Flesinoxan pretreatment differentially affects corticosterone, prolactin and behavioural responses to a flesinoxan challenge

To determine whether alterations in 5-HT1A receptor mediated responses induced by a single injection with a selective 5-HT1A receptor agonist is a transient effect, or whether the (de)sensitisation is more persistent, rats were pretreated with the selective and full 5-HT1A receptor agonist, flesinoxan (3 mg/kg SC once daily) for either 1 day or 1 week. Twenty-four hour after the last pretreatment injection, rats were challenged with flesinoxan (3 mg/kg SC), and the effects on plasma corticosterone and prolactin levels, lower lip retraction and behaviour in the shock-probe burying test were determined. Several 5-HT1A receptor mediated responses were modified differentially following the flesinoxan pretreatment. However, all changes induced by a single flesinoxan injection remained present upon repeated flesinoxan administration. The differential changes in the responses to flesinoxan cannot easily be explained by differences in pre- or postsynaptically 5-HT1A mediated responses. The prolactin response to flesinoxan, which is thought to be mediated postsynaptically, was enhanced, whereas the corticosterone response to flesinoxan, which is also mediated postsynaptically, was attenuated. The presynaptically mediated lower lip retraction response was attenuated as well, whereas the behavioural effects of flesinoxan remained relatively unaffected following repeated flesinoxan administration. Upon prolonged flesinoxan pretreatment, the changes induced by a single flesinoxan injection remained present or increased further. Although repeated flesinoxan administration (1 day and 1 week) resulted in 20% lower plasma flesinoxan concentrations, this effect could not explain the neuroendocrine and behavioural findings.