MOTONEURON MORPHOLOGY IN THE DORSOLATERAL NUCLEUS OF THE RAT SPINAL-CORD - NORMAL DEVELOPMENT AND ANDROGENIC REGULATION

The rat lumbar spinal cord contains two sexually dimorphic motor nucle i, the spinal nucleus of the bulbocavernosus (SNB), and the dorsolater al nucleus (DLN). These motor nuclei innervate anatomically distinct p erineal muscles that are involved in functionally distinct copulatory reflexes. The motoneurons in the SNB and DLN have different dendritic morphologies. The dendrites of motoneurons in the medially positioned SNB have a radial, overlapping arrangement, whereas the dendrites of t he laterally positioned DLN have a bipolar and strictly unilateral org anization. During development, SNB motoneuron dendrites grow exuberant ly and then retract to their mature lengths. In this experiment we det ermined whether the adult difference in SNB and DLN motoneuron morphol ogy was reflected in different patterns of dendritic growth during nor mal development. Furthermore, the development of both these nuclei is under androgenic control. In the absence of androgens, SNB dendrites f ail to grow; testosterone replacement supports normal dendritic growth . Thus, we also examined the development of DLN dendrites for similar evidence of androgenic regulation. By using cholera toxin-horseradish peroxidase (BHRP) to label motoneurons retrogradely, we measured the m orphology of DLN motoneurons in normal males, and in castrates treated with testosterone or oil/blank implants at postnatal day (P) 7, P28, P49, and P70. Our results demonstrate that in contrast to the biphasic pattern of dendritic development in the SNB, dendritic growth in the DLN was monotonic; the dendritic length of motoneurons increased more than 500% between P7 and P70. However, as in the SNB, development of D LN motoneuron morphology is androgen-dependent. In castrates treated w ith oil/blank implants, DLN somal and dendritic growth were greatly at tenuated compared to those of normal or testosterone-treated males. Th us, while androgens are clearly necessary for the growth of motoneuron s in both the SNB and DLN, their different developmental patterns sugg est that other factors must be involved in regulating this growth. (C) 1993 Wiley-Liss, Inc.