Melis, Miriam and Gessa, Gian Luigi and Diana, Marco (2000) Different mechanisms for dopaminergic excitation induced by opiates and cannabinoids in the rat midbrain. Progress in Neuro-Psychopharmacology and Biological Psychiatry, Vol. 24 (6), p. 993-1006. ISSN 0278-5846. Article.
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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 CB1 receptor antagonist SR 141716A (1 mg/kg i.v.).
3. The intravenous administration of cumulative doses (0.125–0.5 mg/kg) of Δ9-tetrahydrocannabinol (Δ9-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 Δ9-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 Δ9-THC on dopamine cells, did alter the activity of SNr neurons.
5. The results indicate that morphine and Δ9-THC activate dopaminergic neurons through distinct receptor-mediated mechanisms; morphine may act by removing the inhibitory input from substantia nigra pars reticulata neurons (an effect mediated by μ-opioid receptors). Alternatively, the Δ9-THC-induced excitation of dopaminergic neurons seems to be mediated by CB1 cannabinoid receptors, while neither μ-opioid receptors nor substantia nigra pars reticulata neurons are involved.
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