Different mechanisms for dopaminergic excitation induced by opiates and cannabinoids in the rat midbrain

1. The mechanism underlying morphine and cannabinoid-induced excitation of meso-accumbens and nigro-striatal dopaminergic neurons was investigated by extracellular single unit recording techniques coupled with antidromic activation from the nucleus accumbens and striatum respectively, in unanesthetized rats. 2. The intravenous administration of cumulative doses (1-4 mg/kg) of morphine, dose-dependently increased the firing rate of dopaminergic neurons projecting to the nucleus accumbens and neostriatum, while the same doses inhibited the activity of pars reticulata neurons of the substantia nigra. Both effects were antagonized by naloxone (0.1 mg/kg i.v.) but not by the selective CBI receptor antagonist SR 141716A (1 mg/kg i.v.). 3. The intravenous administration of cumulative doses (0.125-0.5 mg/kg) of Delta(9)-tetrahydrocannabinol (Delta(9)-THC) also increased the firing rate of meso-accumbens and nigro-striatal dopaminergic neurons; this effect was antagonized by SR 141716A (1 mg/kg i.v.), but not by naloxone. 4. Furthermore, nor Delta(9)-THC up to a dose of 1 mg/kg, maximally effective in stimulating dopamine neurons, neither SR 141716A (1 mg/kg i.v.) at a dose able to reverse the stimulatory effect of Delta(9)-THC on dopamine cells, did alter the activity; of SNr neurons. 5. The results indicate that morphine and Delta(9)-THC activate dopaminergic neurons through distinct receptor-mediated mechanisms; morphine may act by removing the inhibitory input from substantia nigra pars reticulata neurons tan effect mediated by mu-opioid receptors). Alternatively, the Delta(9)-THC-induced excitation of dopaminergic neurons seems to be mediated by CB1 cannabinoid receptors, while neither mu-opioid receptors nor substantia nigra pars reticulata neurons are involved.