Abstracts

Adensine is an inhibitory synaptic modulator in the central nervous system (CNS). The main adenosine receptor in the CNS is A1R and its activation inhibits neurotransmitter release and hyperpolarizes neurons. Since adenosine release appears to be activity dependent, it may play an important role in regulating synaptic activity during periods of heightened neuronal firing such as occurs during epileptiform activity. We investigated the effects of modulating calcium influx on adenosine release during spontaneuous CA3 hippocampal network interictal discharges. We used extracellular and intracellular recordings to investigate the effects of decreasing postsynaptic calcium entry through NMDA receptors and L-type calcium channels on the probability of spontaneuous CA3 hippocampal network interictal discharges before and after pharmacological block of the adenosine A1Rs (theophylline (250 microM) of DPCPX (100nM)). Hippocampal coronal slices were prepared from 4-6 week old Sprague-Dauley rats. Spontaneous bursting of the CA3 network was induced by blockade of GABA[sub]A[/sub] and [sub]B[/sub] conductances with 100mM picrotoxin and 1mM CGP55845A, respectively. Intracellular recordings were performed in the presence of picrotoxin (100microM), CGP55845A (1 microM). Lowering of postsynaptic calcium influx (through block of NMDA and/or L-type calcium channels) resulted in a significant increase in the frequency of CA3 interictal discharges (n=5, p[lt]0.01, one tailed t-test). This frequency increase was reversed by prior blockade of adenosine A1Rs (n=5, p[lt]0.001, one tailed t-test), suggesting that postsynaptic calcium entry in the CA3 pyramidal cells plays a modulatory role in adenosine A1R activity. Intracellular voltage clamp recordings revealed that the time constant of recovery of spontaneuous EPSCs following an interictal discharge is dependent on adenosine A1R activity. The frequency, but not the amplitude recovery of spontaneous EPSCs was markedly affected by adenosine A1R activation, consistent with a presynaptic locus of adenosine action. These data suggest that postsynaptic calcium entry during hippocampal CA3 synchronous network activity results in significant modulation of adenosine release and, consequently, glutamate release. The dependence of adenosine release on postsynaptic calcium suggests that adenosine may act as a trans-synaptic neuromodulator at CA3 pyramidal synapses. Further, the adenosine modulation of post-burst synaptic recovery may control hippocampal CA3 network timing of synchronization. (Supported by NIH.)