Molecular Diversity of Neuron Types in the Salamander Amygdala and Implications for Amygdalar Evolution.

The amygdala is a complex brain structure in the vertebrate telencephalon, essential for regulating social behaviors, emotions, and (social) cognition. In contrast to the vast majority of neuron types described in the many nuclei of the mammalian amygdala, little is known about the neuronal diversity in non-mammals, making reconstruction of its evolution particularly difficult. Here, we characterize glutamatergic neuron types in the amygdala of the urodele amphibian Pleurodeles waltl. Our single-cell RNA sequencing data indicate the existence of at least ten distinct types and subtypes of glutamatergic neurons in the salamander amygdala. These neuron types are molecularly distinct from neurons in the ventral pallium (VP), suggesting that the pallial amygdala and the VP are two separate areas in the telencephalon. In situ hybridization for marker genes indicates that amygdalar glutamatergic neuron types are located in three major subdivisions: the lateral amygdala, the medial amygdala, and a newly defined area demarcated by high expression of the transcription factor Sim1. The gene expression profiles of these neuron types suggest similarities with specific neurons in the sauropsid and mammalian amygdala. In particular, we identify Sim1+ and Sim1+ Otp+ expressing neuron types, potentially homologous to the mammalian nucleus of the lateral olfactory tract (NLOT) and to hypothalamic-derived neurons of the medial amygdala, respectively. Taken together, our results reveal a surprising diversity of glutamatergic neuron types in the amygdala of salamanders, despite the anatomical simplicity of their brain. These results offer new insights on the cellular and anatomical complexity of the amygdala in tetrapod ancestors.

Cell-type profiling in salamanders identifies innovations in vertebrate forebrain evolution.

The evolution of advanced cognition in vertebrates is associated with two independent innovations in the forebrain: the six-layered neocortex in mammals and the dorsal ventricular ridge (DVR) in sauropsids (reptiles and birds). How these innovations arose in vertebrate ancestors remains unclear. To reconstruct forebrain evolution in tetrapods, we built a cell-type atlas of the telencephalon of the salamander Pleurodeles waltl. Our molecular, developmental, and connectivity data indicate that parts of the sauropsid DVR trace back to tetrapod ancestors. By contrast, the salamander dorsal pallium is devoid of cellular and molecular characteristics of the mammalian neocortex yet shares similarities with the entorhinal cortex and subiculum. Our findings chart the series of innovations that resulted in the emergence of the mammalian six-layered neocortex and the sauropsid DVR.

Testosterone replacement causes dose-dependent improvements in spatial memory among aged male rats.

Testosterone has been shown to have dose-dependent effects on spatial memory in males, but the effects of aging upon this relationship remain unclear. Additionally, the mechanism by which testosterone regulates memory is unknown, but may involve changes in brain-derived neurotrophic factor (BDNF) within specific brain regions. We tested the effects of age and testosterone on spatial memory among male rats using two spatial memory tasks: an object-location memory task (OLMT) and the radial-arm maze (RAM). Castration had minimal effect on performance on the RAM, but young rats (2 months) performed significantly fewer working memory errors than aged rats (20 months), and aged rats performed significantly fewer reference memory errors. Both age and castration impaired performance on the OLMT, with only the young rats with intact gonads successfully performing the task. Subsequent experiments involved daily injections of either drug vehicle or one of four doses of testosterone propionate (0.125, 0.250, 0.500, and 1.00 mg/rat) given to castrated aged males. On the RAM, a low physiological dose (0.125 mg) and high doses (0.500-1.000 mg) of testosterone improved working memory, while an intermediate dose (0.250 mg) did not. On the OLMT, only the 0.250 mg T group showed a significant increase in exploration ratios from the exposure trials to the testing trials, indicating that this group remembered the position of the objects. Brain tissue (prefrontal cortex, hippocampus, and striatum) was collected from all subjects to assay BDNF. We found no evidence that testosterone influenced BDNF, indicating that it is unlikely that testosterone regulates spatial memory through changes in BDNF levels.

Seasonal and sex differences in cell proliferation, neurogenesis, and cell death within the dentate gyrus of adult wild-caught meadow voles.

Past research indicates that female meadow voles (Microtus pennsylvanicus) show decreased neurogenesis within the hippocampus during the breeding season relative to the non-breeding season, whereas male voles show no such seasonal changes. We expanded upon these results by quantifying a variety of endogenous cell proliferation and neurogenesis markers in wild-caught voles. Adult male and female voles were captured in the summer (breeding season) or fall (non-breeding season), and blood samples and brain tissue were collected. Four cellular markers (pHisH3, Ki67, DCX, and pyknosis) were labeled and then quantified using either fluorescent or light microscopy. The volume of the cell layers within the dentate gyrus (hilus and granule cell layer) was significantly larger in males than in females. In both sexes, all the cellular markers decreased significantly in the dentate gyrus during the breeding season relative to the non-breeding season, indicating decreased cell proliferation, neurogenesis, and pyknosis. Only the pHisH3 marker showed a sex difference, with females having a greater density of this cell proliferation marker than males. During the breeding season relative to the non-breeding season, males and females showed the predicted significant increases in testosterone and estradiol, respectively. Overall, these results suggest higher levels of neuronal turn-over during the non-breeding season relative to the breeding season, possibly due to seasonal changes in sex steroids.