Differential regulation of parvocellular neuronal activity in the paraventricular nucleus of the hypothalamus following single vs. repeated episodes of water restriction-induced drinking

Abstract

Acute activation of the hypothalamic-pituitary-adrenal (HPA) axis releases glucocorticoids to maintain homeostasis, whereas prolonged exposure to elevated glucocorticoids has deleterious effects. Due to the potential benefits of limiting stress-induced glucocorticoid secretion, the present study uses drinking in dehydrated rats as a model to delineate mechanisms mobilized to rapidly inhibit HPA activity during stress. Using Fos expression as an indicator of neuronal activation, the effect of a single or repeated episode of dehydration-induced drinking on the activity of magnocellular and parvocellular neurons in the paraventricular nucleus (PVN) of the hypothalamus was examined. Adult male rats underwent a single episode or repeated (six) episodes of water restriction and were sacrificed before or after drinking water in the AM. Plasma osmolality, vasopressin (AVP), adrenocorticotropic hormone (ACTH) and corticosterone were elevated by water restriction and reduced after drinking in both models. Fos expression was elevated in AVP-positive magnocellular PVN neurons and AVP- and corticotropin releasing hormone (CRH)-positive parvocellular PVN neurons after water restriction. Fos expression was reduced in magnocellular AVP neurons after both models of restriction-induced drinking. In contrast, Fos expression did not change in AVP and CRH parvocellular neurons after a single episode of restriction-induced drinking, but was reduced after repeated episodes of restriction-induced drinking. These data indicate that drinking-induced decreases in glucocorticoids in dehydrated rats involve multiple factors including reduction in magnocellular release of vasopressin and reduction in parvocellular neuronal activity. The differential inhibition of PVN parvocellular neurons after repeated rehydration may reflect a conditioned response to repeated stress reduction.

Figure 1

Double-immunolabeling for Fos (black) and CRH (brown, left panels) or AVP (brown, right panels) in medial parvocellular (mp) and posterior magnocellular (pm) PVN neurons. High magnification of neurons double-labeled for Fos and CRH (A, area denoted by box in panel E) and for Fos and AVP (B, area denoted by box in panel F) from rats that were water restricted one time. Labeling of neurons in rats that had water ad libitum (C and D), in rats that were water restricted one time (E and F), in rats that were water restricted one time and received water for 30 min (G and H) and in rats that were water restricted six times and received water for 30 min (I and J). White arrow indicates Fos-positive parvocellular neurons phenotypically labeled for CRH (A) or AVP (B). Black arrow indicates Fos-positive magnocellular neuron labeled for AVP (B). Scale bar = 25μM (A and B); bar = 100 μM (C-J).

Figure 2

The effect of single and repeated 23.5h episodes of water restriction and drinking on the percentage of Fos-positive AVP or CRH neurons in the PVN. Both a single episode and repeated episodes of water restriction increased Fos expression in AVP magnocellular (pmPVN) (A), CRH parvocellular (mpPVN) (B), and AVP parvocellular (mpPVN) neurons (C). Drinking for 15 min after a single episode of water restriction reduced Fos expression in AVP magnocellular neurons (A), but not in CRH (B) or AVP parvocellular neurons (C). Drinking after repeated episodes of water restriction reduced Fos expression in AVP magnocellular neurons (A) as well as CRH (B) and AVP parvocellular neurons (C). Values represent means ± SEM; n=6/group. * = p < 0.05 vs. AL, ^ = p < 0.05 vs. WR, Single Episode, # = p < 0.05 vs. WR+Water, Single Episode, + = p < 0.05 vs. WR, Repeated Episode.

Figure 3

The effect of a single 47.5h episode of water restriction and drinking on the percentage of Fos-positive AVP or CRH neurons in the PVN. A single 47.5h episode of water restriction increased Fos expression in AVP magnocellular (pmPVN) (A), CRH parvocellular (mpPVN) (B), and AVP parvocellular (mpPVN) neurons (C). Drinking for 15 min reduced Fos expression in AVP magnocellular neurons (A) but not in CRH (B) or AVP parvocellular neurons (C). Values represent means ± SEM; AL, n=4; WR and WR+Water, n=8. * = p < 0.05 vs. AL, ^ = p < 0.05 vs. WR.