Faculty Bibliography

This article introduces a new experimental paradigm for the study of temporal lobe epilepsy. This approach utilizes the isolated guinea pig brain in vitro preparation, which generates a pattern of hypersynchronous neuronal activity similar to the peculiar 8-30 Hz rhythm characterizing stereoelectroencephalographic hippocampal recordings in human temporal lobe epilepsy. The present report describes an attempt to identify the functional events underlying the epileptiform activities observed in this preparation. Rhythmic epileptiform discharges (EDs), here defined as population spikes (PSs) recorded from somata or dendritic layers, were induced in the hippocampal formation of the isolated guinea pig brain maintained in vitro by tetanic stimulation of the entorhinal cortex (EC). Two patterns of EDs were distinguished by performing simultaneous field potential recordings along the dentate gyrus (DG), EC, CA1, and CA3. During stage 1, the first self-sustained EDs were isolated PSs occurring at a frequency of 2-3 Hz at all levels of the entorhinal-hippocampal loop, the only exception being the DG, where no signs of synchronized neuronal discharge could be found. Over the next 30-50 sec, the temporal organization of these EDs changed dramatically. During stage 2, at all levels of the entorhinal-hippocampal loop, EDs occurred in 0.3-0.5 sec trains of 16-25 Hz population spikes interrupted by 0.7-1.3 sec silent periods. The transition between stages 1 and 2 coincided with the occurrence of population spikes in the DG. Laminar analyses and multiple simultaneous field potential recordings revealed that the trains of EDs observed in stage 2 resulted from the repetitive, sequential activation of the hippocampal-entorhinal loop. In the transverse axis, the earliest event usually occurred in the CA3 region. Thereafter, population spikes occurred sequentially in the CA1 region, EC, DG, and back to the CA3 region. Intracellular recordings confirmed that the EDs recorded extracellularly resulted from the synchronous activation of the cells in phase with the locally recorded field potentials. Dentate granule cells, layer II entorhinal cells, as well as CA1 pyramids displayed large-amplitude EPSPs crowned by an isolated action potential phase locked to the locally recorded field potential. In contrast, the activity of CA3 pyramids consisted of typical paroxysmal depolarization shifts on which bursts of action potentials of decreasing amplitude were observed. These results suggest that reentrant loop activity in the hippocampal-entorhinal circuit represents the central event in the functional organization of hippocampal epileptic discharges

To better understand the functional organization of the mammillary nuclei, we investigated the afferents to this nuclear complex in the rat with iontophoretically injected wheat germ agglutinin conjugated to horseradish peroxidase. Particular attention was paid to tracing local hypothalamic afferents to these nuclei. Injections into the medial mammillary nucleus (MMN) revealed strong projections from the subicular region, and weaker projections from the prefrontal cortex, medial septum, and the nucleus of the diagonal band of Broca. Other descending subcortical projections to the MMN arise from the anterior and the lateral hypothalamic area, the medial preoptic area, and the bed nucleus of the stria terminalis. Ascending afferents to the MMN were found to originate in the raphe and various tegmental nuclei. Following all injections into the MMN, labelled neurons were found in nuclei surrounding the mammillary body. The lateral and posterior subdivisions of the tuberomammillary nucleus projected mainly to the pars medianus and pars medialis of the MMN. The dorsal and ventral premammillary nuclei projected to the pars lateralis of the MMN. The supramammillary nucleus at rostral level had a small projection to the pars medialis and lateralis of the MMN. However, the most obvious projection from this nucleus was to the pars posterior of the MMN, chiefly from the lateral part of the caudal supramammillary nucleus. Injections into the lateral mammillary nucleus revealed inputs from the presubiculum, parasubiculum, septal region, dorsal tegmental nucleus, dorsal raphe nucleus, and periaqueductal gray. In addition, the lateral mammillary nucleus was found to receive a moderate projection from the medial part of the supramammillary nucleus and stronger projections from the lateral part of the caudal supramammillary nucleus. A very light projection was also seen from the lateral and posterior subdivisions of the tuberomammillary nucleus. These findings add to our knowledge of the extensive and complex connectivity of the mammillary nuclei. In particular, the local connections we have demonstrated with the supramammillary and tuberomammillary nuclei indicate the existence of significant local circuits as well as circuits involving more distant brain regions such as the septal nuclei, subiculum, prefrontal cortex, and brain stem tegmentum

The presence and distribution of dopaminergic neurons and terminals in the hypothalamus of the rat were studied by tyrosine hydroxylase (TH) immunohistochemistry. Strongly labelled TH-immunoreactive neurons were seen in the dorsomedial hypothalamic nucleus, periventricular region, zona incerta, arcuate nucleus, and supramammillary nucleus. A few TH-positive neurons were also identified in the dorsal and ventral premammillary nucleus, as well as the lateral hypothalamic area. TH-immunoreactive fibres and terminals were unevenly distributed in the mammillary nuclei; small, weakly labelled terminals were scattered in the medial mammillary nucleus, while large, strongly labelled, varicose terminals were densely concentrated in the internal part of the lateral mammillary nucleus. A few dorsoventrally oriented TH-positive axon bundles were also identified in the lateral mammillary nucleus. A dopaminergic projection to the mammillary nuclei from the supramammillary nucleus and lateral hypothalamic area was identified by double labelling with retrograde transport of wheat germ agglutinin-horseradish peroxidase and TH-immunohistochemistry. The lateral mammillary nucleus receives a weak dopaminergic projection from the medial, and stronger projections from the lateral, caudal supramammillary nucleus. The double-labelled neurons in the lateral supramammillary nucleus appear to encapsulate the caudal end of the mammillary nuclei. The medial mammillary nucleus receives a very light dopaminergic projection from the caudal lateral hypothalamic area. These results suggest that the supramammillary nucleus is the principal source of the dopaminergic input to the mammillary nuclei, establishing a local TH-pathway in the mammillary complex. The supramammillary cell groups are able to modulate the limbic system through its dopaminergic input to the mammillary nuclei as well as through its extensive dopaminergic projection to the lateral septal nucleus