Taste responses in the parabrachial pons of decerebrate rats

Abstract

1. Behavioral studies have shown that chronic decerebrate rats retain the capacity to react appropriately to gustatory stimuli (12), but do not form taste-illness associations (13). Little is known, however, about the effects of decerebration on the processing of gustatory information. The present experiment was designed to investigate this issue in the parabrachial nucleus of the pons (PbN). 2. Rats were decerebrated at the supracollicular level under ketamine and ether anesthesia and were prepared for electrical recording in the PbN. Thereafter, animals were maintained under Flaxedil, and wound edges were frequently treated with lidocaine. Heart rate, core temperature, and CO2 were monitored throughout each experiment. Control subjects were treated identically, except that they were not decerebrated. 3. Sapid solutions of NaCl (0.1 M), HCl (0.01 M), sucrose (0.5 M), saccharin sodium (0.004 M), and quinine HCl (.01 M) were used as taste stimuli. After a 10-s base line, each stimulus was bathed over the tongue for 10 s followed by a 10-s wait and a 20-s rinse of distilled water. The intertrial interval was at least 2 min. 4. Gustatory responses from 32 parabrachial units in 13 decerebrate rats were recorded. These were compared with responses in 31 units from the PbN of 16 intact rats. 5. Analysis of response profiles of PbN units in decerebrate rats showed that these units produced smaller responses to NaCl and HCl and larger responses to saccharin sodium compared with units in intact rats. 6. Despite changes in response magnitude, the temporal patterns of response (phasic-tonic relationships) were not different in PbN units in decerebrate rats compared with controls. Differences in the length of responses were, however, apparent. Responses to saccharin sodium were longer, response to NaCl, HCl and sucrose were shorter, and responses to quinine HCl were unchanged. 7. Results of a multidimensional scaling analysis of the response profiles across units showed that "taste spaces" for decerebrate and intact rats were similar. Units in each group were meaningfully placed near stimuli that evoked the best response in a given unit. Units that did not respond well to any stimulus were placed close together regardless of their best stimulus in both taste spaces. 8. Responses to the termination of the taste stimulus (OFF-responses) were observed in PbN units in the decerebrate rat but not in units from the intact rat. Twenty-one OFF-responses were recorded in 14 units; 6 of these occurred in the absence of a response to the stimulus.(ABSTRACT TRUNCATED AT 400 WORDS)