[NADPH-diaphorase activity and Fos expression in brainstem nuclei involved in cardiovascular regulation following intracarotid injection of capsaicin].

ABSTRACT

The present study was undertaken to define whether intracarotid injection of capsaicin induces Fos expression associated with the activation of NOS-containing neurons in brainstem nuclei by combining the immunocytochemical method for Fos with NADPH-d histochemical technique for NOS. The results obtained are as follows (1) Intracarotid injection of capsaicin caused a significant increase of Fos-like immunoreactive neurons in area postrema (AP), nucleus tractus solitarius (NTS), paragigantocellularis lateralis (PGL) and locus coeruleus (LC), without influence upon the neurons of raphe nuclei (RN) and periaqueductal gray (PAG). (2) NO-containing neurons in PGL and NTS and the double-labeled neurons in PGL were also increased significantly following intracarotid injection of capsaicin. Small numbers of NO-containing neurons were found in LC, but there was no change in the number of NO-containing neurons in RN and PAG. No NADPH-d histochemical activity could be found in AP. (3) The above responses to capsaicin were significantly inhibited by pretreatment with either a capsaicin receptor antagonist ruthenium red or a NMDA receptor antagonist MK-801. The above results indicate that intracarotid injection of capsaicin may activate the neurons in brainstem nuclei involved in cardiovascular regulation, and that NO only plays an indirect role in the modulation of the responses of brainstem nuclei to capsaicin. These effects of capsaicin are mediated by capsaicin receptors with involvement of glutamate.