Pubertal hormones mediate sex differences in levels of myelin basic protein in the orbitofrontal cortex of adult rats

Abstract

Previous work from our lab revealed that adult female rats have increased levels of myelin basic protein (MBP), a marker for myelination, in the orbitofrontal cortex (OFC) as compared to adult males. The goal of the present study was to determine the role of gonadal hormones, acting either in adulthood or at puberty, in the development of an adult sex difference in OFC levels of MBP. In an initial experiment, we replicated our previous results demonstrating that gonadally intact female rats have increased levels of MBP in the OFC as compared to males. In a second experiment, gonadectomy in adulthood did not alter MBP levels in rats of either sex. In a third experiment, gonadectomy immediately prior to pubertal onset resulted in significant reduction of levels of MBP in adult females but not males. This reduction eliminated the sex difference in adult MBP levels in the OFC. These results reveal puberty to be an organizational time point for a sex difference in the OFC of adult rats in levels of a marker of myelination. This neuroanatomical difference may contribute to observed sex differences in OFC-associated behaviors including in inhibitory control.

Keywords: gonadal hormone; impulsivity; inhibitory control; myelin basic protein; orbitofrontal cortex; sex difference.

Fig. 1.

Summary of experimental protocols for Experiments 1–3. GDX, gonadectomy.

Fig. 2.

Graphical representation of region dissected out for Western Blotting, bolded in black outline. Coronal sections containing the orbitofrontal cortex (OFC) were cut in a cryostat at −20°C using coordinates from Paxinos and Watson (1998) (OFC: AP +4.2 mm to +2.7 mm). Using a scalpel and visual cues from natural boundaries, the OFC was dissected at −20°C. DLO, dorsolateral orbitofrontal cortex; LO, lateral orbitofrontal cortex; MO, medial orbitofrontal cortex, VO, ventral orbitofrontal cortex.

Fig. 3.

Effect of sex on levels of myelin basic protein (MBP) in the orbitofrontal cortex (OFC) of gonadally intact adult male and female rats. (A) Representative western blot images for MBP and the loading control β-actin. MBP is expressed in four isoforms at approximately 21.5kDa, 18.5kDa, 17.0 kDa, and 14.0 kDa. (B) MBP isoforms isolated. Log10 transformation of mean density × area (±SEM) expressed relative to control β-actin. (C) MBP isoforms averaged. Overall main effect of sex: *p = .036.

Fig. 4.

Effects of sex and adult gonadectomy on levels of myelin basic protein (MBP) in the adult orbitofrontal cortex (OFC). Male and female rats were gonadectomized at 70 days of age (GDX) or underwent sham surgery (Intact) and were killed at ~90 days of age. (A) Representative western blot images for MBP and the loading control β-actin. MBP is expressed in four isoforms at approximately 21.5kDa, 18.5kDa, 17.0 kDa, and 14.0 kDa. (B) MBP isoforms isolated. Log10 transformation of mean density × area (±SEM) expressed relative to control β-actin. (C) MBP isoforms averaged. Main effect of sex; *p < .034 vs males. GDX, gonadectomy.

Fig. 5.

Western blot data showing the effect of sex, adult gonadectomy, and pre-pubertal gonadectomy on levels of myelin basic protein (MBP) in the adult orbitofrontal cortex (OFC). Male and female rats were gonadectomized at 28 days of age (Pre-pub GDX), 70 days of age (Adult GDX), or underwent sham surgeries (Intact) and were killed 2.5 months later. (A) Representative western blot images for MBP and the loading control β-actin. MBP is expressed in four isoforms at approximately 21.5kDa, 18.5kDa, 17.0 kDa, and 14.0 kDa. (B) MBP isoforms isolated. Log10 transformation of mean density × area (±SEM) expressed relative to control β-actin. (C) MBP isoforms averaged. Main effects of sex and surgery; * p < .025 vs Female Intact. GDX, gonadectomy; Pre-pub, pre-pubertal.