Transactivation of TrkB Receptors by Oxytocin and Its G Protein-Coupled Receptor

Brain-derived Neurotrophic Factor (BDNF) binds to the TrkB tyrosine kinase receptor, which dictates the sensitivity of neurons to BDNF. A unique feature of TrkB is the ability to be activated by small molecules in a process called transactivation. Here we report that the brain neuropeptide oxytocin increases BDNF TrkB activity in primary cortical neurons and in the mammalian neocortex during postnatal development. Oxytocin produces its effects through a G protein-coupled receptor (GPCR), however, the receptor signaling events that account for its actions have not been fully defined. We find oxytocin rapidly transactivates TrkB receptors in bath application of acute brain slices of 2-week-old mice and in primary cortical culture by increasing TrkB receptor tyrosine phosphorylation. The effects of oxytocin signaling could be distinguished from the related vasopressin receptor. The transactivation of TrkB receptors by oxytocin enhances the clustering of gephyrin, a scaffold protein responsible to coordinate inhibitory responses. Because oxytocin displays pro-social functions in maternal care, cognition, and social attachment, it is currently a focus of therapeutic strategies in autism spectrum disorders. Interestingly, oxytocin and BDNF are both implicated in the pathophysiology of depression, schizophrenia, anxiety, and cognition. These results imply that oxytocin may rely upon crosstalk with BDNF signaling to facilitate its actions through receptor transactivation.

Oxytocin Transforms Firing Mode of CA2 Hippocampal Neurons

Oxytocin is an important neuromodulator in the mammalian brain that increases information salience and circuit plasticity, but its signaling mechanisms and circuit effect are not fully understood. Here we report robust oxytocinergic modulation of intrinsic properties and circuit operations in hippocampal area CA2, a region of emerging importance for hippocampal function and social behavior. Upon oxytocin receptor activation, CA2 pyramidal cells depolarize and fire bursts of action potentials, a consequence of phospholipase C signaling to modify two separate voltage-dependent ionic processes. A reduction of potassium current carried by KCNQ-based M channels depolarizes the cell; protein kinase C activity attenuates spike rate of rise and overshoot, dampening after-hyperpolarizations. These actions, in concert with activation of fast-spiking interneurons, promote repetitive firing and CA2 bursting; bursting then governs short-term plasticity of CA2 synaptic transmission onto CA1 and, thus, efficacy of information transfer in the hippocampal network.

Sex-Specific Differences in Oxytocin Receptor Expression and Function for Parental Behavior

Parental care is among the most profound behavior expressed by humans and other animals. Despite intense interest in understanding the biological basis of parental behaviors, it remains unknown how much of parenting is encoded by the genome and which abilities instead are learned or can be refined by experience. One critical factor at the intersection between innate behaviors and experience-dependent learning is oxytocin, a neurohormone important for maternal physiology and neuroplasticity. Oxytocin acts throughout the body and brain to promote prosocial and maternal behaviors and modulates synaptic transmission to affect neural circuit dynamics. Recently we developed specific antibodies to mouse oxytocin receptors, found that oxytocin receptors are left lateralized in female auditory cortex, and examined how oxytocin enables maternal behavior by sensitizing the cortex to infant distress sounds. In this study we compare oxytocin receptor expression and function in male and female mice. Receptor expression is higher in adult female left auditory cortex than in right auditory cortex or males. Developmental profiles and mRNA expression were comparable between males and females. Behaviorally, male and female mice began expressing parental behavior similarly after cohousing with experienced females; however, oxytocin enhanced parental behavior onset in females but not males. This suggests that left lateralization of oxytocin receptor expression in females provides a mechanism for accelerating maternal behavior onset, although male mice can also effectively co-parent after experience with infants. The sex-specific pattern of oxytocin receptor expression might genetically predispose female cortex to respond to infant cues, which both males and females can also rapidly learn.

Consequences of brain-derived neurotrophic factor withdrawal in CNS neurons and implications in disease

Growth factor withdrawal has been studied across different species and has been shown to have dramatic consequences on cell survival. In the nervous system, withdrawal of nerve growth factor (NGF) from sympathetic and sensory neurons results in substantial neuronal cell death, signifying a requirement for NGF for the survival of neurons in the peripheral nervous system (PNS). In contrast to the PNS, withdrawal of central nervous system (CNS) enriched brain-derived neurotrophic factor (BDNF) has little effect on cell survival but is indispensible for synaptic plasticity. Given that most early events in neuropsychiatric disorders are marked by a loss of synapses, lack of BDNF may thus be an important part of a cascade of events that leads to neuronal degeneration. Here we review reports on the effects of BDNF withdrawal on CNS neurons and discuss the relevance of the loss in disease.

Oxytocin Enhances Social Recognition by Modulating Cortical Control of Early Olfactory Processing

Oxytocin promotes social interactions and recognition of conspecifics that rely on olfaction in most species. The circuit mechanisms through which oxytocin modifies olfactory processing are incompletely understood. Here, we observed that optogenetically induced oxytocin release enhanced olfactory exploration and same-sex recognition of adult rats. Consistent with oxytocin's function in the anterior olfactory cortex, particularly in social cue processing, region-selective receptor deletion impaired social recognition but left odor discrimination and recognition intact outside a social context. Oxytocin transiently increased the drive of the anterior olfactory cortex projecting to olfactory bulb interneurons. Cortical top-down recruitment of interneurons dynamically enhanced the inhibitory input to olfactory bulb projection neurons and increased the signal-to-noise of their output. In summary, oxytocin generates states for optimized information extraction in an early cortical top-down network that is required for social interactions with potential implications for sensory processing deficits in autism spectrum disorders.

A New Population of Parvocellular Oxytocin Neurons Controlling Magnocellular Neuron Activity and Inflammatory Pain Processing

Oxytocin (OT) is a neuropeptide elaborated by the hypothalamic paraventricular (PVN) and supraoptic (SON) nuclei. Magnocellular OT neurons of these nuclei innervate numerous forebrain regions and release OT into the blood from the posterior pituitary. The PVN also harbors parvocellular OT cells that project to the brainstem and spinal cord, but their function has not been directly assessed. Here, we identified a subset of approximately 30 parvocellular OT neurons, with collateral projections onto magnocellular OT neurons and neurons of deep layers of the spinal cord. Evoked OT release from these OT neurons suppresses nociception and promotes analgesia in an animal model of inflammatory pain. Our findings identify a new population of OT neurons that modulates nociception in a two tier process: (1) directly by release of OT from axons onto sensory spinal cord neurons and inhibiting their activity and (2) indirectly by stimulating OT release from SON neurons into the periphery.

A Distributed Network for Social Cognition Enriched for Oxytocin Receptors

Oxytocin is a neuropeptide important for social behaviors such as maternal care and parent-infant bonding. It is believed that oxytocin receptor signaling in the brain is critical for these behaviors, but it is unknown precisely when and where oxytocin receptors are expressed or which neural circuits are directly sensitive to oxytocin. To overcome this challenge, we generated specific antibodies to the mouse oxytocin receptor and examined receptor expression throughout the brain. We identified a distributed network of female mouse brain regions for maternal behaviors that are especially enriched for oxytocin receptors, including the piriform cortex, the left auditory cortex, and CA2 of the hippocampus. Electron microscopic analysis of the cerebral cortex revealed that oxytocin receptors were mainly expressed at synapses, as well as on axons and glial processes. Functionally, oxytocin transiently reduced synaptic inhibition in multiple brain regions and enabled long-term synaptic plasticity in the auditory cortex. Thus modulation of inhibition may be a general mechanism by which oxytocin can act throughout the brain to regulate parental behaviors and social cognition.