Competitive NMDA antagonists enhance the catalepsy induced by delta 9-tetrahydrocannabinol in mice

Effects of opioids, cannabinoids, and vanilloids on body temperature

Cannabinoid and opioid drugs produce marked changes in body temperature. Recent findings have extended our knowledge about the thermoregulatory effects of cannabinoids and opioids, particularly as related to delta opioid receptors, endogenous systems, and transient receptor potential (TRP) channels. Although delta opioid receptors were originally thought to play only a minor role in thermoregulation compared to mu and kappa opioid receptors, their activation has been shown to produce hypothermia in multiple species. Endogenous opioids and cannabinoids also regulate body temperature. Mu and kappa opioid receptors are thought to be in tonic balance, with mu and kappa receptor activation producing hyperthermia and hypothermia, respectively. A particularly intense research focus is TRP channels, where TRPV1 channel activation produces hypothermia whereas TRPA1 and TRPM8 channel activation causes hyperthermia. The marked hyperthermia produced by TRPV1 channel antagonists suggests these warm channels tonically control body temperature. A better understanding of the roles of cannabinoid, opioid, and TRP systems in thermoregulation may have broad clinical implications and provide insights into interactions among neurotransmitter systems involved in thermoregulation.

Effects of opioids, cannabinoids, and vanilloids on body temperature

Cannabinoid and opioid drugs produce marked changes in body temperature. Recent findings have extended our knowledge about the thermoregulatory effects of cannabinoids and opioids, particularly as related to delta opioid receptors, endogenous systems, and transient receptor potential (TRP) channels. Although delta opioid receptors were originally thought to play only a minor role in thermoregulation compared to mu and kappa opioid receptors, their activation has been shown to produce hypothermia in multiple species. Endogenous opioids and cannabinoids also regulate body temperature. Mu and kappa opioid receptors are thought to be in tonic balance, with mu and kappa receptor activation producing hyperthermia and hypothermia, respectively. A particularly intense research focus is TRP channels, where TRPV1 channel activation produces hypothermia whereas TRPA1 and TRPM8 channel activation causes hyperthermia. The marked hyperthermia produced by TRPV1 channel antagonists suggests these warm channels tonically control body temperature. A better understanding of the roles of cannabinoid, opioid, and TRP systems in thermoregulation may have broad clinical implications and provide insights into interactions among neurotransmitter systems involved in thermoregulation.

N-methyl-D-aspartate antagonists and WIN 55212-2 [4,5-dihydro-2-methyl-4(4-morpholinylmethyl)-1-(1-naphthalenyl-carbonyl)-6H-pyrrolo[3,2,1-i,j]quinolin-6-one], a cannabinoid agonist, interact to produce synergistic hypothermia

CB(1) cannabinoid receptors mediate profound hypothermia when cannabinoid agonists are administered to rats. Glutamate, the principal excitatory neurotransmitter in the central nervous system (CNS), is thought to tonically increase body temperature by activating N-methyl-D-aspartate (NMDA) receptors. Because NMDA antagonists block cannabinoid-induced antinociception and catalepsy, intimate glutamatergic-cannabinoid interactions may exist in the CNS. The present study investigated the effect of two NMDA antagonists on the hypothermic response to WIN 55212-2 [4,5-dihydro-2-methyl-4(4-morpholinylmethyl)-1-(1-naphthalenyl-carbonyl)-6H-pyrrolo[3,2,1-i,j]quinolin-6-one], a selective cannabinoid agonist, in rats. WIN 55212-2 (1-10 mg/kg i.m.) produced dose-dependent hypothermia that peaked 60 to 180 min postinjection. Dextromethorphan (5-75 mg/kg i.m.), a noncompetitive NMDA antagonist, or LY 235959 [(-)-6-[phosphonomethyl-1,2,3,4,4a,5,6,7,8,8a-decahydro-isoquinoline-2-carboxylate]](1-4 mg/kg i.m.), a competitive and highly selective NMDA antagonist, evoked hypothermia in a dose-sensitive manner, suggesting that endogenous glutamate exerts a hyperthermic tone on body temperature. A dose of dextromethorphan (10 mg/kg) that did not affect body temperature by itself potentiated the hypothermic response to WIN 55212-2 (1, 2.5, or 5 mg/kg). The enhancement was strongly synergistic, indicated by a 2.7-fold increase in the relative potency of WIN 55212-2. Similarly, a dose of LY 235959 (1 mg/kg) that did not affect body temperature augmented the hypothermia associated with a single dose of WIN 55212-2 (2.5 mg/kg), thus confirming that NMDA receptors mediated the synergy. We have demonstrated previously that CB(1) receptors mediate WIN 55212-2-evoked hypothermia in rats. The present data are the first evidence that NMDA antagonists exert a potentiating effect on cannabinoid-induced hypothermia. Taken together, these data suggest that interactions between NMDA and CB(1) receptors produce synergistic hypothermia.

Motor actions of cannabinoids in the basal ganglia output nuclei

The levels of CB1 cannabinoid receptors in the basal ganglia are the highest in the brain, comparable to the levels of dopamine receptors, a major transmitter in the basal ganglia. This localization of receptors is consistent with the profound effects on motor function exerted by cannabinoids. The output nuclei of the basal ganglia, the globus pallidus (GP) and substantia nigra reticulata (SNr), apparently lack intrinsic cannabinoid receptors. Rather, the receptors are located on afferent terminals, the striatum being the major source. Cannabinoids blocked the inhibitory action of the striatal input in the SNr. Furthermore, cannabinoids blocked the excitatory effect of stimulation of the subthalamic input to the SNr revealing, along with data from in situ hybridization studies, that this input is another likely source of cannabinoid receptors to the SNr. Similar actions of cannabinoids were observed in the GP. Behavioral studies further revealed that the action of cannabinoids differs depending upon which input to the output nuclei of the basal ganglia is active. The inhibitory striatal input is quiescent and the cannabinoid action is observable only upon stimulation of the striatum, while the noticeable effect of cannabinoids under basal conditions would be on the tonically active subthalamic input. These data suggest that the recently discovered endogenous cannabinergic system exerts a major modulatory action in the basal ganglia by its ability to block both the major excitatory and inhibitory inputs to the SNr and GP.

Effects of intrapallidal cannabinoids on rotational behavior in rats: interactions with the dopaminergic system

The effect of unilateral intrapallidal cannabinoid receptor stimulation on rotational behavior in rats was explored. The potent cannabinoid agonist CP55,940 (5 microg/0.5 microl) induced ipsilateral turning when microinjected unilaterally into the globus pallidus. The D2 dopamine agonist quinpirole reversed this ipsilateral rotation but failed to affect motor behavior on its own. Finally, the D1 dopamine agonist SKF 82958 inhibited movement when administered into the globus pallidus, and this effect was not additive with CP55,940.

NMDA antagonists: antiepileptic-neuroprotective drugs with diversified neuropharmacological profiles

In conclusion, NMDA antagonists as anticonvulsants are especially active in preventing the generalization of the behavioural and electrical seizures and display a typical spectrum of in vitro antiepileptiform activities. In addition, based on in vitro and in vivo limbic kindled studies, the drugs should be regarded more as an antiepileptiform than as an anticonvulsant drugs. As neuroprotective drugs, NMDA antagonists are effective against many types of neuronal injury and show a window of activity which does not exceed 1-2 h, thus suggesting an influence of NMDA receptors in the 'early' or 'acute' mechanisms of brain damage. Among NMDA antagonists, glycine antagonists or the morphinans dextromethorphan and dextrorphan showed a spectrum of antiepileptiform and neuroprotective activities broader than other NMDA antagonists. The primary pharmacological activities of NMDA antagonists are accompanied by some effects including perturbation of many sensory, psychological or motor processes. Typical behavioural and EEG changes were also induced by the drugs. In spite of the side-effects elicited by the drugs, differential effects detected among the various classes of NMDA antagonists (i.e. lack of induction of typical EEG-behavioural effects and of typical cortical neurotoxicity) might render some of these suitable for full clinical application as anticonvulsant-neuroprotective drugs.