Differential expression of G proteins in the mouse olfactory system.

Transmembrane signaling events at the dendrites and axons of olfactory receptor neurons mediate distinct functions. Whereas odorant recognition and chemosensory transduction occur at the dendritic membranes of olfactory neurons, signal propagation, axon sorting and target innervation are functions of their axons. The roles of G proteins in transmembrane signaling at the dendrites have been studied extensively, but axonal G proteins have not been investigated in detail. We used immunohistochemistry to visualize expression of alpha subunits of G(o) and G(i2) in the mouse olfactory system. G(o) is expressed ubiquitously on axons of olfactory receptor neurons throughout the olfactory neuroepithelium and in virtually all glomeruli in the main olfactory bulb. In contrast, expression of G(i2) is restricted to a sub-population of olfactory neurons, along the dorsal septum and the dorsal recess of the nasal cavity, which projects primarily to medial regions of the olfactory bulb, with the exception of glomeruli adjacent to the pathway of the vomeronasal nerve. In contrast to the overlapping expression patterns of G(o) and G(i2) in the main olfactory system, neurons expressing G(o) and those expressing G(i2) in the accessory olfactory bulb are more clearly separated, in agreement with previous studies. Vomeronasal axons terminating in glomeruli in the rostral region of the accessory olfactory bulb express G(i2), whereas those projecting to the caudal region express G(o). Characterization of the expression patterns of G(i2) and G(o) in the olfactory projection is essential for future studies aimed at relating transmembrane signaling events to signal propagation, axon sorting and target innervation.

Pheromone regulated production of inositol-(1, 4, 5)-trisphosphate in the mammalian vomeronasal organ.

Social behaviors of most mammals are profoundly affected by chemical signals, pheromones, exchanged between conspecifics. Pheromones interact with dendritic microvilli of bipolar neurons in the vomeronasal organ (VNO). To investigate vomeronasal signal transduction pathways, microvillar membranes from porcine VNO were prepared. Incubation of such membranes from prepubertal females with boar seminal fluid or urine results in an increase in production of inositol-(1, 4, 5)-trisphosphate (IP3). The dose response for IP3 production is biphasic with a GTP-dependent component at low stimulus concentrations and a nonspecific increase in IP3 at higher stimulus concentrations. The GTP-dependent stimulation is mimicked by GTPgammaS and blocked by GDPbetaS. Furthermore, the GTP-dependent component of the stimulation of IP3 production is sex specific and tissue dependent. Studies with monospecific antibodies reveal a G alpha(q/11)-related protein in vomeronasal neurons, concentrated at their microvilli. Our observations indicate that pheromones in boar secretions act on vomeronasal neurons in the female VNO via a receptor mediated, G protein-dependent increase in IP3. These observations set the stage for further investigations on the regulation of stimulus-excitation coupling in vomeronasal neurons. The pheromone-induced IP3 response also provides an assay for future purification of mammalian reproductive pheromones.

Androgen exposure and reproductive behavior of an induced ovulator, the pine vole (Microtus pinetorum).

The actions of steroid hormones on brain and behavior are classically divided into organizational effects that are permanent and occur early in development and activational effects that are temporary and occur throughout life. Here, we test the hypothesis that in an induced ovulator, testosterone defeminizes only those neural tissues that rely on synergistic interactions of estrogen and progesterone for normal function in adulthood. Female voles, Microtus pinetorum, injected with testosterone (T) or oil neonatally were paired with males for an 8-week period. During the pairing, androgenized and oil-treated females spent a similar amount of time investigating the caudal and rostral regions of the males. Males spent significantly less time investigating the caudal and rostral regions of androgenized females. Androgenized females mounted males, did not exhibit lordosis, and were less likely to be mounted by males. Moreover, none of the 10 androgenized females gave birth, whereas 8 of 9 control females gave birth. Androgenized females were also not capable of being stimulated into reproductive condition by males. Injection of 0.5 microg of estradiol benzoate for 4 consecutive days resulted in reduced uterine hypertrophy in androgenized females. These results support the original organizational-activational hypothesis by showing that neonatal androgenization defeminizes and masculinizes female pine voles.

Chemical cues are necessary but insufficient for reproductive activation of female pine vole (Microtus pinetorum).

Among various arvicoline rodents, reproduction is influenced to varying degrees by social factors, including behavioral or chemical cues. Since previous research suggested that chemosignals from adult males reproductively activate female pine voles (Microtus pine-torum), we sought to determine specifically what types of stimuli promote the activation response. In these experiments, female were exposed to unfamiliar adult males, or to some combination of cues from males, or were housed alone. Using uterine mass as a measure of reproductive activation, we found that females were not activated by exposure either to male urine by itself or to male-soiled bedding by itself, but full contact with a male clearly resulted in heavier uteri. Females whose vomeronasal organs were surgically excised failed to undergo reproductive activation when housed with males. Finally, females allowed physical contact by being housed directly underneath males had heavier uteri than did females whose housing allowed contact only with the chemical cues from males. Among female arvicoline rodents, it appears that there exists a physiological continuum between absolute dependence on both contact and chemical cues from males vs. absolute independence for reproductive activation. The present results place female pine voles closer to the former extreme than to the latter.

Removal of the vomeronasal organ reduces reproductive performance and aggression in male prairie voles.

Several short-duration tests have demonstrated that the surgical removal of the vomeronasal organ (VNX) from sexually-inexperienced male rodents results in a reduction in copulatory behavior, compared to the effects of sham surgery (SHAM). We extended these studies to adult male prairie voles, Microtus ochrogaster, and substantially increased the duration of the tests. During the initial interactions with females, VNX males spent significantly less time with their noses in close proximity to the females than did SHAM males. Moreover, only two of nine VNX males sired offspring after having been paired with females for 8 weeks, whereas nine of 12 SHAM males sired offspring in that interval. We also found that VNX and SHAM males were equivalently non-aggressive to an anesthetized stimulus-male prior to being paired with females. However, after spending 2 weeks paired with a female, the VNX males were significantly less aggressive than were the SHAM males, possibly as a result of having copulated less often. In a later test, nearly all of the VNX and SHAM males that sired offspring were vigorously aggressive to a stimulus male. We conclude that the stimulation of the vomeronasal system in sexually-inexperienced male prairie voles is important for maximal reproductive performance and that the VNX-induced impairment in reproduction is associated with a decrease in inter-male aggression. The possible sensory effects of the vomeronasal system on the neural and endocrine control of reproduction and behavior are discussed.