Transsynaptic trophic effects of steroid hormones in an avian model of adult brain plasticity

Abstract

The avian song control system provides an excellent model for studying transsynaptic trophic effects of steroid sex hormones. Seasonal changes in systemic testosterone (T) and its metabolites regulate plasticity of this system. Steroids interact with the neurotrophin brain-derived neurotrophic factor (BDNF) to influence cellular processes of plasticity in nucleus HVC of adult birds, including the addition of newborn neurons. This interaction may also occur transsynpatically; T increases the synthesis of BDNF in HVC, and BDNF protein is then released by HVC neurons on to postsynaptic cells in nucleus RA where it has trophic effects on activity and morphology. Androgen action on RA neurons increases their activity and this has a retrograde trophic effect on the addition of new neurons to HVC. The functional linkage of sex steroids to BDNF may be of adaptive value in regulating the trophic effects of the neurotrophin and coordinating circuit function in reproductively relevant contexts.

Activity Dependency and Aging in the Regulation of Adult Neurogenesis

“…Estrogen substitution therapy in postmenopausal women might have a positive effect on these problems, but no consistent link between hormone-replacement therapy and cognitive performance has been found (Hogervorst et al 2002;Low and Anstey 2006). In songbirds, a fundamental link between testosterone signaling (including via synapses) and the control of adult neurogenesis has been described (Goldman and Nottebohm 1983;Louissaint et al 2002;Brenowitz 2014).…”

“Seasonal changes in the neuroendocrine system”: Some reflections

“…It was also the unambiguous example of photoperiod regulating neurogenesis in the adult vertebrate brain (Goldman & Nottebohm, 1983). Brenowitz (2015) discusses the role of sex steroids in controlling seasonal changes in song nuclei. This suggests that the changes are indirect through gonadotropin-driven steroid release (see also Balthazart et al, 2010) but photoperiod also has a direct effect upon the size of the song nuclei (which is then amplified by steroids) (e.g.…”

Rapamycin blocks the neuroprotective effects of sex steroids in the adult birdsong system

“…Neuron size in RA is a reliable and sensitive measure of seasonal plasticity of the song control system in songbirds (Brenowitz, ). For the present study, RA neuron size is an interesting phenotypic measure because the changes in this trait are mediated entirely by changes in transsynaptic trophic support of RA neurons from HVC neurons (Brenowitz, ); any changes observed in RA reflect the effects of presynaptic manipulations in HVC.…”

“…RA neuronal soma area is larger in breeding than nonbreeding condition birds (Brenowitz, ). The increase in the RA neuron size is mediated entirely by changes in transsynaptic trophic support of RA neurons from HVC neurons (Brenowitz, ; Brenowitz & Larson, ). As sparrows transition from breeding to nonbreeding conditions, T drops to basal levels and an equal number of HVC neurons die rapidly through apoptosis (Thompson, Bentley, & Brenowitz, ; Thompson & Brenowitz, ), and RA neuronal soma area decreases.…”