Capsaicin-induced excitation of locus coeruleus neurons

The noradrenergic pontine nucleus locus coeruleus (LC) seems to be involved in sensory processing. In the present study, the effect of capsaicin, a drug which specifically interferes with chemosensitive primary afferents, on LC firing rate was analysed, utilizing electrophysiological techniques. In control rats, low doses of capsaicin (1-8 micrograms kg-1 i.v.) caused a marked excitation of LC units. The effect was instantaneous in onset but short-lasting and no signs of tachyphylaxis were observed. The excitation was maintained in adult rats treated as neonates with high doses of capsaicin (50 mg kg-1 s.c.) but almost totally prevented by pretreatment of adult rats with high doses of capsaicin (300 mg kg-1 s.c.). According to our histological data, using selective silver impregnation techniques, the LC seems not to receive innervation by sensory primary afferents. It is proposed that the capsaicin-induced excitation of LC neurons is a centrally mediated effect and might, in part, be involved in the analgetic effect induced by the drug.