ACTION OF ZOLPIDEM ON RESPONSES TO GABA IN RELATION TO MESSENGER-RNASFOR GABA(A) RECEPTOR-ALPHA SUBUNITS WITHIN SINGLE CELLS - EVIDENCE FOR MULTIPLE FUNCTIONAL GABA(A) ISORECEPTORS ON INDIVIDUAL NEURONS

The relationship between zolpidem sensitivity and GABA(A) receptor alp ha subunits was studied in individual dissociated neurons from rat bra in. Using whole-cell recording, similar EC50 values were demonstrated for the effect of gamma-aminobutyric acid (GABA) on gated-chloride cur rents from substantia nigra reticulata (SNR) and lateral septal neuron s. Subsequently, many neurons from both the SNR or lateral septum were found to exhibit enhanced GABA-gated chloride currents across concent rations of zolpidem ranging from 10 to 300 nM. Some neurons exhibited a greater than 20 % increase in responsiveness to GABA at 30 nM of zol pidem without further increase at higher concentrations of zolpidem. C onversely, zolpidem enhancement of GABA from another group of neurons was not observed at 30 nM zolpidem, but between 100 and 300 nM the res ponse to GABA increased greater than 20 %. Finally, a third group of n eurons reached both of these criteria for zolpidem enhancement of GABA . This latter spectrum of responses to GABA after varying concentratio ns of zolpidem was consistent with the presence of either two GABA(A) receptors or a single receptor with differing affinities for zolpidem on an individual neuron. Following determination of the sensitivity of neurons from SNR or lateral septum to zolpidem, cytoplasm was extract ed from some individual cells to allow identification of cellular mRNA s for the alpha 1, alpha 2 and alpha 3 GABA(A) receptor subunits with RT-PCR. Those neurons that responded to the 30 nM zolpidem concentrati on invariably expressed the alpha 1-GABA(A) receptor subunit. This res ult is consistent with the GABA(A) alpha 1-receptor subunit being an i ntegral part of a functional high-affinity zolpidem type 1-BZD recepto r complex on neurons in brain. Those neurons which showed enhancement of GABA from 100 to 300 nM zolpidem contained mRNAs for the alpha 2 an d/or the alpha 3 receptor subunits, a finding consistent with these al pha subunits forming type 2-BZD receptors. Some individual dissociated SNR neurons were sensitive to both low and high concentrations of zol pidem and contained mRNAs for all three alpha-receptor subunits. These latter individual neurons are proposed to have at least two functiona l GABA(A) receptor subtypes. Thus, the present investigation emphasize s the importance of characterizing the relationship between endogenous GABA(A) receptor function and the presence of specific structural com ponents forming GABA(A) receptor subtypes on neurons. (C) 1998 Elsevie r Science Ltd. All rights reserved.