Delta FosB and AP-1-mediated transcription modulate cannabinoid CB1 receptor signaling and desensitization in striatal and limbic brain regions

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Graphical abstractRepeated Δ9-tetrahydrocannabinol (THC) administration produces cannabinoid type 1 receptor (CB1R) desensitization and downregulation, as well as tolerance to its in vivo pharmacological effects. However, the magnitude of CB1R desensitization varies by brain region, with CB1Rs in the striatum and its output nuclei undergoing less desensitization than other regions. A growing body of data indicates that regional differences in CB1R desensitization are produced, in part, by THC-mediated induction of the stable transcription factor, ΔFosB, and subsequent regulation of CB1Rs. The purpose of the present study was to determine whether THC-mediated induction of ΔFosB in the striatum inhibits CB1R desensitization in the striatum and output nuclei. This hypothesis was tested using bitransgenic mice with inducible expression of ΔFosB or ΔcJun, a dominant negative inhibitor of AP-1-mediated transcription, in specific forebrain regions. Mice were treated repeatedly with escalating doses of THC or vehicle for 6.5 days, and CB1R-mediated G-protein activation was assessed using CP55,940-stimulated [35S]GTPγS autoradiography. Overexpression of ΔFosB in striatal dopamine type 1 receptor-containing (D1R) medium spiny neurons (MSNs) attenuated CB1R desensitization in the substantia nigra, ventral tegmental area (VTA) and amygdala. Expression of ΔcJun in striatal D1R- and dopamine type 2 receptor (D2R)-containing MSNs enhanced CB1R desensitization in the caudate-putamen and attenuated desensitization in the hippocampus and VTA. THC-mediated in vivo pharmacological effects were then assessed in ΔcJun-expressing mice. Tolerance to THC-mediated hypomotility was enhanced in ΔcJun-expressing mice. These data reveal that ΔFosB and possibly other AP-1 binding proteins regulate CB1R signaling and adaptation in the striatum and limbic system.

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