Chronic desipramine treatment selectively potentiates somatostatin-induced dopamine release in the nucleus accumbens

Ventral striatal islands of Calleja neurons bidirectionally mediate depression-like behaviors in mice

The ventral striatum is a reward center implicated in the pathophysiology of depression. It contains islands of Calleja, clusters of dopamine D3 receptor-expressing granule cells, predominantly in the olfactory tubercle (OT). These OT D3 neurons regulate self-grooming, a repetitive behavior manifested in affective disorders. Here we show that chronic restraint stress (CRS) induces robust depression-like behaviors in mice and decreases excitability of OT D3 neurons. Ablation or inhibition of these neurons leads to depression-like behaviors, whereas their activation ameliorates CRS-induced depression-like behaviors. Moreover, activation of OT D3 neurons has a rewarding effect, which diminishes when grooming is blocked. Finally, we propose a model that explains how OT D3 neurons may influence dopamine release via synaptic connections with OT spiny projection neurons (SPNs) that project to midbrain dopamine neurons. Our study reveals a crucial role of OT D3 neurons in bidirectionally mediating depression-like behaviors, suggesting a potential therapeutic target.

Nucleus accumbens in the pathogenesis of major depressive disorder: A brief review

Major depressive disorder (MDD) is the most prevalent mental disorder characterized by anhedonia, loss of motivation, avolition, behavioral despair and cognitive abnormalities. Despite substantial advancements in the pathophysiology of MDD in recent years, the pathogenesis of this disorder is not fully understood. Meanwhile,the treatment of MDD with currently available antidepressants is inadequate, highlighting the urgent need for clarifying the pathophysiology of MDD and developing novel therapeutics. Extensive studies have demonstrated the involvement of nuclei such as the prefrontal cortex (PFC), hippocampus (HIP), nucleus accumbens (NAc), hypothalamus, etc., in MDD. NAc,a region critical for reward and motivation,dysregulation of its activity seems to be a hallmark of this mood disorder. In this paper, we present a review of NAc related circuits, cellular and molecular mechanisms underlying MDD and share an analysis of the gaps in current research and possible future research directions.

Endocannabinoid Modulation of Nucleus Accumbens Microcircuitry and Terminal Dopamine Release

The nucleus accumbens (NAc) is located in the ventromedial portion of the striatum and is vital to valence-based predictions and motivated action. The neural architecture of the NAc allows for complex interactions between various cell types that filter incoming and outgoing information. Dopamine (DA) input serves a crucial role in modulating NAc function, but the mechanisms that control terminal DA release and its effect on NAc neurons continues to be elucidated. The endocannabinoid (eCB) system has emerged as an important filter of neural circuitry within the NAc that locally shapes terminal DA release through various cell type- and site-specific actions. Here, we will discuss how eCB signaling modulates terminal DA release by shaping the activity patterns of NAc neurons and their afferent inputs. We then discuss recent technological advancements that are capable of dissecting how distinct cell types, their afferent projections, and local neuromodulators influence valence-based actions.

A role for the neuropeptide somatostatin in the neurobiology of behaviors associated with substances abuse and affective disorders

In recent years, neuropeptides which display potent regulatory control of stress-related behaviors have been extensively demonstrated to play a critical role in regulating behaviors associated with substance abuse and affective disorders. Somatostatin (SST) is one neuropeptide known to significantly contribute to emotionality and stress behaviors. However, the role of SST in regulating behavior has received relatively little attention relative to other stress-involved peptides, such as neuropeptide Y or corticotrophin releasing factor. This review characterizes our current understanding of the role of SST and SST-expressing cells in general in modulating several behaviors intrinsically linked to substance abuse and affective disorders, specifically: anxiety and fear; stress and depression; feeding and drinking; and circadian rhythms. We further summarize evidence of a direct role for the SST system, and specifically somatostatin receptors 2 and 4, in substance abuse disorders. This article is part of the special issue on 'Neuropeptides'.

Role of endocannabinoid system in dopamine signalling within the reward circuits affected by chronic pain

The association between chronic pain, depression and anxiety has gained particular attention due to high rates of comorbidity. Recent data demonstrated that the mesolimbic reward circuitry is involved in the pathology of chronic pain. Interestingly, the mesolimbic reward circuit participates both in pain perception and in pain relief. The endocannabinoid system (ECS) has emerged as a highly relevant player involved in both pain perception and reward processing. Targeting ECS could become a novel treatment strategy for chronic pain patients. However, little is known about the underlying mechanisms of action of cannabinoids at the intersection of neurochemical changes in reward circuits and chronic pain. Because understanding the benefits and risks of cannabinoids is paramount, the aim of this review is to evaluate the state-of-art knowledge about the involvement of the ECS in dopamine signalling within the reward circuits affected by chronic pain.

Acupuncture – Self-Appraisal and the Reward System

Acupuncture is an ancient therapy with a variety of different explanatory models. A cascade of physiological effects has been reported, both in the peripheral and the central nervous system, following the insertion of a needle or light tapping of the skin. Clinical trials testing the specific claims of acupuncture have generally tried to focus on testing the efficacy of applying specific techniques and/or specified points. However, different conditions may respond differently to different modes of stimulation.

Recently, it was demonstrated that both superficial and deep needling (with de qi/Hibiki) resulted in amelioration of patellofemoral pain and unpleasantness. The pleasurable aspect of the acupuncture experience has largely been ignored as it has been considered secondary to its pain alleviating effects. This aspect of acupuncture treatment is likely to be related to activation of self-appraisal and the reward system.

When a patient seeks a therapist there are expectations of a specific effect. These expectations are partly based on self-relevant phenomena and self-referentia introspection and constitute the preference. Also, when asked about the effect of the treatment, processes that orientate pre-attentive anticipatory or mnemonic information and processes that mediate self-reflection and recollection are integrated together with sensory detection to enable a decision about the patient's perception of the effect of acupuncture treatment. These ‘self-appraisal’ processes are dependent on two integrated networks: a ventral medial prefrontal cortex paralimbic limbic ‘affective’ pathway and a dorsal medial prefrontal cortex cortical hippocampal ‘cognitive’ pathway.

The limbic structures are implicated in the reward system and play a key role in most diseases and illness responses including chronic pain and depression, regulating mood and neuromodulatory responses (eg sensory, autonomic, and endocrine). The pleasurable and neuromodulatory aspects of acupuncture as well as ‘placebo needling’ may partly be explained by the activation or deactivation of limbic structures including the hippocampus, amygdala, and their connections with the hypothalamus.

In patients with patellofemoral pain, the effects of superficial and deep needling remained for six months. These long term pain-alleviating effects have been attributed to activation of pain inhibiting systems in cortical and subcortical pathways. When considering long term effects the cortical cerebellar system needs to be taken into account. The cortical cerebellar system is probably central to the development of neural models that learn and eventually stimulate routinely executed (eg motor skills) and long term (eg pain alleviation) cognitive processes. These higher order cognitive processes are initially mediated in prefrontal cortical loci but later shift control iteratively to internal cerebellar representations of these processes. Possibly part of the long term healing effects of acupuncture may be attributed to changes in the cerebellar system thereby sparing processing load in cortical and subcortical areas.

As cortical and subcortical structures are activated and/or de-activated following stimulation of receptors in the skin, disregarding site, ‘placebo or sham needling’ does not exist and conclusions drawn on the basis that it is an inert control are invalid.

‘Self’ may be seen as a shifting illusion, ceaselessly constructed and deconstructed, and the effect of acupuncture may reflect its status (as well as that of the therapist).

The Dopamine Receptor D3 Regulates Lipopolysaccharide-Induced Depressive-Like Behavior in Mice

BACKGROUND

The altered expression and function of dopamine receptor D3 (D3R) in patients and animal models have been correlated with depression disease severity. However, the morphological alterations and biological effects of D3R in the brain after inflammation-induced depressive-like behavior remain elusive.

METHODS

In the present study, we ascertained the changes of D3R expression in the brain regions after depressive-like behavior induced by peripheral administration of lipopolysaccharide (LPS). Protein levels of proinflammatory cytokines, brain-derived neurotrophic factor (BDNF), and extracellular signal-regulated kinase (ERK1/2)-cAMP-response element-binding protein (CREB) signaling pathway after activation or inhibition of D3R in the brain of depressive mice were also investigated.

RESULTS

LPS caused a significant reduction of D3R in the ventral tegmental area (VTA), medial prefrontal cortex (mPFC), and nucleus accumbens (NAc), which are areas related to the mesolimbic dopaminergic system. Pretreatment with pramipexole (PPX), a preferential D3R agonist, showed antidepressant effects on LPS-induced depression-like behavior through preventing changes in LPS-induced proinflammatory cytokines (tumour necrosis factor-α, interleukin-1β, and interleukin-6), BDNF, and ERK1/2-CREB signaling pathway in the VTA and NAc. In opposition, treatment with a D3R selective antagonist NGB 2904 alone made mice susceptible to depression-like effects and caused changes in accordance with the LPS-induced alterations in proinflammatory cytokines, BDNF, and the ERK1/2-CREB signaling pathway in the mPFC and NAc.

CONCLUSIONS

These findings provide a relevant mechanism for D3R in LPS-induced depressive-like behavior via its mediation of proinflammatory cytokines and potential cross-effects between BDNF and the ERK1/2-CREB signaling pathway.

Regulation of somatostatin receptor 2 in the context of antidepressant treatment response in chronic mild stress in rat

RATIONALE

The role of somatostatin and its receptors for the stress-related neuropsychiatric disorders has been widely raised. Recently, we have also demonstrated the involvement of somatostatin receptor type 2-sst2R and dopamine receptor type 2-D2R in stress.

OBJECTIVE

In this context, we decided to find if these receptors are involved in response to antidepressant treatment in animal model of depression-chronic mild stress (CMS).

METHODS

Here, we report data obtained following 7-week CMS procedure. The specific binding of [125I]Tyr3-Octreotide to sst2R and [3H]Domperidone to D2R was measured in the rat brain, using autoradiography. Additionally, the level of dopamine and metabolites was measured in the rat brain.

RESULTS

In the final baseline test after 7 weeks of stress, the reduced consumption of sucrose solution was observed (controls vs the stressed animals (6.25 0.16 vs. 10.39 0.41; p < 0.05). Imipramine was administered for the next 5 weeks, and it reversed anhedonia in majority of animals (imipramine-reactive); however, in some animals, it did not (imipramine-non-reactive). Two-way repeated measures ANOVA revealed significant effects of stress and treatment and time interaction [F(16, 168) = 3.72; p < 0.0001], n = 10 per groups. We observed decreased binding of [125I]Tyr3-Octreotide in most of rat brain regions in imipramine non-reactive groups of animals. The decrease of D2R after stress in striatum and nucleus accumbens and no effect of imipramine were observed. In the striatum and prefrontal cortex, the significant role of stress and imipramine in dopamine levels was observed.

CONCLUSIONS

The results obtained in binding assays, together with dopamine level, indicate the involvement of sst2R receptors for reaction to antidepressant treatment. Besides, the stress context itself changes the effect of antidepressant drug.

Antidepressants promote formation of heterocomplexes of dopamine D2 and somatostatin subtype 5 receptors in the mouse striatum

The interaction between the dopaminergic and somatostatinergic systems is considered to play a potential role in mood regulation. Chronic administration of antidepressants influences release of both neurotransmitters. The molecular basis of the functional cooperation may stem from the physical interaction of somatostatin receptor subtypes and dopamine D2 receptors since they colocalize in striatal interneurons and were shown to undergo ligand-dependent heterodimerization in heterologous expression systems. In present study we adapted in situ proximity ligation assay to investigate the occurrence of D2-Sst5 receptor heterocomplexes, and their possible alterations in the striatum of mice treated acutely and repeatedly (21days) with antidepressant drugs of different pharmacological profiles (escitalopram and desipramine). Additionally we analysed number of heterocomplexes in primary striatal neuronal cultures incubated with both antidepressant drugs for 1h and 6days. The studies revealed that antidepressants increase formation of D2-Sst5 receptors heterodimers. These findings provide interesting evidence that dopamine D2 and somatostatin Sst5 heterodimers may be considered as potential mediators of antidepressant effects, since the heterodimerization of these receptors occurs in native brain tissue as well as in primary striatal neuronal cultures where receptors are expressed at physiological levels.

Frequency-dependent, cell type-divergent signaling in the hippocamposeptal projection

Hippocampal oscillations are critical for information processing, and are strongly influenced by inputs from the medial septum. Hippocamposeptal neurons provide direct inhibitory feedback from the hippocampus onto septal cells, and are therefore likely to also play an important role in the circuit; these neurons fire at either low or high frequency, reflecting hippocampal network activity during theta oscillations or ripple events, respectively. Here, we optogenetically target the long-range GABAergic projection from the hippocampus to the medial septum in rats, and thereby simulate hippocampal input onto downstream septal cells in an acute slice preparation. In response to optogenetic activation of hippocamposeptal fibers at theta and ripple frequencies, we elicit postsynaptic GABAergic responses in a subset (24%) of septal cells, most predominantly in fast-spiking cells. In addition, in another subset of septal cells (19%) corresponding primarily to cholinergic cells, we observe a slow hyperpolarization of the resting membrane potential and a decrease in input resistance, particularly in response to prolonged high-frequency (ripple range) stimulation. This slow response is partially sensitive to GIRK channel and D2 dopamine receptor block. Our results suggest that two independent populations of septal cells distinctly encode hippocampal feedback, enabling the septum to monitor ongoing patterns of activity in the hippocampus.

Potential role of G protein-coupled receptor (GPCR) heterodimerization in neuropsychiatric disorders: a focus on depression

G protein-coupled receptors (GPCRs) represent the largest family of membrane proteins in the human genome and are the target of approximately half of all therapeutic drugs. For many years, GPCRs were thought to exist and function as monomeric units. However, during the past two decades, substantial biochemical, structural and functional evidence have indicated that GPCRs can associate and form heteromers that exhibit functional properties distinct from the corresponding monomers. The understanding of the unique pharmacological and functional properties of such heteromers is a major challenge for neuroscience, particularly given the abundant evidence suggesting that GPCR heteromers may play a crucial role in neuropsychiatric disorders. Herein, we present current data on the role of GPCR heterodimerization in neuropsychiatric disorders, with a focus on its potential implications in depression. The presented examples of pairs of receptors, with their specific pharmacological and functional properties, are likely to lead to novel effective strategies in antidepressant drug development. The currently available techniques for studying GPCR heterodimerization, both in vitro as well as in situ in native tissue, are also described.

Effects of chronic antidepressant drug administration and electroconvulsive shock on activity of dopaminergic neurons in the ventral tegmentum

Increasing attention is now focused on reduced dopaminergic neurotransmission in the forebrain as participating in depression. The present paper assessed whether effective antidepressant (AD) treatments might counteract, or compensate for, such a change by altering the neuronal activity of dopaminergic neurons in the ventral tegmental area (VTA-DA neurons), the cell bodies of the mesocorticolimbic dopaminergic system. Eight AD drugs or vehicle were administered to rats for 14 d via subcutaneously implanted minipumps, at which time single-unit electrophysiological activity of VTA-DA neurons was recorded under anaesthesia. Further, animals received a series of five electroconvulsive shocks (ECS) or control procedures, after which VTA-DA activity was measured either 3 d or 5 d after the last ECS. Results showed that the chronic administration of all AD drugs tested except for the monoamine oxidase inhibitor increased the spontaneous firing rate of VTA-DA neurons, while effects on 'burst' firing activity were found to be considerably less notable or consistent. ECS increased both spontaneous firing rate and burst firing of VTA-DA neurons. It is suggested that the effects observed are consistent with reports of increased dopamine release in regions to which VTA neurons project after effective AD treatment. However, it is further suggested that changes in VTA-DA neuronal activity in response to AD treatment should be most appropriately assessed under conditions associated with depression, such as stressful conditions.

Somatostatin-28 modulates prepulse inhibition of the acoustic startle response, reward processes and spontaneous locomotor activity in rats

Somatostatins have been shown to be involved in the pathophysiology of motor and affective disorders, as well as psychiatric disorders, including schizophrenia. We hypothesized that in addition to motor function, somatostatin may be involved in somatosensory gating and reward processes that have been shown to be dysregulated in schizophrenia. Accordingly, we evaluated the effects of intracerebroventricular administration of somatostatin-28 on spontaneous locomotor and exploratory behavior measured in a behavioral pattern monitor, sensorimotor gating, prepulse inhibition (PPI) of the acoustic startle reflex, and brain reward function (measured in a discrete trial intracranial self-stimulation procedure) in rats. Somatostatin-28 decreased spontaneous locomotor activity during the first 10 min of a 60 min testing session with no apparent changes in the exploratory activity of rats. The highest somatostatin-28 dose (10 microg/5 microl/side) induced PPI deficits with no effect on the acoustic startle response or startle response habituation. The somatostatin-induced PPI deficit was partially reversed by administration of SRA-880, a selective somatostatin 1 (sst(1)) receptor antagonist. Somatostatin-28 also induced elevations in brain reward thresholds, reflecting an anhedonic-like state. The non-peptide sst(1) receptor antagonist SRA-880 had no effect on brain reward function under baseline conditions. Altogether these findings suggest that somatostatin-28 modulates PPI and brain reward function but does not have a robust effect on spontaneous exploratory activity. Thus, increases in somatostatin transmission may represent one of the neurochemical mechanisms underlying anhedonia, one of the negative symptoms of schizophrenia, and sensorimotor gating deficits associated with cognitive impairments in schizophrenia patients.

GABA antagonists reverse the somatostatin dependent attenuation of rat locomotor activity

Somatostatin infusion in rat ventral pallidum (VP) led to the attenuation of locomotor activity (Marazioti, A., Kastellakis, A., Antoniou, K., Papasava, D., Thermos, K., 2005. Somatostatin receptors in the ventral pallidum/substantia innominata modulate rat locomotor activity. Psychopharmacology 181, 319-326). In the present study, we investigated the putative circuitry involved in somatostatin's actions by examining the involvement of GABAergic neurotransmission in locomotor activity subsequent to somatostatin's infusion into the VP. Male Sprague-Dawley rats, 300-350 g, were used for all experiments. Saline or somatostatin (240 ng/0.5 microl/side) in the absence or presence of bicuculline (GABA-A antagonist; 5 mg/kg/ml, i.p.; 120 ng/side nucleus accumbens (NAc)) or phaclofen (GABA-B antagonist; 10 mg/kg/ml, i.p.; 120 ng/side NAc) were infused bilaterally, and the locomotor activity measured for 60 min using a rectangular activity cage. Somatostatin infused in the VP decreased the locomotor activity of the rat in a statistically significant manner. Bicuculline (i.p., and in the NAc) and phaclofen (only i.p.) reversed SRIF's actions, when administered prior to somatostatin's infusion in the VP. The present study provides further information on somatostatin's involvement in the VP-NAc circuitry, and implicates the GABAergic system in somatostatin's actions in the VP.

Antidepressants influence somatostatin levels and receptor pharmacology in brain

This study investigated how the administration (acute and chronic) of the antidepressants citalopram and desmethylimipramine (DMI) influences somatostatin (somatotropin release inhibitory factor, SRIF) levels and SRIF receptor density (sst(1-5)) in rat brain. Animals received either of the following treatments: (1) saline for 21 days (control group), (2) saline for 20 days and citalopram or DMI for 1 day (citalopram or DMI acute groups), (3) citalopram or DMI for 21 days (citalopram or DMI chronic groups). Somatostatin levels were determined by radioimmunoassay. [(125)I]LTT SRIF-28 binding in the absence (labeling of sst(1-5)) or presence of 3 nM MK678 (labeling of sst(1/4)) and [(125)I]Tyr(3) octreotide (labeling of sst(2/5)) binding with subsequent autoradiography was performed in brains of rats treated with both antidepressants. Somatostatin levels were increased after citalopram, but not DMI administration, in the caudate-putamen, hippocampus, nucleus accumbens, and prefrontal cortex. Autoradiography studies illustrated a significant decrease in receptor density in the superficial and deep layers of frontal cortex (sst(2)), as well as a significant increase in the CA1 (sst(1/4)) hippocampal field in brains of chronically citalopram-treated animals. DMI administration increased sst(1/4) receptors levels in the CA1 hippocampal region. These results suggest that citalopram and to a lesser extent DMI influence the function of the somatostatin system in brain regions involved in the emotional, motivational, and cognitive aspects of behavior.

Activation of somatostatin receptors in the globus pallidus increases rat locomotor activity and dopamine release in the striatum

RATIONALE

Somatostatin and its receptors have been localized in brain nuclei implicated in motor control, such as the striatum, nucleus accumbens, ventral pallidum, and globus pallidus (GP).

OBJECTIVES

The objective of this study was to investigate the role of somatostatin receptors (sst(1,2,4)) in the GP on dopamine (DA)-mediated behaviors, such as locomotor activity, and to examine the GP-striatum circuitry by correlating the effect of somatostatin in the GP with the release of DA in the striatum.

MATERIALS AND METHODS

Animals received saline, somatostatin (60, 120, 240 ng/0.5 microl per side) or the following selective ligands: L-797,591 (sst(1) analog, 60, 120, 240 ng/0.5 microl per side), L-779,976 (sst(2) analog, 120, 240, 480 ng/0.5 microl per side), L-803,087 (sst(4) analog; 120, 240, 480 ng/0.5 microl per side), L-796,778 (sst(3) analog, 240 ng/0.5 microl per side), SRA-880 (sst(1) selective antagonist + somatostatin, 120 ng/0.5 microl per side), CYN154806 (sst(2) selective antagonist + somatostatin, 120 ng/0.5 microl per side) bilaterally in the GP of the rat. Locomotor activity was measured for 60 min. The effect of somatostatin, administered intrapallidally, on the extracellular concentrations of DA, 3,4-dihydroxyphenylacetic acid, and homovanillic acid in the striatum was also studied in the behaving rat using in vivo microdialysis methodology.

RESULTS

Somatostatin increased the locomotor activity of the rat in a dose-dependent manner. This effect was mediated by activation of the sst(1), sst(2), and sst(4) receptors. Selective sst agonists increased locomotor activity in a statistical significant manner, while selective sst(1) and sst(2) antagonists reversed the somatostatin-mediated locomotor activity to control levels. DA levels increased in the striatum after intrapallidal infusion of somatostatin (240 ng/side).

CONCLUSIONS

These data provide behavioral and neurochemical evidence of the functional role of somatostatin receptors in the GP-striatum circuitry.

Somatostatin increases rat locomotor activity by activating sst(2) and sst (4) receptors in the striatum and via glutamatergic involvement

The involvement of striatal somatostatin receptors (sst(1), sst(2) and sst(4)) in locomotor activity was investigated. Male Sprague-Dawley rats, 280-350 g, received in the striatum bilateral infusions of saline, somatostatin, and selective sst(1), sst(2), and sst(4) ligands. Spontaneous locomotor activity was recorded for 60 min. The involvement of excitatory amino acid receptors (AMPA and NMDA) on somatostatin's actions was also examined. Western blot analysis was employed for the identification of somatostatin receptors in striatal membranes. Somatostatin, sst(2) and sst(4), but not sst(1), selective ligands increased rat locomotor activity in a dose-dependent manner. Blockade of AMPA and NMDA receptors reversed somatostatin's actions. In conclusion, striatal somatostatin receptor activation differentially influence rat locomotor activity, while glutamatergic actions underlie the behavioral actions of somatostatin.

Behavioural and physiological effects of electrical stimulation in the nucleus accumbens: a review

Electrical stimulation (ES) in the brain is becoming a new treatment option in patients with treatment-resistant obsessive-compulsive disorder (OCD). A possible brain target might be the nucleus accumbens (NACC). This review aims to summarise the behavioural and physiological effects of ES in the NACC in humans and in animals and to discuss these findings with regard to neuroanatomical, electrophysiological and behavioural insights. The results clearly demonstrate that ES in the NACC has an effect on reward, activity, fight-or-flight, exploratory behaviour and food intake, with evidence for only moderate physiological effects. Seizures were rarely observed. Finally, the results of ES studies in patients with treatment-resistant OCD and in animal models for OCD are promising.

The mesolimbic dopamine reward circuit in depression

The neural circuitry that mediates mood under normal and abnormal conditions remains incompletely understood. Most attention in the field has focused on hippocampal and frontal cortical regions for their role in depression and antidepressant action. While these regions no doubt play important roles in these phenomena, there is compelling evidence that other brain regions are also involved. Here we focus on the potential role of the nucleus accumbens (NAc; ventral striatum) and its dopaminergic input from the ventral tegmental area (VTA), which form the mesolimbic dopamine system, in depression. The mesolimbic dopamine system is most often associated with the rewarding effects of food, sex, and drugs of abuse. Given the prominence of anhedonia, reduced motivation, and decreased energy level in most individuals with depression, we propose that the NAc and VTA contribute importantly to the pathophysiology and symptomatology of depression and may even be involved in its etiology. We review recent studies showing that manipulations of key proteins (e.g. CREB, dynorphin, BDNF, MCH, or Clock) within the VTA-NAc circuit of rodents produce unique behavioral phenotypes, some of which are directly relevant to depression. Studies of these and other proteins in the mesolimbic dopamine system have established novel approaches to modeling key symptoms of depression in animals, and could enable the development of antidepressant medications with fundamentally new mechanisms of action.

Somatostatin modulates dopamine release in rat retina

The aim of the present study was to determine the possible role of somatostatin as a modulator of dopamine release in rat retina. Basal release of dopamine, and how this is influenced by somatostatin receptor (sst) selective ligands, was examined ex vivo in rat retinal explants. Dopamine levels were quantified by high-pressure liquid chromatography (HPLC) with electrochemical detection. Basal levels of dopamine were measured over 120 min of tissue incubation and found to be 1.17+/-0.35 ng/ml. Somatostatin (10(-6), 10(-5), 10(-4)M) increased dopamine levels in a concentration-dependent manner, while the sst(2) antagonist CYN154806 (10(-4)M) reversed its actions. BIM23014 (sst(2) agonist) increased dopamine levels in a statistically significant manner only at the concentration of 10(-5)M. The sst(1) agonist L797.591 (10(-5), 10(-4)M) also increased dopamine levels, while activation of the sst(3) receptor (sst(3) agonist, L796.778, 10(-4)M) had no effect. These data substantiate a neuromodulatory role for sst(1) and sst(2) somatostatin receptors in the retina and show for the first time somatostatin's influence on dopamine release.

Chronic antidepressant treatment modulates the release of somatostatin in the rat nucleus accumbens

This study investigated the in vivo neuronal release of somatostatin in the rat nucleus accumbens (NAc), and the effect of chronic administration of antidepressants. Microdialysis studies were performed on male Sprague-Dawley rats, in accordance with the EU guidelines (EEC Council 86/609). Somatostatin levels were quantified by radioimmunoassay (RIA) or enzyme linked immuno sorbent assay (ELISA). A high concentration of potassium ions (K(+), 100 mM) was used to ascertain the neuronal release of somatostatin. Antidepressant treatments involved the administration of citalopram (20 mg/2 ml/kg, i.p., once daily) or desipramine (DMI, 5 mg/2 ml/kg, i.p., twice daily) for 21 days. Control groups received saline (2 ml/kg for 21 days, i.p.) once or twice daily respective of the antidepressant treatment. Basal levels of somatostatin released were found to be 20.01+/-0.52 fmol/sample. K(+) (100 mM) increased somatostatin levels at 205% of basal. Chronic citalopram and desipramine treatments also increased the somatostatin levels by 83+/-32% and 40+/-6% of basal, respectively. These findings indicate that somatostatin is released neuronally in the NAc. Antidepressants influence its release in a positive manner, suggesting the necessity of further studies for the elucidation of the involvement of somatostatin in the putative therapeutic effects of these agents.

The somatostatin sst1 receptor: an autoreceptor for somatostatin in brain and retina?

The sst1 receptor was the first of the 5 somatostatin receptors to be cloned by homology with the glucagon receptor 13 years ago. It is a 7-transmembrane domain G-protein-coupled receptor that is negatively coupled to adenylyl cyclase, but can also trigger other transduction pathways. The distribution of sst1 mRNA, immunolabeling, and radioligand binding are not entirely overlapping, but the recent availability of knockout (KO) mice and a (still limited) number of selective agonists/antagonists has increased our knowledge about this receptor. These new tools have helped to reveal a role for the sst1 receptor in hippocampal, hypothalamic, basal ganglia, and retinal functions. In at least the latter 3 structures, the sst1 receptor appears to act as an inhibitory autoreceptor located on somatostatin neurons, whereas in the hippocampus such a role is still based on circumstantial evidence.

Somatostatin receptors in the ventral pallidum/substantia innominata modulate rat locomotor activity

RATIONALE

Somatostatin and its receptors (sst(1) and sst(2)) have been localized in brain nuclei implicated in motor control, such as the nucleus accumbens, ventral pallidum (VP) and substantia innominata (SI).

OBJECTIVES

The objective of the study is to investigate the effect of somatostatin and selective sst(1) and sst(2) analogs infused in the VP/SI on the locomotor activity of the rat.

METHODS

Somatostatin (15, 30, 60, 120 and 240 ng/0.5 microl/side), CH275 (sst(1) analog; 60, 180, 240 and 480 ng/0.5 microl/side), MK678 (sst(2) analog; 120, 240 and 480 ng/0.5 microl/side), L-809,087 (sst(4) agonist, 240 ng/0.5 microl/side) or saline (vehicle) were infused bilaterally in the VP/SI of the rat and locomotor activity measured for 60 min. The effect of SRA-880 (sst(1) antagonist) and CYN-154806 (sst(2) antagonist) on somatostatin-, CH275- and MK678-mediated locomotor activity was also ascertained.

RESULTS

Somatostatin decreased locomotor activity in the first 30 min after its infusion in the VP/SI and in a dose-dependent manner. The sst(1) and sst(2) antagonists, SRA-880 and CYN-154806, respectively, reversed the somatostatin effect. The sst(1) and sst(2) agonists CH275 and MK678, respectively, mimicked somatostatin's actions, while the selective sst(4) agonist L-809,087 had no effect. Moreover, SRA-880 and CYN-154806 reversed the respective agonist action on locomotor activity.

CONCLUSION

The present study provides functional evidence for the presence of sst(1) and sst(2) receptors in the VP/SI and their implication in motor control. The mechanism via which somatostatin and agonists mediate the attenuation of locomotor activity is presently being investigated.

Voice acoustical measurement of the severity of major depression

A number of empirical studies have documented the relationship between quantifiable and objective acoustical measures of voice and speech, and clinical subjective ratings of severity of Major Depression. To further explore this relationship, speech samples were extracted from videotape recordings of structured interviews made during the administration of the 17-item Hamilton Depression Rating Scale (HDRS; ). Pilot data were obtained from seven subjects (five males, two females) from videotapes that have been used to train expert raters on the administration and scoring of the HDRS. Several speech samples were isolated for each subject and processed to obtain the acoustic measurements. Acoustic measures were selected on the basis that they were correlated with HDRS ratings of symptom severity as seen under ideal voice recording conditions in previous studies. Our findings corroborate earlier reports that speaking rate is well correlated (negatively) with HDRS scores, with a strong correlation and nearly significant trend seen for the measure of pitch variability. A moderate pairwise correlation between percent pause time and HDRS score was also revealed, although this relationship was not statistically significant. The results from this cross-sectional study further demonstrate the ability of voice and speech signal analyses to objectively track severity of depression. In the present case, it is suggested that this relationship is robust enough to be found despite the less than ideal recording conditions and equipment used during the original videotape recording. Voice acoustical analyses may provide a powerful compliment to the standard clinical interview for depression. Use of such measures increases the range of techniques that are available to explore the neurobiological substrates of Major Depression, its treatment, and the dynamic interplay of the systems that govern the motor, cognitive, and emotional aspects of speech production.

The somatostatin receptor (sst1) modulates the release of somatostatin in the nucleus accumbens of the rat

The aim of the present study was to examine the function of the somatostatin receptor (sst(1)) in the nucleus accumbens (NAc) of the basal ganglia. Radioligand binding studies were performed in rats to assess the presence of the receptor, while in vivo microdialysis studies were performed to examine its role in somatostatin release. CH-275, which is selective for sst(1), MK-678, selective for sst(2) and L-803,087, selective for sst(4) receptors displaced [(125)I]-Tyr(11)-somatostatin specific binding in a concentration-dependent manner with IC(50) values of 75, 0.21 and 11 nM, respectively. Infusion of CH-275 (10(-5), 10(-6) or 10(-7) M) in the NAc of freely moving rats resulted in a decrease in somatostatin levels only at the concentration of 10(-5) M. This effect was reversed by 10(-5) M of the selective sst(1) antagonist SRA-880. The sst(1) agonist L-797,591 (10(-5) M) mimicked the effect of CH-275, while MK-678 and L-803,087 at the same concentration were unable to influence somatostatin levels. These results provide functional evidence to demonstrate that the sst(1) receptor modulates somatostatin release in the basal ganglia.

Brain-derived neurotrophic factor in the ventral midbrain-nucleus accumbens pathway: a role in depression

BACKGROUND

Previous work has shown that brain-derived neurotrophic factor (BDNF) and its receptor, tyrosine kinase receptor B (TrkB), are involved in appetitive behavior. Here we show that BDNF in the ventral tegmental area-nucleus accumbens (VTA-NAc) pathway is also involved in the development of a depression-like phenotype.

METHODS

Brain-derived neurotrophic factor signaling in the VTA-NAc pathway was altered in two complementary ways. One group of rats received intra-VTA infusion of vehicle or BDNF for 1 week. A second group of rats received intra-NAc injections of vehicle or adeno-associated viral vectors encoding full-length (TrkB.FL) or truncated (TrkB.T1) TrkB; the latter is kinase deficient and serves as a dominant-negative receptor. Rats were examined in the forced swim test and other behavioral tests.

RESULTS

Intra-VTA infusions of BDNF resulted in 57% shorter latency to immobility relative to control animals, a depression-like effect. Intra-NAc injections of TrkB.T1 resulted in and almost fivefold longer latency to immobility relative to TrkB.FL and control animals, an antidepressant-like effect. No effect on anxiety-like behaviors or locomotion was seen.

CONCLUSIONS

These data suggest that BDNF action in the VTA-NAc pathway might be related to development of a depression-like phenotype. This interpretation is intriguing in that it suggests a role for BDNF in the VTA-NAc that is opposite of the proposed role for BDNF in the hippocampus.

Physiological release of striatal acetylcholine (in vivo): effect of somatostatin on dopaminergic–cholinergic interaction

The effects of somatostatin (SOM) on the release of acetylcholine (ACh) and dopamine (DA) from striatum of freely moving rats were studied by transversal microdialysis. Acetylcholine (ACh) and dopamine (DA) were detected by high performance liquid chromatography (HPLC) with electrochemical detection. Somatostatin (0.1, 0.5 and 1 microM), administered locally through the microdialysis probe to the striatum, was able to release dose-dependently ACh from the cholinergic neurons of the striatum. The increase in the extracellular levels of ACh produced by 1 microM SOM in the striatum reached a maximum of 200%. ACh-releasing effect of SOM was completely inhibited by tetrodotoxin indicating that neuronal firing is involved in its effect. Local infusion of sulpiride, 10 microM, D(2) receptor antagonist, potentiated (about 100%) the SOM (1 microM)-induced release of ACh. SOM, 1 microM, was more effective in enhancing the release of ACh in the striatum (two-fold increase) after degeneration of the nigrostriatal DA pathway with 6-hydroxydopamine (6-OHDA) (250 microg/animal, i.c.v.). The D(2) receptor agonists bromcriptine, 10 microM, or apomorphine, 10 microM, completely antagonize SOM-induced release. SOM, 1 microM, enhanced the release of DA (about 400%). These findings indicate that SOM is capable of releasing both ACh and DA in the striatum, however, its effect on ACh release is partially masked unless the D(2) receptor-mediated tonic inhibitory effect of released DA from the nigro-striatal pathway is attenuated.

Somatostatin receptor 2 knockout/lacZ knockin mice show impaired motor coordination and reveal sites of somatostatin action within the striatum

The peptide somatostatin can modulate the functional output of the basal ganglia. The exact sites and mechanisms of this action, however, are poorly understood, and the physiological context in which somatostatin acts is unknown. Somatostatin acts as a neuromodulator via a family of five 7-transmembrane G protein-coupled receptors, SSTR1-5, one of which, SSTR2, is known to be functional in the striatum. We have investigated the role of SSTR2 in basal ganglia function using mice in which Sstr2 has been inactivated and replaced by the lacZ reporter gene. Analysis of Sstr2lacZ expression in the brain by beta-galactosidase histochemistry demonstrated a widespread pattern of expression. By comparison to previously published in situ hybridization and immunohistochemical data, Sstr2lacZ expression was shown to accurately recapitulate that of Sstr2 and thus provided a highly sensitive model to investigate cell-type-specific expression of Sstr2. In the striatum, Sstr2 expression was identified in medium spiny projection neurons restricted to the matrix compartment and in cholinergic interneurons. Sstr2 expression was not detected in any other nuclei of the basal ganglia except for a sparse number of nondopaminergic neurons in the substantia nigra. Microdialysis in the striatum showed Sstr2-null mice were selectively refractory to somatostatin-induced dopamine and glutamate release. In behavioural tests, Sstr2-null mice showed normal levels of locomotor activity and normal coordination in undemanding tasks. However, in beam-walking, a test of fine motor control, Sstr2-null mice were severely impaired. Together these data implicate an important neuromodulatory role for SSTR2 in the striatum.

Neurobiology of depression

Current treatments for depression are inadequate for many individuals, and progress in understanding the neurobiology of depression is slow. Several promising hypotheses of depression and antidepressant action have been formulated recently. These hypotheses are based largely on dysregulation of the hypothalamic-pituitary-adrenal axis and hippocampus and implicate corticotropin-releasing factor, glucocorticoids, brain-derived neurotrophic factor, and CREB. Recent work has looked beyond hippocampus to other brain areas that are also likely involved. For example, nucleus accumbens, amygdala, and certain hypothalamic nuclei are critical in regulating motivation, eating, sleeping, energy level, circadian rhythm, and responses to rewarding and aversive stimuli, which are all abnormal in depressed patients. A neurobiologic understanding of depression also requires identification of the genes that make individuals vulnerable or resistant to the syndrome. These advances will fundamentally improve the treatment and prevention of depression.