Neural distribution of the nuclear progesterone receptor in the túngara frog, Physalaemus pustulosus.

The gonadal steroid hormone progesterone plays an important role across all vertebrates in mediating female reproductive physiology and behavior. Many effects of progesterone are mediated by a nuclear progesterone receptor (PR), which is crucial for integration of external signals and internal physiological cues in the brain to produce an appropriate behavioral output. The túngara frog, Physalaemus pustulosus, is an excellent model system for the study of mechanisms by which sensory signals, such as auditory communication, are processed within neural circuits where mate choice decisions are made. To establish a framework for studying the neural basis of mate choice and social behavior in this species, we first describe the cytoarchitecture of the brain using Nissl-stained sections. Then, in order to better understand where progesterone acts to regulate social decisions, we determined the distribution of PR protein throughout the brain of P. pustulosus by immunohistochemistry. We found PR immunoreactivity in key brain regions known to modulate the processing of auditory cues and social behavior in other vertebrates. Due to its widespread distribution, PR likely also plays important roles in non-limbic brain regions that mediate non-social information processing. Further, we have colocalized PR with tyrosine hydroxylase, providing a functional context for the role of progesterone in mediating motivation and motor behavior. Our results significantly extend our understanding of hormonal modulation in the anuran brain and support the important role of the nuclear progesterone receptor in modulating female mate choice and receptivity in amphibians and across vertebrates.