Social isolation in the rat enhances α 2-autoreceptor function in the hippocampus in vivo

Abstract

This study investigated the effects of isolation rearing from weaning in rats on extracellular noradrenaline in the dorsal hippocampus in vivo, measured using microdialysis. Male Lister hooded rats were obtained at weaning and reared in social isolation or in groups for six to eight weeks. Basal noradrenaline efflux did not differ between isolation- and group-reared rats. Local K + stimulation (50 and 100 mM) increased noradrenaline efflux in the hippocampus of both groups of rats; however, this effect was greater in group-reared rats (50 mM K +). The α 2-adrenoceptor agonist, clonidine (0.3 mg/kg, i.p.), reduced noradrenaline efflux in both groups of rats, but this decrease was greater in isolates. Systemic (1.0 mg/kg, i.p.) and local (via the probe; 100 μM) administration of the α 2-adrenoceptor antagonist, idazoxan, increased noradrenaline efflux, but these responses were also greater in isolation-reared rats. The magnitudes of the idazoxan-induced increases in noradrenaline efflux were similar for both systemic and local administration, indicating that presynaptic terminal α 2-adrenoceptors were predominantly involved. Furthermore, although tail pinch increased noradrenaline efflux in both isolation- and group-reared rats, there was a significant attenuation in the response in the isolation- compared to group-reared rats. Taken together with previous findings, the present results provide evidence in favour of an isolation-induced enhancement in the function of the presynaptic terminal α 2-autoreceptor in the dorsal hippocampus in vivo, resulting in decreased functional responsiveness of hippocampal noradrenergic nerve terminals.