Prokineticin Receptor 2 Expressing Neurons of the Posterior Amygdala and Their Role in Reproductive Physiology

Abstract

The reproductive neuroendocrine axis is tightly regulated by feedback loops controlling appropriate hormone levels for sexual maturation and reproductive success. Neuronal inputs from physiological processes that can affect reproduction (e.g., metabolism and sleep) converge to stimulate gonadotropin-releasing hormone (GnRH) neurons. Pulsatile GnRH release stimulates the pituitary biosynthesis and secretion of gonadotropins, luteinizing hormone (LH) and follicle-stimulating hormone (FSH). Gonadotropins in turn stimulate the gonadal steroidogenesis and gametogenesis.

During development, GnRH neurons must migrate from the olfactory epithelium to the hypothalamus. Deficits in this developmental process results in Kallmann syndrome (KS). KS is characterized by GnRH deficiency and impaired or complete loss of sense of smell (hyposmia or anosmia, respectively). The prokineticin 2 and prokineticin receptor 2 (PROK2/PROKR2) system has been identified as a key component in migratory process of GnRH neurons. In fact, mice with global deletion of Prokr2 replicate the phenotype of KS patients by displaying olfactory bulb dysgenesis and significant decrease in number of GnRH neurons in the hypothalamus. Though PROK2/PROKR2 system has been implicated in this developmental process, many aspects remain unclear and knowledge on neuronal networks of the PROK2/PROKR2 system is limited.

Aside from a developmental role, Prokr2 is widely expressed in many reproductive control sites. Our lab generated a ProkR2-Cre mouse model allowing for functional studies and the neural circuitry. We hypothesized that ProkR2 expressing neurons have a role in adult reproductive function. To test our hypothesis, we set out to determine if ProkR2-Cre cells co-express gonadal steroid receptors relevant for reproductive success (estrogen receptor alpha and the androgen receptor). We found that the only brain site co-expressing Prokr2 and either receptor is the posterior amygdala. Building on this, we described the projection pattern of ProkR2-Cre neurons in the posterior amygdala of male and female mice using an adeno-associated viral vector (AAV) for molecular mapping. We found projections to several reproductive control sites such as the lateral septum, the medial preoptic area and the ventral premammillary nucleus. Lastly, we did a series of functional studies (i.e., sexual behavior, response to opposite and same sex odors, chemogenetic activation) to uncover the role of ProkR2 expressing neurons in the posterior amygdala. We found these neurons are highly responsive to opposite sex odor and have a role in opposite sex recognition in males. We also found that chemogenetic activation of these neurons can induce luteinizing hormone secretion. Our findings reveal that the Prokr2-expressing neurons of the posterior amygdala are part of a neuron circuitry that can sense the gonadal steroid milieu, integrate olfactory cues and modulate the neuroendocrine responses.