to determine whether cb2 receptor activation alters intravenous cocaine self - administration , we used jwh133 , a highly selective cb2 receptor agonist ( 200-fold selectivity for cb2 
 versus cb1 ) , and am630 , a highly selective cb2 receptor antagonist ( 160-fold selectivity for cb2 
 versus cb1 ) , as pharmacological tools .
we found that over 50% of wild - type ( wt ) ( 20 of 34 ) and cb2 ( 22 of 36 ) mice , while only about 30% of cb1 ( 10 of 36 ) mice acquired stable intravenous cocaine self - administration , defined as 20 or more infusions per 3-h session , with a regular pattern of self - administration achieved after 10 days of training ( supplementary fig .
strikingly , cb1 mice displayed a significant reduction in both total number and rate ( infusions per h ) of cocaine infusions on days 15 , compared to wt or cb2 mice ( supplementary fig .
, the majority of cb1 mice ( 7 of 10 ) displayed a distinct  burst - like  drug - taking pattern with long inter - burst intervals , while wt and cb2 mice displayed evenly - paced drug - taking without significant difference between the two strains ( supplementary fig .
these findings suggest that deletion of cb1 receptors may lower cocaine s rewarding efficacy , leading to a compensatory increase in drug intake during each individual drug - taking episode .
this is further supported by the finding that cb1 mice displayed a significant reduction in break - point level for cocaine self - administration under progressive - ratio ( pr ) reinforcement , compared to wt mice ( supplementary fig .
since pr break - point , defined as maximal work performed by the animal to get a cocaine infusion , is cocaine dose - dependent and positively correlated to reward strength , the reduction in pr break - point observed in cb1 mice suggests a reduction in cocaine s reward strength and/or motivation for cocaine - taking behavior .
this is consistent with previous findings that cb1 receptor deletion impairs cocaine s rewarding , locomotor - stimulating , and da - elevating effects .
intraperitoneal ( i.p . ) administration of jwh133 ( 10 , 20 mg / kg ) produced a significant and dose - dependent reduction in cocaine self - administration and cocaine intake in both wt and cb1 mice , but not in cb2 mice ( fig .
this inhibition lasted for no longer than 24 hrs after 20 mg / kg jwh133 ( fig .
pretreatment with am630 , a selective cb2 receptor antagonist , but not with am251 , a selective cb1 receptor antagonist , significantly attenuated jwh133-induced inhibition of cocaine self - administration ( fig .
this suggests that jwh133 s attenuating effect is mediated by activation of cb2 , not cb1 , receptors .
this conclusion is further supported by the additional finding that systemic administration of gw405833 ( 3 , 10 mg / kg , i.p . ) , another highly selective but structurally distinct cb2 receptor agonist , also inhibited cocaine self - administration in wt mice ( fig .
2a ) . to determine whether jwh133-induced attenuation of cocaine self - administration was due to a reduction in cocaine s rewarding efficacy , we studied jwh133 s effect on i.v .
we found that systemic administration of jwh133 ( 10 , 20 mg / kg , i.p . )
significantly lowered the pr break - point for cocaine self - administration in wt mice ( fig .
2b ) , suggesting a reduction in cocaine s reward strength and/or motivation for drug - taking behavior after jwh133 administration .
we previously showed that cb1 receptor blockade by am251 significantly lowered the pr break - point for cocaine self - administration in rats .
we therefore also tested am251 in the present study , and found that am251 ( 3 mg / kg ) lowered the pr break - point for cocaine self - administration in wt mice ( fig .
these data suggest that the jwh133-induced reduction in cocaine self - administration resulted from a reduction in cocaine s rewarding efficacy . to further determine whether jwh133 s action was mediated by activation of brain or peripheral cb2 receptors
extensive studies have shown that a wide variety of compounds that can not penetrate the blood - brain barrier can be delivered directly from nose into brain .
we found that intranasal microinjections of jwh133 ( 50 , 100 g/10 l / side ) dose - dependently inhibited i.v .
cocaine self - administration ( fig . 2c ) . to explore the possibility that effects of intranasal jwh133 might be mediated by drug absorption into the nasal vasculature with subsequent venous delivery of drug to pharmacological site(s ) of action
injection of the same micro - quantities of jwh133 as used intranasally on cocaine self - administration .
microinjections of jwh133 ( 100 , 200 g ) had no effect on cocaine self - administration ( fig .
these data suggest that intranasal jwh133-induced pharmacological effects are mediated by activating brain rather than peripheral cb2 receptors . to further explore this issue , we observed the effects of local administration of jwh133 into the nucleus accumbens ( nac ) on cocaine self - administration .
we found that intra - nac microinjections of jwh133 ( 0.3 , 1 , 3 g / side ) significantly and dose - dependently inhibited cocaine self - administration in wt mice ( fig .
this inhibition was blocked by intra - nac co - administration of am630 ( 3 g / side ) .
we further examined whether jwh133 itself has cocaine - like rewarding effects . to address this issue , we first trained mice to acquire stable cocaine self - administration , and then cocaine was replaced by jwh133 ( 1 mg / kg / infusion ) or vehicle .
we found that neither jwh133 nor vehicle sustained stable self - administration in mice previously trained for cocaine self - administration ( supplementary fig .
in fact , the self - administration behavior underwent gradual extinction over the 5 days of substitution testing .
this extinction pattern was essentially identical to that seen when vehicle was substituted for cocaine .
however , when jwh133 or vehicle was replaced by cocaine , self - administration behavior returned to levels previously observed during stable cocaine self - administration .
in addition , we also found that cocaine ( 10 , 20 mg / kg , i.p . )
produced a significant conditioned place preference , while jwh133 , at the same doses , produced neither conditioned place preference nor place aversion in wt mice ( supplementary fig .
these findings suggest that jwh133 has no cocaine - like reinforcing nor aversive effects in mice . to determine whether jwh133 s effect on cocaine self - administration generalizes to other cocaine actions , we investigated the effects of jwh133 on cocaine - enhanced locomotion .
systemic administration of 10 mg / kg cocaine produced a significant increase in locomotion in all 3 mouse strains ( fig .
3 ) . pretreatment with jwh133 ( 10 , 20 mg / kg , 30 min prior to cocaine ) dose - dependently attenuated cocaine - enhanced locomotion in wt ( fig .
systemic administration of the same doses of jwh133 alone also significantly inhibited locomotion in a dose - dependent manner in wt and cb1 mice , but not in cb2 mice ( fig .
since the same doses of jwh133 alone failed to alter locomotor performance on a fast - running rotarod device in all 3 mouse strains ( supplementary fig .
3 ) , we infer that jwh133 s inhibition of cocaine self - administration or locomotion is not produced by nonspecific impairment of locomotor capacity . to further determine whether such locomotor inhibition is mediated by activation of brain cb2 receptors , we observed the effects of intra - nac jwh133 and/or am630 on locomotion .
we found that intra - nac microinjections of jwh133 ( 1 , 3 g / side ) significantly inhibited locomotion in wt or cb1 mice , but not in cb2 mice ( fig .
4b ) , in a dose - dependent manner similar to systemic administration ( fig .
we note that systemic administration of am630 failed to alter locomotion in any mouse strain tested ( fig .
however , when locally administered into the nac , am630 ( 1 , 3 , 10 g / side ) significantly increased locomotor activity ( fig .
a higher brain am630 level , achieved by local rather than by systemic administration , appears to be required to block endocannabinoid action on brain cb2 receptors .
given the crucial role of the mesolimbic da system in cocaine self - administration and modulation of locomotion , we further investigated the effects of jwh133 on basal and cocaine - enhanced da in the nac by in vivo microdialysis .
we did not see significant differences in basal levels of extracellular nac da between wt and cb2 mice ( supplementary fig .
however , cb1 mice displayed a significant basal reduction , compared to wt mice ( supplementary fig .
consistent with the findings in cocaine self - administration and locomotion , systemic administration of jwh133 ( 3 , 10 , 20 mg / kg , i.p . )
this reduction in nac da was blocked by am630 ( 10 mg / kg , i.p . ) in cb1 mice ( fig .
5b ) , suggesting that jwh133 s da - inhibiting effect is mediated by activation of cb2 receptors .
moreover , pretreatment with the same doses of jwh133 also significantly attenuated cocaine - enhanced nac da in wt ( fig .
5f ) mice . to determine whether this inhibition is mediated by activation of brain or peripheral cb2 receptors
, we also observed the effects of intranasal or intra - nac local administration of jwh133 on extracellular da .
we found that intranasal administration of jwh133 ( 100 g / nostril ) produced a significant reduction in extracellular nac da in wt and cb1 mice , but not in cb2 mice ( fig .
similarly , intra - nac local perfusion of jwh133 ( 11000 m ) significantly lowered extracellular da in both wt and cb1 mice , but not in cb2 mice ( fig .
in fact , an unexpected increase in extracellular da was observed in cb2 mice after local administration of jwh133 .
one possibility is that jwh133 may bind to other ( non - cb2 ) receptors in cb2 mice , producing an increase in extracellular da .
congruent with this finding , intra - nac local perfusion of am630 ( 1 , 10 , 100 m ) elevated extracellular da in a concentration - dependent manner in both wt and cb1 mice , but not in cb2 mice ( fig .
6c ) , suggesting that endocannabinoids tonically modulate nac da release by activation of brain cb2 receptors .
further , am630-enhanced extracellular da appears more robust in cb1 mice than in wt mice ( fig .
moreover , intra - nac local perfusion of am630 also blocked the reduction in extracellular nac da produced by systemic administration of jwh133 seen in wt and cb1 mice ( fig .
6c , d ) , suggesting that jwh133-induced inhibition of da release is mediated by activation of nac cb2 receptors .
the locations of microdialysis probes or microinjection cannulae were within the nac ( supplementary fig .
to determine whether cb2 receptor activation alters intravenous cocaine self - administration , we used jwh133 , a highly selective cb2 receptor agonist ( 200-fold selectivity for cb2 
 versus cb1 ) , and am630 , a highly selective cb2 receptor antagonist ( 160-fold selectivity for cb2 
 versus cb1 ) , as pharmacological tools .
we found that over 50% of wild - type ( wt ) ( 20 of 34 ) and cb2 ( 22 of 36 ) mice , while only about 30% of cb1 ( 10 of 36 ) mice acquired stable intravenous cocaine self - administration , defined as 20 or more infusions per 3-h session , with a regular pattern of self - administration achieved after 10 days of training ( supplementary fig .
strikingly , cb1 mice displayed a significant reduction in both total number and rate ( infusions per h ) of cocaine infusions on days 15 , compared to wt or cb2 mice ( supplementary fig .
, the majority of cb1 mice ( 7 of 10 ) displayed a distinct  burst - like  drug - taking pattern with long inter - burst intervals , while wt and cb2 mice displayed evenly - paced drug - taking without significant difference between the two strains ( supplementary fig .
these findings suggest that deletion of cb1 receptors may lower cocaine s rewarding efficacy , leading to a compensatory increase in drug intake during each individual drug - taking episode .
this is further supported by the finding that cb1 mice displayed a significant reduction in break - point level for cocaine self - administration under progressive - ratio ( pr ) reinforcement , compared to wt mice ( supplementary fig .
since pr break - point , defined as maximal work performed by the animal to get a cocaine infusion , is cocaine dose - dependent and positively correlated to reward strength , the reduction in pr break - point observed in cb1 mice suggests a reduction in cocaine s reward strength and/or motivation for cocaine - taking behavior .
this is consistent with previous findings that cb1 receptor deletion impairs cocaine s rewarding , locomotor - stimulating , and da - elevating effects .
intraperitoneal ( i.p . ) administration of jwh133 ( 10 , 20 mg / kg ) produced a significant and dose - dependent reduction in cocaine self - administration and cocaine intake in both wt and cb1 mice , but not in cb2 mice ( fig .
this inhibition lasted for no longer than 24 hrs after 20 mg / kg jwh133 ( fig .
pretreatment with am630 , a selective cb2 receptor antagonist , but not with am251 , a selective cb1 receptor antagonist , significantly attenuated jwh133-induced inhibition of cocaine self - administration ( fig .
this suggests that jwh133 s attenuating effect is mediated by activation of cb2 , not cb1 , receptors .
this conclusion is further supported by the additional finding that systemic administration of gw405833 ( 3 , 10 mg / kg , i.p . ) , another highly selective but structurally distinct cb2 receptor agonist , also inhibited cocaine self - administration in wt mice ( fig .
2a ) . to determine whether jwh133-induced attenuation of cocaine self - administration was due to a reduction in cocaine s rewarding efficacy , we studied jwh133 s effect on i.v .
we found that systemic administration of jwh133 ( 10 , 20 mg / kg , i.p . )
significantly lowered the pr break - point for cocaine self - administration in wt mice ( fig .
2b ) , suggesting a reduction in cocaine s reward strength and/or motivation for drug - taking behavior after jwh133 administration .
we previously showed that cb1 receptor blockade by am251 significantly lowered the pr break - point for cocaine self - administration in rats .
we therefore also tested am251 in the present study , and found that am251 ( 3 mg / kg ) lowered the pr break - point for cocaine self - administration in wt mice ( fig .
these data suggest that the jwh133-induced reduction in cocaine self - administration resulted from a reduction in cocaine s rewarding efficacy .
to further determine whether jwh133 s action was mediated by activation of brain or peripheral cb2 receptors , we first studied the effects of intranasal microinjections of jwh133 on i.v . cocaine self - administration .
extensive studies have shown that a wide variety of compounds that can not penetrate the blood - brain barrier can be delivered directly from nose into brain .
we found that intranasal microinjections of jwh133 ( 50 , 100 g/10 l / side ) dose - dependently inhibited i.v .
cocaine self - administration ( fig . 2c ) . to explore the possibility that effects of intranasal jwh133 might be mediated by drug absorption into the nasal vasculature with subsequent venous delivery of drug to pharmacological site(s ) of action , we observed the effects of i.v .
injection of the same micro - quantities of jwh133 as used intranasally on cocaine self - administration .
microinjections of jwh133 ( 100 , 200 g ) had no effect on cocaine self - administration ( fig .
these data suggest that intranasal jwh133-induced pharmacological effects are mediated by activating brain rather than peripheral cb2 receptors . to further explore this issue , we observed the effects of local administration of jwh133 into the nucleus accumbens ( nac ) on cocaine self - administration .
we found that intra - nac microinjections of jwh133 ( 0.3 , 1 , 3 g / side ) significantly and dose - dependently inhibited cocaine self - administration in wt mice ( fig .
this inhibition was blocked by intra - nac co - administration of am630 ( 3 g / side ) .
we further examined whether jwh133 itself has cocaine - like rewarding effects . to address this issue , we first trained mice to acquire stable cocaine self - administration , and then cocaine was replaced by jwh133 ( 1 mg / kg / infusion ) or vehicle .
we found that neither jwh133 nor vehicle sustained stable self - administration in mice previously trained for cocaine self - administration ( supplementary fig .
in fact , the self - administration behavior underwent gradual extinction over the 5 days of substitution testing .
this extinction pattern was essentially identical to that seen when vehicle was substituted for cocaine .
however , when jwh133 or vehicle was replaced by cocaine , self - administration behavior returned to levels previously observed during stable cocaine self - administration .
in addition , we also found that cocaine ( 10 , 20 mg / kg , i.p . ) produced a significant conditioned place preference , while jwh133 , at the same doses , produced neither conditioned place preference nor place aversion in wt mice ( supplementary fig .
these findings suggest that jwh133 has no cocaine - like reinforcing nor aversive effects in mice .
to determine whether jwh133 s effect on cocaine self - administration generalizes to other cocaine actions , we investigated the effects of jwh133 on cocaine - enhanced locomotion .
systemic administration of 10 mg / kg cocaine produced a significant increase in locomotion in all 3 mouse strains ( fig .
3 ) . pretreatment with jwh133 ( 10 , 20 mg / kg , 30 min prior to cocaine ) dose - dependently attenuated cocaine - enhanced locomotion in wt ( fig .
systemic administration of the same doses of jwh133 alone also significantly inhibited locomotion in a dose - dependent manner in wt and cb1 mice , but not in cb2 mice ( fig .
4a ) , suggesting an effect mediated by activation of cb2 receptors . since the same doses of jwh133 alone failed to alter locomotor performance on a fast - running rotarod device in all 3 mouse strains ( supplementary fig .
3 ) , we infer that jwh133 s inhibition of cocaine self - administration or locomotion is not produced by nonspecific impairment of locomotor capacity . to further determine whether such locomotor inhibition is mediated by activation of brain cb2 receptors , we observed the effects of intra - nac jwh133 and/or am630 on locomotion .
we found that intra - nac microinjections of jwh133 ( 1 , 3 g / side ) significantly inhibited locomotion in wt or cb1 mice , but not in cb2 mice ( fig .
4b ) , in a dose - dependent manner similar to systemic administration ( fig .
we note that systemic administration of am630 failed to alter locomotion in any mouse strain tested ( fig .
4c ) . however , when locally administered into the nac , am630 ( 1 , 3 , 10 g / side ) significantly increased locomotor activity ( fig .
a higher brain am630 level , achieved by local rather than by systemic administration , appears to be required to block endocannabinoid action on brain cb2 receptors .
given the crucial role of the mesolimbic da system in cocaine self - administration and modulation of locomotion , we further investigated the effects of jwh133 on basal and cocaine - enhanced da in the nac by in vivo microdialysis .
we did not see significant differences in basal levels of extracellular nac da between wt and cb2 mice ( supplementary fig .
however , cb1 mice displayed a significant basal reduction , compared to wt mice ( supplementary fig .
consistent with the findings in cocaine self - administration and locomotion , systemic administration of jwh133 ( 3 , 10 , 20 mg / kg , i.p . )
this reduction in nac da was blocked by am630 ( 10 mg / kg , i.p . ) in cb1 mice ( fig .
5b ) , suggesting that jwh133 s da - inhibiting effect is mediated by activation of cb2 receptors .
moreover , pretreatment with the same doses of jwh133 also significantly attenuated cocaine - enhanced nac da in wt ( fig .
5f ) mice . to determine whether this inhibition is mediated by activation of brain or peripheral cb2 receptors
, we also observed the effects of intranasal or intra - nac local administration of jwh133 on extracellular da .
we found that intranasal administration of jwh133 ( 100 g / nostril ) produced a significant reduction in extracellular nac da in wt and cb1 mice , but not in cb2 mice ( fig .
similarly , intra - nac local perfusion of jwh133 ( 11000 m ) significantly lowered extracellular da in both wt and cb1 mice , but not in cb2 mice ( fig .
in fact , an unexpected increase in extracellular da was observed in cb2 mice after local administration of jwh133 .
one possibility is that jwh133 may bind to other ( non - cb2 ) receptors in cb2 mice , producing an increase in extracellular da .
congruent with this finding , intra - nac local perfusion of am630 ( 1 , 10 , 100 m ) elevated extracellular da in a concentration - dependent manner in both wt and cb1 mice , but not in cb2 mice ( fig .
6c ) , suggesting that endocannabinoids tonically modulate nac da release by activation of brain cb2 receptors .
further , am630-enhanced extracellular da appears more robust in cb1 mice than in wt mice ( fig .
moreover , intra - nac local perfusion of am630 also blocked the reduction in extracellular nac da produced by systemic administration of jwh133 seen in wt and cb1 mice ( fig .
6c , d ) , suggesting that jwh133-induced inhibition of da release is mediated by activation of nac cb2 receptors .
the locations of microdialysis probes or microinjection cannulae were within the nac ( supplementary fig .
here we report that systemic administration of the cb2 receptor agonist jwh133 significantly and dose - dependently inhibits intravenous cocaine self - administration under both fr1 and pr reinforcement and inhibits cocaine - enhanced locomotion and extracellular nac da in wt and cb1 mice , but not in cb2 mice .
this effect was mimicked by gw405833 ( another selective cb2 receptor agonist ) , and blocked by am630 , a selective cb2 receptor antagonist , but not by am251 , a selective cb1 receptor antagonist , suggesting an effect mediated by activation of cb2 receptors .
further , intranasal microinjections of jwh133 , but not intravenous injections of the same micro - quantities of jwh133 as injected intranasally , also significantly and dose - dependently inhibited intravenous cocaine self - administration , suggesting an effect mediated by activation of brain , not peripheral , cb2 receptors .
this is further supported by the finding that local intra - nac administration of jwh133 also significantly inhibited cocaine self - administration in a dose - dependent manner , an effect that was blocked by intra - nac co - administration of am630 .
in addition , intra - nac local administration of jwh133 dose - dependently lowered , while am630 elevated , basal levels of locomotion and extracellular nac da .
intra - nac local perfusion of am630 blocked the reduction in cocaine self - administration and nac da produced by systemic administration of jwh133 .
these data suggest that both the behavioral and neurochemical effects of jwh133 are mediated by activation of brain cb2 receptors .
we note that systemic administration of am630 failed to alter , while intra - nac local administration of am630 elevated , extracellular da and locomotion , suggesting that local am630 administration is more effective than systemic administration .
this may be related to am630 s relatively poor pharmacokinetic properties and/or blood - brain barrier passage .
in addition , we also note that intra - nac am630 significantly elevated extracellular da and locomotion , but failed to alter cocaine self - administration .
this may be related to previous findings that locomotion is largely da - dependent , while cocaine self - administration is dependent on both da and non - da mechanisms .
we also note that pharmacological blockade of nac cb2 receptors elevated , while genetic deletion of cb2 receptors did not alter , basal extracellular da in the nac .
one possibility is that cb2 receptor deletion - induced disinhibition of nac da release may be compromised by actions in other brain loci that modulate the mesolimbic da system .
another possibility is that neuroadaptative processes may antagonize cb2 receptor inactivation - induced da neuronal disinhibition after cb2 receptor deletion .
whatever the exact mechanism(s ) , the present data strongly suggest that brain cb2 receptors functionally modulate the mesolimbic da system and da - related functions .
activation of brain cb2 receptors by jwh133 inhibits both the behavioral and neurochemical effects of cocaine . since jwh133 neither alters locomotor performance as assessed by the rotarod test , nor produces drug rewarding or aversive effects as assessed by i.v .
self - administration and conditioned place preference , jwh133-induced inhibition of cocaine self - administration is most likely mediated by attenuation of cocaine s rewarding efficacy secondary to the reduction in cocaine - enhanced nac da rather than by nonspecific locomotor impairment or malaise .
we fully recognize that these findings challenge the currently accepted opinion that selective cb2 receptor agonists have no cns effects .
this opinion is largely based upon previous reports that the selective cb2 receptor agonist am1241 neither inhibits locomotion or rotarod performance , nor produces catalepsy or hypothermia in rats or mice .
in addition , am1241 also failed to alter brain functional activity as assessed by pharmacological mri .
the ineffectiveness of am1241 may be related to the relatively lower doses ( 30 g-3.3 mg / kg ) used in those studies , relatively poor selectivity , and species differences in cb2 receptor response to am1241 .
for example , am1241 is reported to act as a full or partial agonist at human cb2 receptors , while acting as an inverse agonist at rodent cb2 receptors .
further , the analgesic effects produced by am1241 are reported to be blocked by the opioid receptor antagonist naloxone , suggesting that am1241 may interact with other , non - cannabinoid , receptors .
however , the cb2 receptor agonist gw405833 , at high doses ( 30100 mg / kg ) , produces significant cns effects such as analgesia , sedation and catalepsy , consistent with our finding that gw405833 ( 310 mg / kg ) significantly inhibits cocaine self - administration in mice .
the presence of cb2 receptors in the cns , in particular on neurons , has been subject to debate .
previous studies using in situ hybridization and northern blot assays failed to detect cb2 receptor mrna in brain .
however , recent studies with more sensitive rt - pcr and immunolabeling techniques have claimed to find significant cb2 receptor expression in microglia and subpopulations of neuronal cells in brain . by using highly sensitive and specific taqman probes ,
we have recently identified two cb2 receptor isoforms ( cb2a , cb2b ) in both brain and peripheral tissues , which display significant species differences in both structure and expression between humans , rats and mice .
it is now well accepted that cb2 receptors are expressed on microglia and a subset of neurons with levels increasing under certain pathological conditions such as neuroinflammation and brain injury .
first , a low density of cb2 receptors may be expressed on mesolimbic da neurons . since
cb2 receptors are gi / o coupled , activation of cb2 receptors on da neurons in the midbrain ventral tegmental area ( vta ) may directly inhibit vta da neurons and decrease nac da release , and therefore inhibit intravenous cocaine self - administration and cocaine - enhanced locomotion as observed in the present study .
although direct evidence of cb2 receptor expression in the mesolimbic da neurons is lacking at present , functional cb2 receptors are found in other neurons .
for example , cb2 receptor mrna is expressed on striatal gabaergic neurons in non - human primates , and activation of cb2 receptors inhibits gabaergic neurotransmission in the medial entorhinal cortex of the rat .
in addition , cb2 receptors are also found on neurons in the dorsal - root ganglion ( drg ) and spinal cord ( sc ) , and activation of cb2 receptors on drg - sc neurons inhibits neuronal response to noxious stimuli , thereby contributing to the antinociceptive effects of cb2 receptor agonists .
the second possibility is that activation of cb2 receptors located on microglial cells or astrocytes in the vta and/or nac may indirectly inhibit nac da release by releasing cytokines and inflammatory factors , thereby inhibiting cocaine self - administration and cocaine - enhanced locomotion as observed in the present study .
whatever the mechanisms , the present findings , for the first time , suggest that activation of brain cb2 receptors inhibits cocaine s rewarding and psychomotor - stimulating effects , which is congruent with a rapidly expanding corpus of published reports implicating brain cb2 receptors in modulating a variety of cns functions such as locomotion , pain , emesis , neurogenesis , and neuroprotection .
this finding not only challenges current views that cb2 receptors are absent from the cns and that cb2 receptor ligands lack cns effects , but also suggests that brain cb2 receptors may be a novel target for the pharmacotherapy of drug abuse and addiction .
 the presence and function of cannabinoid cb2 receptors in the brain have been subject to debate . 
 we report here that systemic , intranasal or intra - accumbens local administration of jwh133 , a selective cb2 receptor agonist , dose - dependently inhibits intravenous cocaine self - administration , cocaine - enhanced locomotion , and cocaine - enhanced accumbens dopamine ( da ) in wild - type ( wt ) and cb1 receptor - knockout ( cb1/ ) , but not cb2/ , mice . 
 this inhibition is mimicked by gw405833 , another cb2 receptor agonist with a different chemical structure , and is blocked by am630 , a selective cb2 receptor antagonist . 
 intra - accumbens jwh133 alone dose - dependently decreases , while intra - accumbens am630 elevates , extracellular da and locomotion in wt and cb1/ mice , but not in cb2/ mice . 
 intra - accumbens am630 also blocks the reduction in cocaine self - administration and extracellular da produced by systemic administration of jwh133 . 
 these findings , for the first time , suggest that brain cb2 receptors modulate cocaine s rewarding and locomotor - stimulating effects , likely by a da - dependent mechanism . 
RESULTS
JWH133 inhibits intravenous cocaine self-administration
JWH133 inhibits cocaine self-administration by activation of brain CB
JWH133 itself has no reinforcing or aversive effects
JWH133 inhibits cocaine-enhanced locomotion
JWH133 inhibits cocaine-enhanced extracellular DA in the nucleus accumbens
DISCUSSION
Supplementary Material