Laura J. Sim-Selley

Δ⁹-Tetrahydrocannabinol (THC), the main psychoactive component of marijuana, produces motor and motivational effects via interactions with the dopaminergic system in the caudate-putamen and nucleus accumbens. However, the molecular events that underlie these interactions after THC treatment are not well understood. Our study shows that pretreatment with dopamine D₁ receptor (D₁R) antagonists before repeated administration of THC attenuated induction of Δ FBJ murine osteosarcoma viral oncogene homolog B (ΔFosB) in the nucleus accumbens, caudate-putamen, amygdala, and prefrontal cortex. Anatomical studies showed that repeated THC administration induced ΔFosB in D₁R-containing striatal neurons. Dopamine signaling in the striatum involves phosphorylation-specific effects of the dopamine- and cAMP-regulated phosphoprotein Mr 32 kDa (DARPP-32), which regulates protein kinase A signaling. Genetic deletion of DARPP-32 attenuated ΔFosB expression measured after acute, but not repeated, THC administration in both the caudate-putamen and nucleus accumbens. THC was then acutely or repeatedly administered to wild-type (WT) and DARPP-32 knockout (KO) mice, and in vivo responses were measured. DARPP-32 KO mice exhibited enhanced acute THC-mediated hypolocomotion and developed greater tolerance to this response relative to the WT mice. Agonist-stimulated guanosine 5′-O-(3-[³⁵S]thio)triphosphate ([³⁵S]GTPγS) binding showed that cannabinoid-stimulated G-protein activity did not differ between DARPP-32 KO and WT mice treated with vehicle or repeated THC. These results indicate that D₁Rs play a major role in THC-mediated ΔFosB induction in the forebrain, whereas the role of DARPP-32 in THC-mediated ΔFosB induction and modulation of motor activity appears to be more complex.

Repeated administration of Δ⁹-tetrahydrocannabinol (THC), the primary psychoactive constituent of Cannabis sativa, induces profound tolerance that correlates with desensitization and downregulation of CB₁ cannabinoid receptors in the CNS. However, the consequences of repeated administration of the endocannabinoid N-arachidonoyl ethanolamine (anandamide, AEA) on cannabinoid receptor regulation are unclear because of its rapid metabolism by fatty acid amide hydrolase (FAAH). FAAH^−/− mice dosed subchronically with equi-active maximally effective doses of AEA or THC displayed greater rightward shifts in THC dose–effect curves for antinociception, catalepsy, and hypothermia than in AEA dose–effect curves. Subchronic THC significantly attenuated agonist-stimulated [³⁵S]GTPγS binding in brain and spinal cord, and reduced [³H]WIN55,212-2 binding in brain. Interestingly, AEA-treated FAAH^−/− mice showed less CB₁ receptor downregulation and desensitization than THC-treated mice. Experiments examining tolerance and cross-tolerance indicated that the behavioral effects of THC, a low efficacy CB₁ receptor agonist, were more sensitive to receptor loss than those of AEA, a higher efficacy agonist, suggesting that the expression of tolerance was more affected by the intrinsic activity of the ligand at testing than during subchronic treatment. In addition, the CB₁ receptor antagonist, rimonabant, precipitated a markedly reduced magnitude of withdrawal in FAAH^−/− mice treated subchronically with AEA compared with mice treated repeatedly with THC. The findings that repeated AEA administration produces lesser adaptive changes at the CB₁ receptor and has reduced dependence liability compared with THC suggest that pharmacotherapies targeting endocannabinoid catabolic enzymes are less likely to promote tolerance and dependence than direct acting CB₁ receptor agonists.

Sphingosine‐1‐phosphate (S1P) is a ubiquitous, lipophilic cellular mediator that acts in part by activation of G‐protein‐coupled receptor. Modulation of S1P signaling is an emerging pharmacotherapeutic target for immunomodulatory drugs. Although multiple S1P receptor types exist in the CNS, little is known about their function. Here, we report that S1P stimulated G‐protein activity in the CNS, and results from [³⁵S]GTPγS autoradiography using the S1P₁‐selective agonist SEW2871 and the S1P_1/3‐selective antagonist VPC44116 show that in several regions a majority of this activity is mediated by S1P₁receptors. S1P receptor activation inhibited glutamatergic neurotransmission as determined by electrophysiological recordings in cortical neuronsin vitro, and this effect was mimicked by SEW2871 and inhibited by VPC44116. Moreover, central administration of S1P producedin vivoeffects resembling the actions of cannabinoids, including thermal antinociception, hypothermia, catalepsy and hypolocomotion, but these actions were independent of CB₁receptors. At least one of the central effects of S1P, thermal antinociception, is also at least partly S1P₁receptor mediated because it was produced by SEW2871 and attenuated by VPC44116. These results indicate that CNS S1P receptors are part of a physiologically relevant and widespread neuromodulatory system, and that the S1P₁receptor contributes to S1P‐mediated antinociception.