Insights into the Sigma-1 receptor chaperone's cellular functions: a microarray report

We previously demonstrated that Sig-1Rs are critical regulators in neuronal morphogenesis and development via the regulation of oxidative stress and mitochondrial functions. In the present study, we sought to identify pathways and genes that are affected by Sig-1R. Gene expression profiles were examined in rat hippocampal neurons that had been cultured for 18 days in vitro (DIV). The cells were transduced with AAV siRNA targeting Sig-1R on DIV 10 for 7 days, followed by gene expression analysis using a rat genome cDNA array. The gene array results indicated that Sig-1R knockdown hampered cellular functions including steroid biogenesis, protein ubiquitination, actin cytoskeleton network, and Nrf-2 mediated oxidative stress. Many of the cellular components important for actin polymerization and synapse plasticity, including F-actin capping protein and neurofilaments, were significantly changed in AAV-siSig-1R neurons. Further, cytochrome c was reduced in AAV-Sig-1R neurons whereas free-radical generating enzymes including cytochrome p450 and cytochrome b-245 were increased. The microarray results also suggest that Sig-1Rs may regulate genes that are involved in the pathogenesis of many CNS diseases including Alzheimer's disease and Parkinson's disease. These data further confirmed that Sig-1Rs play critical roles in the CNS and thus these findings may aid in future development of therapeutic treatments targeting neurodegenerative disorders.     

Neuroprotective effects of cutamesine,a ligand of the sigma‐1 receptor chaperone,against noise‐induced hearing loss

The sigma‐1 receptor, which is expressed throughout the brain, provides physiological benefits that include higher brain function. The sigma‐1 receptor functions as a chaperone in the endoplasmic reticulum and may control cell death and regeneration within the central nervous system. Cutamesine (1‐(3,4‐dimethoxyphenethyl)?4‐(3‐phenylpropyl) piperazine dihydrochloride) is a ligand selective for this receptor and may mediate neuroprotective effects in the context of neurodegenerative disease. We therefore assessed whether cutamesine protects the inner ear from noise‐induced or aging‐associated hearing loss. Immunohistochemistry and Western blotting revealed that the sigma‐1 receptor is present in adult cochlea. We treated mice with 0, 3, or 30 mg/kg cutamesine from 10 days before noise exposure until the end of the study. All subjects were exposed to a 120‐dB, 4‐kHz octave‐band noise for 2 hr. We assessed auditory thresholds by measuring the auditory‐evoked brainstem responses at 4, 8, and 16 kHz, prior to and 1 week, 1 month, or 3 months following noise exposure. For the aging study, measurements were made before treatment was initiated and after 3 or 9 months of cutamesine treatment. Damage to fibrocytes within the cochlear spiral limbus was assessed by quantitative histology. Cutamesine significantly reduced threshold shifts and cell death within the spiral limbus in response to intense noise. These effects were not dose or time dependent. Conversely, cutamesine did not prevent aging‐associated hearing loss. These results suggest that cutamesine reduces noise‐induced hearing loss and cochlear damage during the acute phase that follows exposure to an intense noise. © 2015 Wiley Periodicals, Inc.     

Possible involvement of the sigma‐1 receptor chaperone in chemotherapeutic‐induced neuropathic pain

Previous studies have shown that ligands of the sigma‐1 receptor chaperone (Sig‐1R) regulate pain‐related behaviors. Clinical use of chemotherapeutics is often compromised due to their adverse side effects, particularly those related to neuropathy. Previous studies have shown that repeated administration of oxaliplatin and paclitaxel produces neuropathy in rodents. Therefore, the aim of the present study was to clarify the involvement of the Sig‐1R in chemotherapeutic‐induced neuropathy by examining the effects of oxaliplatin and paclitaxel on the Sig‐1R levels in the spinal cord, and by examining the effects of Sig‐1R agonist and antagonist on oxaliplatin‐ and paclitaxel‐induced neuropathy in rats. Chemotherapeutic‐induced neuropathic pain was accompanied by a significant reduction of the Sig‐1R level in the spinal cord. Furthermore, the administration of paclitaxel to CHO cells that stably overexpressed Sig‐1Rs induced the clustering of Sig‐1Rs. We also found that the Sig‐1R agonist SA4503 potently inhibited the neuropathy induced by oxaliplatin‐ and paclitaxel, whereas this action was abolished by the Sig‐1R antagonist NE‐100. These results suggest that the reduction of Sig‐1R activity is involved in chemotherapeutic‐induced neuropathy, and the Sig‐1R agonist SA4503 could serve as a potential candidate for the treatment of chemotherapeutic‐induced neuropathy. Synapse 69:526–532, 2015 . © 2015 Wiley Periodicals, Inc.     

Plasma exosomes from depression ameliorate inflammation-induced depressive-like behaviors via sigma-1 receptor delivery

A growing body of studies indicated that exosomes are one of vital players in pathological process of neuropsychiatric diseases, but their role in major depressive disorder (MDD) remains poorly understood. Here we purified plasma exosomes from depression including lipopolysaccharide (LPS)-challenged depression, chronic restraint stress (CRS)-induced depression, MDD subjects, and from control mice or volunteers. The therapeutic effect of these exogenous exosomes was assessed utilizing behavioral tests and biochemical approaches in the LPS-caused depression or microglial BV2 cells. The expression of exosomal sigma-1 receptor (Sig-1R) was evaluated by western blotting. The role of Sig-1R in the biological function of exosomes was determined using Sig-1R knockout mice and HEK 293 cells. Our results revealed that injection of exosomes from depression models or patients rather than normal controls significantly ameliorated depressive-like behaviors, deficiency of BDNF expression and neuro-inflammation in LPS-challenged mice. In addition, co-culture with exosomes from depression models or patients instead of from controls prevented LPS-induced inflammation responses in microglial BV2 cells. Moreover, Sig-1R was demonstrated for the first time to significantly be enriched in exosomes from depression models or patients compared with that from normal controls. However, Sig-1R null exosomes no longer emerged antidepressant-like action in LPS-challenged mice. Thus, we demonstrated that plasma exosomes from depression exerted antidepressant-like effects in a Sig-1R dependent manner in the LPS-induced depression. This work improves our understanding of the exosomes in depression, suggesting a novel exosomes-based approach for MDD treatment.     

The secreted brain-derived neurotrophic factor precursor pro-BDNF binds to TrkB and p75NTR but not to TrkA or TrkC

The neurotrophin brain-derived neurotrophic factor (BDNF) binds to two cell surface receptors: TrkB receptors that promote neuronal survival and differentiation and p75NTR that induces apoptosis or survival. BDNF, as well as the other members of the neurotrophin family, is synthesized as a larger precursor, pro-BDNF, which undergoes posttranslational modifications and proteolytic processing by furin or related proteases. Both mature neurotrophins and uncleaved proneurotrophins are secreted from cells. The bioactivities of proneurotrophins could differ from those of mature, cleaved neurotrophins; therefore, we wanted to test whether pro-BDNF would differ from mature BDNF in its neurotrophin receptor binding and activation. A furin-resistant pro-BDNF, secreted from COS-7 cells, bound to TrkB-Fc and p75NTR-Fc, but not to TrkA-Fc or TrkC-Fc. Likewise, pro-BDNF elicited prototypical TrkB responses in biological assays, such as TrkB tyrosine phosphorylation, activation of ERK1/2, and neurite outgrowth. Moreover, mutation of the R103 residue of pro-BDNF abrogated its binding to TrkB-Fc but not to p75NTR-Fc. Taken together, these data indicate that pro-BDNF binds to and activates TrkB and could be involved in TrkB-mediated neurotrophic activity in vivo.     

Human immunodeficiency virus type 1 alters brain-derived neurotrophic factor processing in neurons

The molecular mechanisms leading to synaptic simplification and neuronal apoptosis in human immunodeficiency virus type 1 (HIV-1)-positive subjects are unknown. The HIV protein gp120 reduced the length of neuronal processes similarly to the proneurotrophin pro-brain-derived neurotrophic factor (proBDNF). Intriguingly, the effects of both proBDNF and gp120 were blocked by inhibitors of the p75 neurotrophin receptor, suggesting that proBDNF and gp120 share a similar mechanism of neurotoxicity. Therefore, we tested the hypothesis that gp120 affects the release of proBDNF. Using rat primary neurons, we observed that gp120 promotes a time-dependent intracellular and extracellular accumulation of proBDNF concomitantly with a decrease in mature BDNF. A similar imbalance in the ratio proBDNF/mature BDNF was confirmed in postmortem brains of HIV-positive subjects cognitively impaired and motor impaired. Therefore, it is conceivable to formulate the hypothesis that HIV neurotoxicity includes a gp120-mediated alteration of BDNF processing. To determine the cellular mechanism whereby gp120 produces an accumulation of proBDNF, we examined the levels of intracellular and extracellular enzymes that proteolytically cleave proBDNF furin and tissue plasminogen, respectively. In rat neurons exposed to gp120, intracellular furin levels decreased before cell death, whereas tissue plasminogen changed only during apoptosis. Our data suggest that HIV, through gp120, reduces proBDNF processing by affecting furin levels, and therefore causes an altered balance between antiapoptotic and proapoptotic neurotrophins. Our studies identify a new mechanism that may explain how HIV promotes neuronal injury.     

Activation of astrocytic sigma-1 receptor exerts antidepressant-like effect via facilitating CD38-driven mitochondria transfer

Despite sigma-1 receptor (Sig-1R) is a promising therapeutic target in depression, little is known regarding the cellular mechanisms underlying its antidepressant responses. Here, we demonstrated that astrocyte can be a direct cellular target of Sig-1R exerting antidepressant-like effect. In multiple behavioral models including forced swimming test (FST), tail suspension test (TST), open field test (OFT), and chronic unpredictable mild stress (CUMS), inhibition of astrocyte function blocked pharmacological Sig-1R activation-induced antidepressant-like effect, while specific activation of astrocytc Sig-1R by adeno-associated virus (AAV) was sufficient to produce antidepressant-like effect. In depression-related cellular tests, Sig-1R agonist or lentivirus-stimulated astrocyte conditioned medium (ACM) promoted neuronal neurite outgrowth, dendritic branch, and survival. Mechanismly, stimulation of Sig-1R enhanced the expression of CD38 via activation of extracellular regulated protein kinases 1/2 (ERK1/2), resulting in facilitating mitochondrial transfer from astrocyte. Furthermore, blockage of CD38-driven astrocyte transferring mitochondria in vivo and in vitro reversed the antidepressant-like effect of pharmacological Sig-1R activation. Thus, this study sheds light on the cellular mechanism of Sig-1R activation producing antidepressant-like effect. These data present the first evidence that enhancement of Sig-1R action on astrocytes entirely exerts antidepressant-like effect, indicating that specific activation of astrocytic Sig-1R may provide a new approach for antidepressant drug development.     

A riot of rhythms: neuronal and glial circadian oscillators in the mediobasal hypothalamus

Background

Proneurotrophins and mature neurotrophins elicit opposite effects via the p75 neurotrophin receptor (p75^NTR) and Trk tyrosine kinase receptors, respectively; however the molecular roles of proneurotrophins in the CNS are not fully understood.

Results

Based on two rare single nucleotide polymorphisms (SNPs) of the human brain-derived neurotrophic factor (BDNF) gene, we generated R125M-, R127L- and R125M/R127L-BDNF, which have amino acid substitution(s) near the cleavage site between the pro- and mature-domain of BDNF. Western blot analyses demonstrated that these BDNF variants are poorly cleaved and result in the predominant secretion of proBDNF. Using these cleavage-resistant proBDNF (CR-proBDNF) variants, the molecular and cellular roles of proBDNF on the CNS neurons were examined. First, CR-proBDNF showed normal intracellular distribution and secretion in cultured hippocampal neurons, suggesting that inhibition of proBDNF cleavage does not affect intracellular transportation and secretion of BDNF. Second, we purified recombinant CR-proBDNF and tested its biological effects using cultured CNS neurons. Treatment with CR-proBDNF elicited apoptosis of cultured cerebellar granule neurons (CGNs), while treatment with mature BDNF (matBDNF) promoted cell survival. Third, we examined the effects of CR-proBDNF on neuronal morphology using more than 2-week cultures of basal forebrain cholinergic neurons (BFCNs) and hippocampal neurons. Interestingly, in marked contrast to the action of matBDNF, which increased the number of cholinergic fibers and hippocampal dendritic spines, CR-proBDNF dramatically reduced the number of cholinergic fibers and hippocampal dendritic spines, without affecting the survival of these neurons.

Conclusion

These results suggest that proBDNF has distinct functions in different populations of CNS neurons and might be responsible for specific physiological cellular processes in the brain.     

BDNF, NT-3 and NGF induce distinct new Ca2+ channel synthesis in developing hippocampal neurons

Neurotrophins exert short- and long-term effects on synaptic transmission. The mechanism underlying these forms of synaptic plasticity is unknown although it is likely that intracellular Ca2+ and presynaptic Ca2+ channels play a critical role. Here we show that BDNF, NGF and NT-3 (10-100 ng/mL) exhibit a selective long-term up-regulation of voltage-gated Ca2+ current densities in developing hippocampal neurons of 6-20 days in culture. NGF and NT-3 appear more effective in up-regulating L-currents, while BDNF predominantly acts on non-L-currents (N, P/Q and R). The effects of the three neurotrophins were time- and dose-dependent. The EC50 was comparable for BDNF, NGF and NT-3 (10-16 ng/mL) while the time of half-maximal activation was significantly longer for NGF compared to BDNF (58 vs. 25 h). Despite the increased Ca2+ current density, the neurotrophins did not alter the voltage-dependence of channel activation, the kinetics parameters or the elementary properties of Ca2+ channels (single-channel conductance, probability of opening and mean open time). Neurotrophin effects were completely abolished by coincubation with the nonspecific Trk-receptor inhibitor K252a, the protein synthesis blocker anisomycin and the MAP-kinase inhibitor PD98059, while cotreatment with the PLC-gamma blocker, U73122, was without effect. Immunocytochemistry and Western blotting revealed that neurotrophins induced an increased MAP-kinase phosphorylation and its translocation to the nucleus. The present findings suggest that on a long time scale different neurotrophins can selectively up-regulate different Ca2+ channels. The action is mediated by Trk-receptors/MAP-kinase pathways and induces an increased density of newly available Ca2+ channels with unaltered gating activity.     

Conversion of psychological stress into cellular stress response: Roles of the sigma‐1 receptor in the process

Psychiatrists empirically recognize that excessive or chronic psychological stress can result in long‐lasting impairments of brain functions that partly involve neuronal cell damage. Recent studies begin to elucidate the molecular pathways activated/inhibited by psychological stress. Activation of the hypothalamic–pituitary–adrenal axis under psychological stress causes inflammatory oxidative stresses in the brain, in part due to elevation of cytokines. Psychological stress or neuropathological conditions (e.g., accumulation of β‐amyloids) trigger ‘cellular stress responses’, which promote upregulation of molecular chaperones to protect macromolecules from degradation. The unfolded protein response, the endoplasmic reticulum (ER)‐specific cellular stress response, has been recently implicated in the pathophysiology of neuropsychiatric disorders and the pharmacology of certain clinically used drugs. The sigma‐1 receptor is an ER protein whose ligands are shown to exert antidepressant‐like and neuroprotective actions. Recent studies found that the sigma‐1 receptor is a novel ligand‐operated ER chaperone that regulates bioenergetics, free radical generation, oxidative stress, unfolded protein response and cytokine signaling. The sigma‐1 receptor also regulates morphogenesis of neuronal cells, such as neurite outgrowth, synaptogenesis, and myelination, which can be perturbed by cellular stress. The sigma‐1 receptor may thus contribute to a cellular defense system that protects nervous systems against chronic psychological stress. Findings from sigma receptor research imply that not only cell surface monoamine effectors but also intracellular molecules, especially those at the ER, may provide novel therapeutic targets for future drug developments.     

Effects of (+)‐pentazocine on the antinociceptive effects of (–)‐pentazocine in mice

Previous studies have shown that sigma‐1 receptor chaperone (Sig‐1R) ligands can regulate pain‐related behaviors, and Sig‐1R itself can regulate μ‐opioid receptor functions as well as signal transduction. Even though (±)‐pentazocine has been used clinically for the treatment of pain through opioid receptors, (+)‐pentazocine is known to be a selective Sig‐1R agonist. To the best of our knowledge, there is no information available regarding the involvement of Sig‐1R agonistic action in the antinociceptive effects of (±)‐pentazocine. Therefore, the present study was designed to investigate the effects of (+)‐pentazocine on the antinociceptive effects of (–)‐pentazocine in mice. Both and (–)‐pentazocine induced biphasic antinociceptive effects as measured by the warm‐plate test. The early phase, but not the delayed phase, of the antinociceptive effects induced by (–)‐pentazocine, which are mediated by the activation of μ‐opioid receptors, were suppressed by pretreatment with (+)‐pentazocine. These results suggest that the innate antinociceptive action of (±)‐pentazocine could be marginally reduced by the effects of (+)‐pentazocine, but (+)‐pentazocine can suppress the antinociceptive effects of (–)‐pentazocine at certain time points. Synapse 69:166–171, 2015. © 2015 Wiley Periodicals, Inc.     

Sigma-1 receptors potentiate epidermal growth factor signaling towards neuritogenesis in PC12 cells: potential relation to lipid raft reconstitution

We previously demonstrated that overexpression of sigma-1 receptors (sigma-1R) potentiated neurite sprouting caused by nerve growth factor in PC12 cells (Takebayashi et al. 2002 J Pharmacol Exp Ther 202:1227-1237). In this study we examined if sigma-1R may be involved in the action of epidermal growth factor (EGF). EGF is conventionally recognized as a mitogenic factor that stimulates only the proliferation of various types of cells, including PC12 cells. We found here that in sigma-1 receptor-overexpressing PC12 cells (sigma-1R OE cells), EGF markedly stimulates neuritogenesis without affecting cellular proliferation. EGF receptors (EGFR) are largely reduced in lipid rafts and are enriched in non-raft regions in sigma-1R OE cells. The enrichment of EGFR in the non-raft region is correlated with enhanced downstream signaling of EGFR including the phosphorylation of both EGFR and extracellular signal-regulated kinases (ERKs). Destruction of cholesterol-containing rafts by treating cells with methyl-beta-cyclodextrin also causes a reduction of EGFR in lipid rafts, a concomitant increase in the phosphorylation of both EGFR and ERK, and an increase in the EGF-induced neurite sprouting in wildtype cells. Furthermore, while overexpression of sigma-1R increases the level of lipid raft-associated cholesterol, the overexpression alters the levels of gangliosides in lipid rafts: GM1 and GM2 are decreased, whereas GD1a is increased. We conclude that sigma-1R cause the remodeling of lipid rafts, at least by increasing the level of lipid raft-associated cholesterol and by altering the levels of certain critical lipid raft-forming gangliosides. sigma-1R may thus play an important role in directing EGF signaling towards neuritogenesis, perhaps by shifting EGFR from the lipid raft into non-raft regions.     

Distinct usages of phospholipase C gamma and Shc in intracellular signaling stimulated by neurotrophins

Nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3), members of the neurotrophin family, bind to and activate TrkA, TrkB and TrkC, respectively, members of the Trk receptor tyrosine kinase family, to exert various effects including promotion of differentiation and survival, and regulation of synaptic plasticity in neuronal cells. Many reports have suggested that different neurotrophins show distinct biological functions, although molecular mechanisms by which neurotrophins exert their different functions remain unclear. In the present study, we found distinct usages of phospholipase Cgamma (PLCgamma) and Shc in intracellular signaling stimulated by neurotrophins. BDNF stimulated much stronger interactions of PLCgamma with Trk than NGF and NT-3 in PC12 cells stably expressing TrkB and cultured cerebral cortical neurons, respectively, although BDNF, NGF and NT-3 induced similar levels of tyrosine phosphorylation of Trk. Furthermore, the cultured cortical neurons showed large PLCgamma-dependent increases in intracellular Ca(2+) levels in response to BDNF compared with NT-3. In Shc signaling, NGF, but not BDNF, displayed interactions between Trk and Shc in a phenylarsine oxide (PAO; an inhibitor of tyrosine phosphatase)-dependent manner in TrkB-expressing PC12 cells. These results indicated that neurotrophins stimulate distinct kinds of interactions between Trk and PLCgamma and between Trk and Shc. These differences may lead to the distinct biological functions of neurotrophins.     

Brain-derived neurotrophic factor and tyrosine kinase receptor TrkB in rat brain are significantly altered after haloperidol and risperidone administration

The antipsychotics haloperidol and risperidone are widely used in the therapy of schizophrenia. The former drug mainly acts on the dopamine (DA) D(2) receptor whereas risperidone binds to both DA and serotonin (5HT) receptors, particularly in the neurons of striatal and limbic structures. Recent evidence suggests that neurotrophins might also be involved in antipsychotic action in the central nervous system (CNS). We have previously reported that haloperidol and risperidone significantly affect brain nerve growth factor (NGF) level suggesting that these drugs influence the turnover of endogenous growth factors. Brain-derived neurotrophic factor (BDNF) supports survival and differentiation of developing and mature brain DA neurons. We hypothesized that treatments with haloperidol or risperidone will affect synthesis/release of brain BDNF and tested this hypothesis by measuring BDNF and TrkB in rat brain regions after a 29-day-treatment with haloperidol or risperidone added to chow. Drug treatments had no effects on weight of brain regions. Chronic administration of these drugs, however, altered BDNF synthesis or release and expression of TrkB-immunoreactivity within the brain. Both haloperidol and risperidone significantly decreased BDNF concentrations in frontal cortex, occipital cortex and hippocampus and decreased or increased TrkB receptors in selected brain structures. Because BDNF can act on a variety of CNS neurons, it is reasonable to hypothesize that alteration of brain level of this neurotrophin could constitute one of the mechanisms of action of antipsychotic drugs. These observations also support the possibility that neurotrophic factors play a role in altered brain function in schizophrenic disorders.     

Characterization of neurotrophin receptors by affinity crosslinking

Neurotrophic factors regulate the developmental survival and differentiation of specific neuronal populations. Brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) are members of the nerve growth factor (NGF) protein family, also known as the neurotrophins. Insights into the different roles of neurotrophins can be gained by studying the expression of their functional receptors. Here we report the development of procedures for their radiolabeling and efficient crosslinking to specific cell-surface receptors. BDNF and NT-3 receptors in cell lines and tissue preparations expressing receptors for the 2 neurotrophins have been identified using this affinity crosslinking procedure. Like NGF, BDNF and NT-3 crosslinking to the low affinity NGF receptor (p75^NGFR) on PC12 cells. BDNF and NT-3 also crosslinked to cells expressing p145^trkB protein, producing an approximately 160 kD neurotrophin-receptor complex. Crosslinking of the 2 neurotrophins in vivo to specific trk family members in many areas of the central nervous system also produced a 160 kD receptor complex. However, in all brain regions a complex of approx. 100 kD could also be identified, all or most of which represents crosslinking to a truncated from of trkB. The broad distribution of BDNF and NT-3 receptors throughout the CNS suggests that neurotrophins may have yet unrecognized functions on specific neuronal populations. BDNF and NT-3 receptors were also found in brain areas in which the neurotrophins themselves are also synthesized, suggesting that beyond long-range trophic effects, these proteins may also act as autocrine or short-range paracrine regulators. © 1993 Wiley-Liss, Inc.     

Brain derived neurotrophic factor and neurotrophin-4 employ different intracellular pathways to modulate norepinephrine uptake and release in rat hypothalamus

Classical actions of the neurotrophin family are related to cellular survival and differentiation. Moreover, acute effects of neurotrophins have been reported. Although neurotrophins effects on synaptic transmission at central nervous system level have been largely studied, acute effects of neurotrophins on hypothalamic noradrenergic transmission are still poorly understood. Thus, we have studied the effects of the neurotrophin family members nerve growth factor (NGF), brain derived neurotrophic factor (BDNF) and neurotrophin-4 (NT-4) on norepinephrine (NE) neuronal uptake and its evoked release, as well as the receptor and the intracellular pathways involved in these processes in rat hypothalamus.Present results indicate that BDNF increased NE uptake and decreased its evoked release through a mechanism that involve Trk B receptor and phospholipase C. Moreover, NT-4, also through the Trk B receptor, decreased NE uptake and its evoked release by activating phosphatidylinositol 3-OH-kinase. These effects were observed in whole hypothalamus as well as in the anterior hypothalamic zone. On the other hand, NGF did not modify noradrenergic transmission.In conclusion, we showed for the first time that BDNF and NT-4 activate two different intracellular signalling pathways through a Trk B receptor dependent mechanism. Furthermore, present findings support the hypothesis that BDNF and NT-4 acutely applied, could be considered as modulators of noradrenergic transmission and thus may regulate hypothalamic physiological as well as pathophysiological responses.     

TrkB receptor ligands promote activity-dependent inhibitory synaptogenesis.

Organotypic cerebellar cultures derived from newborn mice were simultaneously exposed to activity-blocking agents and neurotrophins for 2 weeks. Activity-blocked explants treated with the TrkB receptor ligands BDNF and neurotrophin-4 (NT-4) developed a full complement of Purkinje cell inhibitory axosomatic synapses, as defined ultrastructurally, and displayed control spontaneous cortical discharge rates after recovery from activity blockade. Otherwise untreated activity-blocked cultures and activity-blocked cultures exposed to the TrkC receptor ligand NT-3 had reduced inhibitory synapse development and persistent cortical hyperactivity after recovery. The added TrkB receptor ligands did not induce axonal sprouting to account for increased inhibitory synaptogenesis. Addition of neurotrophins to untreated cerebellar cultures did not increase the complement of Purkinje cell axosomatic synapses. Exposure of cerebellar cultures to a combination of antibodies to BDNF and NT-4 resulted in reduced inhibitory synapse formation, similar to the effects of activity blockade, indicating the necessity for endogenous neurotrophins for development of the full complement of inhibitory synapses in the presence of neuronal activity. Application of antibodies to BDNF and NT-4 to cerebellar explants exposed to picrotoxin to increase neuronal activity prevented the hyperinnervation of Purkinje cell somata by inhibitory terminals characteristic of cultures exposed to picrotoxin alone. These results are consistent with the concept that TrkB receptor ligands promote inhibitory synaptogenesis. The ability of neurotrophins to substitute for neuronal activity in encouraging development of inhibitory synapses may have therapeutic implications.     

Insulin-like growth factor-I enhances the biological activity of brain-derived neurotrophic factor on cerebrocortical neurons

Insulin-like growth factor (IGF)-I and brain-derived neurotrophic factor (BDNF) act within the brain to enhance neuronal survival and plasticity. We extend these findings by showing that the presence of both neurotrophins is required to depress the rise in intracellular Ca2+ caused by glutamate in primary cultures of cerebrocortical neurons. IGF-I enhanced expression of BDNF receptors (Trk-B) and increased the ability of BDNF to induce ERK1/2 phosphorylation. This IGF-I-induced increase in BDNF responsiveness describes a new interaction between these peptides in the brain.     

Dynamic regulation of GABAA receptors at synaptic sites

γ-Aminobutyric acid type A receptors (GABA_ARs) mediate fast synaptic inhibition in brain and spinal cord. They are ligand-gated ion channels composed of numerous distinct subunit combinations. For efficient synaptic transmission, GABA_ARs need to be localized to and anchored at postsynaptic sites in precise apposition to presynaptic nerve terminals that release the neurotransmitter GABA. Neurons therefore require distinct mechanisms to regulate intracellular vesicular protein traffic, plasma membrane insertion, synaptic clustering and turnover of GABA_ARs. The GABA_A receptor-associated protein GABARAP interacts with the γ2 subunit of GABA_ARs and displays high homology to proteins involved in membrane fusion underlying Golgi transport and autophagic processes. The binding of GABARAP with NSF, microtubules and gephyrin together with its localization at intracellular membranes suggests a role in GABA_AR targeting and/or degradation. Growth factor tyrosine kinase receptor activation is involved in the control of GABA_AR levels at the plasma membrane. In particular insulin recruits GABA_ARs to the cell surface. Furthermore, the regulation of GABA_AR surface half-life can also be the consequence of negative modulation at the proteasome level. Plic-1, a ubiquitin-like protein binds to both the proteasome and GABA_ARs and the Plic1-GABA_AR interaction is important for the maintenance of GABA-activated current amplitudes. At synaptic sites, GABA_ARs are clustered via gephyrin-dependent and gephyrin-independent mechanisms and may subsequently become internalized via clathrin-mediated endocytosis underlying receptor recycling or degradation processes. This article discusses these recent data in the field of GABA_AR dynamics.