The effects of capsaicin pre-treatment on the responses of single neurones to sensory stimuli in the trigeminal nucleus caudalis of the rat: evidence against a role for substance P as the neurotransmitter serving thermal nociception.

Rats were systemically pre-treated with capsaicin either on the first day of life or at an age of 1 month. Both treatments were found to deplete substance P levels in the trigeminal nucleus caudalis (55.6% and 57.9% depletions, respectively). Extracellular single neurone recordings in the trigeminal nucleus caudalis revealed that neither type of capsaicin treatment greatly altered the proportions of neurones responding to non-noxious or noxious mechanical stimulation of the face. However, the proportion of mechanically-nociceptive neurones also responding to noxious thermal stimulation was greatly reduced in neonatally-treated, but not adult-treated rats. As both methods of capsaicin treatment caused similar depletions of substance P, it is concluded that this peptide may not be the neurotransmitter of afferent fibres to the trigeminal nucleus caudalis signalling thermal nociception.