The effects of type of lesions and percentage body weight loss on measures of motivated behavior in rats with hypothalamic lesions

Abstract

Three experiments were conducted to determine the effect of direct current (DC) or radio frequency (RF) current lesions of the ventromedial hypothalamus (VMN) and percentage body weight loss on measures of motivated behavior and physiology. In experiment one, Ss were pretrained to bar press, then lesioned, half with DC, half with RF current. The Ss reduced to control weight levels (Hn Ss) did not differ from control Ss, but DC Ss reduced to 90% of static weight (Hs Ss) responded reliably less than control Ss. The RF-Hs Ss started responding at control group levels, but reached DC-Hs levels during the second half of the test session. In the second experiment, the Ss were pretrained to consume food pellets. No reliable differences were found in the number of food cup entries among the groups on rewarded trials, but Hn Ss made a larger number of entries during non-rewarded trials. The RF-Hn Ss initially made as many non-rewarded entries as the DC-Hn Ss but decreased their rate of response to control group levels after 50 min. The latencies of control and Hs Ss to the first food cup entry were not reliably different on either rewarded or non-rewarded trials. The Hn Ss had shorter latencies than other groups on all trials, but during both rewarded and non-rewarded trials, the RF-Hn Ss started at the same level as the DC-Hn -Ss, but after 50 min., RF-Hn Ss increased their latencies and during the last 40 min. were not reliably different than control or Hs Ss. The Ss in the third experiment were not pretrained. When each s reached its appropriate weight, serum free fatty acid (FFA), serum glucose, rate of FFA mobilization, and change in anal-nasal length from the time of surgery were measured. Weight loss levels and type of lesion had effects on most of the measures. The results of the three studies led to the conclusion that percentage body weight loss or effort could not account for the effects observed. It was clear that the interaction between type of lesion, weight loss levels, and the task was of major importance in this type of explanation. The physiological measures indicated differences among the groups, but although a number of previous findings were confirmed, the physiological data, itself, did not suggest any additional hypotheses. It was concluded that Reynolds' "irritative hypothesis" was incorrect since it could not account for many of the studies designed to test it. It was equally clear that while RF and DC lesions could both produce obesity, their other effects were not always the same. After a review of the literature dealing with hypophysectomy and its effects in comparison with VMN lesions, it was suggested that while hypophysectomy does not produce the weight gain and obesity characteristic of VMN lesions, it does have similar metabolic effects. In addition, it was suggested that if measures other than food intake and weight gain were made, differences between VMN lesions, with and without the presence of hypophysis, might be detected.