Experimental Brain Research

, Volume 99, Issue 1, pp 75–83 | Cite as

Quiescence and hyporeactivity evoked by activation of cell bodies in the ventrolateral midbrain periaqueductal gray of the rat

  • Antoine Depaulis
  • Kevin A. Keay
  • Richard Bandler
Original Paper


Much evidence suggests that the midbrain periaqueductal gray region (PAG) plays a pivotal role in mediating an animal's responses to threatening, stressful, or painful stimuli. Active defensive reactions, hypertension, tachycardia and tachypnea are coordinated by a longitudinally oriented column of cells, found lateral to the midbrain aqueduct, in the caudal two-thirds of the PAG. In contrast, microinjections of excitatory amino acid (EAA) made in the ventrolateral region of the PAG in anesthetized or isolated animals evoke hypotension, bradycardia, and behavioral arrest. The aim of the present study was to examine further the effects of activation of neurons in the ventrolateral PAG. By injecting into this region low doses (40 pmol) of kainic acid (KA), a long-acting EAA, it was possible to observe a freely moving rat's behavior in a social situation (i.e., paired with a weight-matched, untreated partner). Such injected rats become quiescent, i.e., there was a cessation of all ongoing spontaneous activity. These rats were also hyporeactive: the investigative approaches of the partner failed to evoke orientation, startle reactions, or vocalization. Electroencephalographic measurements indicated that the effects of injections of KA in the ventrolateral PAG were not secondary to seizure activity. In addition to the quiescence and hyporeactivity reported here, and the hypotension and bradycardia reported previously, the ventrolateral PAG is a part of the brain from which analgesia has been readily evoked by electrical stimulation, or microinjections of either EAA or morphine. As a reaction to “deep” or “inescapable” pain, chronic injury, or defeat, animals often reduce their somatomotor activity, become more solitary, and are generally much less responsive to their environment. These data, and those from other recent studies, suggest that neurons in the ventrolateral PAG may play an important role in integrating such a passive behavioral response of which quiescence and hyporeactivity are the major components.

Key words

Periaqueductal gray Stress Defense Passive coping behavior Rat 


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Copyright information

© Springer-Verlag 1994

Authors and Affiliations

  • Antoine Depaulis
    • 1
  • Kevin A. Keay
    • 2
  • Richard Bandler
    • 2
  1. 1.Neurobiologie et Neuropharmacologie des épilepsies généralisées, U.398 INSERM, Centre de NeurochimieStrasbourg CedexFrance
  2. 2.Department of Anatomy and HistologyThe University of SydneyNSWAustralia

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