Differential expression and functions of GABAB receptor isoforms in the hippocampus

Joshua Foster, Irina Vinogradova, Ying Chen

University of Surrey, Guildford, UK

GABAB receptor activation results in synaptic inhibition in neural circuitries. The GABAB receptors are heterodimers comprised of GABAB1 and GABAB2 subunits. The GABAB1 subunit has two major isoforms, GABAB1a and GABAB1b, producing two main receptor subtypes with GABAB2 subunits. No subtype specific pharmacological agents are currently available to distinguish these receptor subtypes, but GABAB1a -/- and GABAB1b -/- mice have been generated. Using these mice, it was demonstrated that GABAB(1a, 2) and GABAB(1b, 2) were expressed preferentially on the presynaptic glutamatergic terminals and postsynaptic sites, respectively (Vigot et al., 2006). The aim of this study is to investigate the differential expression patterns and functional effects of GABAB receptor isoforms in the hippocampus.

Male adult BALB/c wild-type (WT), GABAB1a -/- or GABAB1b -/- mice were used for electrophysiological or immunohistochemical investigations. Free floating immunoperoxidase labelling of the GABAB1 and GABAB2 receptor subunits was performed on coronal brain sections using antibodies directed against the GABAB1 or GABAB2 subunits. Field excitatory postsynaptic potentials (fEPSPs) from CA1 stratum radiatum were recorded in response to paired stimuli using a multi-electrode device (MED64).

A loss of GABAB1 and GABAB2 immuno-positive staining was seen in the stratum lucidum of the CA3 in GABAB1a -/- mice, but was not seen in GABAB1b -/- or WT mice, in agreement with the findings of Vigot et al. (2006). Intense GABAB2 staining was found in the proximal region of the molecular layer of the dentate gyrus in the GABAB1b -/- and WT mice, but absent in the GABAB1a -/- mice. These results agree with the suggestion that the GABAB1a is located in regions associated with presyantpic glutamate terminals. The fEPSPs recorded in the CA1 region of the slices from mutant and WT mice showed no significant differences in the input-output relationship. GABAB receptor activation by baclofen suppressed the fEPSPs initial slope and amplitude in the WT and GABAB1b -/- but not GABAB1a -/- mice. At 50 μM baclofen, fEPSP slope in GABAB1a-/- mice was 95.0 ± 11.7 % of control, compared to 34.6 ± 4.3 % and 16.3 ± 3.2 % for GABAB1b -/- and WT, respectively. The reduction of the fEPSPs by baclofen was accompanied by a large increase in the paired-pulse facilitation. Recordings of the population spikes (stimulus intensity 100-140 μA) in the CA1 area showed a decrease in paired-pulse depression in response to baclofen as described previously (Chen et al., 2006). The results show that the GABAB1a but not the GABAB1b receptor subtype is important in modulating the CA1 fEPSPs in hippocampal slices, agreeing with a presynaptic localisation of the GABAB1a receptor isoform.

Both the anatomical and electrophysiological studies have thus provided further evidence to support the idea that GABAB receptor isoforms are expressed at different synaptic locations to modulate different aspects of synaptic transmission.

Vigot et al., 2006, Neuron, 50, 589-60

Chen et al., 2006, JPET, 317, 1170- 1177