Ventrolateral periaqueductal gray matter and the control of tonic immobility

Tonic immobility is an inborn defensive behavior characterized by a temporary state of profound and reversible motor inhibition elicited by some forms of physical restraint. The periaqueductal gray matter (PAG) contains neural circuits involved in descending pain modulation, as well as in the modulation of TI. We have reported previously that the cholinergic stimulation of the ventrolateral PAG increases the duration of TI in guinea pigs. In the present study, we attempted to characterize further the modulation of TI by pharmacological alteration of the neurochemistry of the ventrolateral PAG circuitry. We observed that both cholinergic (carbachol, 5.4 nmol/0.2 microl) and opioidergic stimulations (morphine, 4.48 nmol/0.2 microl) of the ventrolateral PAG increase the duration of TI and that these effects can be reversed by pre-treatment with naloxone (2.74 nmol/0.2 microl). Our results also showed that microinjection of the GABAergic agonist muscimol (1, 0.5, and 0.26 nmol/0.2 microl) decreased the duration of TI episodes, while microinjection of the GABAergic antagonist bicuculline (1 nmol/microl) increased it. Moreover, we observed that preadministration of muscimol (0.13 nmol/0.2 microl) at a dose that had no effect per se at this site antagonized the potentiating effect of morphine. Our results suggest that this modulation of TI from the ventrolateral PAG circuitry is accomplished by a complex interaction of cholinergic, opioidergic, and GABAergic mechanisms, similar to that proposed for descending antinociceptive circuits.