Sexual activity increases the number of newborn cells in the accessory olfactory bulb of male rats

Abstract

In rodents, sexual behavior depends on the adequate detection of sexually relevant stimuli. The olfactory bulb (OB) is a region of the adult mammalian brain undergoing constant cell renewal by continuous integration of new granular and periglomerular neurons in the accessory (AOB) and main (MOB) olfactory bulbs. The proliferation, migration, survival, maturation, and integration of these new cells to the OB depend on the stimulus that the subjects received. We have previously shown that 15 days after females control (paced) the sexual interaction an increase in the number of cells is observed in the AOB. No changes are observed in the number of cells when females are not allowed to control the sexual interaction. In the present study we investigated if in male rats sexual behavior increases the number of new cells in the OB. Male rats were divided in five groups: (1) males that did not receive any sexual stimulation, (2) males that were exposed to female odors, (3) males that mated for 1 h and could not pace their sexual interaction, (4) males that paced their sexual interaction and ejaculated one time and (5) males that paced their sexual interaction and ejaculated three times. All males received three injections of the DNA synthesis marker bromodeoxyuridine at 1h intervals, starting 1 h before the beginning of the behavioral test. Fifteen days later, males were sacrificed and the brains were processed to identify new cells and to evaluate if they differentiated into neurons. The number of newborn cells increased in the granular cell layer (GrCL; also known as the internal cell layer) of the AOB in males that ejaculated one or three times controlling (paced) the rate of the sexual interaction. Some of these new cells were identified as neurons. In contrast, no significant differences were found in the mitral cell layer (also known as the external cell layer) and glomerular cell layer (GlCL) of the AOB. In addition, no significant differences were found between groups in the MOB in any of the layers analyzed. Our results indicate that sexual behavior in male rats increases neurogenesis in the GrCL of the AOB when they control the rate of the sexual interaction.

Keywords: accessory olfactory bulb and main olfactory bulb; neurogenesis; sexual behavior.

Figure 1

Representation of the different groups (A–D) and sequence of BrdU injections for all subjects. The control group (A) was placed in the mating cage for 1 h; in group (B) males were exposed without physical contact to a sexually receptive female; in group (C) the males mated but could not control the rate of the sexual interaction, while males in group (D) were allowed to pace the sexual interaction until 1 ejaculation or until 3 ejaculations. BrdU treatment (100 mg/kg per injection; 3 injections) was administered 1 h before the behavioral test (day 0), at the end of the test, and 1 h after the test. Subjects were sacrificed 15 days after the behavioral test.

Figure 2

Photomicrographs showing the layers analyzed in the sagittal (A) plane in the AOB and MOB, glomerular cell layer (GlCL), mitral cell layer (MCL), and granular cell layer (GrCL). Representative photomicrographs (10×) showing BrdU positive cells in the GrCL of the AOB in control (B), exposed (C), non-paced (D), 1 ejaculation (E) and 3 ejaculation (F) groups. A 40× magnification of the 3 ejaculation group is shown in (G). lo, lateral olfactory tract; r, rostral; c, caudal; d, dorsal and v, ventral.

Figure 3

Number of BrdU positive cells in the sagittal plane in the accessory olfactory bulb (AOB; A) and main olfactory bulb (MOB; B) in control (C), exposed (EXP), non-paced (NP), 1 ejaculation (1E), and 3 ejaculation (3E) groups. The areas analyzed were the glomerular (GlCL), mitral (MCL) and granular cell layer (GrCL). Data are expressed as mean ± SEM. N = 7. ^*Different from all other groups in the same layer. P < 0.05. +Different from C and NP in the same layer. P < 0.05.

Figure 4

Confocal images of cells in the GrCL of the AOB double-labeled with NeuN (red) and BrdU (green), taken at 20× magnification. Projection and orthogonal plane in control (A–C), and 3E (D–F) groups. The images were rotated in orthogonal planes to verify double labeling throughout its extent (B,E). Photomicrographs C and F were taken at 63× magnification. r, rostral; c, caudal; d, dorsal and v, ventral.