Antagonizing effects of beta-adrenergic blockers on lucus coeruleus-induced inhibition of trigeminal nucleus neurons.

Effects of alpha- and beta-adrenergic blocking agents applied into the lateral ventricle were studied on the relay neuron in the rostral part of spinal trigeminal nucleus (STN) of cats. Conditioning stimulation of the locus coeruleus (LC) and sensory cortex (SC) inhibited the orthodromic spike generation in STN relay neuron without affecting the antidromic spike, as already reported, and re-confirmed herein. The LC-induced inhibition of orthodromic spike was significantly reduced by intraventricular administration of beta-blockers, MJ 1999 (5 mg) and propranolol (0.5 mg), while the SC-induced inhibition of orthodromic one was not modified by the beta-blockers. The antidromic spike in STN relay neuron per se remained unaffected by these treatments. Intraventricular administration of alpha-blockers such as phentolamine and phenoxybezamine produced no alterations of the LC- and SC-induced inhibition of orthodromic spike. As the beta-blockers produced a selective antagonism, noradrenaline originating in LC probably acts as an inhibitory transmitter on the STN relay neuron and is mediated by beta-receptor.