D2 autoreceptor switches CB2 receptor effects on [3 H]-dopamine release in the striatum.

CB2 receptors (CB2 R) are expressed in midbrain neurons. To evidence the control of dopamine release in dorsal striatum by CB2 R, we performed experiments of [3 H]-dopamine release in dorsal striatal slices. We found a paradoxical increase in K+ -induced [3 H]-dopamine release by CB2 R activation with GW 833972A and JWH 133 two selective agonist. To understand the mechanism involved, we tested for a role of the D2 autoreceptor in this effect; because in pallidal structures, the inhibitory effect of CB1 receptors (CB1 R) on GABA release is switched to a stimulatory effect by D2 receptors (D2 R). We found that the blockade of D2 autoreceptors with sulpiride prevented the stimulatory effect of CB2 R activation; in fact, under this condition, CB2 R decreased dopamine release, indicating the role of the D2 autoreceptor in the paradoxical increase. We also found that the effect occurs in nigrostriatal terminals, since lesions with 6-OH dopamine in the middle forebrain bundle prevented CB2 R effects on release. In addition, D2 -CB2 R interaction promoted cAMP accumulation, and the increase in [3 H]-dopamine release was prevented by PKA blockade. D2 -CB2 R coprecipitation and proximity ligation assay studies indicated a close interaction of receptors that could participate in the observed effects. Finally, intrastriatal injection of CB2 R agonist induced contralateral turning in amphetamine-treated rats, which was prevented by sulpiride, indicating the role of the interaction in motor behavior. Thus, these data indicate that the D2 autoreceptor switches, from inhibitory to stimulatory, the CB2 R effects on dopamine release, involving the cAMP â†’ PKA pathway in nigrostriatal terminals.

Presynaptic cannabinoid CB2 receptors modulate [3 H]-Glutamate release at subthalamo-nigral terminals of the rat.

Recent studies suggested the expression of CB2 receptors in neurons of the CNS, however, most of these studies have only explored one aspect of the receptors, i.e., expression of protein, messenger RNA, or functional response, and more complete studies appear to be needed to establish adequately their role in the neuronal function. Electron microscopy studies showed the presence of CB2r in asymmetric terminals of the substantia nigra pars reticulata (SNr), and its mRNA appeared is expressed in the subthalamic nucleus. Here, we explore the expression, source, and functional effects of such receptors by different experimental approaches. Through PCR and immunochemistry, we showed mRNA and protein for CB2rs in slices and primary neuronal cultures from subthalamus. GW833972A, GW405833, and JHW 133, three CB2r agonists dose-dependent inhibited K+ -induced [3 H]-Glutamate release in slices of SNr, and the two antagonist/inverse agonists, JTE-907 and AM630, but not AM281, a CB1r antagonist, prevented GW833972A effect. Subthalamus lesions with kainic acid prevented GW833972A inhibition on release and decreased CB2r protein in nigral synaptosomes, thus nigral CB2rs originate in subthalamus. Inhibition of [3 H]-Glutamate release was PTX- and gallein-sensitive, suggesting a Gißγ -mediated effect. P/Q Ca2+ -type channel blocker, ω-Agatoxin-TK, also inhibited the [3 H]-Glutamate release, this effect was occluded with GW833972A inhibition, indicating that the ßγ subunit effect is exerted on Ca2+ channel activity. Finally, microinjections of GW833972A in SNr induced contralateral turning. Our data showed that presynaptic CB2rs inhibit [3 H]-Glutamate release in subthalamo-nigral terminals by P/Q-channels modulation through the Gißγ subunit and suggested their participation in motor behavior.

Dopamine abnormalities in the neocortex of patients with temporal lobe epilepsy.

Experiments were designed to evaluate different variables of the dopaminergic system in the temporal cortex of surgically treated patients with temporal lobe epilepsy (TLE) associated with mesial sclerosis (MTLE, n=12) or with cerebral tumor or lesion (n=8). In addition, we sought to identify dopaminergic abnormalities in those patients with epilepsy that had comorbid anxiety and depression. Specifically, we investigated changes in dopamine and its metabolites, D1 and D2 receptors, tyrosine hydroxylase (TH) and dopamine transporter. Results obtained from patients with epilepsy were compared with those found in experiments using autopsy material. The neocortex of patients with MTLE demonstrated high D1 expression (1680%, p<0.05) and binding (layers I-II, 31%, p<0.05; layers V-VI, 28%, p<0.05), and decreased D2 expression (77%, p<0.05). The neocortex of patients with TLE secondary to cerebral tumor or lesion showed high expression of D1 receptors (1100%, p<0.05), and D2-like induced activation of G proteins (layers I-II, 503%; layers III-IV, 557%; layers V-VI, 964%, p<0.05). Both epileptic groups presented elevated binding to the dopamine transporter and low tissue content of dopamine and its metabolites. Analysis revealed the following correlations: a) D1 receptor binding correlated negatively with seizure onset age and seizure frequency, and positively with duration of epilepsy; b) D2 receptor binding correlated positively with age of seizure onset and negatively with duration of epilepsy; c) dopamine transporter binding correlated positively with duration of epilepsy and frequency of seizures; d) D2-like induced activation of G proteins correlated positively with the age of patients. When compared with autopsies and patients with anxiety and depression, patients without neuropsychiatric disorders showed high D2-like induced activation of G proteins, an effect that correlated positively with age of patient and seizure onset age, and negatively with duration of epilepsy. The present study suggests that alterations of the dopaminergic system result from epileptic activity and could be involved in the physiopathology of TLE and the comorbid anxiety and depression.