The effects of microiontophoretically applied capsaicin and substance P on single neurones in the rat and cat brain.

Microiontophoretically applied capsaicin (10-480 nA) excited neurones in trigeminal nucleus caudalis (ntV) or potentiated their amino acid-induced excitation (20 of 23 neurones); inhibited one neurone; and had no effect on 2 neurones. Substance P (SP) excited 8 of 9 ntV neurones, and of these 8 neurones 6 were excited, one was depressed, and one was unaffected by capsaicin. Of 13 cerebellar neurones, 5 were depressed by capsaicin and 8 were unaffected. SP excited 3 of 5 cerebellar neurones. It is concluded that the excitatory action of capsaicin in ntV may be due to release of SP from neuronal structures and that the lack of excitatory effects seen in the cerebellum may reflect the absence of SP-containing neurones in this structure.