gamma-Aminobutyric acid responses in rat locus coeruleus neurones in vitro: 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.
Osmanović
S S
SS
Department of Physiology and Biophysics, University of Illinois College of Medicine, Chicago 60612.
Shefner
S A
SA
D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D001640:Bicuculline / Q000494:pharmacology; D002712:Chlorides / Q000494:pharmacology; D018756:GABA Antagonists; D066298:In Vitro Techniques; D008125:Locus Coeruleus / Q000187:drug effects* / Q000502:physiology; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects; D009474:Neurons / Q000187:drug effects*; D051381:Rats; D011916:Rats, Inbred F344; D011919:Rats, Inbred Strains; D005680:gamma-Aminobutyric Acid / Q000494:pharmacology*
D002712:Chlorides; D018756:GABA Antagonists; D005680:gamma-Aminobutyric Acid; D001640:Bicuculline