Increased catecholamine levels in specific brain regions of a rat model of depression: normalization by chronic antidepressant treatment

Brain metabolic changes following 4-week citalopram infusion: increased 18FDG uptake and gamma-amino butyric acid levels

We used 2-week and 4-week citalopram infusion (10 mg/kg/day) to determine how this selective serotonin reuptake inhibitor (SSRI) would alter 2-deoxy-2-[18F]-fluoro-D-glucose (18FDG) uptake and neurotransmitter tissue levels in male Sprague-Dawley rodents. A weekly time course of 18FDG uptake altered by chronic citalopram treatment was determined in vivo with small animal positron emission tomography (microPET). Additionally, end of study monoamine levels were measured ex vivo using high pressure liquid chromatography (HPLC) and amino acid levels were determined ex vivo with proton nuclear magnetic resonance spectroscopy (1H-NMRS). We found increased striatal 18FDG uptake, reduced tissue levels of noradrenaline and serotonin in the striatum and prefrontal cortex, and increased striatal gamma-amino-butyric acid following 4-week citalopram infusion.

Rats selectively-bred for behavior related to affective disorders: proclivity for intake of alcohol and drugs of abuse, and measures of brain monoamines

Several lines of rats potentially useful for studying affective disorders have been developed in our laboratory though selective breeding for behavioral characteristics. The propensity of these lines to consume alcohol and other drugs of abuse (amphetamine and cocaine) was examined. Also, measurement of the concentration of brain monoamines - norepinephrine, dopamine, and serotonin - as well as estimation of their metabolism by measurement of the major extracellular metabolites of these monoamines was carried out to examine possible relationships of brain chemistry to the behavioral characteristics shown by these lines, as well as to their propensity for drug usage. The lines of rats are: Swim Low-active (SwLo) and Swim High-active (SwHi), which show either very low (SwLo) or very high (SwHi) amounts of motor activity in a swim test; Swim-test Susceptible (Susceptible or SUS) and Swim-test Resistant (Resistant or RES), which are highly susceptible (SUS) or highly resistant (RES) to having their swim-test activity depressed by being exposed to a stressful condition prior to the swim test; and Hyperactive (HYPER), which show spontaneous nocturnal hyperactivity compared to non-selectively bred (i.e., normal) rats as well as both extreme hyperactivity and behavioral depression after being exposed to a stressful condition. Regarding alcohol and drug usage, SUS rats readily consume alcohol while all other lines including non-selected, normal rats do not, and SwLo rats show a strong tendency to consume amphetamine and cocaine. Marked differences in brain monoamines were found between the various lines and normal rats, with salient differences seen in norepinephrine, particularly in the hippocampus, and in dopamine in forebrain regions (striatum and nucleus accumbens).

Modulation of stress consequences by hippocampal monoaminergic, glutamatergic and nitrergic neurotransmitter systems

Several findings relate the hippocampal formation to the behavioural consequences of stress. It contains a high concentration of corticoid receptors and undergoes plastic modifications, including decreased neurogenesis and cellular remodelling, following stress exposure. Various major neurotransmitter systems in the hippocampus are involved in these effects. Serotonin (5-HT) seems to exert a protective role in the hippocampus and attenuates the behavioural consequences of stress by activating 5-HT1A receptors in this structure. These effects may mediate the therapeutic actions of several antidepressants. The role of noradrenaline is less clear and possibly depends on the specific hippocampal region (dorsal vs. ventral). The deleterious modifications induced in the hippocampus by stress might involve a decrease in neurotrophic factors such as brain derived neurotrophic factor (BDNF) following glutamate N-methyl-D-aspartate (NMDA) receptor activation. In addition to glutamate, nitric oxide (NO) could also be related to these effects. Systemic and intra-hippocampal administration of nitric oxide synthase (NOS) inhibitors attenuates stress-induced behavioural consequences. The challenge for the future will be to integrate results related to these different neurotransmitter systems in a unifying theory about the role of the hippocampus in mood regulation, depressive disorder and antidepressant effects.

Modification of inherent and drug-induced dopaminergic activity after exposure to benzo(alpha)pyrene

The aim of this study was to investigate the effect of benzo(alpha)pyrene (B(alpha)P), a representative polycyclic aromatic hydrocarbon (PAH), on dopaminergic activity in brain. (B(alpha)P) altered dopaminergic activity in discrete regions of the rat brain, including the hippocampus, hypothalamus, caudate putamen and nucleus accumbens. Specifically, B(alpha)P increased DA levels in the hippocampus and DA turnover in the caudate putamen. In addition, B(alpha)P suppressed DA levels in the caudate putamen and DA turnover in the nucleus accumbens. B(alpha)P also altered the effect of several dopaminergic agents, L-DOPA, sulpiride and bromocriptine, on DA activity. In particular, B(alpha)P enhanced the L-DOPA-induced increase in the DA turnover ratio in the caudate putamen and increased DA levels in the nucleus accumbens. B(alpha)P also reversed the sulpiride-induced increase of DA turnover in the nucleus accumbens and the bromocriptine-induced increase of DA turnover in the hippocampus. In addition, DA turnover was increased by B(alpha)P in the nucleus accumbens and caudate putamen and DA levels were suppressed in the nucleus accumbens of bromocriptine treated rats, though the drug alone had no effect. These changes indicate that exposure to B(alpha)P and related compounds may affect dopaminergic function in discrete brain regions that are implicated in cognitive functions, psychosis, depression and Parkinson's disease, and may possibly interfere with their pharmacological intervention.

Assessment of antidepressant and anxiolytic properties of NK1 antagonists and substance P in Wistar Kyoto rats

In an attempt to explore the involvement of substance P in depression and anxiety and its' potential therapeutic effects, we measured basal plasma and hypothalamic levels of substance P in a well-studied animal model of depression--adult male Wistar Kyoto (WKY) rats and their controls, Wistar rats. We also studied the influence of a substance P receptor (NK1) antagonist (SPA) on "anxiety-like" and "depressive-like" behaviors exhibited by the WKY rats in the open field and swim test paradigms, compared to controls. WKY rats exhibited lower levels of substance P compared to controls in the hypothalamus. Though the WKY strain exhibited less rearing behavior in the open field compared to controls, SPA did not influence this pattern of behavior. In contrast, SPA had a significant effect on a depressive-like behavior exhibited by the WKY strain--it reduced significantly the immobility duration of WKY rats in the swim test. Thus it seems that depression involves alterations in levels of substance P, and that NK1 antagonists may be effective in the relief of depressive, but not anxiety symptoms.

Omega-3 fatty acids and rodent behavior

This paper reviews the role of the n-3 fatty acids in the regulation of cognitive functions, locomotor and exploratory activity and emotional status in rodents. There are disparate data on the performance of n-3 fatty acid deficient animals in the open field test and elevated plus maze. Results obtained in our laboratory indicated slower habituation to the open field in deficient mice, which affects total locomotor and exploratory parameters. We also observed no change in plus maze performance of deficient mice under low-stress but elevated anxiety under high-stress conditions. There is some evidence of elevated aggression and increased immobility time in the forced swimming test caused by n-3 fatty acid deficiency in rodents. Effects of n-3 fatty acid deficiency and supplementation on learning in several tests such as the Morris water maze, two odor olfactory discriminations, radial arm maze performance and avoidance tasks are reviewed in detail. There is some evidence of an enhanced vulnerability to stress of n-3 fatty acid deficient animals and this factor can influence performance in a variety of tests. Thus, behavioral tasks that involve a higher level of stress may better differentiate behavioral effects related to brain docosahexaenoic acid (DHA) status. It is suggested that a fruitful area for future investigations of functional alterations related to brain DHA status will be the delineation of the factors underlying changes in performance in behavioral tasks. The possible role of non-cognitive factors like emotionality and attention in the impaired performance of n-3 fatty acid deficient animals also requires further investigation.

Dehydroepiandrosterone (DHEA) attenuates cocaine-seeking behavior in the self-administration model in rats

The aim of this study was to determine the possible involvement of the neurosteroid dehydroepiandrosterone (DHEA) in cocaine-seeking behavior in a self-administration model in rats. DHEA pretreatment (continued thereafter concomitantly with cocaine self-administration) attenuated cocaine-seeking behavior and elevated the levels of dopamine and serotonin in several brain regions relevant to cocaine addiction. Chronic cocaine self-administration induced elevation in brain DHEA, its sulfate ester, DHEAS, and pregnenolone. The increased brain DHEA following cocaine self-administration may serve as a compensatory protective mechanism geared to attenuate the craving for cocaine. Such anti-craving activity is further enhanced by DHEA treatment before and during cocaine self-administration.

Effects of maternal separation on neuropeptide Y and calcitonin gene-related peptide in "depressed" Flinders Sensitive Line rats: a study of gene-environment interactions

Interactions between genetic vulnerability to stress/depression and early life experience may play a crucial role in the pathogenesis of mood disorders. Here we explore this hypothesis by superimposing early life trauma in the form of maternal deprivation for 180 min per day from postnatal day 2 to 14 onto a genetic model of depression/susceptibility to depression, Flinders Sensitive Line (FSL) and their controls, Flinders Resistant Line (FRL) rats. We investigate effects on neuropeptide Y (NPY) and calcitonin gene-related peptide (CGRP) like immunoreactivity (LI) in 10 brain regions as these neuropeptides are affected by antidepressants and are altered in cerebrospinal fluid of depressed patients. NPY-LI was reduced while CGRP-LI was elevated in hippocampus and frontal cortex of "genetically depressed" FSL rats. The two peptides displayed a significant negative correlation in these regions that was strongest in the FSL strain. Maternal deprivation exacerbated the strain difference in hippocampal CGRP-LI, while it was without effect on NPY-LI. FSL rats had higher tissue concentration of both neuropeptides in periaqueductal grey and higher NPY-LI in caudate/putamen. Maternal deprivation selectively raised CGRP-LI in amygdala of the FRL control stain. Thus, in two brain regions implicated in the neurobiology of depression, hippocampus and frontal cortex, changes in CGRP-LI and NPY-LI were in opposite direction, and CGRP-LI appears to be more responsive to adverse experience. Our findings thus support the hypothesis that genetic disposition and developmental stress may contribute to the susceptibility to depression by exerting selective neuropeptide- and brain region-specific effects on adult neurobiology.

Swim test immobility in a genetic rat model of depression is modified by maternal environment: A cross-foster study

The Flinders sensitive line (FSL) genetic animal model of depression exhibits marked immobility during forced swimming, an accepted index of depressive like behavior in rodent depression models. The present experiment tested the hypothesis that swim test behavior in the FSL rats is influenced in part by early experience, specifically maternal environment. Male FSL and control Flinders resistant line (FRL) pups were cross fostered onto dams of the same or complementary strain. Nest quality and dam behavior during pup retrieval were measured on PN5 and PN8, and swim test behavior assessed in the adult males on PN60. FSL rats reared by foster FRL dams were significantly less immobile than FSL rats raised by FSL dams, but still significantly more immobile that the two FRL groups, which did not differ from each other. FSL dams took significantly longer to retrieve their pups and dropped them more often than the FRL control dams. Moreover, strain differences in maternal retrieval behavior significantly predicted later swim test immobility in the FSL animals. These findings suggest that swim test immobility in the FSL rats is modified by maternal environment. In contrast, the FRL control rats were relatively insensitive to the influence of maternal environment. The FSL model offers promise for understanding the interactions of genetic vulnerabilities and environmental influences in the etiology of clinical depression.

A hypothalamic endorphinic lesion attenuates acquisition of cocaine self-administration in the rat

The present study explores the role of beta-endorphin-producing neurons of the arcuate nucleus in the behavioral effects of cocaine (i.e. acquisition of cocaine self-administration). Eight-week-old female rats were treated with a single estradiol valerate injection that causes a progressive lesion that is specific to beta-endorphin-producing neurons throughout the arcuate nucleus. Cocaine acquisition was suppressed following estradiol valerate pretreatment, while water reinforced behavior was similar to controls. Since estradiol valerate treated rats exhibit low estrogen plasma levels, estrogen replacement was performed but cocaine self-administration acquisition remained suppressed. In addition, analysis of beta-endorphin, dopamine, and DOPAC tissue levels confirmed the specificity of the endorphinic lesion resulting from estradiol valerate treatment. The suppression of cocaine self-administration acquisition following estradiol valerate treatment provides evidence for a significant role for beta-endorphin in cocaine reward.

Overexpression of dopamine receptor genes and their products in the postnatal rat brain following maternal n-3 fatty acid dietary deficiency

A combination of PCR-Select cDNA subtraction and gene array hybridization was used to identify differentially expressed genomic markers in brains of rats fed for 3 weeks in utero and 2 weeks after birth on an n-3 polyunsaturated fatty acid (PUFA)-deficient diet supplied to dams. Total RNA was isolated, switch mechanism at 5'-end of the RNA transcripts (SMART) applied and used for PCR-Select subtraction of PUFA-deficient and adequately-fed control preparations. Subtracted and amplified ds-cDNA end-products were fragmented, terminally labeled with biotin-ddUTP and hybridized with a RN-U34A gene array. A 10-fold increase in potential genes with log2(Tester/Driver) = 1.4 was found compared with traditional gene array technology when the same chip was tested using non-subtracted targets. Reverse transcription-real-time relative PCR confirmed 30% of the transcripts. Among the validated transcripts, D1 and D2 receptors for dopamine (DA), were most prominent among a number of over-expressed neurotransmitter receptors and retinoic acid receptor (RXR alpha-2 and alpha-1). Immunohistochemical staining of brain sections from 2-week-old pups revealed a substantial enrichment of the D2 receptor in discrete regions of the mesolimbic and mesocortical pathways as well as in a large number of brain areas from the n-3 PUFA-deficient pups. Punches of the same areas run on western blots showed similar results. The overwhelming expression of D1 and D2 receptors may be attributed to a behavioral hypersensitivity caused by the possible impairment of DA production during brain development, which may have implications in certain disorders of the nervous system.

Immobility in the swim test and observations of maternal behavior in lactating flinders sensitive line rats

In order to elucidate the relationship between maternal behavior and depression, the Flinders sensitive line (FSL) model of depression was studied and compared to Sprague-Dawley (SD) controls. Immobility in the swim test was measured, as an index for depressive-like behavior, and frequencies of maternal and non-maternal behaviors were recorded using short un-intrusive observations in the home cage. Lactating FSL rats displayed higher levels of immobility in the swim test compared to controls, indicating depressive-like behavior. In addition, compared to SD rats, FSL dams showed less frequent pup licking and non-nutritive contact with pups during the first and third weeks of lactation. In the third postpartum week, FSL dams showed less frequent nursing postures and more frequent self-directed behaviors. Thus, lactating FSL dams exhibit both depressed-like behavior and some abnormalities in maternal behavior.

In vivo up-regulation of brain-derived neurotrophic factor in specific brain areas by chronic exposure to Delta9-tetrahydrocannabinol

Cannabinoids are widely abused drugs. Here we show that chronic administration of Delta(9)-tetrahydrocannabinol (Delta(9)-THC), the active psychotropic agent in marijuana and hashish, at 1.5 mg per kg per day intraperitoneally for 7 days, increases the expression, at both mRNA and protein levels, of brain-derived neurotrophic factor (BDNF), in specific rat brain areas, notably in those involved in reward and addiction. Real-time PCR revealed a 10-fold up-regulation of BDNF mRNA in the nucleus accumbens (NAc) upon chronic Delta(9)-THC treatment, but there was no change at 3 or 24 h after a single injection. Smaller increases in mRNA levels were found in the ventral tegmental area (VTA), medial prefrontal cortex and paraventricular nucleus (PVN). Immunohistochemistry showed large increases in BDNF-stained cells in the NAc (5.5-fold), posterior VTA (4-fold) and PVN (1.7-fold), but no change was observed in the anterior VTA, hippocampus or dorsal striatum. Altogether, our study indicates that chronic exposure to Delta(9)-THC up-regulates BDNF in specific brain areas involved with reward, and provides evidence for different BDNF expression in the anterior and posterior VTA. Moreover, BDNF is known to modulate synaptic plasticity and adaptive processes underlying learning and memory, leading to long-term functional and structural modification of synaptic connections. We suggest that Delta(9)-THC up-regulation of BDNF expression has an important role in inducing the neuroadaptive processes taking place upon exposure to cannabinoids.

The Flinders Sensitive Line rat: a selectively bred putative animal model of depression

The Flinders Sensitive Line (FSL) rats were originally selectively bred for increased responses to an anticholinesterase agent. The FSL rat partially resembles depressed individuals because it exhibits reduced appetite and psychomotor function but exhibits normal hedonic responses and cognitive function. The FSL rat also exhibits sleep and immune abnormalities that are observed in depressed individuals. Neurochemical and/or pharmacological evidence suggests that the FSL rat exhibits changes consistent with the cholinergic, serotonergic, dopaminergic, NPY, and circadian rhythm models but not the noradrenergic, HPA axis or GABAergic models of depression. However, evidence for the genetic basis of these changes is lacking and it remains to be determined which, if any, of the neurochemical changes are primary to the behavioral alterations. The FSL rat model has been very useful as a screen for antidepressants because known antidepressants reduced swim test immobility when given chronically and psychomotor stimulants did not. Furthermore, rolipram and a melatonin agonist were shown to have anti-immobility effects in the FSL rats and later to have antidepressant effects in humans. Thus, the FSL rat model of depression exhibits some behavioral, neurochemical, and pharmacological features that have been reported in depressed individuals and has been very effective in detecting antidepressants.

Increased arachidonic acid concentration in the brain of Flinders Sensitive Line rats, an animal model of depression

Depression may be associated with impaired membrane PUFA composition, especially decreased n-3 PUFA. This assumption has not been tested at the level of brain tissue. Moreover, most studies were confounded by dietary variability. We examined the FA composition of selected brain areas in an animal model of depression, the Flinders Sensitive Line (FSL) rat, and compared the findings with those in controls fed identical diets. In all brain regions studied, the concentration of arachidonic acid (AA) was significantly higher in the FSL rats: in the hypothalamus by 21%, in the nucleus accumbens by 24%, in the prefrontal cortex by 31%, and in the striatum by 23%. No significant differences were observed for n-3 PUFA or for the saturated and monounsaturated FAs. Our results confirm the existence of altered brain PUFA composition in an animal model of depression. The finding of increased AA, an n-6 PUFA, rather than decreased n-3 PUFA, emphasizes the importance of both PUFA families in the pathophysiological processes underlying depression. The FSL rat is a useful tool for further elucidation of the FA disturbances in depression.

Variability of the mesolimbic neuronal activity in a rat model of depression

The Flinders Sensitive Line of rats is a widely accepted and validated model of depression. These rats demonstrate abnormalities in limbic dopamine neurotransmission, suggesting disturbed neuronal activity in the ventral tegmental area. Interspike interval time series were recorded from the ventral tegmental area of the control Sprague-Dawley and Flinders Sensitive Line rats. These data were analyzed for the variance of interspike interval for each group of animals. We found that FSL rats show a significant decrease in the variance of 0.25-0.5-s-long interspike intervals. Moreover, these abnormalities were normalized following 14-day treatment with desipramine. We suggest that the interspike intervals at this range may have an important role in the information encoding of mesolimbic dopaminergic activity. Impaired variance of the length of interspike intervals in this area may correspond to the pathophysiology of depression, and hence be a possible marker for the analysis of the efficiency of antidepressant treatment.

Hyperfunctionality of serotonin-2C receptor-mediated inhibition of accumbal dopamine release in an animal model of depression is reversed by antidepressant treatment

Dopamine release in the nucleus accumbens mediates motivation and reward, making it a likely candidate to be involved in anhedonia, one of the major symptoms of depression. In the current study, alterations in basal extracellular dopamine levels and 5HT2C receptor-mediated inhibition of accumbal dopamine release in Flinders Sensitive Line (FSL) rats, an animal model of depression, were investigated. We found that FSL rats have decreased extracellular dopamine levels in the nucleus accumbens and an increased inhibitory-like effect of 5HT2C receptors on accumbal dopamine release. However, neither basal 5HT levels nor the accumbal 5HT response to the local 5HT2C receptor antagonist (RS 102221) differed between Sprague-Dawley and FSL rats. Seven-day treatment with the nefazodone (a serotonin/noradrenaline reuptake inhibitor and 5HT2C antagonist) as well as 7-day and 14-day treatments with a tricyclic antidepressant desipramine increased extracellular dopamine levels in the nucleus accumbens of FSL rats. However, only 14-day treatment with desipramine or 7-day treatment with nefazodone, but not 7-day treatment with desipramine, decreased 5HT2C receptor-mediated inhibition of accumbal dopamine release. Based on a possible correlation between the onset of 5HT2C receptor-mediated inhibition and the behavioral effects of desipramine and nefazodone treatment that was described in our previous studies, we suggest that 5HT2C receptor activation may be important for the onset of the behavioral effects of antidepressant treatment.

Blunted response to cocaine in the Flinders hypercholinergic animal model of depression

The Flinders sensitive line (FSL) rat is a proposed genetic hypercholinergic animal model of human depression. Considering the strong comorbidity between depression and cocaine dependence we investigated the well-documented behavioral and molecular effects of cocaine in the FSL and their control Flinders resistant line (FRL) rats. First, we found no difference between the two lines to establish cocaine self-administration; both lines reached stable responding within 10 days of training at a fixed ratio-1 schedule of reinforcement (1.5 mg/kg/injection). However, the FSL rats exhibited reduced cocaine intake at a dose of 0.09 mg/kg/injection in a within-session dose-response curve (0.02, 0.09, 0.38, 1.5 mg/kg/injection). Second, we examined the effects of repeated cocaine administration on locomotor activity, dopamine overflow and striatal prodynorphin mRNA expression. We found the FSL rats to be low responders to novelty and to exhibit less locomotor activation after repeated cocaine administration (30 mg/kg, i.p., daily injections for 10 days) than their controls. Microdialysis sampling from the nucleus accumbens shell revealed no significant difference in the dopamine overflow between the rat lines, neither during baseline nor after cocaine stimulation. Postmortem analyses of striatal prodynorphin mRNA expression (using in situ hybridization histochemistry) revealed a differentiated response to the cocaine exposure. In contrast to control FRL rats, the FSL rats showed no typical cocaine-evoked elevation of prodynorphin mRNA levels in rostral subregions of the striatum whereas both strains expressed increased prodynorphin mRNA levels in the caudal striatum after cocaine administration. In conclusion, the FSL animal model of depression demonstrates marked blunting of the locomotor and dynorphin neuroadaptative responses to cocaine in accordance with its enhanced cholinergic sensitivity.

Dimensional complexity of the neuronal activity in a rat model of depression

The Flinders sensitive line of rats is a widely accepted and validated model of depression. These rats demonstrate abnormalities in limbic dopamine neurotransmission, suggesting disturbed neuronal activity in the ventral tegmental area. Interspike interval time-series were recorded from the ventral tegmental area of control Sprague-Dawley and Flinder sensitive line rats. These data were analyzed using standard measures (mean firing rate, bursting activity and spectral analysis) as well as the pointwise correlation dimension, a nonlinear measurement characterizing the complexity degree of dynamic systems. Pointwise correlation dimension, but not standard analysis revealed a significant difference between the animal lines. Our results suggest that nonlinear analyses can detect dysregulation of the mesolimbic dopaminergic system and shed light on the pathophysiology of depression.

Antidepressant effects of citalopram and CRF receptor antagonist CP-154,526 in a rat model of depression

Due to the interest in the antidepressant potential of nonpeptide corticotropin-releasing factor (CRF)(1) receptor antagonists, the present investigation examined the antidepressant-like effects of the CRF(1) receptor antagonist CP-154,526 on the exaggerated swim test immobility in the Flinders Sensitive Line (FSL) rat, a genetic animal model of depression. Chronic treatment with CP-154,526 (10 mg/kg; 2x day) for 14 days increased swimming in the Flinders Sensitive Line rats. Citalopram (5 and 10 mg/kg; 2x day) and desipramine (5 mg/kg; 1x day) also significantly increased swimming in the Flinders Sensitive Line rats, as expected. However, neither CP-154,526 nor citalopram (10 mg/kg) altered swimming times in the control Flinders Resistant Line (FRL) rats. Citalopram (10 mg/kg) and CP-154,526 also increased the abnormally low level of social interaction behavior in the Flinders Sensitive Line rats. These findings indicate that citalopram and CP154,526, a CRF(1) receptor antagonist, have both antidepressant and anxiolytic effects that can be detected in an experimental model of depression only and not in "normal" control animals.

The serotonin-dopamine interaction is critical for fast-onset action of antidepressant treatment: in vivo studies in an animal model of depression

In the last decade, many new antidepressants have been developed that display a more rapid onset to clinical effects than classical antidepressants. However, the mechanism that enables some drugs to have a faster onset of action than others is poorly understood. The aim of the present study was to determine neural alterations that are specific to fast-acting antidepressant action using Flinders Sensitive Line (FSL) rats, an animal model of depression. Because of the central role of accumbal dopamine in the mediation of motivation and reward, our measurements were focused on dopaminergic neurotransmission in the nucleus accumbens (NAC). The authors found that 7-day treatment with nefazodone (a putative fast-onset antidepressant) but not with desipramine (a classical antidepressant) normalized immobility time in the swim test in FSL rats. Serotonin (5-HT)-induced dopamine release but not basal dopamine levels correlated with the improvement of depressive-like behavior. The authors conclude that the 5-HT-dopamine interaction is critical to the fast-onset action of antidepressant treatment.

Transplantation of glial cell line-derived neurotrophic factor-expressing cells into the striatum and nucleus accumbens attenuates acquisition of cocaine self-administration in rats

Neurotrophic factors, such as glial cell line-derived neurotrophic factor (GDNF), may play a role in drug-induced biochemical and behavioural adaptations that characterize addiction. We found that GDNF mRNA levels are lower in the striatum of rats that chronically self-administered cocaine. Therefore, we examined the effect of transplanted cells used as a biodelivery system for GDNF on cocaine self-administration in rats. A human astrocyte-like cell line, which produces and excretes GDNF, was transplanted into the striatum and nucleus accumbens of rats. These rats showed a significantly lower number of active lever presses in the cocaine self-administration paradigm compared with control rats. Moreover, rats that received a chronic infusion of GDNF via a micro-osmotic pump also exhibited weak cocaine self-administration. Therefore, we conclude that exogenous augmentation of GDNF repositories may be useful in suppressing cocaine self-administration.

Endocrine withdrawal syndromes

Hypersecretion of endogenous hormones or chronic administration of high doses of the same hormones induces varying degrees of tolerance and dependence. Elimination of hormone hypersecretion or discontinuation of hormone therapy may result in a mixed picture of two syndromes: a typical hormone deficiency syndrome and a generic withdrawal syndrome. Thus, hormones with completely different physiological effects may produce similar withdrawal syndromes, with symptoms and signs reminiscent of those observed with drugs of abuse, suggesting shared mechanisms. This review postulates a unified endocrine withdrawal syndrome, with changes in the hypothalamic-pituitary-adrenal axis and the central opioid peptide, in which noradrenergic and dopaminergic systems of the brain act as common links in its pathogenesis. Long-term adaptations to hormones may involve relatively persistent changes in molecular switches, including common intracellular signaling systems, from membrane receptors to transcription factors. The goals of therapy are to ease withdrawal symptoms and to expedite weaning of the patient from the hormonal excess state. Clinicians should resort to the fundamentals of tapering hormones down over time, even in the case of abrupt removal of a hormone-producing tumor. In addition, the prevention of stress and concurrent administration of antidepressants may ameliorate symptoms and signs of an endocrine withdrawal syndrome.

Decreased limbic vesicular monoamine transporter 2 in a genetic rat model of depression

Psychopharmacological and neurochemical research suggests that alterations in monoamine transporters may be involved in the etiology of depression. We studied the expression of the brain-type vesicular monoamine transporter (VMAT2) in the Flinders sensitive line (FSL) rats, which represent a genetic animal model for clinical depression in humans. VMAT2 expression at the protein level was assessed by autoradiography using [(3)H]dihydrotetrabenazine ([(3)H]TBZOH) binding, in the prefrontal cortex, the striatum and its subregions, nucleus accumbens (NAC), ventral tegmental area (VTA) and the substantia nigra pars compacta (SNC). The VMAT2 mRNA level was analyzed by in situ hybridization, in the VTA, SNC and the dorsal raphe (DR) nucleus. Reduced levels of [(3)H]TBZOH binding were detected in the striatum and its subregions, NAC shell but not in the NAC core. A marked reduction of 21% was found in the VTA while only a slight reduction (13%) was observed in the SNC. The reduced levels of VMAT2 protein binding capacity were not accompanied by a parallel alteration in VMAT2 mRNA levels in the VTA, the SNC and the DR. Since the VMAT2 is responsible for the intracellular storage and regulated release of monoamines, the reduced [(3)H]TBZOH binding levels in limbic brain regions of FSL rats may imply a reduced density of vesicular monoamine transporters, which can result in reduced monoamine transmission. Such reduction in the limbic neurotransmission, especially in NAC shell and VTA regions, may be involved in the depressive features of anhedonia and lack of motivation reported in the FSL rats.

Electroconvulsive stimuli alter nerve growth factor but not brain-derived neurotrophic factor concentrations in brains of a rat model of depression

Nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) are proteins involved in neuronal survival and plasticity of dopaminergic, cholinergic and serotonergic neurons in the central nervous system (CNS). Moreover, it has been hypothesized that these molecules play a role in the pathophysiology as well as treatment of depression. Using an animal model of depression, the Flinders Sensitive Line (FSL) rats and their controls, the Flinders Resistant Line (FRL), we investigated the effects of electroconvulsive stimuli (ECS) on brain NGF and BDNF. ECS or SHAM ECS were administered eight times, with a 48-h interval between each treatment. NGF and BDNF were measured with enzyme-linked immunosorbent assay (ELISA). In the hippocampus ECS increased NGF concentration in FSL but not FRL rats. ECS decreased NGF concentration in the frontal cortex of FSL rats. In both FSL and FRL rats ECS increased NGF levels in the striatum. In contrast, ECS did not change BDNF concentration in hippocampus, frontal cortex and striatum of FSL and FRL rats. Our data support the notion that neurotrophin concentrations may be altered by ECS.

Impaired release of beta-endorphin in response to serotonin in a rat model of depression

Involvement of both the serotonergic and the endogenous opioid systems in the onset of depressive behavior has been suggested. Previously we showed that serotonin (5-hydroxytryptamine) facilitates beta-endorphin release in the nucleus accumbens (NAcc). Herein, the microdialysis method was used to assess in vivo the effects of serotonin on beta-endorphin release in a rat model of depressive behavior (the Flinders sensitive line, FSL), before and after antidepressant treatment. The basal extracellular level of beta-endorphin in the NAcc of FSL rats did not differ significantly from that in control rats. However, serotonin-induced beta-endorphin release was impaired in FSL rats. Chronic treatment (18 days) with desipramine or paroxetine did not significantly affect the extracellular levels of beta-endorphin in the NAcc of either the FSL or control rats. However, the chronic antidepressant treatment did normalize the serotonin-induced release of beta-endorphin in FSL rats, as well as their behavioral manifestation of depressive behavior. Our results show that depressive behavior may relate to an impaired effect of serotonin on beta-endorphin release in the NAcc in a rat model of depression, and suggest a possible new mode of action of antidepressant drugs.

Can't shake that feeling: event-related fMRI assessment of sustained amygdala activity in response to emotional information in depressed individuals

BACKGROUND

Previous research suggests that depressed individuals engage in prolonged elaborative processing of emotional information. A computational neural network model of emotional information processing suggests this process involves sustained amygdala activity in response to processing negative features of information. This study examined whether brain activity in response to emotional stimuli was sustained in depressed individuals, even following subsequent distracting stimuli.

METHODS

Seven depressed and 10 never-depressed individuals were studied using event-related functional magnetic resonance imaging during alternating 15-sec emotional processing (valence identification) and non-emotional processing (Sternberg memory) trials. Amygdala regions were traced on high-resolution structural scans and co-registered to the functional data. The time course of activity in these areas during emotional and non-emotional processing trials was examined.

RESULTS

During emotional processing trials, never-depressed individuals displayed amygdalar responses to all stimuli, which decayed within 10 sec. In contrast, depressed individuals displayed sustained amygdala responses to negative words that lasted throughout the following non-emotional processing trials (25 sec later). The difference in sustained amygdala activity to negative and positive words was moderately related to self-reported rumination.

CONCLUSIONS

Results suggest that depression is associated with sustained activity in brain areas responsible for coding emotional features.

Depression as a spreading adjustment disorder of monoaminergic neurons: a case for primary implication of the locus coeruleus

A model for the pathophysiology of depression is discussed in the context of other existing theories. The classic monoamine theory of depression suggests that a deficit in monoamine neurotransmitters in the synaptic cleft is the primary cause of depression. More recent elaborations of the classic theory also implicitly include this postulate, other theories of depression frequently prefer to depart from the monoamine-based model altogether. We suggest that the primary defect emerges in the regulation of firing rates in brainstem monoaminergic neurons, which brings about a decrease in the tonic release of neurotransmitters in their projection areas, an increase in postsynaptic sensitivity, and concomitantly, exaggerated responses to acute increases in the presynaptic firing rate and transmitter release. It is proposed that the initial defect involves, in particular, the noradrenergic innervation from the locus coeruleus (LC). Dysregulation of the LC projection activities may lead in turn to dysregulation of serotonergic and dopaminergic neurotransmission. Failure of the LC function could explain the basic impairments in the processing of novel information, intensive processing of irrational beliefs, and anxiety. Concomitant impairments in the serotonergic neurotransmission may contribute to the mood changes and reduction in the mesotelencephalic dopaminergic activity to loss of motivation, and anhedonia. Dysregulation of CRF and other neuropeptides such as neuropeptide Y, galanin and substance P may reinforce the LC dysfunction and thus further weaken the adaptivity to stressful stimuli.

An automated analysis of rat behavior in the forced swim test

The Porsolt forced swim test (FST) is a commonly used paradigm to evaluate antidepressant activity of drugs. This test is based on visual measurement of the rat's floating time (FT) in a tank filled with water. Here, we present an automated, accurate and faster method for estimating FT by the distance moved (DM) by the animal via the use of the Ethovision software in three separate experiments. Experiment 1 investigated the effect of varying delays (24-h and 7-day) between pretest and test on FT and DM. Experiment 2 aimed at examining the effects of a 2-day withdrawal period in rats sensitized to amphetamine and cocaine, on FT and DM. Finally, Experiment 3 looked at the effects of desipramine and fluoxetine on FT and DM. The results of these experiments show that increasing the delay between pretest and test reduced FT during subsequent exposure (test). In addition, rats sensitized to and then withdrawn from either amphetamine or cocaine did not differ in FT or DM compared with control rats. Finally, both desipramine and fluoxetine reduced FT and increased DM. Furthermore, DM was consistently significantly negatively correlated with FT. These results support the use of an automated method for the evaluation of rat behavior in FST.

Limbic dopaminergic adaptation to a stressful stimulus in a rat model of depression

The dopaminergic mesolimbic system has a key role in motivation and reward, and stressful stimuli appear to alter its functionality. Since stress is considered to be one of the primary factors that mediate the expression of depressive behavior, dopamine and its metabolites in the nucleus accumbens of control and Flinders Sensitive Line rats, an animal model of depression, were examined prior to and after a forced swim test. In both types of rats, the levels of dopamine metabolites markedly decreased after the forced swimming, albeit to different extents. In contrast, 60 min after the swim test, dopamine levels were elevated only in the control rats. The accumbal dopaminergic activity is discussed in relation to the behavior of 'depressed' and normal rat lines subjected to a stressful event.

A genetic rat model of cholinergic hypersensitivity: implications for chemical intolerance, chronic fatigue, and asthma

The fact that only some individuals exposed to environmental chemicals develop chemical intolerance raises the possibility that genetic factors could be contributing factors. The present communication summarizes evidence from a genetic animal model of cholinergic supersensitivity that suggests that an abnormal cholinergic system could be one predisposing genetic factor. The Flinders Sensitive Line (FSL) rats were established by selective breeding for increased responses to an organophosphate. It was subsequently found that these FSL rats were also more sensitive to direct-acting muscarinic agonists and had elevated muscarinic receptors compared to the selectively bred parallel group, the Flinders Resistant Line (FRL) rats, or randomly bred control rats. Increased sensitivity to cholinergic agents has also been observed in several human populations, including individuals suffering from chemical intolerance. Indeed, the FSL rats exhibit certain behavioral characteristics such as abnormal sleep, activity, and appetite that are similar to those reported in these human populations. In addition, the FSL rats have been reported to exhibit increased sensitivity to a variety of other chemical agents. Peripheral tissues, such as intestinal and airway smooth muscle, appear to be more sensitive to both cholinergic agonists and an antigen, ovalbumin. Hypothermia, a centrally mediated response, is more pronounced in the FSL rats after nicotine and alcohol, as well as agents that are selective for the dopaminergic and serotonergic systems. In some cases, the increased sensitivity has been detected in the absence of any changes in the receptors with which the drugs interact (dopamine receptors), while receptor changes have been seen in other cases (nicotine receptors). Therefore, there may be multiple mechanisms underlying the multiple chemical sensitivity-chemical intolerance of the FSL rats. An elucidation of these mechanisms may provide useful clues to those involved in chemical intolerance in humans.

The flinders sensitive line rats, a genetic model of depression, show abnormal serotonin receptor mRNA expression in the brain that is reversed by 17beta-estradiol

The possible link between estrogen and serotonin (5-HT) in depression was investigated using a genetic animal model of depression, the Flinders Sensitive Line (FSL) rats, in comparison to control Flinders Resistant Line rats. The mRNA levels of the estrogen receptor (ER) alpha and beta subtypes and the 5-HT(1A) and 5-HT(2A) receptors were analyzed in several limbic-related areas of ovariectomized FSL and FRL rats treated with 17beta-estradiol (0.15 microg/g) or vehicle. The FSL animals were shown to express significantly lower levels of the 5-HT(2A) receptor transcripts in the perirhinal cortex, piriform cortex, and medial anterodorsal amygdala and higher levels in the CA 2-3 region of the hippocampus. The only significant difference between the rat lines in ER mRNA expression was found in the medial posterodorsal amygdala, where the FSL rats showed lower ERalpha expression levels. Overall, estradiol treatment increased 5-HT(2A) and decreased 5-HT(1A) receptor mRNA levels in several of the examined regions of both lines. Thus, in many areas, estradiol was found to regulate the 5-HT receptor mRNA expression in the opposite direction to the alterations found in the FSL rats. These findings further support the implication of 5-HT receptors, in particular the 5-HT(2A) subtype, in the etiology of affective disorders. Moreover, the ability of estradiol to regulate the expression of the 5-HT(1A) and 5-HT(2A) receptor genes might account for the reported influence of gonadal hormones in mood and depression.

Serotonin-mediated increases in the extracellular levels of beta-endorphin in the arcuate nucleus and nucleus accumbens: a microdialysis study

Although the involvement of both endogenous opioid and serotonergic systems in modulation of pain and emotion was suggested, the neurochemical interaction between these systems in the brain has not previously been studied directly. Herein, the effects of the local application of serotonin (5-HT) and fluoxetine (a 5-HT reuptake inhibitor) on extracellular levels of beta-endorphin in the arcuate nucleus and nucleus accumbens were assessed in freely moving rats using in vivo microdialysis. The mean basal concentrations of beta-endorphin in dialysates obtained from the arcuate nucleus and nucleus accumbens were 259.9 and 143.3 pM, respectively. Specific lesion of the serotonergic system by 5,7-dihydroxytryptamine (5,7-DHT) caused a significant decrease in these dialysate beta-endorphin levels. When 5-HT (0.25-5 microM) was added to the perfusion solution, the levels of beta-endorphin in the dialysate from the arcuate nucleus increased (186-296% of baseline), in a concentration-dependent manner. In the nucleus accumbens, 0.5 and 2 microM 5-HT in the perfusion fluid did not affect the levels of beta-endorphin in the dialysate, whereas 5 and 10 microM 5-HT caused an increase of approximately 190% of baseline. When fluoxetine (250 microM) was present in the perfusing solution, the levels of beta-endorphin in the dialysates from the arcuate nucleus and nucleus accumbens increased two- to threefold. This effect was not obtained in the 5,7-DHT-lesioned rats. Thus, 5-HT, either endogenously or exogenously delivered, appears to facilitate the release of beta-endorphin in the arcuate nucleus and nucleus accumbens. This indication of an interaction between serotonergic and endorphinic systems may be relevant for assessing pain and mood disorder circuits and the mode of action of antidepressant drugs.

Sidedness of the ATP-Na+-K+ interactions with the Na+ pump in squid axons

Using dialysed squid axons we have been able to control internal and external ionic compositions under conditions in which most of the Na+ efflux goes through the Na+ pump. We found that (i) internal K+ had a strong inhibitory effect on Na+ efflux; this effect was antagonized by ATP, with low affinity, and by internal Na+, (ii) a reduction in ATP levels from 3 mM to 50 microM greatly increased the apparent affinity for external K+, but reduced its effectiveness compared with other monovalent cations, as an activator of Na+ efflux, and (iii) the relative effectiveness of different K+ congeners as external activator of the Na+ efflux, though affected by the ATP concentration, was not affected by the Na+/K+ ratio inside the cells. These results are consistent with the idea that the same conformation of the (Na+ + K+)-ATPase can be reached by interaction with external K+ after phosphorylation and with internal K+ before rephosphorylation. They also stress a nonphosphorylating regulatory role of ATP.