Studies on neural mechanisms of the gustatory-salivary reflex in rabbits

1. Submandibular salivary secretion and the electrical activity of the parasympathetic preganglionic fibres innervating the submandibular gland were recorded in decerebrated rabbits in response to taste stimulation of the tongue. The electrical activity of a taste nerve (chorda tympani) responding to varying taste stimuli was also recorded in the deeply anaesthetized rabbits. These data representing input and output information were compared with each other. 2. Sucrose, quinine, tartaric acid, NH4Cl and KCl which induced a long-lasting response in the taste nerve evoked a continuous salivary secretion, and those chemicals such as NaCl, CaCl2 and MgCl2 which induced a transient activity of the nerve evoked a transient salivary secretion. 3. The magnitude of responses of the whole taste nerve to moderate concentrations of chemical stimuli applied to the anterior part of the tongue was statistically significantly correlated with the volume of reflex submandibular salivation. 4. Parasympathetic preganglionic fibres to the salivary gland were classified into two types according to their responsiveness to taste stimuli; taste-sensitive and taste-insensitive fibres. The magnitude of electrical activity of the taste nerve fibres was significantly correlated with that of the taste-sensitive preganglionic fibres to stimulation of the tongue with varying taste stimuli. 5. By calculating correlations between responses of the taste fibres to each of the four basic taste stimuli and all the stimuli tested, it was concluded that afferent inputs from the taste of sucrose and NaCl were different, while those of HCl and quinine were similar. On the other hand, it was found by the same procedure for the preganglionic fibres that sucrose and NaCl, and HCl and quinine produced a similar response profile, respectively. This result means that the afferent taste inputs are processed into appropriate outputs (perhaps on a hedonic basis) in the lower brain stem without involvement of higher central nervous mechanisms.