In vitro down-regulation predicts agonist efficacy at central muscarinic cholinergic receptors

Reduced severity of ischemic stroke and improvement of mitochondrial function after dietary treatment with the anaplerotic substance triheptanoin

Triheptanoin, an oily substance, consists of glycerol bound to three molecules of heptanoic acid, a C7 odd-chain fatty acid. A triheptanoin-rich diet has anaplerotic effects because heptanoate metabolism yields succinate which delivers substrates to the Krebs cycle. While previous studies on the effects of triheptanoin focused on metabolic disorders and epilepsy, we investigated triheptanoin's effect on ischemic stroke. Mice were fed a triheptanoin-enriched diet for 14days; controls received soybean oil. Only mice fed triheptanoin had measurable quantities of odd-numbered fatty acids in the plasma and brain. Transient ischemia was induced in the brain by occlusion of the middle cerebral artery (MCAO) for 60min. One day later, mice were tested for neurological function (chimney, rotarod and corner tests) which was found to be better preserved in the triheptanoin group. Microdialysis demonstrated that the strong, neurotoxic increase of extracellular glutamate, which was observed in the mouse striatum during MCAO, was strongly reduced in triheptanoin-fed mice while glucose levels were not affected. Triheptanoin diet reduced the infarct area in stroked mice by about 40%. In ex vivo-experiments with isolated mitochondria, ischemia was found to cause a reduction of mitochondrial respiratory activity. This reduction was attenuated by triheptanoin diet in complex II and IV. In parallel measurements, ATP levels and mitochondrial membrane potential were reduced in control animals but were preserved in triheptanoin-fed mice. We conclude that triheptanoin-fed mice which sustained an experimental stroke had a significantly improved neurological outcome. This beneficial effect is apparently due to an improvement of mitochondrial function and preservation of the cellular energy state. Our findings identify triheptanoin as a promising new dietary agent for neuroprotection.

Qualitative and quantitative assessment of relative agonist efficacy

Historically, the ability of a ligand to bind to its receptor and the ability to subsequently activate that receptor have been described as the properties of affinity and intrinsic efficacy, respectively. These properties were originally believed to be independent of one another; both are possessed by ligands classed as "agonists," and they have served as the quantitative foundation of the drug and receptor classification process. Although affinity has been interpreted readily in physicochemical terms, equivalent molecular models for efficacy remain elusive. In recent times, there has been a significant paradigm shift in our understanding of the interrelationship between affinity and intrinsic efficacy, particularly on theoretical grounds, yet the actual methods available to measure these parameters remain largely operational. Nevertheless, a number of approaches, based on both functional measurements and radioligand binding studies, are available to quantify agonist efficacy on a relative scale and, to date, these remain the most practical. This commentary discusses the most common of these methods, their advantages and limitations, the dependence of the expression of agonism on the chosen assay system, and the impact of recent biochemical and molecular biological advances on the study of efficacy. Additionally, some of the more contemporary theories regarding the molecular nature of efficacy are briefly discussed, as well as the caveats that always must be borne in mind when any determinations of relative agonist efficacy are made.

Cholinergic REM induction test: muscarinic supersensitivity underlies polysomnographic findings in both depression and schizophrenia

Disinhibition of rapid eye movement (REM) sleep (e.g. shortening of REM latency, heightened REM density) is frequently encountered in patients with a major depressive disorder (MDD). Administration of cholinomimetics prior to or during sleep leads to a more pronounced advance of REM sleep in depressed patients compared to healthy controls and patients with other psychiatric disorders. The present study tested whether the cholinergic REM induction test (CRIT) with 1.5 mg RS 86 (an orally acting muscarinic agonist) differentiates patients with MDD (n = 40) from those with schizophrenia (n = 43) and healthy controls (n = 36). The most pronounced shortening of REM latency after cholinergic stimulation occurred in patients with MDD. However, a significant number of patients with schizophrenia also displayed short REM latencies (REM latency < 25 minutes) under placebo conditions and after cholinergic stimulation. REM density measures more clearly differentiated patients with MDD from those with schizophrenia. It is concluded that a subgroup of patients suffering from schizophrenia displays signs of a muscarinic receptor supersensitivity.

Muscarinic receptor characteristics and regulation in rat cerebral cortex: changes during development, aging and the oestrous cycle

The effects of postnatal development, aging and the oestrous cycle on muscarinic acetylcholine receptor (mAChR) properties were examined in in vitro living slices of rat neocortex. Using the hydrophilic antagonist ([3H]NMS) to label cell surface mAChRs, an increase in both Bmax and Kd was found during the first postnatal weeks. These values peaked at between 20-40 days postnatally and then declined to adult levels. After 3 months of age, a steady decline in receptor number started: it was 10.1% lower at 10 months and 38.7% lower at 17 months of age. In contrast, Kd values increased, being 31.7 and 20% higher respectively at these ages. Carbachol-induced (4 h at 37 degrees C) down-regulation of receptor number was approximately 22.2% in newborn and 26.1% in adult (3-month-old) rats, but only 16.3% at 20-40 days of age. The degree of carbachol-induced down-regulation of mAChR was not affected in the older animals. Veratridine, which increases neural activity, also induced a significant reduction in [3H]NMS binding sites of 11.4% in rats aged 0-20 days and 22.4% in 3-month-old rats, but at 20-40 and 40-60 days of age no significant down-regulation of receptor number was observed. Furthermore, down-regulation was absent in the 10-month-old rats as well. Since a great variation in Bmax and Kd values was seen in 3-month-old females but not in male rats, we investigated mAChR characteristics during the oestrous cycle of female rats. In pro-oestrus, mACh receptor number was increased and affinity decreased in comparison with di-oestrus.(ABSTRACT TRUNCATED AT 250 WORDS)

Sleep in depression: the influence of age, gender and diagnostic subtype on baseline sleep and the cholinergic REM induction test with RS 86

One hundred and eight healthy controls and 178 patients with a major depressive disorder according to DSM-III were investigated in the sleep laboratory after a 7-day drug wash-out period. Subsamples of 36 healthy controls and 56 patients additionally took part in the cholinergic rapid eye movement (REM) sleep induction test with RS 86. Data analysis revealed that age exerted powerful influences on sleep in control subjects and depressed patients. Sleep efficiency and amount of slow wave sleep (SWS) decreased with age, whereas the number of awakenings, early morning awakening, and amounts of wake time and stage 1 increased with age. REM latency was negatively correlated with age only in the group of patients with a major depression. Statistical analysis revealed group differences for almost all parameters of sleep continuity with disturbed indices in the depressed group. Differences in SWS were not detected. REM latency and REM density were altered in depression compared to healthy subjects. Sex differences existed for the amounts of stage 1 and SWS. The cholinergic REM induction test resulted in a significantly more pronounced induction of REM sleep in depressed patients compared with healthy controls, provoking sleep onset REM periods as well in those depressed patients showing baseline REM latencies in the normal range. Depressed patients with or without melancholia (according to DSM-III) did not differ from each other, either concerning baseline sleep or with respect to the results of the cholinergic REM induction test. The results stress the importance of age when comparing sleep patterns of healthy controls with those of depressed patients. Furthermore they underline the usefulness of the cholinergic REM induction test for differentiating depressed patients from healthy controls and support the reciprocal interaction model of nonREM-REM regulation and the cholinergic-aminergic imbalance hypothesis of affective disorders.

The potent free radical scavenger alpha-lipoic acid improves memory in aged mice: putative relationship to NMDA receptor deficits

alpha-Lipoic acid (alpha-LA) improved longer-term memory of aged female NMRI mice in the habituation in the open field test at a dose of 100 mg/kg body weight for 15 days. In a separate experiment, no such effect could be found for young mice. alpha-LA alleviated age-related NMDA receptor deficits (Bmax) without changing muscarinic, benzodiazepine, and alpha 2-adrenergic receptor deficits in aged mice. The carbachol-stimulated accumulation of inositol monophosphates was not changed by the treatment with alpha-LA. These results give tentative support to the hypothesis that alpha-LA improves memory in aged mice, probably by a partial compensation of NMDA receptor deficits. Possible modes of action of alpha-LA based on its free radical scavenger properties are discussed in relation to the membrane hypothesis of aging.

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.

High-affinity kainate binding sites in living slices of rat neocortex: characterization and regulation

We have characterized a high-affinity kainate binding site in in vitro living rat neocortical slices using [3H]kainate. [3H]Kainate labelled at least two binding sites, the higher affinity site with a Kd of 7.1 nM and a Bmax of 71.2 fmol/mg protein. This high-affinity binding site showed a pharmacology consistent with a kainate receptor with competition by kainate and domoic acid, as well as the (RS)-alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate antagonist 6-cyano-2,3-dihydroxy-7-nitroquinoxaline. Increases in cellular depolarization induced by 2-h preincubations in veratridine and glutamate led to a significant 55% average decrease in [3H]kainate binding in adult cortex. Similarly, preincubation in kainate led to a significant average 26% decrease in binding. In both instances, Eadie-Hofstee analysis of saturation binding data revealed that the decreased binding reflected changes in receptor number. At different postnatal ages, increases in cellular depolarization significantly decreased binding (< 20 days postnatal age, -86%; > 60 days, -48%). Kainate treatment also significantly decreased binding at all ages (-64% at < 20 days; > 60 days, -18%), with significant differences noted between ages. These age-dependent effects are unlike those previously described for either N-methyl-D-aspartate [Lanius and Shaw (1992) Anat. Rec. 232, 54(A)] or (RS)-alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate high affinity receptors [Shaw and Lanius (1992) Devl Brain Res. 68, 225-233].(ABSTRACT TRUNCATED AT 250 WORDS)

Aging enhances the calcium sensitivity of central neurons of the mouse as an adaptive response to reduced free intracellular calcium

Age-related changes in Ca(2+)-homeostasis have been investigated in mechanically dissociated neurons from young and aged mice. In aged animals, basal intracellular calcium ([Ca2+]i) was significantly reduced and depolarization (KCl)-induced rise in [Ca2+]i was lower, probably as a result of increased activation of Ca(2+)-dependent mechanisms terminating Ca2+ influx. Additionally, depolarization-induced inositol-phosphate (IP) accumulation in aged animals was found to be significantly increased. Both findings suggest that Ca(2+)-dependent intracellular processes become more sensitive to Ca2+ in aged animals due to decreased Ca2+ availability.

Effects of subchronic administration of pyritinol on receptor deficits and phosphatidylinositol metabolism in the brain of the aged mouse

The effect of pyritinol, a commonly used nootropic drug, on receptor properties and function was investigated in different neuronal systems, possibly associated with age-related decline in brain function. Chronic treatment (15 days) of aged (22 months) female NMRI mice with pyritinol (200 mg/kg) restored the reduced density of N-methyl-D-aspartate receptors in the aged mouse brain. Furthermore, the total number of binding sites of the alpha 2-receptor ([3H]yohimbine binding) decreased after treatment with drug, while the number of high-affinity agonist binding sites ([3H]UK 14304 binding) was not changed. In both systems, receptor affinity was not influenced. The densities of other receptors investigated (muscarinic-cholinergic, benzodiazepine and beta-adrenergic) were not altered by treatment with pyritinol. Additionally, the effect of pyritinol on phosphatidylinositol (PI) metabolism was investigated in dissociated neurones from young and aged mice. Muscarinic-cholinergic induced accumulation of phosphatidylinositol and the inositol phosphate response due to activation of G-protein by fluoride was increased in aged animals, treated with drug. The inositolphosphate response after stimulation with pilocarpine was slightly but not significantly increased. The metabolism of phosphatidylinositol in young animals was not altered by treatment with drug. These results support the hypothesis of a nootropic-mediated restoration of age-related brain deficits. Changes caused by pyritinol may be due to beneficial effects on age-related alterations of the properties of the neuronal membrane.

Muscarinic receptor regulation and 2nd messenger responses in rat neocortex cultures

Primary cultures of dissociated cerebral cortex cells were used to characterize the muscarinic acetylcholinergic receptors (mAChR) present and to study receptor down-regulation and receptor mediated 2nd messenger responses induced by muscarinic agonists. Binding of the hydrophilic antagonist [3H]N-methyl scopolamine ([3H]NMS) to the cultured cells was saturated after one hour at 4 degrees C with a Kd of 93 pM and a Bmax of 958 fmol/mg protein. Competition binding studies with several antagonists and agonists indicated that the mAChR present in the culture were of a mixed M1/M3 subtype. The number of muscarinic receptors at the cell surface decreased by 60% after one hour pre-incubation of the cultures with 10 microM carbachol or oxotremorine. After down-regulation with carbachol affinity for pirenzepine was decreased, while low affinity sites for 4-DAMP were lost, indicating that especially M1 subtypes are sensitive to this type of regulation. Carbachol and oxotremorine-M induced a 2-3 fold increase in phosphatidyl inositide (PI) turnover, which was blocked with high affinity by both pirenzepine and 4-DAMP. Down-regulation of the mAChR and stimulation of PI-turnover by agonists with different potency and intrinsic activity appeared highly correlated. These data suggest that activation of the PI second-messenger system is involved in the desensitization and down-regulation of the muscarinic acetylcholine receptor.

Age-related deficits of central muscarinic cholinergic receptor function in the mouse: partial restoration by chronic piracetam treatment

The effect of aging on muscarinic cholinergic receptor function in dissociated cell aggregates of the mouse brain was investigated using two biochemical models, i.e., carbachol-induced accumulation of inositol monophosphates and carbachol-induced desensitization of muscarinic cholinergic receptors as measured by the sequestration of specific 3H-N-methyl-scopolamine binding. While aging strongly reduced carbachol-induced inositol monophosphate accumulation, desensitization was not affected in the brains of aged animals. Chronic treatment of aged mice with the nootropic drug piracetam (500 mg/kg daily PO) significantly elevated the agonist-induced accumulation of inositol monophosphates possibly by increasing the available number of muscarinic cholinergic receptors not being in a desensitized state. The results support the hypothesis that nootropics like piracetam might act in part by restoring age-related deficits of central muscarinic cholinergic receptor function.