Chronic but not acute intracerebroventricular administration of amyloid ß-peptide(25-35) decreases somatostatin content, adenylate cyclase activity, somatostatin-induced inhibition of adenylate cyclase activity, and adenylate cyclase I levels in the rat hippocampus

Abstract

Although alterations in adenylate cyclase (AC) activity and somatostatin (SRIF) receptor density have been reported in Alzheimer's disease, the effects of amyloid β-peptide (Aβ) on these parameters in the hippocampus are unknown. Our aim was to investigate whether the peptide fragment Aβ(25–35) can affect the somatostatinergic system in the rat hippocampus. Hence, Aβ(25–35) was injected intracerebroventricularly (i.c.v.) to Wistar rats in a single dose or infused via an osmotic minipump connected to a cannula implanted in the right lateral ventricle during 14 days. The animals were decapitated 7 or 14 days after the single injection and 14 days after chronic infusion of the peptide. Chronic i.c.v. infusion of Aβ(25–35) decreased SRIF-like immunoreactive content without modifying the SRIF receptor density, SRIF receptor expression, or the Giα1, Giα2, and Giα3 protein levels in the hippocampus. This treatment, however, caused a decrease in basal and forskolin-stimulated AC activity as well as in the capacity of SRIF to inhibit AC activity. Furthermore, the protein levels of the neural-specific AC type I were significantly decreased in the hippocampus of the treated rats, whereas an increase in the levels of AC V/VI was found, with no alterations in type VIII AC. A single i.c.v. dose of Aβ(25–35) exerted no effect on SRIF content or SRIF receptors but induced a slight decrease in forskolin-stimulated AC activity and its inhibition by SRIF. Because chronic Aβ(25–35) infusion impairs learning and memory whereas SRIF facilitates these functions, the alterations described here might be physiologically important given the decreased cognitive behavior previously reported in Aβ-treated rats.