Subcellular localization of GABA receptors in the central nervous system using post-embedding immunohistochemistry

Loss of cholecystokinin-containing terminals in temporal lobe epilepsy

Altered GABA-mediated inhibition is proposed to play a role in the pathogenesis of epilepsy. Previous studies have demonstrated a loss of somatostatin-containing GABAergic interneurons innervating granule cells in epileptic animals. However, the reorganization of synapses between interneurons and granule cells has not been investigated. We studied synapse organization in an animal model of temporal lobe epilepsy (TLE) using continuous hippocampal stimulation. The distribution of axon terminals and inhibitory synapses on granule cell dendrites was studied using a combination of immunohistochemistry and pre-embedding electron microscopy techniques. A whole-cell patch-clamp technique was applied to study the functional changes in GABAergic input from different interneurons. In epileptic animals, the density of cholecystokinin (CCK)-immunoreactive (IR) fibers and α2 subunit containing GABAA receptors in the inner molecular layer of the dentate gyrus was reduced. Quantitative immuno-electron microscopy study revealed that the ratio of CCK-containing symmetric synapses to the total symmetric synapses was reduced. The frequency of GABAergic synaptic currents (sIPSC) was decreased and their amplitude was increased. The inhibitory effect of the activation of cannabinoid 1 (CB1) receptors was also reduced in epileptic animals. Isolation of CCK- and parvalbumin (PV)-containing GABAergic inputs by N- and P/Q-type calcium channel blockers respectively suggested that GABA release from CCK-containing interneurons was selectively reduced in epileptic rats. This study found that there was a loss of CCK-containing GABAergic synapses to granule cells both morphologically and functionally. These studies add to our understanding of the mechanisms that contribute to altering GABAergic inhibition of granule cells in TLE.

Diminished neurosteroid sensitivity of synaptic inhibition and altered location of the alpha4 subunit of GABA(A) receptors in an animal model of epilepsy

In animal models of temporal lobe epilepsy (TLE), neurosteroid sensitivity of GABA(A) receptors on dentate granule cells (DGCs) is diminished; the molecular mechanism underlying this phenomenon remains unclear. The current study investigated a mechanism for loss of neurosteroid sensitivity of synaptic GABA(A) receptors in TLE. Synaptic currents recorded from DGCs of epileptic animals (epileptic DGCs) were less frequent, larger in amplitude, and less sensitive to allopregnanolone modulation than those recorded from DGCs of control animals (control DGCs). Synaptic currents recorded from epileptic DGCs were less sensitive to diazepam and had altered sensitivity to benzodiazepine inverse agonist RO 15-4513 (ethyl-8-azido-6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5alpha][1,4]benzodiazepine-3-carboxylate) and furosemide than those recorded from control DGCs. Properties of synaptic currents recorded from epileptic DGCs appeared similar to those of recombinant receptors containing the alpha4 subunit. Expression of the alpha4 subunit and its colocalization with the synaptic marker GAD65 was increased in epileptic DGCs. Location of the alpha4 subunit in relation to symmetric (inhibitory) synapses on soma and dendrites of control and epileptic DGCs was examined with postembedding immunogold electron microscopy. The alpha4 immunogold labeling was present more commonly within the synapse in epileptic DGCs compared with control DGCs, in which the subunit was extrasynaptic. These studies demonstrate that, in epileptic DGCs, the neurosteroid modulation of synaptic currents is diminished and alpha4 subunit-containing receptors are present at synapses and participate in synaptic transmission. These changes may facilitate seizures in epileptic animals.

Selective loss of dentate hilar interneurons contributes to reduced synaptic inhibition of granule cells in an electrical stimulation-based animal model of temporal lobe epilepsy

Neuropeptide-containing hippocampal interneurons and dentate granule cell inhibition were investigated at different periods following electrical stimulation-induced, self-sustaining status epilepticus (SE) in rats. Immunohistochemistry for somatostatin (SOM), neuropeptide Y (NPY), parvalbumin (PV), cholecystokinin (CCK), and Fluoro-Jade B was performed on sections from hippocampus contralateral to the stimulated side and studied by confocal laser scanning microscopy. Compared to paired age-matched control animals, there were fewer SOM and NPY-immunoreactive (IR) interneurons in the hilus of the dentate gyrus in animals with epilepsy (40-60 days after SE), and 1, 3, and 7 days following SE. In the hilus of animals that had recently undergone SE, some SOM-IR and NPY-IR interneurons also stained for Fluoro-Jade B. Furthermore, there was electron microscopic evidence of the degeneration of SOM-IR interneurons following SE. In contrast, the number of CCK and PV-IR basket cells in epileptic animals was similar to that in controls, although it was transiently diminished following SE; there was no evidence of degeneration of CCK or PV-IR interneurons. Patch-clamp recordings revealed a diminished frequency of inhibitory postsynaptic currents in dentate granule cells (DGCs) recorded from epileptic animals and animals that had recently undergone SE compared with controls. These results confirm the selective vulnerability of a particular subset of dentate hilar interneurons after prolonged SE. This loss may contribute to the reduced GABAergic synaptic inhibition of granule cells in epileptic animals.

Early GABA(A) receptor clustering during the development of the rostral nucleus of the solitary tract

While there is an abundance of gamma-aminobutyric acid (GABA) in the gustatory zone of the nucleus of the solitary tract of the perinatal rat, we know that GABAergic synapse formation is not complete until well after birth. Our recent results have shown that GABA(B) receptors are present at birth in the cells of the nucleus; however, they do not redistribute and cluster at synaptic sites until after PND10. The present study examined the time course of appearance and redistribution of GABA(A) receptors in the nucleus. GABA(A) receptors were also present at birth. However, in comparison to GABA(B) receptors, GABA(A) receptors underwent an earlier translocation to synaptic sites. Extrasynaptic label, for example, of GABA(A) receptors was non-existent compared to GABA(B) receptors at PND10 and well-defined clusters of GABA(A) receptors could be seen as early as PND1. We propose that while GABA(A), receptors may play an early neurotransmitter role at the synapse, GABA(B) receptors may play a non-transmitter neurotrophic role.

Calcium-dependent maintenance of agrin-induced postsynaptic specializations

Although much progress has been made in understanding synapse formation, little is known about the mechanisms underlying synaptic maintenance and loss. The formation of agrin-induced AChR clusters on cultured myotubes requires both activation of the receptor tyrosine kinase MuSK and intracellular calcium fluxes. Here, we provide evidence that such AChR clusters are maintained by agrin/MuSK-induced intracellular calcium fluxes. Clamping intracellular calcium fluxes after AChR clusters have formed leads to rapid MuSK and AChR tyrosine dephosphorylation and cluster dispersal, even in the continued presence of agrin. Both the dephosphorylation and the dispersal are inhibited by the tyrosine phosphatase inhibitor pervanadate. In contrast, clamping intracellular calcium at the time of initial agrin stimulation has no effect on agrin-induced MuSK or AChR phosphorylation, but blocks AChR cluster formation. These findings suggest an avenue by which postsynaptic stability can be regulated by modification of intracellular signaling pathways that are distinct from those used during synapse formation.