Sexual Differentiation of a Sexually Dimorphic Spinal Motor Pool
Strains of Rats
S. Walley, K.
Zahller, A. Wilson, R. Craft*, and C. Ulibarri, Integrative Physiology and Neuroscience (IPN), *Dept of Psych, College of Veterinary Medicine, Pullman WA 99164-6520.
This research tested the
hypothesis that genotype will influence sexual differentiation of the sexual
dimorphic motor pool, the spinal nucleus of the bulbocavernosus (SNB). The SNB
innervates the bulbocavernosus, the levator ani, and the anal sphincter muscles.
The SNB is an example of a sexually dimorphic structure in the CNS. The SNB is
located on the dorsal-lateral aspect of the ventral funiculus at the lumbosacral
transition zone of the spinal cord. Genotype is suggested to play a role in SNB
characteristics. However, no research has been conducted comparing strain
differences of the SNB in rats. Rat strains such as F344, Long Evans, and
Sprague Dawley show differences in reproductive behavior, so it is likely that
these rat strains will show differences in their neuroananatomy such as the SNB.
Also, sexual dimorphisms in the CNS generated in response to early exposure to
androgens and/or adulthood gonadal steroids have been reported. Therefore
offspring of F344 albino rats, Long Evans outbred pigmented rats, and Sprague
Dawley outbred albino rats were assigned same-strain breeding pairs. When pups
were born they were assigned to one of five groups (male neonatal gonadectomy,
male neonatal sham gonadectomy, neonatal females injected with 100 µg
testosterone, neonatal females injected with 1000 µg testosterone, or neonatal
females injected with oil as control). As adults, rats were anesthetized,
castrated if necessary, and implanted with either testosterone or empty
capsules. Four weeks later rats were anesthetized, perfused, and their spinal
cords removed. After removal, spinal cords were imbedded, frozen, coronally
sectioned, and mounted onto gelatin-chrom-alum coated slides and allowed to air
dry. They were then processed for Nissl staining. Slides were coded so they
could be examined without knowledge of treatment. The slides were examined by
two independent observers using light microscopy. SNB motoneurons were
identified, counted, and measured. Differences across groups were determined by
analyses of variance.
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