AXON TERMINAL HYPEREXCITABILITY ASSOCIATED WITH EPILEPTOGENESIS IN-VITRO .2. PHARMACOLOGICAL REGULATION BY NMDA AND GABA(A) RECEPTORS

1. The preceding report presented evidence that the kindling-like induction of electrographic seizures (EGSs) in the hippocampal slice is accompanied by a lasting increase in the excitability of CA3 axon terminals, which is manifested by an increase in action-potential initiation at this site. In this report we explore the role of the N-methyl-D-aspartate (NMDA) receptor in the induction and maintenance of this antidromic firing, as well as the role of the gamma-aminobutyric acid type A (GABA(A)) receptor in regulating this activity once it has been induced. 2. Kindling-like stimulus trains (60 Hz, 2s) were delivered to s. radiatum of CA3 at 10-min intervals. As EGSs developed in control artificial cerebrospinal fluid (ACSF), the frequency of axon terminal firing increased markedly (by 10.33 +/- 3.29 spikes/min, mean +/- SE P much less than 0.01). The prior application of the competitive NMDA antagonist D-2-amino-5-phosphonovaleric acid (D-APV, 50 or 100 muM) prevented the induction of EGSs and suppressed the increase in terminal firing seen in control ACSF (mean increase 1.06 +/- 1.11 spikes/min, P < 0.02). However, when D-APV was applied only after EGSs and antidromic spikes were induced in control ACSF, it failed to alter the frequency of terminal firing (mean 6.44 +/- 2.03 in control ACSF, 8.89 +/- 2.31 in APV; P much greater than 0.1). Thus the NMDA receptor is required for the induction but not maintenance of increased axon terminal firing, as we previously have shown to be the case for EGSs. 3. Measurements of antidromic stimulus threshold at several points along the course of the Schaffer collateral axons of CA3 cells showed that EGS induction is accompanied by a significant decrease in threshold at synaptic terminal regions but not at nonterminal regions. This decrease in threshold in the terminal region did not occur in separate experiments during which D-APV (50 mum) was preapplied to block the induction of EGSs. Considering the similar effects of D-APV on the induction and maintenance of antidromic firing, we conclude that during EGS induction the NMDA receptor mediates a selective increase in the excitability of CA3 axon terminals, leading to antidromic firing. Once induced, this axon terminal hyperexcitability is maintained by mechanisms independent of the NMDA receptor. These results also further strengthen the link between axon terminal hyperexcitability and EGS expression. 4. Spontaneous antidromic action potentials were frequently observed in association with spontaneous or evoked GABA(A) receptor-mediated inhibitory postsynaptic potentials (IPSPs). Therefore, in our search for an NMDA receptor-independent mechanism that maintains axon terminal backfiring, we applied the GABA(A) antagonists bicuculline methochloride (BIC, 1 or 10 muM) and picrotoxin (PTX, 1 muM). These agents completely suppressed antidromic firing in all of seven cells tested. (+/-)-Baclofen (0.5-1 muM), which suppresses the release of GABA via activation of presynaptic GABA(B) receptors on interneurons, also suppressed antidromic firing, although less dramatically (mean frequency in ACSF: 16.9 +/- 8.81; in baclofen: 5.16 +/- 4.81; P = 0.1). These results demonstrate that the axon terminal backfiring induced by EGS induction is subsequently regulated by GABA(A) receptors, raising the possibility that GABAergic presynaptic inhibition is present in the hippocampus and may be upregulated in some forms of neural plasticity. 5. We discuss the pharmacological regulation of the processes leading to axon terminal backfiring as well as those maintaining this activity once induced. We give special attention to the location of the NMDA receptors responsible for this change, to the possible role of presynaptic inhibition in these processes, and to the relationship of terminal backfiring to epileptogenesis and other forms of neural plasticity.