DISTRIBUTION AND KINETICS OF GALANIN INFUSED INTO THE VENTRAL HIPPOCAMPUS OF THE RAT - RELATIONSHIP TO SPATIAL-LEARNING

A recent study has shown that ventral hippocampal galanin plays a role in spatial learning and that it has an inhibitory effect on basal ace tylcholine release [Ogren S. O. ei al. (1996) Neuroscience 75, 1127-11 40]. The present studies were designed to compare the in vivo tissue d istribution and kinetics of infused galanin (porcine) with the tempora l effect of galanin on spatial learning in the rat. Daily bilateral mi croinfusions of galanin (1.5 mmol/side for five days) via chronic cann ulae placed in the ventral hippocampus produced a significant impairme nt of acquisition of the spatial task when infused 20 min, but not 5 o r 60 min, before the daily training session. No overall impairment of memory retention (examined 24 h after the last training session) was o bserved in the galanin-treated rats. These results indicate that galan in given in the ventral hippocampus produces a time-dependent effect o n acquisition. Using an antibody to porcine galanin and immunohistoche mistry, galanin infused in the ventral hippocampus was found to be dis tributed mainly within the ventral part of the hippocampus and around the infusion site. The infused galanin was rapidly cleared from the ex traceilular space between 5 and 20 min after infusion. Five minutes af ter infusion of galanin, a number of cells in the ventral hippocampus, both within and outside the zone of extracellularly located galanin, showed a positive galanin-like immunoreactivity. These cells appear mo rphologically to be medium-sized neurons with a similar position as ce lls showing neuropeptide Y-like immunoreactivity; At 20 and 60 min aft er infusion of galanin, no cells with detectable levels of galanin-lik e immunoreactivity could be seen. These results indicate that the temp oral kinetics and distribution of infused galanin are of major importa nce for its behavioural effect in the ventral hippocampus. The rapid c learance of the infused galanin and its internalization by neuronal en docytotic mechanisms may be important for its effect on cognition. (C) 1997 IBRO. Published by Elsevier Science Ltd.