Seizures induced by fever (febrile seizures) are the most frequent seizures affecting infants and children; however, their impact on the developing hippocampal formation is not completely understood. Such understanding is highly important because of the potential relationship of prolonged febrile seizures to temporal lobe epilepsy. Using an immature rat model, we have previously demonstrated that prolonged experimental febrile seizures render the hippocampus hyperexcitable throughout life. Here we examined whether (1) neuronal loss, (2) altered neurogenesis, or (3) mossy fiber sprouting, all implicated in epileptogenesis in both animal models and humans, were involved in the generation of a pro-epileptic, hyperexcitable hippocampus by these seizures. The results demonstrated that prolonged experimental febrile seizures did not result in appreciable loss of any vulnerable hippocampal cell population, though causing strikingly enhanced sensitivity to hippocampal excitants later in life. In addition, experimental febrile seizures on postnatal day 10 did not enhance proliferation of granule cells, whereas seizures generated by kainic acid during the same developmental age increased neurogenesis in the immature hippocampus. However, prolonged febrile seizures resulted in long-term axonal reorganization in the immature hippocampal formation: Mossy fiber densities in granule cell- and molecular layers were significantly increased by 3 months (but not 10 days) after the seizures. Thus, the data indicate that prolonged febrile seizures influence connectivity of the immature hippocampus long-term, and this process requires neither significant neuronal loss nor altered neurogenesis. In addition, the temporal course of the augmented mossy fiber invasion of the granule cell and molecular layers suggests that it is a consequence, rather than the cause, of the hyperexcitable hippocampal network resulting from these seizures.

Febrile seizures (FSs) typically occur at the onset of fever and do not recur within the same febrile episode despite enduring or increased hyperthermia. Recurrent seizures during the same febrile episode are considered "complex," with potentially altered prognosis. A characterized immature rat model of FS was used to test the hypotheses that (1) a first FS influences the threshold temperature for subsequent ones, and (2) the underlying mechanisms involve the release and actions of the endogenous inhibitory hippocampal neuropeptide Y (NPY). Experimental FSs were induced two or three times, at 3- to 4-h intervals, and threshold temperatures measured. To determine the potential effects of seizure-induced endogenous NPY on thresholds for subsequent seizures, an antagonist of the major hippocampal NPY receptor (type 2) was infused prior to induction of the second seizure. As an indicator of NPY release, NPY expression was determined 4 and 24 h later. Threshold core and brain temperatures for hyperthermic seizures were consistent with those observed during human fever. Threshold temperatures for a second and third seizure were significantly and progressively higher than those required for the first. This "protective" effect involved induction of endogenous NPY because it was abolished by the NPY antagonist. In addition, NPY mRNA expression was increased in dentate gyrus, CA3 and CA1, after an experimental FS, consistent with peptide release. Collectively these data indicate that the absence of repetitive seizures during a febrile episode involves the inhibitory actions of endogenous NPY, suggesting that the signaling cascade triggered by this peptide might provide targets for therapeutic intervention.

The principal form of synchronized network activity in neonatal hippocampus consists of low frequency 'giant depolarizing potentials' (GDPs). Whereas contribution of both GABA and glutamate to their generation has been demonstrated, full understanding of the mechanisms underlying these synchronized activity bursts remains incomplete. A contribution of the h-current, conducted by HCN channels, to GDPs has been a topic of substantial interest. Here we focus on HCN1, the prevalent HCN channel isoform in neonatal hippocampus, and demonstrate an HCN1 spatiotemporal expression pattern in both CA3 principal cells and interneurons that correlates with the developmental profile of GDPs. Abrogation of HCN physiological function in CA3, via the selective I(h)-blocker ZD7288, disrupts GDP generation. Furthermore, ZD7288 specifically abolishes spontaneous bursting of the CA3 pyramidal cells at frequencies typical of GDPs without major influence on interneuronal firing. These findings support a pivotal role for HCN channels expressed by CA3 neurons, and particularly CA3 pyramidal cells, in GDP-related network synchronization.