γ‐Aminobutyric acidA ρ receptor subunits in the developing rat hippocampus

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Abstract
The RT‐PCR approach was used to estimate the expression of γ‐aminobutyric acid (GABA)A ρ receptor subunits in the hippocampus of neonatal and adult rats. All three ρ subunits were detected at postnatal day (P) 2, the ρ3 subunit being expressed at an extremely low level. The ρ1 and ρ2 products appeared to be developmentally regulated; they were found to be more pronounced in adulthood. In another set of experiments, to correlate gene expression with receptor function, GABAA ρ subunit mRNAs were detected with single‐cell RT‐PCR in CA3 pyramidal cells (from P3–P4 hippocampal slices), previously characterized with electrophysiological experiments for their bicuculline‐sensitive or ‐insensitive responses to GABA. In 6 of 19 cells (31%), pressure application of GABA evoked at –70 mV inward currents that persisted in the presence of 100 μM bicuculline (314 ± 129 pA). RT‐PCR performed in two of these neurons revealed the presence of ρ1 and ρ2 subunits, the latter being present with the α2 subunit. A ρ2 subunit was also found in 1 neuron (among 9) exhibiting a response to GABA, which was completely abolished by bicuculline. This might be due to the lack of putative accessory GABAA subunits that can coassemble with ρ2 to make functional receptors. Similar experiments from 10 P15 CA3 pyramidal cells failed to reveal any ρ1–3 transcripts. However, these neurons abundantly express α3 subunits. It is likely that in CA3 pyramidal cells of neonatal and adult hippocampus GABAA ρ subunits are present but at very low levels of expression.