Fimbria-fornix lesions in aged rats cause increased carbachol-stimulated phosphoinositide hydrolysis in the hippocampus, but no change in muscarinic receptor binding

Modest cholinergic deafferentation fails to alter hippocampal G-proteins

The integrity of hippocampal G-protein mediated signalling following ibotenate induced lesion of the medial septum was examined. The lesion was confined histologically to the septum and induced a 23% reduction in hippocampal choline acetyltransferase (ChAT) activity and G-proteins levels and related enzyme activities were measured in the hippocampus following a 21 day survival period. The relative levels of five G-protein subunits (Gbeta, G(alpha)o, G(alpha)i1, G(alpha)i2, and G(alpha)s-L), basal GTPase, the degree of carbachol- or baclofen-stimulated GTPase activities, and the basal and fluoroaluminate-stimulated adenylate cyclase activities were apparently unaffected. To determine if our assay methodology was sensitive to changes in pre-synaptic signalling, we compared G-protein density in synaptosomes with total hippocampal homogenates. The concentration of G(alpha)q/11, G(alpha)i1, and G(alpha)i2. were significantly lower in synaptosomes, while G(alpha)o, was only marginally reduced. Thus, modest lesions of the medial-septal nucleus fail to alter G-protein signalling. However, our findings that G-protein density is lower in synaptosomal membranes than in total homogenates, indicates that the analysis of signalling events in synaptosomes following deafferentation could clarify adaptive changes which may occur at the presynaptic level.

Effects of postmortem interval, age, and Alzheimer's disease on G-proteins in human brain

Heterotrimeric G-proteins are critical components in many receptor-coupled signal transduction systems, and altered levels and functions of G-proteins have been implicated in several neurological disorders, including Alzheimer's disease. Investigations in postmortem human brain provide a direct approach to study G-protein involvement in neurological disorders. Therefore, the effects of postmortem interval, aging, and Alzheimer's disease on G-protein levels were determined in postmortem human brain and an assay to measure activation of G-proteins was developed. Within the postmortem interval range of 5 to 21 h, the levels of G alpha i1, G alpha i2, G alpha s, and G beta were stable, whereas G alpha q and G alpha o decreased slightly, in human prefrontal cortex. In subjects aged 19 to 100 y, decreased levels of G alpha q and G alpha o were significantly correlated with increased age, but levels of the other G-protein subunits did not vary. In Alzheimer's disease prefrontal cortex, superior temporal gyrus, and occipital cortex, all G-protein subunit levels were equivalent to those in matched controls except for a slight deficit in G alpha i1. An ELISA assay using selective antibodies was used to measure [35S]GTP gamma S binding to G alpha o and G alpha i1. Binding was proportional to the concentration of GTP-gamma S and was concentration-dependently stimulated by mastoparan equivalently in control and Alzheimer's disease prefrontal cortical membranes.

Cholinergic deafferentation of dorsal hippocampus by fimbria-fornix lesioning differentially regulates subtypes (m1-m5) of muscarinic receptors

Unilateral aspiration lesions of the rostral supracallosal stria/cingulum bundle and fimbria-fornix were performed on adult female rats. Ten and 24 days post lesioning, an elevation (17%; p < 0.01) of total muscarinic receptors was observed in lesioned versus control hippocampi. By using antisera selective for each of the five molecularly defined subtypes (m1-m5) of muscarinic receptors, significant changes were observed in the levels of expression for at least four receptor proteins. Three receptor subtypes increased in density: m1 by 14% (from 943 to 1,078 fmol/mg); m3 by 77% (from 150 to 268 fmol/mg); and m4 by 29% (from 220 to 285 fmol/mg). In contrast, a 22% decrease in the density of m2 receptors was found (from 220 to 173 fmol/mg). Detectable levels of m5 receptors were low in the hippocampus (approximately 1% of total receptors), and reliable measurements were not obtained. The directions of these changes are likely to be related to the pre- or postsynaptic localization of these receptor subtypes.

Effect of chronic cholinergic denervation on the m1 muscarinic receptor mechanism

The first three parts of the transduction mechanism for m1 muscarinic receptors (m1 receptors, receptor-G protein coupling, and the activation of phospholipase C) were studied in the rat hippocampus following unilateral or bilateral surgical lesions of the fimbria/fornix. One nM 3H-pirenzepine was used to label m1 receptors selectively. No changes in m1 receptor numbers were found between age 1.7 and 29 months old during normal aging or one year after cholinergic denervation. The interaction between m1 receptors and their associated G protein was examined by competition between 1 nM 3H-pirenzepine and oxotremorine-M in the presence and absence of a guanine nucleotide. The percentage of guanine nucleotide-sensitive high affinity binding sites for the agonist was similar in rats 1.7-29 months old and in rats 1 year after denervation. The ability of oxotremorine-M to activate phospholipase C, via m1, m3, and m5 receptors was also unchanged more than a year after cholinergic denervation of the hippocampus. We concluded that the initial steps in the m1 receptor transduction mechanism remain remarkably stable after denervation.