Infusions of an excitant amino acid, N-methyl-D-aspartate (NMDA) into the inferior colliculus (IC) render normal rats susceptible to audiogenic seizures (AGS) and/or spontaneous audiogenic-like seizures without tonic components. The excess excitant amino acid in the IC and the anticonvulsant effects of NMDA antagonists in genetically epilepsy-prone rats (GEPRs), along with innate norepinephrine (NE) deficits and anticonvulsant effects of NE agonists in these animals suggest a mutual role of excitant amino acids and NE in regulating AGS in GEPRs. Saline or 6-hydroxydopamine (6-OHDA, 4 micrograms/side in 2 microliters) was infused bilaterally into the locus coeruleus (LC) of normal male rats and guide cannulas were implanted into the IC. Two weeks later, NMDA was infused bilaterally into the IC (0.5 microliters; 10 nmol/side) and 10 min later the rats were subjected to an electric bell (110 db, 60 s) unless preceded by spontaneous tonic seizures. Tonic seizures were not observed in male rats following NMDA infusions in rats with LC infusions of saline. However, a marked increase in the incidence of tonic seizures was observed in the 6-OHDA-treated rats which were markedly depleted of brain NE as determined by HPLC. These findings indicate that a NE deficit greatly enhances the incidence of tonic convulsions and support the hypothesis that an excitant amino acid excess in the GEPR IC may act to initiate AGS, whereas the NE deficit may allow expression of the tonic components of AGS seen in some GEPRs.