Electrophysiological actions of somatostatin (SRIF) in hippocampus: an in vitro study

Neuropeptide System Regulation of Prefrontal Cortex Circuitry: Implications for Neuropsychiatric Disorders

Neuropeptides, a diverse class of signaling molecules in the nervous system, modulate various biological effects including membrane excitability, synaptic transmission and synaptogenesis, gene expression, and glial cell architecture and function. To date, most of what is known about neuropeptide action is limited to subcortical brain structures and tissue outside of the central nervous system. Thus, there is a knowledge gap in our understanding of neuropeptide function within cortical circuits. In this review, we provide a comprehensive overview of various families of neuropeptides and their cognate receptors that are expressed in the prefrontal cortex (PFC). Specifically, we highlight dynorphin, enkephalin, corticotropin-releasing factor, cholecystokinin, somatostatin, neuropeptide Y, and vasoactive intestinal peptide. Further, we review the implication of neuropeptide signaling in prefrontal cortical circuit function and use as potential therapeutic targets. Together, this review summarizes established knowledge and highlights unknowns of neuropeptide modulation of neural function underlying various biological effects while offering insights for future research. An increased emphasis in this area of study is necessary to elucidate basic principles of the diverse signaling molecules used in cortical circuits beyond fast excitatory and inhibitory transmitters as well as consider components of neuropeptide action in the PFC as a potential therapeutic target for neurological disorders. Therefore, this review not only sheds light on the importance of cortical neuropeptide studies, but also provides a comprehensive overview of neuropeptide action in the PFC to serve as a roadmap for future studies in this field.

Excitatory neurons are more disinhibited than inhibitory neurons by chloride dysregulation in the spinal dorsal horn

Neuropathic pain is a debilitating condition caused by the abnormal processing of somatosensory input. Synaptic inhibition in the spinal dorsal horn plays a key role in that processing. Mechanical allodynia - the misperception of light touch as painful - occurs when inhibition is compromised. Disinhibition is due primarily to chloride dysregulation caused by hypofunction of the potassium-chloride co-transporter KCC2. Here we show, in rats, that excitatory neurons are disproportionately affected. This is not because chloride is differentially dysregulated in excitatory and inhibitory neurons, but, rather, because excitatory neurons rely more heavily on inhibition to counterbalance strong excitation. Receptive fields in both cell types have a center-surround organization but disinhibition unmasks more excitatory input to excitatory neurons. Differences in intrinsic excitability also affect how chloride dysregulation affects spiking. These results deepen understanding of how excitation and inhibition are normally balanced in the spinal dorsal horn, and how their imbalance disrupts somatosensory processing.

Molecular and Cellular Mechanisms Underlying Somatostatin-Based Signaling in Two Model Neural Networks, the Retina and the Hippocampus

Neural inhibition plays a key role in determining the specific computational tasks of different brain circuitries. This functional "braking" activity is provided by inhibitory interneurons that use different neurochemicals for signaling. One of these substances, somatostatin, is found in several neural networks, raising questions about the significance of its widespread occurrence and usage. Here, we address this issue by analyzing the somatostatinergic system in two regions of the central nervous system: the retina and the hippocampus. By comparing the available information on these structures, we identify common motifs in the action of somatostatin that may explain its involvement in such diverse circuitries. The emerging concept is that somatostatin-based signaling, through conserved molecular and cellular mechanisms, allows neural networks to operate correctly.

Somatostatin immunoreactivity in axon terminals in rat nucleus tractus solitarii arising from central nucleus of amygdala: coexistence with GABA and postsynaptic expression of sst2A receptor

Axon terminals synapsing on neurones in the nucleus tractus solitarii (NTS) that originate from the central nucleus of the amygdala (CeA) have been shown to contain gamma-aminobutyric acid (GABA) immunoreactivity. Here we investigated whether such terminals also contain somatostatin (SOM), a neuropeptide found in axons distributed throughout the NTS and in somata in the CeA, and known to modulate cardiovascular reflexes when microinjected into the NTS. With fluorescence microscopy, SOM immunoreactivity was seen in the varicosities of some axons throughout the NTS that were anterogradely labelled with biotin dextran amine injected into the CeA. Such varicosities were frequently observed in close proximity to dendrites of NTS neurones that were immunoreactive for the SOM receptor sst(2A) subtype, and in many cases also for catecholamine synthesising enzymes. In the caudal, cardioregulatory zone of NTS, SOM immunoreactivity was localised by electron microscopic pre-embedding gold labelling to boutons containing dense-cored and clear pleomorphic vesicles and forming symmetrical synapses, mostly onto dendrites. Additional post-embedding gold labelling for GABA suggested that a subpopulation (29%) of GABAergic terminals sampled in this area of NTS contained SOM. Almost all boutons anterogradely labelled from the amygdala were GABA-immunoreactive (-IR) and 21% of these were SOM-IR. A similar proportion of these boutons (22%) formed synapses onto dendrites containing immunoreactivity for the SOM receptor sst(2A) subtype. These observations provide evidence that some of the GABAergic projection neurones in the CeA that inhibit baroreceptor reflex responses in the NTS in response to fear or emotional stimuli could release SOM, which might modulate the activity of NTS neurones via an action on sst(2A) receptors.

Peptidergic transmission: from morphological correlates to functional implications

Like non-peptidergic transmitters, neuropeptides and their receptors display a wide distribution in specific cell types of the nervous system. The peptides are synthesized, typically as part of a larger precursor molecule, on the rough endoplasmic reticulum in the cell body. In the trans-Golgi network, they are sorted to the regulated secretory pathway, packaged into so-called large dense-core vesicles, and concentrated. Large dense-core vesicles are preferentially located at sites distant from active zones of synapses. Exocytosis may occur not only at synaptic specializations in axonal terminals but frequently also at nonsynaptic release sites throughout the neuron. Large dense-core vesicles are distinguished from small, clear synaptic vesicles, which contain "classical' transmitters, by their morphological appearance and, partially, their biochemical composition, the mode of stimulation required for release, the type of calcium channels involved in the exocytotic process, and the time course of recovery after stimulation. The frequently observed "diffuse' release of neuropeptides and their occurrence also in areas distant to release sites is paralleled by the existence of pronounced peptide-peptide receptor mismatches found at the light microscopic and ultrastructural level. Coexistence of neuropeptides with other peptidergic and non-peptidergic substances within the same neuron or even within the same vesicle has been established for numerous neuronal systems. In addition to exerting excitatory and inhibitory transmitter-like effects and modulating the release of other neuroactive substances in the nervous system, several neuropeptides are involved in the regulation of neuronal development.

Facilitatory effects of somatostatin on reduced uptake of 2-deoxyglucose in cerebral cortical and hippocampal slices from aged rats

The aim of the present study was to determine whether or not the reduction of 2-deoxyglucose uptake by the cerebral cortical slices in aged rats (22-23 months old) was attenuated by somatostatin or carbachol. In 8-week-old rats, somatostatin and carbachol produced concentration-dependent increases in 2-deoxyglucose uptake. 2-Deoxyglucose uptake of the cortical slices in 22-23-month-old rats was significantly facilitated by treatment with 0.1-1 microM somatostatin or 1-100 microM carbachol. Metabolic responses to somatostatin or carbachol were quite similar in young and aged rats. The present results demonstrated that 2-deoxyglucose uptake by the cerebral cortex was facilitated by somatostatin and carbachol in both young and old rats.

The distribution of somatostatin binding sites in the brain of gymnotiform fish, Apteronotus leptorhynchus

The neuropeptide somatostatin (SS) and its binding sites display a wide distribution in the central nervous system of vertebrates. By employing semi-quantitative autoradiography, we identified such binding sites in the brain of the weakly electric fish Apteronotus leptorhynchus (Gymnotiformes, Teleostei). Whereas (SS1) binding sites for the octapeptide analogue Tyr3-SMS-201-995 appear to be absent in the gymnotiform brain, (SS2) binding sites for the analogue [Tyr0-D-Trp8]-somatostatin-14 were found in many brain regions and showed a similar distribution to that observed by other authors in the amphibian and mammalian central nervous system. Telencephalon While binding in the ventral telencephalon was typically low, all cell groups of the dorsal portion displayed a high degree of binding. The highest density of binding sites was found in the dorsal and caudal subdivision 2 of the dorsomedial telencephalon. Diencephalon Many cell groups of the diencephalon showed a medium to high degree of binding density. The highest level was seen in the habenula. Mesencephalon All layers of the optic tectum contained a medium number of binding sites, except the stratum marginale. In the torus semicircularis, the different layers displayed distinct binding density. While laminae 7-8 showed the highest degree of binding, the lowest density was found in lamina 6. Rhombencephalon Binding was generally low or absent in the tegmentum. Low levels of binding density were observed in the electrosensory lateral line lobe. Cerebellum Extremely high levels of binding were found in the eminentia granularis medialis and the eminentia granularis posterior. Throughout most regions of the brain, the relative density of binding sites and the relative amount of somatostatin immunoreactivity in fibres, as determined in previous studies, were in good agreement.

Somatostatin-immunoreactivity in the hippocampus of mouse, rat, guinea pig, and rabbit

The hippocampi of species commonly used for in vitro physiologic studies were examined to determine if there were species-specific and regional differences in somatostatin immunoreactivity. The distributions of somatostatin-immunoreactive somata and fiber plexuses were determined, and the concentration of somatostatin along the septotemporal axis of the hippocampus was measured using a radioimmunoassay. There are many similarities in the patterns of somatostatin immunoreactivity in the hippocampi of mice, rats, guinea pigs, and rabbits. All species had a relatively even distribution of somatostatin-positive perikarya across three fields of the hippocampus (dentate gyrus, CA3, and CA1-2), a similar distribution of somatostatin-immunoreactive perikarya across the strata of the CA1-2 field and the dentate gyrus; and more somatostatin-positive cells in temporal than in septal hippocampus. However, there are species-specific differences in the distribution of somatostatin-immunoreactive perikarya across the strata of CA3. In addition, unlike the other species examined, mice appeared not to have a somatostatin-immunoreactive fiber plexus in the molecular layer of the dentate gyrus. The functional significance of these differences remains to be determined.

Actions of a long-acting somatostatin analog SMS201-995 (sandostatin) on rat locus coeruleus neurons

The actions of sandostatin involving rat locus coeruleus (LC) neurons were examined using an intracellular recording in a brain slice preparation. Bath application of sandostatin (5-100nM) reversibly decreased the firing rate of all neurons tested in a dose-dependent manner. Sandostatin was 9.5 times more potent than somatostatin in light of an inhibition of the spontaneous firing rate. In addition to the inhibition of spontaneous firing, larger concentrations of sandostatin (30-100nM) also hyperpolarized the neurons of the locus coeruleus and simultaneously caused a reduction in input resistance. At the highest concentration (100nM) applied, sandostatin produced complete inhibition of firing of all neurons tested (n = 49); the inhibition was associated with a 11.6mV hyperpolarization (range 2.2-26.4 mV, n = 49) and a 21.8% reduction in input resistance (range 2.6-55.2%, n = 39). The voltage-current relationship of the resting cell revealed an inward-going rectification that became enhanced after the perfusion of sandostatin (100nM) for 5 min. The reversal potential for the sandostatin-induced hyperpolarization was -111 +/- 1 mV (n = 10), which is approximately the potassium equilibrium potential. This hyperpolarization was blocked by both caesium chloride and barium chloride. Our results also showed that sandostatin displayed an antagonistic action on mu opiate receptors. Both the somatostatin-agonistic and opiate-antagonistic activities of sandostatin were conclusively found to be stronger than those of somatostatin; in addition, the inhibitory actions of sandostatin on LC neurons were due to an opening of the inward-going rectification potassium channels.

Cysteamine potentiates entorhinal activation of dentate gyrus granule cells in rats

A dense plexus of somatostatin-positive fibers and varicosities is observed in the outer two-thirds of the dentate gyrus molecular layer where the glutamatergic perforant path afferents from the entorhinal cortex terminate. To test for a functional interaction between these two pathways, we examined the effects of cysteamine, which enhances somatostatin release for a few hours after administration but produces subsequent depletion of somatostatin lasting several days, on perforant path evoked potentials recorded in the dentate gyrus. Cysteamine (50-400 mg/kg, IP) increased the population spike dose-dependently both in anesthetized and in awake rats, but the slope of the population excitatory postsynaptic potential (EPSP) was left unchanged or even decreased. The antidromic population spike evoked by mossy fiber stimulation was not changed by cysteamine. The change is thought to be due to the increase in slope of the EPSP-spike relationship. In the hippocampal slice preparation, a similar effect of the drug (1-5 mM) on dentate evoked potentials was observed, suggesting that cysteamine acts through its effects on somatostatin in the hippocampus itself. In chronically implanted awake animals, the perforant path population spike was increased 1 h after cysteamine but returned to the predrug level by 24 h when somatostatin seemed to be depleted. These results suggest that hippocampal somatostatin released by cysteamine potentiates the response of dentate granule cells to perforant path input, without directly affecting synaptic transmission or general cell excitability.

Role of somatostatin in the augmentation of hippocampal long-term potentiation by FR121196, a putative cognitive enhancer

N-(4-Acetyl-1-piperazinyl)-4-fluorobenzenesulfonamide (FR121196), a newly introduced putative cognitive enhancer of a derivative of piperazine, was investigated for its effects on long-term potentiation in guinea-pig hippocampal slices. The magnitude of long-term potentiation of population spikes recorded in CA3 pyramidal neurons was significantly augmented by perfusing FR121196 (10(-9)-10(-6) M) for 25 min before and during tetanic stimulation of the mossy fibers; the basal amplitude of population spikes before tetanus was hardly affected by the drug. The dose-response curve was bell-shaped with a maximal augmentation at 10(-7) M. Similar activity and bell-shaped dose-response curve were observed with methamphetamine (10(-8)-10(-6) M). Physostigmine (10(-8)-10(-6) M) also facilitated long-term potentiation of this pathway and the magnitude of augmentation was concentration-dependent. Scopolamine (10(-6) M) per se had little effect on the magnitude of long-term potentiation in the mossy fiber-CA3 pathway, but significantly attenuated its enhancement by FR121196 (10(-7) M) and physostigmine (10(-6) M), although it failed to influence that by methamphetamine (10(-7) M). In hippocampal slices from animals treated with cysteamine, which was shown to deplete hippocampal somatostatin, FR121196 (10(-7) M) hardly affected long-term potentiation generation, whereas physostigmine (10(-6) M) and methamphetamine (10(-7) M) augmented it significantly. These results suggest that FR121196 enhances the development of long-term potentiation in the mossy fiber-CA3 pathway through activation of somatostatinergic neurons in the hippocampal formation.

Facilitation of 2-deoxyglucose uptake in rat cortex and hippocampus slices by somatostatin is independent of cholinergic activity

2-Deoxyglucose (2-DG) uptake is an index of regional glucose utilization which reflects predominantly activity in the axonal terminal of neuronal pathways. The present experiments showed that somatostatin elevated 2-DG uptake in rat cortex and hippocampus slices. Treatment with somatostatin-14 and somatostatin-28 markedly enhanced 2-DG uptake, whereas the amino-terminal fragment of somatostatin-28 did so only slightly. This effect appeared to be mediated by an interaction with somatostatin receptors because cyclo-somatostatin, a somatostatin antagonist, abolished the effect of somatostatin-14. The increase in 2-DG uptake caused by somatostatin-14 was blocked by the calcium channel antagonist, nifedipine, but not by tetrodotoxin, suggesting that the action of somatostatin does not require the initiation of impulse activity, somatostatin enhanced the KCl-induced release of acetylcholine, suggesting that a cholinergic mechanism is involved in the somatostatin-induced cellular responses. We therefore examined whether acetylcholine receptor antagonists block the somatostatin-induced increase in 2-DG uptake. Neither muscarinic nor nicotinic receptor antagonists affected the somatostatin-14-induced response. The present results suggest that somatostatin has a stimulatory effect on energy metabolism and that this effect is independent of acetylcholine receptor mechanism.

Neurons in the ventral subiculum, amygdala and entorhinal cortex which project to the nucleus accumbens: their input from somatostatin-immunoreactive boutons

Neurons in the hippocampus, amygdala and entorhinal cortex which project to the nucleus accumbens were labelled retrogradely following injection of horseradish peroxidase. The injections were targetted on the medial part of the nucleus accumbens, but some injection sites included the whole nucleus. Projection neurons in all three areas were found to be spiny, and from the entorhinal cortex and ventral subiculum of the hippocampus they were pyramidal neurons. Somatostatin (S28(1-12)-immunoreactive neurons were found in all parts of the three limbic areas examined. They were found to have various morphologies, but in the electron microscope all had the ultrastructural characteristics of interneurons. In the hippocampus the stratum lacunosum was found to contain the most immunoreactive fibres while most cells lay in the stratum oriens. In the amygdala the densest staining for both cells and fibres was in the central nucleus. In the entorhinal cortex somatostatin-immunoreactive fibres and cells seemed to have no preferential distribution. Examination of somatostatin-immunoreactive profiles in the electron microscope revealed that the majority of synaptic contacts were made with dendrites, many of which were spine-bearing. In the light microscope somatostatin-immunoreactive fibres could be seen to lie near the somata and proximal dendrites of neurons that projected to the nucleus accumbens. In the electron microscope it was found that somatostatin-immunoreactive boutons were in symmetrical synaptic contact with the somata and proximal dendrites of neurons in the ventral subiculum, entorhinal cortex and amygdala which project to the nucleus accumbens.

Somatostatin binding reduced by ammonium acetate in the rat hippocampus can be reversed by treatment with N-carbamyl-L-glutamate plus L-arginine

The effects of short-term (90 min), mid-term (5 days), and long-term (15 days) administration of ammonium acetate (5 mmol/Kg day i.p.) on the somatostatinergic neurotransmitter system of the rat hippocampus have been studied. Scatchard analysis of the binding of 125I-Tyr11-somatostatin to hippocampal dissociated cells indicated that administration of ammonium acetate at the times studied were associated with a decrease in the number of somatostatin receptors in this brain area, whereas the affinity of the same receptors remained unchanged. Administration of ammonium acetate did not affect the levels of somatostatin-like immunoreactivity in the hippocampus. Treatment with N-carbamyl-L-glutamate (1 mmol/Kg, i.p.) plus L-arginine (1 mmol/kg), which lead to the conversion of ammonia into urea, prevented the ammonium acetate-induced changes in somatostatin binding in this brain area. N-carbamyl-L-glutamate plus L-arginine alone had no observable effect on the somatostatinergic system. The decrease in the number of somatostatin receptors induced by ammonium acetate might reflect a decreased sensitivity of the target cells to somatostatin, a phenomenon that could contribute to the depressed neuronal excitability induced by ammonia in the rat hippocampus.

Changes in gamma-aminobutyric acid and somatostatin in epileptic cortex associated with low-grade gliomas

✓ The role of specific neuronal populations in epileptic foci was studied by comparing epileptic and nonepileptic cortex removed from patients with low-grade gliomas. Epileptic and nearby (within 1 to 2 cm) nonepileptic temporal lobe neocortex was identified using electrocorticography. Cortical specimens taken from four patients identified as epileptic and nonepileptic were all void of tumor infiltration. Somatostatin- and γ-aminobutyric acid (GABAergic)-immunoreactive neurons were identified and counted. Although there was no significant difference in the overall cell count, the authors found a significant decrease in both somatostatin- and GABAergic-immunoreactive neurons (74% and 51 %, respectively) in the epileptic cortex compared to that in nonepileptic cortex from the same patient. It is suggested that these findings demonstrate changes in neuronal subpopulations that may account for the onset and propagation of epileptiform activity in patients with low-grade gliomas.

A facilitatory role of endogenous somatostatin in long-term potentiation of the mossy fiber-CA3 system in guinea-pig hippocampus

To estimate the functional role of endogenous somatostatin in the production of long-term potentiation (LTP) in mossy fiber-CA3 system, an influence of depletion of somatostatin on the magnitude of it was examined in guinea-pig hippocampal slices. Administration of cysteamine (200 mg/kg, s.c.), a depletor of somatostatin, to guinea-pigs 13 h prior to preparing slices resulted in a significant decrease in the magnitude of LTP of population spikes in mossy fiber-CA3 system, being associated with a significant depletion of the content of somatostatin in the hippocampus. Furthermore, bath-applied somatostatin (1-14) at a concentration, at which the substance did not influence LTP in slices prepared from saline-treated animals, significantly augmented LTP in slices from cysteamine-treated animals. Cyclo-somatostatin (0.32 and 3.2 microM), a putative antagonist of somatostatin receptors, failed to affect the magnitude of LTP of mossy fiber-CA3 system when applied alone; however, the combined application of cyclo-somatostatin with somatostatin (0.32 microM) significantly inhibited the augmenting action of somatostatin on LTP. From these observations, it is suggested that endogenous somatostatin plays a facilitatory role in the production of LTP in mossy fiber-CA3 system in guinea-pig hippocampus.

Somatostatin augments long-term potentiation of the mossy fiber-CA3 system in guinea-pig hippocampal slices

The effect of exogenously applied somatostatin (1-14), which is one of the candidates of neuromodulators in the hippocampus, on long-term potentiation (LTP) was investigated in the CA1 and CA3 subfields of guinea-pig hippocampal slices. In the mossy fiber-CA3 pyramidal cell system, the magnitude of LTP of both population excitatory postsynaptic potential (pEPSP) and population spike was significantly augmented by somatostatin (10(-7)-10(-6) M) perfused before and during tetanic stimulation which never affected basal amplitude of population spikes before tetanus. The enhancement of LTP by somatostatin lasted for at least one hour after washout. On the other hand, somatostatin at the most effective concentration (3.2 x 10(-7) M) in the above described system failed to affect the magnitude of the LTP of population spikes in Schaffer collateral-CA1 pathway. The enhancing effect of somatostatin on LTP in the mossy fiber CA3 system was inhibited either by a muscarinic antagonist, scopolamine (10(-6) M), or a beta-adrenoceptor antagonist, timolol (10(-6) M). These results suggest that somatostatin enhances the production of LTP in the mossy fiber-CA3 pathway of the guinea-pig hippocampus through the intervention of cholinergic and noradrenergic neurons.

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.

Somatostatin inhibits nicotinic cholinoceptor mediated-excitation in rat ambigual motoneurons in vitro

The interaction between somatostatin and acetylcholine, two putative transmitters in the nucleus ambiguus, was investigated on single ambigual neurons in a brainstem slice preparation. Somatostatin reversibly inhibited the nicotinic cholinoceptor-mediated depolarization and inward current induced by acetylcholine. This inhibition persisted in the presence of tetrodotoxin (TTX) or Mn2+. In contrast, somatostatin enhanced both the glutamate-evoked depolarization and spiking discharges generated by current injection. These results suggest that somatostatin exerts a differential action in modulating excitatory inputs to the nucleus ambiguus at the level of postsynaptic receptors.

Evidence that somatostatin enhances endogenous acetylcholine release in the rat hippocampus

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

Somatostatin immunohistochemistry of hippocampal slices with lucifer yellow-stained pyramidal neurons responding to somatostatin

We have combined electrophysiology and immunohistochemistry to study the somatostatin (SS) innervation of neurons in the rat hippocampal slice. After recording the intracellular response of a pyramidal CA1 neuron in vitro to SS, Lucifer Yellow was injected into the cell and the slice fixed and processed for immunohistochemical localization of SS in the vicinity of the recorded neuron. Most pyramidal neurons (70%) responded to SS with a hyperpolarization associated with marked slowing of spontaneous discharge and reduced input resistance. SS-containing elements either crossed, ran parallel or seemingly terminated on the Lucifer Yellow-filled SS-responsive cell. These occurrences of close proximity of apparent pre- and postsynaptic elements were observed in all layers of the CA1 region and may represent synaptic terminations of SS elements on a pyramidal neuron that are likely to elicit membrane hyperpolarizations.

Behavioral and electrophysiological effects of crustacean neurohormone on freely moving cats

The behavior of freely moving cats was assessed in an observation chamber during prolonged periods of time. Four patterns of behavior were consistently scored during the mid-day period: a) exploration, b) attention, c) grooming and d) drowsiness. Intracerebroventricular injections of crustacean neurodepressing hormone (NDH) greatly extended the time spent in drowsiness. The threshold dose of NDH for this effect was 300 units. The effect was established a few minutes after the injections and lasted for several hours. During this time the animals sat quietly and showed complete or semicomplete closure of the eyelids. Conspicuous changes in brain electrical activity were also observed under NDH. At low doses, the predominant electrophysiological pattern matches the activity recorded under spontaneous lapses of drowsiness, i.e., spindle bursts in trains of 8-16 Hz in cortical areas and mesencephalic reticular formation. At higher doses, the brain electrical activity changes into a nonconvulsive spiking activity in limbic areas. The time course of the effects differs in the various structures recorded. These results suggest a multiple substrate of NDH activity.

Actions of Somatostatin on Rat Neocortical Neurons in vitro

The effects of somatostatin (SS14) on neocortical neurons of the rat were investigated in an in vitro slice preparation. Intracellular recordings were performed in neurons (n=30) in layers 2 and 3 of the frontal cortex. Iontophoretically applied SS14 reduced the responses evoked by iontophoretically applied L-glutamate (GLU) and gamma-aminobutyric acid (GABA). The blocking effect of SS14 was apparent 1 - 2 min after onset of SS14 application and recovery required 2 - 3 min. The conductance increase evoked by GLU or GABA was reduced by SS14. In the majority of neurons, SS14 did not produce any measurable changes in passive membrane properties, spike threshold or on orthodromically evoked synaptic potentials. In 5 cells, SS14 induced a slight hyperpolarization (<3 mV). These results lend further support to claims that SS14 plays a neuromodulatory role in the neocortex.

Selective depression of GABA-mediated IPSPs by somatostatin in area CA1 of rabbit hippocampal slices

In area CA1 of hippocampus, a subpopulation of gamma-aminobutyric acid (GABA)-containing interneurons that make synaptic contacts on pyramidal cells also contains the neuropeptide, somatostatin. The effects of GABA and somatostatin on hippocampal pyramidal cells have been investigated separately, but it is not known whether an interaction occurs between these co-localized substances. We demonstrate that somatostatin has a potent inhibitory effect on GABA-mediated synaptic potentials which hyperpolarize pyramidal cells. This effect may be relevant to the well-documented epileptogenicity of the hippocampus, as well as the phenomenon of long-term potentiation, which is a well-studied example of synaptic plasticity.

Somatostatin blocks a calcium current in rat sympathetic ganglion neurones

1. The effects of somatostatin and somatostatin analogues on a Ca2+ current from acutely isolated and short-term (24-48 h) cultured adult rat superior cervical ganglion (SCG) neurones were studied using the whole-cell variant of the patch-clamp technique. 2. [D-Trp8]Somatostatin (SOM) produced a rapid, reversible and concentration-dependent reduction of the Ca2+ current. Ca2+ current amplitude was reduced over the voltage range -15 to +40 mV with the greatest reduction occurring where the amplitude was maximal (ca +10 mV). In the presence of SOM, the Ca2+ current rising phase was slower and biphasic at potentials between 0 and +40 mV. 3. Application of 0.1 microM-SOM for greater than 10 s resulted in a desensitization of the response. During a 4 min application of 0.1 microM-SOM, Ca2+ current amplitude returned to about 90% of control. A second application of 0.1 microM-SOM produced less block than the initial application. 4. Concentration-response curves for SOM, somatostatin-14 (SOM-14) and somatostatin-28 (SOM-28) were fitted to a single-site binding isotherm. The concentrations producing half-maximal block and the maximal attainable blocks of the Ca2+ current for SOM, SOM-14 and SOM-28 were 3.3, 5.4 and 35 nM, respectively and 55, 51 and 54%, respectively. SOM-14 and SOM-28 slowed the Ca2+ current rising phase in a manner similar to that of SOM. Somatostatin-28 had no effect on the Ca2+ current at 1 microM. 5. The magnitude of the Ca2+ current block produced by 0.1 microM-SOM was not significantly altered in the presence of 1 microM-idazoxan, atropine, naloxone or the somatostatin antagonist aminoheptanoyl-Phe-D-Trp-Lys-O-benzyl-Thr. 6. Internal dialysis with solutions containing 500 microM-guanylyl-imidodiphosphate (Gpp(NH)p) or guanosine-5'-O-(3-thiotriphosphate)(GTP-gamma-S) decreased the Ca2+ current amplitude by 36 and 41%, respectively, and induced a biphasic rising phase in the Ca2+ current. Under these conditions, application of 0.1 microM-SOM produced significantly less block of Ca2+ current amplitude (7.1 and 14.7%, respectively) when compared with controls. 7. Internal dialysis with solutions containing 500 microM-guanosine-5'-O-(2-thiodiphosphate)(GDP-beta-S) had no significant effect on either the Ca2+ current amplitude or block produced by 0.1 microM-SOM. 8. Internal dialysis with solutions containing 500 microM-cyclic adenosine 3',5'-monophosphate (cyclic AMP) and 3-isobutyl-1-methylxanthine had no significant effect on either the Ca2+ current block produced by 0.1 microM-SOM or the Ca2+ current amplitude.(ABSTRACT TRUNCATED AT 400 WORDS)

Further studies of the effects of somatostatin and related peptides in area CA1 of rabbit hippocampus

1. In slice studies of mature and immature CA1 hippocampal pyramidal cells from rabbit, somatostatin 14 (SS14), the related peptide somatostatin 28(1-12) [SS(1-12)], and the synthetic analogue of somatostatin 14, SMS-201995 (SMS), had similar effects. When pressure-ejected onto cell somata, these peptides elicited depolarizations, often accompanied by action potential discharge. When applied to dendrites, the peptides produced depolarizations or hyperpolarizations. 2. When a large amount of one of the three somatostatin-related (SS) peptides was applied to the slice at some distance from the impaled cell, hyperpolarizations were observed that were not always blocked by tetrodotoxin (TTX) or low Ca2+. Since SS peptides were also found to depolarize interneurons in area CA1, it seems likely that the hyperpolarizations that were blocked by TTX or low Ca2+ were mediated via excitation of interneurons that in turn hyperpolarized pyramidal cells. 3. All SS peptides also had long-lasting effects on CA1 pyramidal cells that led to spontaneous firing of action potentials and an increase in the number of action potentials discharged in response to a given depolarizing current pulse; the spontaneous discharge effect was blocked by TTX or low Ca2+ plus Mn2+ and, thus, appeared to have a presynaptic mechanism. However, the increase in discharge in response to a constant depolarizing current pulse was not dependent on intact synaptic transmission and, therefore, was attributable to a direct postsynaptic effect of the SS peptides.

Somatostatin(14) and -(28) but not somatostatin(1-12) hyperpolarize CA1 pyramidal neurons in vitro

The three major prosomatostatin-derived peptides found within CNS neurons are a 28-amino acid peptide (SS28), a cyclic 14 amino acid peptide (SS14) and a 12 amino acid peptide (SS1-12). Immunohistochemical studies demonstrate a differential distribution of these related forms of somatostatin within CNS neurons and have led to the suggestion that SS1-12 may represent the predominant neurotransmitter form of this family of peptides. Intracellular recordings from CA1 pyramidal neurons in the in vitro rat hippocampal slice revealed that application of SS14 and SS28 in nanomolar concentration produced neuronal hyperpolarization; synaptic responses, recorded extracellularly, were also reduced. In contrast, we were unable to demonstrate a pre- or postsynaptic action of SS1-12 on these neurons. These results do not support the hypothesis that SS1-12 functions as a central neurotransmitter in area CA1 of the hippocampus.

Somatostatin depresses excitability in neurons of the solitary tract complex through hyperpolarization and augmentation of IM, a non-inactivating voltage-dependent outward current blocked by muscarinic agonists

The synaptic function of somatostatin-containing fibers in the nervous system is controversial. Therefore, we used a slice preparation of the rat brain stem to test the electrophysiological effects of prosomatostatin-derived peptides on neurons of the solitary tract complex, which contains an abundance of somatostatin-containing fibers and cell bodies. Superfusion of both somatostatin-14 and somatostatin-28 (the precursor for somatostatin-14), but not somatostatin-28-(1-12) or -(1-10), predominantly inhibited spontaneous spike and subthreshold (probably synaptic) activity. In intracellular recordings, somatostatin-14 and -28 hyperpolarized most neurons in association with a slight (10-35%) but reproducible decrease in input resistance. These hyperpolarizing responses were augmented in depolarized cells and persisted in cells in which spontaneous inhibitory postsynaptic potentials became depolarizing after Cl- injection. These data suggest that somatostatin receptors regulate a K+ conductance. In voltage-clamp studies, somatostatin-28 and -14 induced a steady outward current and augmented the voltage-dependent, nonactivating outward K+ conductance (IM) shown to be blocked by activation of muscarinic cholinergic receptors. These results suggest (i) that somatostatin-containing elements in the solitary tract complex play an inhibitory role through the activation of postsynaptic permeability to potassium ions and (ii) that the same ion channel type may be coregulated by two neurotransmitter candidates, somatostatin and acetylcholine, through a reciprocal control mechanism.

Somatostatin augments the M-current in hippocampal neurons

Immunocytochemical and electrophysiological evidence suggests that somatostatin may be a transmitter in the hippocampus. To characterize the ionic mechanisms underlying somatostatin effects, voltage-clamp and current-clamp studies on single CA1 pyramidal neurons in the hippocampal slice preparation were performed. Both somatostatin-28 and somatostatin-14 elicited a steady outward current and selectively augmented the noninactivating, voltage-dependent outward potassium current known as the M-current. Since the muscarinic cholinergic agonists carbachol and muscarine antagonized this current, these results suggest a reciprocal regulation of the M-current by somatostatin and acetylcholine.

Ultrastructural characterization and GAD co-localization of somatostatin-like immunoreactive neurons in CA1 of rabbit hippocampus

Immunocytochemical techniques have been used to identify a striking interneuronal population which is immunoreactive for the peptide, somatostatin. The cell population, which is seen most densely in stratum oriens and at the oriens/alveus border of the CA1 region of rabbit hippocampus, was characterized in light and electron microscopic observations. The cells have dendrites which extend parallel to and into the alveus, with occasional processes ascending through stratum pyramidale toward the hippocampal fissure. The dendrites receive numerous synaptic contacts directly onto aspinous dendritic shafts. Axon collaterals ramify profusely within the pyramidale region, and among the proximal apical and basal pyramidal cell dendrites in areas of stratum radiatum and stratum oriens. Somatostatin-like immunoreactive terminals make synaptic contact, primarily of the symmetric type, with the somata and proximal dendrites of pyramidal neurons. Somatostatin-like neurons are found at approximately equal density in the hippocampus of immature (8 days postnatal) and mature (30 days postnatal) rabbit. Double-labelling techniques, to identify both somatostatin-like and glutamic acid decarboxylase (GAD) immunoreactive neurons, demonstrated that a large proportion of the somatostatin neurons were also GABAergic.

Somatostatin blocks a calcium current in acutely isolated adult rat superior cervical ganglion neurons

Somatostatin-like immunoreactivity has been reported to occur in the postganglionic neurons of sympathetic ganglia. We therefore investigated the effect of somatostatin (SOM) on the Ca2+ current in sympathetic neurons. Voltage-clamp recordings, using the whole-cell patch-clamp technique, were made from acutely isolated adult rat superior cervical ganglion (SCG) neurons in solutions (external and internal) designed to isolate Ca2+ currents. Application of 0.001-1.0 microM [D-Trp8]SOM resulted in a rapid, reversible and concentration-dependent decrease in the amplitude of the Ca2+ current evoked from a holding potential of -80 mV. The concentration-response relationship for SOM could be fitted to a single-site binding model with an apparent dissociation constant of 11 nM; the maximal attainable block of Ca2+ current by SOM was 50%. SOM also produced a pronounced slowing of the Ca2+ current rising phase, especially at more depolarized potentials. At higher concentrations (0.03-1.0 microM), prolonged application of SOM resulted in a progressive decrease in blocking ability. The results are consistent with a neurotransmitter and/or neuromodulator role for SOM in the sympathetic nervous system.