The effect of GABA and its antagonists on midbrain periaqueductal gray neurons in the rat.

Injection of GABA into the midbrain periaqueductal gray (PAG) activates medullary neurons that are involved in pain inhibition and potentiates morphine-induced analgesia. These observations suggest that GABAergic mechanisms in the PAG may modulate the descending pain inhibitory system that arises from this structure. In the present study, the effects of GABA and GABA antagonists on membrane properties and baseline activity of PAG neurons were examined using both in vitro and in vivo preparations. Application of bicuculline methiodide (BICM), at a dose that blocked the response to GABA, potently increased the baseline firing rate in 53% of cells recorded in vitro and 74% of cells recorded in the intact preparation. Application of BICM often yielded multiple or burst spiking episodes in both preparations. In 69% of cells the effect of BICM was diminished or totally abolished when the slice was perfused with high-magnesium, calcium-free, physiological saline solution. Intracellular recordings revealed that bicuculline caused depolarization of the membrane (70% of cells), increased the firing frequency (94% of cells) and increased the frequency of excitatory postsynaptic potentials (18% of cells). The effect of bicuculline on membrane resistance was not pronounced and in 64% of neurons it did not cause any measurable change in the resting membrane resistance. PAG neurons responsive to GABA and its antagonists were observed in all regions of the PAG. However, the highest number of neurons that responded to GABA and its antgonists was found in the medial and medioventral parts of the PAG. These results indicate that PAG may contain a tonically active GABAergic network that operates, at least in part, through GABAA receptors. This GABAergic system may modulate activity in descending pain inhibitory pathways emanating from PAG.