Distribution of substance P-responsive and nociceptive neurones in relation to substance P-immunoreactivity within the caudal trigeminal nucleus of the rat.

Substance P is a peptide which is found in small diameter primary afferent fibres and may have a function in nociceptive afferent transmission. In order to study the role of substance P in sensory processes in depth, we have compared the distributions of nociceptive neurones and substance P-responsive neurones with the distribution of substance P in the caudal trigeminal nucleus of the rat. It was found that substance P-like immunoreactivity was located primarily in the superficial layers of nucleus caudalis (equivalent to laminae I and II of the dorsal horn) and in more ventromedially located areas (equivalent to laminae V and VI). The distribution was found to be in good agreement with the distribution of nociceptive neurones. Iontophoretically applied substance P had predominantly excitatory actions on both nociceptive and non-nociceptive nucleus caudalis neurones, although the peptide did appear to be slightly more likely to excite nociceptive neurones. Similarly, the peptide appeared slightly more likely to be excitatory in areas of nucleus caudalis showing substance P staining, but excitations were also predominantly seen in areas containing little or no apparent substance P staining. These results are consistent with the proposed role for substance P as a nociceptive afferent neurotransmitter. However, it is also possible that the peptide performs other functions in the processing of sensory information.