Abstract
Seasonal affective disorder (SAD) involves a number of psychological and behavioral impairments that emerge during the low daytime light intensity associated with winter, but which remit during the high daytime light intensity associated with summer. One symptom frequently reported by SAD patients is reduced sexual interest and activity, but the endocrine and neural bases of this particular impairment during low daylight intensity is unknown. Using a diurnal laboratory rodent, the Nile grass rat (Arvicanthis niloticus), we determined how chronic housing under a 12:12 h day/night cycle involving dim low-intensity daylight (50 lux) or bright high-intensity daylight (1,000 lux) affects males’ copulatory behavior, reproductive organ weight, and circulating testosterone. We also examined the expression of mRNAs for the aromatase enzyme, estrogen receptor 1 (ESR1), and androgen receptor (AR) in the medial preoptic area (mPOA; brain site involved in the sensory and hormonal control of copulation), and mRNAs for the dopamine (DA) D1 and D2 receptors in both the mPOA and nucleus accumbens (NAC; brain site involved in stimulus salience and motivation to respond to reward). Compared to male grass rats housed in high-intensity daylight, males in low-intensity daylight displayed fewer mounts and intromissions when interacting with females, but the groups did not differ in their testes or seminal vesicle weights, or in their circulating levels of testosterone. Males in low-intensity daylight unexpectedly had higher ESR1, AR and D1 receptor mRNA in the mPOA, but did not differ from high-intensity daylight males in D1 or D2 mRNA expression in the NAC. Reminiscent of humans with SAD, dim winter-like daylight intensity impairs aspects of sexual behavior in a male diurnal rodent. This effect is not due to reduced circulating testosterone and is associated with upregulation of mPOA steroid and DA receptors that may help maintain some sexual motivation and behavior under winter-like lighting conditions.
Highlights
Seasonal affective disorder (SAD) is a recurrent major depressive disorder with a seasonal pattern that in most cases worsens in fall and winter, but remits in spring and summer (Rosenthal et al, 1984; American Psychiatric Association, 2013)
Daytime light intensity did not affect males’ testes weights (1.40 ± 0.03 vs. 1.45 ± 0.44 g/g bodyweight × 100; t(13) = 0.96, p = 0.35) or seminal vesicle weights (0.91 ± 0.12 vs. 0.89 ± 0.07 g/g bodyweight × 100; t(13) = 0.18, p = 0.85). It did not affect their levels of circulating testosterone (F(1,34) = 1.83, p = 0.18), there was a significant effect of when blood was taken, such that male grass rats sacrificed during the day had higher circulating testosterone compared to males sacrificed at night (F(1,34) = 4.23, p = 0.049; Figure 2)
While we found no effect of daytime light intensity on gonadal function, we did find significantly higher circulating testosterone in male grass rats sacrificed in the morning compared to those sacrificed at night
Summary
Seasonal affective disorder (SAD) is a recurrent major depressive disorder with a seasonal pattern that in most cases worsens in fall and winter, but remits in spring and summer (Rosenthal et al, 1984; American Psychiatric Association, 2013). Many studies have reported seasonal variation in circulating gonadal steroids in humans (Smals et al, 1976; Ronkainen et al, 1985; Kauppila et al, 1987a,b; Kivelä et al, 1988; Dabbs, 1990; Meriggiola et al, 1996; Valero-Politi and FuentesArderiu, 1998; Garde et al, 2000; Wisniewski and Nelson, 2000; van Anders et al, 2006; Stanton et al, 2011; Demir et al, 2016), but there is no evidence that SAD patients have atypical gonadal hormone levels at any time of year ( they do for some pituitary and adrenal hormones—Jacobsen et al, 1987; Avery et al, 1997; Martiny et al, 2004; Thorn et al, 2011). There is some evidence that the winter-time reduction in libido does not depend on the presence of a mood disorder (Bossini et al, 2009)
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