Visualization, Properties, and Functions of GABAergic Hippocampal Neurons Containing Calcium-Permeable Kainate and AMPA Receptors

Abstract

Calcium-permeable kainate (CP-KARs) and AMPA (CP-AMPARs) receptors of the brain neurons are active participants of synaptic plasticity and neurotransmitter release trigger. In this paper, CP-KARs and CP-AMPARs were identified in hippocampal neuroglial culture on 14–17 day of cultivation by a characteristic Ca2+ response to a selective agonist of CP-KARs and CP-AMPARs, domoic acid (DA), and to a selective agonist of CP-KARs, ATPA. It was shown that DA at a concentration of 300 nM caused a rapid intracellular Ca2+ concentration increase in two minor subpopulations of neurons. Both subpopulations were found to be GABAergic neurons that were positively stained with antibodies against glutamate decarboxylase 65 and 67 (GAD65/67). The antagonist of CP-AMPARs, NASPM, did not suppress Ca2+ response to DA in the neurons of the first subpopulation. The selective agonist of CP-KARs, ATPA, increased [Ca2+]i to the same extent as DA only in the first subpopulation of GABAergic neurons. An inhibitor of GABA(A) receptors, bicuculline, did not increase the amplitude of Ca2+ response to DA in this subpopulation, indicating the absence of CP-KARs in the postsynaptic membrane, where GABA(A) receptors are located. Thus, these GABAergic neurons can be attributed to neurons containing CP-KARs, which are apparently located in the presynaptic membrane of the GABAergic neurons. The [Ca2+]i increase caused by the DA application in the second subpopulation was completely suppressed by NASPM, an inhibitor of CP-AMPARs. NASPM reduced the Ca2+ oscillations amplitude in the same subset, indicating the involvement of CP-AMPARs in the Ca2+ impulse formation during synchronous calcium activity. For this reason, the neurons of this subpopulation can be attributed to the GABAergic neurons containing CP-AMPARs. Most of the neurons in the hippocampal cell culture (70–85%) were not stained with antibodies against GAD65/67 and responded to the DA by increasing the calcium oscillations frequency with a delay. The amplitude of DA-induced oscillations increased in the presence of NASPM in the subpopulation of inhibitory neurons containing CP-KARs, indicating their innervation by inhibitory neurons containing CP-AMRARs. This increase in the Ca2+ oscillation amplitude in the inhibitory neurons containing CP-KARs correlated with a decrease in the amplitude of synchronous calcium activity in a large subpopulation (42 ± 6% of cells) of glutamatergic neurons, suggesting innervation of the latter by inhibitory neurons containing CP-KARs. Thus, GABAergic neurons containing CP-KARs and CP-AMPARs can work in tandem, controlling the activity of individual subpopulations of neurons.