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.

A glue-based, screw-free method for implantation of intra-cranial electrodes in young mice.

Intra-cranial electroencephalographic recordings are increasingly employed in mice because of the availability of genetically manipulated mouse models. Currently, dental acrylic and anchoring screws are used to cement implanted electrodes. This technique works well for adult animals but often encounters difficulty when employed in young mice because their skulls are not strong enough to bear the anchoring screws. Here we describe a novel method favorable for implantation of intra-cranial electrodes in mice as young as postnatal 18 days and suitable for long-term intra-cranial electroencephalographic recordings. Our approach is to construct a multi-electrode assembly according to the desired stereotaxic coordinates of intra-cranial recordings and to secure the implanted electrode assembly to the skull via glue rather than dental acrylic/anchoring screws. The surgical operation for such electrode implantation is relatively quick and rarely associated with complications such as infection, bleeding, neurological deficits, spontaneous seizures or behavioral disturbances. The implanted electrodes are stable, allowing repeated monitoring for several months. Data obtained by simultaneous intra-hippocampal and intra-cortical recordings indicate that our method is suitable for the examination of behaviorally related electroencephalographic activities and experimentally induced seizures. Technical aspects of our methods are discussed, and the procedures for constructing the electrode assembly are presented in detail.

Regulation of hippocampal sharp waves by Ca2+-dependent slow after hyperpolarization.

In rodent hippocampal pyramidal neurons, repetitive discharges are followed by a slow afterhyperpolarization (sAHP) as a result of activation of a Ca2+-dependent K+ current. The sAHP is sensitive to activation of several G-protein coupled neurotransmitter receptors and downstream signal cascades. Modulations of the sAHP have been shown to be closely associated with synaptic plasticity, learning, and aging processes. However, it is presently unclear whether the sAHP generation is involved in hippocampal network activities. We explored this issue using an in vitro (thick-slice) model of mouse hippocampal sharp waves. Our data show that the sAHP occurs in CA3 pyramidal neurons following each sharp wave event and sAHP suppression is associated with a large increase in occurrence frequency of spontaneous sharp waves. Considering that sharp waves are important for hippocampal-cortical communication and memory processes, we postulate that the sAHP serves as an intrinsic regulatory mechanism of sharp waves and plays a significant role in hippocampus-dependent cognitive functions.