Elsevier

Physiology & Behavior

Volume 68, Issues 1–2, 1–15 December 1999, Pages 257-261
Physiology & Behavior

Brief communication
Female-soiled bedding induced Fos immunoreactivity in the ventral part of the premammillary nucleus (PMv) of the male mouse

https://doi.org/10.1016/S0031-9384(99)00160-2Get rights and content

Abstract

Previous studies have indicated that the ventral part of the premammillary nucleus (PMv) of rodents is involved in the regulation of aggressive and male mating behavior, although the precise physiological function of the PMv is still unclear. To analyze the physiological role of the PMv in male mating behavior, the effects of exposure to bedding soiled by female mice on Fos immunoreactivity (Fos-ir), an early marker of neuronal activation, were studied in the PMv and some sex-related nuclei. We observed that exposure to female-soiled bedding induced Fos-ir expression in the PMv of the male mouse. Although Fos-ir positive cells were found in the posterodorsal part of the medial amygdaloid nucleus and in the posteromedial cortical amygdaloid nucleus, which are terminals of the neuronal projections from the main and accessory olfactory bulbs, the numbers of Fos-ir cells in those nuclei were not affected by exposure to female-soiled bedding. Moreover, Fos-ir was not detected in the ventromedial hypothalamic nucleus. It is well established that soiled bedding is useful as a source of chemosensory substances, which include “pheromones.” Thus, our findings, in agreement with previous behavioral and anatomical data, suggest that the PMv plays a role in initiating male copulative behavior that is induced by a female mice pheromone(s).

Introduction

The ventral part of the premammillary nucleus (PMv) of the rodent is located in the ventral portion of the most posterior region of the hypothalamus 1, 2. Anatomical and physiological data suggested that the PMv of the rat has important functions including the regulation of aggressive behavior [3] and gonadotrophin secretion 4, 5, 6. To fulfill these functions, the PMv of the rat produces many kinds of neuropeptides such as substance P, vasoactive intestinal polypeptide, neurotensin, and corticotropin-releasing hormone [7]. The PMv has direct neuronal connections with several reproductive function-related brain regions: the medial preoptic nucleus, the bed nucleus of the stria terminalis, the ventromedial hypothalamic nucleus (VMH), and the posterodorsal part of the medial amygdaloid nucleus (MePD) [8]. In addition, the PMv has a large population of neurons with androgen receptors in the rat 9, 10, 11 and the Syrian hamster [38]. These facts strongly suggest that the PMv plays an important role in the initiation and/or expression of the mating behavior of male rodents.

According to many reports and articles, pheromone signals from estrous females can alter the physiological function, and initiate mating behavior, of the males 13, 14, 15. It is well established that male- and/or female-soiled bedding is useful as a source of sex-related pheromones. For example, Vandenbergh [16] reported that exposure to male-soiled bedding accelerates the onset of puberty in juvenile female mice. Keverne and de la Riva [17] showed that the frequency of pregnancy block increases when female mice are housed in cages that contain bedding soiled by alien male mice. Exposure to female-soiled bedding induces the expression of Fos immunoreactivity (Fos-ir) [18], and increases the size of the synapses [19], in the accessory olfactory bulb in the mice and rat, respectively.

Mice provide important and useful models for studies of pheromone-inducible reproductive behaviors 20, 21, 22. However, the action of female pheromones on the hypothalamus of the male mouse has not been reported. Although its physiological function is still unclear, the PMv may be involved in initiating male mating behavior by receiving sex pheromone-related neural activities. Therefore, we have examined whether the number of Fos-positive cells within the PMv of the male mouse increases following exposure to female-soiled bedding.

Examination of the medial preoptic area and the bed nucleus of the stria terminalis, which form the so-called “classical” chemosensory pathway, was beyond the scope of the present study.

Section snippets

Animal treatments and tissue preparation

Sexually inexperienced 7- to 8-week-old ICR strain male and female mice (CREA Japan Inc., Tokyo, Japan) were studied. All of the animals were housed (four to five animals/cage, male and female separately) in standard laboratory cages (27 × 16 × 14 cm) with wood-chip bedding on the floor, and were provided with food and water ad lib. They were housed in an air-conditioned room (24°C) with a controlled light–dark cycle (12-h light/12-h dark).

For approximately 48 h before the experiment began, the

Results

Fos-ir was clearly greater in throughout the PMv region of the male mice that had been exposed to the female mice-soiled bedding compared to the controls (Fig. 1B vs. 1C). Exposure to the soiled bedding elicited Fos-ir in the male PMv that was significantly greater than that seen in mice exposed to clean bedding (Fig. 3; clean versus soiled = 12.2 ± 7.9 vs. 380.3 ± 60.3 cells, p < 0.05). However, Fos-ir levels in the MePD (clean versus soiled = 16.3 ± 2.0 vs. 20.1 ± 3.2 cells) and in the PMCo

Discussion

In this study, we observed that exposure to female-soiled bedding increased Fos-ir in the PMv, but not in the VMH, MePD, and PMCo, of the male mouse. It has been reported that male-soiled bedding and/or male urine increase c-fos mRNA expression and/or Fos-ir in the main and accessory olfactory bulbs of the female 18, 24. However, to our knowledge, there has been no report of altered c-fos mRNA expression and/or Fos-ir in the male's hypothalamic area in response to pheromones from the female.

Acknowledgments

This work was supported in part by CREST of Japan Science and Technology Corporation to M. Ichikawa, by a Research Fellowship of the Japan Society for the Promotion of Science to M.M., and by a Grant-in Aid for the Encouragement of Young Scientists from The Ministry of Education, Science, Sports and Culture of Japan to M.Y. (910740391).

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