Activation of kappa-opioid receptors depresses electrically evoked excitatory postsynaptic potentials on 5-HT-sensitive neurones in the rat dorsal raphé nucleus in vitro.

Intracellular recordings from dorsal raphé neurones in slices from rat brains were used to study the actions of kappa-opioid receptor agonists on an excitatory postsynaptic potential (epsp) evoked by local electrical stimulation of afferent terminals. The epsp was observed on all 5-HT-sensitive neurones and was blocked by 1 microM TTX. The epsp was reduced in a dose-dependent manner by the specific kappa-opioid receptor agonist [5R-(5 alpha,7 alpha,8 beta)]-N-methyl-N-[7-(1-pyrrolidinyl)-1- oxaspiro[4.5]dec-8-yl]-4-benzofuranacetamide monohydrochloride (CI-977) (1-100 nM). The effects of CI-977 were blocked by the specific kappa-opioid receptor antagonist norbinaltorphimine (NorBNI) (0.1-1 microM). In the presence of the GABAA receptor antagonists picrotoxin and bicuculline (30 microM), CI-977 still had its depressant action on the epsp. Application of the excitatory amino acid receptor antagonists either kynurenic acid (0.5-1 mM) or 6-cyano-2,3-dihydro-7-nitro-quinoxaline-2,3-dione (CNQX) (30 microM) and DL-2-amino-5-phosphonovaleric acid (APV) reduced both the peak and area of the epsp suggesting that the main component of the epsp evoked by electrical stimulation was largely due to release of excitatory amino acids from afferent terminals. Using potassium chloride-filled recording electrodes an epsp which was only partially occluded by kynurenic acid or CNQX and APV was seen on some neurones, this residual epsp was insensitive to CI-977 but was blocked by 30 microM picrotoxin and bicuculline. The specific mu-opioid receptor agonist, DAGOL, had no consistent effect on the fast epsp. Longer duration electrical stimuli produced a slow inhibitory postsynaptic potential (ipsp) and a long duration increase in firing. CI-977 did not affect either the slow 5-HT-mediated ipsp which was blocked by spiperone or the slow noradrenaline-mediated increase in firing which was sensitive to prazosin. CI-977 did not change the depolarizing response to brief applications of either glutamic acid or N-methyl-D-aspartic acid (NMDA). CI-977, NorBNI, naloxone, DAGOL, picrotoxin, bicuculline and kynurenic acid had no consistent effects on the resting postsynaptic membrane potential or conductance. Under voltage-clamp conditions CI-977 had no effect on a membrane current resembling IA. These results suggest that kappa-opioid receptors are present on the terminals of afferents which release excitatory amino acids onto the 5-HT-sensitive neurones in the raphé.