Contingent tolerance to carbamazepine: alterations in TRH mRNA and TRH receptor binding in limbic structures

Abstract

Tolerance to carbamazepine's anticonvulsant effects on amygdala kindled seizures occurs contingently, that is, only when carbamazepine is given prior to, but not after the seizure occurs. Biological correlates of contingent tolerance were examined using in situ hybridization and receptor binding techniques for thyrotropin-releasing hormone (TRH) mRNA and TRH receptor binding. Rats were fully kindled and given daily injections of carbamazepine (15 mg/kg, i.p.) either 15 min before (CBZ-before) or after (CBZ-after) amygdala stimulation until the CBZ-before rats became tolerant. Kindled rats were matched so that the two groups had an equal number of seizures and doses of CBZ. Three other groups were also used for comparison: kindled rats that received vehicle injections, and sham-kindled animals treated with either vehicle or CBZ. Rats were sacrificed 4 h after the last seizure or sham stimulation. Both sham-kindled rat groups had barely detectable levels of TRM mRNA. In the CBZ-after (non-tolerant) and vehicle-kindled rats, TRH mRNA levels were increased in the dentate gyrus, pyriform, entorhinal, and perirhinal cortices. In contrast to the other kindled animals, the CBZ-before rats (tolerant) had dramatically diminished TRH mRNA levels bilaterally in the dentate gyrus and pyriform cortex, and ipsilateral to the stimulation in the entorhinal cortex. Decreases in TRH receptor binding were demonstrated autoradiographically in the dentate gyrus and perirhinal cortex in all of the kindled groups with no differences between tolerant and non-tolerant rats. The reported TRH mRNA alterations are, thus, specifically associated with the development of contingent tolerance and are not attributable to either the occurence of seizures or repeated drug exposure alone. The role of TRH inn carbamazepine's anticonvulsant effects and the functional consequences of the failure of CBZ-tolerant animals to show seizure-induced increases in TRH mRNA warrants further investigation.