Lateral hypothalamic modulation of the gustatory-salivary reflex in rats

It is well recognized that the basic mechanism for the gustatory-salivary reflex is located in the lower brainstem and that suprabulbar structures possibly influence this mechanism. This study is designed to evaluate the neurophysiological mechanism underlying the effect of the lateral hypothalamic area (LHA) on the bulbar gustatory-salivary reflex. Submandibular salivary secretion and the electrical activity of the preganglionic parasympathetic fibers innervating the submandibular and sublingual glands were recorded in anesthetized rats. Stimulation of the tongue with high concentrations of chemical solutions (1 M and 2 M NaCl and 0.01 M and 0.05 M HCl) and/or pinching the tongue with a small clamp induced a profuse salivary secretion (3 to 28.5 microliter/5 min) recorded from a unilateral submandibular gland. The preganglionic fibers consisted of three types: taste-related fibers, which increased their firing rate by taste stimuli; pinch-related fibers, which increased their firing rate by pinching; and unidentified fibers, which did not respond to taste or pinching stimulations of the tongue. Electrical stimulation of the ipsilateral LHA caused the secretion of a small amount of saliva (1.5 microliter/5 min), and it appeared that taste-related fibers more often received polysynaptic connections from the LHA than other types of fibers. Conditioning stimulation of the LHA increased the magnitude of impulse discharges by the test stimulation of the tongue in the taste-related fibers. Therefore, the results of this study demonstrate that the LHA enhances the activity of the gustatory-salivary reflex in the lower brainstem.