STRUCTURE AND FUNCTION OF GUSTATORY NEURONS IN THE NUCLEUS OF THE SOLITARY TRACT .2. RELATIONSHIPS BETWEEN NEURONAL MORPHOLOGY AND PHYSIOLOGY

This study employed intracellular recording and labeling techniques to examine potential relationships between the physiology and morphology of brainstem gustatory neurons. When we considered the neuronal respo nse to the four ''prototypic'' tastants, we were able to demonstrate a positive correlation between breadth of responsiveness and the number of dendritic branch points. An analysis of the response to eight tast ants also revealed an association between dendritic spine density and the breadth of responsiveness, with more narrowly tuned neurons exhibi ting more spines. Interestingly, a neuron's ''best response'' was a re latively poor predictor of neuronal morphology. When we focused on tho se neurons that responded to only one tastant, however, a number of po tentially important relationships became apparent. We found that the c ells that only responded to quinine were smaller than the neurons that only responded to NaCl, HCl, or sucrose. The HCl-only neurons, howeve r, were more widespread in the rostrocaudal dimension than the neurons that only responded to NaCl. A number of additional structure-functio n relationships were identified when we examined the neuronal response to selected tastants. We found that neurons that responded to sucrose but not quinine, as well as neurons that responded to quinine but not sucrose, were more widespread in the mediolateral dimension than neur ons that responded to bath sucrose and quinine. We also discovered tha t the neurons that responded to NaCl, but not to NH4Cl or KCl, were la rger than neurons that responded to all three salts. We believe that t hese results support the hypothesis that there are relationships betwe en the structure and function of gustatory neurons in the nucleus of t he solitary tract, with the data highlighting the importance of three themes: 1) the relationship between dendritic specializations and tuni ng, 2) the relationship between dendritic arbor orientation and respon se properties, and 3) the potential importance of stimulus-specific ne urons. (C) 1996 Wiley-Liss, Inc.