Patients with Alzheimer's disease show an increased content of 15 Kdalton somatostatin precursor and a lowered level of tetradecapeptide in their cerebrospinal fluid

Immunohistochemical expression and colocalization of somatostatin, carboxypeptidase-E and prohormone convertases 1 and 2 in rat brain

The processing of many peptides for their maturation in target tissue depends upon the presence of sorting receptor. Several previous studies have predicted that carboxypeptidase-E (CPE), prohormone convertase 1 (PC1) and prohormone convertase 2 (PC2) may function as sorting elements for somatostatin (SST) for its maturation and processing to appropriate targets. However, nothing is currently known about whether brain, neuronal culture or even endocrine cells express SST, CPE, PC1 and PC2 and exhibit colocalization. Accordingly, in the present study using peroxidase immunohistochemistry, double-labeled indirect immunofluorescence immunohistochemistry and Western blot analysis, we mapped the distributional pattern of SST, CPE, PC1 and PC2 in different rat brain regions. Additionally, we also determined the colocalization of SST with CPE, PC1 and PC2 as well as colocalization of CPE with PC1 and PC2. The localization of SST, CPE, PC1 and PC2 reveals a distinct and region specific distribution pattern in the rat brain. Using an indirect double-label immunofluorescence method we observed selective neuron specific colocalization in a region specific manner in cortex, striatum and hippocampus. These studies provide the first evidence for colocalization between SST, CPE, PC1 and PC2 as well as CPE with PC1 and PC2. SST in cerebral cortex colocalized in pyramidal and non-pyramidal neurons with CPE, PC1 and PC2. Most importantly, in striatum and hippocampus colocalization was mostly observed selectively and preferentially in interneurons. CPE is also colocalized with PC1 and PC2 in a region specific manner. The data presented here provide a new insight into the distribution and colocalization of SST, CPE, PC1 and PC2 in rat brain. Taken together, our data anticipate the possibility that CPE, PC1 and PC2 might be potential target for the maturation of SST.

Low cerebrospinal fluid concentrations of peptide histidine valine and somatostatin-28 in Alzheimer's disease: altered processing of prepro-vasoactive intestinal peptide and prepro-somatostatin

Recent studies have indicated that deposition of beta amyloid peptide in the brains of patients with senile dementia of the Alzheimer type (SDAT) is a consequence of abnormal processing of the beta amyloid protein precursor. In addition, reduced concentrations of various peptides have been measured in post-mortem brain tissue and cerebrospinal fluid (CSF) of patients with SDAT. We determined concentrations of the peptides derived from prepro-vasoactive intestinal peptide (VIP)--peptide histidine methionine-27 (PHM-27), peptide histidine valine (PHV) and VIP--and peptides derived from prepro-somatostatin (prepro-SS), SS-14 and SS-28, in CSF of patients with SDAT by radioimmunoassay combined with high performance liquid chromatography. We found significantly reduced levels of total PHM-immunoreactivity (IR) and PHV, and unaltered levels of PHM-27 and VIP in SDAT, compared with those in controls. Total SS-IR and SS-28 concentrations were significantly reduced in SDAT, while SS-14 levels did not differ from those of controls. These results suggest that an altered processing of the prepro-peptides of VIP and SS may occur in SDAT and that these alterations might have a significant role in the pathogenesis of SDAT.

A long-term follow-up study of cerebrospinal fluid somatostatin in delirium

Cerebrospinal fluid somatostatin-like immunoreactivity (CSF SLI) was determined for elderly delirious patients during the acute stage and after 1- and 4-year follow-up periods, and the SLI levels were compared with age-equivalent controls. As a whole group, and also when the group was subdivided according to the severity of cognitive decline at the acute stage, type of delirium or the central nervous system disease, delirious patients showed significant reduction of SLI as compared with the controls. In the follow-up, we observed a further reduction of CSF SLI together with significant correlations in the second, third and fourth samples between SLI levels and Mini-Mental State Examination scores. Our results suggest a role for somatostatinergic dysfunction in the genesis of some symptoms of delirium, and this dysfunction may be linked to the long-term prognosis of delirious patients.

Somatostatin in Alzheimer's disease and depression

Somatostatin (somatotropin release-inhibiting factor, SRIF) was originally discovered (1) during the purification of growth hormone-releasing factor from rat hypothalamus and was subsequently isolated and characterized (2) in 1972 from ovine hypothalamus. Since its initial characterization, SRIF has been shown to fulfill criteria for a neurotransmitter and to directly modulate neuronal activity as well as acting as an inhibitory factor regulating endocrine and exocrine secretion. Alterations in cerebrospinal fluid (CSF) concentrations of SRIF have been reported in several diseases exhibiting prominent cognitive dysfunction, including Alzheimer's disease (AD), major depression, Huntington's chorea, multiple sclerosis, schizophrenia and Parkinson's disease, while evidence for regional brain tissue concentration deficits in SRIF are more specific for AD. This mini-review will focus on the studies reporting alterations in CSF and postmortem tissue concentrations of SRIF in AD and depression.

Somatostatin inhibition of adenylate cyclase activity in different brain areas

Somatostatin receptors have been identified in different brain areas but the characterization of their postreceptor effect is still lacking. In this study we analyze the somatostatin effect on adenylate cyclase activity in different brain regions, namely frontal cortex, striatum, hypothalamus and hippocampus. Somatostatin inhibited basal adenylate cyclase activity in all brain areas, being maximally effective in the frontal cortex (-42%). Moreover, somatostatin inhibited both dopamine- and norepinephrine-stimulated adenylate cyclase activity in the examined cerebral regions showing a higher effectiveness than in basal conditions. VIP stimulation of adenylate cyclase was also reduced by somatostatin. The peptide inhibited by 50% forskolin-stimulated (10 nM to 10 microM) enzyme activity in frontal cortex and hypothalamus (in hippocampus the inhibition reached only -25%) showing a non-competitive pattern of inhibition. Finally, pertussis toxin pretreatment abolished the somatostatin inhibition of forskolin-stimulated frontal cortex adenylate cyclase activity. These results show that brain somatostatin receptors are coupled in an inhibitory way with adenylate cyclase enzyme that may represent one of the postreceptor mechanisms mediating the somatostatin modulation of brain functions.

Subpopulations of somatostatin 28-immunoreactive neurons display different vulnerability in senile dementia of the Alzheimer type

We tested whether the vulnerability of somatostatin (SST) neurons in senile dementia of the Alzheimer type (SDAT) depended upon their co-localization with neuropeptide Y (NPY). Density estimates of SST28- and NPY-immunoreactive neurons and percentage of double-labeled SST-NPY neurons were obtained in the cortex (areas 9 and 25) and the bed nucleus of stria terminalis (BST), in 6 SDAT and 5 control cases. Counts of senile plaques (SP) and neurofibrillary tangles (NFT) were done on thioflavin S stains. In both cortical areas, a decrease in the density of SST28-IR neurons was found in SDAT cases (-60% in area 25 and -80% in area 9), whereas density of NPY-IR neurons was unchanged. Accordingly, the proportion of single-labeled SST neurons decreased; this decrease was significantly correlated with SP (r = -0.89, P less than 0.001). We conclude that single SST-IR neurons, in cortical layers II-III, and V, are preferentially lost relative to co-localized SST-NPY neurons. In the BST, no significant reduction of SST-IR, NPY-IR neurons nor of the percentage of single labeled SST neurons was found, despite the presence of SP. Thus one subpopulation of SST neurons, defined by associated neurochemical characters (not co-localized with NPY nor with NADPH diaphorase) and by topography (cortical layers III and V) appears to be particularly vulnerable in SDAT. The potential importance of their position in neural circuitry is emphasized.

Brain and CSF somatostatin concentrations in patients with psychiatric or neurological illness. An overview

Somatostatin was originally isolated as a 14-amino-acid peptide from the ovine hypothalamus. The peptide has a widespread regional distribution within the central and peripheral nervous systems, as well as in peripheral organs. Preservation of the chemical structure over a wide range of vertebral species indicates important functional roles of the peptide. Recent results about the role of somatostatin and related peptides in different psychiatric (depression, schizophrenia, Alzheimer's disease) and neurological (Huntington's disease, multiple sclerosis, Parkinson's disease) diseases, and the effects on the hypothalamic-pituitary-adrenal axis are summarized. Also, the influence of some psychotropic drugs (halo-peridol, carbamazepine) on somatostatin levels in cerebrospinal fluid is discussed.

Somatostatin and SMS 201-995 reverse the impairment of cognitive functions induced by cysteamine depletion of brain somatostatin

The involvement of somatostatin in the organization of cognitive functions was studied. We assessed changes in learning and memory processes by studying the effects of cysteamine, a compound that decreases somatostatin-like immunoreactivity in the brain, somatostatin and the potent somatostatin analogue, SMS 201-995, on active avoidance behaviour, assessed with a shuttle box apparatus, or on passive avoidance behaviour. Cysteamine induced a loss of the conditioned active avoidance response acquired after 3 weeks of daily trials. The effect was observed 2 h (-29%) and 4 h (-51%) after cysteamine treatment (300 mg/kg s.c.) and disappeared after 24 h. Intracerebroventricular administration of somatostatin or SMS 201-995 to cysteamine-treated rats significantly reversed the cysteamine effects on the conditioned avoidance responses. Similar results were obtained on passive avoidance behaviour. We also investigated the effect of cysteamine treatment on brain somatostatin-sensitive adenylate cyclase. We observed that adenylate cyclase activity in the frontal cortex of cysteamine-pretreated animals was more sensitive to inhibition by the SRIF analogue, SMS 201-995, than it was in control animals. This effect was observed at concentrations of SMS 201-995 that were ineffective in control tissue. These results show that disruption of somatostatinergic transmission affects cognitive functions of rats.