Effects of repeated cold stress on activity of hypothalamic neurons in rats during performance of operant licking task

The present study investigated the effects of repeated cold stress on singl e neuron activity in the lateral hypothalamic area (LHA) and medial hypotha lamic area (MHA) of behaving rats. The rats were trained to lick a protrudi ng spout in response to one of several cue-tone stimuli (CTSs) to ingest wa ter, or amino acid, NaCl or glucose solution. Following this training, the rats were raised under either stressed (repeated temperature changes betwee n -3 and 24 degreesC) or control (24 degreesC) condition for 2 mo. During t his period, neuronal activity was recorded in the LHA and MHA. For rats rai sed under the stressed condition, mean spontaneous firing rate of LHA neuro ns was significantly greater than for rats under the control condition. Mor e LHA neurons in the stressed rats responded, with an accompanying decrease in activity (inhibitory response), to CTSs than in the control rats. Durin g extinction learning, some LHA neurons enhanced or reversed the responses to CTSs in the stressed rats, whereas no LHA neurons showed such response c hanges in the control rats. In contrast to the effects of the stressed cond ition on LHA neuron activity, mean spontaneous firing rate of MHA neurons i n the stressed rats was significantly smaller than in the control rats. Few er MHA neurons in the stressed rats responded to CTSs and/or ingestion of s apid solutions. The preceding results suggested that repeated cold stress p roduces a specific pattern of changes in spontaneous activity and responses to sensory stimuli in LHA and MHA neurons; this could underlie the behavio ral changes induced by repeated cold stress such as hyperphagia and hyper-r eactivity to sensory stimuli.