The inhibitory neurotransmitter GABA activates several types of receptors that are therapeutic targets for the treatment of epilepsy and other neuropsychiatric illness. GABAA receptors are linked to a Cl ion channel and produce electrical signals when activated at synapses. GABAB receptors are G- protein coupled receptors (GPCRs) that regulate intracellular signaling pathways. In addition to synaptic GABAA receptors, there are unique GABAA receptors localized to extrasynaptic sites. These high-affinity extrasynaptic GABAA receptors are tonically activated by the low levels of ambient GABA in the brain. The tonic form of inhibition produced by extrasynaptic GABAA receptors importantly affects cellular and network excitability. Emerging data indicate that extrasynaptic GABAA receptors in several brain regions are regulated by postsynaptic GABAB receptors. Because the intracellular signaling pathways affected by GABAB receptors are shared by many GPCRs, other neurotransmitter receptors may also regulate extrasynaptic GABAA receptors. This proposal describes three hypotheses to investigate the regulation of extrasynaptic GABAA receptors in health and disease. It is hypothesized that multiple neurotransmitters acting through GPCRs regulate extrasynaptic GABA receptors. Tonic currents produced by extrasynaptic GABAA receptors will be recorded from dentate gyrus granule cells (DGGCs) and CA1 pyramidal neurons in acute hippocampal brain slices using standard whole cell techniques. The effects of intracellular signaling pathways on tonic currents will be investigated. Using selective agonists and antagonists, the effects of other GPCRs (i.e. - adrenoreceptors, D2 dopamine receptors, and 5-HT2 receptors) on tonic currents will be determined. It is predicted that tonic currents will be enhanced by PKA inhibition (D2 receptors) and PKC activation (5HT2 receptors). Activation of PKA by -adrenoreceptors is predicted to reduce tonic currents. Biochemical studies will be done to determine if intracellular signaling pathways enhance tonic currents by increasing surface expression of GABAA receptors. Spontaneous physiological or pathophysiological neural activity can produce periodic and transient increases in ambient GABA levels. It is hypothesized that periodic increases in ambient GABA will activate postsynaptic GABAB receptors and enhance extrasynaptic GABAA receptor function. Tonic currents will be repeatedly measured before and during procedures to periodically increase ambient GABA. Three approaches will be used to produce transient increases in ambient GABA; 1. periodic application of exogenous GABA, 2. stimulating release of endogenous GABA (electrical stimulation, high K+), and 3. inducing spontaneous interictal discharges with 4-AP. It will be confirmed that the effects of these procedures resulted from postsynaptic GABAB receptor activation using the GABAB receptor antagonist CGP55845 and pipette solutions containing a G protein inhibitor (GDP--s). Traumatic brain injury (TBI) alters GABAA receptor function in the hippocampus. It is hypothesized that GPCRs regulate extrasynaptic GABAA receptors following experimental TBI. We will determine the effects of TBI on basal tonic currents and their regulation by GPCRs. Specifically, it is hypothesized that tonic currents of DGGCs will be enhanced by TBI, due in part to increased activation of GPCRs.
