Actions of excitatory amino acids on somatostatin release from cortical neurons in primary cultures

Somatostatin: Linking Cognition and Alzheimer Disease to Therapeutic Targeting

Over 4 decades of research support the link between Alzheimer disease (AD) and somatostatin [somatotropin-releasing inhibitory factor (SRIF)]. SRIF and SRIF-expressing neurons play an essential role in brain function, modulating hippocampal activity and memory formation. Loss of SRIF and SRIF-expressing neurons in the brain rests at the center of a series of interdependent pathological events driven by amyloid-β peptide (Aβ), culminating in cognitive decline and dementia. The connection between the SRIF and AD further extends to the neuropsychiatric symptoms, seizure activity, and inflammation, whereas preclinical AD investigations show SRIF or SRIF receptor agonist administration capable of enhancing cognition. SRIF receptor subtype-4 activation in particular presents unique attributes, with the potential to mitigate learning and memory decline, reduce comorbid symptoms, and enhance enzymatic degradation of Aβ in the brain. Here, we review the links between SRIF and AD along with the therapeutic implications. SIGNIFICANCE STATEMENT: Somatostatin and somatostatin-expressing neurons in the brain are extensively involved in cognition. Loss of somatostatin and somatostatin-expressing neurons in Alzheimer disease rests at the center of a series of interdependent pathological events contributing to cognitive decline and dementia. Targeting somatostatin-mediated processes has significant therapeutic potential for the treatment of Alzheimer disease.

Cell-Type-Specific Effects of Somatostatin on Synaptic Transmission in the Anterior Cingulate Cortex

Inhibitory modulation of glutamatergic information processing is a prerequisite for proper network function. Among the many groups of interneurons (INs), somatostatin-expressing interneurons (SOM-INs) play an important role in the maintenance of physiological brain activity. We have previously shown that somatostatin (SOM) causes a reduction in pyramidal cell (PC) excitability. However, the mechanisms of action of the peptide on cortical synaptic circuits are still unclear. To understand the effects of the neuropeptide SOM on cortical synaptic circuits, we performed a detailed side-by-side comparison of its postsynaptic effects on PCs, SOM-INs, and layer 1 interneurons (L1-INs) in the anterior cingulate cortex of male and female mice and found that SOM produced pronounced postsynaptic effects in PCs while having little to no effect on either IN type. This comparison allowed us to link the observed postsynaptic effects to SOM-induced modulations of glutamatergic and GABAergic synaptic transmission and to trace the impact of the neuropeptide on the neuronal circuitry between these three cell types. We show here that SOM depresses glutamatergic synaptic transmission via a presynaptic mechanism while exerting a differential impact on GABAA receptor- and GABAB receptor-mediated transmission at the pre- and postsynaptic level resulting in a shift of inhibition in L2/3 PCs from L1-INs to SOM-INs. In summary, this study unravels a novel aspect by which SOM modulates synaptic signaling between PCs, L1-INs, and SOM-INs.

The role of neuropeptide somatostatin in the brain and its application in treating neurological disorders

Somatostatin (SST) is a well-known neuropeptide that is expressed throughout the brain. In the cortex, SST is expressed in a subset of GABAergic neurons and is known as a protein marker of inhibitory interneurons. Recent studies have identified the key functions of SST in modulating cortical circuits in the brain and cognitive function. Furthermore, reduced expression of SST is a hallmark of various neurological disorders, including Alzheimer's disease and depression. In this review, we summarize the current knowledge on SST expression and function in the brain. In particular, we describe the physiological roles of SST-positive interneurons in the cortex. We further describe the causal relationship between pathophysiological changes in SST function and various neurological disorders, such as Alzheimer's disease. Finally, we discuss potential treatments and possibility of novel drug developments for neurological disorders based on the current knowledge on the function of SST and SST analogs in the brain derived from experimental and clinical studies.

Somatostatin and Somatostatin-Containing Neurons in Shaping Neuronal Activity and Plasticity

Since its discovery over four decades ago, somatostatin (SOM) receives growing scientific and clinical interest. Being localized in the nervous system in a subset of interneurons somatostatin acts as a neurotransmitter or neuromodulator and its role in the fine-tuning of neuronal activity and involvement in synaptic plasticity and memory formation are widely recognized in the recent literature. Combining transgenic animals with electrophysiological, anatomical and molecular methods allowed to characterize several subpopulations of somatostatin-containing interneurons possessing specific anatomical and physiological features engaged in controlling the output of cortical excitatory neurons. Special characteristic and connectivity of somatostatin-containing neurons set them up as significant players in shaping activity and plasticity of the nervous system. However, somatostatin is not just a marker of particular interneuronal subpopulation. Somatostatin itself acts pre- and postsynaptically, modulating excitability and neuronal responses. In the present review, we combine the knowledge regarding somatostatin and somatostatin-containing interneurons, trying to incorporate it into the current view concerning the role of the somatostatinergic system in cortical plasticity.

Protective effect of BDNF against beta-amyloid induced neurotoxicity in vitro and in vivo in rats

We examined the potential protective effect of BDNF against beta-amyloid-induced neurotoxicity in vitro and in vivo in rats. In neuronal cultures, BDNF had specific and dose-response protective effects on neuronal toxicity induced by Abeta(1-42) and Abeta(25-35). It completely reversed the toxic action induced by Abeta(1-42) and partially that induced by Abeta(25-35). These effects involved TrkB receptor activation since they were inhibited by K252a. Catalytic BDNF receptors (TrkB.FL) were localized in vitro in cortical neurons (mRNA and protein). In in vivo experiments, Abeta(25-35) was administered into the indusium griseum or the third ventricle and several parameters were measured 7 days later to evaluate potential Abeta(25-35)/BDNF interactions, i.e. local measurement of BDNF release, number of hippocampal hilar cells expressing SRIH mRNA and assessment of the corpus callosum damage (morphological examination, pyknotic nuclei counting and axon labeling with anti-MBP antibody). We conclude that BDNF possesses neuroprotective properties against toxic effects of Abeta peptides.

β-Amyloid increases somatostatin expression in cultured cortical neurons

In beta-amyloid (Abeta)-induced neurotoxicity, activation of the NMDA receptor, increased Ca2+ and oxidative stress are intimately associated with neuronal cell death as normally seen in NMDA-induced neurotoxicity. We have recently shown selective sparing of somatostatin (SST)-positive neurons and increased SST expression in NMDA agonist-induced neurotoxicity. Accordingly, the present study was undertaken to determine the effect of Abeta25-35-induced neurotoxicity on the expression of SST in cultured cortical neurons. Cultured cortical cells were exposed to Abeta25-35 and processed to determine the cellular content and release of SST into medium by radioimmunoassay and SST mRNA by RT-PCR. Abeta25-35 induces neuronal cell death in a concentration- and time-dependent fashion, increases SST mRNA synthesis and induces an augmentation in the cellular content of SST. No significant changes were seen on SST release at any concentration of Abeta25-35 after 24 h of treatment. However, Abeta25-35 induces a significant increase of SST release into medium only after 12 h in comparison with other time points. Most significantly, SST-positive neurons are selectively spared in the presence of a lower concentration of Abeta25-35, whereas, in the presence of higher concentrations of Abeta25-35 for extended time periods, SST-positive neurons decrease gradually. Furthermore, Abeta25-35 induces apoptosis at lower concentrations (5 and 10 micromol/L) and necrosis at higher concentrations (20 and 40 micromol/L). Consistent with the increased accumulation of SST, these data suggest that Abeta25-35 impairs cell membrane permeability. Selective sparing of SST-positive neurons at lower concentrations of Abeta25-35 at early time points directly correlates with the pathophysiology of Alzheimer's disease.

Gly-Pro-Glu protects beta-amyloid-induced somatostatin depletion in the rat cortex

The effect of Gly-Pro-Glu (GPE) on the somatostatinergic system of the temporal cortex in amyloid beta-peptide (Abeta) treated rats was investigated. Intracerebroventricular Abeta25-35 administration for 14 days (300 pmol/day) to ovariectomized rats produced a marked reduction in somatostatin (SRIF) content, SRIF receptor density and reduced the inhibitory effect of SRIF on adenylyl cyclase activity. I.p. injection of three doses (300 microg) of GPE on days 0, 6 and 12 resulted in a partial recovery of the parameters affected by Abeta25-35 administration. These results indicate that GPE may have an in vivo effect protecting the temporal cortical somatostatinergic system from Abeta insult.

Somatostatin inhibits glutamate release from mouse cerebrocortical nerve endings through presynaptic sst2 receptors linked to the adenylyl cyclase-protein kinase A pathway

The effects of somatostatin (SRIF, somatotropin release inhibiting factor) on the release of glutamate have been investigated using superfused mouse cerebrocortical synaptosomes. SRIF-14 inhibited the K+ (12 mM)-evoked overflow of preaccumulated [3H]D-aspartate as well as that of endogenous glutamate. Cyanamid 154806, a selective sst2 receptor antagonist, but not BIM-23056, an antagonist at sst5 receptors, prevented the SRIF-14 effect. Octreotide and L779976, selective agonists at sst2 receptors, mimicked SRIF-14, whereas L797591, L796778, L803087 and L362855, selective agonists at sst1, sst3, sst4 and sst5 receptor subtypes, were inactive. Activation of sst2 receptors seems to involve inhibition of the adenylyl cyclase-protein kinase A pathway present in glutamatergic terminals since the adenylyl cyclase inhibitor MDL-12,330A and the protein kinase A inhibitor H89 prevented the K+-evoked [3H]D-aspartate overflow. Consistent with the involvement of adenylyl cyclase, depolarization with 12 mM K+ increased synaptosomal cyclic AMP (cAMP) content, while forskolin, an adenylyl cyclase activator, potentiated basal [3H]D-aspartate release in an octreotide-, MDL-12,330A- and H89-sensitive manner. To conclude, glutamatergic cerebrocortical nerve endings possess release-inhibiting sst2 receptors which represent potential targets for new drugs able to mitigate the effects of excessive glutamate transmission.

Acute stress and dexamethasone rapidly increase hippocampal somatostatin synthesis and release from the dentate gyrus hilus

Somatostatin is a neuropeptide whose facilitatory action in the generation of long-term potentiation (LTP) in the hippocampal dentate gyrus has been associated with memory processes. Since stress and memory seem to share some neural pathways, we studied somatostatin release from dentate gyrus hilar cells of the hippocampus in unanesthetized free-moving rats subjected to stress or dexamethasone treatments. In parallel, the number of dentate gyrus hilar cells expressing somatostatin mRNA was quantified by nonradioactive in situ hybridization in these two experimental conditions. Rats were stereotaxically implanted with a push-pull cannula in the dentate gyrus hilar region. Animals were perfused 1 week later in basal or stress (30 min immobilization stress) conditions. The other group was intraperitoneally injected with the synthetic glucocorticoid dexamethasone (3 mg/kg b.w.). Samples were collected every 15 min for somatostatin radioimmunoassay. In parallel, in other groups of animals undergoing the same treatments, brains were removed for in situ hybridization studies with an oligonucleotide labeled with digoxigenin that recognizes somatostatin-14. The results showed that stress induced a significant increase in somatostatin release from dentate gyrus hilar cells 30-45 min after immobilization stress application. Dexamethasone-injected animals exhibited a similar response 45 min after drug administration. In situ hybridization analysis revealed that the two treatments significantly increased the number of cells expressing somatostatin mRNA in the hilar region. In conclusion, somatostatin interneurons of the hippocampal hilar region appear to be a novel stress stimulus target. Their rapid reactivity, expressed as modifications of both somatostatin release and number of cells expressing somatostatin mRNA, provides an interesting model of neuronal plasticity.

Glutamate receptors mediate regulation of Na pump isoform activities in neurons

Rat cerebral neurons matured in culture were stimulated with glutamate analogues, and the K+ uptake activities of Na pump isoforms were measured. Ionotropic receptor agonists, kainate, AMPA, and NMDA, increased total K+ uptake activity via activation of the alpha2/alpha3 isoforms and an inhibition of the alpha1 isoform as reported previously for glutamate. The effects of kainate or AMPA were antagonized by CNQX and those of NMDA were by APV or MK-801. In contrast, metabotropic receptor agonist ACPD had no effects on the Na pump isoform activities. Glutamate transporter substrate, PDC, was effective, but NMDA receptor antagonists abolished the effects of PDC. These results suggest that the ionotropic glutamate receptors mediate the regulation of Na pump isoform activities in neurons.

N-methyl-D-aspartate (NMDA)-mediated corticotropin-releasing factor (CRF) release in cultured rat amygdala neurons

Corticotropin-releasing factor (CRF) plays an important role in the activation of centrally mediated responses to stress. The amygdala, a limbic structure involved in the stress response, has a significant number of CRF cell bodies and CRF receptors. Activation of glutamatergic projections to the amygdala has been implicated in the stress response. Few studies have evaluated neurotransmitter-stimulated CRF release in the amygdala. We measured the effects of glutamate (0.1-1000 microM) and N-methyl-D-aspartate (NMDA, 0.1-1000 microM) on CRF release from the amygdala using primary neuronal cultures from embryonic rat brains (E18-19). Experiments were performed after the cultures grew for 17-20 days. CRF was measured using radioimmunoassay. The excitatory amino acid neurotransmitters, glutamate and NMDA, stimulated CRF release in a concentration-dependent manner. The apparent EC50 values for glutamate and NMDA were 17.5 microM and 12 microM, respectively. Consistent with a NMDA receptor-driven event, glutamate-stimulated CRF release was blocked by the NMDA antagonist, 2-amino-5-phosphonovaleric acid (AP-5, 1-100 microM) and antagonized by the addition of 1.2 mM MgCl2 to the incubation medium. These results implicate an inhibition of CRF release in the amygdala as a possible mechanism for the reported anxiolytic effects of NMDA antagonists.

Evidence for functional native NMDA receptors activated by glycine or D-serine alone in the absence of glutamatergic coagonist

In this study we have examined the effects of N-methyl-D-aspartate (NMDA) receptor activation on the release of cholecystokinin and somatostatin from rat neocortical nerve endings. The release of cholecystokinin-like immunoreactivity (CCK-LI) and of somatostatin-like immunoreactivity (SRIF-LI) elicited by 12 mM K+ from superfused synaptosomes, but not the spontaneous release, was increased by NMDA in a concentration-dependent manner. The effects of NMDA could be prevented by antagonists selective for the glutamate recognition site, the receptor channel and the glycine site of the NMDA receptor. In the absence of NMDA, glycine increased on its own and in a concentration-dependent manner the depolarization-evoked release of both CCK-LI and SRIF-LI. This effect of glycine was strychnine-insensitive and could be mimicked by D-serine, a stereoselective agonist at the NMDA receptor glycine site. Antagonists selective for the glycine site or for the NMDA receptor channel prevented the effects of glycine/D-serine; these effects were, however, insensitive to blockade of the glutamate recognition site of the NMDA receptor, suggesting that glutamate released from synaptosomes or present as contaminant was not involved. The neuropeptide release elicited by D-serine was strongly inhibited by ifenprodil (0.3 microM) and by Zn2+ ions (50 nM), selective ligands at the NR2B and NR2A subunits of NMDA receptors, respectively. It is concluded that nerve terminals of CCK- and SRIF-releasing neurons possess non-conventional NMDA receptors whose channels can be operated by glycine or D-serine without apparent activation of the glutamatergic coagonist site. These receptors may display the triple subunit combination NR1/NR2A/NR2B.

Glutamate pathways mediate somatostatin responses to glucose in normal and diabetic rat hypothalamus

We investigated the role of hypothalamic glutamate receptors in mediating the stimulatory effect of low glucose (< 5 mM) on somatostatin release. We also studied whether alteration in glutamate release might contribute to the reduced hypothalamic somatostatin response to low glucose observed in diabetic (Goto-Kakizaki) rat hypothalami. Hypothalamic somatostatin release in response to incubation with 1 mM D-glucose was inhibited by the ionotropic glutamate receptor antagonists MK801, D-AP5 and DNQX but not by the metabotropic antagonists L-AP3 or MCPG. The release of somatostatin was increased by the ionotropic agonists NMDA, AMPA and kainate but not by metabotropic agonists t-ACPD or L-AP4. Basal and peak glutamate release in response to incubation with 1 mM glucose, were significantly lower from GK hypothalami There were no significant differences in the basal or stimulated release of serine and GABA. These data indicate that ionotropic NMDA/AMPA/kainate receptors and not metabotropic receptors mediate the effects of glucose on rat hypothalamic somatostatin release. Reduced hypothalamic somatostatin release in response to low glucose in diabetic (Goto-Kakizaki) rats may well be secondary, at least in part, to reduced glutamate release.

Role of glutamate receptor subtypes in the differential release of somatostatin, neuropeptide Y, and substance P in primary serum-free cultures of striatal neurons

The spiny and aspiny neuronal populations of the striatum display differential vulnerability to the toxic effects of glutamatergic agonists. Substance P-containing spiny neurons appear to be more vulnerable to NMDA-receptor-mediated toxicity and less susceptible to kainate toxicity than the somatostatin- and neuropeptide Y (NPY)-containing aspiny population. We studied whether selective glutamatergic agonists might have similar differential effects on neuropeptide release from the substance P- and somatostatin/NPY-containing neuronal populations. After collection of a baseline sample, striatal neurons in primary culture were treated with one of the following: phosphate-buffered saline, 56 mM potassium chloride (KCl), 100 microM N-methyl-D-aspartate (NMDA), 100 microM quisqualate, 100 microM kainate, or 100 microM glutamate. Baseline and treatment samples were measured by radioimmunoassay for somatostatin, NPY, and substance P. KCl and kainate provoked a selective release of somatostatin and NPY, whereas substance P measured in the same samples showed no response. By contrast, NMDA elicited a selective release of substance P without a similar increase of either somatostatin or NPY. Quisqualate evoked comparable responses in the three peptides. These results indicate that the glutamatergic regulation of somatostatin and NPY release from aspiny striatal neurons in primary culture is preferentially mediated by the kainate receptor, whereas substance P release is selectively mediated by the NMDA receptor. These findings suggest a preferential expression of functional kainate receptors on the aspiny somatostatin/NPY neurons and of NMDA receptors on the substance-P-containing spiny neurons.

Depolarization-induced release of corticotropin-releasing factor (CRF) in primary neuronal cultures of the amygdala

The 41-amino acid neuropeptide, corticotropin-releasing factor (CRF) is distributed throughout the central nervous system and appears to play a pivotal role in stress, anxiety and depression. CRF is present in high concentrations in the limbic brain region, the amygdala, an area important in emotional and autonomic responses to stress. In this report, primary neuronal cultures of amygdala from fetal rat brains (E18-E19) were used to study depolarization-induced CRF release. Immunocytochemical analyses of the cultures revealed a bead-like distribution of CRF immunoreactivity (CRFir) in about 1% of the neurons. Time course studies showed that 56 mM KCl-evoked CRF release occurred with an initial burst during the first minute that was maintained over 30 min; basal CRF release slightly increased over a 30-min period. CRF release in response to depolarization increased with increasing cell density and with increasing days in culture. Multiple serial incubations alternating basal and depolarizing conditions caused a depletion of the releasable pool of CRF. Potassium-evoked CRF release was calcium-dependent. These data suggest that primary neuronal cultures of fetal rat amygdala are an effective model system to study CRF release in this brain region.

Differential responses of rat cerebral somatostatinergic and cholinergic cells to glutamate agonists

Reductions in cortical somatostatin (SRIH) and choline acetyl-transferase (ChAT) are major biochemical deficits in Alzheimer disease (AD). SRIH and ChAT were measured in fetal rat cerebral neurons after exposure to the glutamate agonists N-methyl-D-aspartate (NMDA), kainate (KA), and quisqualate (Q). NMDA (96 h incubation) stimulated SRIH release and content in a dose-dependent manner with a Bmax of 10(-5)M and EC50 of 2-3 x 10(-6)M. KA showed a small stimulation in SRIH levels at 10(-5)M, but produced marked inhibition at 10(-4)M. Q decreased both intracellular and secreted SRIH. KA (51-76% of basal) and Q (27-56% of basal) but not NMDA (91-114% of basal) also inhibited the incorporation of [35S]methionine into proteins. In similar experiments 10(-4)M Q (23 +/- 9% of basal) and KA (20 +/- 3% of basal) but not NMDA (80 +/- 16% of basal) reduced ChAT levels in hypothalamic/septal cultures. These inhibitory actions on ChAT activity by KA and Q were reversed by gamma-glutamyltaurine (GT) but not by 2-amino-5-phosphonopentanoic acid (AP5). Chronic NMDA exposure partially inhibited muscarinic acetylcholine receptor (mAChR) mediated inositol phospholipid (PI) turnover, whereas it was abolished after KA and Q pretreatment. These findings suggest that in cerebral cell cultures, NMDA has a stimulatory action on somatostatinergic neurons and non-NMDA receptor agonism could play an important role in EAA-mediated neural damage.

NMDA receptor activation stimulates phospholipase A2 and somatostatin release from rat cortical neurons in primary cultures

We have recently shown that glutamate exerts a stimulatory action on somatostatin secretion in cortical neurons essentially through NMDA receptor sites. Here, we investigated whether arachidonic acid release could be modified after NMDA receptor activation in cortical neurons in primary culture. We also studied whether pharmacological manipulation of phospholipase A2 could modify somatostatin release. We found that both glutamate and NMDA (N-methyl-D-aspartate) stimulated [3H]arachidonic acid release. NMDA-evoked arachidonic acid release was inhibited by MK-801 and TCP (two NMDA receptor-type antagonists), or by mepacrine, an inhibitor of phospholipase A2. NMDA-induced somatostatin release was inhibited by MK-801, mepacrine and by another phospholipase A2 inhibitor, p-bromophenacylbromide (pBPB). However, responses to NMDA were unaffected by H7, NDGA (nordihydroguaiaretic acid), indomethacin or by RHC 80267 (inhibitors of protein kinase C, lipooxygenase, cyclooxygenase and diacylglycerol lipase, respectively). Mepacrine (greater than or equal to 100 microM) decreased NMDA-stimulated phosphatidylinositol (PI) hydrolysis and at higher concentrations (250 microM) was also able to inhibit basal release whereas pBPB had no effect in the range of concentrations tested. Neomycin (which inhibits phosphatidylinositol metabolism by binding strongly and selectively to inositol phospholipids) reduced by 30% the NMDA-stimulated somatostatin release, although chronic treatment of neurons with the phorbol ester 12-myristate, 13-acetate (PMA) had no effect on this response. Melittin, an activator of phospholipase A2, was able to stimulate both arachidonic acid release and somatostatin secretion. High-performance liquid chromatography (HPLC) analysis of tritiated metabolites released from cortical neurons under basal or NMDA-stimulated conditions revealed that [3H]arachidonic acid was the only metabolite detectable. Furthermore, external addition of arachidonic acid increased somatostatin secretion. Our results show a correlation between the two parameters studied.

Phospholipase A and Somatostatin Release are Activated in Response to N-Methyl-D-Aspartate Receptor Stimulation in Hypothalamic Neurons in Primary Culture

Abstract We have recently shown that glutamate primarily induces somatostatin release in hypothalamic neurons through N-methyl-D-aspartate (NMDA)-type receptor sites. Here we report that glutamate and NMDA also stimulate the release of [(3)H]arachidonic acid in a dose-dependent manner. The NMDA-induced effects (arachidonic acid release and somatostatin secretion) were both inhibited by MK-801, an NMDA receptor-type antagonist, or mepacrine, a phospholipase A(2) inhibitor. In addition, mepacrine was able to inhibit A23187-stimulated arachidonic acid release and somatostatin secretion. p-Bromophenacylbromide, another phospholipase A(2) inhibitor, also blocked NMDA-induced secretion of somatostatin. However, responses to NMDA were unaffected by H7 (inhibitor of protein kinase C), nordihydroguaiaretic acid or indomethacin (inhibitors of lipoxygenase and cyclooxygenase). Melittin, a phospholipase A(2) activator, was found to stimulate both responses, but omission of extracellular Ca(2+) from the incubation media strongly reduced melittin-induced somatostatin release. Six-h pertussis toxin pretreatment did not significantly reduce the action of NMDA on either of the two parameters studied. High-performance liquid chromatography analysis of [(3)H]metabolites released in the medium after NMDA stimulation revealed that [(3)H]arachidonic acid was the only detectable metabolite. External addition of arachidonic acid increased the release of somatostatin, whereas E(2) and F(2)alpha prostaglandins had no effect. Our results show a close correlation between arachidonic acid release and somatostatin secretion, the two parameters we investigated.

Prolonged exposure to N-methyl-D-aspartate increases intracellular and secreted somatostatin in rat cortical cells

Somatostatin (SRIF) release from fetal rat cortical cells was stimulated by exposure to 10(-5) M N-methyl-D-aspartate (NMDA) (250 +/- 20% of basal at 96 h). A similar but much less potent effect was seen with kainate (KA) but not with quisqualate (Q) which inhibited SRIF release (KA 150 +/- 13%, Q 65 +/- 18% of basal at 96 h). Similar data were obtained for intracellular levels of SRIF. Dose-dependent experiments showed that the EC50 for the stimulatory action of NMDA was 2-3 x 10(-6) M with a Bmax of around 10(-5) M. At 10(-4) M KA and Q but not NMDA reduced tissue content and release of SRIF (KA: 47 +/- 14, 67 +/- 17%; Q: 36 +/- 13, 42 +/- 6% of basal for content and release, respectively). These findings indicate that cortical SRIF content and release is enhanced by exposure to NMDA but not by KA or Q. We suggest that SRIF-containing neurones are sensitive to glutamate damage through the activation of non-NMDA rather than NMDA receptors.

Vigabatrin pre-treatment prevents hilar somatostatin cell loss and the development of interictal spiking activity following sustained simulation of the perforant path

Somatostatin-containing neurons in the hilus of the dentate gyrus are known to be exceptionally vulnerable in experimental models of epilepsy, as well as in human temporal lobe epilepsy. The position of these cells in the circuitry of the dentate gyrus is ideal for gating the activation evoked by afferents from the entorhinal cortex. In the present study we have shown that the loss of hilar somatostatin-containing neurons, and the development of interictal spiking activity induced by sustained perforant pathway stimulation can be prevented by high doses (500 mg/kg), but not by low doses (100 mg/kg) of vigabatrin, an irreversible inhibitor of GABA-transaminase.

N-methyl-D-aspartate evokes the release of somatostatin from striatal interneurons in primary culture

Indirect immunocytochemistry of striatal neurons in primary culture, generated from the embryonic mouse brain, suggested that 2-4% of the neurons contained somatostatin-like immunoreactivity; the majority of these cells also contained neuropeptide Y immunoreactivity, characteristic of a subset of striatal interneurons. Although 10-15% of cultured striatal neurons showed moderate or intense immunoreactivity for calbindin-D28k, the majority of neurons with somatostatin-like immunoreactivity did not contain calbindin-D28k-like immunoreactivity; parvalbumin immunoreactivity was absent from the culture preparation. A highly sensitive radioimmunoassay was used to examine the actions of depolarizing agents and excitatory amino acids on the release of endogenous somatostatin-like immunoreactivity from striatal interneurons. During a 15 min incubation period, 47 +/- 10 fmol of somatostatin-like immunoreactivity were released from 14 days in vitro striatal neurons, cultured in 35 mm dishes. Depolarization with 56 mM KCl or 10 micrograms/ml veratrine resulted in an additional 105 +/- 9 and 56 +/- 5 fmol, respectively, of somatostatin-like immunoreactivity released; the release evoked by veratrine was blocked by 1 microM tetrodotoxin. In the presence of 100 microM N-methyl-D-aspartate, 112 +/- 21 fmol of somatostatin-like immunoreactivity (above basal) were released (+238%); the N-methyl-D-aspartate-evoked release was dose-dependent (EC50, 20 microM), attenuated in the absence of added Ca2+, potentiated in the absence of added Mg2+ and unaffected by the presence of 1 microM tetrodotoxin. The selective antagonists 2-amino-5-phosphonovalerate (100 microM) and MK-801 (1 microM) blocked the N-methyl-D-aspartate-evoked release of somatostatin-like immunoreactivity; KCl-evoked release was unaffected. Kainate was slightly more effective, yet five-fold less potent (EC50, 100 microM), than N-methyl-D-aspartate in evoking somatostatin-like immunoreactivity release; quisqualate was marginally effective. The results of this study suggest that N-methyl-D-aspartate and kainate receptors are present on striatal somatostatinergic interneurons in primary culture.

Functional and histological consequences of quinolinic and kainic acid-induced seizures on hippocampal somatostatin neurons

Changes in endogenous somatostatin after quinolinic and kainic acids were investigated by measuring somatostatin-like peaks by in vivo voltammetry and by assessing the distribution of somatostatin-positive neurons by immunocytochemistry. Kainic acid (0.19 nmol/0.5 microliter) or quinolinic acid (120 nmol/0.5 microliter) in doses inducing comparable electroencephalographic seizure patterns, were injected into the hippocampus of freely moving rats. Somatostatin-like peaks were measured every 6 min for 3 h by a carbon fiber electrode implanted in the proximity of the injection needle. Kainic acid kept somatostatin-like peaks significantly higher than saline from 48 min after the injection till the end of the recording. Somatostatin-like peaks were dramatically elevated by quinolinic acid, reaching a maximum of 482% 60 min after the injection. Three days later, administration of kainic acid resulted in selective degeneration of CA3 pyramidal neurons but did not affect the number of somatostatin-positive cells, while quinolinic acid induced cell loss in all pyramidal layers and complete degeneration of somatostatin-positive cells in the whole hippocampus. Thus, the quantitative difference in somatostatin release in response to doses of kainic and quinolinic acids inducing comparable electroencephalographic seizure patterns was reflected in a substantial difference in the neurodegenerative consequences. In both models, the release of somatostatin in response to seizures may be interpreted as a "defense" mechanism aimed at reducing the spread of excitation in the tissue.

Pharmacological properties of the NMDA receptor involved in somatostatin release from cortical neurons

Glutamate increases somatostatin release from cultured cerebral cortical neurons, presumably through a N-methyl-D-aspartate (NMDA) receptor type. We report here that the NMDA response was potentiated by D-serine (10 microM) and that this potentiation was blocked by kynurenic acid (4-hydroxyquinoline-2-carboxylic acid; KYN). A higher concentration of D-serine (100 microM) reduced the antagonistic effect of KYN. Furthermore, the NMDA response exhibited another characteristic property of the NMDA receptor: it was decreased by low concentrations of Zn2+ (50 microM). In contrast, Zn2+ slightly but significantly potentiated the quisqualate (QA)- and kainate (KA)-induced responses.