The life span of new neurons in a song control nucleus of the adult canary brain depends on time of year when these cells are born.

Abstract

The number of high vocal center (HVC) neurons labeled in adult male caries by systemic Injections of [3H]thymidine depended on season and survival time. This was true for HVC neurons projecting to the robust nucleus of the archistriatum and for other HVC neurons that could not be retrogradely filled from the robust nucleus of the archistriatum. Birds Injected in October and killed 40 days later had twice as many labeled HVC neurons as birds injected in May and killed 40 days later. However, this difference became much larger (5 times) when the birds were allowed to survive for 4 months. Whereas more than half of the spring-born neurons disappeared between 40 days and 4 months, there was no reduction in the number of fall-born neurons present at the 4-month survival point. We infer that seasonal variables affect the life span of HVC neurons born in adulthood.It has been known for many years that the developing nervous system overproduces neurons. Many parts of the developing central nervous system retain only a fraction of the neurons originally produced (1-5). The conditions that determine this selective death are under intensive study (e.g., refs. 6-9). Neuronal death is also seen in some diseases ofthe adult human brain-e.g., Alzheimer and Parkinson diseases. Several studies (e.g., refs. 10-12) have looked at the factors or conditions that may prevent or delay neuronal death during development or in the adult diseased brain. There is hope that factors that prevent selective death during development may also rescue neurons that die during disease. But such studies need not be restricted to the developing brain. Neurogenesis and neuronal replacement occur spontaneously in a part of the song system of adult songbirds. This offers the opportunity to study neuronal death and replacement in a healthy adult vertebrate brain and in a system that controls a well-characterized behavior.Oscine songbirds acquire their song by reference to auditory information (13)(14)(15). The circuits involved in the acquisition and production of learned song have been described (16-19). One ofthe telencephalic nuclei, the high vocal center (HVC), necessary for the production of learned song (16,20,21), is of particular interest because it incorporates and replaces neurons in adulthood (22)(23)(24)(25)(26). More than half of the adult-formed neurons added to the HVC (26-28) grow an axon that reaches nucleus robustus archistriatalis (RA), on the efferent pathway for song production (16). The number of HVC neurons, including those that project to RA, ceases to increase in adulthood and neurogenesis in this system is part of a process of neuronal replacement (25,28,29).Earlier work (27) GBq; New England Nuclear). These birds received the PHIthymidine treatment at two times of the year, May (spring) or September (fall), and were divided into four groups as described in Table 1. All birds were weighed, anesthetized with 50 A1 of a dilution of Nembutal (Abbott; 10 mg/ml), and given FluoroGold [2% (wt/vol) solution in saline] microinj...

Timing of brain-derived neurotrophic factor exposure affects life expectancy of new neurons

“…Brains were embedded in polyethylene glycol and sectioned sagittally at 6-m intervals. A set of six sections, obtained from the middle of HVC at a spacing of 60 m, was mounted on slides and stained with Cresyl violet, which, other studies have shown, can be used to reliably identify neuronal nuclei (3,6,15). Sections were then treated for autoradiography with Kodak NTB2 nuclear track emulsion, incubated for 4 weeks in the dark, and developed with D19 (Kodak) as described (7,11).…”

“…Thereafter, the life expectancy of the neurons depends on the time of year when they are born. Whereas the number of fall-born neurons is virtually the same 30 and 120 days after their birth, this number drops by one-half at the later survival time in spring-born neurons (6). However, the mechanism that regulates this season-dependent survival͞attrition has not been worked out in detail.…”

Behavioral development in the honey bee: toward the study of learning under natural conditions.

“…Analyses of the cellular changes resulting in changes in total regional volume suggested that changes in processes of intrinsic neurons (specifically, increases in dendritic length) led to changes in synaptic density in HVC (DeVoogd and Nottebohm 1981 ) and RA (Canady et al 1988). More recent studies have uncovered a relationship between seasonal changes in singing, testosterone production, and neuronal number in HVC (Kirn et al 1994;Nottebohm et al 1994;Rasika et al 1994). Gonadal steroids thus coordinate changes in the structure of the brain with reproductive status.…”

Neurogenesis in the Adult Avian Song-Control System

“…Unlike mammals, in which substantial neurogenesis is primarily limited to two regions of the brain (HC and olfactory bulb), new neurons are added throughout most of the avian telencephalon, and this form of plasticity, therefore, seems to be a fundamental feature of the forebrain in birds (AlvarezBuylla et al 1994). …”