Behavioral Effects of Systemic, Infralimbic and Prelimbic Injections of a Serotonin 5-HT Antagonist in Carioca High- and Low-Conditioned Freezing Rats.

The role of serotonin (5-hydroxytryptamine [5-HT]) and 5-HT receptors in anxiety has been extensively studied, mostly without considering individual differences in trait anxiety. Our laboratory developed two lines of animals that are bred for high and low freezing responses to contextual cues that are previously associated with footshock (Carioca High-conditioned Freezing [CHF] and Carioca Low-conditioned Freezing [CLF]). The present study investigated whether ketanserin, a preferential 5-HT receptor blocker, exerts distinct anxiety-like profiles in these two lines of animals. In the first experiment, the animals received a systemic injection of ketanserin and were exposed to the elevated plus maze (EPM). In the second experiment, these two lines of animals received microinjections of ketanserin in the infralimbic (IL) and prelimbic (PL) cortices and were exposed to either the EPM or a contextual fear conditioning paradigm. The two rat lines exhibited bidirectional effects on anxiety-like behavior in the EPM and opposite responses to ketanserin. Both systemic and intra-IL cortex injections of ketanserin exerted anxiolytic-like effects in CHF rats but anxiogenic-like effects in CLF rats. Microinjections of ketanserin in the PL cortex also exerted anxiolytic-like effects in CHF rats but had no effect in CLF rats. These results suggest that the behavioral effects of 5-HT receptor antagonism might depend on genetic variability associated with baseline reactions to threatening situations and 5-HT receptor expression in the IL and PL cortices. -CHF and CLF rats are two bidirectional lines that are based on contextual fear conditioning.-CHF rats have a more "anxious" phenotype than CLF rats in the EPM.-The 5-HT receptor antagonist ketanserin had opposite behavioral effects in CHF and CLF rats.-Systemic and IL injections either decreased (CHF) or increased (CLF) anxiety-like behavior.-PL injections either decreased (CHF) anxiety-like behavior or had no effect (CLF).