HYPERPOLARIZATION BY GABA(B) RECEPTOR AGONISTS IN MID-BRAIN PERIAQUEDUCTAL GRAY NEURONS IN-VITRO

1 The effects of GABA(B) receptor stimulation on membrane properties of rat periaqueductal gray neurones were studied by use of intracellular recordings from single neurones in superfused brain slices. Intracellular staining with biocytin was used to characterize the anatomical location of each impaled neurone. 2 The GABA(B) receptor agonist, baclofen, directly hyperpolarized or produced an outward current (single electrode voltage-clamp) in all 66 neurones tested. Baclofen-induced hyperpolarizations were concentration-dependent with an EC(50) of approximately 0.6 mu M and maximum hyperpolarization with 10 mu M baclofen. Hyperpolarizations persisted in the presence of tetrodotoxin (1 mu M, n = 2). 3 2-OH-saclofen, a selective GABA(B) receptor antagonist, competitively antagonized baclofen-induced hyperpolarizations (n = 4)with equilibrium dissociation constants estimated in two neurones to be 6 and 23 mu M. Naloxone (1 mu M) did not prevent hyperpolarizations induced by baclofen (n = 34). 4 Hyperpolarizations induced by baclofen were associated with an increased inwardly rectifying potassium conductance. Ba2+ superfusion (5 to 10 mM) blocked this conductance increase (n = 4). Elevation of extracellular potassium concentration (from 2.5 to 6.5 mM) shifted the reversal potential in agreement with predictions of the Nernst equation. 5 Hyperpolarizations produced by baclofen (10 mu M) desensitized (>5% inhibition of the maximum response) in 7/22 neurones during continuous superfusion for 5 min. Strong desensitization (>25% inhibition of the maximum response) was observed in only 2/22 neurones in the ventrolateral periaqueductal gray. In contrast 6/9 neurones in the laterodorsal tegmental nucleus displayed strong desensitization. 6 These studies demonstrate that baclofen acting on GABA, receptors increases potassium conductance in all lateral and ventrolateral periaqueductal gray neurones. The neurones hyperpolarized by baclofen are likely to be involved in the well-established antinociceptive actions of baclofen in the ventrolateral periaqueductal gray, but might also be involved in other functions because many of them. lie outside the main 'antinociceptive' zone of this region. The cellular mechanisms underlying baclofen-induced antinociception presumably differ from the postulated antinociceptive action of opioids, thought to be mediated via disinhibition of periaqueductal gray neurones which project to the ventromedial medulla.