Participation of the periaqueductal gray matter in the modulation of tonic immobility in the guinea pig.

Unilateral microinjection of carbachol (CCh, 1 microgram/0.2 microliter) into the specific sites in the ventral and ventrolateral portions of the periaqueductal gray (PAG) matter, which is known to be involved in analgesia, increases the duration of restraint-induced tonic immobility (TI) episodes induced in 23 adult male guinea pigs (Cavia porcellus). Mean duration of TI episodes was 107 +/- 16.38 s in the control group and increased to 220.7 +/- 40.24 s in the group microinjected with CCh. The potentiating effect of carbachol on TI duration was blocked by pretreatment with atropine (7.6 micrograms/0.4 microliter). These data suggest that PAG and the cholinergic system are involved in the modulation of the motor inhibition characteristic of TI which may be activated by the same stimuli that induce defensive analgesia.