In addition to effects on brain protein synthesis, neurotransmitter release, and electrophysiology, estrogens alter neurite outgrowth and synaptogenesis. This study examined in the adult rat the effects of estrogen and sex on the expression of the GAP-43 gene; encoding a phosphoprotein structurally and physiologically linked to these two processes in the rat CNS. Ovariectomized (OVX) rats were injected with vehicle or estrogen, or male and female rats were either gonadectomized or left intact. Brains were dissected to obtain ventromedial hypothalamus (VMH), posterior hypothalamus (PH), or frontal cortex (CTX). Total RNA from these areas were extracted, and slot-blots of equal masses of total RNA were hybridized to P-32-labeled cDNAs for GAP-43 and beta-actin, and also to synthetic poly-dT. Resultant autoradiograms were scanned by laser densitometry, quantitated, and ratios of the gray scale generated by each probe were compared between experimental groups. GAP-43 mRNA expression, when compared to expression of either beta-actin mRNA or total poly(A)-containing RNA (poly(A) RNA), was higher in VMH and PH as compared to CT-A. Estrogen treatment of OVX rats resulted in a 48-74% increase in GAP-43 mRNA levels in the VMH-in one experiment, this increase was noted after 2 h of estradiol treatment, and in another after 3 days of estradiol benzoate treatment; but PH and CTX were unaffected by either estrogen regimen. Conversely, ovariectomy of intact rats decreased GAP-43 mRNA expression by 45% in the VMH, but not in the CTX. Males exhibited 1.5-2 fold higher GAP-43 gene expression in both VMH and CTX as compared to females, and this increase was unaffected by gonadectomy. The results of these studies document: (1) the estrogenic regulation of GAP-43 mRNA levels in an estrogen-sensitive area of the female rat brain; and (2) a sex difference in GAP-43 gene expression in both VMH and CTX, which is unaffected by gonadectomy. These results support the notion that GAP-43 gene expression is sexually dimorphic and hormonally regulated, and may help to account for sexually dimorphic structural differences in various limbic system areas thought to be important in the process of neuroendocrine maturation.
