Peptide receptors on astrocytes

Transcriptomic alterations in cortical astrocytes following the development of post-traumatic epilepsy

Post-traumatic epilepsy (PTE) stands as one of the numerous debilitating consequences that follow traumatic brain injury (TBI). Despite its impact on many individuals, the current landscape offers only a limited array of reliable treatment options, and our understanding of the underlying mechanisms and susceptibility factors remains incomplete. Among the potential contributors to epileptogenesis, astrocytes, a type of glial cell, have garnered substantial attention as they are believed to promote hyperexcitability and the development of seizures in the brain following TBI. The current study evaluated the transcriptomic changes in cortical astrocytes derived from animals that developed seizures as a result of severe focal TBI. Using RNA-Seq and ingenuity pathway analysis (IPA), we unveil a distinct gene expression profile in astrocytes, including alterations in genes supporting inflammation, early response modifiers, and neuropeptide-amidating enzymes. The findings underscore the complex molecular dynamics in astrocytes during PTE development, offering insights into therapeutic targets and avenues for further exploration.

Dexamethasone increases production of C-type natriuretic peptide in the sheep brain

Although C-type natriuretic peptide (CNP) has high abundance in brain tissues and cerebrospinal fluid (CSF), the source and possible factors regulating its secretion within the central nervous system (CNS) are unknown. Here we report the dynamic effects of a single IV bolus of dexamethasone or saline solution on plasma, CSF, CNS and pituitary tissue content of CNP products in adult sheep, along with changes in CNP gene expression in selected tissues. Both CNP and NTproCNP (the amino-terminal product of proCNP) in plasma and CSF showed dose-responsive increases lasting 12-16 h after dexamethasone, whereas other natriuretic peptides were unaffected. CNS tissue concentrations of CNP and NTproCNP were increased by dexamethasone in all of the 12 regions examined. Abundance was highest in limbic tissues, pons and medulla oblongata. Relative to controls, CNP gene expression (NPPC) was upregulated by dexamethasone in 5 of 7 brain tissues examined. Patterns of responses differed in pituitary tissue. Whereas the abundance of CNP in both lobes of the pituitary gland greatly exceeded that of brain tissues, neither CNP nor NTproCNP concentration was affected by dexamethasone, despite an increase in NPPC expression. This is the first report of enhanced production and secretion of CNP in brain tissues in response to a corticosteroid. Activation of CNP secretion within CNS tissues by dexamethasone, not exhibited by other natriuretic peptides, suggests an important role for CNP in settings of acute stress. Differential findings in pituitary tissues likely relate to altered processing of proCNP storage and secretion.

Diencephalic and septal structures containing the avian vasotocin receptor (V1aR) involved in the regulation of food intake in chickens, Gallus gallus

Recently, it was found that the avian central vasotocin receptor (V1aR) is associated with the regulation of food intake. To identify V1aR-containing brain structures regulating food intake, a selective V1aR antagonist SR-49059 that induced food intake was administrated intracerebroventricularly in male chickens followed by detection of brain structures using FOS immunoreactivity. Particularly, the hypothalamic core region of the paraventricular nucleus, lateral hypothalamic area, dorsomedial hypothalamic nucleus, a subnucleus of the central extended amygdalar complex [dorsolateral bed nucleus of the stria terminalis], medial septal nucleus and caudal brainstem [nucleus of the solitary tract] showed significantly increased FOS-ir cells. On the other hand, the supraoptic nucleus of the preoptic area and the nucleus of the hippocampal commissure of the septum showed suppressed FOS immunoreactivity in the V1aR antagonist treatment group. Further investigation revealed that neuronal activity of arginine vasotocin (AVT-ir) magnocellular neurons in the supraoptic nucleus, preoptic periventricular nucleus, paraventricular nucleus and ventral periventricular hypothalamic nucleus and most likely corticotropin releasing hormone (CRH-ir) neurons in the nucleus of the hippocampal commissure were reduced following the antagonist treatment. Dual immunofluorescence labeling results showed that perikarya of AVT-ir magnocellular neurons in the preoptic area and hypothalamus were colabeled with V1aR. Within the nucleus of the hippocampal commissure, CRH-ir neurons were shown in close contact with V1aR-ir glial cells. Results of the present study suggest that the V1aR plays a role in the regulation of food intake by modulating neurons that synthesize and release anorectic neuropeptides in the avian brain.

Multifactorial Effects on Different Types of Brain Cells Contribute to Ammonia Toxicity

Effects of ammonia on astrocytes play a major role in hepatic encephalopathy, acute liver failure and other diseases caused by increased arterial ammonia concentrations (e.g., inborn errors of metabolism, drug or mushroom poisoning). There is a direct correlation between arterial ammonia concentration, brain ammonia level and disease severity. However, the pathophysiology of hyperammonemic diseases is disputed. One long recognized factor is that increased brain ammonia triggers its own detoxification by glutamine formation from glutamate. This is an astrocytic process due to the selective expression of the glutamine synthetase in astrocytes. A possible deleterious effect of the resulting increase in glutamine concentration has repeatedly been discussed and is supported by improvement of some pathologic effects by GS inhibition. However, this procedure also inhibits a large part of astrocytic energy metabolism and may prevent astrocytes from responding to pathogenic factors. A decrease of the already low glutamate concentration in astrocytes due to increased synthesis of glutamine inhibits the malate-aspartate shuttle and energy metabolism. A more recently described pathogenic factor is the resemblance between NH₄⁺ and K⁺ in their effects on the Na⁺,K⁺-ATPase and the Na⁺,K⁺, 2 Cl^- and water transporter NKCC1. Stimulation of the Na⁺,K⁺-ATPase driven NKCC1 in both astrocytes and endothelial cells is essential for the development of brain edema. Na⁺,K⁺-ATPase stimulation also activates production of endogenous ouabains. This leads to oxidative and nitrosative damage and sensitizes NKCC1. Administration of ouabain antagonists may accordingly have therapeutic potential in hyperammonemic diseases.

Glial modulators as potential treatments of psychostimulant abuse

Glia (including astrocytes, microglia, and oligodendrocytes), which constitute the majority of cells in the brain, have many of the same receptors as neurons, secrete neurotransmitters and neurotrophic and neuroinflammatory factors, control clearance of neurotransmitters from synaptic clefts, and are intimately involved in synaptic plasticity. Despite their prevalence and spectrum of functions, appreciation of their potential general importance has been elusive since their identification in the mid-1800s, and only relatively recently have they been gaining their due respect. This development of appreciation has been nurtured by the growing awareness that drugs of abuse, including the psychostimulants, affect glial activity, and glial activity, in turn, has been found to modulate the effects of the psychostimulants. This developing awareness has begun to illuminate novel pharmacotherapeutic targets for treating psychostimulant abuse, for which targeting more conventional neuronal targets has not yet resulted in a single, approved medication. In this chapter, we discuss the molecular pharmacology, physiology, and functional relationships that the glia have especially in the light in which they present themselves as targets for pharmacotherapeutics intended to treat psychostimulant abuse disorders. We then review a cross section of preclinical studies that have manipulated glial processes whose behavioral effects have been supportive of considering the glia as drug targets for psychostimulant-abuse medications. We then close with comments regarding the current clinical evaluation of relevant compounds for treating psychostimulant abuse, as well as the likelihood of future prospects.

Electrophysiological and structural aspects in the frontal cortex after the bee (Apis mellifera) venom experimental treatment

The aim of this study is to evaluate the bioelectrical and structural-functional changes in frontal cortex after the bee venom (BV) experimental treatments simulating both an acute envenomation and a subchronic BV therapy. Wistar rats were subcutaneously injected once with three different BV doses: 700 μg/kg (T(1) group), 2100 μg/kg (T(3) group), and 62 mg/kg (sublethal dose-in T(SL) group), and repeated for 30 days with the lowest dose (700 μg/kg-in T(S) group). BV effects were assessed by electrophysiological, histological, histochemical, and ultrastructural methods. Single BV doses produced discharges of negative and biphasic sharp waves, and epileptiform spike-wave complexes. The increasing frequency of these elements suggested a dose-dependent neuronal hyperexcitation or irritation. As compared to the lower doses, the sublethal dose was responsible for a pronounced toxic effect, confirmed by ultrastructural data in both neurons and glial cells that underwent extensive, irreversible changes, triggering the cellular death. Subchronic BV treatment in T(S) group resulted in a slower frequency and increased amplitude of cortical activity suggesting neuronal loss. However, neurons were still stimulated by the last BV dose. Structural-functional data showed a reduced cellular density in frontal cortex of animals in this group, while the remaining neurons displayed both specific (stimulation of neuronal activity) and unspecific modifications (moderate alterations to necrotic phenomena). Molecular mechanisms involved in BV interactions with the nervous tissue are also discussed. We consider all these data very important for clinicians who manage patients with multiple bee stings, or who intend to set an appropriate BV therapy.

Astrocytic beta(2)-adrenergic receptors: from physiology to pathology

Evidence accumulates for a key role of the beta(2)-adrenergic receptors in the many homeostatic and neuroprotective functions of astrocytes, including glycogen metabolism, regulation of immune responses, release of neurotrophic factors, and the astrogliosis that occurs in response to neuronal injury. A dysregulation of the astrocytic beta(2)-adrenergic-pathway is suspected to contribute to the physiopathology of a number of prevalent and devastating neurological conditions such as multiple sclerosis, Alzheimer's disease, human immunodeficiency virus encephalitis, stroke and hepatic encephalopathy. In this review we focus on the physiological functions of astrocytic beta(2)-adrenergic receptors, and their possible impact in disease states.

Major depressive disorder viewed as a dysfunction in astroglial bioenergetics

For many years scientists and physicians have pondered upon the apparent connection between depressive disorder and diabetes mellitus. Several epidemiologic studies confirm that diabetics have increased incidence of depression, and vice versa. In addition: depressive, non-diabetic patients have several insulin- and glucose-metabolism disturbances, probably exerting a compensatory reaction to the malfunction in the depressed brain as these disturbances are normalised in remission. After the discovery of PET-scanning, such studies have shown that patients with depressive disorder have reduced glucose metabolism in frontal parts of the brain. The present hypothesis regards the PET findings as observations of the primary pathophysiology of depression. Furthermore: two studies of post mortem samples from depressed patients show reduced numbers of astroglia. This is in accordance to the mentioned insulin disturbances, as only astroglia, not neurons, have insulin-sensitive glucose metabolism. Hence: the astroglia, not necessarily the neurons, are proposed to be the type of cells in which the disease resides. Most probably depressive disorder is a multitude of diseases, explaining the apparent multitude of symptoms, and the fact that different patients do respond to different drugs. Therefore: one can only formulate the hypothesis by mentioning a common denominator to these specific malfunctions, namely: disturbed glucose metabolism in the depressed brain. The present paper reviews several findings and proposes that attenuated cerebral glucose metabolism in frontal parts of the brain, in the astroglia, is the cause of depressive disorder.

Glial Fibrillary Acidic Protein in Astrocytes in the Human Neocortex

Glial fibrillary acidic protein (GFAP), expressed in the brain by astrocytes, is one of the major immunocytochemical markers of these cells. The aim of the present work was to study the structure of GFAP-positive astrocytes in the human neocortex. Immunocytochemical confocal laser microscopy was used to characterize the main types of GFAP-immunoreactive astrocytes in the human neocortex. These were astrocytes of layer I, forming the superficial glial delimiting membrane, along with transmembrane astrocytes of layer I, with very long processes penetrating several layers of the cortex, astrocytes of the middle layers of the neocortex, mostly of the protoplasmic type and involved in forming perivascular delimiting membranes, and typical white matter fibrous astrocytes. These data may help unify assessments of histopathological processes seen in various types of gliosis in the CNS.

‘Neuro’-peptides in glia: Focus on NPY and galanin

The term neuropeptide was advanced by de Wied and collaborators in the early seventies. At that time, they defined neuropeptides as endogenous substances synthesized in nerve cells and involved in nervous system functions. Since then, several studies have revealed that the very same 'neuropeptides' are also expressed in non-neuronal cells. It is therefore generally accepted that the original definition of these peptides was too limited and, consequently, it has recently been revised. Among the non-neuronal cells that synthesize neuropeptides are several glial cell types.

Pituitary adenylate cyclase-activating polypeptide (PACAP) stimulates endozepine release from cultured rat astrocytes via a PKA-dependent mechanism

Astroglial cells synthesize and release endozepines, neuropeptides that are related to the octadecaneuropeptide ODN. Glial cells also express PACAP/VIP receptors. We have investigated the possible effect of PACAP on the release of ODN-like immunoreactivity (ODN-LI) by cultured rat astrocytes. Administration of PACAP27 and PACAP38 induced a concentration-dependent increase in secretion of ODN-LI whereas VIP was approximately 1000-fold less potent. The maximum effect of PACAP38 occurred after 5 min, then gradually declined during the next 10 min. The stimulatory effects of PACAP and VIP were abrogated by the PACAP antagonist PACAP6-38. PACAP38 stimulated cAMP formation, activated polyphosphoinositide turnover, and provoked calcium mobilization from IP3-sensitive pools. The PKA inhibitor H89 suppressed PACAP-induced secretion of ODN-LI, whereas PLC inhibitor U73122 and the PKC inhibitor chelerythrine had no effect. In contrast, U73122 restored the stimulatory action of PACAP on ODN-LI release and cAMP formation during prolonged (15 min) incubation with the peptide, and this effect was prevented by PMA. The present results demonstrate that PACAP stimulates endozepine release through activation of PAC1 receptors coupled to the AC/PKA pathway. Our data also show that activation of the PLC/PKC pathway down-regulates the effect of PACAP on endozepine release.

The Astrocentric Hypothesis: proposed role of astrocytes in consciousness and memory formation

Consciousness is self-awareness. This process is closely associated with attention and working memory, a special form of short-term memory, which is vital when solving explicit task. Edelman has equated consciousness as the "remembered present" to highlight the importance of this form of memory (G.M. Edelman, Bright Air, Brilliant Fire, Basic Books, New York, 1992). The majority of other memories are recollections of past events that are encoded, stored, and brought back into consciousness if appropriate for solving new problems. Encoding prior experiences into memories is based on the salience of each event (A.R. Damasio, Descartes' Error, G.P. Putnam's Sons, New York, 1994; G.M. Edelman, Bright Air, Brilliant Fire, Basic Books, New York, 1992). It is proposed that protoplasmic astrocytes bind attended sensory information into consciousness and store encoded memories. This conclusion is supported by research conducted by gliobiologist over the past 15 years.

Corticotropin releasing factor induces proliferation of cerebellar astrocytes

In the adult cerebellum, corticotropin releasing factor (CRF), that is localized in climbing fibers, mossy fibers, and a fine varicose plexus along the Purkinje cell layer, modulates the responsiveness of Purkinje cells to excitatory amino acids. During development, CRF has been detected in the primitive cerebellar anlage as early as embryonic day (E)10, and is continuously expressed throughout embryonic and postnatal cerebellar ontogeny. To investigate a possible trophic role for CRF during cerebellar development, cerebellar culture studies using E18 mouse embryos were carried out. In our culture paradigm, that used serum-free defined medium to suppress cell proliferation, CRF induced proliferation of cells in a dose-dependent manner in a range of concentrations between 0.1-10 microM. The proliferating cells were identified as astrocytes based on their expression of vimentin and GFAP. BrdU incorporation studies supported the proposed mitogenic effect of CRF on developing astrocytes. The mitogenic effects of CRF seemed to be primarily on immature astrocytes determined by their differential expression of vimentin and GFAP. Astrocytes at more advanced stages of development, as determined by the extent of process outgrowth and GFAP expression, incorporated less BrdU compared to immature astrocytes. CRF receptors were localized in astrocytes, and the proliferation of astrocytes induced by CRF was inhibited by astressin, a competitive CRF receptor antagonist. In conclusion, CRF induces proliferation of astrocytes derived from the developing cerebellum, that suggests a gliotrophic role for CRF during cerebellar development.

Astroglial modulation of neurotransmitter/peptide release from the neurohypophysis: present status

Reviewed in this article are those studies that have contributed heavily to our current conceptualizations of glial participation in the functioning of the magnocellular hypothalamo-neurohypophysial system. This system undergoes remarkable morphological and functional reorganization induced by increased demand for peptide synthesis and release, and this reorganization involves the astrocytic elements in primary roles. Under basal conditions, these glia appear to be vested with the responsibility of controlling the neuronal microenvironment in ways that reduce neuronal excitability, restrict access to neuronal membranes by neuroactive substances and deter neuron neuron interactions within the system. With physiological activation, the glial elements, via receptor-mediated mechanisms, take up new positions. This permissively facilitates neuron neuron interactions such as the exposure of neuronal membranes to released peptides and the formation of gap junctions and new synapses, enhances and prolongs the actions of those excitatory neurotransmitters for which there are glial uptake mechanisms, and facilitates the entry of peptides into the blood. In addition, subpopulations of these glia either newly synthesize or increase synthesis of neuroactive peptides for which their neuronal neighbors have receptors. Release of these peptides by the glia or their functional roles in the system have not yet been demonstrated.