GAMMA-AMINOBUTYRIC-ACID RESPONSES IN RAT LOCUS COERULEUS NEURONS INVITRO - A CURRENT-CLAMP AND VOLTAGE-CLAMP STUDY

1. Intracellular recordings were made from locus coeruleus (LC) neurones in a totally submerged brain slice preparation from adult rats. The effect of gamma‐aminobutyric acid (GABA) on LC neurones was studied under current‐clamp and voltage‐clamp conditions. GABA caused inhibition of spontaneous firing and a large conductance increase in LC neurones. These effects could be accompanied by depolarization, hyperpolarization or little change in membrane potential depending on the presence or absence of Cl‐ in the recording microelectrode. 2. The reversal potential for GABA‐induced changes in membrane potential (EGABA) was ‐71.3 +/‐ 1.1 mV (S.E.M., n = 21) in cells impaled with potassium acetate electrodes and ‐47.5 +/‐ 1.4 mV (S.E.M., n = 15) in cells impaled with KCl electrodes. When the external Cl‐ concentration was reduced EGABA was shifted in the depolarizing direction by 51.5 mV per tenfold change in external Cl‐ which is close to the shift predicted by the Nernst equation for a selective increase in CL‐ conductance. 3. GABA effects on LC neurones result from a direct action since they persist in low‐Ca2+ and high‐Mg2+ media which block synaptic transmission. 4. The effects of GABA were concentration dependent and antagonized by bicuculline (10 microM) and bicuculline methiodide (80‐100 microM) indicating that they were mediated predominantly by an action on GABAA receptors. In the presence of bicuculline, EGABA was shifted towards the K+ equilibrium potential which indicated a residual bicuculline‐resistant action at GABAB receptors. 5. GABA‐induced responses were membrane potential dependent. GABA conductance was observed to decrease with membrane hyperpolarization in a linear manner. GABA‐induced current showed outward rectification. In the voltage range studied (rest to ‐110 mV) the extent of this rectification was predicted by the Goldman‐Hodgkin‐Katz equation, suggesting that it was due to the unequal distribution of Cl‐ across the membrane. In addition, the time constant of decay of GABA current was decreased by membrane hyperpolarization; this could be due to a voltage‐dependent change in receptor or channel kinetics. 6. These data suggest that the primary action of GABA on LC neurones is to increase Cl‐ conductance by activation of bicuculline‐sensitive GABAA receptors. Due to the voltage dependence of GABA responses, GABA will exert a stronger inhibitory effect on LC neurones at depolarized than at hyperpolarized membrane potentials. This could serve as a negative feedback mechanism to control excitability of these neurones. © 1990 The Physiological Society