Mapping of somatostatin receptor localization in rat brain: forebrain and diencephalon

Cortistatin radioligand binding in wild-type and somatostatin receptor-deficient mouse brain

Cortistatin-14 (CST-14) is a recently discovered member of the somatostatin family of neuropeptides. It shares 11 of its 14 amino acids with somatostatin-14 (SRIF-14). In the present study, binding sites for cortistatin-14 in the mouse brain were examined and compared to those for somatostatin using iodinated cortistatin-14 and iodinated somatostatin-14. By in vitro receptor autoradiography, high densities of cortistatin-14 and somatostatin-14 specific binding sites were detected in the cortex, hippocampal formation, basolateral amygdala and medial habenula. Unlabeled 100 nM cortistatin-14 inhibited iodinated somatostatin-14 binding in the hippocampus, but not in the cortex or amygdaloid nuclei. In somatostatin receptor subtype-2 knock-out (KO) mice, autoradiographic iodinated somatostatin-14 binding was observed in the hippocampus and habenula but was removed in the cortex and amygdaloid nuclei, specific iodinated cortistatin-14 binding sites were found in the hippocampus, habenula and throughout the cortex. We conclude that the somatostatin receptor subtype-2 is responsible for somatostatin binding in cortical and amygdaloid regions and that cortistatin predominantly interacts with the same receptors as somatostatin.

The somatostatin receptors in the normal and epileptic hippocampus of the gerbil: subtype-specific localization and its alteration

We investigated the distribution of somatostatin receptors (SSTs) in the hippocampi of SR (seizure-resistant) and SS (seizure-sensitive) gerbils in order to characterize the alterations in SST expressions induced by seizure activity. SST2A immunodensity in the hippocampus of SS gerbils was lower than that of SR gerbils, though its localization in the hippocampus was similar in both SR and SS gerbils. SST3 immunodensity in the hippocampus of SS gerbils was lower than in SR gerbils. In SR gerbils, strong SST4 immunoreactivity was detected in the dentate gyrus and in the CA3 region, in contrast little immunoreactivity was detected in these regions in SS gerbils. In SR and SS gerbils, the strong SST5 immunoreactivity in the hippocampus was also detected in the stratum oriens of the CA2-3 regions and the septal area of CA1 region. However, SST5 immunodensity in the stratum radiatum in SS gerbils was lower than in SR gerbils. These results are the first comprehensive description of the distribution of SSTs in the normal and epileptic hippocampus of gerbils, and suggest that these alterations in the hippocampus of the SS gerbil may be related with a regulatory mechanism for seizure activity in these seizure prone animals.

Immunohistochemical localization of the somatostatin sst2(b) receptor splice variant in the rat central nervous system

Somatostatin is a neuromodulator in the mammalian CNS. To date, genes for at least five different somatotrophin release inhibiting factor receptors, termed sst1-sst5, have been cloned. The rat sst2 receptor exists in two splice variants, sst(alpha)a) and sst2(b), which differ in their carboxy-termini. When heterologously expressed in Chinese hamster ovary-K1 cells, these splice variants show little difference in their operational characteristics. Recently, the distribution of the sst2(a) receptor was documented, yet at present no data are available about the distribution of the sst2(b) receptor in the CNS. Here, we present the characterization of a novel polyclonal anti-peptide antibody that is selective for the sst2(b) receptor splice variant. The antibody was raised against the unique intracellular carboxy-terminal portion of the receptor protein. Using this affinity-purified antibody in western blotting experiments, the sst2(b) receptor expressed in Chinese hamster ovary-K1 cells was shown to be a glycoprotein with a molecular weight centred at about 85,000. The antibody showed no cross-reactivity to any of the recombinant human sst1-5 receptors, the rat sst2(a) receptor or wild-type Chinese hamster ovary-K1 cells. Employing immunohistochemistry, we investigated the distribution of the sst2(b) receptor in the brain and spinal cord of adult rats. A distinct distribution was found throughout the rostrocaudal axis of the CNS. Somatodendritic as well as axonal staining was observed. Somatodendritic labelling was particularly obvious in the olfactory bulb, cerebral cortex, hippocampal formation, mesencephalic trigeminal nucleus and cerebellum, as well as in cranial and spinal motor areas. The results show that the distribution of the sst2(b) receptor partially overlaps with that of the sst2(b) receptor, although there were differences in a number of brain areas. The location of the sst2(b) receptor implies that it may mediate a modulatory role of somatostatin inhibitory releasing factor on sensory as well as motor functions.

Outward current produced by somatostatin (SRIF) in rat anterior cingulate pyramidal cells in vitro

1. A high density of receptors for somatostatin (SRIF) exists in the anterior cingulate cortex but their function is unknown. Whole-cell patch clamp recordings were made from visualized deep layer pyramidal cells of the rat anterior cingulate cortex contained in isolated brain slices to investigate the putative effects of SRIF and to identify the receptor subtype(s) involved. 2. SRIF (1-1000 nM) produced a concentration-dependent outward current which was associated with an increased membrane conductance, was sensitive to Ba2+ (300 microM - 1 mM), and was absent in the presence of a maximal concentration of the GABA(B) receptor agonist, baclofen (100 microM). These observations suggest the outward current was carried by K+ ions. 3. SRIF analogues also elicited outward currents with a rank potency order of (EC50, nM): octreotide (1.8)>BIM-23027 (3.7)>SRIF (20)=L-362,855 (20). BIM-23056 was without agonist or antagonist activity. Responses to L-362,855 were unlike those to the other agonists since they were sustained for the duration of the application. 4. The sst2 receptor antagonist, L-Tyr8Cyanamid 154806 (1 microM), had no effect alone but partially reversed responses to submaximal concentrations of SRIF (100 nM, 44+/-6% reversal) and L-362,855 (100 nM, 70+/-6% reversal) and fully reversed the response to BIM-23027 (10 nM). In contrast, L-Tyr8Cyanamid 154806 did not antagonize the response to baclofen (10 microM). 5. We conclude that SRIF activates a K+ conductance in anterior cingulate pyramidal neurones via an action predominantly at sst2 receptors.

Immunohistochemical localization of the somatostatin SST2(A) receptor in the rat brain and spinal cord

The neuropeptide somatostatin is widely distributed in the CNS and is believed to play a role as a neurotransmitter or a neuromodulator. Somatostatin mediates its actions by the binding of the peptide to high affinity membrane receptors. The genes for five somatostatin receptor types have been cloned recently and Northern blotting and in situ hybridization studies have shown that the transcripts of all five types are expressed in the CNS. Here we report the cellular distribution of somatostatin sst2(a) receptor protein in the adult rat CNS, using a polyclonal anti-peptide antibody directed against a portion of the C-terminal domain of the receptor. The specificity of the affinity-purified antibody was demonstrated by Western blotting and immunolabelling of cells transfected with a hemagglutinin epitope-tagged version of the sst2(a) receptor. Immunohistochemistry showed a distinct distribution of the receptor protein in the rat brain. Cells and processes were labelled in a number of areas, including the basolateral amygdala, the locus coeruleus, the endopiriform nucleus, the deep layers of the cerebral cortex, the subiculum, the claustrum, the habenula, the interpenduncular nucleus, the hippocampus and the central grey. In the spinal cord, the substantia gelatinosa showed strongly-labelled cell bodies and their processes. This study provides an improved understanding of the distribution of the sst2(a) receptor in rat brain.

Functional activation of somatostatin- and neuropeptide Y-containing neurons in the entorhinal cortex of chronically epileptic rats

The in vitro release of somatostatin and neuropeptide Y, their tissue concentration and immunocytochemical pattern were examined in the entorhinal cortex of chronically epileptic rats. A systemic administration of 12 mg/kg kainic acid causing generalized tonic-clonic seizures for at least 3 h after injection was used to induce, 60 days later, a chronically enhanced susceptibility to seizures in the rats. The release of both peptides under depolarizing conditions was significantly reduced by 15% on average from slices of the entorhinal cortex two days after kainic acid-induced status epilepticus. At 60 days, the spontaneous and 30 mM KCl-induced release of somatostatin was significantly enhanced by 30% on average. The release induced by 100 mM KCl was raised by 70%. The spontaneous, 30 mM and 100 mM KCl-induced release of neuropeptide Y from the same slices was increased, respectively, by 120%, 76% and 36%. The late changes were associated with an increased tissue concentration of neuropeptide Y but not of somatostatin. This was confirmed by immunocytochemical evidence showing that neuropeptide Y-, but not somatostatin-immunoreactive neurons were increased in the entorhinal cortex of kainic acid-treated rats. These results indicate that neurotransmission mediated by somatostatin and neuropeptide Y, two peptides previously shown to play a role in limbic epileptogenesis, is enhanced in the entorhinal cortex of chronically epileptic rats.

Somatostatin receptors in the central nervous system

Somatostatin was first identified chemically in 1973, since when much has been established about its synthesis, storage and release. It has important physiological actions, including a tonic inhibitory effect on growth hormone release from the pituitary. It has other central actions which are not well understood but recent cloning studies have identified at least five different types of cell membrane receptor for somatostatin. The identification of their genes has allowed studies on the distribution of the receptor transcripts in the central nervous system where they show distinct patterns of distribution, although there is evidence to indicate that more than one receptor type can co-exist in a single neuronal cell. Receptor selective radioligands and antibodies are being developed to further probe the exact location of the receptor proteins. This will lead to a better understanding of the functional role of these receptors in the brain and the prospect of determining the role, if any, of somatostatin in CNS disorders and the identification of potentially useful medicines.

Relationship of thalamic basal forebrain projection neurons to the peptidergic innervation of the midline thalamus

To better understand the input-output organization of the midline thalamus, we compared the distribution of its peptidergic and monoaminergic afferents, which were visualized by using immunocytochemistry, with the distribution of neurons projecting to different basal forebrain structures, which were mapped using retrograde fluorescent tracers. Serotonin and most of the peptides were found throughout paraventricular thalamic nucleus (PV) and in other midline and intralaminar nuclei (type 1 pattern). Neuropeptide Y, alpha MSH and the catecholamine synthetic enzymes were largely restricted to dorsolateral PV (type 2 pattern). Vasopressin was found in dorsomedial PV and intermediodorsal nucleus in a pattern complementary to the type 2 distribution (type 3 pattern). Neurons projecting to accumbens core were present in paraventricular, intermediodorsal, and other midline nuclei. Neurons projecting to accumbens shell and to central amygdaloid nucleus were found in dorsal PV. The peptidergic zones were only loosely correlated with the distribution of different classes of projection neurons. The type 2 pattern overlapped best with neurons projecting to accumbens shell, and to a lesser extent to central amygdaloid nucleus, while the type 3 pattern overlapped best with neurons projecting to core of accumbens. This partial overlap suggests that some brainstem and hypothalamic nuclei preferentially affect different basal forebrain targets through the midline thalamus, and may allow, for example, information about stress to specifically influence accumbens shell and central amygdaloid nucleus. Nevertheless, most of the peptidergic afferents (type 1 pattern) to midline thalamus cover neurons projecting throughout the basal forebrain, which suggests that all of these neurons receive a variety of brainstem and hypothalamic inputs.

Decrease of somatostatin receptor binding in the rat cerebral cortex after ibotenic acid lesion of the nucleus basalis magnocellularis: a quantitative autoradiographic study

The specific binding of 125I-Tyr11-somatostatin-14 (125I-Tyr11-SS-14) was measured in different cortical regions after unilateral ibotenic acid lesion of the rat nucleus basalis magnocellularis (NBM). A marked loss of acetylcholinesterase-positive fibers was observed in the frontal, parietal, temporal and occipital cortices ipsilateral to the lesion. The loss of cholinergic cell bodies in the NBM was further investigated with choline-acetyltransferase (ChAT) immunohistochemistry which indeed demonstrated a loss of ChAT-positive magnocellular perikarya. Autoradiographic analyses of specific binding of 125I-Tyr11-SS-14 demonstrated a significant reduction in binding density in the denervated parts of the neocortex. The decrease in specific binding was most pronounced (40-50%) in the superficial layers (I-III) of the frontal, parietal and temporal cortices 2 and 4 weeks after lesion. A significant loss in 125I-Tyr11-SS-14 binding in the deeper layers was only observed in the frontal cortex after 2 and 4 weeks. In the occipital cortex a significant decrease was measured in the superficial layers only after 4 weeks. The specific binding in all cortical regions returned to normal after 6 weeks. The results suggested that 125I-Tyr11-SS-14 binding sites are localized on cholinergic afferents in the rat neocortex and that an up-regulation of number of binding sites, alternatively an increased binding affinity occurred with time after lesion.

Somatostatin release in rat neocortex during gamma-hydroxybutyrate-provoked seizures: Microdialysis combined with EEG recording

Gamma-hydroxybutyrate (GHB) was intracortically applied in two doses (first 10 and then 20 mg/ml) to awake Wistar rats using microdialysis. Simultaneously, EEG and the release of somatostatin-like immunoreactivity (SLI) were measured from the frontal cortex. Intracerebrally administered GHB induced cortical epileptogenic spikes, but not high voltage spindles (HVS) as reported after systemic administration, and seizures with myoclonic jerks and contraversive head movements. Compared to the basal level, GHB (10 mg/ml) initially increased the release of SLI (p < 0.05). However, when the frequency of spikes and seizures rose rapidly (p < 0.001), SLI release decreased significantly (p < 0.001). Minimum release of SLI occurred when seizures were most frequent (during perfusion with 20 mg/ml GHB), while after removal of the drug it rose above the basal level (p < 0.05). According to these results, intracortically applied GHB increases the release of SLI in the surrounding tissue. However, further exposure of GHB leads to a manifestation of epileptic spikes and seizures, during which the release of SLI is significantly attenuated. This suggests that release of somatostatin is affected during epileptic phenomena induced also by intracortical GHB application.