The acoustic startle reflex (ASR) is a short and intense defensive reaction in response to a loud and unexpected acoustic stimulus. In the rat, a primary startle pathway encompasses three serially connected central structures: the cochlear root neurons, the giant neurons of the nucleus reticularis pontis caudalis (PnC), and the spinal motoneurons. As a sensorimotor interface, the PnC has a central role in the ASR circuitry, especially the integration of different sensory stimuli and brain states into initiation of motor responses. Since the basal ganglia circuits control movement and action selection, we hypothesize that their output via the substantia nigra (SN) may interplay with the ASR primary circuit by providing inputs to PnC. Moreover, the pedunculopontine tegmental nucleus (PPTg) has been proposed as a functional and neural extension of the SN, so it is another goal of this study to describe possible anatomical connections from the PPTg to PnC. Here, we made 6-OHDA neurotoxic lesions of the SN pars compacta (SNc) and submitted the rats to a custom-built ASR measurement session to assess amplitude and latency of motor responses. We found that following lesion of the SNc, ASR amplitude decreased and latency increased compared to those values from the sham-surgery and control groups. The number of dopamine neurons remaining in the SNc after lesion was also estimated using a stereological approach, and it correlated with our behavioral results. Moreover, we employed neural tract-tracing techniques to highlight direct projections from the SN to PnC, and indirect projections through the PPTg. Finally, we also measured levels of excitatory amino acid neurotransmitters in the PnC following lesion of the SN, and found that they change following an ipsi/contralateral pattern. Taken together, our results identify nigrofugal efferents onto the primary ASR circuit that may modulate motor responses.
