Transient expression of a novel type of GABA response in rat CA3 hippocampal neurones during development.

1. Intracellular recordings were used to study the effects of gamma‐aminobutyric acid (GABA) on rat CA3 hippocampal neurones during the first two weeks of postnatal life. 2. In the presence of tetrodotoxin (TTX, 1 microM), from postnatal day 0 (P0) to P12 both associated with an increase in input conductance whereas baclofen (30‐100 microM) produced a membrane hyperpolarization. 3. Bicuculline (50 microM) reduced the effects of GABA and abolished the response to isoguvacine without affecting the response to baclofen. 4. This novel bicuculline‐insensitive GABA response was chloride dependent and was blocked by picrotoxin (10‐100 microM) in an uncompetitive way. In bicuculline and picrotoxin, a GABAB‐mediated hyperpolarization appeared. 5. Towards the end of the second postnatal week, bicuculline blocked the GABA‐induced depolarization and revealed a small hyperpolarizing response which was blocked by the GABAB antagonist CGP 35348 (0.5‐1 mM). 6. It is suggested that, during development, the GABA response was mediated through the conventional GABAA and GABAB receptors as well as a new bicuculline‐baclofen‐insensitive type of receptor.

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