Synapsin Ia, synapsin Ib, protein IIIa, and protein IIIb, four related synaptic vesicle-associated phosphoproteins, share regional and cellular localization in rat brain

Effects of propofol on presynaptic synapsin phosphorylation in the mouse brain in vivo

The commonly used general anesthetic propofol can enhance the γ-aminobutyric acid-mediated inhibitory synaptic transmission and depress the glutamatergic excitatory synaptic transmission to achieve general anesthesia and other outcomes. In addition to the actions at postsynaptic sites, the modulation of presynaptic activity by propofol is thought to contribute to neurophysiological effects of the anesthetic, although potential targets of propofol within presynaptic nerve terminals are incompletely studied at present. In this study, we explored the possible linkage of propofol to synapsins, a family of neuron-specific phosphoproteins which are the most abundant proteins on presynaptic vesicles, in the adult mouse brain in vivo. We found that an intraperitoneal injection of propofol at a dose that caused loss of righting reflex increased basal levels of synapsin phosphorylation at the major representative phosphorylation sites (serine 9, serine 62/67, and serine 603) in the prefrontal cortex (PFC) of male and female mice. Propofol also elevated synapsin phosphorylation at these sites in the striatum and S9 and S62/67 phosphorylation in the hippocampus, while propofol had no effect on tyrosine hydroxylase phosphorylation in striatal nerve terminals. Total synapsin protein expression in the PFC, hippocampus, and striatum was not altered by propofol. These results reveal that synapsin could be a novel substrate of propofol in the presynaptic neurotransmitter release machinery. Propofol possesses the ability to upregulate synapsin phosphorylation in broad mouse brain regions.

Synapsin I in Intraocular Hippocampal Transplants during Maturation and Aging: Effects of Brainstem Cografts

The role of target innervation for maintenance of synaptic proteins in the hippocampal formation during aging was investigated. Fetal CA1 tissue and brainstem tissue containing the nucleus locus coeruleus was dissected from albino rats and grafted sequentially into the anterior chamber of the eye of adult rat recipients. Synapsin protein distribution and levels were evaluated by immunohistochemistry and quantitative immunolabeling in single hippocampal grafts or brainstem-hippocampal double grafts at 6,12, or 24 mo postgrafting. The synapsin levels in 6-mo-old single hippocampal transplants were significantly lower than those in situ, and remained at these lower levels at 12 and 24 mo. On the contrary, synapsin levels were close to normal in the hippocampal portion of double grafts in the 6- and the 12-mo-group. However, in the 24-mo-old double transplants the levels had declined significantly, approaching levels seen in single hippocampal grafts. The immunoblot results were supported by morphological observations with synapsin antibodies and immunohistochemistry. The present data demonstrate that hippocampal tissue maintained near normal synapsin levels when grafted together with brainstem tissue, as compared to the lower levels seen in single hippocampal grafts. This normalization of synapsin levels was, however, not seen in the aged hippocampal-brainstem double grafts.

Functions of synapsins in corticothalamic facilitation: important roles of synapsin I

KEY POINTS

The synaptic vesicle associated proteins synapsin I and synapsin II have important functions in synaptic short-term plasticity. We investigated their functions in cortical facilitatory feedback to neurons in dorsal lateral geniculate nucleus (dLGN), feedback that has important functions in state-dependent regulation of thalamic transmission of visual input to cortex. We compared results from normal wild-type (WT) mice and synapsin knockout (KO) mice in several types of synaptic plasticity, and found clear differences between the responses of neurons in the synapsin I KO and the WT, but no significant differences between the synapsin II KO and the WT. These results are in contrast to the important role of synapsin II previously demonstrated in similar types of synaptic plasticity in other brain regions, indicating that the synapsins can have different roles in similar types of STP in different parts of the brain.

ABSTRACT

The synaptic vesicle associated proteins synapsin I (SynI) and synapsin II (SynII) have important functions in several types of synaptic short-term plasticity in the brain, but their separate functions in different types of synapses are not well known. We investigated possible distinct functions of the two synapsins in synaptic short-term plasticity at corticothalamic synapses on relay neurons in the dorsal lateral geniculate nucleus. These synapses provide excitatory feedback from visual cortex to the relay cells, feedback that can facilitate transmission of signals from retina to cortex. We compared results from normal wild-type (WT), SynI knockout (KO) and SynII KO mice, in three types of synaptic plasticity mainly linked to presynaptic mechanism. In SynI KO mice, paired-pulse stimulation elicited increased facilitation at short interpulse intervals compared to the WT. Pulse-train stimulation elicited weaker facilitation than in the WT, and also post-tetanic potentiation was weaker in SynI KO than in the WT. Between SynII KO and the WT we found no significant differences. Thus, SynI has important functions in these types of synaptic plasticity at corticothalamic synapses. Interestingly, our data are in contrast to the important role of SynII previously shown for sustained synaptic transmission during intense stimulation in excitatory synapses in other parts of the brain, and our results suggest that SynI and SynII may have different roles in similar types of STP in different parts of the brain.

Monocular deprivation delays the dynamic changes of phosphorylated synapsin Ia/b at site-1 in contralateral visual cortex of juvenile mice

Synapsins as a family of presynaptic terminal phosphoprotein participates in neuronal development, but their role in the synaptic plasticity of visual cortex is unclear. In this study, the impact of monocular deprivation (MD) on dynamic changes of isoform-specific protein expression and site 1 phosphorylation of synapsins in visual cortex of the postnatal mice were observed by using the technique of Western blot analysis. The results showed that the total (T-) protein levels of synapsins including the isoform of Ia/b, IIa/b and IIIa were about 21-26% of adult level in visual cortex of mice at postnatal 7 days (P7), and then the T-synapsin Ia/b and IIb could quickly reach adult level at P35. However, the T-synapsin IIa and IIIa increased more slowly (71-74% at P35), and then kept increasing in the visual cortex of mice at P60. Unlike to the changes of T-synapsins, the level of phosphorylated (P-) synapsin Ia/b (not IIa/b and IIIa) at site 1 increased with development to the highest level at P21, and then decreased rapidly to a low level in visual cortex of mice at P35-60. In addition, we found that the levels of P-synapsin Ia/b increased significantly in left visual cortex of P28 and P35 (not P21 and P42) mice with 1-week MD of right eye; and no significant changes of T-synapsins were observed in both left and right sides of visual cortex in P21-42 mice with MD treatment. These results suggested that the isoform-specific protein expression and site-1 phosphorylation of synapsins might play a different role in the synaptic plasticity of visual cortex, and MD delays the dynamic changes of phosphorylated synapsin Ia/b at site-1 in contralateral visual cortex of juvenile mice.

Behavioral effects of non-viral mediated RNA interference of synapsin II in the medial prefrontal cortex of the rat

Synapsin II is a synaptic vesicle-associated phosphoprotein that has been implicated in the pathophysiology of schizophrenia. Researchers have demonstrated reductions in synapsin II mRNA and protein in post-mortem prefrontal cortex and hippocampus samples from patients with schizophrenia. Synapsin II protein expression has been shown to be regulated by dopamine D(1) and D(2) receptor activation. Furthermore, behavioral testing of the synapsin II knockout mouse has revealed a schizophrenic-like behavioral phenotype in this mutant strain, suggesting a relationship between dysregulated and/or reduced synapsin II and schizophrenia. However, it remains unknown the specific regions of the brain of which perturbations in synapsin II play a role in the pathophysiology of this disease. The aim of this project was to evaluate animals with a selective knock-down of synapsin II in the medial prefrontal cortex through the use of siRNA technology. Two weeks after continuous infusion of synapsin II siRNAs, animals were examined for the presence of a schizophrenic-like behavioral phenotype. Our results reveal that rats with selective reductions in medial prefrontal cortical synapsin II demonstrate deficits in sensorimotor gating (prepulse inhibition), hyperlocomotion, and reduced social behavior. These results implicate a role for decreased medial prefrontal cortical synapsin II levels in the pathophysiology of schizophrenia and the mechanisms of aberrant prefrontal cortical circuitry, and suggest that increasing synapsin II levels in the medial prefrontal cortex may potentially serve as a novel therapeutic target for this devastating disorder.

Distinct changes in neuronal and astrocytic amino acid neurotransmitter metabolism in mice with reduced numbers of synaptic vesicles

The relations between glutamate and GABA concentrations and synaptic vesicle density in nerve terminals were examined in an animal model with 40-50% reduction in synaptic vesicle numbers caused by inactivation of the genes encoding synapsin I and II. Concentrations and synthesis of amino acids were measured in extracts from cerebrum and a crude synaptosomal fraction by HPLC and (13)C nuclear magnetic resonance spectroscopy (NMRS), respectively. Analysis of cerebrum extracts, comprising both neurotransmitter and metabolic pools, showed decreased concentration of GABA, increased concentration of glutamine and unchanged concentration of glutamate in synapsin I and II double knockout (DKO) mice. In contrast, both glutamate and GABA concentrations were decreased in crude synaptosomes isolated from synapsin DKO mice, suggesting that the large metabolic pool of glutamate in the cerebral extracts may overshadow minor changes in the transmitter pool. (13)C NMRS studies showed that the changes in amino acid concentrations in the synapsin DKO mice were caused by decreased synthesis of GABA (20-24%) in cerebral neurons and increased synthesis of glutamine (36%) in astrocytes. In a crude synaptosomal fraction, the glutamate synthesis was reduced (24%), but this reduction could not be detected in cerebrum extracts. We suggest that lack of synaptic vesicles causes down-regulation of neuronal GABA and glutamate synthesis, with a concomitant increase in astrocytic synthesis of glutamine, in order to maintain normal neurotransmitter concentrations in the nerve terminal cytosol.

Synapsin-dependent development of glutamatergic synaptic vesicles and presynaptic plasticity in postnatal mouse brain

Inactivation of the genes encoding the neuronal, synaptic vesicle-associated proteins synapsin I and II leads to severe reductions in the number of synaptic vesicles in the CNS. We here define the postnatal developmental period during which the synapsin I and/or II proteins modulate synaptic vesicle number and function in excitatory glutamatergic synapses in mouse brain. In wild-type mice, brain levels of both synapsin I and synapsin IIb showed developmental increases during synaptogenesis from postnatal days 5-20, while synapsin IIa showed a protracted increase during postnatal days 20-30. The vesicular glutamate transporters (VGLUT) 1 and VGLUT2 showed synapsin-independent development during postnatal days 5-10, following which significant reductions were seen when synapsin-deficient brains were compared with wild-type brains following postnatal day 20. A similar, synapsin-dependent developmental profile of vesicular glutamate uptake occurred during the same age periods. Physiological analysis of the development of excitatory glutamatergic synapses, performed in the CA1 stratum radiatum of the hippocampus from the two genotypes, showed that both the synapsin-dependent part of the frequency facilitation and the synapsin-dependent delayed response enhancement were restricted to the period after postnatal day 10. Our data demonstrate that while both synaptic vesicle number and presynaptic short-term plasticity are essentially independent of synapsin I and II prior to postnatal day 10, maturation and function of excitatory synapses appear to be strongly dependent on synapsin I and II from postnatal day 20.

Absence of synapsin I and II is accompanied by decreases in vesicular transport of specific neurotransmitters

Studies of synapsin-deficient mice have shown decreases in the number of synaptic vesicles but knowledge about the consequences of this decrease, and which classes of vesicles are being affected, has been lacking. In this study, glutamatergic, GABAergic and dopaminergic transport has been analysed in animals where the genes encoding synapsin I and II were inactivated. The levels of the vesicular glutamate transporter (VGLUT) 1, VGLUT2 and the vesicular GABA transporter (VGAT) were decreased by approximately 40% in adult forebrain from mice devoid of synapsin I and II, while vesicular monoamine transporter (VMAT) 2 and VGLUT3 were present in unchanged amounts compared with wild-type mice. Functional studies on synaptic vesicles showed that the vesicular uptake of glutamate and GABA was decreased by 41 and 23%, respectively, while uptake of dopamine was unaffected by the lack of synapsin I and II. Double-labelling studies showed that VGLUT1 and VGLUT2 colocalized fully with synapsin I and/or II in the hippocampus and neostriatum, respectively. VGAT showed partial colocalization, while VGLUT3 and VMAT2 did not colocalize with either synapsin I or II in the brain areas studied. In conclusion, distinct vesicular transporters show a variable degree of colocalization with synapsin proteins and, hence, distinct sensitivities to inactivation of the genes encoding synapsin I and II.

Changes in signaling pathways regulating neuroplasticity induced by neurokinin 1 receptor knockout

Neurokinin 1 (NK-1) receptor knockout mice showed behavioral responses similar to animals chronically treated with antidepressants. The aim of this study was to analyse, in NK-1 receptor knockout, the molecular modifications of signaling pathways involved in the pathophysiology of depression and antidepressant mechanism. We found, in total cell cytosol from the prefrontal/frontal cortex, hippocampus and striatum, a marked up-regulation of Ca(2+)-independent enzymatic activity and Thr(286) autophosphorylation of Ca(2+)/calmodulin-dependent protein kinase (CaMK) II. Similar changes in CaMKII regulation were previously observed in rats chronically treated with antidepressants. In striatum, up-regulation of the activity and phosphorylation of CaMKII was also found in the homogenate and synaptosomes. No major changes were observed in the Ca(2+)-dependent kinase activity, with the exception of homogenate from the prefrontal/frontal cortex. We also analysed the expression and phosphorylation of presynaptic proteins, which modulate synaptic vesicle trafficking and exocytosis, and found a marked decrease in synapsin I total expression and basal phosphorylation of Ser(603) (the phosphorylation site for CaMKII) in the prefrontal/frontal cortex. Accordingly, the Ca(2+)/calmodulin-dependent posthoc endogenous phosphorylation of synapsin I in the same area was increased. The knockout of NK-1 receptor had no consequences on the expression or phosphorylation levels of the transcription factor cAMP-responsive element-binding protein and its regulating kinase CaMKIV. However, phosphorylation of ERK1/2-mitogen-activated protein kinases was reduced in the hippocampus and striatum, again resembling an effect previously observed in antidepressant-treated rats. These results show similarities between NK-1 knockouts and animals chronically treated with antidepressants and support the putative antidepressant activity of NK-1 receptor antagonists.

Hypoxia-induced endocytosis of Na,K-ATPase in alveolar epithelial cells is mediated by mitochondrial reactive oxygen species and PKC-zeta

During ascent to high altitude and pulmonary edema, the alveolar epithelial cells (AEC) are exposed to hypoxic conditions. Hypoxia inhibits alveolar fluid reabsorption and decreases Na,K-ATPase activity in AEC. We report here that exposure of AEC to hypoxia induced a time-dependent decrease of Na,K-ATPase activity and a parallel decrease in the number of Na,K-ATPase alpha(1) subunits at the basolateral membrane (BLM), without changing its total cell protein abundance. These effects were reversible upon reoxygenation and specific, because the plasma membrane protein GLUT1 did not decrease in response to hypoxia. Hypoxia caused an increase in mitochondrial reactive oxygen species (ROS) levels that was inhibited by antioxidants. Antioxidants prevented the hypoxia-mediated decrease in Na,K-ATPase activity and protein abundance at the BLM. Hypoxia-treated AEC deficient in mitochondrial DNA (rho(0) cells) did not have increased levels of ROS, nor was the Na,K-ATPase activity inhibited. Na,K-ATPase alpha(1) subunit was phosphorylated by PKC in hypoxia-treated AEC. In AEC treated with a PKC-zeta antagonist peptide or with the Na,K-ATPase alpha(1) subunit lacking the PKC phosphorylation site (Ser-18), hypoxia failed to decrease Na,K-ATPase abundance and function. Accordingly, we provide evidence that hypoxia decreases Na,K-ATPase activity in AEC by triggering its endocytosis through mitochondrial ROS and PKC-zeta-mediated phosphorylation of the Na,K-ATPase alpha(1) subunit.

Reduction of synapsin in the hippocampus of patients with bipolar disorder and schizophrenia

Several studies suggest that decreased expression of presynaptic proteins may be characteristic of schizophrenia. We examined one such protein, synapsin, in schizophrenia and bipolar disorder. Samples of hippocampal tissue from controls (n = 13), patients with schizophrenia (n = 16), or bipolar disorder (n = 6), and suicide victims (n = 7) were used. The membrane and cytosolic fractions were analyzed by Western immunoblotting for synapsin using an antibody that detects synapsin Ia, IIa, and IIIa proteins. Synaptophysin was also measured for comparison. Total synapsin was decreased significantly in patients with schizophrenia (P = 0.034) and in bipolar disorder (P = 0.00008) as compared to controls. The synapsin/synaptophysin ratios were decreased in schizophrenia and bipolar disorder, and additionally in suicide victims (P = 0.014). Age, postmortem interval, percentage of protein extracted, and pH of brain were not different between groups. No changes in total synapsin or synaptophysin in the hippocampus were produced by injecting rats with either lithium or haloperidol for 30 days. Reductions in synapsin in both patients with schizophrenia (synapsin IIa and IIIa) and bipolar disorder (synapsin Ia, IIa and IIIa) imply that altered or reduced synaptic function in the hippocampus may be involved in these disorders.

Neuron-specific and developmental regulation of the synapsin II gene expression in transgenic mice

Synapsin II, a major phosphoprotein of synaptic vesicles, is believed to function in neurotransmitter release as well as in synapse formation. The expression of the synapsin II gene is neuron-specific, and correlates temporally with synaptogenesis. To understand the mechanisms by which the expression of the synapsin II gene is regulated in vivo, we generated transgenic mice carrying a 5.1-kb 5'-flanking sequence of the murine synapsin II gene fused to the firefly luciferase reporter gene. The synapsin II-luciferase transgene is specifically expressed in neural tissues, such as brain and spinal cord, but not in non-neural tissues. Throughout the brain, the expression of the transgene is widely distributed, and restricted only to neuronal cells. Moreover, the expression of the transgene is developmentally regulated, with a temporal profile similar to that of endogenous synapsin II expression. These results indicate that the 5.1-kb flanking sequence of the murine synapsin II gene contains cis-regulatory elements that are required for directing neuron-specific and synaptogenesis-regulated expression in vivo.

Differential distribution of synapsin IIa and IIb mRNAs in various brain structures and the effect of chronic morphine administration on the regional expression of these isoforms

Quantitative reverse transcriptase-polymerase chain reaction and in situ hybridization techniques were used to determine the regional distribution of synapsin IIa and IIb mRNAs in rat central nervous system and to assess the effect of chronic morphine administration on the gene expression of these two isoforms of synapsin II. These isoforms are members of a family of neuron-specific phosphoproteins thought to be involved in the regulation of neurotransmitter release. Our data demonstrate the widespread distribution, yet regionally variable expression, of synapsin IIa and IIb mRNAs throughout the adult rat brain and spinal cord. The ratios of the relative abundance of synapsins IIa and IIb differed by up to 4.5-fold among the various regions studied. Synapsin IIa and IIb mRNAs were shown to be highly concentrated in the thalamus and in the hippocampus, whereas lower concentrations were found in most other central nervous system structures. In this study, we show differential regulation by morphine of synapsins IIa and IIb in various regions of the brain. In the striatum, a 2.4-fold increase was observed in the levels of synapsin IIa mRNA following chronic morphine regime, whereas no change was found for synapsin IIb. On the other hand, mRNA levels of synapsin IIb in spinal cord of chronically treated rats were markedly decreased (by 62%), while no alterations were observed in synapsin IIa. Selective regulation by morphine has also been demonstrated in several other central nervous system structures. The opiate-induced regulation of the gene expression of synapsin II isoforms could be viewed as one of the cellular adaptations to the persistent opiate effects and may be involved in the molecular mechanism underlying opiate tolerance and/or dependence.

Temporal onset of synapsin I gene expression coincides with neuronal differentiation during the development of the nervous system

Synapsin I is the best characterized member of a family of nerve terminal-specific phosphoproteins implicated in the regulation of neurotransmitter release. During development, the expression of synapsin I correlates temporally and topographically with synapse formation, and recent physiological studies (Lu et al. [1992] Neuron 8:521-529.) have suggested that synapsin I may participate in the functional maturation of synapses. To better understand the temporal relationship between synapsin I gene expression and particular cellular events during neuronal development, we have used in situ hybridization histochemistry to localize synapsin I mRNA throughout the rat central and peripheral nervous systems during embryonic and postnatal development. From the earliest embryonic time points assayed (E12), the expression of the synapsin I gene was detectable in both the central and peripheral nervous systems. While, in general, levels of synapsin I mRNAs were high in utero, synapsin I cDNA probes revealed specific patterns of hybridization in different regions of the embryonic nervous system. To determine precisely the temporal onset of expression of the synapsin I gene during neuronal development, we examined in detail the appearance of synapsin I mRNA during the well characterized postnatal development of granule cells of the rat cerebellum and hippocampus. In both regions, the onset of synapsin I gene expression correlated with the period of stem cell commitment to terminal differentiation. Finally, our data demonstrate that, in a second phase, synapsin I gene expression increases to a maximum for a given neuronal population during a particular phase of differentiation, i.e., synaptogenesis.

Synapsin I gene expression in the adult rat brain with comparative analysis of mRNA and protein in the hippocampus

Synapsin I is the best characterized member of a family of neuron-specific phosphoproteins thought to be involved in the regulation of neurotransmitter release. In this report, we present the first extensive in situ hybridization study detailing the regional and cellular distribution of synapsin I mRNA in the adult rat brain. Both the regional distribution and relative levels of synapsin I mRNA established by in situ hybridization were confirmed by RNA blot analysis. Our data demonstrate the widespread yet regionally variable expression of synapsin I mRNA throughout the adult rat brain. The greatest abundance of synapsin I mRNA was found in the pyramidal neurons of the CA3 and CA4 fields of the hippocampus, and in the mitral and internal granular cell layers of the olfactory bulb. Other areas abundant in synapsin I mRNA were the layer II neurons of the piriform cortex and layer II and V neurons of the entorhinal cortex, the granule cell neurons of the dentate gyrus, the pyramidal neurons of hippocampal fields CA1 and CA2, and the cells of the parasubiculum. In general, the pattern of expression of synapsin I mRNA paralleled those encoding other synaptic terminal-specific proteins, such as synaptophysin, VAMP-2, and SNAP-25, with noteworthy exceptions. To determine specifically how synapsin I mRNA levels are related to levels of synapsin I protein, we examined in detail the local distribution patterns of both synapsin I mRNA and protein in the rat hippocampus. These data revealed differential levels of expression of synapsin I mRNA and protein within defined synaptic circuits of the rat hippocampus.

Neuronal localization of the tyrosine-specific protein kinase p62c-yes in rat basal ganglia

The cellular localization of the tyrosine-specific protein kinase p62c-yes, the product of the proto-oncogene c-yes, has been examined in the striatonigral neurons which interconnect the rat neostriatum and substantia nigra. Although p62c-yes was more enriched in the neostriatum than in the substantia nigra, excitotoxin-induced necrosis of nerve cells in the neostriatum led to 50-60% decreases of p62c-yes both in the lesioned neostriatum and in the ipsilateral substantia nigra. Hence, the p62c-yes tyrosine kinase is present both in the cell body region and in the axonal and nerve terminal region of the striatonigral neurons. This localization indicates that the enzyme may be involved in both presynaptic and postsynaptic functions in mammalian forebrain neurons.

Bimodal regulation of protein phosphorylation by a ganglioside in rat brain membrane

Whether or not a ganglioside influences the protein phosphorylation in the rat brain membrane fraction was investigated. Phosphorylation of the 72 kDa protein was significantly affected by the addition of 80 nM GQ1b in vitro, which is far below the reported concentration of gangliosides that affects protein phosphorylation in the neuronal membrane fraction. This action of GQ1b was bimodal: it being not only stimulatory as to the incorporation of phosphate into the 72 kDa protein on incubation of 20 sec, but also as to the release of phosphate from or breakdown of the 72 kDa protein on incubation for more than 5 min. Eighty nM GQ1b did not noticeably affect ATPase in the same fraction. These results suggest that the transphosphorylation of the 72 kDa protein is affected by the interaction of GQ1b with either the responsible enzymes or the 72 kDa protein as a substrate.

Release of angiotensin in response to ionic stimulation: a possible role for calmodulin in the secretory event

Although the synthesis and distribution of the putative neurotransmitter angiotensin (ANG) has been well established, little is known about the mechanism for its release. In this study, we utilized dissociated cell cultures of fetal rat brain to examine the cellular and ionic properties of angiotensin release. Graded concentrations of 0-60 mM KCl in the presence of 5 mM CaCl2 were added to the cultured cells and the resulting angiotensin release was measured by radioimmunoassay and high-performance liquid chromatography. Levels of angiotensin release increased from 13.85 +/- 1.53 pg/mg protein to 172.64 +/- 17.49 pg/mg protein with increasing concentrations of K+. Cultures incubated with 60 mM KCl buffer that did not contain CaCl2 released 39.87 +/- 15.74 pg ANG/mg protein. To further show the link between ionic stimulation and angiotensin release and determine the potential role of extracellular sodium ions on angiotensin release, cultures were incubated with the Na(+)-channel blocker tetrodotoxin (300 nM TTX) prior to maximal stimulation with 60 mM KCl/5 mM CaCl2 in the presence of the channel antagonist. Release was attenuated following incubation with stimulating buffer containing TTX (300 nM) to 5.49 +/- 4.37 pg/mg protein. Finally, to determine the role of the calcium binding protein calmodulin in the release event, the cells were incubated with graded concentrations of W-7 (2.5-250 microM) and subsequently stimulated with 60 mM KCl/5 mM CaCl2 in the presence of the calmodulin antagonist. W-7, which displays specificity for inhibition of the Ca2+/calmodulin complex below 0.2 mM, decreased angiotensin release in a dose-dependent manner.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of a synaptic antigen of interest in neuropsychiatric illness

Monoclonal antibodies exhibiting relative differences in binding to brain homogenates from diseased versus control brains may be useful probes into the molecular pathology of neuropsychiatric illness. To be of value, the antibodies must be useful in characterization of the antigens putatively involved in the illness. An antibody called EP10 showed some differences in binding to homogenates from a small sample of schizophrenia brains compared with controls. In the present study, the antigen for this antibody was characterized in control brains using quantitative, immunocytochemical, and biochemical techniques. The antigen is a 38,000 dalton synaptic vesicle protein, which is identical to synaptophysin by immunological criteria. However, the EP10 epitope is of interest, as this binding site does not appear to be present in rat or bovine synaptophysin. Brain regional studies using EP10 indicate that the antigen may be present in only a subset of synaptic terminals. Further studies are required to fully characterize the epitope, and to determine the significance of the earlier findings related to schizophrenia.

Isoproterenol increases the phosphorylation of the synapsins and increases synaptic transmission in dentate gyrus, but not in area CA1, of the hippocampus

Previous studies have shown that either norepinephrine (NE) or isoproterenol (ISO) enhances the slope of the field excitatory postsynaptic potential (EPSP) in the dentate gyrus of the rat hippocampal formation. In contrast, NE and ISO cause no increase in excitatory transmission in area CA1 of the hippocampus. The molecular mechanism underlying this brain region-specific increase in synaptic transmission is not known. The phosphorylation of synapsin I and synapsin II, two homologous presynaptic vesicle-associated proteins, is thought to promote neurotransmitter release. The authors have observed previously NE- and ISO-enhanced phosphorylation of synapsins I and II in the dentate gyrus. The purpose of this study was to determine whether ISO-stimulated phosphorylation also occurs in the CA1, where ISO has no effect on excitatory neurotransmission. These studies were correlated with electrophysiological studies in in vitro hippocampal slices. Superfusion of slices with ISO resulted in an increase in EPSP slope in the dentate but not in area CA1. The enhanced dentate EPSP returned to baseline levels within 30 minutes of washout of the drug. Isoproterenol produced corresponding increases in the phosphorylation of the synapsins in dentate slices but had no effect on these proteins in CA1 slices. Moreover, in dentate slices exposed to a 30-minute wash following incubation with ISO, phosphorylation of the synapsins returned to control levels. This close temporal and brain regional correlation between ISO stimulation of both synapsin phosphorylation and synaptic transmission suggests that the synapsin proteins may play a role in the synaptic potentiation produced by ISO in the dentate.

Increased synthesis of two polypeptides in area CA1 of the hippocampus in response to repetitive electrical stimulation

Changes in the pattern of newly synthesized polypeptides were investigated in the in vitro hippocampal slice following exposure to repetitive stimulation with and without the induction of long-term potentiation. Using [35S]methionine labeling of polypeptides and two-dimensional gel electrophoresis, we detected an increase in the rate of synthesis of two polypeptides (48 kDa and 89 kDa) in CA1 in response to repetitive stimulation of the Schaffer collaterals. The synthesis of the 48 kDa polypeptide (pI approximately 6.6) increased 240% in response to high-frequency stimulation (100 Hz) relative to the same protein from unstimulated slices (n = 14), and increased 220% in response to low-frequency stimulation (1 Hz) (n = 5). Blockade of the N-methyl-D-aspartate (NMDA) receptor induced the protein 180%, with no further increase following tetanic stimulation. An 89 kDa doublet (pI approximately 6.8) increased 150% following high-frequency and 140% following low-frequency stimulation. Blockade of the NMDA receptor increased this protein as well (180% of the unstimulated control) and no further increase was observed following high-frequency stimulation. Based on physicochemical and electrophysiological properties, these proteins are not identifiable as any of those previously associated with long-term potentiation or repetitive electrical stimulation.

Norepinephrine and isoproterenol increase the phosphorylation of synapsin I and synapsin II in dentate slices of young but not aged Fisher 344 rats

A number of recent reports have suggested that norepinephrine (NE) produces a form of synaptic enhancement that resembles long-term potentiation (LTP). LTP, thought to be an electrophysiological correlate of memory, in part involves an augmentation of transmitter release. Although the effects of NE have not been unequivocally linked to LTP, it is clear that NE can produce increased transmitter release in the dentate gyrus of the hippocampus. The purpose of this study was to determine whether NE was capable of enhancing the phosphorylation of synapsin I and synapsin II, two homologous phosphoproteins thought to be involved in modulation of neurotransmitter release. NE (10 microM) and isoproterenol (250 nM) produced an increase in the phosphorylation of synapsin I and synapsin II in dentate slices from young rats. Phosphorylation site analysis of synapsin I, performed by limited proteolysis, indicated that NE and isoproterenol increased the phosphorylation of synapsin I at sites modified by Ca2+/calmodulin-dependent protein kinase II as well as cAMP-dependent protein kinase. These data demonstrate that NE stimulates the phosphorylation of synapsin I at its Ca2+/calmodulin-dependent protein kinase II site, which is a site that has been shown to regulate the effect of synapsin I on neurotransmitter release. We have also examined the effects of NE and isoproterenol on synapsin phosphorylation in dentate slices prepared from aged animals. Such animals have previously been shown to exhibit deficits in NE sensitivity as well as significant impairment in their ability to exhibit LTP. Neither NE nor isoproterenol stimulated synapsin phosphorylation in slices prepared from aged animals. Interestingly, the basal level of phosphorylation of the synapsin proteins was higher in slices prepared from aged animals. This higher basal level of phosphorylation may underlie the failure of aged animals to exhibit NE-stimulated increases in phosphorylation of the synapsin proteins. We hypothesize that the beta-adrenergic agonist-stimulated phosphorylation of synapsin I and synapsin II in young rats plays a role in the increase in transmitter release produced by NE in the dentate. Thus, the failure of the aged rats to show such phosphorylation may underlie, in part, their failure to exhibit normal responsiveness to NE. Moreover, these deficits in synapsin phosphorylation may also play some role in the deficits in plasticity seen in aged rats.

Protein phosphorylation in guinea-pig myenteric ganglia and brain: presence of calmodulin kinase II. protein kinase C and cyclic AMP kinase and characterization of major phosphoproteins

The aim of this study was to demonstrate the presence of calmodulin-stimulated protein kinase II, protein kinase C, and cyclic AMP-stimulated protein kinase in isolated myenteric ganglia and to characterize the major ganglia phosphoproteins using biochemical and immunochemical techniques. Ganglia from the small intestine of guinea-pigs were isolated, disrupted by sonication in Triton X-100, and phosphorylated. The phosphoprotein patterns obtained were compared with those of synaptosomes from guinea-pig and rat cerebral cortex. Myenteric ganglia were as rich in protein kinase C and cyclic AMP-stimulated protein kinase as brain tissue, but the level of calmodulin-stimulated protein kinase II was relatively lower. The alpha subunit of calmodulin-stimulated protein kinase II was detected by immunoblotting and the beta subunit by autophosphorylation. The ratio of beta to alpha subunit was considerably higher in ganglia than in brain and ganglia beta subunit had a lower apparent molecular weight than the brain enzyme. A number of neuronal phosphoproteins were found in ganglia including the 87,000 mol. wt phosphoprotein, synapsins 1a and 1b, and proteins IIIa and IIIb. A phosphoprotein of 48,000 mol. wt had many of the characteristics of the B-50 protein but was not the same. In addition, a number of other phosphoproteins not previously identified in neurons were found in ganglia including those with apparent molecular weights of 60,000 and 58,000 that were the major calmodulin kinase substrates. The guinea-pig enteric nervous system has been extensively studied but, unlike other parts of the mammalian nervous system, little is known about the intracellular mechanisms underlying its functions. A technique for isolating myenteric ganglia is now available and we have used this preparation to characterize the major protein kinase and phosphoproteins present in this tissue. The results obtained will allow the phosphorylation of the various proteins to be investigated after physiological or pharmacological manipulation of myenteric ganglia in situ and in vivo.

A role for cAMP in the development of functional neuromuscular transmission

We have found that the incidence of functionally connected neuron-myotube pairs in chick ciliary-myotube cultures increases from 58% to more than 90% when the cells are treated for several hours with 8-bromo-cyclic adenosine monophosphate (8-br-cAMP) or with agents known to increase intracellular cAMP. The increase in connectivity was not accompanied by a change in neuron survival, or in the length of neurite-myotube contact. Moreover, there was no change in the shape of the presynaptic action potential, in mean end plate potential (epp) amplitude or in the sensitivity of postsynaptic acetylcholine receptors (AChRs). One interpretation of these results in that a cAMP-dependent phosphorylation acts as a trigger to activate a previously "silent" synapse.

Torpedo synaptophysin: evolution of a synaptic vesicle protein

Synaptophysin is an integral membrane protein of synaptic vesicles found in neurons and endocrine cells. Synaptophysin monomers associate into hexamers forming a large conductance channel. We present an analysis of synaptophysin from the nervous system of the marine ray Torpedo californica. Analysis of cDNA clones reveals a 62% amino acid similarity between the Torpedo and rat sequences. The 4 hydrophobic membrane spanning domains and the glycosylation site are conserved. In contrast, the two intravesicular loops connecting the membrane spanning regions, show varying degrees of sequence conservation, suggesting that portions of these domains may play critical functional roles. The carboxyterminal tail has been proposed to bind calcium and is a major site for tyrosine phosphorylation. The precise sequence of this region has almost completely diverged while the proline-tyrosine rich nature is maintained. Blotting studies reveal the RNA and the protein in nervous system tissues and demonstrate that the molecule copurifies with cholinergic synaptic vesicles.

An analysis of postmortem brain samples from 32 alcoholic and nonalcoholic individuals for protein III, a neuronal phosphoprotein

Protein phosphorylation is a primary mechanism of intracellular signal transduction, and abnormalities in protein phosphorylation have been implicated in the pathogenesis of several specific diseases. Protein III is a neuronal phosphoprotein that is associated with synaptic vesicles and is probably involved in the regulation of neurotransmitter release. Analysis of 32 postmortem brains has confirmed our previous report that variant forms of protein III with higher apparent molecular weights are found frequently in the brains of alcoholic individuals but rarely in the brains of nonalcoholic individuals who did not suffer from any other medical or neuropsychiatric disorders. Eight of 14 (57%) brain samples from alcoholic individuals and four of eight (50%) brain samples from suspected alcoholic individuals had variant forms, while none of 10 samples from nonalcoholic individuals had variant forms. Previous data indicate that variant forms of protein III are also associated with other neurodegenerative conditions, including various dementias, and, possibly, normal aging.

Synapsins: mosaics of shared and individual domains in a family of synaptic vesicle phosphoproteins

Synapsins are neuronal phosphoproteins that coat synaptic vesicles, bind to the cytoskeleton, and are believed to function in the regulation of neurotransmitter release. Molecular cloning reveals that the synapsins comprise a family of four homologous proteins whose messenger RNA's are generated by differential splicing of transcripts from two genes. Each synapsin is a mosaic composed of homologous amino-terminal domains common to all synapsins and different combinations of distinct carboxyl-terminal domains. Immunocytochemical studies demonstrate that all four synapsins are widely distributed in nerve terminals, but that their relative amounts vary among different kinds of synapses. The structural diversity and differential distribution of the four synapsins suggest common and different roles of each in the integration of distinct signal transduction pathways that modulate neurotransmitter release in various types of neurons.

Dopamine-regulated phosphorylation of synaptic vesicle-associated proteins in rat neostriatum and substantia nigra

Dopamine, acting through dopamine D1 receptors and cyclic AMP-dependent protein kinase, has been found to increase the state of phosphorylation of the synaptic vesicle-associated phosphoproteins synapsin I and protein III in slices of rat neostriatum and substantia nigra. In the neostriatum, the effect of dopamine was mimicked by SKF 38393, a D2 receptor agonist, and was abolished by preincubation of the slices with fluphenazine or SCH 23390, antipsychotic drugs which are potent D1 receptor antagonists, but not by the D2 receptor antagonists l-sulpiride or spiroperidol. The maximal effect of dopamine in the neostriatum represented approximately 30-35% of the maximal effect induced by 8-bromo cyclic AMP, suggesting that a similar fraction of nerve terminals in the neostriatum may express the dopamine D1 receptor. Evidence for a small population of beta-adrenergic receptors regulating nerve terminal protein phosphorylation in the neostriatum, distinct from the D1 dopamine receptors, was also obtained. In the substantia nigra, the effect of dopamine also appeared to be mediated through a D1 dopamine receptor, since it was abolished by fluphenazine and SCH 23390. The maximal effect of dopamine in the substantia nigra represented approximately two-thirds of the effect induced by 8-bromo cyclic AMP, suggesting that a similar fraction of nerve terminals in the substantia nigra may express the dopamine D1 receptor. The ability of dopamine D1 receptor activation to stimulate both synapsin I and protein III phosphorylation and GABA release in both the neostriatum and substantia nigra may be causally linked.