Neural Substrates for Reflex Salivation Induced by Taste, Mechanical, and Thermal Stimulation of the Oral Region in Decerebrate Rats

In order to investigate the neural mechanisms of reflex salivary secretion, experiments were carried out on anesthetized, decerebrate rats from which the volumes of submandibular salivary secretion and the efferent discharges in the preganglionic parasymapathetic fibers innervating the submandibular gland were recorded. Salivary secretion was induced by either infusing a taste solution, or an aliquot of hot water (45–55°C) into the oral cavity, or by pinching the frontal parts of the oral region with a pair of forceps. The reflex salivation induced by noxious thermal and mechanical stimuli was markedly reduced by lesioning either the caudal (VC), or the interpolar (VI) trigeminal sensory nuclei. Taste-elicited salivary secretion was significantly reduced by lesioning the nucleus of the tractus solitarius (NTS). Of 43 preganglionic parasympathetic fibers sampled, 27 responded to both noxious mechanical and thermal stimulation of the oral region, and to electrical stimulation of the VC. Ten fibers responded only to taste stimulation and to electrical stimulation of the NTS. The remaining 6 fibers responded to both taste and noxious thermal stimulation of the oral region. These fibers responded well to NTS stimulation, but gave only a slight response to VC stimulation. These results suggest that two distinct neural pathways exist which mediate reflex salivation in the lower brain stem of the rat, i.e., the taste pathway via the NTS and the nociceptive pathway via the trigeminal sensory nuclei.