Cerebral tryptophan hydroxylase activity, and 5-HT1A receptor, 5-HT2A receptor, and 5-HT transporter binding in grouped and isolated Roman RHA and RLA rats: relationships with behaviours in two models of anxiety

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Abstract
Male Roman low-(RLA) and high-avoidance (RHA) rats differ when tested in the elevated plus-maze and the black/white box, but not when (isolated and) tested for their social interaction. Herein, we have analysed the impact of prior isolation on male Roman rats tested in the first two models of anxiety; moreover, because central serotonin (5-HT) systems in Roman rats have been scarcely studied, we have also analysed several anxiety-related indices of central serotonergic activity in grouped/isolated Roman rats. Group-housed RLA rats tested in the elevated plus-maze and the black/white box were less anxious than their RHA counterparts, thereby confirming our previous study. Isolation had anxiogenic (and hypolocomotor) effects, these being significant in RLA rats only. Tryptophan hydroxylase activity in midbrain (but not in cortex, hippocampus or hypothalamus) was lower in group-housed (but not in isolated) RLA rats than in RHA rats, a difference independent from changes in the regulatory properties of the enzyme. Neither midbrain and hippocampal [3H]8-hydroxy-2-(di-n-propylamino)-tetrlin binding at 5-HT1A receptors, nor midbrain [3H] citalopram binding at the 5-HT transporter was different between grouped/isolated RHA/RLA rats. Alternatively, a trend toward a lower hypothalamic [3H]citalopram binding in (group-housed) RLA rats than in RHA rats could be noted, whereas cortical [3H]ketanserin binding at 5-HT2A receptors was lower in RLA rats than in RHA rats, a difference prevented by prior isolation. This study opens the possibility that inter-line differences in 5-HT2A receptors partly (or totally) underlie the respective behaviours of RHA and RLA rats in the elevated plus-maze and the black/white box.