Eileen C. Hoyt

GABAB receptor antagonist SGS742 improves spatial memory and reduces protein binding to the cAMP response element (CRE) in the hippocampus

Memory storage in the brain requires protein synthesis initiated through signaling pathways that control transcription. Such mechanisms are under active investigation for therapies in disorders involving cognitive dysfunction. Long-term memory can be improved by inhibiting activation or reducing expression of transcription factors such as ATF4/CREB2 and some C/EBP family members which appear to serve as memory suppressors. Here, we provide evidence that GABAB receptor antagonists may enhance cognition, at least in part, by this mechanism. We tested a GABAB receptor antagonist, SGS742 (CGP36742), on hippocampal-dependent memory and hippocampal nuclear CRE-binding activity in rats. As a result, acute in vivo administration of SGS742 both improved memory and reduced total hippocampal CRE-binding activity of which a large proportion in the basal state could be immunoneutralized with CREB2 antibodies. Consistent with its activity on information storage mechanisms, acute SGS742 effectively improved long-term memory in retrograde protocols, in which drug was given at times when memory formation can be interrupted by blocking new protein production. In conclusion, GABAB antagonists may provide a pharmacological therapy for cognitive impairment, sharing mechanistic features with genetic approaches to reduce CREB2 activity and to augment long-term memory.

Effects of aging on the hippocampal formation in a naturally occurring animal model of mild cognitive impairment

Vertical integration is being used to great advantage in neurobiological research on the basis of age-related cognitive decline. Such research bridges analysis between the molecular and cellular levels and the outcome of impaired psychological functions. Current use of animals models within this paradigm has defined mild cognitive impairment in a subpopulation of outbred aged Long-Evans rats by assessment of hippocampal-dependent spatial cognition. Aged rats with cognitive impairment exhibited no loss of neurons in the hippocampus. Current research is focused on the functional alterations in neurons by methods which assess transcriptional mechanisms and signaling pathways.

Effects of Fasting, Refeeding, and Intraluminal Triglyceride on Proglucagon Expression in Jejunum and Ileum

Intestinal proglucagon is thought to be synthesized primarily by the distal gut, although the role of proglucagon-derived glucagon-like peptide I (GLP-I) as a major physiological incretin would seem to be associated with production in proximal small bowel. To better characterize the sites of production of proglucagon and GLP-I in the small intestine and evaluate nutrient regulation of small bowel proglucagon and derived peptides, we evaluated the effects of fasting for 72 h and subsequent refeeding or jejunal infusion of long-chain triglyceride (LCT) for 24 h on local expression of proglucagon in proximal and distal small bowel. Proglucagon mRNA abundance and cellular localization were determined and correlated with wet weight of bowel. In jejunum, proglucagon mRNA abundance decreased by 40% with fasting (P < 0.005) and increased with refeeding to levels similar to those of ad libitum-fed animals. In ileum, fasting resulted in a 20% decrease in proglucagon mRNA (P < 0.005); in contrast to jejunum, refeeding did not result in a significant rise in ileal proglucagon mRNA abundance from fasting values. In jejunum, signal intensity of proglucagon mRNA per cell, determined by in situ hybridization, decreased with fasting (P < 0.05) and increased with refeeding (P < 0.005) in proportion to changes in mRNA abundance. Plasma enteroglucagon and GLP-I levels correlated with jejunal proglucagon mRNA. Intrajejunal infusion of LCT increased expression of proglucagon to a greater extent in jejunum than in ileum. In conclusion, enteral nutrient intake stimulates small bowel proglucagon expression; this effect is greater in jejunum than ileum, consistent with greater intraluminal nutrient exposure and the role of jejunum as a source of the major incretin GLP-I.

Effect of age and cognitive status on basal level AP-1 activity in rat hippocampus

Activator protein-1 (AP-1) was examined at multiple levels (mRNA, DNA binding, composition) in hippocampus of young and aged rats that were behaviorally characterized for spatial memory. GFAP mRNA was measured as a gene product known to increase with aging and to be regulated by AP-1. The activity of Jun-amino terminal-kinase (JNK) was also assessed. Levels of c-jun and c-fos mRNAs were unchanged with aging or spatial learning ability. Abundance of GFAP mRNA was significantly increased in aged hippocampus but did not correlate with spatial learning. Total AP-1 binding activity was unaltered with age or cognitive ability. In hippocampus of young, aged unimpaired and aged impaired rats, AP-1 consists mainly of c-Jun, phosphorylated c-Jun (p-c-Jun), JunD, and smaller amounts of c-Fos. JNK is constitutively active in young and aged hippocampus. We conclude that the basal expression of c-fos and c-jun mRNA, overall AP-1 binding activity and AP-1 composition are not influenced by aging or cognitive ability.

Transcriptional mechanisms of hippocampal aging.

Aging related cognitive decline is an increasing health problem but affects only a subset of elderly humans. This research uses outbred young (Y) and aged rats. Behavioral characterization distinguishes aged rats with impaired spatial learning (AI) and aged rats with unimpaired learning ability (AU), mimicking the varied susceptibility of the human population to age-associated learning impairment. Studies are testing a hypothesis that hippocampal transcriptional mechanisms and gene expression profiles linked to activator protein-1 (AP-1) and glucocorticoid receptor (GR), mineralocorticoid receptor (MR) or cyclic AMP response element binding protein (CREB) families of transcription factors distinguish successful or unsuccessful aging and cognition. Results from mRNA assays, in situ hybridization, electromobility shift assays and western immunoblot indicate changes in GR and CREB in AI rats. State of the art future approaches to define downstream transcription targets are described.

Increased ileal proglucagon expression after jejunectomy is not suppressed by inhibition of bowel growth

After jejunectomy, a rapid and sustained increase in the abundance of proglucagon mRNA occurs in residual ileum and is accompanied by increases in plasma intestinal proglucagon-derived peptides. This response may be a component of adaptive growth, or proglucagon-derived peptides may regulate adaptive growth. To distinguish these possibilities, rats were treated with difluoromethylornithine, blocking ornithine decarboxylase activity and thereby adaptive bowel growth. Three groups fedad libitum were compared: (1) resect: rats with 80% proximal small bowel resection; (2) resect + difluoromethylornithine: resected rats given difluoromethylornithine in drinking water; and (3) transect: transected controls. Six days after surgery, the resect + difluoromethylornithine group demonstrated inhibition of adaptive bowel growth. Abundance of ileal proglucagon mRNA in resect and resect + difluoromethylornithine groups was double that in the transect group (P<0.02), whereas ornithine decarboxylase mRNA levels did not differ. Plasma enteroglucagon and glucagon-like peptide-I levels were greater in resect than transect groups (P<0.002) and did not differ between resect and resect + difluoromethylornithine groups. The rise in ileal proglucagon mRNA after proximal small bowel resection is not inhibited by difluoromethylornithine despite blocking bowel growth and, therefore, is not merely a component of adaptive growth. Proglucagon-derived peptides are possible modulators of adaptive bowel growth but cannot stimulate growth when ornithine decarboxylase activity is inhibited.

Rodent Studies on the Potential Relevance of Insulin-Like Growth Factor (IGF-I) to Ovarian Physiology

The recurring process of ovarian follicular growth is an exponential rather than a linear process characterized by substantial dramatic proliferation and differentiation of the granulosa cell. Although the pivotal role(s) of gonadotropins and of gonadal steroids in this explosive agenda is well recognized, the variable fate of follicles subject to comparable gonadotropic support suggests the existence of additional intraovarian regulatory mechanism(s). Among potential novel intraovarian regulators, insulin-like growth factor I (IGF-I) has been receiving increasingly intense scrutiny (1–2). Taken together these studies strongly suggest the existence of an intraovarian autocrine control mechanism, wherein IGF-I may serve as the central signal, and the granulosa cell its site of production, reception, and action. Viewed in this light, IGF-I may promote the growth and/or differentiation of the granulosa cell, acting for the most part as an amplifier of gonadotropin action. Granulosa cell-derived IGF-I may also provide paracrine input to the nearby theca-interstitial cell compartment in the interest of coordinated follicular function.

Structural and Functional Characterization of IGF-I RNA 3’ Variants

In rat and other mammalian species a single insulin-like growth factor I (IGF-I) gene gives rise to a heterogeneous and complex family of IGF-I mRNAs (1,2).