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Anticonvulsant Actions of Glutamate Receptor Antagonists Against Audiogenic Seizures in Adult Rats with Neonatal Hypothyroidism.

It has been reported that rats exposed neonatally to propylthiouracil (PTU), an antithyroid agent, develop marked susceptibility to audiogenic seizures after maturation. The immediate early gene, c-fos, is rapidly and transiently induced by a variety of stimuli, and seizures represent one of the stimuli that induce drastic expression of c-fos in the brain. We have reported the specific expression of c-fos messenger RNA (mRNA) after generalized tonicclonic seizures in PTU rats. In this study, we examined the effect of an N-methyl-D-aspartate (NMDA)receptor antagonist on c-fos mRNA expression after audiogenic seizures by means of in situ hybridization. Six pregnant Sprague-Dawley rats were housed individually and allowed to litter normally. On the day of delivery (day 0), all pups were collected, randomized, and then redelivered to each dam (9-10 pups per dam). Four dams received 0.02% PTU in the drinking water from days 0 through 19 of postparturition (PTU rats), and two dams received water only (control). At age 7-8 weeks, the seizure susceptibility was confirmed by auditory stimulation for 80 s. Whereas control rats showed no behavioral change, all the PTU rats exhibited running fits followed by tonic-clonic seizures. At age 4 months, six PTU-treated and six control rats were killed 30 rnin and 60 min (n = 3 each) after the initiation of auditory stimulation. Furthermore, six PTU-treated rats were pretreated intraperitoneally with MK-801 (1 mg/kg) or the vehicle (n = 3 each) 30 rnin before auditory stimulation and killed 30 rnin after the auditory stimulation. The brains were quickly removed and stored at -70°C until in situ hybridization. Sixteen-millimeter sections were hybridized with a [35S]UTP-labeled antisense riboprobe, which was synthesized with SP6 RNA polymerase from a 405 base-pair fragment (nucleotides 171 1-21 16) of rat clfos cDNA. The specificity of the probes was confirmed by the absence of signals in sections hybridized with sense probes and Northern blot hybridization. After hybridization, the sections were rinsed, air dried, and then exposed to Kodak X-OMAT film for 1 week at room temperature. Relative optical density values for the autoradiograms were determined by computer-assisted densitometry . The robust seizure-induced expression of c-fos mRNA both 30 rnin and 60 rnin after auditory stimulation was confirmed in the central gray, peripeduncular nucleus, inferior colliculus, septal nucleus, red nucleus of stria terminalis, and dorsomedial hypothalamus. In the neocortex, dorsal hippocampus, and medial geniculate body, the c-fos mRNA induction was comparable in control and PTU-treated rats after auditory stimulation. Pretreatment with MK801 abolished both the behavioral manifestation of generalized seizures and the expression of c-fos mRNA in specific areas, such as the inferior colliculus, septal nucleus, bed nucleus of stria terminalis, and amygdaloid nuclei. Although preliminary, our results suggest that NMDA receptors in the inferior colliculus and related structures are involved in the mechanisms underlying the development of audiogenic seizures.