Autoradiographic analysis of cannabinoid receptor binding and cannabinoid agonist-stimulated [S]GTPγS binding in morphine-dependent mice

The present study was designed to test the possible existence of changes in brain cannabinoid receptors in morphine-dependent mice. To this end, we compared cannabinoid receptor binding and WIN 55,212-2-stimulated ³⁵S]guanylyl-5′-O-(γ-thio)-triphosphate (³⁵S]GTPγS) binding in several brain regions of mice chronically exposed to morphine or saline. The existence of opiate dependence in morphine-injected mice was assessed by analyzing the well-known jumping behavior induced by the blockade of opioid receptors with naloxone, whereas these animals were unresponsive to the blockade of cannabinoid receptors with SR141716. The different structures analyzed exhibited similar cannabinoid receptor binding levels in morphine-dependent and control mice, with the only exception of the globus pallidus, which exhibited a very small, but statistically significant, increase. In addition, the activation of cannabinoid receptors with WIN 55,212-2 increased ³⁵S]GTPγS binding in most of the structures examined. The increase was of similar magnitude in morphine-dependent and control mice, except in the substantia nigra, where morphine-dependent mice exhibited lesser ³⁵S]GTPγS binding levels in basal conditions, although a significantly higher WIN 55,212-2-stimulated binding. Other structures, such as the central gray substance, where there was a poor agonist-induced stimulation in control mice, exhibited, however, higher levels of WIN 55,212-2-stimulated ³⁵S]GTPγS binding in morphine-dependent mice, whereas these animals tended to exhibit a higher ³⁵S]GTPγS binding levels in basal conditions, although a lesser and not statistically significant WIN 55,212-2-stimulated binding, in the deep layers of the cerebral cortex. Thus, the data support the potential existence of a specific effect of morphine in the coupling of cannabinoid receptors to GTP-binding proteins, rather than on receptor binding, although this was observed only in the substantia nigra and central gray substance.