width, 1 ms). Respiration was monitored with a cantilevered piezoelectric film (LDT1 028K; Measurement Specialties) resting on the rat’s abdomen just caudal to the torso. All signals were sampled at 10 kHz and logged on a computer using the Labchart acquisition system (AD Instruments). Optogenetics Three weeks after injection of AAV2/1-hSyn-hChR2-eYFP in the KF/PBc (three rats), we used a carbon-tip optrode (Kation Scientific) to simultaneously stimulate labeled axons and record interpolaris cells. As a test for KF-induced inhibition, we drove background spiking by jiggling a single vibrissa with a piezoelectric stimulator. Histology Following perfusion brains were postfixed for 1 hr, and cryoprotected overnight in 30% (w/w) sucrose in PBS. Brains were cut at thickness of 50 µm on a freezing microtome. Labeled material was processed for either fluorescence or brightfield microscopy. For fluorescence microscopy, sections were immunoreacted with a chicken anti-GFP antibody (1:1000; Abcam), and a donkey anti-chicken Alexa 488 IgG (1:500; Jackson ImmunoResearch Labs). NeuN immunostaining was carried out with a mouse anti-NeuN antibody (1:1000; MilliporeSigma) and a goat anti-rabbit IgG conjugated to Alexa 594 (1:500; Abcam). For brightfield microscopy, sections were first counterstained for cytochrome oxidase (MilliporeSigma), and then immunoreacted with a rabbit anti-GFP antibody (1:1000; Novus Biological), a biotinylated horse anti-rabbit IgG (1:200; Vector Labs), the avidin/biotin complex (Vectastain ABC Kit; Vector Labs), and the SG peroxidase substrate (ImmPACT SG Substrate; Vector Labs). In three rats, brainstem sections were first immunoreacted with a goat anti-choline acetyltransferase antibody (1:1000; MilliporeSigma) and a rabbit anti-goat IgG conjugated to horseradish peroxidase (1:500; Abcam), which was revealed with diaminobenzidine (brown reaction product). Sections were then immunoreacted with a mouse anti-GFP antibody (1:2000; Abcam), a biotinylated goat anti-mouse IgG (1:200; Vector Labs), which was revealed with streptavidin horseradish peroxidase conjugate (Vector Labs) and the Ni-DAB substrate (black reaction product) (ImmPACT DAB Substrate; Vector Labs). Finally, the extent of electrolytic lesions was assessed on material stained with Neutral Red. Sections were scanned at a resolution of 0.5 µm/pixel (TissueScope LE; Huron Digital Pathology) and imported in Fiji or Photoshop for color and contrast adjustments. Data analysis Three-dimensional maps of retrogradely labeled KF/PBc cells were constructed with the Neurolucida software (Microbrightfield). Peristimulus time histograms were built using the LabChart 8.0 spike histogram module (AD Instruments), and the Matlab Chronux toolbox ( http://www.chronux.org ) was used to compute the spectral coherence between respiration and the firing rate of KF/PBc cells. Data ‘Data sets (anatomical images and physiological time series) supporting this study, and an associated “read me” file, are deposited at Dryad, doi:10.6076/D15C7X.’. 

Experiments were carried out in Long Evans rats of both sexes and included five juvenile rats (P30–P35) and 36 adult rats (P80–P100; 250–350 g in mass). All experiments were carried out according to the National Institutes of Health Guidelines. All experiments were approved by the Institutional Animal Care and Use Committees at Laval University and at the University of California at San Diego. 

G-pseudotyped Lenti-Cre virus was designed and produced as described . AAV2/1-EF1a-DIO-hChR2-EYFP-WPRE-HGHpA, AAV2/1-hSyn-Cre WPRE, AAV2/5-hSyn-eGFP-WPRE, retroAAV-CAG-tdTomato, retroAAV-CAG-eGFP, and AAV2/8-hSyn-DIO-eGFP were obtained from Addgene. 

All but three rats were anesthetized with ketamine (75 mg/kg) and xylazine (5 mg/kg), and body temperature was maintained at 37°C with a thermostatically controlled heating pad. Three rats were anesthetized with urethane (1.4 g/kg) to record the local field potential evoked in the amygdala by vibrissa deflection. All virus injections were carried out after electrophysiological identification of the target regions. When AAV injections were made in the interpolaris nucleus, we targeted the lateral most sector of the nucleus based on the well-known somatotopic representation of the vibrissae in this nucleus. This precaution proved useful to avoid infecting cells in neighboring regions. After 3–4 weeks of survival, based on the results of , animals were either perfused with saline and paraformaldehyde (4% (w/v) in PBS), or anesthetized with ketamine (75 mg/kg) and xylazine (5 mg/kg) for optogenetic stimulation. In three rats, the vibrissa-responsive sector of the KF/PBc was first located by electrophysiological recording and then lesioned by passing 0.5 mA through a stainless steel electrode with a tip (10 μm) de-insulated over a length of 500 μm. 

Single-unit recordings were carried out in the KF/PBc and MdD with micropipettes (tip diameter, 1 µm) filled with either 0.5 M potassium acetate and 2% (w/v) neurobiotin, or 0.5 M potassium acetate and 4% (w/v) Chicago Sky Blue. To maximize the chance of cell recovery after juxtacellular labeling in the KF/PBc, we injected one cell per animal and perfused the rat thereafter. A hand-held probe was first used to identify the vibrissal receptive field of MdD and KF/PBc neurons. Then we used an air jet (1.5 s duration) to deflect the vibrissae in the rostro-caudal direction. Air jets were generated by a Picospitzer (General Valve, Brookshire, TX) connected to a pipette (tip diameter, 500 μm). The pipette was positioned at a distance of 4–5 cm of the vibrissae, in front of the animal, and the air jet was directed away from the rat’s face to avoid as much as possible stimulating other orofacial afferents. This created a cone-shaped air displacement that deflected six to seven whiskers. The delay between the command voltage and the actual motion of the vibrissae was measured by placing a piezoelectric film (Measurement Specialties, Fairfield, NJ) at the same distance from the tip of the pipette . This delay was subtracted from the recordings to build PSTHs of sensory-evoked responses (20 responses; bin width, 1 ms). Respiration was monitored with a cantilevered piezoelectric film (LDT1 028K; Measurement Specialties) resting on the rat’s abdomen just caudal to the torso. All signals were sampled at 10 kHz and logged on a computer using the Labchart acquisition system (AD Instruments). 

Three weeks after injection of AAV2/1-hSyn-hChR2-eYFP in the KF/PBc (three rats), we used a carbon-tip optrode (Kation Scientific) to simultaneously stimulate labeled axons and record interpolaris cells. As a test for KF-induced inhibition, we drove background spiking by jiggling a single vibrissa with a piezoelectric stimulator. 

Following perfusion brains were postfixed for 1 hr, and cryoprotected overnight in 30% (w/w) sucrose in PBS. Brains were cut at thickness of 50 µm on a freezing microtome. Labeled material was processed for either fluorescence or brightfield microscopy. For fluorescence microscopy, sections were immunoreacted with a chicken anti-GFP antibody (1:1000; Abcam), and a donkey anti-chicken Alexa 488 IgG (1:500; Jackson ImmunoResearch Labs). NeuN immunostaining was carried out with a mouse anti-NeuN antibody (1:1000; MilliporeSigma) and a goat anti-rabbit IgG conjugated to Alexa 594 (1:500; Abcam). For brightfield microscopy, sections were first counterstained for cytochrome oxidase (MilliporeSigma), and then immunoreacted with a rabbit anti-GFP antibody (1:1000; Novus Biological), a biotinylated horse anti-rabbit IgG (1:200; Vector Labs), the avidin/biotin complex (Vectastain ABC Kit; Vector Labs), and the SG peroxidase substrate (ImmPACT SG Substrate; Vector Labs). In three rats, brainstem sections were first immunoreacted with a goat anti-choline acetyltransferase antibody (1:1000; MilliporeSigma) and a rabbit anti-goat IgG conjugated to horseradish peroxidase (1:500; Abcam), which was revealed with diaminobenzidine (brown reaction product). Sections were then immunoreacted with a mouse anti-GFP antibody (1:2000; Abcam), a biotinylated goat anti-mouse IgG (1:200; Vector Labs), which was revealed with streptavidin horseradish peroxidase conjugate (Vector Labs) and the Ni-DAB substrate (black reaction product) (ImmPACT DAB Substrate; Vector Labs). Finally, the extent of electrolytic lesions was assessed on material stained with Neutral Red. Sections were scanned at a resolution of 0.5 µm/pixel (TissueScope LE; Huron Digital Pathology) and imported in Fiji or Photoshop for color and contrast adjustments. 

Three-dimensional maps of retrogradely labeled KF/PBc cells were constructed with the Neurolucida software (Microbrightfield). Peristimulus time histograms were built using the LabChart 8.0 spike histogram module (AD Instruments), and the Matlab Chronux toolbox ( http://www.chronux.org ) was used to compute the spectral coherence between respiration and the firing rate of KF/PBc cells. 

‘Data sets (anatomical images and physiological time series) supporting this study, and an associated “read me” file, are deposited at Dryad, doi:10.6076/D15C7X.’. 

