The Role of Calcium-Permeable Kainate and Ampa Receptors in The Leading Reaction of Gabaergic Neurons to Excitation

While all neurons can intrinsically generate action potentials upon depolarization, those in the central nervous system do not fire constantly in response to numerous excitatory signals. This stability is attributed to control by an inhibitory GABAergic system. For this system to effectively prevent hyperexcitability, GABAergic neurons must generate inhibitory signals before glutamatergic excitation occurs, implying a faster response capability. This study demonstrates that in mature hippocampal cultures (14 DIV) and in brain slices from two-month-old rats, neurons do not respond simultaneously to general depolarization (induced by KCl, NH4Cl, or cAMP) or to glutamate receptor agonists (domoic acid, glutamate). Consistent with the above hypothesis, most GABAergic neurons fire before glutamatergic neurons. The delay in glutamatergic response is inversely proportional to stimulus intensity and can last dozens seconds. Crucially, this delay is abolished by GABA(A) receptor inhibitors, indicating a mechanism of preliminary GABA release. We show that early-responding GABAergic neurons express low-threshold, calcium-permeable kainate and AMPA receptors (CP-KARs and CP-AMPARs), which they use to generate a rapid response. However, when domoic acid application was repeated in the presence of the AMPA receptor inhibitor NBQX, the response delay was significantly increased. This result confirms that low-threshold CP-KARs localized on the presynaptic terminals of GABAergic neurons are responsible for the delay in glutamatergic neuron excitation. In hippocampal slices from two-month-old rats, overall depolarization with 50 mM KCl elicited distinct calcium responses in two neuronal populations. The majority of neurons (presumably glutamatergic) showed fluctuating Ca²⁺ signal, while a smaller group (presumably GABAergic) exhibited a steady, advancing [Ca²⁺]i increase. The subsequent application of domoic acid reinforced this distinction. Neurons that displayed an early, advancing Ca²⁺ response to KCl also responded to domoic acid with a similar advancing increase. In contrast, neurons that responded to KCl with delayed fluctuations also showed fluctuating responses to domoic acid, but with an even longer delay (80 s). Therefore, these experiments identify a subgroup of hippocampal neurons—both in slices and in culture—that respond with an early [Ca²⁺]i signal to both depolarization and glutamate receptor agonists (domoic acid, glutamate). Consistent with findings from cell cultures, we conclude that these early-responding neurons are GABAergic. Their early GABA release likely explains the delayed response observed in the glutamatergic neuron.