Fos expression in the olfactory pathway of high- and low-sexually performing rams exposed to urine from estrous or ovariectomized ewes

Highlights

• Sexually active and inactive rams have equal olfactory response to sexual stimuli.

• Sexual olfactory cue increase neural activity in the cortical amygdala of active rams.

• Sexual inactivity was associated with decreased neural activity in the BNST and POA.

Abstract

Exposure to estrous ewe urine stimulates investigation and mounting activity in sexually active but not sexually inactive rams. It was hypothesized sexual indifference may result from an inability to detect olfactory cues or an interruption of the pathway from detection of the olfactory stimulus to the motor response. Sexually active (n = 4) and inactive (n = 3) rams were exposed to urine from estrous ewes. An additional group of sexually active rams (n = 3) were exposed to urine from ovariectomized ewes. Rams were exsanguinated following 1 h of exposure to stimulus. Neural activity was determined in tissues of interest by the presence of fos and fos-related proteins detected by immunohistochemistry procedures. Sexually active rams exposed to urine from ovariectomized ewes had more (P ≤ 0.05) fos-positive cells in the olfactory bulb, but fewer (P = 0.03) fos-positive cells in the cortical amygdala compared to sexually active rams exposed to urine from estrous ewes. Sexually inactive rams had similar (P ≥ 0.13) numbers of fos positive neurons in the olfactory bulb and medial amygdala but fewer (P ≤ 0.04) in the central amygdala, bed nucleus of the stria terminalis and the medial preoptic area compared to sexually active rams exposed to urine from estrous ewes. Sexual inactivity was not associated with decreased hypothalamic function since fos activity was similar (P ≥ 0.14) among groups in the suprachiasmatic and ventral medial nucleus. Sexual inactivity is not likely due to an impaired ability to detect or process olfactory stimuli by the main olfactory bulb and medial-cortical amygdala. Sexually inactive rams may have reduced attentiveness to sexual stimuli and/or decreased responsiveness of regions in the brain which regulate reproductive behaviors.

Introduction

Rams use olfaction to identify ewes in estrus, and display a preference for urine from ewes during the peri-estrual period (Blissitt et al., 1994). Olfactory sexual cues facilitate sexual interest in rams and shorten the interval to mounting and ejaculation (Maina and Katz, 1999). However, regardless of the sexual stimuli, approximately 20% of rams are either slow to initiate sexual activity or fail to mount sexually-receptive females (Alexander et al., 1999).

Both the main and accessory olfactory organs detect chemosensory cues. Although the vomeronasal organ is not required for sexually experienced rams to discriminate among estrous and non-estrous females, a functional vomeronasal organ does enhance the expression of reproductive behaviors (Blissitt et al., 1990). As opposed to rodents (Coquelin et al., 1984, Meredith, 1986, Keverne, 1999), the main olfactory bulb identifies sexually relevant odors in rams and is required for the expression of sexual behavior (Blissitt et al., 1990). Olfactory sexual cues are likely processed directly through the amygdala (Martel and Baum, 2009) influencing the bed nucleus of the stria terminalis (BNST) and preoptic area of the hypothalamus (Petrulis, 2013).

Immediate early gene expression is used as a marker of neural activity as a first and transient indicator of gene expression following sensory stimulation in the central nervous system (Halpern and Martinez-Marcos, 2003). Expression of fos and fos-related proteins is used to evaluate neural activity following exposure to a broad range of stimuli (Bialy and Kaczmarek, 1996). Basal levels of c-fos expression are low in the brain and increase precipitously in brain regions known to be important for male sexual behaviors following sexual stimulation (Bialy and Kaczmarek, 1996).

This study was designed to determine if sexual disinterest was a result of an inability to detect sexually relevant olfactory cues, or whether the processing of those cues is compromised in sexually inactive rams.