Inositol phospholipids and neurotransmitter-receptor signalling mechanisms

Complex Permittivity Characterization of Liquid Samples Based on a Split Ring Resonator (SRR)

A complex permittivity characterization method for liquid samples has been proposed. The measurement is carried out based on a self-designed microwave sensor with a split ring resonator (SRR), the unload resonant frequency of which is 5.05 GHz. The liquid samples in capillary are placed in the resonant zone of the fabricated senor for high sensitivity measurement. The frequency shift of 58.7 MHz is achieved when the capillary is filled with ethanol, corresponding a sensitivity of 97.46 MHz/μL. The complex permittivity of methanol, ethanol, isopropanol (IPA) and deionized water at the resonant frequency are measured and calibrated by the first order Debye model. Then, the complex permittivity of different concentrations of aqueous solutions of these materials are measured by using the calibrated sensor system. The results show that the proposed sensor has high sensitivity and accuracy in measuring the complex permittivity of liquid samples with volumes as small as 0.13 μL. It provides a useful reference for the complex permittivity characterization of small amount of liquid chemical samples. In addition, the characterization of an important biological sample (inositol) is carried out by using the proposed sensor.

Lipid signature of neural tissues of marine and terrestrial mammals: consistency across species and habitats

Marine mammals are exposed to O₂-limitation and increased N₂ gas concentrations as they dive to exploit habitat and food resources. The lipid-rich tissues (blubber, acoustic, neural) are of particular concern as N₂ is five times more soluble in lipid than in blood or muscle, creating body compartments that can become N₂ saturated, possibly leading to gas emboli upon surfacing. We characterized lipids in the neural tissues of marine mammals to determine whether they have similar lipid profiles compared to terrestrial mammals. Lipid profiles (lipid content, lipid class composition, and fatty acid signatures) were determined in the neural tissues of 12 cetacean species with varying diving regimes, and compared to two species of terrestrial mammals. Neural tissue lipid profile was not significantly different in marine versus terrestrial mammals across tissue types. Within the marine species, average dive depth was not significantly associated with the lipid profile of cervical spinal cord. Across species, tissue type (brain, spinal cord, and spinal nerve) was a significant factor in lipid profile, largely due to the presence of storage lipids (triacylglycerol and wax ester/sterol ester) in spinal nerve tissue only. The stability of lipid signatures within the neural tissue types of terrestrial and marine species, which display markedly different dive behaviors, points to the consistent role of lipids in these tissues. These findings indicate that despite large differences in the level of N₂ gas exposure by dive type in the species examined, the lipids of neural tissues likely do not have a neuroprotective role in marine mammals.

Whole-cell voltage-clamp investigation of the role of PKC in muscarinic inhibition of IAHP in rat CA1 hippocampal neurons

Muscarinic, cholinergic inputs, largely from the medial septum, have pronounced effects on hippocampal cell excitability. A major effect of synaptically released ACh is block of the slow Ca(2+)-dependent potassium current, called IAHP. Protein kinase C exists in the hippocampus in high concentrations, its activation blocks IAHP, and it has been suggested as a mediator of the muscarinic-receptor-(mAChR)-mediated actions. Using conditions that produce a stable postspike afterhyperpolarizing current (IAHP) in whole-cell recordings from CA1 hippocampal pyramidal neurons in the slice preparation, we have investigated the role of PKC in the cholinergic inhibition of IAHP mediated by mACHRs. Bath application of the general kinase inhibitor, H7, had no effect on inhibition of IAHP by carbachol, although H7 dramatically reduced inhibition of IAHP by the phorbol ester, phorbol-12, 13-diacetate (PDA). Another muscarinic response thought to be mediated by PKC-inhibition of GABAB-mediated hyperpolarization-was reduced by extracellular H7 treatment, suggesting that the coupling between mAChRs and protein kinase activity was maintained in whole-cell recordings. We also discovered that PDA does not mediate its effects on IAHP directly. Intracellular perfusion of high concentrations of H7 (10 mM) or the specific PKC inhibitor, PKCI(19-31) (1 mM), did not prevent inhibition of IAHP by PDA. These results are consistent with an indirect, presynaptic action of phorbol esters on IAHP, possibly mediated through enhanced release of neurotransmitter from surrounding cells.

Muscarinic receptor-dependent activation of phospholipase C in human fetal central nervous system organotypic tissue culture

The coupling of muscarinic-cholinergic receptors (mAchR) with the phospholipase C (PLC) second messenger system has been demonstrated in central nervous system (CNS) tissue of many animal species. However, little information exists regarding this association in the developing human CNS. Due to the suggested role of acetylcholine in the regulation of development and differentiation of neural cells, the knowledge of these relationships during human fetal development acquires singular importance. Because of this, we examined the cholinergic stimulation of PLC in human fetal CNS organotypic tissue cultures. Agonist treatment of cultures, in the presence of lithium, resulted in a 4-6-fold increase in inositol phosphates formation. This increase was caused principally by the formation of inositol phosphate (IP). However, kinetic studies demonstrated that the levels of IP2, IP3 and IP4 also increased rapidly after stimulation reaching maximum levels before IP. These results support the hypothesis that muscarinic receptor activation results in an increase in the hydrolysis of PIP2. The inositol phosphate formation was dependent on agonist concentration. The obtained EC50 values were approximately 57 +/- 15 microM for carbachol, 8 +/- 2 microM for acetylcholine and 49 +/- 15 microM for oxotremorine. The agonist-dependent formation of inositol phosphates was inhibited by the muscarinic antagonists atropine and pirenzepine. Pirenzepine inhibited carbachol stimulation with high affinity (Ki = 2.90 +/- 1.15 nM), indicating that PLC activation is the result of activation of the m1 subtype of muscarinic receptors. Treatment of cultures with pertussis toxin did not result in inhibition of agonist-dependent activation of PLC. This result suggests that the m1 muscarinic receptor is coupled to PLC through Gq.

Muscarinic receptor-dependent activation of phospholipase C in the developing human fetal central nervous system

The coupling of muscarinic-cholinergic receptors (mAChR) to adenylate cyclase and phospholipase C (PLC) second messenger systems has been demonstrated in many animal species. However, little is known about this association in the developing human central nervous system. Because of the proposed role of acetylcholine in the regulation of development and differentiation of neural cells, an understanding of these relationships during human fetal development gains importance. We report, in this communication, the coupling of mAChR with PLC in the human fetal brain. This coupling was determined using two independent approaches that relied upon estimating the accumulation of inositol phosphates (IPs) and cytidine diphosphate diacylglycerol (CDP-DAG). Carbachol treatment of brain slices, in the presence of lithium, resulted in the accumulation of IPs. Analysis of the kinetics of this accumulation showed that IP3 and IP2 increased rapidly, reaching a peak or plateau before IP. The results also showed that agonist-stimulated PLC produced two second messengers, IP3 and DAG. The production of DAG was strongly supported by the carbachol-dependent increase of CDP-DAG. The accumulation of IP and CDP-DAG was dependent on agonist concentration. The obtained EC50 values were approximately: carbachol 47 microM; acetylcholine 6 microM; and oxotremorine 25 microM. Unexpectedly, all three agonists demonstrated a similar efficacy. The cholinergic stimulation of inositide hydrolysis appears to be the result of activation of the m1 muscarinic receptor.

Roles of second-messenger systems and neuronal activity in the regulation of lordosis by neurotransmitters, neuropeptides, and estrogen: a review

Many neurotransmitters and neuropeptides can affect the rodent feminine sexual behavior, lordosis, when administered in the ventromedial hypothalamus (VMH), midbrain central gray (MCG), or other brain regions. A survey of the electrophysiological and biochemical actions of these neural agents revealed that there is a very consistent association between lordosis facilitation with both the activation of the phosphoinositide (PI) pathway and the excitation of VMH and MCG neurons. In contrast, lordosis inhibition is associated, less consistently, with alterations of the adenylate cyclase (AC) system and the inhibition of neuronal activity. The findings that lordosis could be facilitated by going beyond membrane receptors and directly activating the PI pathway, suggest that this second-messenger pathway is a common mediator for the lordosis-facilitating agents. Furthermore, as in the case of stimulating membrane receptors, direct activation of this common mediator also requires estrogen priming for lordosis facilitation. Therefore, it is likely that the PI pathway is modulated by estrogen in the permissive action of estrogen priming. Indeed, a literature review shows that estrogen can affect selective isozymes of key enzyme families of the PI pathway at various levels. Such selective modulations, at several levels, could easily alter the course of a PI cascade; thence, the eventual functional outcome. These findings prompt us to propose that estrogen enables lordosis to be facilitated by a selective modulation of the PI pathway.

Use of pharmacological agents to implicate a role for phosphoinositide hydrolysis products in malaria gamete formation

The kinetics of phosphoinositol 4,5 bisphosphate hydrolysis products in activated Plasmodium falciparum gametocytes suggests a role for inositol trisphosphate [Ins(1,4,5)P3] and diacylglycerol (DAG) in the signal transduction pathway of malaria gametocytes. To investigate further this role, compounds that have an effect on the metabolism and biologic functions of these second messengers were tested in an in vitro system. Gentamycin, 2,3 diphosphoglycerate (2,3 DPG) and magnesium ion (Mg2+), inhibitors of Ins(1,4,5)P3 5' phosphatase, all stimulated gametocytes to exflagellate in suspended animation buffer, pH 7.4, at room temperature. In addition, methylxanthines, caffeine and theobromine, calcium ionophore (A-23187), and external calcium also stimulated exflagellation. In contrast, neomycin, an aminoglycoside that inhibits phospholipase C activity, and heparin, an antagonist of Ins(1,4,5)P3 binding to its receptor, inhibited microgamete formation. Quinine and chloroquine which can inhibit both phospholipase A and C activity also inhibited gametocyte exflagellation. The consistent manner in which these various compounds affect gametocyte activation further implicates phosphoinositol turnover in the signal transduction pathway of falciparum gametocytes.

Increase in cytoplasmic free Ca2+ elicited by noradrenalin and serotonin in cultured local interneurons of mouse olfactory bulb

Effects of noradrenalin and serotonin on cytoplasmic free Ca2+ concentrations ([Ca2+]i) were studied by using the fluorescent indicator fura-2 in cultured local interneurons of mouse olfactory bulb. Application of noradrenalin (0.1-100 microM) caused a rapid and concentration-dependent rise in [Ca2+]i, while isoproterenol was ineffective at concentrations up to 100 microM. The noradrenalin (1 microM)-induced increase in [Ca2+]i was completely inhibited by pretreatment with alpha 1-antagonist, prazosin (100 nM), whereas the inhibitory effect of alpha 2-antagonist, yohimbine, was about 100-times less potent. Serotonin (0.1-100 microM) also caused the dose-dependent rise in [Ca2+]i, which was inhibited by serotonin2 antagonist, ketanserin. Even in the absence of the extracellular calcium, the noradrenalin- or serotonin-induced increase in [Ca2+]i was observed. These results indicate that both noradrenalin and serotonin elicit the rise in [Ca2+]i in local interneurons of the olfactory bulb. They also suggest that the rise in [Ca2+]i is mediated by alpha 1-adrenergic and serotonin2 receptors, and that the increased calcium is mainly derived from intracellular calcium storage sites. The above results provide evidence to suggest that in the olfactory bulb, noradrenergic and serotonergic centrifugal fibers exert modulatory influences on synaptic interactions between mitral/tufted cells and local interneurons by increasing cytoplasmic Ca2+ in local interneurons.

Increased turnover of platelet phosphatidylinositol in schizophrenia

The potential role of receptor-stimulated phosphatidylinositol (PI) hydrolysis in a signal transduction mechanism has been increasingly recognized. Earlier studies have suggested a defect in alpha-adrenergic receptor function in the platelets of schizophrenic patients. Little is known, however, about the mechanisms for PI synthesis, breakdown, and regulation in schizophrenia. The present study was undertaken to investigate the metabolic turnover of inositol phospholipids and inositol phosphates by incorporation of [3H]myoinositol or [32P]orthophosphate into resting and activated platelets of normal controls and schizophrenic patients with and without neuroleptic treatment. After 5 h incubation at 37 degrees C, the majority of [3H]myoinositol was incorporated into platelet PI. Following thrombin-induced platelet activation, there was rapid formation of 3H-labeled inositol phosphates (IPs) with inositol monophosphate (IP1) being the most abundant product. The thrombin-induced formation of platelet IPs was found significantly higher in both haloperidol-stabilized and drug-free schizophrenics than in normal control subjects. When platelets were prelabeled with [32P]orthophosphates, thrombin-induced formation of phosphatidic acid (PA) was also significantly higher in haloperidol-stabilized schizophrenics than in normal controls. It is thought that thrombin-induced platelet activation is mediated through hydrolysis of polyphosphoinositides (poly-PI). The present data thus may reflect an increased signal transduction in schizophrenia, which is mediated through neuroleptic-regulated inositol phospholipid hydrolysis.

Alterations of 45Ca accumulation and [3H]inositol 1,4,5-trisphosphate binding using autoradiography in the exo-focal postischemic brain areas of the rat

We studied the alterations of calcium accumulation and intracellular signal transduction using autoradiography of the second messenger system in order to clarify the mechanisms of the delayed neuronal changes in the remote areas of rat brain after transient focal ischemia. Chronological changes of 45Ca accumulation and [3H]inositol 1,4,5-trisphosphate (IP3) binding sites were determined after 90 min of right middle cerebral artery (MCA) occlusion and after such occlusion followed by different periods of recirculation. After the ischemic insult, 45Ca accumulation extended to the lateral segment of the caudate putamen and to the cerebral cortex, both supplied by the occluded MCA. One day after the ischemia, [3H]IP3 binding sites decreased significantly compared with the control values in these ischemic areas. Moreover, 3 days after the ischemia, 45Ca accumulation was first detected in the ipsilateral thalamus and the substantia nigra, which lay outside the ischemic areas. In the substantia nigra, a significant decrease of [3H]IP3 binding sites and concurrent 45Ca accumulation were observed. In the thalamus, however, there was not alteration until 1 week after the ischemia, and then [3H]IP3 binding sites increased significantly 2 weeks (P less than 0.05) and 4 weeks (P less than 0.01) after the ischemia. Based on the present study, we speculate that different mechanisms associated with signal transduction systems may be responsible for exo-focal postischemic delayed neuronal changes in the thalamus and the substantia nigra. The increase of [3H]IP3 binding sites of the thalamus in the chronic stage may be new evidence of plasticity related to neurotransmission.

Insulin stimulates membrane phospholipid metabolism by enhancing endogenous alpha 1-adrenergic activity in the rat hippocampus

Insulin stimulates membrane phospholipid metabolism and activates protein kinase C (PKC) in its peripheral target tissues. Additionally, insulin can stimulate PKC activity in cultured fetal chick neurons. In the present study, we tested whether insulin can stimulate membrane phospholipid metabolism in the rat hippocampus, a CNS region in which insulin has been reported to stimulate the phosphorylation of a PKC substrate protein and to suppress spontaneous electrical activity of pyramidal cells. Concentrations of 1, 10 and 100 nM insulin significantly stimulated the accumulation of [3H]inositol phosphate ([3H]IP1) and [3H]IP2 in hippocampal slices labelled with [3H]myoinositol. Significant (P less than 0.05) increases of hippocampal diacylglycerol (a product of phosphoinositol hydrolysis) content were observed at 1, 5 or 10 min of incubation with 50 or 100 nM insulin. Addition of tetrodotoxin resulted in a suppression of insulin stimulation of [3H]IP1 release, suggesting that insulin effects may be indirect and mediated via release of an endogenous neuronal transmitter within the hippocampus. Norepinephrine has been shown to both stimulate PI turnover and suppress the spontaneous electrical activity of pyramidal cells via alpha 1-adrenergic receptors. Therefore, we tested whether the effects of insulin were mediated by norepinephrine. We measured [3H]IP1 release in the presence or absence of the alpha 1-adrenergic antagonists prazobind and prazosin. These compounds blocked insulin stimulation of IP1 accumulation, suggesting that the action of insulin to stimulate PI turnover is secondary to enhancement of endogenous noradrenergic activity within the hippocampus.

Bright light does not alter muscarinic receptor binding parameters

Seasonal Affective Disorders (SADs) are disorders of mood characterized by recurrent episodes of illness with a fixed relationship to season. Winter depression is characterized by recurrent onset of depression in the fall or winter followed by spontaneous recovery in the spring. This syndrome is responsive to treatment with bright light. The pathophysiology of depressive disorders may involve central muscarinic mechanisms. This possibility led to a series of physiological studies. The authors now report that contrary to expectation, treatment with bright light did not decrease the density of muscarinic receptors in either the hypothalamus or striatum.

Lithium stimulation of membrane-bound phospholipase C from PC12 cells exposed to nerve growth factor

LiCl stimulated the formation of inositol monophosphate in PC12 cells that had been exposed to nerve growth factor (NGF) for 4-5 days. Half-maximal accumulation was observed at approximately 8 mM LiCl. Stimulation of formation of inositol bisphosphate plus inositol trisphosphate was half-maximal at approximately 1 mM LiCl. With membranes isolated from PC12 cells differentiated with NGF, the hydrolysis of added phosphatidylinositol 4,5-bisphosphate (PIP2) was stimulated by LiCl in a biphasic manner, with the first stimulation half-maximal at approximately 0.7 mM and the second half-maximal at approximately 15 mM LiCl. The apparent Km for PIP2 was lowered in the presence of 1.1 mM LiCl from approximately 200 to approximately 70 microM. Membranes from cells grown in the absence of NGF did not respond to LiCl. Although observations with intact cells are difficult to interpret without ambiguity, the results obtained with isolated membranes support our interpretation of the stimulatory action of lithium in the intact PC12 cells.

Increased polyphosphoinositide responsiveness in the cerebral cortex induced by cholinergic denervation

Lesion of the nucleus basalis in the basal forebrain of the rat results in the degeneration of the large cholinergic neurones which innervate the cortex. Parameters of cholinergic function, namely, acetylcholinesterase activity, muscarinic acetylcholine receptor number, and the depolarisation-induced release of acetylcholine, fall in ipsilateral cortex subsequent to lesion. These deficits are likely to reflect the loss of the presynaptic input to the cortex. A reversal in these deficits is seen 1 month after lesion, and a full recovery is seen after 150 days. This is thought to be due to a process of "spared axon sprouting" followed by the reestablishment of synapses. To examine the integrity of the cortical muscarinic receptor response following denervation, an assay of the polyphosphoinositide response was carried out. Cortical tissue slices, prelabelled with [3H]inositol, were incubated for 40 min with carbachol in the presence of Li+; the accumulation of [3H]inositol monophosphate ([3H]IP1) was used as an index of this response. A 92% increase in the carbachol-stimulated production of [3H]IP1 was seen 5 days after lesion compared to normal cortex. Sham-operated animals showed no change in [3H]IP1 accumulation at this time point. Dose-response experiments showed that this increase was due to an increase in the maximal response to carbachol after lesion with no change in EC50 values. Two weeks after lesion, this increased response was much attenuated; tissue slices from denervated cortex showing a strong acetylcholinesterase decrease (36-66%) showed an increase of just 30% above normal.(ABSTRACT TRUNCATED AT 250 WORDS)

5-Hydroxytryptamine-stimulated inositol phospholipid hydrolysis in the mouse cortex has pharmacological characteristics compatible with mediation via 5-HT2 receptors but this response does not reflect altered 5-HT2 function after 5,7-dihydroxytryptamine lesioning or repeated antidepressant treatments

5-Hydroxytryptamine (5-HT; 3 x 10(-8)-1 x 10(-5)M) produced a dose-dependent increase in phosphatidylinositol/polyphosphoinositide (PI) turnover in mouse cortical slices, as measured by following production of 3H-labelled inositol phosphates (IPs) in the presence of 10 mM LiCl. Analysis of individual IPs, in slices stimulated for 45 min, indicated substantial increases in inositol monophosphate (IP1; 140%) and inositol bisphosphate (IP2; 95%) contents with smaller increases in inositol trisphosphate (IP3; 51%) and inositol tetrakisphosphate (IP4; 48%) contents. The increase in IP3 level was solely in the 1,3,4-isomer. This response was inhibited by the nonselective 5-HT antagonists methysergide, metergoline, and spiperone. It was also inhibited by the selective 5-HT2 antagonists ketanserin and ritanserin but not by the 5-HT1 antagonists isapirone, (-)-propranolol, or pindolol. 5-HT-stimulated IP formation was also unaltered by atropine, prazosin, and mepyramine. Lesioning brain 5-HT neurones using 5,7-dihydroxytryptamine (5,7-DHT; 50 micrograms i.c.v.) produced a 210% (p less than 0.01) increase in the number of 5-HT2-mediated head-twitches induced by 5-methoxy-N,N-dimethyltryptamine (2 mg/kg). However, 5,7-DHT lesioning had no effect on 5-HT-stimulated PI turnover in these mice. Similarly, an electroconvulsive shock (90 V, 1 s) given five times over a 10-day period caused an 85% (p less than 0.01) increase in head-twitch responses but no change in 5-HT-stimulated PI turnover. Decreasing 5-HT2 function by twice-a-day injection of 5 mg/kg of zimeldine or desipramine (DMI) produced 50% (p less than 0.01) and 56% (p less than 0.01), respectively, reductions in head-twitch behaviour.(ABSTRACT TRUNCATED AT 250 WORDS)

Inositol phospholipid hydrolysis in cultured astrocytes and oligodendrocytes

Cultures of astrocytes and oligodendrocytes were prelabeled with 3H-inositol and the accumulation of 3H-inositol phosphates was determined following stimulation with a number of neuroactive substances. In astrocytes, norepinephrine (NE) produced the greatest stimulation with significant increase also observed with bradykinin. In oligodendrocytes, the greatest stimulation was produced by carbachol with significant increase also produced by bradykinin, histamine and NE. Carbachol was found to be ineffective in producing stimulation in astrocytes. The accumulation of 3H-inositol phosphates in astrocytes in response to NE was found to be dependent on the presence of Li+. The NE stimulation in astrocytes was dose-dependent and had an EC50 of 1.2 microM. This stimulation was blocked by the low concentration of the alpha 1-adrenergic antagonist prazosin but not by the alpha 2-adrenergic antagonist yohimbine. The NE-stimulated accumulation of 3H-inositol phosphates in astrocytes was inhibited by the cyclic nucleotide phosphodiesterase inhibitor isobutylmethylxanthine as well as by the cAMP analog dibutyryl cAMP.

Neurotensin and substance P receptors expressed in Xenopus oocytes by messenger RNA from rat brain

Xenopus oocytes were induced to acquire sensitivity to neurotensin and substance P, by injecting them with a fraction of poly(A)+ mRNA from rat brain. Non-injected oocytes, and oocytes injected with other brain mRNAs, failed to show responses, suggesting that receptors to these peptides were expressed by specific brain mRNAs. Responses to substance P and neurotensin comprised an oscillatory chloride current, and a smooth current having different ionic basis. These currents resembled those seen during activation of muscarinic and serotonergic receptors, but were not blocked by the corresponding antagonists atropine and methysergide. The responses to substance P, and to a lesser extent to neurotensin, showed a long-lasting desensitization. Similarities between the oscillatory currents evoked by the peptides acetylcholine and serotonin suggest that all these receptors may 'link in' to a common intracellular messenger pathway.

Phospholipid requirement of Ca2+-stimulated, Mg2+-dependent ATP hydrolysis in rat brain synaptic membranes

The phospholipid requirement for Ca2+-stimulated, Mg2+-dependent ATP hydrolysis (Ca2+/Mg2+-ATPase) and Mg2+-stimulated ATP hydrolysis (Mg2+-ATPase) in rat brain synaptosomal membranes was studied employing partial delipidation of the membranes with phospholipase A2 (Hog pancreas), phospholipase C (Bacillus cereus) and phospholipase D (cabbage). Treatment with phospholipase A2 caused an increase in the activities of both Ca2+/Mg2+-ATPase and Mg2+-ATPase whereas with phospholipase C treatment both the enzyme activities were inhibited. Phospholipase D treatment had no effect on Ca2+/Mg2+-ATPase but Mg2+-ATPase activity was inhibited. Inhibition of Mg2+-ATPase activity after phospholipase C treatment was relieved with the addition of phosphatidylinositol-4,5-bisphosphate (PIP2) and to a lesser extent with phosphatidylinositol-4-phosphate (PIP) and phosphatidylcholine (PC). Phosphatidylserine (PS), phosphatidic acid (PA), PIP and PIP2 brought about the reactivation of Ca2+/Mg2+-ATPase. Phosphatidylinositol (PI) and PA inhibited Mg2+-ATPase activity. Kms for Ca2+ (0.47 microM) and Mg2+ (60 microM) of the enzyme were found to be unaffected after treatment with the phospholipases.

Raising the ambient potassium ion concentration enhances carbachol stimulated phosphoinositide hydrolysis in rat brain hippocampal and cerebral cortical miniprisms

The influence of the ambient potassium ion concentration ([K+]) upon agonist stimulated hydrolysis of phosphoinositides (PI) has been studied in isolated miniprisms of rat hippocampus and cerebral cortex. When the external [K+] was raised from 6 to 18 mmol/l, there was little or no increase in the hydrolysis of PI in the absence of agonist, however, carbachol (100 mumol/l) stimulated hydrolysis was greatly enhanced in both brain regions studied. Thus, carbachol stimulated the hydrolysis of PI to 146% and 386% of control levels at potassium concentrations of 5.88 and 18.2 mmol/l, respectively, in the rat hippocampus. A similar enhancement of muscarine (100 mumol/l) stimulation was observed in cortical miniprisms with 18 mmol/l [K+]. A further enhancement was seen at higher ambient [K+], although basal hydrolysis of PI was then also increased. The carbachol-stimulated hydrolysis of PI found at both 6 and raised [K+] was prevented by atropine (1 and 10 mumol/l) and tetraethylammonium (20 mmol/l), but not by 10 mmol/l Mg2+. Pirenzepine (50 nmol/l) also reduced this response. The ions Cs+ and Rb+ (but not Li+ or Tris+) produced a similar enhancement of the carbachol stimulation to that found with K+. At a buffer [K+] of 6 mmol/l, noradrenaline (100 mumol/l) produced a 2-fold increase in the hydrolysis of PI whereas 5-hydroxytryptamine (100 mumol/l) and histamine (500 mumol/l) had little or no effect. However, histamine and 5-hydroxytryptamine did stimulate the hydrolysis of PI when [K+] was increased. Miniprism ATP content was not changed by a rise in [K+] to 18 mmol/l. The significance of these results is discussed in terms of the postsynaptic cellular events following cholinergic stimulation.

Cholinergic mechanisms in depression

Evidence supporting a cholinergic hypothesis of depression is presented. First, cholinergic overdrive produces behavioral, neuroendocrine, and polysomnographic features of melancholia, and melancholics exhibit state-independent supersensitivity to cholinergic overdrive. Drugs inducing up-regulation and supersensitivity of cholinergic systems produce behavioral, polysomnographic, and neuroendocrine effects of melancholia when withdrawn. These observations also implicate cholinergic system supersensitivity as a factor in the pathophysiology of certain affective disorders. Cholinergic and monoaminergic mechanisms reciprocally regulate drive-reduction, and substances of abuse either activate monoaminergic networks or antagonize cholinergic systems. These points are consistent with the hypothesis that dynamic interaction between cholinergic and monoaminergic systems is involved in the regulation of mood and affect. Finally, antimuscarinic agents have antidepressant effects. Thus, the hypothesis that supersensitivity of cholinergic systems is involved in the pathophysiology of affective disorders is supported by several lines of evidence. This evidence is reviewed; directions for future research and promising methods of investigation are discussed.

Aluminum impairs glucose utilization and cholinergic activity in rat brain in vitro

The effects of AlCl3 on the production of 14CO2 from [U-14C]glucose and high affinity choline transport in rat brain synaptosomes, and on carbachol-stimulated hydrolysis of phosphoinositides in cortical slices were studied. In buffer containing either high K+ (50 mM) or low K+ (4.9 mM), 1 mM AlCl3 significantly depressed the synaptosomal production of 14CO2 from [U-14C]glucose to 54% and 44% of control rates, respectively. At a concentration of 0.1 mM, AlCl3 depressed the evolution of 14CO2 from [U-14C]glucose from synaptosomes incubated in the high K+ buffer, but did not significantly change 14CO2 production from synaptosomes in the low K+ buffer. Aluminum chloride also inhibited high affinity choline transport in synaptosomes prepared from rat cortex and from hippocampus with an IC50 of approximately 0.5 mM. In brain slices the carbachol-stimulated hydrolysis of phosphoinositides was inhibited by AlCl3 in a dose-dependent manner. One millimolar, 0.5 mM and 0.1 mM AlCl3 inhibited the carbachol-stimulated release of inositol phosphates by 75%, 44% and 33%, respectively. These same concentrations of AlCl3 inhibited the incorporation of [3H]inositol into phospholipids. This inhibitory effect was not dose-dependent as all 3 concentrations of AlCl3 inhibited phospholipid labelling to the same extent (27-37%). These results are discussed in relation to the in vivo neurotoxicity of aluminum.

Pathophysiology of "cholinoceptor supersensitivity" in affective disorders

Phenomenological and physiological variables demonstrate supersensitive changes to cholinergic challenge in affective disorder subjects. Theorists generally assume the primary defect is the postsynaptic muscarinic receptor. However, in addition to defectiveness or up-regulation of this receptor, the appearance of postsynaptic "cholinoceptor supersensitivity" can result from abnormal presynaptic mechanisms, membrane "pathology," derangement of intrasystolic mechanisms that amplify effects of receptor-agonist coupling, or aberrant cholinergic-monoaminergic interaction. This article discusses abnormalities of the postsynaptic receptor, regulation of postsynaptic receptor density, the presynaptic muscarinic receptor, and other mechanisms regulating the release of acetylcholine, membrane dynamics, and "cascade" mechanisms-specifically the phosphatidylinositol (PI) cycle, Ca2+ mobilization, and cyclic guanosine monophosphate (GMP) generation-as causes of cholinergic system "supersensitivity." It is suggested that an approach to the topic emphasizing site of abnormality will encourage greater clarity of thought in the study of the cholinergic component of the pathophysiology of affective illness.

Noradrenaline-stimulated inositol phospholipid breakdown in rat dorsal lateral geniculate nucleus neurones

Noradrenaline-stimulated inositol phospholipid breakdown in matched vibratome sections through the rat dorsal lateral geniculate nucleus (dLGN). The response was measured as a large accumulation of [3H]inositol labelled inositol monophosphate and was mediated via activation of alpha 1-adrenergic receptors. Accumulation of [3H]inositol phosphates was reduced in kainic acid-lesioned animals, indicating that this response occurred within dLGN neurones and not afferent terminals. The results implicate inositol phospholipid breakdown as part of the mechanism of noradrenergic neurotransmission within the dLGN.

The role of myo-inositol in multiple sclerosis

Myo-inositol was given orally to nine multiple sclerosis patients and nine healthy control subjects. Pattern reversal evoked potential testing was used to assess its effect. The principal positive wave increased in amplitude, duration and area in a dose-dependent manner in the multiple sclerosis group compared with controls. Cerebrospinal fluid concentrations of myo-inositol in multiple sclerosis and controls were similar. The significance of these observations is discussed in relation to recent discoveries in inositol phospholipid function.

Excitatory amino acids inhibit stimulation of phosphatidylinositol metabolism by aminergic agonists in hippocampus

Since the initial observations in the 1950s a large number of neurotransmitters and hormones have been shown to influence phosphatidylinositol (PI) metabolism in brain and peripheral ganglia (see ref. 3 for review). This has led to the suggestion that PI is part of an intracellular second messenger system for some types of diffusible chemical factors. Consistent with this are recent reports that one of the products of PI turnover (diacylglycerol) stimulates the Ca-dependent phospholipid-dependent protein kinase (kinase C) while a second (inositol trisphosphate) causes the release of calcium from intracellular stores. Thus it is possible that at least some brain neurotransmitters utilize the PI system to produce functional effects that are in addition to and which outlast the very brief physiological responses they elicit. Although it had been anticipated that another class of receptors might inhibit receptor-mediated stimulation of PI breakdown, no clear examples of such effects have been described. We now report that acidic amino acids, which are that acidic amino acids, which are thought to be excitatory neurotransmitters at the majority of brain synapses, strongly inhibit the stimulation of PI metabolism elicited by carbachol, histamine, or by potassium-induced depolarization, without changing the response to noradrenaline. As well as indicating a novel function for the excitatory amino acids, these results suggest that the central nervous system possesses cell-cell interactions of a previously unsuspected type.

Differential effects of lithium on muscarinic receptor stimulation of inositol phosphates in rat cerebral cortex slices

The accumulation of labelled inositol mono-, bis-, and trisphosphate in rat cerebral cortex slices was examined following preincubation with [3H]inositol. The muscarinic receptor agonist carbachol produced a rapid and sustained increased accumulation of each labelled inositol phosphate both in the presence and absence of 5 mM lithium. Lithium potentiated carbachol-stimulated accumulation of inositol monophosphate (EC50 0.5 mM) and inositol bisphosphate (EC50 4 mM) in a concentration-dependent manner. However, exposure to lithium in the presence of the muscarinic agonist produced a concentration- and time-dependent inhibition of inositol trisphosphate accumulation that was not related to receptor desensitisation. Although the present data do suggest that polyphosphoinositides are substrates for agonist-stimulated phospholipase C in brain, these results may not be entirely consistent with the production of inositol mono- and bisphosphate through inositol trisphosphate dephosphorylation. Furthermore, these data suggest site(s) additional to inositol monophosphatase that are affected by lithium.

Manganese stimulates the incorporation of [3H]inositol into a pool of phosphatidylinositol in brain that is not coupled to agonist-induced hydrolysis

Mn2+ greatly increases the incorporation of myo-[3H]inositol into phosphatidylinositol (PI) of brain and other tissues by stimulating the activity of a PI-myo-inositol exchange enzyme. This study examined the ability of norepinephrine (NE) and carbachol to stimulate the hydrolysis of [3H]PI formed in the absence and presence of Mn2+-stimulated [3H]inositol exchange. Rat cerebral cortical slices were incubated with myo-[3H]inositol for 60 min in an N-2-hydroxyethyl piperazine-N'-2-ethanesulfonic acid (HEPES) buffer without or with MnCl2 (1 mM). The tissue was washed and further incubated with unlabeled myo-inositol and LiCl (10 mM). Prelabeled slices were then incubated with NE (0.1 mM) or carbachol (1 mM) to induce agonist-stimulated [3H]PI hydrolysis. Mn2+ treatment resulted in eight- and sixfold increases in control levels of [3H]PI and [3H]inositol monophosphate [( 3H]IP), respectively. Both NE and carbachol stimulated [3H]IP formation in tissue prelabeled without or with manganese. However, the degree of stimulation (percentage of control values) was greatly attenuated in the presence of Mn2+. In the absence of Mn2+ treatment, NE decreased [3H]PI radioactivity in the tissue to 80% of control values. However, NE did not decrease [3H]PI radioactivity in the Mn2+-treated tissue. These data demonstrate that Mn2+ stimulates incorporation of myo-[3H]inositol into a pool of PI in brain that has a rapid turnover but is not coupled to agonist-induced hydrolysis.

Gas chromatographic-mass spectrometric determination of myo-inositol in human cerebrospinal fluid

The concentration of free myo-inositol in CSF was determined with a gas chromatographic-mass spectrometric method using deuterated myo-inositol as an internal standard after conversion to the hexa-O-acetyl derivative with acetic anhydride and pyridine. Twenty microliters of CSF is sufficient for the analysis which has a coefficient of variation of 9%. Identical analytical results were obtained on two different mass numbers. Schizophrenic patients were compared with healthy control persons. In addition, patients with rheumatoid arthritis or with neurological illnesses were studied. No consistent differences related to the illness could be found. The mean concentration of myo-inositol was about 25 micrograms/ml. Treatment of schizophrenic patients with chlorpromazine or sulpiride had no significant effect on the concentration of myo-inositol in CSF.

Comparison of postnatal changes in alpha 1-adrenoceptor binding and adrenergic stimulation of phosphoinositide hydrolysis in rat cerebral cortex

Adrenergic stimulation of phosphoinositide hydrolysis is mediated by the alpha 1-adrenoceptor subtype in many tissues including the brain. We have investigated the coupling of alpha 1-adrenoceptors to phosphoinositide hydrolysis during ontogeny. Alpha 1-adrenoceptor number and affinity were measured using [3H]prazosin binding in crude membranes of cerebral cortex and compared to the ability of the adrenergic agonists norepinephrine (NE) and phenylephrine (PE) to stimulate the formation of [3H] inositol phosphates from [3H]myo-inositol in brain slices at various ages. The greatest changes in the developmental expression of both the Bmax for [3H]prazosin binding and maximal (10(-4)M) NE- or PE-stimulated [3H]inositol phosphates were observed during the period of 7-21 days of age. No changes in the KD for [3H]prazosin were observed. However, at 14 days of age the EC50 for NE but not PE stimulation of [3H]inositol phosphates was slightly but significantly lower than at later ages. To quantitatively compare these two parameters during ontogeny, data were expressed as a percentage of the adult (greater than 65 day) value. At early ages (7 and 14 days) but not at later ages (21 and 37 days) the percent expression of [3H]prazosin binding sites was significantly greater than the maximal NE-stimulated [3H]inositol phosphates. This suggests that early in neonatal development alpha 1 adrenoceptors in brain are not tightly coupled to phosphoinositide hydrolysis.

Denervation supersensitivity of the rat pineal to norepinephrine-stimulated [3H]inositide turnover revealed by lithium and a convenient procedure

A convenient procedure for the assay of myo-[2-3H(N)]inositol ([3H]inositol) metabolites in cells or small amounts of tissue was developed. The procedure is a composite of modifications of published methods. After preincubation with [3H]inositol, rat pineal glands were disrupted in an acidified organic solvent mixture. Lipids were separated from the hydrophilic products and precursor using Sephadex G-25 columns and further analyzed by TLC. Hydrophilic products were further analyzed by anion-exchange column chromatography using Dowex AG1-X8 (formate form). In the presence of lithium, increases in inositol phosphates consequent to stimulation of the glands by norepinephrine were apparent within 10 min. The response in denervated glands was considerably greater than in intact pineals.

The role of calcium in diuretic hormone action on locust Malpighian tubules

Fluid secretion by the Malpighian tubules (MTs) of Locusta is drastically reduced in the absence of extracellular calcium. Verapamil (10(-4) M) inhibits basal secretion, whereas the ionophore A23187 (10(-5) M) elevates the secretory rate. Cyclic guanosine monophosphate (cGMP) stimulates fluid secretion at a concentration of 10(-3) M. A factor extractable in methanol from the storage lobes of the corpora cardiaca stimulates increased guanylate cyclase activity in MTs, resulting in a 10-fold elevation in intracellular cGMP levels. Attempts to separate the factor stimulating guanylate cyclase by high-performance size-exclusion chromatography proved unsuccessful. Neither 5-hydroxytryptamine (5-HT) (10(-4) M) nor A23187 (10(-5) M) are able to elevate intracellular cGMP levels. Elevations of both intracellular cAMP and cGMP levels in response to diuretic hormone (DH) are potentiated in the absence of extracellular calcium. Consistent with these elevations are increases in the rates of fluid secretion by tubules deprived of extracellular calcium. It is concluded that, although calcium has an important role in the regulation of fluid secretion, its role in the mechanism of hormone-stimulated secretion may be modulatory rather than regulatory.

Norepinephrine stimulation of phosphoinositide hydrolysis in rat cerebral cortex is associated with the alpha1-adrenoceptor

Norepinephrine (NE) and the selective alpha1-adrenoceptor agonist phenylephrine (PE) both markedly stimulate the formation of [3H]inositol phosphates in a concentration-dependent manner upon incubation with [3H]myo-inositol. The selective alpha2 agonist, clonidine, did not significantly alter [3H]inositol phosphate formation, even at concentrations as high as 10(-3) M. The alpha1 antagonist prazosin (IC50, 0.036 microM) was 300 times more potent than the alpha2 antagonist yohimbine (IC50, 10.7 microM) as an inhibitor of NE (10(-4) M)-stimulated phosphatidylinositol (PI) hydrolysis. These results indicate that the alpha1-, but not the alpha2-adrenoceptor subtype in rat brain is coupled to phosphoinositide hydrolysis.

Selective 5HT-2 antagonists inhibit serotonin stimulated phosphatidylinositol metabolism in cerebral cortex

Evidence suggests that the serotonin 5HT-1 receptor site is functionally linked to adenylate cyclase in the brain, but a biochemical effector system which is linked to the serotonin 5HT-2 receptor site has not been found. In the present paper we report an investigation of 5HT stimulated phosphatidylinositol (PI) hydrolysis in rat cerebral cortex and have found that selective 5HT-2 antagonists (pizotifen and ketanserin) block 5HT's effect upon PI metabolism. These data suggest that 5HT stimulated PI hydrolysis is mediated by the 5HT-2 binding site.

Activation of muscarinic receptors stimulates the release of choline from brain slices

The effect of several agents known to interact with muscarinic acetylcholine receptors on the release of choline from slices of rat corpus striatum into the incubation medium has been investigated. The amount of released choline was increased if choline, acetylcholine, or oxotremorine had been added to the incubation medium. Atropine blocked the effects of acetylcholine and oxotremorine; it was not tested with choline. It is proposed that the increased release of choline is due to an increased hydrolysis of phosphatidylcholine, brought about by the activation of muscarinic acetylcholine receptors.