INITIAL SEX-DIFFERENCES IN NEURON GROWTH AND SURVIVAL WITHIN AN AVIANSONG NUCLEUS DEVELOP IN THE ABSENCE OF AFFERENT INPUT

Only male zebra finches (Poephila guttata) sing, and nuclei implicated in song behavior exhibit marked sex differences in neuron number, In the robust nucleus of the anterior neostriatum (RA), these sex differe nces develop because more neurons die in young females than in males, However, it is not known whether the sexually dimorphic survival of RA neurons is a primary event in sexual differentiation or a secondary r esponse to sex differences in the number of cells interacting trophica lly with RA neurons, In particular, since sexual differentiation of th e RA parallels the development of dimorphisms in the numbers of neuron s providing afferent input from the lateral magnocellular nucleus of t he anterior neostriatum (IMAN) and the high vocal center (HVC), it has been hypothesized that sex differences in the size of these afferent populations trigger differential RA neuron survival and growth. To tes t this hypothesis, we lesioned either the IMAN or both the IMAN and HV C unilaterally in 12-day-old male and female zebra finches. Subsequent ly, RA cell death and RA neuron number and size were measured. Unilate ral IMAN lesions increased cell death and decreased neuron number and size within the ipsilateral RA of both sexes. However, even in the IMA N-lesioned hemisphere, these effects were less pronounced in males tha n in females, so that by day 25 the volume, number, and size of neuron s were sexually dimorphic in both the contralateral and ipsilateral RA , Similarly, the absence of both IMAN and HVC afferents did not preven t the emergence of sex differences in the number and size of RA neuron s by 25 days posthatching. We conclude that these sex differences with in the RA are not a secondary response to dimorphisms in the numbers o f IMAN or HVC neurons providing afferent input. (C) 1995 John Wiley & Sons, Inc.