Stimulation-produced analgesia (SPA) from brain-stem and diencephalic sites in the rat: relationships between analgesia, aversion, seizures and catalepsy.

Electrical stimulation of dorsal and dorsolateral periaqueductal gray (PAG) and internal capsule (IC) sites in the rat elicited tail flick and formalin test stimulation-produced analgesia (SPA). SPA from PAG sites was associated with aversion. SPA from IC sites was associated with aversion, generalized seizures and catalepsy. Ventrobasal nucleus of thalamus (VB) stimulation did not elicit analgesia or aversion but did induce behavior characteristic of limbic seizures. A sub-anesthetic dose of sodium pentobarbital (20 mg/kg) suppressed IC stimulation-produced generalized seizures and catalepsy, and attenuated, but did not eliminate, tail flick test analgesia. These data suggest that SPA from IC sites in the rat is partially confounded with reduced responsivity. The hypothesis that SPA associated with aversion may represent a form of stress-induced analgesia is discussed.