Pheromones are chemosensory cues secreted by an individual, which are subsequently detected by a conspecific and elicit either behavioral or physiological responses to their mutual benefit (Karlson and Luscher, 1959; Meredith, 1983). In rodents, pheromones modulate the expression of a wide variety of social behaviors including mating and maternal behavior as well as intraspecies aggression (reviewed by Estes 1972; Keverne, 1983; Meredith, 1983). In the ferret, a carnivore, pheromones have been implicated in defining territories (Moors and Lavers, 1981; Clapperton et al., 1988) and in identifying perspective mates (Chang et al., 2000; Kindon et al., 1996). Although in some mammalian species (e.g., pig: Dorries et al., 1997; ewes: Cohen-Tannoudji et al., 1989) pheromones appear to be detected and processed via the main olfactory epithelium (MOE) and associated main olfactory bulb (MOB). The vomeronasal organ (VNO) and its associated accessory olfactory bulb (AOB) have more often been linked to pheromone detection and processing in rodent species.
The VNO is a paired organ located bilaterally at the base of the nasal septum where it is partially encased in a bony or cartilaginous capsule. Nonvolatile chemosignals gain access to the VNO either through the nasopalatine canal, which connects to the oral cavity through the incisive foramen (as in most ungulates and carnivores), or directly from the nasal cavity (as in rodents) (Wysocki, 1979). Many pheromones are probably detected by 7-trans-membrane domain receptors expressed in VNO receptor neurons, which differ from those in the main olfactory epithelium (Herrada and Dulac, 1997). The VNO sensory neurons send axonal projections to glomeruli of the AOB, which is typically located dorsally in the most caudal portion of the olfactory bulb and among eutherian mammals is best developed in rodents and lagomorphs (Estes, 1972). In the AOB, olfactory information is passed to mitral cells that project into a multisynaptic pathway comprising the medial amygdala, bed nucleus of the stria terminalis, medial preoptic area, and the lateral aspect of the ventral medial nucleus of the hypothalamus (Kevetter and Winans, 1981).
The morphology of the VNO was recently described in male ferrets (Weiler et al., 1999). These workers found no difference in VNO sensory epithelium volume in gonadally intact male ferrets killed during the reproductive (spring) and non-reproductive (fall) seasons. Furthermore, in preliminary studies (Chang et al., 2001; Wersinger and Baum, 1997b; Kelliher et al., 1998) we reported the presence of an AOB in a different location from that previously described for ferrets (Dennis and Kerr, 1969; Lockard, 1985). In our experiments we showed that either mating (Wersinger and Baum, 1997b), exposure to soiled bedding from male or female ferrets (Kelliher et al., 1998), or exposure to maternal odors (Chang et al., 2001) induced Fos-immunoreactivity (Fos-IR; an index of neuronal activation) in granule cells of the main olfactory bulb (MOB) but not in the AOB. These results suggest that the MOE-MOB system plays a central role in the detection and processing of reproductively significant pheromones in ferrets. In the rat, the VNO-AOB system is sexually differentiated and responsive to adult sex hormone manipulations (reviewed in Guillamon and Segovia, 1997), suggesting that it may have different functions in males and females. Although we cannot rule out a possible role of the VNO-AOB system in pheromonal communication in the ferret (a carnivore), we thought it would be useful to compare the morphology of this system in males and females, determine its responsiveness to hormone manipulation in adults, and contrast these features with those of other mammalian species (e.g., rat, mouse, hamster) in which the VNO-AOB system is known to mediate many pheromonal responses. Therefore, we compared the dimensions of the VNO sensory epithelium and AOB in adult gonadectomized ferrets of both sexes that had received either testosterone propionate (TP) or no steroid (oil vehicle). We studied the possible activational effects of TP treatment on the VNO-AOB morphology because our previous study (Kelliher et al., 1998) showed that this steroid dramatically enhanced neuronal Fos responses to estrous odors in several forebrain regions of both male and female ferrets. Since our recent characterization (e.g., Wersinger and Baum, 1997a,b; Kelliher et al., 1998) of the location of the ferret's AOB does not correspond with that of earlier studies (Dennis and Kerr, 1969; Lockard, 1985), we used several histochemical markers to confirm the location and size of this structure in the adult ferret. These included staining for soybean agglutinin (SBA), which binds specifically to glycoconjugates located on the vomeronasal nerve (Key and Giorgi, 1986). We also immunostained for luteinizing hormone-releasing hormone (LHRH), which has been found in the AOB but not the MOB of rodents (Zheng et al., 1988), and for tyrosine hydroxylase (TH), which is usually concentrated in the glomerular zone of the MOB but not of the AOB (Baker, 1986). Finally, we compared AOB and MOB volumes in Nissl-stained sections from adult ferrets of both sexes.