Nerve growth factor affects defense-related behaviors, but not lordosis, in ovariectomized, estrogen-treated rats

Nerve growth factor induces Fos-like immunoreactivity within identified cholinergic neurons in the adult rat basal forebrain

Immunocytochemical techniques were used to examine and compare the effects of intracerebroventricular administration of nerve growth factor (NGF) on Fos expression within identified cholinergic and non-cholinergic neurons located in different regions of the adult rat basal forebrain. Animals were killed 1, 3, 6, and 12 h after receiving NGF (0.5 or 5.0 microg) or vehicle into the left lateral ventricle and sections through the medial septum, diagonal band of Broca, nucleus basalis magnocellularis, and striatum were processed for the combined immunocytochemical detection of Fos and choline acetyltransferase (a marker for cholinergic neurons), or Fos and parvalbumin (a marker for gamma aminobutyric acid (GABA)-containing neurons). NGF produced a significant increase in the percentage of cholinergic neurons containing Fos-like immunoreactivity within all four regions examined. The largest increases were detected in the medial septum (47.8%) and the horizontal limb of the diagonal band of Broca (67.7%). In these areas, NGF-mediated induction of Fos-like immunoreactivity was detected as early as 3 h, peaked at 6 h, and was reduced by 12 h, postinfusion. Small but significant increases in the percentage of cholinergic neurons containing Fos-like immunoreactivity were also detected in the striatum (4.2%) and in the nucleus basalis magnocellularis (19.2%) 3-12 h following administration of the higher dose of NGF. No evidence for an NGF-mediated induction of Fos within parvalbumin-containing neurons was detected in any of the four regions at any of the time-points examined; however, evidence for an NGF-mediated induction of Fos within epithelial cells lining the lateral ventricle was observed. These data demonstrate that NGF induces Fos expression within cholinergic, and not parvalbumin-containing (GABAergic), neurons in the basal forebrain, and furthermore that intracerebroventricular administration of NGF influences the different subgroups of basal forebrain cholinergic neurons to different degrees.

Estrogen and nerve growth factor-related systems in brain. Effects on basal forebrain cholinergic neurons and implications for learning and memory processes and aging

Estrogen replacement can significantly affect the expression of ChAT and NGF receptors in specific basal forebrain cholinergic neurons. The time-course of the effects is consistent with a direct up-regulation of ChAT followed by either direct or indirect down-regulation of p75NGFR and trkA NGF receptors, possibly due to increased cholinergic activity in the hippocampal formation and cortex and a decrease in hippocampal levels of NGF. Current evidence suggests ChAT, p75NGFR, trkA, and NGF all play a role in regulating cholinergic function in the hippocampal formation and cortex. In addition, all have been implicated in the maintenance of normal learning and memory processes as well as in changes in cognitive function associated with aging and with neurodegenerative disease. It is possible that estrogen may affect cognitive function via effects on NGF-related systems and basal forebrain cholinergic neurons. Effects of estrogen on cognitive function have been reported, as has some preliminary evidence for beneficial effects of estrogen in decreasing the prevalence of and reducing some cognitive deficits associated with Alzheimer's disease. Whether these effects are related to effects on NGF-related systems or basal forebrain cholinergic neurons is currently unknown. Indirect evidence suggests that estrogen interacts with NGF-related systems and that changes in circulating levels of estrogen can contribute to age-related changes in hippocampal levels of NGF. These findings have important implications for consideration of estrogen replacement therapy in pre- and post-menopausal women. Further studies examining effects of different regimens of estrogen replacement as well as estrogen combined with progesterone on NGF and basal forebrain cholinergic neurons in young and aged animals are required. Prospective studies correlating aging and estrogen replacement with numbers of basal forebrain cholinergic neurons and hippocampal and cortical levels of NGF also need to be performed to better assess the potential benefits of estrogen replacement in reducing age- and disease-related cognitive decline.