Post- and presynaptic GABA(B) receptor activation in neonatal rat rostral ventrolateral medulla neurons in vitro.
Whole-cell patch recordings were made from immature (six- to 12-day-old) rat rostral ventrolateral medulla neurons in brainstem slices. GABA or the specific GABA(B) receptor agonist (-)baclofen (10-50 microM) by superfusion or by pressure ejection induced an outward current or a hyperpolarization, which persisted in a tetrodotoxin (0.3 microM)-containing Krebs' solution in nearly every cell tested. The GABA(B) receptor antagonists 2-hydroxy saclofen (50-200 microM) and CGP 35348 (50-200 microM) dose-dependently suppressed baclofen-currents. Baclofen-currents were suppressed by barium (1 mM) but not by tetraethylammonium (20 mM), low Ca2+ (0.24 mM) solution or in a solution containing the Ca2+ chelator BAPTA-AM (10 microM). The outward current had an estimated reversal potential of -98, -77 and -52 mV in 3.1, 7 and 15 mM [K+]o. Pre-incubation of slices with pertussis toxin (500 microg/ml for 5-7 h) or intracellular dialysis with GDP-beta-S (500 microM) markedly reduced baclofen-currents. Baclofen in low concentrations (1-3 microM) that caused slight or no change of holding currents and of inward or outward currents induced by exogenously applied glutamate or glycine/GABA, decreased excitatory and inhibitory postsynaptic currents by an average of 86.5 +/- 4.3% and 78.4 +/- 2.7%. The GABA(B) antagonist CGP 35348 (100 microM) increased the excitatory postsynaptic currents by an average of 64%, without causing a significant change in holding currents in 10/18 cells tested. Our results indicate the presence of post- and presynaptic GABA(B) receptors in the rostral ventrolateral medulla neurons. Activation of postsynaptic GABA(B) receptors induces an outward K+ current which is barium-sensitive, Ca2+-independent and may be coupled to a pertussis-sensitive G-protein. Activation of presynaptic GABA(B) receptors attenuates excitatory or inhibitory synaptic transmission. More importantly, the observation that CGP 35348 enhanced excitatory synaptic currents implies a removal of tonic activation of presynaptic GABA(B) receptors by endogenously released GABA (disinhibition), supporting the hypothesis that these receptors may have a physiological role in regulating the input and output ratio in a subset of rostral ventrolateral medulla neurons in vivo.
Department of Pharmacology, James H. Quillen College of Medicine, East Tennessee State University, Johnson City 37614, USA.
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D000818:Animals; D000831:Animals, Newborn; D001418:Baclofen / Q000031:analogs & derivatives / Q000494:pharmacology; D002118:Calcium / Q000494:pharmacology; D002614:Chelating Agents / Q000494:pharmacology; D004533:Egtazic Acid / Q000031:analogs & derivatives / Q000494:pharmacology; D004558:Electric Stimulation; D019706:Excitatory Postsynaptic Potentials / Q000187:drug effects / Q000502:physiology; D018755:GABA Agonists / Q000494:pharmacology*; D018756:GABA Antagonists / Q000494:pharmacology*; D006153:Guanosine Diphosphate / Q000031:analogs & derivatives / Q000494:pharmacology; D066298:In Vitro Techniques; D008526:Medulla Oblongata / Q000502:physiology*; D009474:Neurons / Q000187:drug effects / Q000502:physiology*; D009943:Organophosphorus Compounds / Q000494:pharmacology; D018408:Patch-Clamp Techniques; D051381:Rats; D017207:Rats, Sprague-Dawley; D018080:Receptors, GABA-B / Q000187:drug effects / Q000502:physiology*; D019789:Tetraethylammonium / Q000494:pharmacology; D013779:Tetrodotoxin / Q000494:pharmacology; D013873:Thionucleotides / Q000494:pharmacology
D002614:Chelating Agents; D018755:GABA Agonists; D018756:GABA Antagonists; D009943:Organophosphorus Compounds; D018080:Receptors, GABA-B; D013873:Thionucleotides; C070379:1,2-bis(2-aminophenoxy)ethane N,N,N',N'-tetraacetic acid acetoxymethyl ester; D006153:Guanosine Diphosphate; D013779:Tetrodotoxin; D004533:Egtazic Acid; D019789:Tetraethylammonium; C023427:guanosine 5'-O-(2-thiodiphosphate); C066430:CGP 35348; D001418:Baclofen; D002118:Calcium; C057491:2-hydroxysaclofen