Incremental conductance levels of GABA(A) receptors in dopaminergic neurones of the rat substantia nigra pars compacta

1. Molecular and biophysical properties of GABA(A) receptors of dopaminergi c (DA) neurones of the pars compacta of the rat substantia nigra were studi ed in slices and after acute dissociation. 2. Single-cell reverse transcriptase-multiplex polymerase chain reaction co nfirmed that DA neurones contained mRNAs encoding for the alpha 3 subunit o f the GABA(A) receptor, but further showed the presence of alpha 4 subunit mRNAs. alpha 2, beta 1 and gamma 1 subunit mRNAs were never detected. Overa ll, DA neurones present a, pattern of expression of GABA(A) receptor subuni t mRNAs containing mainly alpha 3/4 beta 2/3 gamma 3. 3. Outside-out patches were excised from DA neurones and GABA(A) single-cha nnel patch-damp currents were recorded under low doses (1-5 mu M) of GABA o r isoguvacine, a selective GABA(A) agonist. Recordings presented several co nductance levels which appeared to be integer multiples of an elementary co nductance of 4-5 pS. This property was shared by GABA(A) receptors of cereb ellar Purkinje neurones recorded in slices (however, with an elementary con ductance of 3 pS). Only the 5-6 lowest levels were analysed. 4. A progressive change in the distribution of occupancy of these levels wa s observed when increasing the isoguvacine concentration (up to 10 mu M) as well as when adding zolpidem (20-200 nM), a drug acting at the benzodiazep ine binding site: both treatments enlarged the occupancy of the highest con ductance levels, while decreasing that of the smallest ones. Conversely, Zn 2+ (10 mu M), a negative allosteric modulator of GABA(A) receptor channels, decreased the occupancy of the highest levels in favour of the lowest ones . 5. These properties of alpha 3/4 beta 2/3 gamma 3-containing GABA(A) recept ors would support the hypothesis of either single GABA(A) receptor channels with multiple open states or that of a synchronous recruitment of GABA(A) receptor channels that could involve their clustering in the membranes of D A neurones.