Abstracts

RATIONALE: There is emerging evidence that kainate receptors play a role in seizure generation and epileptogenesis in the amygdala. Physiological studies have demonstrated that GluR5 kainate receptors mediate a portion of excitatory transmission in the basolateral amygdala (BLA) and also contribute to seizure induction in response to kainate receptor agonists. Presynaptic kainate receptors, which regulate excitatory and inhibitory transmitter release in other brain areas, could also play a role in amygdala seizures. We used immunoelectron microscopy with subunit specific antibodies to define the pre- and postsynaptic localization of GluR5 at synapses. We also examined the distribution of KA2, which may be an auxiliary subunit in heteromeric GluR5-containing kainate receptors.
METHODS: Brain sections from adult male Sprague-Dawley rats were processed for immunocytochemistry with affinity-purified polyclonal antibodies raised against synthetic peptides corresponding to sequences in the carboxy termini of human/rat GluR5 (18-mer) and KA2 (20-mer). Specific labeling was determined by preadsorption of the antisera with the synthetic peptides. The antibody was visualized with the immunoperoxidase method at the light and electron microscopic levels.
RESULTS: GluR5 immunocytochemistry at the light level revealed specific labeling of neuronal somata and processes in the BLA. Electron microscopy revealed a non-uniform distribution of labeling, with a predominance of labeling confined to dendrites and spines. Of 511 synapses examined, 164 exhibited postsynaptic labeling. Labeling over presynaptic terminals and axons was rare ([lt]5 synapses). Similarly, the KA2 antibody was highly localized to postsynaptic structures and was generally distributed in a similar fashion to GluR5.
CONCLUSIONS: Our results indicate that the GluR5 and KA2 kainate receptor subunits are primarily located on postsynaptic elements in BLA, consistent with physiological studies supporting a role for these receptors as mediators of synaptic excitation in the amygdala. The similar overall pattern of immunostaining with the GluR5 and KA2 antibodies is consistent with the existence of GluR5/KA2 heteromers. Presynaptic GluR5 and KA2 receptors, if they occur, are infrequent.
[Supported by: National Institute of Neurological Disorders and Stroke]