GABAB receptor subtypes differentially modulate chemoconvulsant-induced seizure

Ying Chen1, Joshua Foster1, Charlotte Mann1, Irina Vinogradova1, Ian Kitchen1, Bernhard Bettler2. 1Faculty of Health and Medical Sciences, University of Surrey, Guildford, GU2 7XH, United Kingdom, 2Department of Biomedicine, Institute of Physiology, Pharmazentrum, University of Basel, Basel, CH-4056, Switzerland.

Epilepsy is a neurological disorder caused by imbalanced excitatory and inhibitory activities in the brain. GABAB receptors are the G-protein-coupled receptors for the inhibitory neurotransmitter GABA and its activation modulate both the excitatory and inhibitory synaptic transmission in the brain. GABAB receptors are strongly implicated in the genesis and spread of seizures1, however, the precise roles of GABAB receptor subtypes that are located at different synaptic compartments2 and neuronal networks are yet to be defined. In mice lacking either the GABAB1a or GABAB1b isoforms2, we examined the roles of the GABAB(1a,2) and GABAB(1b,2) receptors in the development of seizure.

Epileptic behaviours in mice were triggered by an injection (subcutaneous route at 60 mg/kg) of a chemoconvulsant pentylenetetrazol (PTZ). Epileptic behaviours were monitored for 45 min and scored according to the modified severity scale (1, hypoactivity; 2, partial seizures; 3, generalised seizures; 4, tonic-clonic seizures; 5, mortality due to seizure). To examine the origin of the differences in epileptic behaviour, distributions of the GABAB(1a,2) and GABAB(1b.2) receptors in the brain were examined by immunolabelling of GABAB1 and GABAB2 proteins. In addition, the Schaffer collateral-CA1 field excitatory postsynaptic potentials (fEPSPs) and population spikes, were recorded using a multi-eletrode array system (MED64) in hippocampal slices and the modulation by GABAB receptor activation was examined 3.

Seizures behaviours developed immediately after the injection of PTZ and progressed with increasing severity according to the scale. The mean maximum seizure scores were found to be 3.2±0.2 for wild-type mice, 4.8±0.2 for GABAB1a-/- and 3.3±0.3 for GABAB1b-/- mice, showing significantly increased seizure severity in the GABAB1a-/- mice (p<0.01, one-way ANOVA). The GABAB(1a,2) receptors may, therefore, be essential for the control of seizure activity. Functional heteromeric GABAB receptors labelled by the GABAB2 antibody showed that the expression of GABAB(1a,2) receptors in the GABAB1b-/- mice was uniform across the brain, but in the GABAB1a-/- mice, the GABAB(1b,2) receptors were absent in the caudate putamen, globus pallidus, amygdala and CA3 stratum lucidum, which are brain areas potentially involved in the genesis and spread of seizure activities. GABAB(1a,2) receptors in these brain regions may, therefore, be essential for the control of seizure. In addition, in GABAB1a-/- mice, neither the fEPSPs or the population spike were significantly inhibited by the GABAB receptor agonist, baclofen (50 µM, 95.0 ± 11.7 % of control); whereas baclofen significantly inhibited these activities in the wildtype (16.3 ± 3.2 % of control) and GABAB1b-/- mice (34.6 ± 4.3 % of control), confirming an essential role for the GABAB(1a,2) receptors in heterosynaptic inhibition.

In conclusion, the GABAB(1a,2) receptor subtype is shown to play an essential role in the control of chemoconvulsant-induced seizure. The anatomical and synaptic localisation of the subtype may be responsible for the action. It is possible that deficits in the transcriptional and posttranslational mechanisms for GABAB1a isoforms could lead to increased seizure susceptibility.

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