Kappa opioid receptor activation depresses excitatory synaptic input to rat locus coeruleus neurons in vitro

Intracellular recordings were made from neurons of the rat locus coeruleus contained within a brain slice maintained in vitro. When applied to the slice in known concentrations the selective kappa opioid receptor agonist trans-(+)-3,4-dichloro-N-methyl-[2-(1-pyrrolidinyl)cyclohexyl] benzeneacetamide methane sulphonate (U50488) (0.01-1 microM) produced a concentration-dependent depression of the excitatory post-synaptic potential evoked by electrical stimulation of afferent inputs to the locus coeruleus. This effect was antagonized by naloxone with an apparent dissociation equilibrium constant (Kd) of 28 nM. U50488 did not completely abolish the EPSP. Over the same concentration range U50488 had no effect on the resting membrane potential, input resistance or action potential waveform of locus coeruleus neurons, nor did U50488 depress the depolarization produced by local application of L-glutamic acid. The mu opioid receptor agonists [D-Ala2, NMe Phe4, Gly-ol5] enkephalin (0.003-1 microM) and [D-Ala2, NMe Phe4, Met(O)5] enkephalinol (0.003-1 microM) caused a membrane hyperpolarization concomitant with a fall in neuronal input resistance. These effects were concentration-dependent and antagonized by naloxone with an apparent Kd of 1.5 nM. Mu agonists also caused a depression of the tetrodotoxin resistant action potential. An in vitro autoradiographic study of [3H]bremazocine binding within the locus coeruleus revealed that, although the majority of binding appears to be to mu sites, a significant proportion was displaceable by unlabelled U50488 and thus represented kappa binding sites. The possibility that kappa opioid receptors may be located pre-synaptically within the locus coeruleus, and that activation of these receptors depresses excitatory synaptic input, is discussed.