We have used gene targeting to generate mice with a homozygous deficiency in trp2, a cation channel expressed in the vomeronasal organ (VNO). Trp2 mutant animals reveal a striking reduction in the electrophysiological response to pheromones in the VNO, suggesting that trp2 plays a central role in mediating the pheromone response. These mutants therefore afford the opportunity to examine the role of the VNO in the generation of innate sexual and social behaviors in mice. Trp2 mutant males and nursing females are docile and fail to initiate aggressive attacks on intruder males. Male-female sexual behavior appears normal, but trp2 mutant males also vigorously mount other males. These results suggest that the cation channel trp2 is required in the VNO to detect male-specific pheromones that elicit aggressive behaviors and dictate the choice of sexual partners.A nimals exhibit behavioral repertoires that are often innate and result in stereotyped sexual and social responses to their environment. Innate behaviors do not require learning or experience and are likely to reflect the activation of developmentally programmed neural circuits. The appropriate expression of an innate behavioral array frequently requires signals from the outside world. Mice rely heavily on olfactory information to sense their environment. In mice, odorants are recognized by two anatomically and functionally distinct sensory organs, the main olfactory epithelium (MOE) and the vomeronasal organ (VNO) (1, 2). The main olfactory epithelium is thought to recognize odors that provide information about the world at large and can result in measured behavioral responses. In contrast, the VNO has traditionally been implicated in the recognition of pheromones, odorants that provide information about the social and sexual status of other individuals within the species (3, 4). Activation of the VNO is thought to result in innate neuroendocrine and behavioral responses.In mammals, the chemical nature of the pheromones that activate the VNO to elicit innate behavioral responses has not been elucidated (5). Moreover, it has been difficult to sort out the relative roles of the MOE and the VNO in mediating specific behaviors. In male hamsters, removal of the olfactory bulb, which receives input from both the MOE and the VNO, abolishes the sexual response (6). Removal of the VNO alone diminishes the robustness of the male mating response, but does not eliminate sexual behavior (7). The consequence of VNO removal is most apparent in sexually naive animals, suggesting that with experience the main olfactory system assumes an increasingly important role in the sexual response (8, 9). Other innate behaviors, including lordosis in female pigs (10) in response to the male hormone, androstenone, or suckling behavior in newborn rabbits (11) in response to mammary secretions, are unaffected by removal of the VNO. These innate behavioral responses are likely to be elicited by pheromones that activate the main olfactory system. Thus, mammals have evolved innate behavioral ...

Activation of cyclic nucleotide-gated channels is thought to involve two distinct steps: a recognition event in which a ligand binds to the channel and a conformational change that both opens the channel and increases the affinity of the channel for an agonist. Sequence similarity with the cyclic nucleotide-binding sites of cAMP-and cGMP-dependent protein kinases and the bacterial catabolite activating protein (CAP) suggests that the channel ligand binding site consists of a ␤-roll and three ␣-helices. Recent evidence has demonstrated that the third (or C) ␣-helix moves relative to the agonist upon channel activation, forming additional favorable contacts with the purine ring. Here we ask if channel activation also involves structural changes in the ␤-roll by investigating the contribution of a conserved arginine residue that, in CAP and the kinases, forms an important ionic interaction with the cyclized phosphate of the bound ligand. Mutations that conserve, neutralize, or reverse the charge on this arginine decreased the apparent affinity for ligand over four orders of magnitude but had little effect on the ability of bound ligand to open the channel. These data indicate that the cyclized phosphate of the nucleotide approaches to within 2-4 Å of the arginine, forming a favorable ionic bond that is largely unaltered upon activation. Thus, the binding site appears to be polarized into two distinct structural and functional domains: the ␤-roll stabilizes the ligand in a state-independent manner, whereas the C-helix selectively stabilizes the ligand in the open state of the channel. It is likely that these distinct contributions of the nucleotide/C-helix and nucleotide/␤-roll interactions may also be a general feature of the mechanism of activation of other cyclic nucleotide-binding proteins.Cyclic nucleotides regulate the activity of a diverse family of proteins involved in cellular signaling. These include a transcription factor (the bacterial catabolite activating protein, CAP), the cAMP-(PKA) 1 and cGMP-dependent protein kinases (PKG) and the cyclic nucleotide-gated (CNG) ion channels involved in visual and olfactory signal transduction (1, 2). Despite obvious divergence among the effector domains of these proteins, the cyclic nucleotide binding (CNB) sites appear to share a common architecture. Solution of the crystal structures of CAP (3) and a recombinant bovine PKA RI␣ subunit (4) has demonstrated that their CNB sites are formed from an ␣-helix (A helix), an 8-stranded ␤-roll, and two more ␣-helices (B and C), with the C-helix forming the back of the binding pocket. Six residues are invariant among all members of the CAP and kinase families: three glycines involved in turns between strands of the ␤-roll, an arginine and a glutamate, each of which contact the cyclic nucleotide, and an alanine whose function is uncertain (1) (see also Fig. 1). Strikingly, these six residues are conserved in the CNG channels. Thus, it has been suggested that the invariant residues play important and conserved roles in the fol...