Based on observed changes in the social context for the display of ultrasonic vocalizations, scent marking, aggression, and mounting behavior by male mice with a null mutation of the transient receptor potential 2 ion channel, it was proposed recently that a primary function of the mouse vomeronasal organ (VNO)/accessory olfactory system is sex discrimination. We tested this hypothesis directly by studying the ability of male mice to discriminate between urinary odors of conspecifics of the two sexes and in different endocrine states using habituation-dishabituation tests. Male mice from which the VNO had been surgically removed (VNOx) resembled sham-operated controls (VNOi) in their ability to discriminate between volatile urinary odors from estrous females versus gonadally intact males, as well as between urinary odors from estrous versus ovariectomized females and from gonadally intact versus castrated males. When physical access to stimuli was permitted, VNOi control males strongly preferred to investigate volatile and nonvolatile urinary odorants from estrous females as opposed to intact males, whereas VNOx males showed no such preference. Mating performance in tests with estrous females was equivalent in VNOi and VNOx subjects. Both groups of males preferred to mount an estrous female instead of a castrated male. Our results suggest that the VNO is not required for sex discrimination but instead detects the nonvolatile components of oppositesex urine that may be used to help prolong contact with individuals that produce these chemosignals.

The main olfactory system, like the accessory olfactory system, responds to pheromones involved in social communication. Whereas pheromones detected by the accessory system are transmitted to the hypothalamus via the medial ('vomeronasal') amygdala, the pathway by which pheromones are detected and transmitted by the main system is not well understood. We examined in female mice whether a direct projection from mitral ⁄ tufted (M ⁄ T) cells in the main olfactory bulb (MOB) to the medial amygdala exists, and whether medial amygdala-projecting M ⁄ T cells are activated by volatile urinary odors from conspecifics or a predator (cat). Simultaneous anterograde tracing using Phaseolus vulgaris leucoagglutinin and Fluoro-Ruby placed in the MOB and accessory olfactory bulb (AOB), respectively, revealed that axons of MOB M ⁄ T cells projected to superficial laminae of layer Ia in anterior and posterodorsal subdivisions of the medial amygdala, whereas projection neurons from the AOB sent axons to non-overlapping, deeper layer Ia laminae of the same subdivisions. Placement of the retrograde tracer cholera toxin B into the medial amygdala labeled M ⁄ T cells that were concentrated in the ventral MOB. Urinary volatiles from male mice, but not from female conspecifics or cat, induced Fos in medial amygdala-projecting MOB M ⁄ T cells of female subjects, suggesting that information about male odors is transmitted directly from the MOB to the 'vomeronasal' amygdala. The presence of a direct MOB-to-medial amygdala pathway in mice and other mammals could enable volatile, opposite-sex pheromones to gain privileged access to diencephalic structures that control mate recognition and reproduction.

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The performance of continuous-time (CT) delta-sigma modulators (16M's) suffers more severely from time jitter in the quantizer clock than discrete-time designs. Clock jitter adds a random phase modulation to the modulator feedback signal, which whitens the quantization noise in the band of interest and hence degrades converter resolution. Even with a perfectly uniform sampling clock, a similar whitening can be caused by metastability in the quantizer: a real quantizer has finite regeneration gain, and thus, quantizer inputs near zero take longer to resolve. This paper quantifies the performance lost due to clock jitter in a practical integrated CT 16M clocked with an on-chip voltage-controlled oscillator. It also characterizes metastability in a practical integrated quantizer using the quantizer output zero-crossing time and rise time as a function of both quantizer input voltage and the slope of the input voltage at the sampling instant, and predicts the maximum-achievable performance of a practical CT 16M given jitter and metastability constraints.

Male urinary pheromones modulate behavioral and neuroendocrine function in mice after being detected by sensory neurons in the vomeronasal organ (VNO) neuroepithelium. We used nuclear Fos protein immunoreactivity (Fos‐IR) as a marker of changes in neuronal activity to examine the processing of male pheromones throughout the VNO projection pathway to the hypothalamus. Sexually naive male and female Balb/c mice were gonadectomized and treated daily with estradiol benzoate (EB) or oil vehicle for 3 weeks. Subjects were then exposed to soiled bedding from gonadally intact Balb/c males or to clean bedding for 90 min prior to sacrifice and processing of their VNOs and forebrains for Fos‐IR. Male pheromones induced similar numbers of Fos‐IR cells in the VNO neuroepithelium of oil‐treated male and female subjects; however, EB‐treated females had significantly more Fos‐IR neurons in the VNO than any other group. There was an equivalent neuronal Fos response to male odors in the mitral and granule cells of the anterior and posterior accessory olfactory bulb of males and females, regardless of hormone treatment. In central portions of the VNO projection pathway (i.e., bed nucleus of the stria terminalis, medial preoptic area) neuronal Fos responses to male pheromones were present in female but absent in male subjects, regardless of hormone treatment. In a separate experiment, mating induced neuronal Fos‐IR in these brain regions at levels in gonadally intact male subjects which were equal to or greater than those seen in ovariectomized females primed with estrogen and progesterone. This suggests that neurons in the central portions of the male's VNO pathway are capable of expressing Fos. Our results suggest that sexually dimorphic central responses to pheromones exist in mice that may begin in the VNO neuroepithelium. © 1999 John Wiley & Sons, Inc. J Neurobiol 39: 249–263, 1999

Four experiments were conducted to determine whether vomeronasal organ (VNO) inputs in male mice mediate the rewarding properties of estrous female urinary odors. Sexually naive male mice with either an intact (VNOi) or lesioned (VNOx) VNO preferred to investigate female urine over water in Y-maze tests. Subsequently, VNOi males ran significantly more quickly and remained in nasal contact longer with estrous female urine than with male urine, whereas VNOx males investigated these odors equally. In home-cage habituation-dishabituation tests, VNOi males also investigated female urine significantly longer than did VNOx males, although both groups investigated female urine longer than other non-body odors. Finally, female urinary odors induced Fos in the nucleus accumbens core of VNOi males but not of VNOx males. Our results suggest that female urinary odors retain some incentive value in VNOx males. However, once direct nasal contact is made with female urine, VNO inputs further activate forebrain mechanisms that amplify the reward salience of this stimulus for the male mouse. Keywordsaccessory olfactory bulb; nucleus accumbens; pheromones; sexual behavior Social communication in rodents occurs via volatile as well as nonvolatile components of urine and other body odorants (Brown, 1979). Several studies (Lin, Zhang, Block, & Katz, 2005;Schaefer, Yamazaki, Osada, Restrepo, & Beauchamp, 2002;Schaefer, Young, & Restrepo, 2001) suggest that volatile components of urine are detected by receptor neurons in the main olfactory epithelium and processed in the main olfactory bulb, whereas other studies suggest that nonvolatile components of urine (Luo, Fee, & Katz, 2003) as well as extraorbital lacrimal gland secretions (Kimoto, Haga, Sato, & Touhara, 2005) are detected by receptor neurons in the vomeronasal organ (VNO) and processed in the accessory olfactory bulb (AOB). Socially relevant olfactory signals from both the main and accessory systems are integrated in the medial amygdala prior to being conveyed to different hypothalamic regions (Boehm, Zou, & Buck, 2005;Kevetter & Winans, 1981a, 1981bLicht & Meredith, 1987). We (Pankevich, Baum, & Cherry, 2004) reported that surgical VNO removal (VNOx) eliminated the normal preference of male mice to remain in nasal contact with estrous female as opposed to male urine in the home cage. Likewise, VNO removal significantly reduced the preference of female mice to directly investigate male as opposed to female urine spots in Y-maze tests (Keller, Pierman, Douhard, Baum, & Bakker, 2006). Taken together, these results raise the possibility that inputs from the VNO-accessory olfactory system are rewarding and thereby enhance the motivation Correspondence concerning this article should be addressed to Michael J. Baum, Department of Biology, Boston University, 5 Cummington Street, Boston, MA 02215. E-mail: baum@bu.edu. Diana E. Pankevich, James A. Cherry, and Michael J. Baum, Departments of Biology and Psychology, Boston University. (Beauchamp, Martin, Wysocki, & Wellington, 1...