Activation of mu-opioid receptors inhibits synaptic inputs to spinally projecting rostral ventromedial medulla neurons.

The rostral ventromedial medulla (RVM) is a major locus for the descending control of nociception and opioid analgesia. However, it is not clear how opioids affect synaptic inputs to RVM neurons. In this study, we determined the effect of mu-opioid receptor activation on excitatory and inhibitory synaptic transmission in spinally projecting RVM neurons. RVM neurons were retrogradely labeled with a fluorescent tracer injected into the dorsal horn of the spinal cord in rats. Whole-cell voltage-clamp recordings were performed on labeled RVM neurons in brain slices in vitro. The mu-receptor agonist [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin (DAMGO, 1 microM) significantly decreased the amplitude of evoked excitatory postsynaptic currents (EPSCs) in 52% (9 of 17) of labeled cells. DAMGO also significantly reduced the amplitude of evoked inhibitory postsynaptic currents (IPSCs) in 69% (11 of 16) of cells examined. Furthermore, DAMGO significantly decreased the frequency of miniature EPSCs in 55% (15 of 27) of cells and significantly decreased the frequency of miniature IPSCs in all 12 cells studied. Although most EPSCs and IPSCs were mediated by glutamate and GABA, the nicotinic and glycine receptor antagonists attenuated EPSCs and IPSCs, respectively, in some labeled RVM neurons. Immunocytochemical labeling revealed that only 35% of recorded RVM neurons were tryptophan hydroxylase-positive, and 15% cells had GABA immunoreactivity. Thus, this study provides important functional evidence that activation of mu-opioid receptors decreases the release of both excitatory and inhibitory neurotransmitters onto most spinally projecting RVM neurons.