Glycogenolysis induced by serotonin in brain: identification of a new class of receptor

A tribute to Leif Hertz: The historical context of his pioneering studies of the roles of astrocytes in brain energy metabolism, neurotransmission, cognitive functions, and pharmacology identifies important, unresolved topics for future studies

Leif Hertz, M.D., D.Sc. (honōris causā) (1930-2018), was one of the original and noteworthy participants in the International Conference on Brain Energy Metabolism (ICBEM) series since its inception in 1993. The biennial ICBEM conferences are organized by neuroscientists interested in energetics and metabolism underlying neural functions; they have had a high impact on conceptual and experimental advances in these fields and on promoting collaborative interactions among neuroscientists. Leif made major contributions to ICBEM discussions and understanding of metabolic and signaling characteristics of astrocytes and their roles in brain function. His studies ranged from uptake of K+ from extracellular fluid and its stimulation of astrocytic respiration, identification, and regulation of enzymes specifically or preferentially expressed in astrocytes in the glutamate-glutamine cycle of excitatory neurotransmission, a requirement for astrocytic glycogenolysis for fueling K+ uptake, involvement of glycogen in memory consolidation in the chick, and pharmacology of astrocytes. This tribute to Leif Hertz highlights his major discoveries, the high impact of his work on astrocyte-neuron interactions, and his unparalleled influence on understanding the cellular basis of brain energy metabolism. His work over six decades has helped integrate the roles of astrocytes into neurotransmission where oxidative and glycogenolytic metabolism during neurotransmitter glutamate turnover are key aspects of astrocytic energetics. Leif recognized that brain astrocytic metabolism is greatly underestimated unless the volume fraction of astrocytes is taken into account. Adjustment for pathway rates expressed per gram tissue for volume fraction indicates that astrocytes have much higher oxidative rates than neurons and astrocytic glycogen concentrations and glycogenolytic rates during sensory stimulation in vivo are similar to those in resting and exercising muscle, respectively. These novel insights are typical of Leif's astute contributions to the energy metabolism field, and his publications have identified unresolved topics that provide the neuroscience community with challenges and opportunities for future research.

Brain glycogen metabolism: A possible link between sleep disturbances, headache and depression

The functions of sleep and its links with neuropsychiatric diseases have long been questioned. Among the numerous hypotheses on sleep function, early studies proposed that sleep helps to replenish glycogen stores consumed during waking. Later studies found increased brain glycogen after sleep deprivation, leading to "glycogenetic" hypothesis, which states that there is a parallel increase in synthesis and utilization of glycogen during wakefulness, whereas decrease in the excitatory transmission creates an imbalance causing accumulation of glycogen during sleep. Glycogen is a vital energy reservoir to match the synaptic demand particularly for re-uptake of potassium and glutamate during intense glutamatergic transmission. Therefore, sleep deprivation-induced transcriptional changes may trigger migraine by reducing glycogen availability, which slows clearance of extracellular potassium and glutamate, hence, creates susceptibility to cortical spreading depolarization, the electrophysiological correlate of migraine aura. Interestingly, chronic stress accompanied by increased glucocorticoid levels and locus coeruleus activity and leading to mood disorders in which sleep disturbances are prevalent, also affects brain glycogen turnover via glucocorticoids, noradrenaline, serotonin and adenosine. These observations altogether suggest that inadequate astrocytic glycogen turnover may be one of the mechanisms linking migraine, mood disorders and sleep.

A thermodynamic function of glycogen in brain and muscle

Brain and muscle glycogen are generally thought to function as local glucose reserves, for use during transient mismatches between glucose supply and demand. However, quantitative measures show that glucose supply is likely never rate-limiting for energy metabolism in either brain or muscle under physiological conditions. These tissues nevertheless do utilize glycogen during increased energy demand, despite the availability of free glucose, and despite the ATP cost of cycling glucose through glycogen polymer. This seemingly wasteful process can be explained by considering the effect of glycogenolysis on the amount of energy obtained from ATP (ΔG'ATP). The amount of energy obtained from ATP is reduced by elevations in inorganic phosphate (Pi). Glycogen utilization sequesters Pi in the glycogen phosphorylase reaction and in downstream phosphorylated glycolytic intermediates, thereby buffering Pi elevations and maximizing energy yield at sites of rapid ATP consumption. This thermodynamic effect of glycogen may be particularly important in the narrow, spatially constrained astrocyte processes that ensheath neuronal synapses and in cells such as astrocytes and myocytes that release Pi from phosphocreatine during energy demand. The thermodynamic effect may also explain glycolytic super-compensation in brain when glycogen is not available, and aspects of exercise physiology in muscle glycogen phosphorylase deficiency (McArdle disease).

Role of Glycogenolysis in Memory and Learning: Regulation by Noradrenaline, Serotonin and ATP

This paper reviews the role played by glycogen breakdown (glycogenolysis) and glycogen re-synthesis in memory processing in two different chick brain regions, (1) the hippocampus and (2) the avian equivalent of the mammalian cortex, the intermediate medial mesopallium (IMM). Memory processing is regulated by the neuromodulators noradrenaline and serotonin soon after training glycogen breakdown and re-synthesis. In day-old domestic chicks, memory formation is dependent on the breakdown of glycogen (glycogenolysis) at three specific times during the first 60 min after learning (around 2.5, 30, and 55 min). The chicks learn to discriminate in a single trial between beads of two colors and tastes. Inhibition of glycogen breakdown by the inhibitor of glycogen phosphorylase 1,4-dideoxy-1,4-imino-D-arabinitol (DAB) given at specific times prior to the formation of long-term memory prevents memory forming. Noradrenergic stimulation of cultured chicken astrocytes by a selective β₂-adrenergic (AR) agonist reduces glycogen levels and we believe that in vivo this triggers memory consolidation at the second stage of glycogenolysis. Serotonin acting at 5-HT_2B receptors acts on the first stage, but not on the second. We have shown that noradrenaline, acting via post-synaptic α₂-ARs, is also responsible for the synthesis of glycogen and our experiments suggest that there is a readily accessible labile pool of glycogen in astrocytes which is depleted within 10 min if glycogen synthesis is inhibited. Endogenous ATP promotion of memory consolidation at 2.5 and 30 min is also dependent on glycogen breakdown. ATP acts at P₂Y₁ receptors and the action of thrombin suggests that it causes the release of internal calcium ([Ca²⁺]_i) in astrocytes. Glutamate and GABA, the primary neurotransmitters in the brain, cannot be synthesized in neurons de novo and neurons rely on astrocytic glutamate synthesis, requiring glycogenolysis.

Monoaminergic Control of Cellular Glucose Utilization by Glycogenolysis in Neocortex and Hippocampus

Brainstem nuclei are the principal sites of monoamine (MA) innervation to major forebrain structures. In the cortical grey matter, increased secretion of MA neuromodulators occurs in response to a wealth of environmental and homeostatic challenges, whose onset is associated with rapid, preparatory changes in neural activity as well as with increases in energy metabolism. Blood-borne glucose is the main substrate for energy production in the brain. Once entered the tissue, interstitial glucose is equally accessible to neurons and astrocytes, the two cell types accounting for most of cellular volume and energy metabolism in neocortex and hippocampus. Astrocytes also store substantial amounts of glycogen, but non-stimulated glycogen turnover is very small. The rate of cellular glucose utilization in the brain is largely determined by hexokinase, which under basal conditions is more than 90 % inhibited by its product glucose-6-phosphate (Glc-6-P). During rapid increases in energy demand, glycogen is a primary candidate in modulating the intracellular level of Glc-6-P, which can occur only in astrocytes. Glycogenolysis can produce Glc-6-P at a rate higher than uptake and phosphorylation of glucose. MA neurotransmitter are released extrasinaptically by brainstem neurons projecting to neocortex and hippocampus, thus activating MA receptors located on both neuronal and astrocytic plasma membrane. Importantly, MAs are glycogenolytic agents and thus they are exquisitely suitable for regulation of astrocytic Glc-6-P concentration, upstream substrate flow through hexokinase and hence cellular glucose uptake. Conforming to such mechanism, Gerald A. Dienel and Nancy F. Cruz recently suggested that activation of noradrenergic locus coeruleus might reversibly block astrocytic glucose uptake by stimulating glycogenolysis in these cells, thereby anticipating the rise in glucose need by active neurons. In this paper, we further develop the idea that the whole monoaminergic system modulates both function and metabolism of forebrain regions in a manner mediated by glycogen mobilization in astrocytes.

Reflections on glycogen and β-amyloid: why does glycogenolytic β2-adrenoceptor stimulation not rescue memory after β-amyloid?

Normally noradrenaline release ~30 min after training in the day-old chick is essential for memory consolidation by simultaneously increasing both glycogenolysis, by its stimulation of β2-adrenergic (AR) receptors, and glycogen synthesis, by its stimulation of α2-AR receptors in astrocytes. At the same time noradrenaline stimulation of β3-AR receptors increases glucose uptake solely in astrocytes. Intracerebral injection of small oligomeric β-amyloid protein (Aβ1-42) (Aβ) 45 min before one-trial bead discrimination learning in day-old chicks abolishes consolidation of memory 30 min post-learning. The ensuing memory loss can be rescued by injection of selective β3- and β(2-AR agonists (CL316243 and zinterol), which also have the ability to consolidate weakly-reinforced learning into long-term memory. However, although CL316243 rescues Aβ-induced memory loss over a similar time period to when it consolidates weak learning (up to 25 min post training), zinterol is effective over a more limited time period and unexpectedly it does not rescue at the time it promotes glycogenolysis. Injection of Aβ into the hippocampus and the locus coeruleus (LoC) also produces similar memory deficits and injection of both AR agonists into a cortical area can rescue memory from LoC Aβ. We have previously shown that β3-AR stimulation increases astrocytic glucose uptake and have suggested there may be sensitization or upregulation of the receptor. Since β2-AR stimulation does not rescue memory at the time it promotes glycogenolysis, but the receptor does not appear to be impaired, it is suggested that Aβ may be causing an impairment in the synthesis of readily available glycogen.

Serotonin mediation of early memory formation via 5-HT2B receptor-induced glycogenolysis in the day-old chick

Investigation of the effects of serotonin on memory formation in the chick revealed an action on at least two 5-HT receptors. Serotonin injected intracerebrally produced a biphasic effect on memory consolidation with enhancement at low doses and inhibition at higher doses. The non-selective 5-HT receptor antagonist methiothepin and the selective 5-HT2B/C receptor antagonist SB221284 both inhibited memory, suggesting actions of serotonin on at least two different receptor subtypes. The 5-HT2B/C and astrocyte-specific 5-HT receptor agonist, fluoxetine and paroxetine, enhanced memory and the effect was attributed to glycogenolysis. Inhibition of glycogenolysis with a low dose of DAB (1,4-dideoxy-1,4-imino-D-arabinitol) prevented both serotonin and fluoxetine from enhancing memory during short-term memory but not during intermediate memory. The role of serotonin on the 5-HT2B/C receptor appears to involve glycogen breakdown in astrocytes during short-term memory, whereas other published evidence attributes the second period of glycogenolysis to noradrenaline.

Brain glycogenolysis, adrenoceptors, pyruvate carboxylase, Na(+),K(+)-ATPase and Marie E. Gibbs' pioneering learning studies

The involvement of glycogenolysis, occurring in astrocytes but not in neurons, in learning is undisputed (Duran et al., 2013). According to one school of thought the role of astrocytes for learning is restricted to supply of substrate for neuronal oxidative metabolism. The present "perspective" suggests a more comprehensive and complex role, made possible by lack of glycogen degradation, unless specifically induced by either (1) activation of astrocytic receptors, perhaps especially β-adrenergic or (2) even small increases in extracellular K(+) concentration above its normal resting level. It discusses (1) the known importance of glycogenolysis for glutamate formation, requiring pyruvate carboxylation; (2) the established role of K(+)-stimulated glycogenolysis for K(+) uptake in cultured astrocytes, which probably indicates that astrocytes are an integral part of cellular K(+) homeostasis in the brain in vivo; and (3) the plausible role of transmitter-induced glycogenolysis, stimulating Na(+),K(+)-ATPase/NKCC1 activity and thereby contributing both to the post-excitatory undershoot in extracellular K(+) concentration and the memory-enhancing effect of transmitter-mediated reduction of slow neuronal afterhyperpolarization (sAHP).

Serotonin-induced brain glycogenolysis in rainbow trout (Oncorhynchus mykiss)

In this study, we evaluated the serotonin-mediated control of cerebral glycogen levels in the rainbow trout, Oncorhynchus mykiss. Intracerebroventricular (i.c.v.) administration of serotonin (5-HT) to normoglycemic trout (time and dose response) decreased glycogen levels in the brain and increased brain glycogen phosphorylase activity (time response). In hypoglycemic fish (that had been fasted for 5 and 10 days), there was a time-dependent decrease in brain glycogen levels; under these conditions, i.c.v. administration of 5-HT also reduced the brain glycogen content in fish that had been fasted for 5 days. In fish with local cerebral hypoglycemia (induced by 2-DG administration), the glycogen levels decreased and, as above, i.c.v. administration of 5-HT also lowered the glycogen content. In hyperglycemic fish, 5-HT did not affect glycogen levels. Administration of receptor agonists 5-HT1A (8-OH-DPAT), 5-HT1B (anpirtoline and CP93129) or 5-HT2 (α-m-5-HT) decreased the brain glycogen levels. This effect was antagonized by the administration of receptor antagonists 5-HT1A (WAY100135 and NAN190), 5-HT1B (NAS181) and 5-HT2B/C (SB206553). Administration of the receptor agonists (±)-DOI (5-HT2A/2C), m-CPP (5-HT2B/2C), BW723C86 (5-HT2B) and WAY 161503 (5-HT2C) led to decreases in the levels of brain glycogen. We found that 5-HT is involved in the modulation of brain glycogen homeostasis in the rainbow trout, causing a glycogenolytic effect when fish are in a normoglycemic or hypoglycemic state, but not when they are in a hyperglycemic state. 5-HT1A, 5-HT1B, 5HT2B and 5-HT2C-like receptors appeared to be involved in the glycogenolytic action of 5-HT, although the effect mediated by 5-HT1A or 5-HT1B was apparently stronger.

Serotonin prolongs survival of encapsulated pond snail embryos exposed to long-term anoxia

Embryos of the pond snail, Helisoma trivolvis, develop bilateral serotonergic neurons that innervate ciliary bands and stimulate cilia-driven rotation. This behaviour is postulated to increase oxygen availability during hypoxia by mixing the capsular fluid. We hypothesised that the stimulation of ciliary-driven rotation by serotonin (5-HT) enhances the survival of embryos during prolonged hypoxia. Embryo rotation and survival were monitored in different levels of oxygen for 24-48 h while in the presence or absence of 5-HT (100 micromol l(-1)) or a 5-HT antagonist (50 micromol l(-1)). Long-term hypoxia caused delayed embryonic development that appeared morphologically normal. Hypoxia also induced a transient increase in rotation rate in embryos exposed to artificial pond water (APW) or 5-HT that lasted around 3 h. 5-HT-treated embryos had an elevated rotation rate over embryos in APW throughout the long-term exposure to hypoxia. Long-term anoxia also induced a transient increase in rotation rate in embryos exposed to APW or 5-HT. Rotation ceased in embryos exposed to APW by 13 h but persisted in 5-HT-treated embryos for up to 40 h. Fifty percent mortality was reached at 9 h of anoxia in embryos in APW and at 24 h in 5-HT-treated embryos. The 5-HT antagonist mianserin partially inhibited the 5-HT enhancement of rotation but not the prolongation of survival in anoxia. The ability of 5-HT to prolong survival in anoxia reveals a 5-HT-activated metabolic pathway that liberates an alternative energy source.

Glycogen stores are impaired in hypothalamic nuclei of rats malnourished during early life

Perinatal nutrition has persistent influences on neural development and cognition. In humans and other animals, protein malnutrition during the perinatal period causes permanent changes, inducing to adulthood metabolic syndrome. Feeding is mainly modulated by neural and hormonal inputs to the hypothalamus. Hypothalamic glycogen stores are a source of glucose in high energetic demands, as during development of neural circuits. As some hypothalamic circuits are formed during lactation, we studied the effects of malnutrition, during the first 10 days of lactation, on glycogen stores in hypothalamic nuclei involved in the control of energy metabolism. Female pregnant rats were fed ad libitum with a normal protein diet (22% protein). After delivery, each dam was kept with 6 male pups. During the first 10 days of lactation, dams from the experimental group received a protein-free diet and the control group a normoprotein diet. By post-natal day 10 (P10), glycogen stores were very high in the arcuate nucleus and median eminence of control group. Glycogen stores decreased during development. In P20 control animals, glycogen stores were lower when compared to P10 control animals. Animals submitted to malnutrition presented a staining even lower than control ones. After P45, it was difficult to determine differences between control and diet groups because glycogen stores were reduced. We also showed that tanycytes were the cells presenting glycogen stores. Our data reinforce the concept that maternal nutritional state during lactation may be critical for neurodevelopment since it resulted in a low hypothalamic glycogen store, which may be critical for establishment of neuronal circuitry.

Epilepsy, regulation of brain energy metabolism and neurotransmission

Seizures are the result of a sudden and temporary synchronization of neuronal activity, the reason for which is not clearly understood. Astrocytes participate in the control of neurotransmitter storage and neurotransmission efficacy. They provide fuel to neurons, which need a high level of energy to sustain normal and pathological neuronal activities, such as during epilepsy. Various genetic or induced animal models have been developed and used to study epileptogenic mechanisms. Methionine sulfoximine induces both seizures and the accumulation of brain glycogen, which might be considered as a putative energy store to neurons in various animals. Animals subjected to methionine sulfoximine develop seizures similar to the most striking form of human epilepsy, with a long pre-convulsive period of several hours, a long convulsive period during up to 48 hours and a post convulsive period during which they recover normal behavior. The accumulation of brain glycogen has been demonstrated in both the cortex and cerebellum as early as the pre-convulsive period, indicating that this accumulation is not a consequence of seizures. The accumulation results from an activation of gluconeogenesis specifically localized to astrocytes, both in vivo and in vitro. Both seizures and brain glycogen accumulation vary when using different inbred strains of mice. C57BL/6J is the most "resistant" strain to methionine sulfoximine, while CBA/J is the most "sensitive" one. The present review describes the data obtained on methionine sulfoximine dependent seizures and brain glycogen in the light of neurotransmission, highlighting the relevance of brain glycogen content in epilepsies.

Idazoxan activates rat forebrain glycogen phosphorylase in vivo: A histochemical study

In vitro experiments show norepinephrine activates glycogen phosphorylase and glycogenolysis in forebrain glia. The present study used idazoxan (5 mg/kg) to elevate NE in vivo and examined patterns of active (aGP) and total (tGP) glycogen phosphorylase reactivity in selected neocortical, hippocampal, diencephalic, and striatal sites using a histochemical method. In somatosensory neocortex, aGP reactivity was highest in Layer 4 with consistent reactivity in the barrel fields in vehicle-treated brains. In the hippocampus, the stratum lacunosum moleculare was highly reactive, while cell layers were least reactive. The dentate gyrus and CA3 were more reactive for aGP than CA1. In the diencephalon, the medial habenula was most reactive followed by the reticular nucleus of the thalamus. In the striatum, globus pallidus was most reactive. Reactivity patterns for tGP were similar to those for aGP, but more intense. The neocortex had the highest overall reactivity for tGP. An estimate of the percentage of aGP relative to tGP suggested the regions sampled had similar levels of median basal activation (approximately 65%). Idazoxan increased aGP reactivity in all regions of the neocortex assessed (layers 3-6 of primary and secondary somatosensory cortex and the barrel fields). The neuropil layers, but not the cell layers, of hippocampus were more reactive following idazoxan treatment. Idazoxan also increased aGP reactivity in the laterodorsal, paraventricular, and reticular nuclei of the thalamus. The largest idazoxan-induced changes, as an estimated percentage of tGP, occurred in the hippocampus (approximately 16% for stratum lacunosum moleculare and for CA1 stratum oriens). Increases ranged from approximately 3 to 6% in neocortex and were less than 3% in the diencephalic and striatal areas. These effects of idazoxan are consistent with a role for norepinephrine in activating forebrain glycogenolyis in vivo and supporting increased brain metabolism. They contrast with earlier evidence showing that idazoxan reduces 2-deoxyglucose uptake in these brain areas. Idazoxan, and norepinephrine, may preferentially recruit glycolytic over oxidative metabolism in the rat forebrain.

The relationship between hyperthermia and glycogenolysis in 3,4-methylenedioxymethamphetamine-induced serotonin depletion in rats

Although the exact mechanisms involved in the serotonergic neurotoxicity produced by substituted amphetamines are not completely known, evidence suggests that oxidative and/or bioenergetic stress may contribute in the mechanism of neurotoxicity of 3,4-methylenedioxymethamphetamine (MDMA). It has been postulated that MDMA-induced hyperthermia also contributes to the MDMA-induced neurotoxicity. MDMA produces brain glycogenolysis, and MDMA-induced hyperthermia appears to mediate this effect. The relationship of MDMA-induced hyperthermia and glycogenolysis in the serotonergic neurotoxicity of MDMA was investigated in the present study. The administration of MDMA (20 mg/kg sc) at an ambient temperature of 24 degrees C produced hyperthermia and brain glycogenolysis in Postnatal Day (PND)21 and PND70 rats; however, long-term reductions in serotonin (5-HT) concentrations in the striatum were detected only in the PND70 rats. Treatment of PND21 and PND70 rats with MDMA at 17 degrees C resulted in neither hyperthermia nor glycogenolysis; nevertheless, long-term reductions in 5-HT concentrations were still evident in the PND70 rats treated with MDMA. These results support the conclusion that hyperthermia, as well as glycogenolysis, are neither necessary nor sufficient in the serotonergic neurotoxicity of MDMA.

Tracing the Neuroanatomical Profiles of Reward Pathways with Markers of Neuronal Activation

Functional neuroanatomical tools have played an important role in proposing which structures underlie brain stimulation reward circuitry. This review focuses on studies employing metabolic markers of neuronal and glial activation, including 2-deoxyglucose, cytochrome oxidase, and glycogen phosphorylase, and a marker of cellular activation, the immediate early gene c-fos. The principles underlying each method, their application to the study of brain stimulation reward, and their strengths and limitations are described. The usefulness of this strategy in identifying candidate structures, and the degree of overlap in the patterns of activation arising from different markers is addressed in detail. How these data have contributed to an understanding of the organization of reward circuitry and directed our thinking towards an alternative framework of neuronal arrangement is discussed in the final section.

Activation of 5-HT2 receptors induces glycogenolysis in the rat brain

The effect of 5-HT(2) receptor activation on brain glycogen and the extracellular concentration of glucose was investigated in the present study. An injection of 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) (2 mg/kg, i.p.) or mescaline (10 mg/kg, i.p.) at an ambient temperature of 29 degrees C produced a 35-45% decrease in brain glycogen that persisted for at least 2 h. DOI also increased the extracellular concentration of glucose in the striatum by 60%. Maintenance of rats at 22 degrees C significantly attenuated DOI-induced glycogenolysis, as well as DOI-induced hyperthermia, and the increase in the extracellular concentration of glucose in the striatum. DOI-induced hyperthermia, glycogenolysis and increase in the extracellular concentration of glucose also were attenuated in rats treated with the 5-HT(2) receptor antagonist, 6-methyl-1-(methylethyl)-ergoline-8beta-carboxylic acid 2-hydroxy-1-methylpropyl ester maleate (LY-53,857) (3 mg/kg, ip). These results support the conclusion that 5-HT(2) receptor activation promotes glycogenolysis and that hyperthermia exerts a prominent role in this process.

High glycogen levels in brains of rats with minimal environmental stimuli: implications for metabolic contributions of working astrocytes

The concentration of glycogen, the major brain energy reserve localized mainly in astrocytes, is generally reported as about 2 or 3 micromol/g, but sometimes as high as 3.9 to 8 micromol/g, in normal rat brain. The authors found high but very different glycogen levels in two recent studies in which glycogen was determined by the routine amyloglucosidase procedure in 0.03N HCl digests either of frozen powders (4.8 to 6 micromol/g) or of ethanol-insoluble fractions (8 to 12 micromol/g). To evaluate the basis for these discrepant results, glycogen was assayed in parallel extracts of the same samples. Glycogen levels in ethanol extracts were twice those in 0.03N HCl digests, suggesting incomplete enzyme inactivation even with very careful thawing. The very high glycogen levels were biologically active and responsive to physiologic and pharmacological challenge. Glycogen levels fell after brief sensory stimulation, and metabolic labeling indicated its turnover under resting conditions. About 95% of the glycogen was degraded under in vitro ischemic conditions, and its "carbon equivalents" recovered mainly as glc, glc-P, and lactate. Resting glycogen stores were reduced by about 50% by chronic inhibition of nitric oxide synthase. Because neurotransmitters are known to stimulate glycogenolysis, stress or sensory activation due to animal handling and tissue-sampling procedures may stimulate glycogenolysis during an experiment, and glycogen lability during tissue sampling and extraction can further reduce glycogen levels. The very high glycogen levels in normal rat brain suggest an unrecognized role for astrocytic energy metabolism during brain activation.

Metabolic signaling between neurons and glial cells: a short review

There is convincing evidence that astrocytes transform blood-born glucose to lactate, alpha-Keto-glutarate and alanine and supply the neurons. There is a tight regulation of this metabolic coupling by means of chemical signals released by functioning neurons. Previous, pioneer, studies have explored several signals-candidates the major being K(+), Ca(++) and several neuromodulators. However, recent results of numerous studies identify glutamate as the major signal that traffics between excited neurons and astrocytes. The excited neurons also produce and release NH(4)(+) in the extracellular space. Both glutamate and ammonium are taken up preferentially by astrocytes and form glutamine. Ammonia fixation by glutamine synthase controls the amount of lactate, glutamine and alanine produced and released by Muller cells in the extracellular space and then taken up by neurons. Thus, there is a tight coupling between function and metabolism in the central neurons system.

3,4-Methylenedioxymethamphetamine produces glycogenolysis and increases the extracellular concentration of glucose in the rat brain

Oxidative and/or bioenergetic stress is thought to contribute to the mechanism of neurotoxicity of amphetamine derivatives, e.g., 3,4-methylenedioxymethamphetamine (MDMA). In the present study, the effect of MDMA on brain energy regulation was investigated by examining the effect of MDMA on brain glycogen and glucose. A single injection of MDMA (10-40 mg/kg, s.c.) produced a dose-dependent decrease (40%) in brain glycogen, which persisted for at least 1 h. MDMA (10 and 40 mg/kg, s.c.) also produced a significant and sustained increase in the extracellular concentration of glucose in the striatum. Subjecting rats to a cool ambient temperature of 17 degrees C significantly attenuated MDMA-induced hyperthermia and glycogenolysis. MDMA-induced glycogenolysis also was prevented by treatment of rats with the 5-hydroxytryptamine(2) (5-HT(2)) antagonists 6-methyl-1-(1-methylethyl)-ergoline-8 beta-carboxylic acid 2-hydroxy-1 methylprophyl ester maleate (LY-53,857; 3 mg/kg i.p.), desipramine (10 mg/kg i.p.), and iprindole (10 mg/kg i.p.). LY-53,857 also attenuated the MDMA-induced increase in the extracellular concentration of glucose as well as MDMA-induced hyperthermia. Amphetamine analogs (e.g., methamphetamine and parachloroamphetamine) that produce hyperthermia also produced glycogenolysis, whereas fenfluramine, which does not produce hyperthermia, did not alter brain glycogen content. These results support the conclusion that MDMA induces glycogenolysis and that the process involves 5-HT(2) receptor activation. These results are supportive of the view that MDMA promotes energy dysregulation and that hyperthermia may play an important role in MDMA-induced alterations in cellular energetics.

A single dose of the serotonin neurotransmission agonist paroxetine enhances motor output: double-blind, placebo-controlled, fMRI study in healthy subjects

Since serotonin (5-HT) stimulates motor function, pharmacological potentiation of 5-HT neurotransmission may improve motor function in healthy subjects and, possibly, recovery in post-stroke patients. Indeed, fluoxetine, a selective serotonin reuptake inhibitor (SSRI), increased activation in executive motor areas of healthy subjects as fenozolone, a releaser of monoamines (including noradrenaline, dopamine, and serotonin) from intracellular stores. This study is intended to test the hypothesis that paroxetine can likewise modulate brain motor activity in a dose-dependent manner in healthy subjects. In a double-blind counterbalanced study, six subjects underwent functional MRI examinations on three sessions 1 week apart (E1, E2, and E3) at the time of peak plasma concentrations (5 h after drug intake, i.e., either 20 or 60 mg of paroxetine or placebo) with a complex sequential opposition task. Rest and activation alternated in a block design. During activation, subjects performed, with the right hand, a 1-Hz-paced task that alternated two fist closings with a sequential opposition task. Paroxetine elicited effects similar to those reported for fluoxetine; notable changes were hyperactivation in the contralateral S1/M1, and posterior SMA and widespread hypoactivation of basal ganglia and cerebellum. There was an inverse correlation between dose and effect: significantly greater effects were observed with the 20-mg dose compared with 60 mg. Paroxetine dose-dependently modulates activation of the entire motor pathway in a way that favors motor output. Thus, a single dose of the SSRI paroxetine reorganized motor processing.

5-ht6 receptors as emerging targets for drug discovery

5-ht6 receptors are the latest serotonin receptors to be identified by molecular cloning. Their high affinity for a wide range of drugs used in psychiatry, coupled with their intriguing distribution in the brain, has stimulated significant interest. Antisense oligonucleotides, antipeptide antibodies, selective radioligands, knockout mice, and selective antagonists of the 5-ht6 receptor have recently become available. Surprisingly, 5-ht6 receptors appear to regulate cholinergic neurotransmission in the brain, rather than the expected interaction as modulators of dopaminergic transmission. This interaction predicts a possible role for 5-ht6 receptor antagonists in the treatment of learning and memory disorders. Furthermore, polymorphisms in the sequence of the 5-ht6 receptor gene may provide a genetic tool to further our understanding of the differential responses of patients to antipsychotic medications.

L-Glutamate and insulin enhance glycogen synthesis in cultured astrocytes from the rat brain through different intracellular mechanisms

The effects of L-glutamate and insulin on glycogen synthesis in astrocytes were examined. L-Glutamate and insulin both stimulated glycogen synthesis in primary cultures of rat astrocytes in a dose-dependent manner, as measured by the incorporation of 14C from [14C]glucose into glycogen. D-Aspartate also increased the incorporation of 14C into glycogen. When insulin and L-glutamate were added together, the glycogen synthesis as well as glycogen content of the cells was additively increased. Wortmannin, an inhibitor of phosphatidylinositol 3-kinase, had little effect on glycogen synthesis induced by L-glutamate, whereas it suppressed the insulin-induced glycogen synthesis. These results suggest that the insulin- and L-glutamate-induced glycogen syntheses are mediated by different intracellular mechanisms. In fact, insulin stimulated the conversion of glycogen synthase b to glycogen synthase a, which was suppressed by wortmannin. L-Glutamate and D-aspartate, however, did not increase the level of glycogen synthase a activity. By contrast, L-glutamate increased 2-deoxy-D-[3H]glucose uptake by the astrocytes, whereas insulin did not affect the uptake. These results suggest that insulin stimulates glycogen synthesis in astrocytes by activating glycogen synthase, which is dependent on a wortmannin-sensitive signaling pathway. L-Glutamate, however, enhances the glucose uptake, which contributes to the increase in glycogen synthesis in the cells.

Serotonin in the regulation of brain microcirculation

Manipulation of brainstem serotonin (5-HT) raphe neurons induces significant alterations in local cerebral metabolism and perfusion. The vascular consequences of intracerebrally released 5-HT point to a major vasoconstrictor role, resulting in cerebral blood flow (CBF) decreases in several brain regions such as the neocortex. However, vasodilatations, as well as changes in blood-brain barrier (BBB) permeability, which are blocked by 5-HT receptor antagonists also can be observed. A lack of relationship between the changes in flow and metabolism indicates uncoupling between the two variables and is suggestive of a direct neurogenic control by brain intrinsic 5-HT neurons on the microvascular bed. In line with these functional data are the close associations that exist between 5-HT neurons and the microarterioles, capillaries and perivascular astrocytes of various regions but more intimately and/or more frequently so in those where CBF is altered significantly following manipulation of 5-HT neurons. The ability of the microvascular bed to respond directly to intracerebrally released 5-HT is underscored by the expression of distinct 5-HT receptors in the various cellular compartments of the microvascular bed. Thus, it appears that while some 5-HT-mediated microvascular functions involve directly the blood vessel wall, others would be relayed through the perivascular astrocyte. The strategic localization of perivascular astrocytes and the different 5-HT receptors that they harbor strongly emphasize their putative pivotal role in transmitting information between 5-HT neurons and microvessels. It is concluded that the cerebral circulation has full capacity to adequately and locally adapt brain perfusion to changes in central 5-HT neurotransmission either directly or indirectly via the neuronal-astrocytic-vascular tripartite functional unit. Dysfunctions in these neurovascular interactions might result in perfusion deficits and might be involved in specific pathological conditions.

Pharmacological characteristics of potassium-induced, glycogenolysis in astrocytes

Elevated extracellular concentrations of the potassium ion ([K+]o) stimulate glycogenolysis in primary cultures of mouse astrocytes that have been grown in the presence of dibutyryl cyclic AMP but not in corresponding cultures which have not been treated in this manner. The response is potently inhibited by nifedipine, suggesting that it is evoked by entry of calcium ions through voltage dependent L-channels. The benzodiazepine midazolam, which is known to enhance calcium entry at concentrations of [K+]o causing submaximum calcium entry, increases the glycogenolytic effect by such levels of [K+]o.

Medial forebrain bundle stimulation in rats activates glycogen phosphorylase in layers 4, 5b and 6 of ipsilateral granular neocortex

Functional activation in human brain produces an increase in glycolytic metabolism. Animal studies suggest activation-induced glycolysis is coupled to brain glycogenolysis. Medial forebrain bundle (MFB) stimulation activates the release of neurotransmitters which promote neocortical glycogenolysis in vitro. In the present study, active glycogen phosphorylase (GP), an index of glycogenolysis, is assessed histochemically in rat brain after 15 min of MFB self-stimulation. Active GP increased significantly in layers 4, 5b and 6 of granular neocortex ipsilateral to MFB self-stimulation. Restriction of increased glycogenolysis to granular neocortex suggests an important functional interaction between sensory neocortical processing and ascending MFB systems.

Activation of glycogen phosphorylase by serotonin and 3,4-methylenedioxymethamphetamine in astroglial-rich primary cultures: involvement of the 5-HT2A receptor

Neurotransmitters, neuropeptides, and ions regulate glycogen levels in the brain by modulating the activity of glycogen synthase (GSase) and glycogen phosphorylase (GPase). GPase is co-localized with glial fibrillary acidic protein (GFAP), an astroglia-specific marker, suggesting that glycogen is localized in astroglial cells. Additionally, functional serotonin (5-HT) receptors are found in both neurons and glia, and 5-HT is known to stimulate glycogenolysis. It is reported that 3,4-methylenedioxymethamphetamine (MDMA), a drug of abuse, stimulates the release and inhibits the reuptake of 5-HT, and selectively inhibits the activity of MAO-A. These biochemical consequences of MDMA lead to increased extra-cellular 5-HT levels. This study investigates the effects of MDMA(+) and serotonin (5-HT) on glycogen metabolism in the rat brain. A histochemical method was designed to visualize active glycogen phosphorylase (GPase) in an astroglial-rich primary culture. Serotonin activated GPase in a concentration-dependent manner (100 nM-100 microM). Maximal activation by 5-HT was achieved by 50 microM and resulted in a 167% increase in the number of reactive sites (P < 0.001). MDMA(+) (500 nM-50 microM) directly stimulated GPase activity with maximal activation induced by 5 microM, which caused a 70% increase in the number of reactive sites (P < 0.001). The 5-HT2 receptor agonist, 1-(2,5-dimethoxy-4-bromophenyl)-2-aminopropane (DOB), also displayed a concentration-dependent increase in the number of GPase reactive sites. Maximal stimulation by DOB occurred at 100 nM which increased the number of reactive sites by 166% (P < 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Daily variation in active glycogen phosphorylase patches in the molecular layer of rat dentate gyrus

A larger number of discrete patches of active glycogen phosphorylase (alpha GP) were found in the molecular layer of the hippocampal dentate gyrus in rats sacrificed during the daily dark phase (mean = 28.7/section) than during the light phase (mean = 7.8/section). Light-dark differences in the patterns of alpha GP may reflect circadian differences in metabolic demand in the hippocampus. Patch sizes were consistent with increased activation of single astrocytes or perisynaptic astrocyte clusters by focal input at night.

Glycogen in astrocytes: possible function as lactate supply for neighboring cells

In order to contribute to the elucidation of the function of astrocyte glycogen in brain, studies on the fate of the glucosyl residues of glycogen were carried out on astroglia-rich primary cultures derived from the brains of newborn rats. On glucose deprivation astroglial cells rapidly deplete their glycogen. In contrast to the situation with hepatocytes, only lactate, but not glucose, is detectable in the medium surrounding the astroglial cells. Besides glucose, astroglial cultures can also use mannose as a substrate for the synthesis of glycogen and the generation of lactate. Although mannose-fed astroglial cells contain glucose-6-phosphate, they do not release a measurable amount of glucose into the culture medium. Instead of glucose the astroglial cells release high amounts of lactate into the culture medium. Gluconolactone or 2-deoxyglucose which prevent glycogen breakdown in astroglial cells after glucose deprivation, allow to discriminate between lactate generated from glycogen and lactate from other sources. The amount of lactate found in the medium in the absence of gluconolactone (or 2-deoxyglucose) exceeds the amount found in the presence of either compound by the lactate equivalents calculated to be contained in the cellular glycogen. In conclusion, glycogen in astrocytes can be considered as a store for lactate rather than for glucose.

Molecular cloning, characterization, and localization of a high-affinity serotonin receptor (5-HT7) activating cAMP formation

By using a strategy based on nucleotide sequence homology, we have cloned a cDNA encoding a functional serotonin (5-HT) receptor. The deduced amino acid sequence of the 5-HT7 receptor displays limited homology with that of other 5-HT receptors. In addition to the seven stretches of hydrophobic amino acids that characterize the superfamily of receptors interacting with guanine nucleotide-binding proteins, the 448-aa sequence of the 5-HT7 receptor contains a hydrophobic domain located at its N-terminal end. Genomic analysis indicated the presence of introns interrupting the coding sequence. The 5-HT7 receptor, stably expressed in transfected CHO cells, bound [3H]5-HT with high affinity (Kd = 1 nM), like receptors of the 5-HT1 subfamily from which, however, it was clearly distinguished by its pharmacology. 5-HT in nanomolar concentrations stimulated cAMP accumulation in these CHO cells by approximately 10-fold, whereas lysergic acid diethylamide displayed low intrinsic agonist activity. These various properties differentiate the 5-HT7 receptor from the four other subfamilies of mammalian 5-HT receptors (i.e., the 5-HT1-, 5-HT2-, 5-HT3-, and 5-HT4-like subfamilies) and, therefore, appear to define another receptor subfamily. Northern blot and in situ hybridization analyses showed the 5-HT7 transcripts to be expressed in discrete areas of the limbic brain (e.g., pyramidal hippocampus cells, tenia tecta, amygdaloid, or mammillary nuclei), suggesting that the receptor mediates serotoninergic controls in functions like mood, learning, or neuroendocrine and vegetative behaviors.

Glucose, insulin, and insulin-like growth factor I regulate the glycogen content of astroglia-rich primary cultures

The glycogen content of astroglia-rich primary cultures derived from the brains of newborn rats depends on the concentration of glucose in the culture medium. After administration of culture medium lacking glucose, the glycogen content decreases with a half-time of 7 min. Readdition of glucose results in replenishment of the glycogen stores within 2-3 h, but fully only if glucose is present in a concentration of at least 4 mM. Insulin, or the more potent insulin-like growth factor I, increases the content of glycogen approximately 1.7-fold, with the half-maximal effects being attained at concentrations of 10 and 0.5 nM, respectively. These results suggest that (a) glucose or a metabolite of it and (b) insulin-like growth factor I or a closely related peptide, but not insulin, are likely to be physiological regulators of the level of glycogen in astrocytes.

Effect of adrenergic agonists on glycogenolysis in primary cultures of astrocytes

A stimulation of glycogenolysis in astrocytes by adrenergic agonists has repeatedly been demonstrated in the literature. However, some confusion exists regarding which type of adrenergic receptor subtype is involved, and little information is available about rates of glycogenolysis and potencies of adrenergic agonists. In the present study, we have investigated these parameters using primary cultures of mouse astrocytes which constitute a reliable model for their in vivo counterparts. Antagonists as well as agonist studies revealed that noradrenaline acts both on a beta- and on an alpha 2-receptor. Isoproterenol and clonidine, agonists acting relatively specifically on only one of these receptor subtypes could, on their own, stimulate glycogenolysis and the effect by noradrenaline could be inhibited by alprenolol (beta-adrenergic antagonist) and/or yohimbine (alpha 2-adrenergic antagonist) but not by prazosin (alpha 1-adrenergic antagonist). Excess potassium also stimulated glycogenolysis but this effect was not antagonized by adrenergic antagonists, alone or in combination. The involvement of an alpha 2-adrenergic receptor in a homogeneous culture of astrocytes provides proof that not all alpha 2-adrenergic receptors in brain are presynaptic. The maximum rate of stimulated glycogenolysis was calculated to be 3-7 nmol/min per mg protein. Computer analysis showed that the EC50 values for noradrenaline, isoproterenol and clonidine were 4.6 x 10(-8) M, 3.0 x 10(-7) M, and 6.5 x 10(-7) M, respectively.

Brain isozyme of glycogen phosphorylase: immunohistological localization within the central nervous system

An antibody specific for the predicted carboxyterminal sequence of the human brain isozyme of glycogen phosphorylase (alpha-1,4-D-glucan:orthophosphate D-glucosyltransferase, EC 2.4.1.1) was generated to verify the carboxyterminal amino acid sequence of this protein. The isozyme-specific antibody was used to examine the localization of this protein in primate and non-primate brain. The highest levels of the brain isozyme in cerebrum and cerebellum were found in fibrous astrocytes, many with glial processes that appear to terminate upon blood vessels.

Correlation between carbohydrate and catecholamine level impairments in methionine sulfoximine epileptogenic rat brain

This work shows that the convulsant methionine sulfoximine induces an increase in glucose and glycogen levels and a parallel decrease in norepinephrine and dopamine levels in rat brain. Among the epileptogenic agents, methionine sulfoximine is known to have a glycogenic property in the central nervous system. The aim of this work is to look for the neurochemical mechanism underlying this property. For this, catecholamines, glucose, and glycogen were measured at the same time in different areas of the brain in rats submitted to methionine sulfoximine. The convulsant induced an increase in glucose and glycogen levels as previously described and a decrease in dopamine and norepinephrine levels in all the areas of the rat brain. These changes were roughly dose dependent. When L-dihydroxyphenylalanine and benserazide (a decarboxylase inhibitor) were administered with methionine sulfoximine, the latter failed to induce seizures in rat up to 8 h after dosing. Moreover, the glucose and glycogen amounts did not increase. In all these experiments, there was an obvious evidence of parallelism between seizures, increase in carbohydrate levels, and decrease in catecholamine levels. These results allow to conclude that the glycogenic property of methionine sulfoximine in the central nervous system probably results from its ability to decrease norepinephrine and dopamine levels. Because the effect of the convulsant on the catecholamine levels persisted for long, it is normal that glucose and glycogen levels increased during preconvulsive, convulsive and postconvulsive period. Methionine sulfoximine is probably glycogenic in rat brain because it decreases catecholamine levels for a long time.

Possible involvement of indolamines in the glycogenic effect of the convulsant methionine sulfoximine in rat brain

The aim of the present investigation was to look for the mechanisms causing disturbances in carbohydrate metabolism during the action of the epileptogenic agent methionine sulfoximine. The levels of glucose, glycogen, and indolamines were measured in seven different regions of rat brain. Methionine sulfoximine induced a decrease in serotonin level which was roughly dose-dependent. There were no obvious changes in tryptophan and 5-hydroxyindoleacetic levels in any area. Methionine sulfoximine induced the known increase in glucose and glycogen levels. The direct precursor of serotonin. 5-hydroxytryptophan, and benserazide (a decarboxylase inhibitor) were then injected into rats in association with methionine sulfoximine. In this case, methionine sulfoximine failed to induce seizures. Moreover, the serotonin level was unchanged and the carbohydrate content did not significantly increase. There was only a rise in 5-hydroxyindoleacetic acid level. This work shows a striking parallelism between serotonin decrease and glycogen increase.

Agonist-induced glycogenolysis in rabbit retinal slices and cultures

1. The effects of different putative retinal transmitters and/or modulators on glycogenolysis in rabbit retinal slices and in retinal Müller cell cultures were examined. 2. Incubation of rabbit retinal slices or primary retinal cultures (either 3-5 day-old or 25-30 day-old) in a buffer solution containing [3H]-glucose resulted in the accumulation of newly synthesized [3H]-glycogen. 3. Noradrenaline (NA), isoprenaline, vasoactive intestinal peptide (VIP), 5-hydroxytryptamine (5-HT) and 8-hydroxy-dipropylaminetetralin (8-OH-DPAT) stimulated the hydrolysis of this newly formed 3H-polymer. The potency order of maximal stimulations was: VIP greater than NA greater than isoprenaline greater than 5-HT greater than 8-OH-DPAT. 4. The putative retinal transmitters, dopamine, gamma-aminobutyric acid (GABA), glycine and taurine and the muscarinic agonist carbachol (CCh) had no effect on [3H]-glycogen content. 5. The glycogenolytic effects of NA/isoprenaline and 5-HT/8-OH-DPAT appear to be mediated by beta-adrenoceptors and 5-HT1 receptors (possibly 5-HT1A), respectively while the VIP-induced response involved another receptor subtype. 6. Agonists which mediated [3H]-glycogen hydrolysis also stimulated an increase in adenosine 3':5'-cyclic monophosphate (cyclic AMP) formation. Both responses are blocked to a similar extent by the same antagonists and so are probably mediated via the same receptor subtypes. Moreover, dibutyryl cyclic AMP (db cyclic AMP) promoted tritiated glycogen breakdown in the three retinal preparations. 7. Not all receptors linked to cyclic AMP production however promote glycogenolysis. Dopamine and apomorphine stimulated cyclic AMP formation via D1-receptors without influencing glycogenolysis. These receptors are exclusively associated with neurones.

Age-dependent supersensitivity to the glycogenolytic effect of K+ in the cerebral cortex of the spontaneously epileptic quaking mouse mutant

K+, at concentrations reached in the extracellular space during neuronal activity (5-10 mM), promotes a time- and concentration-dependent hydrolysis of [3H]glycogen newly synthesized by mouse cerebral cortical slices. In the present study, the glycogenolytic action of K+ was examined in the neocortex of the quaking mouse, a spontaneously epileptic mutant characterized by deficient myelination of the CNS. The potency and efficacy of K+ in eliciting glycogen hydrolysis was greatly enhanced in cerebral cortical slices prepared from homozygous quaking mice (qk/qk) older than 7 weeks of age, indicating a supersensitive response to a metabolic action of the ion. A detailed ontogenic analysis showed an evolution of the supersensitive response to K+ which is reminiscent of the previously described increase in the number of alpha 2-adrenoreceptors in the brainstem of this mutant. In contrast to the altered response to K+, the glycogenolytic action of noradrenaline and vasoactive intestinal peptide reported earlier was equally expressed in qk/qk and in their unaffected littermates.

Effects of neurotransmitters on astrocyte glycogen stores in vitro

We have used receptor binding assays to determine the presence of three neurotransmitter receptors in a crude membrane fraction derived from neonatal rat cortical astrocyte cultures and subsequently determined the effects of transmitter receptor activation on astrocyte glycogen content in vitro. beta-Adrenergic (KD = 88 pM; Bmax = 51 fmol/mg of protein), serotonin (KD = 70 nM; Bmax = 44 pmol/mg of protein), and muscarinic cholinergic receptors (KD = 79 pM; Bmax = 44 fmol/mg of protein) were found to be present on astrocyte membranes using [3H]dihydroalprenolol, [3H]serotonin, and [3H]quinuclidinyl benzilate, respectively, as ligands. Astrocyte cultures exposed to noradrenaline but not specific alpha- and beta-receptor agonists contained 33% less glycogen than controls. Neither serotonin nor carbachol caused alterations in astrocyte glycogen content under normal conditions. Reserpine-treated cultures, however, responded to serotonin with a 28% decrease in glycogen content and contained higher levels of glycogen than non-reserpine-treated controls (a 55% increase). These results show that both noradrenaline and serotonin can evoke astrocyte glycogenolysis and that noradrenergic control of glycogen metabolism is probably exerted through both alpha- and beta-receptors. Neurotransmitter control of astrocyte glycogen turnover may represent a form of neuron-astrocyte signalling in addition to that provided by changes in external potassium concentration.

Spatial relationship of histochemically demonstrable patches in the mouse superior colliculus

Patches of high phosphorylase activity are found in the intermediate and dorsal deep grey layers of the mouse superior colliculus when either coronal or sagittal sections are cut. These patches indicate that the phosphorylase a activity is arranged in a continuous lattice composed of bands of high phosphorylase a activity with a width of 100-200 microns that surround pale islands of low activity. This lattice was demonstrated by cutting surface parallel sections through the partially flattened superior colliculus. An almost identical lattice is observed in sections incubated to demonstrate total phosphorylase or cytochrome oxidase (CYO) activity. This phosphorylase/CYO lattice extends over the entire area of the superior colliculus. A discontinuous staining pattern is also observed in the intermediate and deep grey layers of both sagittal and coronal sections incubated for acetylcholinesterase (AChE) activity. The staining is arranged in two discontinuous sheets of intense activity that are joined together by vertical streamers. In surface parallel sections the AChE activity is found to form a network pattern which extends over the entire extent of the superior colliculus but which becomes fainter at the anterior pole. The phosphorylase/CYO lattice is not in register with the AChE lattice and the two seem to be organized independently of each other despite occurring at the same depth.

Intercellular communication mediated by VIP in the cerebral cortex

A growing number of biologically active peptides is being identified within the central nervous system (CNS). According to currently accepted criteria, several of these peptides, including VIP, can be viewed as neurotransmitters. Recent immunohistochemical and pharmacological investigations have been directed at the characterization of the position of VIP neurons in the circuitry of the cerebral cortex. From these studies some views on the possible function of VIP neurons in this CNS region are beginning to emerge. In the cerebral cortex, VIP neurons constitute a rather homogeneous population of intracortical, bipolar and radially oriented cells, which arborize locally, within cortical columns of 60-100 micron diameter. The cellular actions of VIP in the cerebral cortex include the stimulation of cAMP formation and of glycogen breakdown. A certain degree of cellular resolution of these two actions of VIP has been achieved by using purified preparations. Thus VIP stimulates cAMP formation in cerebral microvessels and in cultured astrocytes; in this latter cell type VIP also promotes glycogenolysis. Furthermore, VIP interacts synergistically with norepinephrine to stimulate cAMP formation and to inhibit the firing rate of spontaneously active identified cortical neurons. VIP neurons appear therefore to be strategically positioned to regulate the coupling between energy metabolism, blood flow and neuronal activity with great spatial selectivity and at a fine level of cortical resolution.

Serotonin receptors in parasitic worms

It is evident from the above review that during the last two decades a great deal of interest in investigating the action of serotonin in parasitic worms has been shown by parasitologists as well as by scientists from several other disciplines. What we have initially reported concerning the effect of serotonin on motility and carbohydrate metabolism of F. hepatica has been pursued on several other parasitic worms. The studies so far indicate that serotonin stimulates motility of every species tested among the phylum Platyhelminthes. The indoleamine also stimulates glycogenolysis in the few flatworm parasites that have been investigated. The information in nematodes is scanty and the role of serotonin in these parasites is still open for experimentation. Recent biochemical investigations on F. hepatica and S. mansoni demonstrated that serotonin and related compounds utilize a common class of receptors in plasma membrane particles which I designate as 'serotonin receptors'. These receptors are linked to an adenylate cyclase that catalyses the synthesis of the second messenger, cyclic 3',5'-AMP. Serotonin and its congeners increase the concentration of cyclic AMP in intact parasites whereas antagonists inhibit such an effect. Cyclic AMP stimulates glycogenolysis, glycolysis and some rate-limiting glycolytic enzymes. It activates a protein kinase that may be involved in activation of glycogen phosphorylase and phosphofructokinase. Serotonin-activated adenylate cyclase in S. mansoni is activated early in the life of the schistosomule. The possibility is discussed that the availability of cyclic AMP through serotonin activation in these parasites may be a prelude to the development processes that take place in the parasite. The different components of the serotonin-activated adenylate cyclase in the parasite are the same as those that have been previously described for the host. Binding characteristics of the receptors indicate that the receptors in F. hepatica appear to be different from those that have been described in the host. The discovery of these receptors and their differences from those in the host offer a new site which is amenable to pharmacological manipulation. The search for new agents that influence serotonin receptors in these parasites could be included in a strategy for the development of new chemotherapeutic agents against these parasites.

Corticosterone prevents the increase in noradrenaline-stimulated adenyl cyclase activity in rat hippocampus following adrenalectomy or metopirone

Corticosterone modulation of the noradrenaline-responsive cyclic AMP generating system was examined in rat hippocampus. Adrenalectomy was found to produce a small but significant elevation in the rate of cyclic AMP formation in response to noradrenaline. Implantation of corticosterone pellets 5 days prior to sacrifice prevented this adrenalectomy-induced increase. Metopirone, an inhibitor of corticosterone synthesis, was also observed to increase cyclic AMP formation. This elevation was seen 2 h following a 50 mg/kg i.p. injection and was completely prevented by corticosterone pellet implantation. Metopirone had no significant effect on cyclic AMP production after 1 h, while a slight but statistically non-significant elevation remained at 4 h. These observations parallel the inhibitory effect of Metopirone on corticosterone synthesis as determined by serum corticosterone levels.

The distribution and morphological characteristics of the intracortical VIP-positive cell: an immunohistochemical analysis

Using a sensitive immunohistochemical procedure, we have undertaken a detailed morphological characterization of the vasoactive intestinal peptide-positive (VIP-positive) neuron in the cerebral cortex of the rat. VIP-positive neurons are present in all regions of cortex, and are usually strongly bipolar, possessing long, radially directed processes with very limited branching in the tangential plane. The most extensive dendritic branching occurs in layers I and deep IV-superficial V, and the density of axonal varicosities is highest in layers II-IV. In the visual cortex, approximately 50% of the labeled cell bodies are in layer II and III and 80% of the labeled cell bodies are contained in layers I-IV (superficial 600 microns of cortex). In order to determine the density and 3-dimensional distribution pattern of these cells, we prepared serial tangential sections through the rat visual cortex, mapped the distribution of all labeled cells in each section on transparent acetate sheets, and compressed these superimposed maps. This analysis demonstrated that: (1) approximately 1% of the cortical neurons are VIP-positive, (2) their distribution is fairly uniform and statistically random, (3) there are no large areas (i.e. with a diameter greater than 100 microns) that lack a VIP-positive cell, (4) on the average, there is one VIP-positive cell per column of 30 microns diameter, and (5) the average nearest neighbor distance on the compressed display is 15 microns. Given the morphological characteristics of VIP-positive cells, these data indicate that each VIP-containing cell is identified with a unique radial volume, which is generally between 15 and 60 microns in diameter, and overlaps with the contiguous domains of neighboring VIP-positive cells. These morphological data support the notion that VIP-containing neurons play an important functional role within radially oriented columns of cerebral cortex.