Effects of AP5 infusion into the lateral ventricle on the activities and hippocampal electrical patterns of sleep episodes in rats

Adenosine as an endogenous regulating factor of hippocampal sharp waves

The rodent hippocampus exhibits population activities called sharp waves (SPWs) during slow wave sleep and wake immobility. SPWs are important for hippocampal-cortical communication and memory consolidation, and abnormal sharp wave-ripple complexes are closely related to epileptic seizures. Although the SPWs are known to arise from the CA3 circuit, the local mechanisms underlying their generation are not fully understood. We hypothesize that endogenous adenosine is a local regulator of hippocampal SPWs. We tested this hypothesis in thick mouse hippocampal slices that encompass a relatively large hippocampal circuit and have a high propensity of generating spontaneous in vitro SPWs. We found that application of adenosine A1 receptor antagonists induced in vitro SPWs and that such induction was sensitive to blockade by NMDA receptor antagonists. By contrast, an increase in endogenous adenosine via pharmacological inhibition of adenosine transporters or adenosine degrading enzymes suppressed spontaneous in vitro SPWs. We thus suggest that the initiation and incidence of sharp wave-like population events are under tight control by the activity of endogenously stimulated A1 receptors.

Cellular mechanisms of cobalt-induced hippocampal epileptiform discharges

PURPOSE

To explore the cellular mechanisms of cobalt-induced epileptiform discharges in mouse hippocampal slices.

METHODS

Hippocampal slices were prepared from adult mice and briefly exposed to a CoCl(2)-containing external solution. Population and single cell activities were examined via extracellular and whole-cell patch recordings.

RESULTS

Brief cobalt exposure induced spontaneous, ictal-like discharges originating from the CA3 area. These discharges were suppressed by anticonvulsants, gap junction blockers, or by raising extracellular Ca(2+), but their generation was not associated with overall hyperexcitability or impairment in GABAergic inhibition in the CA3 circuit. Electroencephalographic ictal discharges of similar waveforms were observed in behaving rats following intrahippocampal cobalt infusion.

DISCUSSION

Mechanisms involving activity-dependent facilitation of gap junctional communication may play a major role in cobalt-induced epileptiform discharges.

Effects of REM deprivation and an NMDA agonist on the extinction of conditioned fear

Rapid eye movement sleep (REM) has been implicated in a number of learning and memory tasks. Previous research has demonstrated that REM deprivation impairs the development of extinction of conditioned fear responses. However, the neurobiological mechanisms of this effect remain unclear. The present study investigated the effects of systemic administration of d-cycloserine (DCS), an NMDA agonist, on the extinction of a conditioned fear response following 6 h of REM deprivation. In experiment 1, rats were administered DCS between fear training and REM deprivation. In experiment 2, rats were administered DCS prior to extinction training. The results of experiment 1 indicated that both DCS alone and REM deprivation alone impaired extinction learning. Administration of DCS to REM deprived animals partially, but not completely, reversed the deficit in extinction. The results of experiment 2 indicated that regardless of prior REM deprivation history, DCS facilitated extinction learning. The results provide further evidence for a role of REM in the extinction of cued fear learning and indicate that this effect appears to be partially mediated by NMDA-dependent mechanisms.