Endogenous neuropeptide Y prevents recurrence of experimental febrile seizures by increasing seizure threshold

Abstract

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.

Fig. 1

(A) Electrophysiological features of hyperthermia-induced seizures in immature rats. EEGs were recorded using bipolar electrodes from dorsal hippocampus of 10-d-old rats. Top: Typical hippocampal activity in a euthermic animal, consisting of low-amplitude, nonrhythmic awake pattern (Dubé et al., 2000; Bender et al., 2003). Bottom: Rhythmic, high-amplitude epileptiform spike waves indicate a seizure. During this electrographic seizure, the animal was essentially motionless, with an occasional jerk (arrow) or oral automatisms. Calibration bars: 1 s; 50 μV. (B,C) Validity and correlation of brain and core temperatures in the euthermic immature rat. (B) Core temperatures of individual rats are tightly clustered within and among individuals. Symbols denote mean ± S.E. of five measurements obtained over 25 min from individual animals kept on a euthermic pad. These were identical to temperatures obtained in the home cage and represent normothermic values for the 10-d-old rat. (Values for rat no. 1 were too clustered to visualize error bars). (C) Brain temperatures, obtained under the same conditions, by direct dural measurements (see Materials and Methods). These temperatures were, on average, 2.8°C higher than core temperatures.

Fig. 2

Progressive increase of temperatures required for further seizure provocation after an experimental FS in the immature rat. (A) Threshold temperatures (measured at seizure onset) were significantly higher for the second seizure and elevated even further for the third (repeated measure ANOVA, p = 0.0015). As explained in the text, at this temperature range, brain and core temperatures converged. (B) Baseline core temperatures, measured immediately prior to induction of hyperthermia, were not influenced by previous seizures, elicited 3–4 h earlier.

Fig. 3

The progressive increase of threshold temperature for a second experimental FS was abolished by a selective antagonist of NPY Y2Rs. (A) Comparison of threshold for a first (1st) and second (2nd) seizure in untreated control rats (CTL) and those infused with vehicle (VEH). The asterisk (*) denotes a significant difference from the first seizure threshold. (B) Comparison of threshold for a first and second seizure in rats infused with vehicle and those infused with NPY receptor antagonist prior to the second seizure. The solid square indicates significantly lower threshold vs that for the second seizure in the vehicle-infused group.

Fig. 4

Experimental FSs induce NPY gene expression in immature rat hippocampus. (A) Hippocampal sections from naive and seizure-experiencing immature rats were subjected to ISH for NPY mRNA (see Materials and Methods). Scale bar = 1.3 mm. (B) Quantitative analysis of the NPY mRNA signal demonstrates increased expression in dentate gyrus interneurons by 4 h after the seizures, and in dentate gyrus, CA3 and CA1 by 24 h. Values are means with standard error bars. * denotes statistically significant difference compared with the naive (control) group. Note that for neuropeptides in general, increased secretion typically results in enhanced expression, so that the latter can be used as an indicator of endogenous peptide release.