Commissural responses of rat retrohippocampal neurons

Hippocampus-mediated activation of superficial and deep layer neurons in the medial entorhinal cortex of the isolated guinea pig brain

The entorhinal cortex (EC) is regarded as the structure that regulates information flow to and from the hippocampus. It is commonly assumed that superficial and deep EC neurons project to and receive from the hippocampal formation, respectively. Anatomical evidences suggest that both the hippocampal output and deep EC neurons also project to superficial EC layers. To functionally characterize the interlaminar synaptic EC circuit entrained the by hippocampal output, we performed simultaneous intracellular recordings and laminar profile analysis in the medial EC (m-EC) of the in vitro isolated guinea pig brain after polysynaptic hippocampal activation by lateral olfactory tract (LOT) stimulation. Optical imaging of voltage-generated signals confirmed that the LOT-evoked hippocampus-mediated response is restricted to the m-EC. The hippocampal output generated an extracellular current sink in layers V-VI, coupled with an EPSP in deep neurons. Deep neuron firing was terminated by a biphasic IPSP. The earliest response observed in superficial layer neurons was characterized by a feedforward IPSP of circa 100 ms (-69 +/- 1.3 mV reversal potential) abolished by local application of 1 mm bicuculline. The feedforward IPSP was followed by a delayed EPSP blocked by AP-5 (100 microM), presumably mediated by deep-to-superficial m-EC connections. Our findings demonstrate that superficial m-EC cells are inhibited by the hippocampal output via a feedforward pathway that prevents activity reverberation in the hippocampal-EC-hippocampal loop. We propose that such inhibition could serve as a protective mechanism to prevent epileptic hyperexcitability.

Secondary epileptogenesis in humans

Secondary epileptogenesis as it applies to humans remains a controversial topic despite 40 years of investigation. Part of the controversy stems from disagreement about the definition of secondary epileptogenesis, and part of the controversy stems from the imperfect fit of animal models to the human epileptic syndromes. It may be that models of secondary epileptogenesis can be useful to describe specific epileptic syndromes such as bitemporal epilepsy and secondary bilateral synchrony, but other models may be required for remitting syndromes such as the Landau-Kleffner syndrome. The concept of secondary epileptogenesis may also provide a useful construct for evaluating patients with partial epileptic syndromes, especially those under consideration for epilepsy surgery, and for the evaluation of preventive strategies in epilepsy.

Basket-like interneurones in layer II of the entorhinal cortex exhibit a powerful NMDA-mediated synaptic excitation

Spiny stellate neurones of layer II of the entorhinal cortex (EC) provide the perforant path input to the dentate gyrus. Previous studies have shown that synaptic responses of these neurones are dominated by GABAergic inhibition. The present study describes intracellular recordings from 'fast-spiking' interneurones in layer II which may be the basis of the synaptic inhibition. Lucifer yellow fills of fast-spiking cells revealed neurones with a widespread axonal arborization forming basket-like complexes around unlabelled cells in layer II. Synaptic activation of the fast-spiking cells evoked long duration excitations which were mediated largely by NMDA receptors. A fast AMPA/kainate EPSP was also detectable. These neurones have morphological and physiological properties which make them well-suited to exert a widespread inhibitory control over the efferent output of layer II to the dentate gyrus.

Physiologic properties of human dentate granule cells in slices prepared from epileptic patients

The neurophysiological properties of human dentate granule cells were studied in hippocampal slices prepared from patients undergoing surgical treatment for medically intractable temporal lobe epilepsy. In 24 neurons which were morphologically identified as dentate granule cells by intracellular staining with biocytin, there were 2 types of synaptic responses to perforant path stimulation: one showed an EPSP-IPSP sequence (n = 10) and the other showed prolonged EPSPs without accompanying hyperpolarizing IPSPs (n = 14). The prolonged EPSPs were markedly retarded by the application of an NMDA receptor antagonist, APV. Membrane properties of neurons showing the different classes of synaptic responses were similar in resting membrane potential (pooled average: -56.2 mV +/- 0.94 SEM) and spike amplitude (pooled average: 65.2 mV +/- 1.69 SEM). However, membrane resistance tended to be lower in neurons with prolonged EPSPs (31.8 M omega +/- 2.63 SEM) than in neurons that showed EPSP-IPSP responses (40.2 +/- 4.33) (P less than 0.05, Fisher). No spontaneous and/or evoked burst firing was observed. These data provide fuller information on the neurophysiological properties of human dentate granule cells in surgically resected epileptogenic hippocampus, implicating a role of NMDA receptor activation in human temporal lobe epilepsy.