Decreased carbachol-stimulated inositol 1,3,4,5-tetrakisphosphate formation in senescent rat cerebral cortical slices

Age-related loss of cholinergic-muscarinic coupling to PLC: comparison with changes in brain regional PLC subtypes mRNA distribution

Activation of phospholipase C (PLC) coupled to phosphoinositide (PtdIns) hydrolysis occurs through one of the two pathways. One of the major pathways for the neurotransmitter signaling involves phosphoinositide (PtdIns) specific and G-protein dependent PLC-beta, which stimulates the formation of inositol triphosphate (IP3) and inositol tetraphosphate (IP4). Another pathway through the stimulation of calcium influx can directly activate all of the PLC isozymes. At least three isozymes of PLC have been characterized in the brain; PLC-A (alpha), PLC-I (beta) and PLC-II (gamma), which are shown to be localized differentially in brain regions. Muscarinic-cholinergic signals are mediated in large part through the hydrolysis of PtdIns by PLC. To investigate changes in muscarinic coupling to PLC during aging, we examined carbachol stimulated and calcium stimulated PtdIns hydrolysis in cerebral cortical membranes in young, middle aged and old rats. In order to determine whether PtdIns hydrolysis changes correspond to PLC isozyme expression in these animals, we examined three subtypes of PLC mRNA expression in brain sections of young and old rats using in situ hybridization technique. Our study indicated decreased carbachol-induced PLC activity in the cerebral cortex and, in contrast, increased PLC-beta mRNA in the frontal cortex and superficial cortical layer of aged rats. PLC-alpha mRNA was decreased in hippocampal regions of older rats. These studies suggest that during aging there is an uncoupling of muscarinic stimulated PtdIns hydrolysis, which is accompanied by an increased PLC-beta mRNA and decreased PLC-alpha mRNA that may represent compensatory changes in PLC expression.

Stimulation of angiotensin II AT1 receptors in rat median eminence increases phosphoinositide hydrolysis

The aim of our study was to determine the second messenger systems for angiotensin II in the rat median eminence. Angiotensin II AT1 receptors are highly expressed in the median eminence and binding is selectively inhibited by the guanine nucleotide GTP gamma S, indicating possible coupling to G-proteins. In male rats, angiotensin II increased phosphatidylinositol hydrolysis about 45% over basal values, with an EC50 of about 2.7 nM. This effect was antagonized by 10 microM losartan, the selective AT1 antagonist, but not by the AT2 competitor PD 123319. Conversely, angiotensin II, 1 microM, did not alter basal or forskolin-stimulated cAMP production, and failed to influence cGMP production. These results support a role for angiotensin II, through stimulation of AT1 receptors and increased phosphatidylinositol hydrolysis, in the median eminence. Angiotensin II increased the phosphatidylinositol hydrolysis not only in male rats but also in ovariectomized rats, with or without estrogen-progesterone replacement. However, angiotensin II (up to 1 microM) failed to increase the phosphatidylinositol hydrolysis in randomly selected intact female rats. Estrogen treatment did not alter the number or affinity of median eminence AT1 receptors in ovariectomized rats. The increase in phosphatidylinositol hydrolysis resulting from stimulation of median eminence AT1 receptors appears to be sexually dimorphic, but hormonal manipulations failed to point to a role for reproductive hormones in this phenomenon.

Hippocampal muscarinic receptor function in spatial learning-impaired aged rats

Efficiency of coupling of hippocampal muscarinic receptors to phosphoinositide (PI) turnover was investigated in behaviorally characterized young and aged Long-Evans rats using hippocampal minces and the method of partial receptor alkylation of Furchgott. Densities of the m1, m2, and m3 receptor proteins were determined using specific antibodies and immunoprecipitation. Spatial learning ability was quantified using a water maze. There were no differences in the levels of muscarinic receptor proteins between young and aged (27 months) rats or in rats with impaired spatial learning. The dissociation constant (KD) for the agonist oxotremorine-M and the KD/EC50 ratio, an indicator of receptor-effector coupling efficiency were similar in young and aged rats. However, the maximal PI turnover response to oxotremorine-M was decreased in impaired aged rats and this parameter was highly correlated with the spatial learning index (R = -0.825; p < 0.001). A reduction in effector stimulation in the absence of changes in receptor protein or coupling efficiency suggests that dysfunction in the hippocampal muscarinic receptor systems occurs at the level of phospholipase C or beyond.

Decreased phospholipase C-beta immunoreactivity, phosphoinositide metabolism, and protein kinase C activation in senescent F-344 rat brain

Phosphoinositide metabolism, phospholipase C immunoreactivity, and protein kinase C translocation were measured in brain slices of 6- and 24-month-old F-344 rats. Basal phosphoinositide labeling and accumulation of [3H]inositol phosphates were reduced in the 24-month-old rats. The cholinergic agonist, carbachol, induced lower net accumulations of inositol phosphates in striatal, hippocampal, and cortical slices of the aged rats. The dose-response curve for carbachol showed a significantly lower maximal response in the striatum of senescent rats, whereas the time course of [3H]inositol incorporation into inositol metabolites and the accumulation of free [3H]inositol in tissues from young and old animals were not different. Quantitative analyses showed marked reductions in endogenous brain levels of the phosphoinositides and in phospholipase C-beta 1 immunoreactivity, but no marked reductions in endogenous brain levels of the phosphoinositides and in phospholipase C-beta 1 immunoreactivity, but no changes in phospholipase C-gamma levels in aged animals. Moreover, basal protein kinase C activity and carbachol-mediated translocation of the enzyme were significantly reduced in the cerebral cortex of the senescent animals. These findings imply that aging is associated with alterations in the brain content, metabolism, and activity of phosphoinositide-derived second messengers in the F-344 rat brain.

Carbachol-induced accumulation of inositol phosphates and its modulation by excitatory amino acids in cortical slices of young and aged rats with down-regulation of muscarinic M-1 receptors

The effects of a subacute intoxication with diisopropyl fluorophosphate (DPF) on total muscarinic acetylcholine receptor sites (mAChRs) and M-1 AChRs were evaluated in the cerebral cortex of young (2-4 months) and aged (22-24 months) Fischer 344 rats. Since M-1 AChRs are coupled to the metabolism of phosphoinositides, carbachol-induced accumulation of inositol phosphates (IP) and its inhibition by glutamate and NMDA was also measured in the cortical slices. DFP treatment caused about 75% inhibition of cholinesterase and 35% down-regulation of mAChRs (measured as [3H]quinuclidinyl benzylate binding) in both young and aged rats. The down-regulation of M-1-ACHRs (measured as [3H]pirenzepine binding) was more pronounced in aged (30%) than in young (17%) DFP-treated rats. There was a significant increase in carbachol-induced IP accumulation in aged, with respect to young, untreated rats. DFP treatment caused a considerable decrease in such IP accumulation in aged but not in young rats. Glutamate and NMDA antagonized carbachol-induced IP accumulation in untreated young and aged rats (and the effects of NMDA were reversed by carboxy-piperazinyl-propyl phosphonic acid). In DFP-treated rats such antagonism was somewhat less pronounced. The data appear of interest in relation to the use of anticholinesterase compounds in the therapy of senile dementia of Alzheimer's type. They suggest that beside their primary action (increasing brain ACh levels) such compounds also act on post-receptor mechanisms and on the interactions between cholinergic and glutamatergic neurotransmitter systems.

Inositol 1,4,5-trisphosphate receptor and ryanodine receptor in the aging brain of Wistar rats

Intracellular Ca2+ release channels are key players in the regulation of Ca2+ homeostasis. In the present study, we investigated the age-related changes of inositol 1,4,5-trisphosphate (IP3) receptor/Ca2+ release channel and ryanodine receptor/Ca2+ release channel in microsomes derived from either cerebellum or cerebrum cortex from male Wistar rats. A significant reduction (about 50%) in density of IP3 receptor/Ca2+ release channels was observed in cerebrum cortex, only, in 8- and 28-month old rats, whereas density and Kd of ryanodine binding sites were unaffected in both cerebellum and cerebrum microsomes. These findings, along with impairment of Ca(2+)-dependent protein kinase C phosphorylation of endogeneous substrates, point to coordinate, quantitative alterations of both targets of phosphoinositide metabolism, i.e., PKC and IP3 receptor, in the cerebrum cortex at least. The relevance of the present findings is discussed in relation to reported changes of neuronal Ca2+ homeostasis during aging.

Effect of aging on myo-inositol and phosphoinositide metabolism in the cochlear and vestibular sensory epithelia of the rat

Neurotransmission and transmembrane signaling are among the cellular mechanisms affected in the aging nervous system. In the inner ear, the phosphoinositide second messenger cascade is of particular interest as a target of the aging process. In both the cochlear (CSE) and vestibular sensory epithelia (VSE), the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PtdInsP2) to the second messenger inositol 1,4,5-trisphosphate (InsP3) is coupled to muscarinic cholinergic and P2y purinergic receptors and may be linked to calcium homeostasis. The present study compared the turnover of phosphoinositides (PtdInsPs), receptor-mediated release of inositol phosphates (InsPs), and concentrations of endogenous myo-inositol in the CSE and VSE of young (3 months) and aged (24 months) Fischer-344 rats. In the aged rat, there was a significant increase in [3H]inositol incorporation (per mass of protein) into PtdInsPs plus InsPs in both sensory epithelia while the protein content remained unchanged. In contrast, no age-dependent differences were found when pre-labeled [3H]PtdInsPs were 'chased' with non-radiolabeled myo-inositol indicating that the turnover of these lipids was unaffected. The cholinergic receptor agonist carbamylcholine and the P2 purinergic receptor agonist adenosine 5'-O-(3-thiotriphosphate) stimulated the release of [3H]InsPs two- to six-fold in both organs. This agonist-stimulated release of [3H]InsPs (per mass of protein) was significantly higher in aged animals. However, when the same stimulation was expressed as per cent of control values, there was no age-dependent difference.(ABSTRACT TRUNCATED AT 250 WORDS)

Cholinergic and serotonergic stimulation of phosphoinositide hydrolysis is decreased in Alzheimer's disease

Agonist-stimulated phosphoinositide (PPI) hydrolysis is a major signal transduction pathway in brain. These studies investigated neurotransmitter stimulated PPI hydrolysis in postmortem human brain. Preliminary studies using rat brain suggested that moderate postmortem delay has little effect on PPI hydrolysis and that human tissue might be reliably studied for differences in receptor-PLC coupling. Studies in human brain membranes (frontal cortex) indicated that the time course for GTP gamma S and carbachol/GTP gamma S-stimulated PPI hydrolysis was linear for at least 20 min. GTP gamma S-stimulated [3H]inositol phosphate (InsP) formation was enhanced by carbachol (232%) and 5-Hydroxytryptamine (5HT-147%). SAX-HPLC separation of [3H]inositol polyphosphates indicated that the major isomer of inositol trisphosphate (InsP3) was Ins(1.4.5)P3, the expected product of PtdIns(4,5)P2 hydrolysis. Ca2+ increased PPI hydrolysis progressively from 100 nM through 50 microM and synergistically enhanced carbachol/GTP gamma S stimulation. Comparisons of age-matched controls with Alzheimer's patients indicated that GTP gamma S, carbachol/GTP gamma S, and 5HT/GTP gamma S-stimulation of PPI hydrolysis is reduced approximately 50% in membranes prepared from Alzheimer's patients. Ca2+ of PPI hydrolysis was not different between controls and Alzheimer's patients suggesting that muscarinic cholinergic and serotonergic receptors are uncoupled from PLC in Alzheimer's disease. These studies indicate that there are changes in cholinergic and serotonergic signal transduction in Alzheimer's disease. Further, this method can be used to study signal transduction events in postmortem human brain.

Age-related changes in receptor-mediated and depolarization-induced phosphatidylinositol turnover in mouse brain

The effect of aging on receptor- and G-protein-activated and on depolarization-induced phosphoinositide (PI) hydrolysis was examined in mechanically dissociated neurons from female NMRI mice. Additionally, age-dependent changes in Ca2+ homeostasis, i.e. changes in basal intracellular calcium ([Ca2+]i) and in depolarization-induced rise in [Ca2+]i were investigated. No age-related differences in PI hydrolysis were found after stimulation of muscarinic cholinergic, alpha 1, serotonin and quisqualate receptors coupled to the phosphoinositide-phospholipase C (PI-PLC) system. PI hydrolysis following stimulation with AMPA ((RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) revealed a significantly increased response in aged animals. Activation of G-proteins with NaF also induced a higher inositol monophosphate (InsP1) accumulation in aged mice. Moreover, InsP1 accumulation due to PLC activation by increased [Ca2+]i after depolarization with KCl was significantly increased in neurons from aged animals. Investigations about age-related changes in Ca2+ homeostasis revealed lower basal [Ca2+]i and lower rise in [Ca2+]i after depolarization with KCl. The data indicate that receptor-mediated and depolarization-induced PI hydrolysis are differentially affected by aging. Decreased availability of [Ca2+]i in aged animals may enhance the sensitivity of Ca(2+)-activated mechanisms. This may explain increased KCl- and AMPA-induced InsP1 accumulation whereas receptor-coupled PLC activation is less affected.

Colocalization of muscarinic acetylcholine receptors and protein kinase C gamma in rat parietal cortex

The present investigation analyzes the cellular distribution of muscarinic acetylcholine receptors (mAChRs) and the gamma isoform of protein kinase C (PKC) in the rat parietal cortex employing the monoclonal antibodies M35 and 36G9, respectively. Muscarinic cholinoceptive neurons were most present in layers 2, 3 and 5, whereas most PKC gamma-positive cells were found in layers 2, 5 and 6. Under normal, non-stimulated conditions, approximately 58% of all muscarinic cholinoceptive neurons were immunoreactive for PKC gamma. Conversely, nearly all PKC gamma-positive neurons were M35-immunoreactive. Although both pyramidal and nonpyramidal neurons express the two types of protein, the pyramidal cell type represents the vast majority. Of all cortical neurons, the large (15-25 microns in diameter) muscarinic cholinoceptive pyramidal neurons in layer 5 express the gamma isoform of PKC most abundantly and most frequently. Approximately 96% of these cells are immunoreactive for PKC gamma. Stimulation of mAChRs by the cholinergic agonist carbachol resulted in a pronounced increase in the intensity of 36G9 immunoreactivity, which may suggest that the mAChRs are functionally linked to the colocalized PKC gamma. No change was found in the number of 36G9-immunoreactive neurons. In contrast, the number of immunocytochemically detectable muscarinic cholinoceptive neurons increased by approximately 38% after carbachol stimulation. The high degree of codistribution in cortical neurons of both transduction proteins suggests a considerable cholinergic impact upon the regulation of PKC gamma, a candidate key enzyme in cortical learning and memory mechanisms.