The role of brain-derived neurotrophic factor in learned fear processing: an awake rat fMRI study

Brain‐derived neurotrophic factor (BDNF) signaling is implicated in the etiology of many psychiatric disorders associated with altered emotional processing. Altered peripheral (plasma) BDNF levels have been proposed as a biomarker for neuropsychiatric disease risk in humans. However, the relationship between peripheral and central BDNF levels and emotional brain activation is unknown. We used heterozygous BDNF knockdown rats (BDNF^+/−) to examine the effects of genetic variation in the BDNF gene on peripheral and central BDNF levels and emotional brain activation as assessed by awake functional magnetic resonance imaging (fMRI). BDNF^+/− and control rats were trained to associate a flashing light (conditioned stimulus; CS) with foot‐shock, and brain activation in response to the CS was measured 24 h later in awake rats using fMRI. Central and peripheral BDNF levels were decreased in BDNF^+/− rats compared with control rats. Activation of fear circuitry (amygdala, periaqueductal gray, granular insular) was seen in control animals; however, activation of this circuitry was absent in BDNF^+/− animals. Behavioral experiments confirmed impaired conditioned fear responses in BDNF^+/− rats, despite intact innate fear responses. These data confirm a positive correlation [r = 0.86, 95% confidence interval (0.55, 0.96); P = 0.0004] between peripheral and central BDNF levels and indicate a functional relationship between BDNF levels and emotional brain activation as assessed by fMRI. The results demonstrate the use of rodent fMRI as a sensitive tool for measuring brain function in preclinical translational studies using genetically modified rats and support the use of peripheral BDNF as a biomarker of central affective processing.

Time-dependent induction of anxiogenic-like effects after central infusion of urocortin or corticotropin-releasing factor in the rat

RATIONALE

Corticotropin-releasing factor (CRF) and urocortin (Ucn) belong to the CRF-related family, share a high degree of structural homology and bind to CRF receptors. However, compared with CRF, Ucn was shown to display either weaker or similar anxiogenic-like effects in vivo.

OBJECTIVE

To compare the anxiogenic-like responses of rats injected intracerebroventricularly (ICV) with different doses of either rat/human CRF (r/hCRF) or rat Ucn (rUcn) at different intervals after injection.

METHODS

Rats were tested on three validated paradigms of emotional behavior [i.e. elevated plus-maze (EPM), defensive withdrawal (DW) and conflict test (CT)] 5 and 30 min after treatment.

RESULTS

In the EPM test only r/hCRF, but not rUcn, produced anxiogenic-like effects at the dose of 1.0 microg, when the peptides were injected 5 min before testing. At 30 min after injection, both peptides caused a significant reduction of open arms exploration, rUcn being effective at 0.01 microg. In the DW test both peptides were equally potent in decreasing the exploratory behavior and increasing the time spent in the chamber at the dose of 1.0 microg when tested 30 min after injection. In the CT both rUcn (0.25-1.0 microg) and r/hCRF (0.75-1.0 microg) decreased significantly the responding in the punished component. However, rUcn reduced food responding also in the unpunished component possibly due to its powerful anorectic activity.

CONCLUSIONS

Comparison of anxiogenic-like activities of r/hCRF and rUcn at doses up to 1.0 microg revealed striking differential effects that depended on the time of testing after ICV peptide injection, and on the paradigm of anxiety used. These results suggest that the onset of r/hCRF and rUcn actions related to behavioral responses to anxiety is likely to depend on brain peptide-specific mechanisms including binding properties to CRF-receptors, differential distribution to specific functional brain sites and the distribution and effectiveness of binding-protein interactions.

A dopaminergic mechanism is involved in the 'anxiogenic-like' response induced by chronic amphetamine treatment: a behavioral and neurochemical study

The purpose of this study was to examine the influence of chronic d-amphetamine (AMPH) treatment (2 mg/kg i.p., for 9 consecutive days) on behavioral and neurochemical responses to a subsequent exposure - 4 days after the last AMPH injection--to the elevated plus-maze (EPM), as well as to determine the involvement of a dopaminergic mechanism in that influence. Results showed that chronic AMPH treatment induced an 'anxiogenic-like' response when animals were evaluated in the EPM test. Pretreatment with either haloperidol (HAL, 1 mg/kg i.p., 20 min prior to each injection) or SCH-23390 (0.1 mg/kg i.p., 10 min prior to each injection) completely abolished the chronic AMPH-induced 'anxiogenic-like' effect displayed in the EPM test. However, sulpiride pretreatment (60 mg/kg i.p., 10 min prior to each AMPH injection) did not modify such effect. In addition, rats treated with AMPH and subsequently exposed to the EPM, showed a decrease in the maximal GABA-stimulated chloride uptake in cortical microsacs. HAL pretreatment restored the maximal chloride uptake induced by chronic AMPH. Altogether, these results suggest that: (1) previous exposure to chronic AMPH treatment induces an increased emotional response following a conflict situation, (2) dopamine D(1) receptors are mainly involved in chronic AMPH-induced changes in the behavior displayed in EPM test, and (3) an interaction between GABAergic and dopaminergic mechanisms may be implicated in neurochemical and behavioral changes induced by chronic AMPH treatment.

Altered amygdalar CRF release and increased anxiety-like behavior in Sardinian alcohol-preferring rats: a microdialysis and behavioral study

BACKGROUND

Dysregulation of the stress-regulatory corticotropin-releasing factor (CRF) system in the central nucleus of the amygdala (CeA) may be a factor in genetically determined alcohol preference.

METHODS

To test this hypothesis, basal and restraint stress-induced CRF efflux in the CeA was determined by microdialysis in Sardinian alcohol-preferring (sP) and nonpreferring (sNP) rats. In addition, differences in anxiety-like behavior between sP and sNP rats were evaluated by using the elevated plus maze and conditioned defensive burying tests.

RESULTS

Basal dialysate CRF levels in the CeA were elevated in the alcohol-preferring line (sP, 281.2+/-83.96 pg/ml; sNP, 70.2+/-16.76 pg/ml; p < 0.05). In contrast, no differences in whole-tissue CRF content in the CeA were observed (sP, 1143+/-142 ng/mg protein; sNP, 1181+/-139 ng/mg protein). Restraint stress elevated CRF dialysate concentrations in both sP and sNP rats. Rats of the sP line exhibited more anxiety-like behavior than sNP rats in the elevated plus maze but not in the conditioned defensive burying test.

CONCLUSIONS

The results suggest that ethanol-preferring sP rats show a dysregulation in basal CRF release within the CeA that may, in turn, heighten ethanol intake and increase susceptibility to anxiogenic stimuli in these animals.

Behavioral effects of central administration of the novel CRF antagonist astressin in rats

Astressin, a novel corticotropin releasing factor (CRF) antagonist, has been found to be particularly potent at inhibiting the hypothalamo-pituitary-adrenal axis. The aim of the present study was to determine the effects in rats of astressin in attenuating the anxiogenic-like response produced by social stress and intracerebroventricular (ICV) CRF administration on the elevated plus-maze, and ICV CRF-induced locomotor activation in the rat. Astressin significantly reversed the anxiogenic-like response induced by both social stress and ICV rat/humanCRF (r/hCRF) on the elevated plus-maze, but failed to block the effects of r/hCRF-induced locomotor activity in a familiar environment. When these results were compared to previous studies performed with the same paradigms using other CRF antagonists, astressin showed effects similar to those of D-PheCRF(12-41) on plus-maze performance. However, contrary to alpha-helicalCRF(9-41) and D-PheCRF(12-41), astressin had no effect on CRF-induced locomotor activity. These results suggest that astressin may have a unique anti-CRF profile compared to previously tested antagonists.

Corticotropin-releasing factor antagonist attenuates the "anxiogenic-like" effect in the defensive burying paradigm but not in the elevated plus-maze following chronic cocaine in rats

RATIONALE

Chronic cocaine abuse is associated with the development of anxiogenic states in humans. Corticotropin-releasing factor (CRF) is an endogenous neurotropic factor well known to modulate stress responses. It has been postulated that CRF is involved in the neurobiological mechanisms underlying the anxiety and/or stress responses associated with removal of cocaine after chronic administration.

OBJECTIVE

The present study investigated the role of endogenous CRF in mediating the "anxiety-like" effect 48 h after the cessation of saline or chronic cocaine treatment in rats, using the defensive burying paradigm and the elevated plus-maze.

METHODS

Rats received daily injections of cocaine (20 mg/kg IP, for 14 consecutive days) or vehicle. Forty-eight hours after the last injection, animals were tested in the plus-maze and then in the defensive burying paradigm. In a second experiment, intracerebroventricular (ICV) cannulae were implanted at the lateral ventricle. Animals were allowed a 1-week period for recovery before starting the chronic drug treatment. The defensive burying testing took place 48 h after cessation of the treatment. The CRF antagonist [DPhe12, Nle21,38, CalphaMe Leu37] r/h CRF(12-41), (also known as D-phe CRF(12-41)) (0.04, 0.2 and 1.0 microg/5 microl) was injected 5 min before the 15-min testing.

RESULTS

An "anxiogenic-like" effect following chronic cocaine treatment was demonstrated with the defensive burying paradigm, but not with the elevated plus-maze. This "anxiety-like" response was attenuated by ICV pretreatment with the CRF antagonist D-Phe CRF(12-41), with the highest dose of the CRF antagonist reversing the observed "anxiogenic-like" response.

CONCLUSIONS

These data suggest that brain CRF may be substantially involved in the development of "anxiety-like" responses related to cocaine withdrawal and could be important for future drug dependence treatments.

Feeding induced by GABA(A) receptor stimulation within the nucleus accumbens shell: regional mapping and characterization of macronutrient and taste preference

This study investigated the areas of the nucleus accumbens shell involved in the modulation of feeding behavior by GABAergic stimulation and characterized this response using macronutrient diets as well as saline, sucrose, and saccharin solutions. The GABA agonist muscimol induced a pronounced feeding response when infused in the medial nucleus accumbens shell but not in the ventral or lateral accumbens shell. In the macronutrient preference study, muscimol increased the intake of both high fat and high carbohydrate diets when presented separately. When both diets were available simultaneously, muscimol stimulated feeding of both diets to the same degree. Muscimol elicited a robust increase in the consumption of sucrose solution. However, no effect of muscimol was demonstrated for water, saline, or saccharin intake. These findings provide evidence for a selective role for GABA-sensitive neurons in the medial accumbens shell in the regulation of ingestive behavior and further suggest that GABA(A) receptors in this region do not modulate palatability, macronutrient selection, or rewarding properties of food.

Chronic amphetamine facilitates immunosuppression in response to a novel aversive stimulus: reversal by haloperidol pretreatment

The effect of chronic d-amphetamine sulfate (AMPH) treatment (nine daily injections, 2 mg/kg i.p.) on subsequent foot shock stress-induced immunological response was investigated. In addition, the potential role of a dopaminergic (DA) mechanism in the development of chronic AMPH-induced changes in stress-influenced immune responses was characterized. Exposure to foot shock stress decreased the percentage of T-lymphocytes, and reduced the delayed-type hypersensitivity reaction (DTH) in chronically AMPH-pretreated rats relative to vehicle-treated controls. Both of those stress-induced immunosuppressive responses were no longer evident when AMPH-pretreated rats were injected with haloperidol (HAL, 1 mg/kg i.p.) 30 min prior to each daily AMPH injection. The present findings are indicative of a modulatory role for dopamine in the facilitating process induced by AMPH on stress-induced immunosuppressive effects.

Appetite-suppressing effects of urocortin, a CRF-related neuropeptide

The neuropeptide corticotropin-releasing factor (CRF) is well known to act on the central nervous system in ways that mimic stress and result in decreases in exploration, increases in sympathetic activity, decreases in parasympathetic outflow, and decreases in appetitive behavior. Urocortin, a neuropeptide related to CRF, binds with high affinity to the CRF2 receptor, is more potent than CRF in suppressing appetite, but is less potent than CRF in producing anxiety-like effects and activation. Doses as low as 10 nanograms injected intracerebroventricularly were effective in decreasing food intake in food-deprived and free-feeding rats. These results suggest that urocortin may be an endogenous CRF-like factor in the brain responsible for the effects of stress on appetite.

Chronic restraint attenuates the immunosuppressive response induced by novel aversive stimuli

The exposure to a novel aversive event, such as foot shock, induced a decrease in the percentage of T lymphocytes and a clear reduction in the delayed-type hypersensitivity reaction (DTH). This immunosuppressive response to an acute stressor was absent in rats that were previously exposed to a chronic immobilization stress regime (2 h daily during 7 consecutive days), but was still present in animals with prior exposure to only one or three restraint sessions. No stress effect was observed in other immunologic parameters, such as the percentage of B lymphocytes or the hemagglutinin titer, in any of the experimental treatments. The possible involvement of central adaptive mechanisms in the attenuation of the immunosuppressive response induced by an acute stress is discussed.

Seven-day variable-stress regime alters cortical beta-adrenoceptor binding and immunologic responses: reversal by imipramine

Rats were submitted daily to a variable stressor for 1 week with or without concurrent imipramine (IMI) administration. One day after the last injection or stressful event, binding of cortical beta-adrenoceptors was determined in all experimental groups. Another group of chronically stressed animals with or without concurrent IMI administration were sacrificed 24 h following the last stress or injection treatment, and several immunologic parameters were evaluated. Chronically stressed rats showed an enhanced number of cortical beta-adrenergic sites without changes in their affinity. This effect was not present following concurrent administration with the antidepressant. In addition, a decreased percentage of T lymphocytes and a reduced delayed-type hypersensitivity reaction was also observed in stressed animals. Both responses were no longer evident when stressed rats were previously administered IMI. A possible link between behavioral, neurochemical, and immunologic alterations due to the stress regime is discussed.

Chronic variable stress facilitates tumoral growth: reversal by imipramine administration

The present study was conducted to examine whether a chronic variable stress procedure (CVS)--an animal model of depression--facilitates tumor growth, and whether this effect can be modified by concurrent administration of the antidepressant imipramine (IMI). Unstressed rats, with or without previous administration of the immunosuppressive agent cyclosporine (CS), were inoculated with 5 x 10(6) cells of a sarcoma. Another group of rats was inoculated with tumoral cells and later exposed to the CVS procedure with or without concurrent administration of IMI (10 mg/kg, i.p.). An additional group of animals was treated with CS and later given daily injections of IMI (10 mg/kg, i.p.) without stress manipulation. A lack of tumoral development was observed in unstressed animals without previous injections of CS, whereas, prior injections of this immunosuppressive agent increased tumoral growth in unstressed animals. Exposure to the CVS procedure facilitated tumoral growth even in animals without CS injections. This effect was clearly attenuated when chronically stressed rats were concurrently given IMI. These findings support the notion that the development of a tumoral process is facilitated when a state of experimental depression is induced and that the reversal of such a state by antidepressant treatment results in the inhibition of tumor development.

Perinatal undernutrition reduced ethanol intake preference in adult recovered rats

Adult female rats submitted to a protein deprivation schedule at perinatal age (from 14th day of fetal life until 50 days of age) were tested for alcohol intake in a preference test. When compared with control animals, experimental rats exhibited higher overall fluid intake. Nevertheless, in terms of ethanol preference these subjects evidenced lower preference to this drug. A test for assessing ethanol olfactory preference did not show any differences between control and experimental rats in basal conditions. However, after repeated exposure to alcohol, deprived rats showed an aversion to ethanol odor, while controls evidenced the opposite effect, i.e., heightened preference. Possible differences to the aversive effects of ethanol between control and experimental animals were assayed by means of two taste aversion tests, by associating alcohol to sucrose or NaCl. No differences were detected between both groups of rats. These results demonstrate that early undernutrition reduces ethanol preference in a free choice situation. Such an effect could be due, at least partially, to odor aversion developed by repeated exposure.

Effect of chronic variable stress on monoamine receptors: influence of imipramine administration

Adult male rats were exposed to a series of unpredictable stressors, a paradigm considered to be a model of experimental depression, with or without concurrent administration of imipramine. One day after the last stress event of the chronic regime, binding of cortical beta-adrenoreceptors and the behavioral serotonin (5-HT) syndrome induced by 5-methoxy-N,N,dimethyltryptamine (5-MeODMT) were determined in all the experimental groups. Stressed rats showed an "up-regulation" of cortical beta-adrenergic sites, while similar values to control rats were observed when stressed animals were administered imipramine. Regarding the behavioral 5-HT syndrome, comparable behavioral scores were observed between controls and chronically stressed rats. The combination of chronic exposure to different stressors with imipramine treatment resulted in a significant increase of forepaw treading and Straub tail scores. The probable facilitation of behavioral deficits induced by this scheme of chronic stress and the recovery following concurrent administration of imipramine are discussed.

Gangliosides enhance the anti-immobility response elicited by several antidepressant treatments in mice

Ganglioside pretreatment enhanced the anti-immobility effect induced in the forced swim test after a chronic treatment with desipramine, mianserin, clomipramine, nialamide or repeated electroconvulsive shock in mice. Gangliosides, which had no effect per se, showed a clear dose-response relationship in enhancing the anti-immobility effect of desipramine. These results suggest that, regardless of their mechanisms of action, gangliosides facilitate the behavioral response of several antidepressant treatments.