Somatostatin and analogs inhibit endogenous synaptic plasma membrane protein phosphorylation in vitro

Role of the vestibular nuclei in endothelin-1-induced barrel rotation in rats

The fourth or lateral ventricular injection of endothelin-1 resulted in a dose-dependent increase in the barrel rotation and produced marked induction of c-Fos-positive cells in the vestibular nuclei. The doses of the former injection were lower and had shorter mean latent periods compared with the later injection. c-Fos expression after endothelin-1 injection was prevented by the pretreatment with the endothelin ET(A) receptor antagonist, cyclo(D-alpha-aspartyl-L-propyl-D-valyl-L-leucyl-D-tryptophyl) (BQ-123), the glutamate NMDA receptor antagonist, dizocilpine maleate (MK-801), or the L-type Ca(2+) channel antagonist, verapamil, in addition to the incidence of the rotational behavior. There was a significant difference in c-Fos expression between the right and left medial vestibular nuclei, and the number of c-Fos-labeled neurons in the medial vestibular nucleus was markedly increased on the opposite side of the rotational direction. These results suggest that the elicitation of the barrel rotation may be mediated by endothelin ET(A) receptors, glutamate NMDA receptors, and L-type Ca(2+) channels. The changes in the receptor and channel systems induced by endothelin-1 injections appeared to exert crucial influences on the vestibular nuclei and then on the maintenance of equilibrium. The direction of the barrel rotation has a deep connection with the imbalance of neuronal activity in the left and right medial vestibular nuclei.

Phorbol ester- and glutamate-sensitive phosphorylation of hippocampal membrane proteins from adult and neonatal rats

The phosphoinositide (PI) second messenger system in the neonatal rat brain is differentially stimulated as compared to that of the adult by agonists such as glutamate. Among the factors that might contribute to the neonatal pattern is the nature of phosphorylated membrane-bound proteins which could regulate this receptor-mediated response. This study was undertaken to compare membrane protein phosphorylation under conditions that affect PI hydrolysis in neonatal and adult rat hippocampus. Two-dimensional gel analysis revealed enhanced basal phosphorylation of two membrane proteins (M(r): 46,000 and 80,000; pI: 4.4 and 4.2, respectively) in the neonatal hippocampus when compared to the adult. The former phosphoprotein is present only in neonatal hippocampus. Phosphorylation of a 48,000 M(r) protein with a pI of 4.5 is prominent in hippocampal membranes from both neonatal and adult rats. After incubation of neonatal hippocampal slices with an active phorbol ester, 12-O-tetradecanoyl phorbol-13-acetate (TPA), all three proteins show decreased post-hoc phosphorylation. Slices from neonatal rats incubated with glutamate demonstrated no alteration in the phosphorylation of any of these proteins, while those from adult rats produced a marked change in phosphorylation of the 80,000 M(r) protein. The data suggest that phosphorylation of this protein from neonates is not yet as efficiently coupled to receptor stimulation as that from the adult. Immunoblot analysis revealed that the 48,000 M(r) protein is the growth-associated protein B-50/GAP-43 and that the 80,000 M(r) protein is a membrane-associated form of the MARCKS protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Somatostatin inhibition of VIP- and isoproterenol-stimulated cyclic AMP accumulation in dissociated cells from rat cerebral cortex

Freshly dissociated cerebral cortex cells from adult rats have been used in the present study to determine if dual regulation of cyclic AMP levels by inhibitory and stimulatory agents can be expressed in the mature brain. Somatostatin, an inhibitory agent, barely affected the basal cyclic AMP metabolism while vasoactive intestinal peptide (VIP) and isoproterenol, two stimulatory agents enhanced cyclic AMP production. However, this increase was depressed by somatostatin, which decreased the efficiency, but not the potency, of the effects of the two stimulatory agents on cyclic AMP accumulation.

Dephosphorylation of B-50 in synaptic plasma membranes

Synaptic plasma membranes from rat brain cortex possess intrinsic ability to dephosphorylate the endogenous protein B-50. At low concentrations of [gamma-32P]ATP, B-50 phosphorylation in synaptic membranes is maximal at 30 seconds, followed by dephosphorylation for an additional 60 minutes. The dephosphorylation of 32P-labeled B-50 is not sensitive to the protease inhibitor leupeptin and not correlated with a loss of the B-50 content of synaptic membranes as measured with immunoblot analysis. Dephosphorylation of membrane-associated B-50 is stimulated to a small extent by Mg2+ but not by Ca2+. Heat-stable protein phosphatase inhibitors prevent dephosphorylation of 32P-labeled B-50. Dephosphorylation of B-50 in synaptic membranes is stimulated by ATP, ADP, or adenosine 5'-O-thiotriphosphate, but not by adenine, adenosine, other adenine or guanine nucleotides, nonhydrolyzable analogs of ATP or GTP, nor by adenosine 5'-O-(2-thiodiphosphate). B-50, phosphorylated by exogenous protein kinase C and purified to homogeneity, has been used as a substrate to follow the purification of B-50 phosphatase activity. B-50 phosphatase activity can be solubilized from synaptic membranes with 0.5% Triton X-100 and 75 mM KCl. Chromatography of the extract on DEAE-cellulose yields enhanced B-50 phosphatase activity.

The influence of substance P and its fragments on endogenous phosphorylation of synaptosomal membrane protein (synapsin) from cerebral cortex of rat brain

1. The effects of substance P and its fragments and analogue of a C-terminal fragment on cyclic AMP-dependent phosphorylation of synapsin I in synaptosomal membranes (SM) from cerebral cortex were investigated. 2. SP(I-II) and SP(1-4) at 10(-3) M caused a marked stimulation of synapsin I phosphorylation. 3. A C-terminal fragment of SP (SP6-11) had no effect on phosphorylation of synapsin 1. 4. Analogue of C-terminal fragment [(Tyr8)SP6-11] at 10(-3) M distinctly inhibits phosphorylation of synapsin I. 5. These data suggest that SPI-II and its C- and N-terminal fragments have a modulator function against the phosphorylation of some rat brain proteins.

Preclinical and clinical studies with somatostatin related to the central nervous system

1. The tetradecapeptide somatostatin (SS) has a widespread, uneven distribution within several organs including the central nervous system (CNS), with particularly high concentration in the hypothalamus. 2. The SS-related peptides (SS28, SS28(1-12), SS28(15-28)) are originated from the precursor pre-prosomatostatin. 3. SS is suggested to be involved in a large number of CNS functions, locomotion, sedation, excitation, catatonia, body temperature, feeding, nociception, paradoxical sleep, self-stimulation, seizure, learning and memory. 4. SS influences central neurochemical processes. 5. It is possible that SS is related to various neurological and psychiatric illnesses, like Huntington's disease, multiple sclerosis, Parkinson's disease, epilepsy, eating disorders, Alzheimer's disease, schizophrenia and major depressive illness.

The growth-associated neuronal phosphoprotein B-50: improved purification, partial primary structure, and characterization and localization of proteolysis products

A reversed phase HPLC procedure is reported that has allowed the separation of the growth-associated, kinase C substrate protein B-50 [previously purified by isoelectric focussing (IEF)] into three components (1-, m- and rB-50). The minor form 1B-50 (probably a proteolysis product) gave a 24-residue N-terminal amino acid sequence, but the major and possibly native form (mB-50) (and also rB-50 which is probably formed during IEF) appeared to be N-terminally blocked. HPLC also separated B-60, the major proteolysis product of B-50, into three components, and the N-terminal sequence of the major B-60 was determined. HPLC peptide mapping of SAP digests of the various B-50 and B-60 protein confirmed their close relationship, and four SAP generated fragments also afforded sequence data. The amino acid sequences obtained (1B-50, B-60 and fragments) are all found in the recently predicted (based on nucleotide sequencing) B-50/GAP43 sequence (226 amino acids), further confirming the identity of B-50 and GAP43, and helping clarify the relationship of B-60 (starting at position 41 of the predicted sequence) to B-50. Correlation of amino acid analyses, SAP fragment data, and the predicted sequence provided additional information on the length of the translated products, including evidence that the N-terminus of the major (blocked) form of B-50 starts at position 1 (Met) of the predicted sequence.

Excessive grooming induced by somatostatin or its analog SMS 201-995

Intracerebroventricular (i.c.v.) administration of somatostatin or SMS 201-995 induces excessive grooming behavior in rats. The grooming inducing effect of somatostatin is rather weak, as doses of 300 ng or less did not result in increased total grooming scores. In contrast a dose of 10 ng SMS 201-995 already significantly increased the total grooming scores. However, doses of 100 ng and more did not further increase the total grooming scores reached with a 50 ng dose of this peptide. Systemic administration of SMS 201-995 in doses up to 900 micrograms did not result in excessive grooming behavior. The patterns of excessive grooming induced by i.c.v. SMS 201-995 and somatostatin were characterized by a predominant display of scratching. Since peptide-induced scratching is mainly due to activation of opiate receptor systems it is suggested that opiate receptors are involved in the behavioral response to SMS 201-995 and somatostatin administration. This suggestion is further supported by the suppressive effect of naloxone on excessive grooming induced by these peptides. Haloperidol and neurotensin also suppress the excessive grooming induced by somatostatin but not that induced by SMS 201-995. Finally, tolerance developed to the grooming-inducing effect of SMS 201-995 and somatostatin. In addition there was cross tolerance between somatostatin and SMS 201-995.

Effects of arginine vasopressin on protein phosphorylation in rat hippocampal synaptic membranes

Our laboratory has reported previously the characteristics of specific AVP binding to rat hippocampal synaptic membranes (SPM) in the presence of Ni2+ [Costantini MG, Pearlmutter AF: J Biol Chem 259: 11739-11745, 1984]. We extended our investigation to determine the effects of Ni2+, (AVP), and AVP analogs on SPM protein phosphorylation. Ni2+ (5 mM) caused a dramatic reduction in phosphorylation of most SPM phosphoproteins. The most prominent protein which is phosphorylated in SPM has a molecular weight of 48 kilodaltons (KDa) and has been named B50 or F1; this protein shows altered phosphorylation in vitro in response to long-term potentiation in vivo as well as changes induced by exposure of SPM to ACTH (1-24), dopamine, and somatostatin. AVP and related peptides reduced phosphorylation of this pre-synaptic phosphoprotein in the following order of potency: AVP = oxytocin greater than DG-AVP greater than dDAVP greater than d(CH2)5Tyr(Me)AVP = [pGlu4,Cyt6]AVP-(4-9). Except for the pressor antagonist d(CH2)5Tyr(Me)AVP, this corresponds to their relative efficacy in displacing 3H-AVP from high-affinity specific binding sites on rat hippocampal synaptic membranes. Ni2+ did not alter the degree of inhibition caused by the peptides. When SPM were treated with AVP after the attainment of maximum 32P incorporation, AVP inhibited dephosphorylation over a 30-min period. Our results show that AVP can alter both phosphorylation and dephosphorylation of hippocampal SPM phosphoproteins in vitro; the direction of these effects depends upon experimental conditions. Since B50/F1 is known to be a substrate for protein kinase C, AVP may act by inhibition of protein kinase C activity, either directly or indirectly.

Protein phosphorylation in rat hippocampal synaptic plasma membranes in response to neurohypophyseal peptides

The effect of arginine vasopressin (AVP) on protein phosphorylation in rat hippocampal synaptic plasma membranes (SPM) was examined. With a crude SPM preparation, AVP (10(-8)-10(-5) M) stimulated phosphorylation of a number of proteins which included a brain-specific protein of 48 kDa called B50 or protein F1, which is thought to be related to synaptic plasticity. Equimolar levels of oxytocin also stimulated B50/F1 phosphorylation. AVP and oxytocin at higher concentrations (10(-4)-10(-3) M) reduced SPM protein phosphorylation. When SPM was treated with both AVP and oxytocin, the effects were not additive; on the other hand, the effects of the phorbol ester (TPA) and AVP were additive. With SPM, partially purified by sucrose density centrifugation, only the inhibitory effect of AVP on B50/F1 phosphorylation was seen. These results suggest that AVP and oxytocin stimulation of B50/F1 phosphorylation requires cellular factors which are removed from SPM during membrane purification. In contrast, the inhibitory mechanism triggered by AVP and oxytocin appears to be associated with, or an integral part of, the synaptic membrane itself. Because the effects on membrane protein phosphorylation with maximal amounts of AVP and oxytocin were not additive, they must bind to the same sites on the membrane. This conclusion is supported by the additivity of the effects of AVP and phorbol ester, since the phorbol ester can act directly on the kinase and does not require a membrane recognition site.

Somatostatin receptors

More insight into the biochemical structure and operation of the somatostatin receptor(s) has been gained in recent years from several approaches. The minimal active structure of the receptor(s) has been identified, and active minisomatostatins have been synthesized. High-affinity binding sites (KDS ranging from 0.1 to 1 nM) have been demonstrated in brain and peripheral organs. In pancreas, stomach, and intestine additional low-affinity sites (or states) have been also suggested Furthermore, cytosolic receptors might be present. Binding affinities of synthetic minisomatostatins, somatostatin-14 and somatostatin-28, show different tissue specificities, suggesting the existence of different receptor subtypes. Two possible interactions of somatostatin with stimulus-secretion coupling in secretory cells have been suggested: a direct activation of the GTP-dependent inhibitory subunit of adenylate cyclase and a distal activation of cytosolic phosphoprotein phosphatases.

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.

Characteristics of [D-Trp8]-somatostatin-sensitive B50 phosphorylation

Inhibition of the phosphorylation of the synaptic plasma membrane (SPM) protein B50 by [D-Trp8]-somatostatin in vitro is time-dependent. Increasing the time of incubation of hippocampal synaptic plasma membranes with the peptide from 15 sec to 30 min prior to addition of 7.5 microM [gamma-32P]ATP results in a complete reduction of B50 phosphorylation. Incubation of synaptic plasma membranes for 30 min in the absence of peptide does not alter basal B50 phosphorylation. Neither ACTH nor beta-endorphin produces similar effects--inhibition of B50 phosphorylation by ACTH is maximal at 15 sec and beta-endorphin produces only a small inhibition, even after 30 min. [D-Trp8]-somatostatin is not activating a membrane-bound protease, since maximal inhibition of B50 phosphorylation by the peptide is seen in the presence of leupeptin or bacitracin. Hippocampal synaptic plasma membranes contain protein phosphatase activity. Assays of B50 phosphorylation in synaptic plasma membranes done under conditions that favor either net phosphorylation or dephosphorylation are consistent with inhibition of protein phosphatase activity by [D-Trp8]-somatostatin. This evidence suggests that [D-Trp8]-somatostatin interacts with SPM binding sites in the hippocampus, which may alter the activity of an endogenous protein phosphatase to determine the degree of B50 phosphorylation.

Neuropeptide-containing neurons of the cerebral cortex are also GABAergic

Neurons in the cat and monkey cerebral cortex were stained immunocytochemically for glutamic acid decarboxylase (GluDCase; L-glutamate 1-carboxy-lyase, EC 4.1.1.15), somatostatin (SRIF), neuropeptide Y (NPY), and cholecystokinin octapeptide (CCK). In all areas of cortex examined (somatic sensory, motor, parietal and visual areas), neurons displaying immunoreactivity for each of these molecules were nonpyramidal cells. Co-localization of GluDCase immunoreactivity with peptide immunoreactivity in the same cells was demonstrated by (i) the antibody elution method, staining the same cells by immunofluorescence, first for a peptide and then for GluDCase; (ii) double staining of the same sections with sheep anti-GluDCase and rabbit anti-peptide antisera, the bound antibodies being localized by rhodamine-conjugated donkey anti-sheep and fluorescein-conjugated swine anti-rabbit secondary antisera. With both procedures, cell bodies immunoreactive for GluDCase and for each of the peptides were found in all areas of cortex examined. With double labeling on single sections, it was found that all CCK-, SRIF-, and NPY-immunoreactive cells in cat cortex and 90%-95% in monkey cortex are also GluDCase positive. Many more cells, however, are immunoreactive for GluDCase alone. GluDCase was co-localized with CCK, SRIF, or NPY not only in cell somata, but also in small punctate structures, which are likely to be axon terminals. From the data gained in previous electron microscopic studies, we postulate that neurons displaying GluDCase- and CCK-like immunoreactivity are a class separate from those displaying GluDCase- and SRIF-like immunoreactivity. NPY, however, is co-localized with SRIF immunoreactivity. These results imply that classes of cortical interneuron contain a conventional neurotransmitter (gamma-aminobutyric acid) and a neuromodulator (one of the peptides).

Alteration of hippocampal B50 phosphorylation after adrenalectomy

Adrenalectomy alters endogenous phosphorylation of the presynaptic protein, B50, in the hippocampus. Three and four days following adrenalectomy decreases are seen, relative to control values, in the in vitro phosphorylation of B50 when a synaptic plasma membrane fraction from the hippocampus is incubated with [gamma-32P]ATP. At four days post-adrenalectomy, the percent decrease in B50 phosphorylation is -49.8 +/- 6.8%. No alteration is seen in the level of B50 phosphorylation when comparing hippocampal membrane preparations from sham-operated and intact animals. Fourteen days following adrenalectomy, hippocampal B50 phosphorylation was restored to normal levels. Hypophysectomy did not alter the degree of in vitro B50 phosphorylation, but the effect of adrenalectomy occurred in hypophysectomized rats. Following adrenalectomy, no differences are seen in the phosphorylation of any hippocampal cytosolic proteins. Changes in B50 phosphorylation seen in hypothalamic synaptic plasma membranes are likely due to the effects of sham-operation. The results indicate that a transient neurochemical or neuroendocrine event following adrenalectomy modulates in vivo the degree of B50 phosphorylation in hippocampal synaptic membranes.