production has been shown to increase in VNO membranes in many animals, while adenylyl cyclase and cyclic adenosine monophosphate (cAMP), the major signaling transduction molecules of the main olfactory system, remain unaltered. This trend has been shown in many animals, such as the hamster, the pig, the rat, and the garter snake upon introduction of vaginal or seminal secretions into the environment. V1Rs and V2Rs are activated by distinct ligands or pheromones. Gi proteins are activated upon stimulation with lipophilic odorants. Go proteins are activated by nonvolatile proteins, such as the major urinary proteins in mice and exocrine gland-secreting peptide 1 (ESP1). Many vomeronasal neurons are activated by chemicals in urine. Some of the active compounds are sulfated steroids. Detecting the types and amounts of different sulfated steroids conveys information about the urine donor's physiological state, and may therefore serve as an honest signal. Recent studies proved a new family of formyl peptide receptor like proteins in VNO membranes of mice, which points to a close phylogenetic relation of signaling mechanisms used in olfaction and chemosensors. Sensory neurons Vomeronasal sensory neurons are extremely sensitive and fire action potentials at currents as low as 1 pA. Many patch-clamp recordings have confirmed the sensitivity of the vomeronasal neurons. This sensitivity is tied to the fact that the resting potential of the vomeronasal neurons is relatively close to that of the firing threshold of these neurons. Vomeronasal sensory neurons also show remarkably slow adaptation and the firing rate increases with increasing current up to 10 pA. The main olfactory sensory neurons fire single burst action potentials and show a much quicker adaptation rate. Activating neurons that have V1 receptors, V1Rs, cause field potentials that have weak, fluctuating responses that are seen the anterior of the accessory olfactory bulb, AOB. Activation of neurons that contain V2 receptors, V2Rs, however, promote distinct oscillations in the posterior of the AOB. Function In mammals, the sensory neurons of the vomeronasal organ detect non-volatile chemical cues, which requires direct physical contact with the source of odor. Notably, some scents act as chemical-communication signals (pheromones) from other individuals of the same species. Unlike the main olfactory bulb that sends neuronal signals to the olfactory cortex, the VNO sends neuronal signals to the accessory olfactory bulb and then to the amygdala, BNST, and ultimately hypothalamus. Since the hypothalamus is a major neuroendocrine center (affecting aspects of reproductive physiology and behavior