Characterization of hippocampal norepinephrine release as measured by microdialysis perfusion: Pharmacological and behavioral studies

The release of endogenous norepinephrine in hippocampus was studied in freely moving rats with microdialysis perfusion. Using a loop-style dialysis probe, the basal amount of norepinephrine collected in 15-min fractions averaged 12pg/25/gml. Correcting for recovery (21%), the concentration of norepinephrine in the extracellular fluid of hippocampus under resting conditions was estimated to be approximately 14nM. The alpha 2 adrenoceptor antagonist yohimbine (5.0mg/kg, i.p.) increased norepinephrine efflux to 230% of basal levels. Clonidine (0.3 mg/kg, i.p.), an alpha 2 adrenoceptor agonist, decreased norepinephrine efflux to 56% of baseline. Addition of the reuptake blocker desipramine (1.0μM) to the perfusate had no significant effect on norepinephrine efflux. However, increasing the K + concentration of the perfusate to 30 mM increased norepinephrine efflux to 196% of baseline, and this effect was increased nearly two-fold by the addition of desipramine to the perfusate (364% of baseline). Restraint stress and intermittent tailshock increased norepinephrine efflux to 213% and 234% of baseline, respectively. The results suggest that microdialysis is a useful way to study norepinephrine release in hippocampus and they permit several conclusions to be drawn. First, the data obtained with systemic administration of alpha 2 adrenoceptor drugs emphasize the fact that a variety of regulatory mechanisms exist that may affect transmitter levels in the extracellular fluid. Second, the ratio of extracellular to intracellular norepinephrine in hippocampal tissue is considerably higher than that reported for dopamine in striatum. Coupled with the small effect of norepinephrine uptake blockade, this suggests that nerve terminal density is an important factor in determining the concentration of catecholamines in the extracellular fluid. Finally, the observed stress-induced increase in norepinephrine efflux supports the hypothesis that the locus coeruleus noradrenergic system exerts an important central influence during stressful conditions.