Neurotoxic lesions centered on the perifornical hypothalamus abolish the cardiovascular and behavioral responses of conditioned fear to context but not of restraint.

The aim of this study was to test the role of the perifornical hypothalamus and adjacent areas in the behavioral and cardiovascular responses to two forms of stress, conditioned fear to context and restraint. Of particular interest was the role of the hypocretin (orexin) containing neurons in these responses. Rats implanted with radio-telemetric probes and fear conditioned to a context received bilateral injections of the neurotoxin hypocretin-2-saporin centered on the perifornical area. One week later, the animals were tested for conditioned fear to context and restraint while recording freezing, 22 kHz ultrasonic vocalizations, activity, mean arterial pressure and heart rate. Histological verification revealed that the lesions were not specific since virtually all the neurons within the injection area were lost. Nevertheless, these lesions, which were centered on the perifornical area, markedly reduced all recorded components of the contextual fear response (by 70%) but had no effect on the response to restraint. The lesions also caused a reduction in body weight and reduced the circadian rhythm of the recorded parameters. The results show (i) that hypocretin-2-saporin was not specific enough to produce lesions restricted to the hypocretin system, (ii) that neurons of the perifornical area are necessary for the expression of both the cardiovascular and behavioral components of conditioned fear to context, and (iii) that the same neurons are not necessary for the cardiovascular response to restraint. Thus, the perifornical hypothalamus is critical for some forms of stress but not others. We propose that it is a necessary relay for emotional responses in which the psychological component is stronger than the sensory component.