Overexpression of androgen receptors in target musculature confers androgen sensitivity to motoneuron dendrites

Abstract

The dendritic arbors of spinal motoneurons are dynamically regulated by a variety of factors, and several lines of evidence indicate that trophic interactions with the target musculature are of central importance. In highly androgen-sensitive motoneuron populations, androgens are thought to regulate motoneuron dendrites through their action at the receptor-enriched target musculature. Using rats transgenically modified to overexpress androgen receptor (AR) in skeletal muscle, we directly tested the hypothesis that the enhanced expression of AR in the target musculature can underlie the androgenic regulation of motoneuron dendritic morphology. The morphology of motoneurons innervating the quadriceps muscle was examined in wild-type (WT) rats as well as in rats that had been transgenically modified to overexpress ARs in their skeletal musculature. Motoneurons innervating the vastus lateralis muscle of the quadriceps in gonadally intact male rats, and castrated males with or without androgen replacement, were labeled with cholera toxin-conjugated horseradish peroxidase, and dendritic arbors were reconstructed in three dimensions. In WT rats, quadriceps motoneuron dendrites were insensitive to hormonal manipulation. In contrast, quadriceps motoneuron dendrites in gonadally intact transgenic males were larger than those of WT males. Furthermore, overexpression of ARs in the quadriceps muscle resulted in androgen sensitivity in dendrites, with substantial reductions in dendritic length occurring after castration; this reduction was prevented with testosterone replacement. Thus, it appears that the androgen sensitivity of motoneuron dendrites is conferred indirectly via the enrichment of ARs in the musculature.

Fig. 1.

Weights of the seminal vesicles in WT males and males transgenically modified to overexpress ARs in skeletal muscle (HSA-AR Tg) in sham castrates (black bars), castrates treated with vehicle (white bars), and castrates treated with testosterone (gray bars). Regardless of genotype, castration of males resulted in marked reductions in seminal vesicle weight, which was prevented by testosterone treatment. Bar heights represent means ± sem. *, Significantly different from WT sham males; †, significantly different from HSA-AR Tg sham males.

Fig. 2.

Darkfield digital micrographs (left) of transverse hemisections and matching computer-generated composites (right) of BHRP-labeled somata and processes after BHRP injection into the left vastus lateralis muscle of WT males after either sham castration, castration without hormone replacement (WT+V), or castration with testosterone replacement (WT+T). For each composite, individual sections were drawn at 480-μm intervals through the entire rostrocaudal extent of the quadriceps motor pool. These composites were selected because they are representative of their respective group average dendritic lengths. Scale bar, 500 μm.

Fig. 3.

Darkfield digital micrographs (left) of transverse hemisections and matching computer-generated composites (right) of BHRP-labeled somata and processes after BHRP injection into the left vastus lateralis muscle of males transgenically modified to overexpress ARs in skeletal muscle (HSA-AR Tg) after either sham castration, castration without hormone replacement (HSA-AR Tg+V), or castration with testosterone replacement (HSA-AR Tg+T). Composites drawn and selected as in Fig. 2. Scale bar, 500 μm.

Fig. 4.

Cross-sectional soma areas (top) and dendritic lengths (bottom) of quadriceps motoneurons in WT males and males transgenically modified to overexpress ARs in skeletal muscle (HSA-AR Tg) in sham castrates (black bars), castrates treated with vehicle (white bars), and castrates treated with testosterone (gray bars). Hormonal manipulation had no effect on quadriceps motoneurons soma size in either WT or HSA-AR Tg males. In transgenic males, dendritic lengths in sham males were longer than those of WT shams. Castration reduced dendritic length in HSA-AR Tg+V castrates, and this reduction was prevented in HSA-AR Tg+T castrates. Bar heights represent means ± sem. *, Significantly different from WT sham males; †, significantly different from HSA-AR Tg sham males; sq, square.

Fig. 5.

Inset, Drawing of spinal gray matter divided into radial sectors for measure of quadriceps motoneuron dendritic distribution. The distribution of dendrites was assessed by determining the total length of quadriceps motoneuron dendrites contained in each radial location in WT males and males transgenically modified to overexpress ARs in skeletal muscle (HSA-AR Tg) in sham castrates (black bars), castrates treated with vehicle (white bars), and castrates treated with testosterone (gray bars). For graphic purposes, dendritic length measures have been collapsed into six bins of 60° each. In both WT and HSA-AR Tg males, quadriceps motoneuron dendritic arbors display a nonuniform distribution, with the majority of the arbor located between 300° and 120°. The distribution of dendrites was not affected by hormonal condition in WT males. Compared with WT sham males, the longer lengths of quadriceps motoneuron dendrites in HSA-AR Tg sham males were present throughout the arbor. Decreases in dendritic length were present throughout the arbor after castration in HSA-AR Tg+V males compared with HSA-AR Tg sham males. These changes were prevented in HSA-AR Tg castrates treated with testosterone. Bar heights represent means ± sem. *, Significantly different from WT sham males; †, significantly different from HSA-AR Tg sham males.

Fig. 6.

Inset, Drawing of spinal gray matter divided into radial sectors for measure of quadriceps motoneuron radial dendritic extent. The maximal distance in the transverse plane that labeled dendrites could be observed at each radial location in WT males and males transgenically modified to overexpress ARs in skeletal muscle (HSA-AR Tg) in sham castrates (black bars), castrates treated with vehicle (white bars), and castrates treated with testosterone (gray bars). For graphic purposes, dendritic extent measures have been collapsed into six bins of 60° each. Maximal extent measures of quadriceps motoneurons did not differ across genotype or hormone manipulation. Bar heights represent means ± sem.