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A study with adult male mice demonstrated that castration increased cell proliferation within the SVZ , suggesting that testosterone suppresses cell proliferation in this region. For both female rats and mice, prolonged exposure (14–21 days) to estradiol caused a decrease in neurogenesis within portions of the olfactory bulbs 80,81, although the relative effects on the accessory and main olfactory bulbs differed between the two species. Testosterone implants given to female canaries caused an increase in neurogenesis within the HVC via enhanced cell survival but not cell proliferation . A better understanding of the effects of testosterone upon adult neurogenesis could shed light on neurological diseases that show sex differences. These experiments have provided clear evidence that testosterone increases adult neurogenesis within the dentate gyrus region of the hippocampus through an androgen-dependent pathway. There is considerable evidence that steroid hormones modulate various stages of adult neurogenesis, and this review provides a focused summary of the effects of testosterone on adult neurogenesis.
These transgenic animals showed a significant increase in neurogenesis within the accessory olfactory bulbs in response to exposure to male urine , a response not observed in wild type males but typical of wild type female mice. Compared to the dentate gyrus, relatively little work has been done testing the effects of testosterone upon neurogenesis in the mammalian olfactory bulbs (Table 1) . Female cowbirds show greater hippocampal neurogenesis than males, and neurogenesis levels peaked after the breeding season was over . However, there is no sex difference in hippocampus size or caching behavior in chickadees and so it seems unlikely that sex steroids regulate seasonal changes in hippocampal neurogenesis in this species. It is also noteworthy that all of the key enzymes for testosterone production have been localized in the rat and human hippocampus, indicating that some testosterone is produced de novo within the brain itself to act as a neurosteroid 51,52,53. A variety of endogenous proteins that are transiently expressed during different stages of neural development can also be used to assess cell proliferation and neurogenesis 24,27.
Experiments with laboratory rodents support the general conclusion from the early studies with voles; namely, that testosterone enhances adult neurogenesis in the dentate gyrus by increasing cell survival, while having little or no effect on cell proliferation (Table 2). In contrast, castration caused a decrease in cell proliferation in the SVZ of juvenile male rats, and proliferation levels were restored by testosterone or estradiol injections but not by DHT injections , which suggests that testosterone is acting through an estrogen-dependent pathway. For example, in a small 2020 study, 10 days of sun exposure and 6 weeks of vitamin D supplementation led to significant increases in vitamin D and testosterone levels in soccer players. Although there are unexplained contradictions, the general conclusion from these experiments is that testosterone enhances neurogenesis by increasing the survival of newly generated neurons, while having minimal influence on levels of cell proliferation. Most work to date has been conducted using rats and mice to test the effects of a wide range of testosterone manipulations upon adult neurogenesis in the dentate gyrus. The ability of testosterone to protect against a stress-induced decrease neurogenesis seems to involve sustaining basal cell survival levels and possibly basal cell proliferation levels as well . Among male rats, physiological levels of testosterone prevented the neurogenesis-reducing effects of social isolation, whereas high doses (0.500 mg/rat) provided no such buffer .
The hypothalamus-pituitary-gonadal axis (HPG axis) plays an essential role in regulating early development, adolescence, and sustaining the adult reproductive functions. Two of the more commonly used markers are Ki67 and doublecortin (DCX), which are expressed in proliferating cells and early developing neurons, respectively 28,29,30,31. BrdU-labeling allows cells to be precisely birth dated based on the timing of injections and brain tissue collection, and co-labeling with neuronal and glial markers allows quantification of cellular differentiation.
However, this article pertains to anyone looking to increase their testosterone levels. Though many supplements claim to help boost testosterone levels, viable research on these products is lacking. This enhanced survival involves an androgen-dependent pathway in males, distinct from the estrogen-dependent pathway that can increase or decrease neurogenesis in females. The experiments summarized here clearly demonstrate that testosterone influences adult neurogenesis, as specifically demonstrated within the HVC of birds and within the olfactory bulbs and dentate gyrus of rodents. These results suggest that physiological levels of testosterone provide antidepressant effects, which are not necessarily mediated by changes in hippocampal neurogenesis.