Beta-receptor involvement in locus coeruleus-induced inhibition of spinal trigeminal nucleus neurons: Microiontophoretic and HRP studies

Abstract

Microiontophoretic and HRP studies were performed on cats anesthetized with α-chloralose to determine whether or not the locus coeruleus (LC)- and noradrenaline (NA)-induced inhibition of relay neurons in the subnucleus oralis of the spinal trigeminal nucleus (STN) is mediated by ß-adrenergic receptors. The inhibition of orthodromic spike generation upon intracranial trigeminal nerve stimulation by LC conditioning stimulation and microiontophoretically applied NA (100–200 nA) was antagonized during microiontophoretic application of sotalol, a ß-adrenergic, but not affected by phentolamine, an α-adrenergic antagonist. When HRP at doses of 300–500 nA was applied for 5–15 min to the immediate vicinity of the STN relay or interneuron, which was electrophysiologically identified by stimulating the ipsilateral trigeminal nerve and contralateral medial lemniscus, the injection site was localized to an area 0.3 mm in diameter and HRP-reactive cells were found in the ipsilateral LC, dorsal raphe nucleus and periaqueductal gray ventral to the aqueduct. These results strongly suggest that NA released from the nerve terminals of LC cells inhibits transmission in the STN relay neuron via ß-adrenergic receptors.