No evidence for functional imidazoline receptors on locus coeruleus neurons

alpha 2-Adrenoceptor agonists inhibit the firing of locus coeruleus (LC) neurons. It was recently observed that the alpha-adrenoceptor agonists clonidine, rilmenidine and cirazoline, when injected intravenously in anaesthetized rats pretreated with the irreversible alpha 2-adrenoceptor antagonist N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ), excite the LC. The effect was attributed to activation of I1 imidazoline receptors. The aim of the present experiments was to characterize the direct effect of alpha 2-adrenoceptor and I1 imidazoline receptor agonists on LC neurons. Electrical activity of LC neurons was extracellularly recorded in midpontine slices prepared from the rat brain. Concentration-response curves were obtained for the alpha 2-agonist noradrenaline and the mixed I1/alpha 2-receptor agonists clonidine, rilmenidine and moxonidine in slices without treatment and in slices treated with 6-chloro-N-methyl-2,3,4,5-tetrahydro-1H-3-benzazepine (SK&F86466) or EEDQ, alpha 2-adrenoceptor antagonists with low affinity for I1 and I2 imidazoline receptors, respectively. All four agonists concentration-dependently reduced the firing rate of the neurons, with full inhibition at higher concentrations. SK&F86466 shifted the concentration-response curves of the agonists to the right; the calculated antagonist dissociation constants are compatible with an effect of the agonists on alpha 2-adrenoceptors. EEDQ completely prevented the inhibition by the agonists. Neither in SK&F86466- nor in EEDQ-treated slices was an excitation by clonidine, rilmenidine and moxonidine observed. We conclude that the LC neurons do not possess functional I1 (and also no I2) imidazoline receptors. The effects of noradrenaline, clonidine, rilmenidine and moxonidine on the neurons can be fully explained with an interaction with inhibitory alpha 2-adrenoceptors (probably of the alpha 2D subtype). The excitation of the LC by imidazoline receptor agonists under in vivo conditions, hence, is not a direct effect on the neurons of the LC.