Effects of rapid eye movement sleep deprivation on hypocretin neurons in the hypothalamus of a rat model of depression

Abstract

Hypocretin (Hcrt, also known as orexin) is a hypothalamic neuropeptide linked to narcolepsy, a disorder diagnosed by the appearance of rapid eye-movement sleep (REMS)-state characteristics during waking. Major targets of Hcrt-containing fibers include the locus coeruleus and the raphe nucleus, areas with important roles in regulation of mood and sleep. A relationship between REMS and mood is suggested by studies demonstrating that REMS-deprivation (REMSD) ameliorates depressive symptoms in humans. Additional support is found in animal studies where antidepressants and REMSD have similar effects on monoamiergic systems thought to be involved in major depression. Recently, we have reported that Wistar-Kyoto (WKY) rats, an animal model of depression, have reduced number and size of hypothalamic cells expressing Hcrt-immunoractivity compared to the parent, Wistar (WIS) strain, suggesting the possibility that the depressive-like attributes of the WKY rat may be determined by this relative reduction in Hcrt cells [Allard, J.S., Tizabi, Y., Shaffery, J.P., Trouth, C.O., Manaye, K., 2004. Stereological analysis of the hypothalamic hypocretin/orexin neurons in an animal model of depression. Neuropeptides 38, 311-315]. In this study, we sought to test the hypothesis that REMSD would result in a greater increase in the number and/or size of hypothalamic, Hcrt-immunoreactive (Hcrt-ir) neurons in WKY, compared to WIS rats. The effect of REMSD, using the multiple-small-platforms-over-water (SPRD) method, on size and number of Hcrt-ir cells were compared within and across strains of rats that experienced multiple-large-platforms-over-water (LPC) as well as to those in a normal, home-cage-control (CC) setting. In accord with previous findings, the number of Hcrt-ir cells was larger in all three WIS groups compared to the respective WKY groups. REMSD produced a 20% increase (p<0.02) in the number of hypothalamic Hcrt-ir neurons in WKY rats compared to cage control WKY (WKY-CC) animals. However, an unexpected higher increase in number of Hcrt-ir cells was also observed in the WKY-LPC group compared to both WKY-CC (31%, p<0.001) and WKY-SPRD (20%, p<0.002) rats. A similar, smaller, but non-significant, pattern of change was noted in WIS-LPC group. Overall the data indicate a differential response to environmental manipulations where WKY rats appear to be more reactive than WIS rats. Moreover, the findings do not support direct antidepressant-like activity for REMSD on hypothalamic Hcrt neurons in WKY rats.

Fig. 1

Mean (±SEM) values for body weight changes after 7 days (n = 6 in each treatment group). Final body weight is expressed as a percentage of the individual weights at the start of the experiment. SPRD, small platform rapid eye movement sleep deprived, LPC, large platform control, CC, dry cage control, WKY, Wistar-Kyoto, WIS, Wistar. Bracketed lines connect significantly different pairs within strains (Bonferroni corrected t-tests). (a) p = 0.002, (b) p = 0.0001.

Fig. 2

Mean (±SEM) values for average number of Hcrt1-ir neurons in the left hypothalamus of each strain of rats (n = 6 except for WKY-CC where n = 5, see text). Group-naming conventions follow those in Fig. 1. Bracketed lines connect significantly different pairs within strains (Bonferroni corrected t-tests). (a) p = 0.02, (b) p = 0.002, (c) p = 0.001. Pairs of like-treatment groups are significantly different across the two strains of rats (WIS-CC vs WKY-CC, p < 0.005, WIS-LPC vs WKY-LPC, p < 0.0001, WIS-SPRD vs WKY-SPRD, p < 0.01).

Fig. 3

Coronal sections showing distribution of hypocretin-1 immunoreactive neurons in the hypothalamus. Frames A–C are sections taken from similar levels of the lateral hypothalamus, taken from WKY rats in the three different treatments (A) control, (B) large platform control, (C) REM sleep deprived. Note that the cumulative differences noted between groups derives from data collected across whole set of slides (15–18 per animal). LH, later hypothalamus; f, perifornical nucleus; 3V, third ventricle; opt, optic tract. Scale bar in C applies to all.

Fig. 4

Mean (±SEM) values for volume of Hcrt1-ir neurons in hypothalamus. There were no systematic differences in cell sizes between any of the groups. Group naming conventions follow those in Fig. 1.