Neuronal apoptosis was observed in the rat dentate gyrus in two experimental models of human limbic epilepsy. Five hours after one hippocampal kindling stimulation, a marked increase of in situ terminal deoxynucleotidyltransferase-mediated dUTP nick-end labeling (TUNEL) of fragmented DNA was observed in nuclei located within and on the hilar border of the granule cell layer and in the polymorphic region. Forty kindling stimulations with 5-min interval produced higher numbers of labeled nuclei compared with one stimulation. The increase of TUNEL-positive nuclei was prevented by the protein synthesis inhibitor cycloheximide but not affected by the N-methyl-D-aspartate receptor antagonist MK-801. Kainic acidinduced seizures lead to a pattern of labeling in the hippocampal formation identical to that evoked by kindling. A large proportion of cells displaying TUNEL-positive nuclei was doublelabeled by the neuron-specific antigen NeuN, demonstrating the neuronal identity of apoptotic cells. Either 1 or 40 kindling stimulations also gave rise to a marked increase of the number of cells double-labeled with the mitotic marker bromodeoxyuridine and NeuN in the subgranular zone and on the hilar border of the dentate granule cell layer. The present data show that single and intermittent, brief seizures induce both apoptotic death and proliferation of dentate gyrus neurons. We hypothesize that these processes, occurring early during epileptogenesis, are primary events in the development of hippocampal pathology in animals and possibly also in patients suffering from temporal lobe epilepsy.

Pathophysiological activity and various kinds of traumatic insults are known to have deleterious long-term effects on neuronal Cl− regulation, which can lead to a suppression of fast postsynaptic GABAergic responses. Brain-derived neurotrophic factor (BDNF) increases neuronal excitability through a conjunction of mechanisms that include regulation of the efficacy of GABAergic transmission. Here, we show that exposure of rat hippocampal slice cultures and acute slices to exogenous BDNF or neurotrophin-4 produces a TrkB-mediated fall in the neuron-specific K+–Cl− cotransporter KCC2 mRNA and protein, as well as a consequent impairment in neuronal Cl− extrusion capacity. After kindling-induced seizures in vivo, the expression of KCC2 is down-regulated in the mouse hippocampus with a spatiotemporal profile complementary to the up-regulation of TrkB and BDNF. The present data demonstrate a novel mechanism whereby BDNF/TrkB signaling suppresses chloride-dependent fast GABAergic inhibition, which most likely contributes to the well-known role of TrkB-activated signaling cascades in the induction and establishment of epileptic activity.

Neural progenitors in the adult dentate gyrus continuously produce new functional granule cells. Here we used whole-cell patch-clamp recordings to explore whether a pathological environment influences synaptic properties of new granule cells labeled with a GFP-retroviral vector. Rats were exposed to a physiological stimulus, i.e., running, or a brain insult, i.e., status epilepticus, which gave rise to neuronal death, inflammation, and chronic seizures. Granule cells formed after these stimuli exhibited similar intrinsic membrane properties. However, the new neurons born into the pathological environment differed with respect to synaptic drive and short-term plasticity of both excitatory and inhibitory afferents. The new granule cells formed in the epileptic brain exhibited functional connectivity consistent with reduced excitability. We demonstrate a high degree of plasticity in synaptic inputs to adult-born new neurons, which could act to mitigate pathological brain function.

The neuropeptide galanin has been shown to suppress epileptic seizures. In cortical and hippocampal areas, galanin is normally mainly expressed in noradrenergic afferents. We have generated a mouse overexpressing galanin in neurons under the plateletderived growth factor B promoter. RIA and HPLC analysis revealed up to 8-fold higher levels of galanin in transgenic as compared with wild-type mice. Ectopic galanin overexpression was detected especially in dentate granule cells and hippocampal and cortical pyramidal neurons. Galanin-overexpressing mice showed retardation of seizure generalization during hippocampal kindling, a model for human complex partial epilepsy. The high levels of galanin in mossy fibers found in the transgenic mice were further increased after seizures. Frequency facilitation of field excitatory postsynaptic potentials, a form of short-term synaptic plasticity assessed in hippocampal slices, was reduced in mossy fiber-CA3 cell synapses of galanin-overexpressing mice, indicating suppressed glutamate release. This effect was reversed by application of the putative galanin receptor antagonist M35. These data provide evidence that ectopically overexpressed galanin can be released and dampen the development of epilepsy by means of receptor-mediated action, at least partly by reducing glutamate release from mossy fibers.G alanin, a 29-aa neuropeptide (1), is widely distributed throughout the rat brain (2, 3) and coexists with classic transmitters in several systems (4), with particularly high levels in noradrenergic locus coeruleus neurons (5, 6). Galanin-binding sites have been mapped (7,8), and three receptor subtypes, GalR1 (9-11), 49), and GalR3 (15), have been cloned. There is evidence for involvement of galanin in multiple neuronal functions (16,17), as also revealed in recent studies on transgenic mice (18)(19)(20).Many studies support an important role of neuropeptides in seizure activity (21)(22)(23). This role is true also for galanin. Thus, intrahippocampal injection of galanin during status epilepticus (SE) in rats has powerful anticonvulsant effects (24). In accordance with these data, mice overexpressing the galanin gene under the dopamine-␤-hydroxylase promoter, presumably in noradrenergic fibers, show increased resistance to SE and kainate-and pentylenetetrazol-evoked seizures (25). Conversely, administration of galanin antagonists, such as M35 (26), and targeted disruption of the galanin gene increase seizure susceptibility in these models (24,25). Taken together, available data indicate that galanin released from noradrenergic fibers (3, 5, 27) can suppress epileptic seizures. Although the underlying mechanisms are not known, it has been shown that galanin can inhibit glutamate release by means of activation of presynaptic ATP-dependent K ϩ channels (28).We speculated that galanin release from neurons not normally containing this neuropeptide, e.g., in the epileptic focus, might be necessary to optimize the seizure-suppressant action of galanin. To explore the functional conseque...