In a 1973 study by Brazeau et al a tetradecapeptide was isolated from the ovine hypothalamus which was found to inhibit in vitro secretion of both rat and human growth hormones from cultured pituitary cells  . Though this somewhat serendipitous discovery was of great significance at the time, it was only the beginning of our knowledge of the peptide which would come to be named 'somatostatin' due to its hypophysiotropic action. In studies on rats, it was shown that somatostatin was most abundant in the hypothalamus, however it was also found to be present in many regions of the rat brain, indicative of a variety of possible roles in the mammalian central nervous system  .
In a 1978 paper Schonbrunn et al first described the somatostatin receptor found in the rat pituitary using whole cell binding studies. This was done using a somatostatin preparation which was shown to bind to a 'limited number of high affinity sites on GH4Cl [rat pituitary] cells', resulting in an inhibition of prolactin and growth hormone production  . Initially different forms of the somatostatin receptor were proposed due to the variable binding properties of SST-14 and SST-28 (the two active structures of somatostatin, one with 14 amino acids and one with 28) to the somatostatin receptor, as well as different actions in the central nervous system and in islet cells  . Furthermore, a study by Srikant et al in 1981 found that SST-28 possessed different affinities for receptors in rat pituitary cells and rat brain cells  . It was shown to be the case that more than one somatostatin receptor existed in subsequent studies which identified 2 pharmacological receptor subtypes  .
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A 1992 paper by Yamada et al uncovered the structure of these two receptor subtypes using molecular cloning and described differing affinities for SST-14 and SST-28  , concluding that the two receptors described were probably part of a larger gene family of somatostatin receptors (based on results of cloning and southern blotting studies). It has been shown more recently that there are 5 different subtypes of somatostatin receptor (sst1-5) with the sst2 receptor having 2 isoforms (a and b)  . The genes for these 5 receptors are all coded for by separate genes on different chromosomes and all 5 receptor groups have been successfully cloned  , validating the idea that the somatostatin receptor family is much larger than was originally thought to be the case  .
In this essay I shall describe the physiological role of the somatostatin receptors in the central nervous system and their distribution throughout the brain according to the findings of recent studies. I shall also discuss the intracellular pathways through which activation of the somatostatin receptors by their natural ligand somatostatin and agonists can bring about a cellular response. The pharmacological significance of the receptors as potential targets for therapeutics shall also be described in the course of the essay.
Distribution of the 5 subgroups
Using techniques of mRNA analysis such as northern blots and reverse-transcriptase PCR the regional expression of somatostatin receptor subtypes in rat and human tissues has been described in the literature  12. However, as well as regional distribution within the rat brain, a 1999 paper by Schulz et al sought to locate all 5 of the different somatostatin receptor subtypes in the rat brain at the cellular level, using subtype specific antibodies generated from the cloned receptors in combination with multicolour immunofluorescence studies  . This study served as something of a review of the 5 receptor subgroups, with much of the data was taken from the previous work by the same researchers.
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The sst1 receptor was found to be located in several regions of the rat brain,'with prominent immunostaining in the main olfactory bulb, nucleus accumbens, globus pallidus, ventral pallidum, medial habenula, lateral septum, amygdala, zona incerta, hypothalamus, eminentia mediana, substantia nigraâ€¦'13, however it has only consistently been described in the retina, basal ganglia and mediobasal hypothalamus  . In most of these regions, the sst1 receptor was predominantly found in terminals which were similar in terms of morphology to varicose axons, suggesting a presynaptic role for the sst1 receptor  . Also, a 2006 paper by Thermos et al asserted that the sst1 receptor serves as a presynaptic inhibitory autoreceptor due to its co-localisation with somatostatin in neurones of the retina, basal ganglia and hypothalamus  .
In two 1998 papers by Shulz et al, the splice variants of the sst2 receptor (a and b) were both found to have a wide distribution throughout the rat brain but were found to be expressed by different neuronal populations  -  . Sst2a receptors were found in a dense network in the superficial dorsal horn whereas sst2b receptors were found to prominently stain throughout the spinal grey matter. In preparations which were stained for both sst2 receptor isoforms as well as SST-14, it became apparent that plasmalemma containing sst2 receptors were frequently apposed by nerve terminals releasing sst-14. This suggests post-synaptic activity of the sst2 receptor17-18. It is interesting to note that somatodendritic staining patterns for the sst2a receptor were found in areas with notable somatostatinergic innervation whereas more diffuse staining patterns are found in regions with less innervation  . This was seen as evidence for the idea that somatostatin regulates the distribution of the sst2a receptor, and the finding that there is upregulation of the sst2 receptor in mice who have had the somatostatin gene invalidated  justifies this claim.
The sst3 receptor was found throughout the brain, in the cortex, amygdala, hippocampus and cerebellum. Interestingly, a 1999 study found that sst3-like immunoreactivity was located in rod-shaped structures which were between 4 and 8 microns long. The sst3 immunoreactivity did not co-localise with specific axon or dendrite markers, implying an extrasynaptic role for the sst3 receptor, which upon elecro-microscopic analysis was found to reside on neuronal cilia  . The sst3 receptor is unique in that it is a G-protein coupled receptor which is found neither pre nor post-synaptically but is instead localized to neuronal cilia. In light of these findings, Handel et al elegantly concluded that the presence of the sst3 receptor on neuronal cilia might indicate that these immotile cilia may not be just a remnant of evolution but might in fact serve as a chemical sensor for the neurone's immediate environment21.
A 2000 paper by Schreff et al located the sst4 receptor mainly in forebrain regions such as the cortex, hippocampus, amygdala and striatum  . Electron-microscopy found the sst4 receptor to be exclusively located post-synaptically on dendritic shafts closely apposed by SST-14 containing terminals. These findings suggest that the sst4 receptor most likely functions post-synaptically at the somato-dendritic region. It has been found that sst2 and sst4 receptors have a tendency to co-localise in regions of the brain involved with learning and memory  .
Interestingly, Schulz et al failed to detect immunoreactivity for sst5 receptor in any parts of the rat central nervous system. It was, however found to very abundant in the anterior pituitary13.
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Though all 5 receptors have been located in the murine central nervous system, it was noted in 2003 by Videau et al that iodinated somatostatin binding was greatly reduced in sst2 knockout mice; moreso than for any of the other receptors  . This finding is significant as it indicates that the sst2 receptor is the most abundant of the somatostatinergic receptors of the murine CNS. This is quite possibly the case for humans, and as a result, much of the research has focused on the sst2 receptor.
Aside from its inhibitory role in the regulation of growth hormone release which has been known for over 25 years1, somatostatin has what we now consider an important role as a neurotransmitter in central nervous system physiology, mediating motor, cognitive and sensory systems23  . The exact effects which somatostatin has on the nervous system are not yet fully understood, but several studies into the location of somatostatin receptors at both the tissue and cellular level have provided clues that somatostatin does not function solely as a neuroendocrine regulator  13  .
The behavioural effects of somatostatin were recognised as early as 1973, when it was found that somatostatin injected into monkeys induced sedation  . Much of the research of the past 10 years regarding somatostatin in animal models has served to highlight its importance for robust central nervous system functioning.
As it has been shown to be the most abundant receptor in the murine central nervous system24, much of the research into the physiological effects of somatostatin has targeted the sst2 receptor. A 2003 study by Allen et al sought to determine the effect of the striatal sst2 receptor on the motor function of mice  . This was done by replacing the inactivated sst2 receptor gene with the lacZ reporter gene. Immunohistochemical studies were used to ensure that the sst2lacz successfully recapitulated sst2 receptor expression in the rat striatum, with sst2 expressed only in medium spiny projection neurones of the matrix compartment and in cholinergic interneurones. These measures ensured that a highly sensitive model was created to investigate exactly how sst2 knockout mice are affected in terms of motor control when compared with wild type mice.
The study measured several parameters with a view to establishing differences in motor control between wild type and sst2 knockout mice. Activity within the cage was measured (to study motor behaviour), and gait abnormalities were assessed by analyzing the footprint pattern of the mice along a narrow corridor. There was no significant difference between the two groups in these, although it had been found in an earlier study that sst2 invalidated mice showed reduced locomotor and exploratory behaviour  . The researchers also investigated the ability of the mice to traverse narrow beams to test fine motor control and balance. This test showed significant differences between the two groups, with the sst2lacz taking approximately twice as long to traverse the narrowest beam. Though the widespread expression of sst2 throughout the brain somewhat complicates interpretation of the motor-control phenotype, the study inferred that the sst2 receptor has an important neuromodulatory role for motor control in the striatum.
A 2001 study by Zeyda et al investigated the role of somatostatin in motor learning using the 'rotarod' test  . According to the study a 'UGO Basile Accelerating rotarod for mice (model 7650, Stoelting)' was used and the time which was taken for the mice to fall off was measured. This test was repeated 3 times with 1 hour intertrial intervals. A non-accelerating trial was also performed in which mice were shown to be able to remain on the rotarod for long periods of time, and therefore that mice were not falling simply due to fatigue. Mice were also tasked to perform the vertical pole test, and a wire-hang test in which the time taken to fall was measured. The study also looked into exploratory behaviour and locomotor activity but as no significant results were drawn from these areas I shan't describe them here. Likewise, no significant differences were observed in the vertical pole and wire-hang tests. The important overall finding was that whilst wild type mice had a tendency towards rather large improvements on consecutive attempts at the rotarod test, the somatostatin null mutant mice showed fewer improvements reflected in smaller increases in the time spent on the rotarod before falling. The study drew the conclusion that the failure of mutant mice to reach the same rotarod performance level as the wild type mice was due to impaired motor learning, with difficulties coordinating movements to remain balanced on the rotarod as the speed (and hence balance demands) increased. Motor coordination (performance of smooth compound movements) and motor learning (adapting these movements to a change in task) are difficult to draw distinctions between experimentally, however they are both considered to be under cerebellar control and a pathology affecting either could lead to impairment on the rotarod test.
In the adult cerebellum somatostatin and its receptors are barely detectable, however this is not the case in the developing murine cerebellum in which both are highly expressed  -  . It has been put forward by Taniwaki et al that somatostatin might act as a cerebellar trophic factor during development  , and the study by Zeyda et al which I described would suggest that somatostatin and its receptors are functionally very relevant during development of the cerebellar cortex31.
Though its function in motor control has been documented in literature, other prospectively very significant roles exist for somatostatin elsewhere in the nervous system. It has been described that somatostatin has a role in nociception  , and studies have shown that it is restricted to unmyelinated sensory afferents with a propensity towards terminating in lamina II of the dorsal horn  -  . This suggests somatostatin has a role in pain modulation within the dorsal horn of the spinal cord, and is by no means the only data linking somatostatin to nociception. Research conducted in 1988 by Morton et al investigated the release of somatostatin in the spinal cords of cats and found that when noxious stimuli were applied peripherally, there was a marked increase in somatostatin levels in the substantia gelatinosa region of the dorsal horn, as well as in overlying pia  , implying a role for somatostatin in spinal cord processing of pain. Furthermore, somatostatin tends to co-localise with the pronociceptive neuropeptides CGRP and substance P in dorsal root ganglion neurones, providing further insight into the role of somatostatin in nociception.
A 2002 study by Song et al sought to understand the role of spinal sst2a receptors in the nociceptive processing of rats  . They noted a significant increase in sst2a receptor content in lamina II of the spinal cord 6 hours after a thermal noxious stimulus was applied, but no significant changes following a mechanical noxious stimulus applied over the same time frame. Hindpaw inflammation was then induced in the rats using complete Freund's adjuvant which resulted in mechanical and thermal hyperalgesia demonstrated by a 'robust decrease' in paw withdrawal latency and pinch threshold. Administration of a polyclonal antiserum to the ss2a receptor led to a significant reduction in thermal, but not mechanical hyperalgesia. The data provided by this study was the first to show that the sst2a receptor in the spinal cord is involved with thermal, but not mechanical nociceptive transmission, and suggests an excitatory role for somatostatin in spinal processing of nociception.
Interestingly, and in light of Song et al's recent findings that somatostatin has an excitory role in spinal pain processing, it has been shown that somatostatin has considerable analgesic properties in humans. A 1983 study into the analgesic effects of somatostatin in cluster headache patients demonstrated a significant reduction in duration and intensity of pain when compared to a placebo  , and a 1994 study by Mollenholt et al showed efficacy of intrathecally/epidurally administered somatostatin in relieving terminal cancer pain in 6 out of the 8 patients in the trial  . However, the analgesic effect of somatostatin tends to rely on continuous central infusion, and it has been found that intrathecal administration of somatostatin in rats leads to hind limb paralysis  -  . This makes it difficult to infer the exact action of somatostatin from the responses alone as somatostatin may in fact have ischaemic neurotoxic effects resulting in reduced nociceptive transmission.
The somatostatin receptors are all G-protein coupled receptors with typical 7 alpha-helical transmembrane spanning domains, and as such elicit their effects via several signaling pathways involving second messengers. All of the receptor types bind somatostatin with a similar affinity, although sst5 binds to SST-28 with a much higher affinity (10x). Somatostatin and its signaling pathways have been extensively studied and it has been found that several transduction mechanisms exist.
All 5 receptors couple with adenylate cyclase upon which they mediate an inhibitory role. This coupling is reliant upon a pertussis sensitive protein, and several G-protein molecules such as GÎ±i1, GÎ±i2, and GÎ±i3 (among others) are also necessary for this coupling  -  .
A 1997 paper by Kreienkamp et al demonstrated the ability of somatostatin receptors to couple with 'G protein gated inwardly rectifying potassium channels (GIRK1)'  . This was shown by stimulating Xenopus oocytes with somatostatinergic agonists and measuring inward current under voltage clamped conditions. The study demonstrated notable inward potassium currents in cells expressing sst2-5 receptors but interestingly not the sst1 receptor which was not shown to couple to GIRK1. There was no inward potassium current in cells expressing somatostatin receptors but no GIRK1 channel, indicating the importance of GIRK1 to this somatostatin mediated potassium influx.
Somatostatin has also been shown to be able to mediate Calcium currents via L type calcium channels  . The sst2 agonist MK678 was shown to inhibit inward Calcium currents in AtT-20 pituitary cell lines, as was the sst5 agonist BIM23052. Pertussis toxin was shown to block this effect in both cases, indicating that the sst2 and sst5 receptors probably couple to L-type calcium channels via a G protein mediated mechanism. A relevant finding was made by Roosterman et al in a 1998 paper in which the sst1 selective ligand CH-257 resulted in a marked inhibition of voltage gated calcium channels of rat insulinoma cells  .
Somatostatin has also been implicated in intracellular signal transduction via the mitogen activated protein kinases Erk1/2, and activation of Erk1/2 via the sst4 receptor can induce phosphorylation of phospholipase A2  . Furthermore, Schweitzer et al suggested in a 1993 work that somatostatin receptors can hyperpolarize hippocampal CA1 pyramidal neurones via mechanisms involving arachidonic acid and its metabolites  . A study by Barber et al described a mechanism by which somatostatin inhibits a sodium/hydrogen ion exchanger in a cAMP/pertussis toxin independent way  . That is to say that whilst pertussis toxin reversed the attenuation of cAMP levels, it had no effect on somatostatin induced Na/H exchange inhibition, indicating a mechanism of action completely separate to that mediated by adenylate cyclase inhibition.
I have described several, but by no means all of the mechanisms by which somatostatin exerts its intracellular effects and in doing so I hope to have shown quite how diverse a neuropeptide somatostatin is. It has actions on several receptor subtypes in many body tissues, and results in a plethora of intracellular signal cascades which are too numerous to describe succinctly in this essay.
Pharmacology and Therapeutics
Sst2 preferring agonists such as octreotide have been widely used in the short term management of acromegalic patients, gastroenterological tumours and several other diseases of the digestive tract, however several possible future applications exist for its use in central nervous system pharmacology and the neuropsychiatric axis. It has been implicated in psychiatric illnesses such as depression and bipolar disorder, as well as in epilepsy and Alzheimer's disease.
Several studies have linked somatostatin to affective disorders, noting that cerebrospinal fluid somatostatin levels are reduced in depression as one of the commonest neuropeptide alterations. This has been enforced by Rubinow's findings that significant reductions in CSF somatostatin were found in 49 patients with unipolar and bipolar depression  . These levels were significantly lower than those found in improving affective disorder patients and in schizophrenia and the study concluded that further research into the role of somatostatin in neuropsychiatric disorders holds great promise for therapeutic opportunities in the future. However, more recently it was found that an increase in CSF somatostatin levels in response to nimodipine (an L-type calcium channel blocker) in patients with affective disorders was seemingly a pharmacological effect of the drug, and did not necessarily correlate with clinical improvement  . Frye et al found that nimodipine increased CSF somatostatin levels in nonresponders as well as responders  , indicating that the role of somatostatin in affective disorders is not a simple one.
CSF somatostatin levels are also reduced in people with epilepsy, Alzheimer's disease and multiple sclerosis, indicating that this dysregulation is not specific to affective disorders54. It has also been shown that somatostatin levels are increased in the CSF of patients with accelerated cognitive states such as mania  , and in ruminative conditions such as obsessive compulsive disorder  , which further implies a role in neuropsychiatric pathology.
In temporal lobe epilepsy, it has been found that serotonergic hilar neurons degenerate significantly in patients with hippocampal sclerosis  . This was linked by Choi et al to the finding that specific enrichment of somatostatinergic hilar interneurones with striatal-enriched phosphatase (enzyme which counters the MAPK neuroprotective pathway) was linked to a high vulnerability to status epilepticus induced excitotoxicity, thus indicating a pathway by which these hilar somatostatinergic neurones degenerate in temporal lobe epilepsy. It has also been demonstrated that somatostatin is preferentially released from neurones under conditions of high activity/ high frequency stimulation such as that which would occur during seizures (reviewed by Vezzani and Hoyer, 1999)  . In a 2002 paper by Buckmaster et al it was described how somatostatin knockout mice experienced more severe kainite induced seizures, with a greater likelihood of mortality, and a shorter latency to more severe (stage 5) seizures  . This indicates an anti-convulsant role for somatostatin and a possible therapeutic target for the future. Although clearly implicated in seizure control, insufficient data is available regarding the contributions of specific receptors to this anti-convulsant activity. The sst1 and sst4 receptors are currently thought to be the main contributors in the murine brain58-  , though more research is necessary to ascertain the exact roles of particular receptors so that pharmacological therapy might accurately target them.
Somatostatin has also been implicated in Alzheimer's disease, and Dournaud et al found that somatostatin and acetyltransferase deficiency in the frontal and temporal cortex correlates with cognitive decline  . It has also been shown that there is reduced somatostatinergic immunoreactivity in the post mortem frontal cortex of Alzheimer's disease patients  . Interestingly, this study found that the somatostatin concentrations were only significantly reduced in patients who carried the epsilon 4 allele, and this group of patients were also found to have lower mini mental state test scores than those who did not carry the allele (66% of epsilon 4 carriers had a score <10 versus 69% of those who do not carry the allele having a score >20). The proportion of pro-somatostatin is significantly reduced in the temporal lobe of Alzheimer's disease patients, with an increase in the proportion of SOM-18 and a reduction in SOM-14  , which suggests abnormalities of somatostatin biosynthesis or cleavage into its active forms are present in the brains of Alzheimer's disease patients. More recently it was found that somatostatin is responsible for promoting amyloid-beta42 degradation via an increase in neprolysin activity  , indicating a possible mechanism through which somatostatin deficiency might induce cognitive decline.
The research which I have described delineates the possible therapeutic importance of monitoring and controlling brain somatostatin levels. Though much research is still necessary, the possibility of creating therapeutics which target somatostatin receptors/systems in the central nervous system has emerged as an exciting prospect in the management of Alzheimer's disease, as well as other neuropsychiatric conditions.
Since its discovery over 35 years ago as a growth hormone regulator, much light has been shed on somatostatin and its role in several body tissues. Animal models have shown its role in motor function, cerebellar development and even nociception, and its relevance to human central nervous system pathology is becoming more pronounced. Much research is still necessary to explore its physiological roles and pathophysiological implications further, however I, along with much of the scientific community, have no doubt that this relatively newly discovered neuropeptide and its receptors will prove to be important targets for future therapeutics of the neuropsychiatric axis.