Activation of the ventrolateral periaqueductal gray reduces locomotion butnot mean arterial pressure in awake, freely moving rats

Activation of the ventrolateral periaqueductal gray produces immobility and antinociception. It has been argued that these behaviors are part of eithe r a defensive fear response to threat or a recuperative quiescence response to deep tissue injury. Data collected in anesthetized animals showing that activation of the ventrolateral peraqueductal gray has a hypotensive effec t supports the quiescence hypothesis. Our objective was to determine whethe r activation of the ventrolateral periaqueductal gray in awake, freely movi ng rats results in a decrease in blood pressure as it does in anesthetized animals. Changes in blood pressure produced by microinjection of the neuroe xcitant D,L-homocysteic acid were measured using radio telemetry while rats were awake and while anesthetized with pentobarbital. Consistent with earl ier reports. microinjection of D,L-homocysteic acid into the ventrolateral periaqueductal gray caused a decrease in blood pressure in anesthetized rat s. In contrast, microinjection at the same ventrolateral periaqueductal gra y sites while rats were awake had no effect on blood pressure. even though the animals became immobile and hear? rare decreased. Thus, the immobility evoked from ventrolateral periaqueductal gray is not associated with a fall in mean arterial pressure. Two conclusions can be drawn from these data. (1) Caution must be used in g eneralizing from data collected in anesthetized animals. (2) The ventrolate ral periaqueductal gray is as likely to contribute to defensive fear as to recuperative quiescence. (C) 2001 IBRO. Published by Elsevier Science Ltd. All rights reserved.