Primary trigeminal afferent neuron of the cat: II. Neuropeptide- and serotonin-like immunoreactivity.

The distribution of several peptides and serotonin in the cat vibrissa primary afferents from the Merkel cell-axon complexes to their central projections in the principal trigeminal sensory nucleus was examined by peroxidase-antiperoxidase immunocytochemical technique. In the trigeminal ganglia, two populations of primary sensory neurons, containing immunoreactive material were identified: a few substance P-, bombesin-, somatostatin-, cholecystokinin-, and vasoactive intestinal polypeptide-like immunoreactive ganglion cells with small and medium-sized somata, and many large non-labelled trigeminal neurons that were surrounded by substance P-, somatostatin-, cholecystokinin-, vasoactive intestinal polypeptide- and serotonin-positive pericellular baskets. Substance P-, bombesin- and vasoactive intestinal polypeptide-like immunoreactive substances were found in the basal side of Merkel cells from sinus hair follicles of the cat, whereas the associated axons were immunonegative. In the principal trigeminal sensory nucleus, cell bodies and fibres, containing substance P and bombesin were observed in both subdivisions. In addition, only nerve fibres and terminals showed somatostatin-, cholecystokinin-, vasoactive intestinal polypeptide- and serotonin-like immunoreactivity. The present results provide direct evidence that an ascending peptidergic system exist and that only substance P and, less likely, bombesin may be primary afferent neuroregulator(s) for the mechanosensory information in the cat trigeminal system.