GABAergic mechanisms are involved in the control of tuberoinfundibular arcuate neurons by the accessory olfactory bulb

Contactin-6-deficient male mice exhibit the abnormal function of the accessory olfactory system and impaired reproductive behavior

INTRODUCTION

Contactin-6 (CNTN6), also known as NB-3, is a neural recognition molecule and a member of the contactin subgroup of the immunoglobulin superfamily. Gene encoding CNTN6 is expressed in many regions of the neural system, including the accessory olfactory bulb (AOB) in mice. We aim to determine the effect of CNTN6 deficiency on the function of the accessory olfactory system (AOS).

METHODS

We examined the effect of CNTN6 deficiency on the reproductive behavior of male mice through behavioral experiments such as urine sniffing and mate preference tests. Staining and electron microscopy were used to observe the gross structure and the circuitry activity of the AOS.

RESULTS

Cntn6 is highly expressed in the vomeronasal organ (VNO) and the AOB, and sparsely expressed in the medial amygdala (MeA) and the medial preoptic area (MPOA), which receive direct and/or indirect projections from the AOB. Behavioral tests to examine reproductive function in mice, which is mostly controlled by the AOS, revealed that Cntn6^-/- adult male mice showed less interest and reduced mating attempts toward estrous female mice in comparison with their Cntn6^+/+ littermates. Although Cntn6^-/- adult male mice displayed no obvious changes in the gross structure of the VNO or AOB, we observed the increased activation of granule cells in the AOB and the lower activation of neurons in the MeA and the MPOA as compared with Cntn6^+/+ adult male mice. Moreover, there were an increased number of synapses between mitral cells and granule cells in the AOB of Cntn6^-/- adult male mice as compared with wild-type controls.

CONCLUSION

These results indicate that CNTN6 deficiency affects the reproductive behavior of male mice, suggesting that CNTN6 participated in normal function of the AOS and its ablation was involved in synapse formation between mitral and granule cells in the AOB, rather than affecting the gross structure of the AOS.

Bicuculline infusion into the accessory olfactory bulb facilitates the induction of maternal behavior in rats

The GABAA antagonist bicuculline, intracranially infused into the accessory olfactory bulb (AOB), facilitated the expression of maternal behavior (MB) in virgin Wistar female rats. Behavioral effects were observed 24 hours after infusion and were injection dependent. Pheromonal stimuli, generated by the pups, are thought to exert an inhibitory effect on vomeronasal nuclei involved in MB in virgin rats. The present study investigated the possibility that a decrement in AOB output, resulting from long-term compensatory synaptic changes to chronic bicuculline infusion, would facilitate the expression of MB. The implications of our findings for the mechanisms involved in the induction of MB and the maternal experience effect are discussed.

Modulation of dendrodendritic interactions and mitral cell excitability in the mouse accessory olfactory bulb by vaginocervical stimulation

When female mice are mated, they form a memory to the pheromonal signal of their male partner. The neural changes underlying this memory occur in the accessory olfactory bulb, depend upon vaginocervical stimulation at mating and involve changes at the reciprocal synapses between mitral and granule cells. However, the action of vaginocervical stimulation on the reciprocal interactions between mitral and granule cells remains to be elucidated. We have examined the effects of vaginocervical stimulation on paired-pulse depression of amygdala-evoked field potentials recorded in the external plexiform layer of the accessory olfactory bulb (AOB) and the single-unit activity of mitral cells antidromically stimulated from the amygdala in urethane-anaesthetized female mice. Artificial vaginocervical stimulation reduced paired-pulse depression (considered to be due to feedback inhibition of the mitral cell dendrites from the granule cells via reciprocal dendrodendritic synapses) recorded in the AOB external plexiform layer. As would be expected from this result, vaginocervical stimulation also enhanced the spontaneous activity of a proportion of the mitral cells tested. These results suggest that vaginocervical stimulation reduces dendrodendritic feedback inhibition to mitral cells and enhances their activity.

Activation of GABA(A) receptors in the accessory olfactory bulb does not prevent the formation of an olfactory memory in mice

When female mice are mated, they form a memory to the pheromonal signal of their male partner. The neural mechanisms underlying this memory involve changes at the reciprocal dendrodendritic synapses between glutamatergic mitral cells and gamma-aminobutyric acid (GABA)-ergic granule cells in the accessory olfactory bulb (AOB). Blockade of GABA(A) receptors in the AOB leads to the formation of an olfactory memory. In an attempt to disrupt memory formation at mating, we used local infusions of the GABA(A) receptor agonist muscimol into the AOB during the critical period for memory formation. Muscimol across a wide range of doses (1-1000 pmol) did not prevent memory formation. The resistance of this memory to GABA(A) receptor activation may reflect the complexity of synaptic microcircuits in the AOB.

Neural mechanisms of mammalian olfactory learning

In this review, we compare the neural basis of olfactory learning in three specialized contexts that occur during sensitive periods of enhanced neural plasticity. Although they involve very different behavioural contexts, they share several common features, including a dependence on noradrenergic transmission in the olfactory bulb. The most extensively characterized of these examples is the learning of pheromonal information by female mice during mating. While this form of learning is unusual in that the neural changes underlying the memory occur in the accessory olfactory bulb at the first stage of sensory processing, it involves similar neural mechanisms to other forms of learning and synaptic plasticity. The learning of newborn lamb odours after parturition in sheep, and the olfactory conditioning in neonatal animals such as rats and rabbits, are mediated by the main olfactory system. Although the neural mechanisms for learning in the main olfactory system are more distributed, they also involve changes occurring in the olfactory bulb. In each case, odour learning induces substantial structural and functional changes, including increases in inhibitory neurotransmission. In the main olfactory bulb, this probably represents a sharpening of the odour-induced pattern of activity, due to increases in lateral inhibition. In contrast, the different morphology of mitral cells in the accessory olfactory bulb results in increased self-inhibition, disrupting the transmission of pheromonal information. Although these examples occur in highly specialized contexts, comparisons among them can enhance our understanding of the general neural mechanisms of olfactory learning.

The importance of calmodulin in the accessory olfactory bulb in the formation of an olfactory memory in mice

Female mice form an olfactory memory to the pheromones of the mating male, during a critical period after mating. Failure to form this memory results in the male being treated as strange, and hence, his pheromones block pregnancy. Previous studies have shown that formation of this memory is dependent on synaptic mechanisms in the accessory olfactory bulb. A number of studies have pointed to calmodulin as a critical mediator of synaptic plasticity. In this study we have examined the effects of local infusions of drugs which block calmodulin-regulated processes, into the accessory olfactory bulb on the formation of this memory. Infusions of the calmodulin antagonist calmidazolium during the critical period prevented memory formation. However, the specific inhibitor of calcium/calmodulin-dependent protein kinase II, KN-62, or the selective inhibitor of calcium/calmodulin-dependent protein phosphatase 2B (calcineurin), FK506, was without effect on memory formation at any of the doses used. Instead of preventing memory formation, FK506 permitted the formation of a non-selective memory to strange male pheromones in the presence of mating, although FK506 alone could not induce a memory without the occurrence of mating. These results suggest that calmodulin in the accessory olfactory bulb is important in the formation of the olfactory memory to male pheromones. However, memory formation may be independent of calmodulin-kinase II. Calcineurin may play a role in processes antagonizing memory formation.

Synaptic mechanisms of olfactory recognition memory

The complexity and inaccessibility of the mammalian brain prevent the localization and description of memory traces and the definition of the processes that produce memories. The model reviewed here is the olfactory recognition memory formed to male pheromones by a female mouse at mating. The memory trace has been localized to the reciprocal dendrodendritic synapse between mitral cells and granule cells in the accessory olfactory bulb. An increase in noradrenaline after mating reduces inhibitory transmission of gamma-aminobutyric acid (GABA) from the granule cells to mitral cells and induces an olfactory memory of pheromones present at mating. Recent work has shown that the activation of mGluR2, a metabotropic glutamate receptor, localized at granule cell dendrites suppresses the GABA inhibition of the mitral cells and permits the formation of a specific olfactory memory that faithfully reflects the memory formed at mating. This simple olfactory memory may provide an excellent model system with which to investigate the molecular mechanisms of the synaptic plasticity involved in learning and memory.

Effective induction of pregnancy block by electrical stimulation of the mouse accessory olfactory bulb coincident with prolactin surges

Electrical stimulation (0.33 Hz, 0.2 mA) applied bilaterally to the accessory olfactory bulb (AOB) of newly mated female mice revealed that stimulation for 4 h was sufficient to produce pregnancy block providing stimulation is given for two separate 2-h periods coincident with prolactin surges. Stimulation for two 1-h periods coincident with prolactin surges or two 2-h periods between prolactin surges was without effect. These results indicate that electrical stimulation of the AOB can faithfully reproduce pheromone-induced pregnancy block and support the view that prolactin is the hormone mainly responsible for the olfactory block to pregnancy.

Sensory processing in the main and accessory olfactory systems: comparisons and contrasts

The vomeronasal organ (VNO) and accessory olfactory system (AOS) are present in most terrestrial vertebrates except birds and higher primates. The receptor neurons of the AOS are sequestered inside the VNO, away from the main airflow to the main olfactory receptor neurons. Mechanisms of stimulus access to the sensory neurons vary across species but in most cases there is a system for delivering stimuli faster than would be possible by diffusion. Vomeronasal (VN) receptor neurons typically lack cilia, the site of most of the transduction apparatus in the main olfactory receptors. The VN receptor neurons have a restricted but privileged pathway to the areas of the brain concerned with reproduction and social behavior. In contrast, the main olfactory neurons have a broad pathway to wide areas of the brain, including the neocortex. Experiments where the VNOs or other parts of the accessory olfactory pathway were ablated indicate that the system is important in many behavioral and physiological responses to pheromones (chemical signals carrying information about gender or reproductive or dominance status), some of which may be proteins. VN sensory neurons respond to both volatile and non-volatile stimuli. There is no evidence in the vertebrate AOS for the extreme sensitivity or selectivity characteristic of insect pheromone detectors, but this has not been adequately tested. There is some evidence for learning, possibly by synaptic modification at the second-order neuron level. Social and reproductive cues stimulating the AOS often elicit an intracerebral release of LHRH--which may act at receptors different from those of the pituitary to facilitate behavior. Whether the LHRH release is necessary for AOS-mediated behavioral response is not yet clear.