Opioid actions on single nucleus raphe magnus neurons from rat and guinea‐pig in vitro.

1. Intracellular recordings were made from neurons of the nucleus raphe magnus (NRM) from rat (n = 128) and guinea‐pig (n = 115). Two types of cells were found in each, primary (103 in rat, 27 in guinea‐pig) and secondary cells (25 in rat, 88 in guinea‐pig). 2. Primary cells had input resistances of 186 +/‐ 9 M omega (n = 9) in rat and 255 +/‐ 50 M omega (n = 11) in guinea‐pig. The action potential in each was about 1.5 ms in duration. Synaptic potentials were evoked by focal electrical stimulation and consisted of both gamma‐aminobutyric acid (GABA) and excitatory amino acid components. 3. Morphine, [Met5]enkephalin (ME) and [D‐Ala2,N‐Me‐Phe4, Gly5‐ol]enkephalin (DAMGO) depressed the amplitude of the GABA‐mediated synaptic potential by a maximum of 50‐65% and had little effect on the excitatory amino acid‐mediated synaptic potential. There was no effect of these opioids on the resting membrane potential or input resistance of primary cells in rat or guinea‐pig. 4. Secondary cells had short duration action potentials (less than 1 ms) and an input resistance of 354 +/‐ 47 M omega in rat (n = 6) and 290 +/‐ 40 M omega in guinea‐pig (n = 15). The synaptic potential observed in the cells of this group was mediated by activation of only excitatory amino acid receptors. 5. ME hyperpolarized and/or abolished the spontaneous firing in sixteen out of twenty‐four neurons in the secondary group from rat and eight out of eighty‐four neurons from guinea‐pig. ME induced an outward current at ‐60 mV that reversed polarity at potentials more negative than ‐92 +/‐ 3 mV in rat (n = 6) and ‐98 +/‐ 2 mV in guinea‐pig (n = 18). The reversal potential of the opioid current was shifted to less negative potentials when the external potassium concentration was increased, as predicted by the Nernst equation. 6. The morphology of the two types of cells were distinguishable in that primary cells were oval (29 x 18 microns in rat; 36 x 19 microns in guinea‐pig) with two to four thick tapering dendrites that branched within 50 microns of the cell body. Secondary cells were generally round or oval (about 24 x 13 microns in rat; 27 x 17 microns in guinea‐pig) with two to five thin non‐tapering dendrites.(ABSTRACT TRUNCATED AT 400 WORDS)