Effects of neurotransmitters and cyclic AMP on somatostatin release from cultured cerebral cortical cells

Reduced numbers of somatostatin receptors in the cerebral cortex in Alzheimer's disease

Somatostatin receptor concentrations were measured in patients with Alzheimer's disease and controls. In the frontal cortex (Brodmann areas 6, 9, and 10) and temporal cortex (Brodmann area 21), the concentrations of somatostatin in receptors in the patients were reduced to approximately 50 percent of control values. A 40 percent reduction was seen in the hippocampus, while no significant changes were found in the cingulate cortex, postcentral gyrus, temporal pole, and superior temporal gyrus. Scatchard analysis showed a reduction in receptor number rather than a change in affinity. Somatostatin-like immunoreactivity was significantly reduced in both the frontal and temporal cortex. Somatostatin-like immunoreactivity was linearly related to somatostatin-receptor binding in the cortices of Alzheimer's patients. These findings may reflect degeneration of postsynaptic neurons or cortical afferents in the patients' cerebral cortices. Alternatively, decreased somatostatin-like immunoreactivity in Alzheimer's disease might indicate increased release of somatostatin and down regulation of postsynaptic receptors.

Vasoactive intestinal peptide and PHI stimulate somatostatin release from rat cerebral cortical and diencephalic cells in dispersed cell culture

To determine the effect of vasoactive intestinal peptide (VIP) on the secretion of somatostatin by neurons, dispersed fetal cerebral cortical and diencephalic cells grown in culture were exposed on day 10 or 11 of culture to various concentrations of VIP, and for comparison to the structurally related peptides PHI (Peptide Histidine Isoleucine-27), growth hormone (GRH1-44-NH2) and secretin and to cholecystokinin. VIP elicited a dose-dependent release of somatostatin from both cortical and diencephalic cells, the lowest effective concentration being 6 X 10(-9) M. PHI also brought about release of somatostatin, but was between 0.06 and 0.1 times as potent as VIP. Placed together in a concentration of 10(-7) M, the two peptides did not have an additive effect. In this system GRH1-44-NH2, secretin and CCK octapeptide were without effect.

The effects of neurotensin, vasoactive intestinal polypeptide and other neuropeptides on the secretion of somatostatin from cerebral cortical cells

We have examined the effects of several neuropeptides on the release of immunoreactive somatostatin from cerebral cortical cells in vitro. Neurotensin and vasoactive intestinal polypeptide induced large increases in somatostatin release, whereas cholecystokinin, gonadotropin releasing hormone and Met-enkephalin induced only modest increases. Thyrotropin releasing hormone and insulin had no effect. These results demonstrate a complex interaction amongst neuropeptides in the cerebral cortex, which must be considered in future studies of the roles of peptides in cortical function.

Thyroid hormones reversibly suppress somatostatin secretion and immunoreactivity in cultured neocortical cells

Thyroid hormone effects on brain somatostatin-like immunoreactivity (SRIF-LI) were studied in an in vitro model system. Serum was removed from the nutrient culture medium of fetal day-18 rat cerebral cortex cells maintained in primary, long-term, dispersed monolayer culture. Chronic administration of either T3 or T4 in serum-free medium was associated with suppressed release of SRIF-LI into the culture medium (36-43 h accumulation), cell content of peptide and acute release in response to potassium-induced depolarization. Suppression was dose-dependent with an IC50 of less than 1 nM for T3. The most dramatic effects were observed for K+-induced release. Thirty-five to 50% suppression was typically observed with T3 at a near maximum dose (3 nM). Reverse T3 and diiodotyrosine were less potent and effective than T3. TRIAC and diiodothyronine also possessed significant suppressive activity. T3 suppression of release depended on duration of pretreatment. Administered for less than 16 h, T3 failed to significantly suppress K+-induced release, but significant suppression was observed for pretreatment periods of 16 h or longer. Indirect fluorescent immunohistochemical examination revealed a reduction in the number of cells positively stained for SRIF-LI in T3-treated dishes relative to controls. Upon removal of T3 and subsequent recovery in serum supplemented medium for 24 h, T3-treated and control cells exhibited similar levels of SRIF-LI release and cell content. T3-treated and control cells incorporated [3H] leucine into trichloracetic acid precipitable counts to similar extents. Dexamethasone and several sex steroids failed to modify the effects of T3 and did not independently influence SRIF-LI levels. Acute cycloheximide administration did not reverse T3 effects. The data indicate that primary brain cell cultures may be useful models to examine direct peripheral hormone actions on nervous tissue. Thyroid hormones suppress SRIF-LI levels in a dose, time and structure-dependent manner, which appears to be reversible. The findings are consistent with a possible integration of peripheral hormone and brain peptide physiology.

Effects of carbamazepine on cerebrospinal fluid somatostatin

This paper reports the reduction of the concentration of the neuropeptide somatostatin in the CSF of patients with affective illness during treatment with the anticonvulsant carbamazepine. None of the other psychotropic agents used in this study similarly affected CSF somatostatin, although zimelidine appeared to increase CSF somatostatin in a small number of patients. The potential mechanism and significance of the effects of carbamazepine on CSF somatostatin are discussed in relation to the psychotropic, anticonvulsant, and analgesic properties of carbamazepine.

Stimulated secretion of pro-opiomelanocortin-related peptides in hypothalamic cells

Although it has been suggested pro-opiomelanocortin (POMC) related peptides in brain may be neurotransmitters or neuromodulators, little is known about their secretion from neurons because it is difficult to study neurosecretion with an in vivo model. To demonstrate the possibility that POMC peptides may be neuroregulators which can be secreted in response to specific stimuli, we studied the secretion of immunoreactive (IR-) adrenocorticotropin (ACTH) and IR-beta-endorphin from dissociated hypothalamic cells during potassium-induced depolarization. Significant increments (p less than 0.025) in secretion of IR-ACTH (267%) and IR-beta-endorphin (88-172%) over basal secretion were stimulated by 60 mM KCl in the presence of calcium.

CONCLUSION

Stimulated secretion of POMC peptides from hypothalamic cells by potassium and calcium follows classical neurosecretory mechanisms and suggests these neuropeptides could be neuroregulators in brain.

Somatostatin and the cerebral cortex

A unique subset of interneurons which are rich in immunoreactive somatostatin (IRS) exists in the cerebral cortex. The regulation of IRS secretion by these cells is reviewed. Acetylcholine, glutamic acid and several neuropeptides including VIP, CCK, and metenkephalin have been identified as IRS secretagogues. The types of molecules which stimulate IRS release, the electrophysiologic effects of somatostatin, and the recognition of abnormal IRS levels in human CNS diseases were all used to formulate a working model of the role of the somatostatinergic cell in ongoing cerebral cortical function.

Lipoprotein lipase is produced, regulated, and functional in rat brain

Lipoprotein lipase (LP lipase, triacylglycero-protein acylhydrolase EC 3.1.1.34) activity was found in four dissimilar brain regions (hypothalamus, cortex, cerebellum, and midbrain) of adult male rats. Progressive accumulation of LP lipase activity in cultured fetal rat hypothalamic cells was also observed, indicating de novo synthesis of the lipase. The brain LP lipase activity was serum-dependent and was inhibited by 1 M NaCl and by protamine sulfate. Kinetic analysis revealed an apparent Km of 0.79 mM very similar to that of rat adipose tissue LP lipase. That the lipase was functioning in the cultured brain cells was indicated by uptake and incorporation of radioactivity from tri[( 1-14C]oleoyl)glycerol into cellular triacylglycerols, and into more polar lipids, such as phosphatidylcholine. Furthermore, brain LP lipase activity in adult rats was decreased in all four regions examined, most significantly in the hypothalamus, after 72 hr of food deprivation. Thus, authentic LP lipase is present in adult rat brain and can be synthesized by isolated brain cells in vitro. LP lipase also mediates the uptake of triacylglycerol fatty acids and their subsequent incorporation into cellular lipids of cultured brain cells. Decreased brain LP lipase activity after fasting suggests that this enzyme may be regulated by metabolic or nutritional factors. Because the largest changes in LP lipase activity in response to food deprivation occurred in the hypothalamus, the enzyme may have a role in hypothalamic control of food intake or in body-weight regulation.

Immunohistochemical distribution of somatostatin-like immunoreactivity in the central nervous system of the adult rat

The localization and distribution of somatostatin (growth hormone release-inhibiting hormone; somatotropin release-inhibiting factor) have been studied with the indirect immunofluorescence technique of Coons and collaborators and the immunoperoxidase method of Sternberger and coworkers using specific and well-characterized antibodies to somatostatin, providing semiquantitative, detailed maps of somatostatin-immunoreactive cell profiles and fibers. Our results demonstrate a widespread occurrence of somatostatin-positive nerve cell bodies and fibers throughout the central nervous system of adult, normal or colchicine-treated, albino rats. The somatostatin cell bodies varied in size from below 10 micron up to 40 micron in diameter and could have only a few or multiple processes. Dense populations of cell somata were present in many major areas including neocortex, piriform cortex, hippocampus, amygdaloid complex, nucleus caudatus, nucleus accumbens, anterior periventricular hypothalamic area, ventromedial hypothalamic nucleus, nucleus arcuatus, medial to and within the lateral lemniscus, pontine reticular nuclei, nucleus cochlearis dorsalis and immediately dorsal to the nucleus tractus solitarii. Extensive networks of nerve fibers of varying densities were also found in most areas and nuclei of the central nervous system. Both varicose fibers as well as dot- or "dust-like" structures were seen. Areas with dense or very dense networks included nucleus accumbens, nucleus caudatus, nucleus amygdaloideus centralis, most parts of the hypothalamus, nucleus parabrachialis, nucleus tractus solitarii, nucleus ambiguus, nucleus tractus spinalis nervi trigemini and the dorsal horn of the spinal cord. One exception is the cerebellum which only contained few somatostatin-positive cell bodies and nerve fibers. It should be noted that somatostatin-positive cell bodies and fibers did not always conform to the boundaries of the classical neuroanatomical nuclei, but could often be found in areas between these well-established nuclei or occupying, in varying concentrations, only parts of such nuclei. It was difficult to identify with certainty somatostatin-immunoreactive axons in the animals studied. Some pathways could, however, be demonstrated, but further experimental studies are necessary to elucidate the exact projections of the somatostatin-immunoreactive neurons in the rat central nervous system.

Identification of glucagon receptors in rat brain

The binding of radiolabeled glucagon to rat brain membranes was investigated. Regional distribution studies indicate higher specific binding of 125I-labeled monoiodoglucagon to olfactory tubercule, hippocampus, anterior pituitary, and amygdala membranes, with somewhat lower binding to membranes from septum, medulla, thalamus, olfactory bulb, and hypothalamus. 125I-labeled glucagon bound to rat brain synaptic plasma membrane fractions with high affinity (KD = 2.24 nM). Specific binding was greater to synaptosomal membrane fractions relative to myelin, mitochondrial nuclear, or microsomal fractions. Inclusion of 0.1 mM GTP in the binding assay reduced the glucagon binding affinity (KD = 44.5 nM). Several neuropeptides and other neuroactive substances tested did not affect binding of labeled glucagon to brain membranes. Three different glucagon analogs inhibited labeled glucagon binding. Synthetic human pancreatic growth hormone-releasing factor, hpGRF-44, also inhibited binding, although the concentration required for half-maximal displacement was 100-fold higher than for native glucagon. Addition of glucagon to brain membranes resulted in approximately equal to 3-fold maximal activation of adenylate cyclase over basal levels. Glucagon at a concentration of 4.74 nM was required for half-maximal activation of pituitary membrane adenylate cyclase. These findings provide evidence for rat brain binding sites that respond to the pancreatic form of glucagon and can transduce this binding into the activation of adenylate cyclase.

Coexistence of acetylcholinesterase and somatostatin-immunoreactivity in neurons cultured from rat cerebrum

Cultures derived from rat cerebral hemispheres were sequentially stained for acetylcholinesterase activity and for either somatostatin-like immunoreactivity or cholecystokinin-like immunoreactivity. Somatostatin-like immunoreactivity was found to coexist with acetylcholinesterase activity in individual neurons of several morphological subtypes, but cholecystokinin-like immunoreactivity and acetycholinesterase activity were never seen in the same neurons. These findings suggest a specific anatomical association, perhaps even an overlap, of the cholinergic and somatostatinergic systems in the mammalian cerebrum, and indicate that the combined deficiencies of somatostatin and cholinergic markers in Alzheimer's dementia and senile dementia of the Alzheimer type may be of pathophysiological importance.

Low levels of somatostatin in human CSF mark depressive episodes

Somatostatin-like immunoreactivity was measured in the cerebrospinal fluid (CSF) of 85 inpatients with current or recent episodes of major depressive disorders, diagnosed according to Research Diagnostic Criteria (RDC) as assessed with the Schedule for Affective Disorders and Schizophrenia (SADS). Several biopsychiatric tests were run during the same week of investigation. Results indicate low levels of CSF somatostatin to be a state marker for episodes of depression characterized by sad appearance, feelings of tiredness, insomnia, and subjective inability to acknowledge any external precipitants for the depression. CSF somatostatin was negatively related to platelet monoamine oxidase (MAO) activity; MAO activity appeared to account better for the degree of melancholic features than did somatostatin. The ratio between 3-methoxy-4-hydroxyphenylglycol (MHPG) and homovanillic acid (HVA) in CSF also correlated negatively with somatostatin. A positive relationship was noted between CSF xanthine and somatostatin. There was a highly significant curvilinear correlation between CSF somatostatin and serum TSH concentrations, but no correlations between CSF somatostatin and serum GH or prolactin, or with plasma cortisol before or after dexamethasone.

Somatostatin release from cerebral cortical cells: influence of amino acid neurotransmitters

We examined the ability of several putative amino acid neurotransmitters to influence immunoreactive somatostatin (IRS) release from cultured rat cerebral cortical cells. The cells were exposed to either or sequential incubations in various concentrations of glutamate (Glu), aspartate (Asp), GABA, glycine, taurine and arginine. Glu and Asp were stimulatory to IRS release, whereas GABA was inhibitory. Glu-induced IRS release was calcium-dependent. Glycine and taurine were weak stimulants.

Glucagon-induced somatostatin release from perifused rat hypothalamus: calcium dependency and effect of cysteamine treatment

Somatostatin (SRIF) release from rat hypothalamus was investigated in vitro with a perifusion system. Glucagon (1 microM) and high potassium concentrations (56 mM) stimulated SRIF release in a calcium-dependent manner. Pretreatment of the rat with cysteamine (30 mg/100 g body weight, 7 h earlier) significantly reduced SRIF release from the hypothalamus in glucagon- and high potassium-stimulated states as well as in the basal state. SRIF release from rat hypothalamus was also stimulated by both dibutyryl cyclic AMP (1 mM) and theophylline (3 mM). These results suggest that glucagon, acting in a calcium-dependent manner and possibly through the adenylate cyclase-cyclic AMP system, stimulates SRIF release from rat hypothalamus and that cysteamine treatment reduces releasable SRIF in the hypothalamus.

Influence of glucose on somatostatin synthesis and secretion in isolated cerebral cortical cells

Somatostatin-producing cerebral cortical cell cultures were grown in either high- (33 mM) or low-glucose (5 mM) medium and then exposed to short repetitive changes of high- or low-glucose Krebs-Ringer's bicarbonate buffer. Equivalent amounts of somatostatin were released in the high-to-high, the low-to-low, and the low-to-high paradigms. The high-to-low experiment produced a rapid rise in somatostatin release, followed by a decline. Cultures exposed to 2-deoxyglucose after high-glucose medium also released much greater amounts of immunoreactive somatostatin. Separate sets of cultures were grown in high- or low-glucose medium for up to 19 days. Cultures grown in high-glucose medium generally contained more somatostatin intracellularly than did those maintained in low glucose, although somatostatin in the medium was only different at day 19. These results identify extracellular glucose as an important determinant of cortical somatostatin production.

Somatostatin release from dispersed hypothalamic cells - effects of membrane depolarization, calcium and glucose deprivation

We have developed a new short term in vitro system to examine hypothalamic somatostatin (SRIF) release. Hypothalamic cells were obtained from normal rats after trypsin or collagenase aided dispersion and released immuno-reactive (IR) SRIF which eluted in 3 molecular weight (MW) forms on gel chromatography. The smallest MW form, which constituted the major peak, co-eluted with synthetic cyclic 1-14 SRIF on gel and reverse phase high pressure liquid chromatography (HPLC). After 24 h in culture in medium containing heat inactivated fetal calf serum, cell viability was demonstrated by two techniques, (1) vital staining with trypan blue, and (2) incorporation of 32Pi into phospholipids. SRIF release was also studied at this time which was optimal in terms of responsivity of the cells to depolarizing stimuli. SRIF release increased in a time dependent manner, over 3 h. Membrane depolarization, induced either by potassium chloride 56 mM or ouabain (the Na+, K+-ATPase inhibitor) 10(-6) M or greater, markedly stimulated SRIF release. Incubation at 4 degrees C, or in the presence of EDTA 0.05 M or verapamil, the calcium channel blocker, 50 microM abolished these stimulatory effects. Glucose deprivation was induced by the addition of 2-deoxy-D-glucose (2-DG) to the experimental medium. 2-DG, at concentrations of up to 200 mg%, had no significant effect on SRIF release during incubation periods of up to 1 h.

Somatostatin depletion of the gut and pancreas induced by cysteamine is not prevented by vagotomy or by dopamine agonists

The role of endogenous somatostatin in the pathogenesis of duodenal was investigated in the present study by using the cysteamine animal model of the disease. Our previous studies showed a rapid and multiorgan depletion of somatostatin immunoreactivity (SIR) in rats given a single dose of duodenal ulcerogen cysteamine. We now determined whether acetylcholinergic and dopaminergic modulation (both known to influence the development of duodenal ulcer) are accompanied by modification of cysteamine-induced SIR depletion in rat organs. Vagotomy performed either 1 or 18 h before cysteamine administration did not interfere with the chemically induced SIR decrease in pancreas, gastric and duodenal mucosa. Vagal denervation alone had no marked influence on SIR levels but if combined with cysteamine, the SIR depletion in the stomach was significantly more pronounced than after the duodenal ulcerogen alone. Pretreatment with the dopamine agonists bromocriptine or lergotrile (known to prevent the chemically induced duodenal ulcers) did not influence the SIR depletion by cysteamine. Thus cysteamine depletes endogenous somatostatin in peripheral organs (e.g., stomach, duodenum, pancreas) by mechanisms independent of both vagus nerve and dopamine agonists. A role of central somatostatin depletion leading to disinhibition of vagus is also considered in the pathogenesis of experimental duodenal ulcer.

Methodological considerations in culturing peptidergic neurons

Both explant and dispersed cell culture preparations of brain tissue provide a means to directly assess the functions of neuropeptide-containing brain cells isolated from the complex influences present in vivo. The validity of the approach depends on reproducibility of observations. In dispersed cell cultures, we find that cell responses, as determined by secretion of the peptide somatostatin, have remained relatively constant both quantitatively and qualitatively over numerous preparations. In addition, for pharmacologic studies on somatostatin secretion, data from several laboratories are in good agreement. The validity of the dispersed cell approach also depends on whether the pharmacologic and physiologic behavior of cells parallels that expected of excitable tissue. The variability of the explant cultures from culture dish to culture dish makes quantitative experiments, such as demonstrated with the dispersed cultures, difficult. On the other hand, explant cultures better maintain the integrity of the tissue components, so that interactions between neurons and glial cells could occur as in vivo. The long-term health and viability of neuropeptidergic cells in explant and dispersed culture make both preparations potentially useful models to examine central nervous system physiology. Future work with such preparations must eventually address the problems of culturing adult brain tissue, the precise nutrient and hormonal requirements of brain cells, so that undefined medium components, such as serum, can be eliminated from the culture environment, and the general question of whether observations made in vitro facilitate our understanding of intact brain physiology.