Activity changes of the cat paraventricular hypothalamus during stressor exposure

Dorso-medial paraventricular hypothalamus (PVH) activity was assessed by light scattering procedures in freely behaving cats during auditory stressor exposure. Acoustic noise (> 95 dB) raised plasma ACTH concentrations, somatic muscle tonus, respiratory frequency and cardiac rates; PVH activity peaked 0.8 s following stimulation, and then markedly declined below baseline to a trough at 9.7 s. Hypothalamic responses were not uniformly distributed across the recorded PVH field. Activity changes emerged from subregions within the visualized area, and were widespread at the overall activity zenith and nadir. Isolated pixels appeared opposite in activity pattern to overall changes. We suggest that transient activity increases represent initial PVH neural stress responses, and that subsequent profound declines result from neural inhibitory feedback.