Effects of substance P on nociceptive and non-nociceptive trigeminal brain stem neurons

As little information is available on the chemistry of synaptic transmission in trigeminal brain stem nuclei, an iontophoretic study was done on the effects of glutamate and substance P on single neurons in trigeminal nuclei oralis and caudalis in cats anesthetized with chloralose and paralyzed. The neurons were additionally studied for their responses to natural noxious and innocuous cutaneous and intraoral stimuli as well as to bipolar stimulation of the ipsilateral and contralateral canine tooth pulps, the exposed infraorbital and superior laryngeal nerves and forepaw. Glutamate excited all units tested. Substance P also had an excitatory effect, but only on some units. The slow time course of this effect was similar to that reported in other CNS regions. Units excited by substance P were located only in nucleus caudalis, and all responded to noxious cutaneous stimuli and/or to stimulation of tooth pulp; units responding only to innocuous orofacial stimulation were not excited by substance P. Levorphanol and opioid peptides were also applied iontophoretically to some of the neurons and were found to have depressant effects on nociceptive units. The data support the possibility that substance P and endogenous opioids play a role in chemical transmission in nociceptive pathways in trigeminal nucleus caudalis. The regional specificity of substance P excitation adds support to the earlier evidence of a differential distribution of sensory inputs to nuclei oralis vs. caudalis, with facial nocicpetive afferents projecting only to caudalis. The functional specificity of substance P excitation also adds to the parallels found between the dorsal horn and nucleus caudalis. In addition, the similarity between the dorsal horn and nucleus caudalis with respect to the effects of substance P and the opioids suggest a parallel in the neurochemistry of synaptic transmission at the two levels.