Physiology & Behavior, Vol. 33, pp. 499-501.Copyright© PergamonPress Ltd., 1984.Printed in the U.S.A. 0031-9384/84$3.00 + .00 BRIEF COMMUNICATION Nociceptive Thresholds Following Food Restriction and Return to Free-Feeding ROBERT J. HAMM AND BRUCE G. LYETH Department of Psychology, Virginia Commonwealth University, Richmond, VA 23284 Received 15 November 1983 HAMM, R. J. AND B. G. LYETH. Nociceptive thresholds following food restriction and return to free-feeding. PHYSIOL BEHAV 33(3) 499-501, 1984.--Nine rats were placed on a restricted food diet for 14 days and nociceptive thresholds were measured by the tall-flick procedure. After 24 hr of food restriction nociceptive thresholds increased. This initial increase in nociceptive threshold was followed by a decrease in pain threshold on the second day of food restriction. Nociceptive thresholds returned to pre-deprivation levels on the remaining 11 days of food restriction. When the rats were given free access to food after 14 days of food restriction, nociceptive thresholds increased 24 hr after the reintroduction of food and decreased during the next 24 hr. Thus, the results of the present experiment demonstrate that both food restriction and a rgXuI:n to free feeding after 14 days of food restriction produced the same biphasic pattern of changes in nociceptive thresholds. Nociceptive threshold Analgesia Pain Food restriction Starvation Free-feeding Stress STRESSFUL stimuli produce a number of autonomic ad- justments in organisms including changes in respiration, body temperature, metabolic rate, and other neuroendocrine responses. Recent research has demonstrated that another component of the stress reaction may be a reduction in sen- sitivity to pain. For example, exposure to a wide variety of stressors such as inescapable electric shock [1], immobiliza- tion [2], rotation [7], and cold water [5] have all produced a reduction in pain sensitivity following stimulation. Acute starvation has also been shown to produce a change in nociceptive thresholds [3]. Starvation produced an increase in nociceptive threshold that was evident for up to 36 hr. This period of decreased pain sensitivity was followed by a decrease in nociceptive threshold. After three days of com- plete food deprivation, rats were given free access to food. Nociceptive thresholds were not reliably influenced by a re- turn to free-feeding conditions. Thus, these results demon- strate that the onset of starvation produces a biphasic change in pain sensitivity, and feasting after a short period of star- vation did not change nociception. Changes in dynorphin, an endogenous opiate peptide implicated in the control of feed- ing, has also been examined as a function of food and water deprivation [11]. As was the case in the Bodnar et al. [3] study, dynorphin levels were sampled only during the first three days of complete food and water deprivation. They found that dynorphin concentrations (measured during the daytime) increased in the hypothalamus as deprivation proceeded. The purpose of the present experiment is to investigate the longer-term consequences of food restriction on no- ciception than the three days previously used [3,11]. In addition, after the animals have adapted to and stabilized on a restricted food diet, free access to food will be given. The rationale for reintroducing food after a period of prolonged deprivation comes from research that has shown that or- ganisms lower their homeostatic set point for metabolic rate, body temperature, and respiratory quotient in response to prolonged fasting [5,10]. The introduction of free access to food when contrasted with a lowered set point for these physiological parameters may be reflected in a change in nociceptive threshold. Thus, the return to free-feeding con- ditions after a long period of food restriction may produce a change in pain sensitivity not detected after only three days of food deprivation. METHOD Subjects Subjects were nine male Sprague-Dawley rats which were 75 days old at the start of the experiment. The average free- feeding weight of the animals was 355 g. Animals were indi- vidually housed and water was freely available. The animal room had a 0600-on/2000-off light-dark cycle. Apparatus To measure pain threshold sensitivity, an Emdie tail-flick apparatus (F'r-6, S/N 2035) was used which consisted of a 421/2× 121/2× 10 cm metal box in which a photo-circuit was located. Mounted on top of the box was an 8 cm metal plate in which a shallow groove was cut in which the rat's tail rested. A small (15 mm) light sensitive optical fiber was lo- 499