Incremental conductance levels of GABAA receptors in dopaminergic neurones of the rat substantia nigra pars compacta

1. Molecular and biophysical properties of GABA(A) receptors of dopaminergic (DA) neurones of the pars compacta of the rat substantia nigra were studied in slices and after acute dissociation. 2. Single-cell reverse transcriptase-multiplex polymerase chain reaction confirmed that DA neurones contained mRNAs encoding for the alpha 3 subunit of 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. Overall, DA neurones present a, pattern of expression of GABA(A) receptor subunit mRNAs containing mainly alpha 3/4 beta 2/3 gamma 3. 3. Outside-out patches were excised from DA neurones and GABA(A) single-channel patch-damp currents were recorded under low doses (1-5 mu M) of GABA or isoguvacine, a selective GABA(A) agonist. Recordings presented several conductance levels which appeared to be integer multiples of an elementary conductance of 4-5 pS. This property was shared by GABA(A) receptors of cerebellar Purkinje neurones recorded in slices (however, with an elementary conductance of 3 pS). Only the 5-6 lowest levels were analysed. 4. A progressive change in the distribution of occupancy of these levels was 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 benzodiazepine binding site: both treatments enlarged the occupancy of the highest conductance levels, while decreasing that of the smallest ones. Conversely, Zn2+ (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) receptors 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 DA neurones.