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

CREB: a stimulus-induced transcription factor activated by a diverse array of extracellular signals

Extracellular stimuli elicit changes in gene expression in target cells by activating intracellular protein kinase cascades that phosphorylate transcription factors within the nucleus. One of the best characterized stimulus-induced transcription factors, cyclic AMP response element (CRE)-binding protein (CREB), activates transcription of target genes in response to a diverse array of stimuli, including peptide hormones, growth factors, and neuronal activity, that activate a variety of protein kinases including protein kinase A (PKA), pp90 ribosomal S6 kinase (pp90RSK), and Ca2+/calmodulin-dependent protein kinases (CaMKs)[corrected]. These kinases all phosphorylate CREB at a particular residue, serine 133 (Ser133), and phosphorylation of Ser133 is required for CREB-mediated transcription. Despite this common feature, the mechanism by which CREB activates transcription varies depending on the stimulus. In some cases, signaling pathways target additional sites on CREB or proteins associated with CREB, permitting CREB to regulate distinct programs of gene expression under different conditions of stimulation. This review discusses the molecular mechanisms by which Ser133-phosphorylated CREB activates transcription, intracellular signaling pathways that lead to phosphorylation of CREB at Ser133, and features of each signaling pathway that impart specificity at the level of CREB activation.

Regulation of the preprosomatostatin gene by cyclic-AMP in cerebrocortical neurons

The gene coding for preprosomatostatin (ppSom), the molecular precursor of somatostatin (Som), is regulated at the level of transcription by calcium ions and cyclic-AMP [F. Baldino, S. Fitzpatrick-McElligott, T. O'Kane, I. Gozes, Hormonal regulation of somatostatin, Synapse 2 (1988) 317-325; M.R. Montminy, M.J. Low, L. Tapia-Arancibia, Cyclic AMP regulates somatostatin mRNA accumulation in primary diencephalic cultures and in transfected fibroblast cells, J. Neurosci. 6 (1986) 1171-1176.], or by agents which increase intracellular levels of cAMP directly, such as forskolin [M.R. Montminy, M.J. Low, L. Tapia-Arancibia, Cyclic AMP regulates somatostatin mRNA accumulation in primary diencephalic cultures and in transfected fibroblast cells, J. Neurosci. 6 (1986) 1171-1176.]. Transcriptional induction of the ppSom gene as examined in PC12 cells, transfected fibroblasts and primary diencephalic cultures, requires the highly conserved cAMP response element (CRE), which confers gene responsiveness to cAMP [M. Comb, N. Mermod, S.E. Hyman, Proteins bound at adjacent DNA elements act synergistically to regulate human proenkephalin cAMP inducible transcription, EMBO J. 7 (1988) 3793-3805; T. Tsukada, J.S. Fink, G. Mandel, Identification of a region in the human vasoactive intestinal polypeptide gene responsible for regulation by cyclic AMP, J. Biol. Chem. 262 (1987) 8743-8747.]. The ppSom gene is subject to stringent regulation during cerebrocortical development in vivo; however, little information is available regarding ppSom gene regulation by neurotransmitters or second-messengers in cortical neurons. We used primary cerebrocortical cell cultures from fetal mice to examine the dose-response and time-course of ppSom gene expression in response to the cyclic-AMP analogs, dibutyrl-cAMP (dbcAMP), and 8-bromo-cAMP (8-BrcAMP). We report a dose-response for both analogs in the range of 0.1-10 mM. Dose-response studies using agents which directly stimulate intracellular cAMP synthesis (forskolin) or inhibit its breakdown (3-isobutyl 1-methyl xanthine) were also performed. We observed an apparent synergistic effect on ppSom expression when used in combination. An increase in ppSom mRNA levels was observed by 4 h, with a maximal response at 12-24 h. No change in ppSom mRNA levels was observed in response to phorbol myristate acetate (PMA). Our findings confirm the specificity of ppSom gene regulation by cAMP and Ca2+ ions, and demonstrate the utility of using primary cerebrocortical cultures for the study of somatostatin gene expression by neurotransmitters and second-messengers as a model of human neurologic disorders.

Cholinergic cell expression in the developing rat medial septal nucleus in vitro is differentially controlled by GABAA and GABAB receptors

The early appearance and relative abundance of GABAergic neurons in basal forebrain cholinergic nuclei like the medial septum suggest that the maturation of the later developing cholinergic neurons in these nuclei may be controlled by GABA. To examine this possibility, the effects of both exogenous GABA and specific GABA receptor agonists, as well as that of endogenous GABA on the phenotypic expression and survival of the cholinergic neurons in primary cultures from the fetal rat medial septum, were studied. Treatment of these cultures for six days with GABA significantly decreased the enzymatic activity of choline acetyltransferase (EC 2.3.1.6) (ChAT) in a dose-dependent manner. This response to exogenous GABA was blocked by bicuculline, mimicked by muscimol and slightly potentiated by saclofen. Consistent with this latter observation, the GABAB receptor agonist, baclofen, dose-dependently increased septal ChAT activity. However, while the effect of baclofen on cholinergic expression was lost in the absence of glia, the suppressive effects of GABA or muscimol were more marked. Acetylcholinesterase (EC 3.1.1.7) (AChE) expression in mixed neuronal-glial cultures, was, like ChAT activity, increased or decreased in intensity with the inclusion of baclofen or muscimol, respectively. Although the number of AChE positive neurons in muscimol-treated cultures was significantly lower than that in controls, no changes in neither neuronal nor general cell viability were noted. Finally, as GABAA or GABAB receptor antagonists bicuculline and picrotoxin or saclofen, when applied alone to mixed cultures, increased or decreased ChAT activity, respectively, it appears that endogenous GABA, tonically released in the developing septum, may, via specific receptor types, differentially control the biochemical maturation of the cholinergic neurons.

Presynaptic modulation by VIP, secretin and isoproterenol of somatostatin release from enriched enteric synaptosomes: role of cAMP

The release of somatostatin-like immunoreactivity was studied in isolated synaptosomes. A significant release of somatostatin-like immunoreactivity was observed in the presence of vasoactive intestinal polypeptide (VIP) (10(-6) M: 53.0 +/- 12.4 pg/mg, basal: 14.3 +/- 1.7 pg/mg, n = 5, P < 0.05), secretin (10(-6) M: 56.1 +/- 3.8 pg/mg, basal: 25.8 +/- 1.6 pg/mg, n = 6, P < 0.01) and isoproterenol (10(-5) M: 54.0 +/- 13.4 pg/mg, basal: 20.0 +/- 3.4 pg/mg, n = 8, P < 0.05). Forskolin, an unspecified activator of the adenylate cyclase, caused a significant release of somatostatin-like immunoreactivity (10(-6) M: 57.3 +/- 13.2 pg/mg, basal: 30.0 +/- 5.8 pg/mg, n = 13, P < 0.01) which was further augmented in the presence of the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX 10(-4) M) (77.0 +/- 17.8 pg/mg, n = 13, P < 0.01). 3-Isobutyl-l-methylxanthine and N6, 2'-O-dibutyryladenosine-3',5'-cyclic monophosphate mimicked at effect of forskolin and VIP. The release of somatostatin was paralleled by an increase of cAMP immunoreactivity in the presence of VIP (10(-6) M: 37.1 +/- 9.4 pmol/mg, basal: 19.8 +/- 4.2 pmol/mg, n = 10, P < 0.05), isoproterenol (10(-5) M: 42.4 +/- 9.8 pmol/mg basal: 16.7 +/- 2.4 pmol/mg, n = 12, P < 0.01) and forskolin (10(-6) M: 47.1 +/- 12.4 pmol/mg, basal: 19.8 +/- 4.2 pmol/mg, n = 10, P < 0.01). The effect of nitric oxide (NO) which acts as an inhibitory neurotransmitter in the enteric nervous system was studied. NO is known to activate guanylate cyclase to induce transmitter release. The NO-generating compound sodium nitroprusside and bromoguanosine-3',5'-cyclic monophosphate (8-Br-cGMP) had no effect on the release of somatostatin-like immunoreactivity. These data demonstrate the stimulatory effect of VIP, secretin and isoproterenol on release of somatostatin-like immunoreactivity from enteric synaptosomes, which is presumably mediated by cAMP-dependent mechanisms. cGMP-dependent mechanisms seem to be of minor relevance.

alpha-Fluoromethylhistidine influences somatostatin content, binding and inhibition of adenylyl cyclase activity in the rat frontoparietal cortex

Slow-wave sleep, wakefulness, locomotor activity and learning and memory are regulated in similar ways by somatostatin (SS) and histamine. To clarify the possible role of endogenous histamine on the somatostatinergic system of the rat frontoparietal cortex, we studied the effect of 50 micrograms of alpha-fluoromethylhistidine (alpha-FMH), a specific inhibitor of histidine decarboxylase, administered intracerebroventricularly (i.c.v.) at 1, 4 and 6 h, on somatostatin-like immunoreactivity (SSLI) content and the SS receptor/effector system. The histamine content in the frontoparietal cortex decreased to about 67, 60 and 72% of control values at 1, 4 and 6 h after alpha-FMH administration, respectively. At 6 h after alpha-FMH injection, there was an increase in SSLI content and a decrease in the number of SS receptors, with no change in the apparent affinity. No significant differences were seen for the basal and forskolin (FK)-stimulated adenylyl cyclase (AC) activities in the frontoparietal cortex of alpha-FMH-treated rats when compared to the control group at all times studied. At 6 h after alpha-FMH administration, however, the capacity of SS to inhibit basal and FK-stimulated AC activity in the frontoparietal cortex was significantly lower than in the control group. The ability of the stable GTP analogue 5'-guanylylimidodiphosphate (Gpp(NH)p) to inhibit FK-stimulated AC activity in frontoparietal cortex membranes was the same in the alpha-FMH-treated (6 h) and control animals. Therefore, the decreased SS-mediated inhibition of AC activity observed in the alpha-FMH-treated rats is not due to an alteration at the guanine nucleotide-binding inhibitory protein (Gi) level but rather may be due to the decrease in the number of SS receptors. Taken together, these data suggest that alpha-FMH influences the sensitivity to SS in the rat frontoparietal cortex.

Glutamate enhances the adenylyl cyclase-cAMP system-induced beta-endorphin secretion and POMC mRNA expression in rat hypothalamic neurons in culture: NMDA receptor-mediated modulation

L-Glutamate, a major excitatory amino acid of the central nervous system, plays important roles as neurotransmitter and neuromodulator in the brain. Increasing evidence suggests that glutamate may also involve in the regulation of the neuroendocrine system at the hypothalamus. Employing long term monolayer hypothalamic cell cultures prepared from neonatal rats, we reported here that whereas glutamate significantly enhanced forskolin-, or N6,2'-O-dibutyryladenosine-3'5'-cyclic monophosphate [(Bu)2cAMP]-stimulated immunoreactive (ir)-beta EP release from cultures treated daily for 4 consecutive days, the excitatory amino acid alone produced little effect. This potentiation of glutamate was time-related and dose-dependent with an Emax value of the amino acid being approximately 50 microM; at this concentration glutamate augmented ir-beta EP secretion about 1.8 times (P < 0.05) that induced by 2 microM forskolin alone. Similar effects were also observed for POMC mRNA levels in cultures subjected to 6 h of the above treatment regime. This potentiating effect of glutamate appears to be mediated specifically through NMDA receptor as it can be mimicked by NMDA but not by kainic acid or quisqualic acid, and blocked by the NMDA receptor antagonist 2-amino-5-phosphonovalerate (APV), but not by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), a non-NMDA glutamate receptor antagonist. Interestingly, glutamate was found not to enhance high doses of forskolin (10 microM) or (Bu)2cAMP (100 microM) stimulated beta EP release and POMC mRNA levels in hypothalamic cell cultures.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential correlation between neurochemical deficits, neuropathology, and cognitive status in Alzheimer's disease

The relationships between neurofibrillary tangles (NFT), senile plaques (SP), and the deficits in somatostatin (SRIH) and choline acetyltransferase (ChAT) levels were determined in Brodmann area 9, 40, 22, and 17/18 in 12 women whose Blessed test score (BTS) ranged from 27 to 1. NFT density correlated with the cognitive decline in areas 9, 40, and 22 and with SP number in area 22 and 17/18. ChAT levels were linked to the BTS in area 9, 40, and 22 and SRIH levels in area 9 only. ChAT, but not SRIH, did correlate with SP (area 22) and NFT (area 40 and 22). Decreases in ChAT and SRIH were correlated in areas 9 and 22. These results indicate that the somatostatinergic deficit in Alzheimer's disease is more regionally restricted than the cholinergic one. The correlation between SRIH and ChAT as observed in area 9 and 22 may indicate that somatostatin- and acetylcholine-containing elements in the frontal and temporal lobes are particularly relevant to the cognitive decline as observed in Alzheimer's disease.

Release of somatostatin-like immunoreactivity from enriched enteric nerve varicosities of rat ileum

Synaptosomes were isolated from rat ileum by various steps of differential centrifugation. The peptide content for somatostatin-like immunoreactivity was used as marker for neuronal membranes. The enriched synaptosomal fraction (P2) showed a good enrichment of somatostatin content (4-fold) in comparison to the post-nuclear supernatant. The basal release of somatostatin-like immunoreactivity was 26 +/- 3 pg/mg tissue protein. KCl-evoked depolarization (65 mM) caused a significant increase of somatostatin-like immunoreactivity release (72 +/- 11 pg/mg, n = 12, P < 0.001) compared to basal release. In Ca(2+)-free medium the evoked release of somatostatin-like immunoreactivity was abolished. A substantial increase of somatostatin-like immunoreactivity release (52 +/- 7 pg/mg, n = 12, P < 0.05) was also observed in the presence of the Ca2+ ionophore A-23187. The cholinergic agonist carbachol elicited a dose-dependent release of somatostatin-like immunoreactivity (10(-7) M: 54 +/- 8 pg/mg, 10(-6) M: 63 +/- 6 pg/mg, 10(-5) M: 53 +/- 5 pg/mg, n = 12, P < 0.001), which was blocked by atropine (10(-6) M: 35 +/- 6 pg/mg, n = 12, P < 0.001), but not by hexamethonium. Other presynaptic modulating substances such as serotonin, the selective neurokinin-B agonist [beta Asp4,MePhe7]neurokinin B-(4-10), neurotensin, cholecystokinin-8, caerulein and pentagastrin had no stimulatory effect on release of somatostatin-like immunoreactivity. In summary, somatostatin-like immunoreactivity can be released from enteric synaptosomes by both depolarization with KCl and cholinergic stimulation via a muscarinic mechanism. The synaptosomes of intrinsic nerves offer an approach to study release of neuronal somatostatin on the subcellular level.

Somatostatin enhances nerve growth factor-induced neurite outgrowth in PC12 cells

The neuropeptide somatostatin has been found to be abundant in numerous developing regions within the central nervous system. In order to understand the role of somatostatin in development, effects of exposure to the neuropeptide were studied in PC12 cells, a well characterized model of neuronal differentiation. Somatostatin increased neurite outgrowth after 2 days in culture and enhanced neurite outgrowth after nerve growth factor (NGF) exposure. This effect was inhibited by somatostatin antibody and pertussis toxin. Somatostatin had no effect on NGF binding or internalization but did cause a decrease in cAMP levels during the time of maximal stimulation of neurite outgrowth. In a protein kinase A-deficient cell line (A126-1B2), somatostatin had no effect on neurite outgrowth. These results indicate that somatostatin may function as a differentiation factor in developing systems through inhibition of cAMP synthesis.

Chronic administration of flumazenil increases life span and protects rats from age-related loss of cognitive functions: a benzodiazepine/GABAergic hypothesis of brain aging

Under barrier condition and with ad lib access to food and water, 20 Fischer-344 rats were chronically treated for 10 months with the benzodiazepine (BDZ) antagonist, flumazenil (FL; 4 mg/kg/day in drinking water acidified to pH = 3.0), beginning at the age of 13 months, while the group of 20 control age-matched rats received plain acidified water. The life span of the first 8 deceased rats treated with FL was significantly longer than that of the first 8 deceased rats in the age-matched control group. In tests for spontaneous ambulation and exploratory behavior in the Holeboard apparatus, conducted during the 3rd and the 8th month of treatment, the FL group, relative to controls, had significantly higher scores for the ambulation and exploratory behavior. In tests for unrewarded spontaneous alternation in the T maze, conducted at days 7, 39, 42, and 47 through 54 after drug withdrawal, i.e., at the age of 24-25 months, the FL-exposed group, compared to age-matched controls, showed a significantly higher percent of alternating choices, a behavior that was statistically comparable to that of the "young" 6-month-old controls. In the Radial Maze tests conducted 2 months after drug withdrawal, the FL group made significantly less "working memory" errors and "reference memory" errors, relative to the age-matched 25-month-old control group, a performance that was comparable to that of the young 7-month-old control group. In conclusion, chronic FL significantly protected rats from age-related loss of cognitive functions. It is postulated that the age-related alterations in brain function may be attributable to the negative metabolic/trophic influences of the "endogenous" benzodiazepine (BDZ) ligands and/or those ingested with food. A BDZ/GABAergic hypothesis of brain aging has been formulated which assumes that age-related and abnormally strong BDZ/GABAergic influences promote neurodegeneration by suppressing trophic functions of the aminergic and peptidergic neurons through opening of chloride channels in soma membrane and axon terminals, causing excessive hyperpolarizing and depolarizing inhibition, respectively. The review of human clinical and animal data indicates that FL has nootropic actions by enhancing vigilance cognitive and habituation processes.

Cell-type specific expression of Na+, K(+)-ATPase catalytic subunits in cultured neurons and glia: evidence for polarized distribution in neurons

Na+,K(+)-ATPase (the sodium pump) is a family of proteins consisting of catalytic (alpha) and glycoprotein (beta) subunit isoforms which are differentially expressed in excitable tissue. To gain insight into the cell-type distribution of sodium pump protein, we determined the expression pattern of fetal rat telencephalic cultures, of telencephalic cultures depleted of neurons, and of pure astrocyte cultures. Isoform-specific antibodies were used for immunoblotting and immunohistochemistry, with supplemental [3H]ouabain binding to assess levels of functional alpha 2/alpha 3 protein. The results show that neurons of mixed telencephalic cultures uniquely express alpha 3 and high levels of alpha 1. The marked similarity in the distribution of microtubule-associated protein-2 and alpha 1 immunocytochemical staining strongly suggests that alpha 1 subunits are enriched in dendrites. Further, highly correlative growth cone-associated protein-43 and alpha 3 staining is consistent with a preferential expression of alpha 3 subunits in axons, which are also characterized by low levels of alpha 1 and no alpha 2 immunoreactivity. Process-bearing glia are intimately associated with neuronal aggregates and express high levels of both alpha 1 and alpha 2 protein, as well as GFAP. Interestingly, polygonal, flat glia not within neuronal aggregates are weakly immunopositive only for alpha 1 and GFAP. Pure astrocytic cultures possess appreciable alpha 1 protein and GFAP, but lack both alpha 2 and alpha 3 immunoreactivity. As predicted by the immunohistochemical findings, [3H]ouabain binding was low in pure astrocytic cultures, and much higher in the neuron-enriched mixed cultures. These observations confirm that neurons express all three catalytic isoforms of the sodium pump. They also suggest that specific alpha-isoforms may be polarized to targeted membrane regions of neurons. Further, glia intimately associated with neurons express alpha 2, bind significant amounts of [3H]ouabain, and possess much higher levels of alpha 1 and GFAP compared to glia not near neurons. Thus, neurons may regulate glial sodium pump expression.

The adenylyl cyclase-cyclic AMP system modulates morphological and functional development of hypothalamic beta-endorphin neurons in culture

In rats, opioidergic beta-endorphin (beta EP1-31) is produced and released from neurons of arcuate nuclei in the hypothalamus. Although the neuropeptide has been implicated in sexual maturation and stress-induced reproductive dysfunction, the intra-hypothalamic regulation of beta EP neurons remains unclear. Employing long-term monolayer cultures of neonatal rat hypothalamic cells, we report here that 4 days of treatment with 10 microM forskolin increased approximately 3-fold (P < 0.01) the proportion of immunoreactive (ir)-beta EP positive neurons bearing neurites. In addition, treatment of forskolin also enhanced ir-beta EP release (634 +/- 59 pg/well; mean +/- SE, n = 4, P < 0.01) by 14-fold and ir-beta EP content (119 +/- 13 pg/well; P < 0.01) by 2-fold above that of vehicle-treated cultures; in both instances, the EC50 and the Emax of forskolin were approximately 10 microM and 100 microM, respectively. The forskolin-stimulated release of ir-beta EP was mimicked by cholera toxin and (Bu)2cAMP treatment in a dose-related manner, but not by pertussis toxin. Although by itself 3-isobutyl-1-methyl-xanthine (100 microM) only doubled ir-beta EP secretion, it markedly potentiated the stimulatory effect of forskolin. This forskolin-induced stimulation was reversible and in cultures re-exposed to the same drug within the first 24 h period, there was a marked increase in the stimulated release of ir-beta EP (P < 0.05); re-challenge of forskolin at later stages, however, induced a smaller but significant secretion of ir-beta EP (P < 0.01) compared to that of vehicle-treated control cultures. Sephadex G-50 gel chromatographic profile of the media prepared from forskolin-treated cultures revealed a major ir-beta EP peak of 3 K M(r). High-performance liquid chromatography analysis showed that ir-beta EP of the 3 K M(r) species was eluted with a retention time similar to that of synthetic rat beta EP1-31. We thus conclude that the adenylyl cyclase-cAMP system plays an important role in the modulation of beta EP1-31 production and release from hypothalamic beta EP neurons in culture. Furthermore, the functional responsiveness and the morphological development of these neurons are affected, at least in part, by the intrinsic activity of the adenylyl cyclase-cAMP system.

Somatostatin release in rat neocortex during gamma-hydroxybutyrate-provoked seizures: Microdialysis combined with EEG recording

Gamma-hydroxybutyrate (GHB) was intracortically applied in two doses (first 10 and then 20 mg/ml) to awake Wistar rats using microdialysis. Simultaneously, EEG and the release of somatostatin-like immunoreactivity (SLI) were measured from the frontal cortex. Intracerebrally administered GHB induced cortical epileptogenic spikes, but not high voltage spindles (HVS) as reported after systemic administration, and seizures with myoclonic jerks and contraversive head movements. Compared to the basal level, GHB (10 mg/ml) initially increased the release of SLI (p < 0.05). However, when the frequency of spikes and seizures rose rapidly (p < 0.001), SLI release decreased significantly (p < 0.001). Minimum release of SLI occurred when seizures were most frequent (during perfusion with 20 mg/ml GHB), while after removal of the drug it rose above the basal level (p < 0.05). According to these results, intracortically applied GHB increases the release of SLI in the surrounding tissue. However, further exposure of GHB leads to a manifestation of epileptic spikes and seizures, during which the release of SLI is significantly attenuated. This suggests that release of somatostatin is affected during epileptic phenomena induced also by intracortical GHB application.

Activation of cyclic AMP second messenger system stimulates secretion of beta-endorphin from fetal hypothalamic cells

Relatively little is known about physiological regulators of hypothalamic beta-endorphin (END) secretion and mechanisms by which they stimulate secretion. We sought to determine whether activation of the cyclic AMP (cAMP) second messenger pathway was involved in stimulating hypothalamic beta-END secretion from dissociated fetal hypothalamic cells in culture. Forskolin (FSK), a direct activator of adenylate cyclase which stimulates cAMP formation, stimulated immunoreactive (IR)-beta-END secretion. Because FSK can also stimulate independent of increased cAMP formation, we studied dibutyryl cAMP and 8-bromo-cAMP, analogues of cAMP, which also stimulated IR-beta-END secretion. From these studies we conclude: (1) activation of the cAMP second messenger system stimulates IR-beta-END secretion from hypothalamic cells and supports the rationale that endogenous regulators which stimulate this pathway could be involved in the physiological regulation of hypothalamic beta-END secretion; (2) coupling between the cAMP second messenger pathway and stimulation of hypothalamic beta-END secretion which is presumably present at maturity (adulthood) originates at early stages of development (fetal life).

Somatostatin receptor elevation in rat striatum after diisopropylfluorophosphate administration

The acute and chronic administration of diisopropylfluorophosphate (DFP), an inhibitor of acetylcholinesterase or of atropine, a blocker of muscarinic cholinergic receptors, did not affect somatostatin-like immunoreactivity (SLI) content in the striatum of rats. Acute and chronic DFP administration increased the number of specific 125I-Tyr11-somatostatin (125I-Tyr11-SS) receptors in cells dissociated from the striatum without changing the affinity constant. Although the increase could be blocked by pretreatment with atropine, it was not due to a direct effect by DFP on somatostatin (SS) receptors, because no rise in 125I-Tyr11-SS binding was produced by high concentrations of DFP (10(-5) M) when added in vitro. The acute administration of atropine alone had no observable effect on the number of SS receptors. However, repeated atropine administration produced a significant decrease in the 125I-Tyr11-SS binding in cells dissociated from the striatum, although the affinity constant was unchanged. The results suggest that interactions between somatostatinergic and cholinergic receptors may be of importance in the rat striatum.

Anticonvulsants inhibit rat neuronal somatostatin release

Somatostatin (SRIF), a peptide widely distributed in the central nervous system, has been implicated in the genesis of seizure activity in a number of animal models of epilepsy. We examined the effects of the anticonvulsants, phenytoin, carbamazepine and diazepam, on the release of SRIF from dispersed adult rat neuronal cells in short-term culture. Each of these agents caused dose-dependent inhibition of ouabain-stimulated SRIF release in a well-characterized hypothalamic dispersed cell system. We also examined the effects of phenytoin on SRIF release from dispersed rat cortical cells and inhibition of stimulated SRIF secretion was again observed. These findings support the hypothesis that the inhibition of neuronal SRIF release may represent a pharmacological mechanism of action of anticonvulsants.

Region-specific immunolocalization of atrial natriuretic peptide in mixed fetal rat brain cell cultures

In adult rodent CNS, atrial natriuretic peptide (ANP) has been localized by immunolabeling and nucleic acid hybridization techniques primarily to hypothalamic neurons and, to a lesser extent, to neurons of the telencephalon and brainstem. In canine brain, ANP immunoreactivity has been reported in neocortical and brainstem astrocytes. Yet, in a recent study using fetal rat tissue, ANP was detected by radioimmunoassay in predominantly neuronal, but not glial, cultures. In the present study ANP was detected by radioimmunoassay on in vitro day 8 in media derived from fetal rat diencephalic and rhombencephalic, but not telencephalic, cultures and in cell homogenates from all 3 regions surveyed. Using indirect immunofluorescence less than one cell per 400x field stained for ANP in the telencephalic cultures and ANP immunopositivity colocalized exclusively to neurons by concomitant anti-neurofilament immunolabeling. In diencephalic monolayers, approximately 1-3 cells per 400x field were ANP-positive; although most of these cells were neurons, small numbers of ANP-positive astrocytes were also detected using anti-glial fibrillary acidic protein (GFAP) immunolabeling. ANP-positive cells were most numerous in rhombencephalic cultures (5-10 cells per 400x field); as in diencephalic cultures, ANP immunoreactivity colocalized to both neurons and astrocytes in this region. In diencephalon and rhombencephalon, less than 1% of all ANP-positive cells were astrocytes and less than 1% of GFAP-positive astrocytes exhibited immunoreactive ANP. In fetal brain, neuronal and astrocytic ANP may play a role in fluid and electrolyte homeostasis. Alternatively, fetal brain cell ANP may subserve entirely different functions (e.g. as trophic factors) as has been suggested for other neuropeptides in the developing nervous system.

Endogenous vasoactive intestinal peptide (VIP) regulates somatostatin secretion by cultured fetal rat cerebral cortical and hypothalamic cells

To determine the possible physiological role of endogenous vasoactive intestinal peptide (VIP) in the control of cerebral somatostatin (SS), we studied the effect of endogenous VIP blockade on immunoreactive SS (IR-SS) accumulation by fetal rat cerebral cortical and hypothalamic cells in culture. Cells were cultured in minimum essential medium (MEM) with 10% fetal calf serum and 10% horse serum. After 7-10 days 'in vitro' media were replaced with MEMs without sera containing anti-VIP immunoglobulins G (IgG) for 1, 3, 6, 24 or 48 h. Controls received the same amount of IgG from normal rabbit serum (NRS). In another group of experiments, cells were incubated with VIP (10(-11) M to 10(-7) M) for 1, 3, 6 or 24 h. Exposure to anti-VIP IgG resulted in a decreased accumulation of IR-SS in both cerebral cortical and hypothalamic cells, whereas the addition of VIP caused a dose-dependent increase in total IR-SS, these effects being evident after 3 h incubation. The stimulatory action VIP on IR-SS was up to 129%, this being decreased to 86% by the addition of anti-VIP to plates containing 10(-7) M VIP. Patterns of IR-SS accumulation throughout prolonged incubation periods were qualitatively similar (in both cerebrocortical and hypothalamic cells) in the presence or absence of anti-VIP IgG. However, in plates containing anti-VIP, the total amount of IR-SS was lower than in the control groups (IgG from NRS). These findings demonstrate that, at this time of brain development, somatostatinergic neurons may be under the physiological regulation of locally produced VIP.

A correlation study of CSF neuropeptides in Alzheimer's and Parkinson's disease

The concentrations of somatostatin (SRIF), vasoactive intestinal polypeptide (VIP), beta-endorphin (beta-EP), adrenocorticotropin (ACTH) and corticotropin-releasing factor (CRF) immunoreactivity were measured in cerebrospinal fluid (CSF) of patients with Alzheimer's disease (AD), patients with Parkinson's disease (PD) and controls. In order to study the mechanisms that regulate peptide levels in CSF and peptide interactions, correlations between CSF peptides were determined. Within all patient groups a number of significant correlations were shown to exist between CSF peptides. The correlations were apparently not coincidental, since there was no such relation between the concentrations of CSF peptides and CSF protein content. Neither age, sex, severity of dementia nor the presence of extrapyramidal signs could explain the number of significant correlations. These results indicate, that the correlations found between CSF peptides may be due to common regulatory mechanisms or general physiological behaviour of peptides in the CSF.

Carbamazepine and phenytoin inhibit somatostatin release from dispersed cerebral cells in culture

To elucidate the mechanism by which carbamazepine lowers somatostatin concentration in cerebrospinal fluid of humans, the effect of carbamazepine on secretion of this peptide was studied in rat cerebral cell cultures. Concentrations of carbamazepine within the therapeutic range (4 x 10(-5) M) inhibited spontaneous release of somatostatin and blocked secretory responses to the epileptogen, picrotoxin, and to the cyclic cAMP stimulator forskolin. One of the proposed mechanisms of carbamazepine action is that it binds to adenosine receptors, but in this study, aminophylline, an adenosine antagonist, in a concentration as high as 2.4 x 10(-4) M, did not reverse carbamazepine effects. Carbamazepine suppression of picrotoxin, however, was overcome by exposure to veratridine, a sodium channel-active compound. This finding supports the hypothesis that carbamazepine acts by binding to sodium channels. Phenytoin, another anticonvulsant with many similar properties, also blocked picrotoxin-induced somatostatin release at a concentration of 10(-4) M, and its effects were also reversed by veratridine at a concentration of 10(-5) M. These findings clarify the mechanism by which carbamazepine and phenytoin act in epilepsy and trigeminal neuralgia.

Coexistence of muscarinic acetylcholine receptors and somatostatin in nonpyramidal neurons of the rat dorsal hippocampus

This study describes the colocalization of muscarinic acetylcholine receptors (mAChRs) and the neuropeptide somatostatin (SOM) in nonpyramidal neurons of the rat dorsal hippocampus. SOM and mAChRs were identified by immunocytochemistry employing antibody S309 and M35, respectively. Half of the SOMergic cell population is found to be immunoreactive for muscarinic receptor protein as obtained by fluorescent double-labeling techniques. These findings provide additional evidence for a direct cholinergic influence upon SOMergic, nonpyramidal neurons, and defines the anatomical distribution of SOMergic, cholinoceptive neurons in the dorsal hippocampus. Concerning the muscarinic cholinoceptive, nonpyramidal neuron population of the dorsal hippocampus, a considerable number (approximately one-third) was found to be colocalized with somatostatin. These results indicate that a significant part of the cholinergic influence upon hippocampal nonpyramidal neurons is relayed via SOMergic neurons.

Corticotropin-releasing hormone and dexamethasone do not alter secretion of immunoreactive beta-endorphin from dissociated fetal hypothalamic cell cultures

Corticotropin-releasing hormone (CRH) and glucocorticoids are major regulators of the hypothalamic-pituitary-adrenal (HPA) axis controlling secretion of beta-endorphin and other pro-opiomelanocortin (POMC)-derived peptides from pituitary. Although previous work has shown that CRH stimulates secretion of beta-endorphin from adult hypothalamic explants, and that glucocorticoids can inhibit basal and stimulated secretion of POMC-derived peptides from pituitary, the role of glucocorticoids on hypothalamic beta-endorphin secretion is not known. Studies were performed to assess the effects of CRH and dexamethasone, a potent glucocorticoid, on secretion of immunoreactive (IR) beta-endorphin from dissociated fetal hypothalamic cell cultures. CRH (10(-9)-10(-6) M) did not stimulate secretion of IR-beta-endorphin from hypothalamic cells which did release IR-beta-endorphin upon potassium-induced depolarization. However, CRH did stimulate IR-beta-endorphin secretion from fetal hypothalamic explants which were similar to hypothalamic tissue from which dissociated hypothalamic cell cultures were derived. Exposure of cells to dexamethasone (10(-6) M) did not inhibit basal or potassium-stimulated release of IR-beta-endorphin. These results indicate that: (1) dissociated fetal hypothalamic cells in culture do not exhibit a functional CRH receptor coupled to stimulation of IR-beta-endorphin secretion; (2) exposure of hypothalamic cells to dexamethasone does not inhibit basal nor depolarization-induced release of IR-beta-endorphin; and (3) dissociated fetal hypothalamic cells may have limited utility in elucidating specific regulatory relationships because of in vitro conditions and/or cytoarchitectural relationships.

Evidence that somatostatin enhances endogenous acetylcholine release in the rat hippocampus

The present experiments show that somatostatin (SS)-like immunoreactive material is present in the hippocampus and that its release can be increased by K+ stimulation of rat hippocampal slices, suggesting that SS-like peptides may be of significance to neurotransmission in the hippocampus. Exogenous SS-28 and SS-14 enhanced the K(+)-evoked release of endogenous acetylcholine (ACh) from rat hippocampal slices, whereas amino-terminal fragments of SS-28 did not. The increased ACh release in the presence of either peptide appeared to be mediated by an interaction with SS receptors because cyclo-SS, a putative SS antagonist, abolished the effects of both SS-28 and SS-14. In addition, the increase in ACh release induced by SS-14 or SS-28 was antagonized by the calcium channel antagonists omega-conotoxin GVIA, nifedipine, and cinnarizine, implicating voltage-sensitive calcium channels in this effect. Moreover, the effect was sensitive to tetrodotoxin, suggesting an indirect action of the peptides at a site distal to cholinergic nerve terminals. Cysteamine, which has been reported to deplete SS content and to increase SS release in brain, augmented the basal and evoked release of ACh from hippocampal slices, without affecting SS-like content and release. Finally, neuropeptide Y, which is colocalized with SS in many neurons of the hippocampal formation, did not alter ACh release, nor did it facilitate the SS-induced increase. The results suggest that in the rat hippocampus, both SS-28 and SS-14 interact with SS receptors to regulate ACh release indirectly by a mechanism that involves alterations of calcium influx during depolarization.

Effect of antiepileptic drugs on somatostatin release in vitro

In the present study we investigated the effect of antiepileptic drugs on high potassium (50 mM) stimulated somatostatin release in rat cortical slices in a superfusion system. The somatostatin-like immunoreactivity (SLI) in superfusate was determined by radioimmunoassay. The antiepileptic drugs studied, vigabatrin, valproate, carbamazepine, phenobarbital, primidone, clonazepam and phenytoin were tested at a concentration range of 1-1000 microM). Of the drugs used vigabatrin had the most significant inhibitory effect on SLI release (IC50 = 240 microM). Vigabatrin also caused a concomitant, dose-dependent increase in superfusate gamma-amino butyric acid (GABA) level. A 30% decrease in the release of SLI followed incubation with valproate and carbamazepine, but only at high drug concentrations (1000 microM). Phenobarbital, primidone, clonazepam and phenytoin did not affect SLI release. Addition of GABA to superfusate caused a dose-dependent decrease in the amount of SLI release (IC50 = 56 microM). In conclusion, at low concentrations the antiepileptic drugs had only minor effects on SLI release. At higher concentrations, however, vigabatrin and valproate decreased the release of SLI, which may relate to their ability to elevate tissue levels of GABA.

Basal and stimulated release of substance P from dissociated cultures of vagal sensory neurons

Substance P, the widely distributed 11 amino acid neuropeptide, is present in up to 20% of vagal sensory cell bodies and the fibers emanating from them. To study the factors regulating the release of SP, vagal sensory (nodose or nodose/jugular) ganglia were obtained from neonatal rats and dissociated using neutral protease. Survival of plated neurons on collagen substrate was 10-20% at 2 weeks and 20-30% when neurons were plated over previously dissociated rat atriacytes. Substance P content was low in cultures for the first several days, then rose linearly to 0.1-0.2 pg/surviving neuron. Substance P was released into a 4.5 mM potassium medium at a steady rate of 0.036%/min. In 50 mM K+ supplemented medium, total release during 20 min increased 5-8-fold and steady-state release increased 4-5-fold to 0.15%/min. The sensory neuron specific excitatory neurotoxin, capsaicin, evoked SP release in similar amounts to 50 mM K+. Both net K(+)- and capsaicin-evoked, but not basal release were completely inhibited by 3.5 mM cobalt chloride. Bradykinin, 1-100 nM, stimulated SP release 2-4 times above basal levels. Forskolin and phorbol ester also increased SP release 1.5-3 times basal amounts. In summary, substance P is present in cultured vagal sensory neurons in amounts similar to in vivo and is released in response to sensory specific stimuli. These cultures should allow exploration of some of the tissue specific factors regulating neurotransmitter release in the sensory vagus nerve.

Gomori-positive astrocytes in primary culture: effects of in vitro age and cysteamine exposure

Gomori-positive astrocytes have been identified in the periventricular brain in situ and in diencephalic explants on the basis of their endogenous peroxidase activity, affinity for chrom alum hematoxylin, and orange-red autofluorescence. To facilitate analyses of their functional properties, we sought to identify these cells in dissociated fetal rat brain cultures. Astrocytes containing cytoplasmic inclusions with the above tinctorial and fluorescent properties represented less than 1% of cultured astrocytes at day 10 in vitro (DIV). There was a marked increase in the fraction of Gomori-positive astrocytes and their granule content between 10 and 46 DIV. As in situ, the peroxidase activity appeared to be non-enzyme-mediated insofar as it catalyzed diaminobenzidine oxidation over a wide range of pH (3-11) and could not be inhibited by tissue preheating or the catalase inhibitor, aminotriazole. Metalloporphyrins probably mediate both the pseudoperoxidase activity and autofluorescence in these cells. Cysteamine and cystamine, but not ethanolamine or L-cysteine, induced a massive accumulation of Gomori-positive astrocytes when administered from DIV 6-18. Alterations of the redox microenvironment or induction of porphyrin/heme biosynthetic enzymes may be the mechanisms responsible for this cyst(e)amine effect. Dissociated rat brain culture enriched for Gomori astroglia should provide ample opportunity to investigate the functional properties of these cells.

Effects of acute and chronic cocaine administration on somatostatin level and binding in the rat brain

The effects of acute and chronic cocaine (40 mg/Kg i.p.) in vivo administration, on 125I-Tyr11-somatostatin binding and somatostatin-like immunoreactivity (SLI) in the rat frontoparietal cortex, hippocampus and olfactory bulb were explored. Acute and chronic cocaine administration did not affect the levels of SLI in the three brain areas studied. Acute cocaine administration resulted in an 55% and 32% decrease in the total number of specific somatostatin receptors in the hippocampus and olfactory bulb respectively, but not in the frontoparietal cortex. Somatostatin receptor affinity increased in the hippocampus and was unaltered in frontoparietal cortex and olfactory bulb. After two weeks of daily cocaine injections the somatostatin binding in the hippocampus and olfactory bulb returned to control values. The in vitro addition of cocaine to a brain membrane preparation obtained from untreated rats did not markedly affect somatostatin binding characteristics. These results are suggestive of a possible role for somatostatin in the limbic structures as a response to cocaine administration.

Estradiol induces rapid remodelling of plasma membranes in developing rat cerebrocortical neurons in culture

Exo-endocytotic images and intramembrane particles were quantitatively assessed in freeze-fracture replicas from the plasma membrane of dispersed fetal rat cortical neurons (day 16 gestation) grown for 24 days in culture. The addition of 10(-10) M 17 beta-estradiol to the culture medium resulted in a significant increase in the numerical density of exo-endocytotic images within 1 min. A further increase of the number of exo-endocytotic images associated to a significant decrease in the number of intramembrane particles was observed in cells exposed for 10 min to 17 beta-estradiol. Similar results were observed when the cells were exposed to 17 beta-estradiol for 17 days. No effects on exo-endocytotic images and intramembrane particles were observed when 17 alpha-estradiol was added, instead of 17 beta-estradiol, to the cultures. These results indicate that physiological levels of 17 beta-estradiol can have rapid effects upon the ultrastructure of the neuronal membrane of developing cerebrocortical neurons.

Production of immunoreactive adrenocorticotropin and beta-endorphin by hypothalamic and extrahypothalamic brain cells

Despite many in vivo studies, little is known about brain regulation of POMC synthesis or regulation of secretion of POMC-related peptides. To test the hypothesis that dissociated brain cells in culture can produce and release POMC-related peptides, immunoreactive (IR)-adrenocorticotropin (ACTH) and beta-endorphin were measured in cells and media of dissociated cell cultures incubated up to 38 days. Fetal rat hypothalamic and extrahypothalamic forebrain cells were maintained in serum free medium. IR-ACTH and beta-endorphin were measured by radioimmunoassay in concentrated cells and media after various incubation times using two ACTH (mid-portion = R4; carboxy-portion directed = KEND) antisera and a beta-endorphin antiserum. IR-ACTH and IR-beta-endorphin in hypothalamic and extrahypothalamic cells and in media (cumulative) were greater than quantities in cells before culture. Peak hypothalamic cellular content of IR-ACTH (5.3 fmol/10(6) cells-R4; 4.7 fmol/10(6) cells-KEND) and content of IR-beta-endorphin (32.0 fmol/10(6) cells) occurred on days 16, 9 and 23, respectively. Peak extrahypothalamic content of IR-ACTH (2.9 fmol/10(6) cells-R4; 1.0 fmol/10(6) cells-KEND) and content of IR-beta-endorphin (10.8 fmol/10(6) cells) was also seen on different days, was lower than hypothalamic content and was not always concurrent with peak hypothalamic content. Gel filtration chromatography revealed that the predominant forms of IR-ACTH and IR-beta-endorphin in hypothalamic cell extracts co-eluted with synthetic ACTH1-39 and beta-endorphin. Changes in molar ratios of IR-ACTH and IR-beta-endorphin also suggested a differential regulation of different POMC derivatives.

CONCLUSIONS

(1) IR-ACTH and IR-beta-endorphin are produced by hypothalamic and extrahypothalamic forebrain cells in culture: and (2) dissociated brain cell cultures can be used as a potential model for studying regulation of POMC-related peptides in brain.

Divergent effects of acute depolarization on somatostatin release and protein synthesis in cultured fetal and neonatal rat brain cells

The influence of membrane depolarization on somatostatin secretion and protein synthesis by fetal and neonatal cerebrocortical neurons was studied. Cortical cells obtained by mechanical dispersion were maintained as monolayer cultures for 8 days. The ability of fetal cerebrocortical and hypothalamic cells to release immunoreactive somatostatin (IR-SRIF) was confirmed. Total protein synthesis was determined by the incorporation of [3H]phenylalanine into trichloroacetic acid-precipitable proteins. To study the effect of acute depolarization on protein synthesis, cells were incubated for 30 min with [3H]phenylalanine or [3H]leucine and the depolarizing agent. In fetal cerebrocortical cells, potassium (30 and 56 mM) decreased protein synthesis and RNA levels and increased IR-SRIF release. Depolarization by veratridine, a sodium channel activator, induced a similar effect. The effect of veratridine on IR-SRIF and protein synthesis was reversed by tetrodotoxin, a sodium channel blocker, or verapamil, a calcium channel blocker. These findings suggest that protein synthesis by cerebrocortical cells is decreased in fetal brain cells by membrane depolarization and is dependent on Na+ and Ca2+ entry into cells. In postnatal (day 7) cerebrocortical cells, depolarization induced by high potassium concentrations led to a concomitant increase in protein synthesis, RNA content, and somatostatin release. These findings indicate that depolarization of the cellular membrane is coupled to an increase in protein synthesis in neonatal, but not in fetal, dispersed brain cells.

Somatostatin in neuropsychiatric disorders

1. Somatostatin is a peptide that is widely and discretely distributed throughout the central nervous system. 2. Its relevance to neuropsychiatric disorders is suggested both by the existence of disease-related alterations in somatostatin content in brain and cerebrospinal fluid as well as by the manifold neuroregulatory capabilities of somatostatin and related peptides. 3. This article will summarize the central nervous system effects of somatostatin, identify those neuropsychiatric disorders that are characterized by changes in somatostatin, and review the evidence for and potential significance of decreases in cerebrospinal fluid somatostatin in depression.

Somatostatin-like immunoreactivity and somatostatin receptor binding in rat brain in pentylenetetrazol-induced kindling

Kindling was induced in Sprague-Dawley rats by repeated injection of pentylenetetrazol (PTZ, 30 mg/kg, IP). Somatostatin-like immunoreactivity (SLI) and 125I-Tyr-somatostatin binding were measured in different areas of the brain in saline-injected controls, rats receiving PTZ but not kindled (prekindled rats), and kindled rats. Compared to SLI levels in controls, SLI increased (p < 0.01) in the frontal cortex, striatum (p < 0.05 in kindled rats), and hippocampus of both prekindled and kindled animals. Hypothalamic levels of SLI remained unchanged. Compared with the controls, no change was found in somatostatin receptor binding in the frontal cortex, striatum or hippocampus in prekindled or kindled animals. These results suggest that an increased level of somatostatin may be connected to the development and maintenance of kindling.

Somatostatin antiserum blocks carbachol-induced increase of paradoxical sleep in the rat

Injection of carbachol into the nucleus tractus solitarius (NTS) produced an increase of paradoxical sleep (PS). When somatostatin antiserum (SRIF-AS) was infused continuously following carbachol injection, the increase in PS was blocked. These results suggest that carbachol-induced increase in PS is mediated by somatostatin. In addition, they confirm the role of somatostatin in the generation of PS.

Long-term neuropathological and neurochemical effects of nucleus basalis lesions in the rat

The long-term effects of excitotoxic lesions in the nucleus basalis magnocellularis of the rat were found to mimic several neuropathological and chemical changes associated with Alzheimer's disease. Neuritic plaque-like structures, neurofibrillary changes, and neuronal atrophy or loss were observed in the frontoparietal cortex, hippocampus, amygdala, and entorhinal cortex 14 months after the lesions were made. Cholinergic markers in neocortex were reduced, while catecholamine and indoleamine metabolism was largely unaffected at this time. Bilateral lesions of the nucleus basalis magnocellularis increased somatostatin and neuropeptide Y in the cortex of the rat by at least 138 and 284 percent, respectively, suggesting a functional interaction between cholinergic and peptidergic neurons that may differ from that in Alzheimer's disease.

Somatostatin, beta-endorphin, and prolactin levels in human cerebrospinal fluid during the gamma-vinyl-GABA treatment of patients with complex partial epilepsy

The anticonvulsant action of the new anticonvulsant drug gamma-vinyl-GABA (GVG) is obviously mediated by elevation of the concentration of GABA in the brain. The effect of GVG administration on other transmitter systems is not fully known in humans. We studied the possible interactions of GVG administration with peptidergic systems. Included in this study were 67 patients with complex partial epilepsy (CPS). The first CSF sample was taken before GVG administration. The second CSF sample was taken after 3 months of GVG treatment (3 g/day). Thereafter half of the responders (50% decrease in seizure frequency or clear improvement in global performance) received 3 g/day and the other half received 1.5 g/day for the next three months, after which the third CSF sample was taken. Somatostatin (SLI), beta-endorphin (beta-EP), and prolactin (PROL) levels in CSF were measured by radioimmunoassay. Total GABA (tGABA) and GVG levels in CSF were measured by high performance liquid chromatography. After 3 months of GVG treatment there was a slight increase in the beta-EP (p = 0.027, Student's paired t-test), which was not found after 6 months of GVG administration. Both SLI and PROL were stable during the study. Peptide levels were not connected to the clinical response to GVG, GVG dosage, or to tGABA levels in the CSF. In conclusion, the elevation of GABA levels in the brain during GVG treatment apparently does not induce long-term interactions with the peptidergic systems studied.

Effect of pentylenetetrazol-induced convulsions on somatostatin-like immunoreactivity in rat cerebrospinal fluid

Somatostatin is a neuropeptide that in several experimental models of epilepsy has been suggested to modulate epileptic activity. The purpose of the present study was to investigate the role of somatostatin in seizure phenomena. We measured the somatostatin-like immunoreactivity (SLI) by radioimmunoassay of the cisternal CSF of rats. A polyethylene cannula had before-hand been inserted into the cisterna magna. Thereafter seizures were induced by pentylenetetrazol (PTZ). The nonconvulsive group of rats received a single subconvulsive dose of PTZ (30 mg/kg, i.p.). This group of rats exhibited only clonic jerks but not generalized clonic-tonic convulsion (GC). The CSF samples were taken 2 and 10 minutes after the jerks began. The convulsive group of rats received a single convulsive dose of PTZ (50 mg/kg, i.p.), and each of those animals had GC. From those rats the CSF samples were collected 5, 30, and 60 minutes and 4 and 24 h after the GC began. The values were compared with the SLI levels in controls, from which CSF was collected 10 minutes after injection of 0.9% NaCl. In the convulsion group the SLI levels increased 241% (p less than 0.01) five minutes after GC and returned to control level in 30 minutes. In the nonconvulsion group, where the rats expressed only jerks but not GC, SLI levels remained constant. These data suggest that somatostatin is released into CSF after the generalized clonic-tonic phase of the PTZ-induced convulsion.

Failure of carbachol to influence the release of somatostatin from slices of rat cerebral cortex

A [K+]-related, Ca2+-dependent efflux of immunoreactive somatostatin (IRS) from superfused slices of rat cerebral cortex has been observed; this release paralleled the release of both [14C]noradrenaline and [14C]GABA. However IRS release in this preparation was not stimulated by the muscarinic agonist carbachol at low (10 microM) or high (500 microM) concentrations. Furthermore, 100 or 500 microM carbachol did not affect the IRS efflux from rat cortex slices incubated in the presence of 12, 25 or 53 mM K+.

Carbamazepine lowering effect on CSF somatostatin-like immunoreactivity in temporal lobe epileptics

The effect of carbamazepine treatment on CSF-somatostatin-like immunoreactivity (SLI) in patients suffering from temporal lobe epilepsy was investigated. A baseline lumbar puncture was performed on 12 patients and 10 normal volunteers. A second tap was repeated only in patients when they were on peak of carbamazepine concentration for 10 days. Levels of CSF-SLI were measured by RIA. No significant differences were found in CSF-SLI basal concentrations between epileptics and controls, whereas a significant decrease (p less than .0002 Duncan's multiple range test) of CSF peptide levels occurred in 9 of 12 patients under medication. Although the neural mechanism through which carbamazepine lowers CSF-SLI is still unknown, the results of the present study suggest that the reported effect might be part of the apparatus by which carbamazepine exerts its anticonvulsant action.

Dopamine stimulates rat cortical somatostatin release

Release of somatostatin from slices of rat frontal cortex was studied. Increasing the potassium ion concentration in the medium from 6 mM to 55 mM resulted in a significantly increased release of somatostatin. Dopamine increased the release of somatostatin from cortex and this effect of dopamine was blocked by haloperidol and other dopaminergic antagonists. Other catecholamines as well as serotonin, histamine and acetylcholine failed to stimulate the release of somatostatin. The stimulatory effect of dopamine on release of somatostatin from cortical slices provides an approach for examination of the receptor properties and function of dopamine in this brain region.

Cerebrospinal fluid somatostatin and psychiatric illness

Somatostatin is a tetradecapeptide that is assuming increasing importance as a regulator of central nervous system activity. Originally identified as the hypothalamic growth hormone release-inhibiting factor, somatostatin has subsequently been shown to be extensively and selectively distributed throughout the central nervous system, to alter neuron excitability, to regulate and be regulated by the activity of classical neurotransmitters and neuropeptides, to exert a number of direct behavioral actions, and to display neuropsychiatric disorder-related alterations. In this article, a three-part study of cerebral spinal fluid (CSF) somatostatin in affective illness and schizophrenia is presented. In part 1, significant reductions in CSF somatostatin were observed in 49 bipolar and unipolar depressed patients relative to 47 controls. Values during depression were also significantly lower than those observed in affective disorder during the improved state or in schizophrenia. Diurnal studies involving paired AM and PM lumbar punctures revealed that depressed patients and normal volunteers had similar somatostatin values in the evening, despite having significantly different values in the morning. In part 2, the effects of several psychopharmacological agents on CSF somatostatin were examined, particularly the tricyclic anticonvulsant carbamazepine. A significant reduction of CSF somatostatin during treatment with carbamazepine was observed. The effect of carbamazepine on somatostatin could be related to its anticonvulsant, analgesic, or psychotropic effects. Part 3 deals with somatostatin as a major regulator of hypothalamic-pituitary-adrenal (HPA) axis activity. Somatostatin affects HPA activity by inhibiting, at a number of cellular levels, the stimulated release of adrenocorticotrophic hormone (ACTH) from the pituitary. A significant negative relationship between CSF somatostatin and the postdexamethasone plasma cortisol level in 22 depressed and 16 schizophrenic patients was observed. This relationship between low CSF somatostatin and escape from dexamethasone suppression was observed irrespective of diagnosis (i.e., depression or schizophrenia). Thus, there is indirect supporting evidence for a role for somatostatin dysregulation in the most consistently observed biological abnormality in depression, escape from dexamethasone suppression. Further study of somatostatin in neuropsychiatric disorders, and particularly depressive illness, offers great promise for better understanding their underlying affective, vegetative, cognitive, and physiological dysregulations.

Intravenous uridine treatment antagonizes hypoglycaemia-induced reduction in brain somatostatin-like immunoreactivity

By means of radioimmunoassay procedures, cholecystokinin-(CCK) and somatostatin-(SRIF) like immunoreactivity have been studied in the dorsal hippocampal formation and in the frontoparietal cortex of the male rat in insulin-induced hypoglycaemia, leading to an isoelectric EEG pattern. It has been demonstrated that severe hypoglycaemia of 40-min-duration produces a disappearance of SRIF but not of CCK-like immunoreactivity in both cortical regions. It was found that an i.v. injection of uridine but not of saline could significantly counteract the disappearance of SRIF-like immunoreactivity induced by severe hypoglycaemia in both cortical areas. Uridine did not by itself change plasma glucose levels. It is suggested that uridine may prevent release and/or increase synthesis of cortical SRIF peptides in severe hypoglycaemia, possibly due to an action on the metabolism (e.g. by enhancing the resynthesis of phosphatidyl inositol) within the tissue of the cerebral cortex and/or on putative pyrimidine binding sites in the brain controlling SRIF synthesis and/or release. It is possible that uridine in this way may improve recovery of neuronal function within SRIF-immunoreactive neurons of the cerebral cortex after severe hypoglycaemia (which also may be true in other states of reduced metabolic support). These findings suggest a possibility to use uridine in the treatment of Alzheimer's disease and Status epilepticus.

Influence of a benzodiazepine, midazolam, and gamma-aminobutyric acid (GABA) on basal somatostatin secretion from cerebral and diencephalic neurons in dispersed cell culture

To investigate the mechanisms of gamma-aminobutyric acid (GABA) and benzodiazepine-induced growth hormone (GH) release, we studied the effects of GABA and a water-soluble benzodiazepine, midazolam, on basal immunoreactive somatostatin secretion from fetal rat brain in dispersed cell culture. Both GABA and midazolam in concentrations of 10(-5) or 10(-6) M inhibited basal somatostatin secretion from either diencephalic or cerebral neurons in culture. Midazolam (10(-5) M) produced a 33.2 +/- 8.6% suppression (P = 0.004) and 10(-5) M GABA produced a 46.0 +/- 4.3% suppression (P = 0.0003) in the diencephalon cultures. When GABA and midazolam were used in combination over the 10(-5)-10(-9) M range, the drugs were shown to act independently (positive main effect, P less than 0.0001 for either drug by two-way analysis of variance); there was a simple additive effect with no statistically significant interaction between the two drugs over the 36 combinations tested. These results suggest that suppression of the GH inhibitory peptide, somatostatin, may be one of the mechanisms by which GABA and benzodiazepines stimulate GH secretion. Based on previous studies of GABA and benzodiazepine receptors, it appears likely that these drugs produce this inhibitory effect by interacting with unassociated lower affinity receptors which require micromolar concentrations of the drugs, and act through calcium-dependent pathways.

Somatostatin in the central nervous system: physiology and pathological modifications

Since its discovery, at the beginning of 1973, somatostatin's multiple actions, in relation to its wide anatomical distribution have been widely documented. Its biochemical pathways have been elucidated with the discovery of other molecular forms as well as the mechanisms of its neuronal release. However, no definite proof is available concerning a neurotransmitter role for any peptide of the somatostatin family other than somatostatin-14. The precise determination of the roles of somatostatin in brain are still hampered by the poor pharmacology of the peptide. New tools are badly needed and in particular a true antagonist at the receptor site. The mechanisms of action of somatostatin are now well under way at least in the pituitary model. More information should come from this model and be applied to brain cells in vitro. The greatest challenge of somatostatin brain function lies in its role in the pathophysiology of neurological diseases such as Alzheimer's dementia and Huntington's disease. Nature has been using somatostatin-related molecules since inhibitory control was first needed in cell functions. Time will tell us if somatostatin is really an old peptide involved in senile dementia.

Somatostatin release from dissociated cerebral cortical cell cultures

Primary monolayer cultures of dispersed fetal cerebral cortical cells can be used to measure the release of the neuropeptide, somatostatin. Three to five percent of cellular IRS is released basally into KRB in 10 min. Basal release is stable for at least 60 min and stimulated levels of release can be induced by introducing ionophores, neurotransmitters, or peptides. The peptide content of the incubation samples is readily measured by a well-characterized, sensitive RIA. Table II summarizes the major factors that must be taken into consideration when developing this system for measuring peptide release.

Effects of haloperidol on somatostatin-like immunoreactivity in the CSF of Schizophrenic patients

The levels of somatostatin-like immunoreactivity (SLI) were determined in the cerebrospinal fluid (CSF) of 14 schizophrenic patients before and after 3 weeks on haloperidol treatment. Baseline levels of SLI correlated negatively with psychopathological items on the Brief Psychiatric Rating Scale related to psychotic productivity. SLI levels increased after haloperidol treatment, but this increase did not correlate with psychopathological improvement. A difference in the ratio of larger and smaller molecular forms of the peptide was found before and after treatment. The drug-free samples showed a preponderance of the larger molecular forms, resulting in a ratio of 4:1, whereas the haloperidol-treated samples showed an equal distribution of both species.