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 single neuron activity in the lateral hypothalamic area (LHA) and medial hypothalamic area (MHA) of behaving rats. The rats were trained to lick a protruding spout in response to one of several cue-tone stimuli (CTSs) to ingest water, or amino acid, NaCl or glucose solution. Following this training, the rats were raised under either stressed (repeated temperature changes between -3 and 24 degrees C) or control (24 degrees C) condition for 2 mo. During this period, neuronal activity was recorded in the LHA and MHA. For rats raised under the stressed condition, mean spontaneous firing rate of LHA neurons was significantly greater than for rats under the control condition. More LHA neurons in the stressed rats responded, with an accompanying decrease in activity (inhibitory response), to CTSs than in the control rats. During extinction learning, some LHA neurons enhanced or reversed the responses to CTSs in the stressed rats, whereas no LHA neurons showed such response changes in the control rats. In contrast to the effects of the stressed condition on LHA neuron activity, mean spontaneous firing rate of MHA neurons in the stressed rats was significantly smaller than in the control rats. Fewer MHA neurons in the stressed rats responded to CTSs and/or ingestion of sapid solutions. The preceding results suggested that repeated cold stress produces a specific pattern of changes in spontaneous activity and responses to sensory stimuli in LHA and MHA neurons; this could underlie the behavioral changes induced by repeated cold stress such as hyperphagia and hyper-reactivity to sensory stimuli.