Presynaptic alterations associated with enhancement of evoked release and synthesis of norepinephrine in hippocampus of chronically cold-stressed rats

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We have previously demonstrated that prior exposure to chronic cold stress does not alter basal levels of norepinephrine (NE) release or synthesis in hippocampus of rat. However, in response to a subsequent novel stressor, an enhancement of both of these noradrenergic parameters is observed in the chronically stressed animals relative to naive controls. In the present experiments, we have examined whether the biochemical sensitization of NE release and synthesis produced by chronic stress can be demonstrated by local depolarization of the noradrenergic nerve terminals with elevated K +. The local application of elevated K + in dorsal hippocampus resulted in a greater increase in extracellular NE and 3,4-dihydroxyphenylacetic acid (DOPAC) in chronically stressed rats than in naive controls. It is proposed that in dorsal hippocampus, extracellular NE and DOPAC provide measures of NE release and biosynthesis, respectively. Therefore, these data suggest that local depolarization, similar to novel stress, elicits both enhanced NE release and synthesis in chronically stressed rats. One factor that is known to modulate both of these processes is the presynaptic α-2 adrenergic receptor. Therefore, we examined whether a change in the sensitivity of these receptors might contribute to the altered noradrenergic responsivity observed in chronically stressed rats. Local administration of clonidine, an α-2 receptor agonist, produced a decrease in extracellular NE and DOPAC in both naive and chronically stressed rats. The dose-response curve for the effect of clonidine on NE was shifted to the left in the latter group. In addition, local administration of idazoxan, an α-2 receptor antagonist, produced a greater increase in extracellular NE and DOPAC in the chronically stressed rats than in naive controls. These data suggest that an increase in the α-2 receptor modulation of NE release and synthesis is operative in the chronically stressed animals. Although this apparent supersensitivity of the α-2 receptors cannot readily explain the enhancement of evoked noradrenergic responses observed in chronically stressed rats, it may underlie the unaltered basal levels of NE release and synthesis observed in these animals.