The neurochemical effects of anxiolytic drugs are dependent on rearing conditions in Fawn-Hooded rats

Repeated social defeat-induced depression-like behavioral and biological alterations in rats: involvement of cholecystokinin

Cholecystokinin (CCK) involvement in depression-like disorders is poorly documented. Here, we investigated whether CCKergic neurotransmission is relevant to depressive-like symptoms and antidepressant therapy using a novel preclinical model based on repeated social defeat over 4 weeks in rats. Repeated social defeat triggers changes that could be considered as behavioral and biological correlates of depressive symptoms in humans, such as a hyperactivity of hypothalamic-pituitary-adrenal (HPA) axis (increase of serum corticosterone levels and of adrenal gland weight), increased immobility time in the forced swimming test (FST), decrease of body weight and of sweet water consumption and reduction of hippocampal volume associated with a decreased cell proliferation in the dentate gyrus. In addition, in vivo microdialysis showed that cortical CCK release was tonically increased in defeated rats. Chronic imipramine treatment (16 mg kg(-1) per day for 25 days) prevented both the repeated social defeat-induced alterations of biological and behavioral parameters and the associated increase of cortical CCK release. Chronic blockade of CCK2 receptors by the specific antagonist CI-988 (1 mg kg(-1) per day for 25 days) also normalized immobility time in the FST and prevented HPA axis hyperactivity, reduction of hippocampal volume and cell proliferation and decreased sweet water intake normally evoked by repeated social defeat. These data showed that the repeated social-defeat paradigm can be considered as a suitable model of 'depression' in rats. The causal link between social defeat-evoked (1) increase in cortical CCKergic neurotransmission and (2) depression-like symptoms that we highlighted here strongly suggests that CCKergic systems may be a relevant target for novel antidepressant therapy.

Persistent downregulation of hippocampal CREB mRNA parallels a Y-maze deficit in adolescent rats following semi-chronic amphetamine administration

BACKGROUND AND PURPOSE

We investigated possible differences in the impact of chronic amphetamine administration during adolescence and adulthood on aspects of behaviour and brain chemistry.

EXPERIMENTAL APPROACH

Adult (n=32) and adolescent (n=32) male Sprague-Dawley rats were given either D-amphetamine sulphate (10 mg kg(-1) daily, i.p.) or saline (1 mL kg(-1), i.p.) for 10 days. Rats were subsequently tested for anxiety-like behaviour, learning and memory, and sensorimotor gating. Nine weeks later, rats received saline (1 mL kg(-1)) or acute amphetamine challenge (1.5 mg kg(-1)) and the expression levels of mRNA for tyrosine kinase B (TrkB) or cAMP response element-binding protein (CREB) were measured in the hippocampus.

KEY RESULTS

The adolescent amphetamine pretreated group revealed a deficit in exploration on the Y-maze during a 6 h retention test. The frequency of visits to the novel arm was 35% lower for the amphetamine group compared with controls. In parallel, a 43% decrease in hippocampal CREB mRNA, but not TrkB mRNA, was observed in periadolescent rats treated chronically with amphetamine 9 weeks earlier. None of the effects were detected in the adult treated cohort.

CONCLUSIONS AND IMPLICATIONS

Chronic amphetamine treatment during periadolescence resulted in altered behaviour on the Y-maze and persistent downregulation of hippocampal CREB mRNA expression. Given that this group had intact spatial learning and reference memory, it would appear that the deficits observed on the Y-maze reflect a dysfunction in response to novelty. Because no effects of amphetamine treatment were observed in the adult cohort, these data suggest idiosyncratic sensitivity of periadolescence to the long-term effects of psychostimulants.

The CRF1 receptor antagonist, antalarmin, reverses isolation-induced up-regulation of dopamine D2 receptors in the amygdala and nucleus accumbens of fawn-hooded rats

We have previously shown that Fawn-Hooded (FH) rats reared in isolation display an anxiety-like phenotype and an enhanced acquisition of ethanol seeking behaviour. Furthermore, antalarmin, a selective corticotrophin-releasing factor type 1 (CRF1) receptor antagonist, reduces isolation-induced acquisition and maintenance of volitional ethanol consumption in this strain. The aim of this study was to investigate the ability of CRF1 receptor antagonism by antalarmin to impact upon brain chemistry in both isolated and group-housed FH rats. To achieve this, FH rats were reared, from weaning, in either group-housed or isolation-housed conditions and at 12 weeks of age were treated with antalarmin (20 mg/kg, i.p; n = 10 per group) or vehicle (1 mL/kg, i.p; n = 10 per group) bi-daily for ten consecutive days before being killed and their brains removed for neurochemical analyses. Autoradiography and in situ hybridization was employed to analyse changes in the dopaminergic and neurotrophin systems. Isolation rearing increased dopamine D2 receptor density in the central amygdala and nucleus accumbens, an effect reversed by antalarmin treatment. Conversely, treatment with antalarmin had no impact upon the isolation-induced alterations of the mRNA encoding brain-derived neurotrophic factor or the TrkB receptor. Collectively, these findings demonstrate that multiple signalling systems are susceptible to modulation by social isolation and that antalarmin can reverse some, but not all, isolation-induced alterations in brain chemistry.

Chronic corticotropin-releasing factor type 1 receptor antagonism with antalarmin regulates the dopaminergic system of Fawn-Hooded rats

Corticotropin-releasing factor is a neuropeptide associated with the integration of physiological and behavioural responses to stress and also in the modulation of affective state and drug reward. The selective, centrally acting corticotropin-releasing factor type 1 receptor antagonist, antalarmin, is a potent anxiolytic and reduces volitional ethanol consumption in Fawn-Hooded rats. The efficacy of antalarmin to reduce ethanol consumption increased with time, suggestive of adaptation to reinforcement processes and goal-directed behaviour. The aim of the present study was to examine the effects of chronic antalarmin treatment on reward-related regions of Fawn-Hooded rat brain. Bi-daily antalarmin treatment (20 mg/kg, i.p.) for 10 days increased tyrosine hydroxylase messenger RNA expression throughout the ventral mesencephalon. Following chronic antalarmin the density of dopaminergic terminals within the basal ganglia and amygdaloid complex were reduced, as was dopamine transporter binding within the striatum. Receptor autoradiography indicated an up-regulation of dopamine D2, but no change in D1, binding in striatum, and Golgi-Cox analysis of striatal medium spiny neurones indicated that chronic antalarmin treatment increased spine density. Thus, chronic antalarmin treatment modulates dopaminergic pathways and implies that chronic treatment with drugs of this class may ultimately alter postsynaptic signaling mechanisms within the basal ganglia.

Reduced 5-HT3 receptor binding and lower baseline plus maze anxiety in the alcohol-preferring inbred fawn-hooded rat

The present investigation sought to explore the relationship between the 5-HT(3) receptor and anxiety-like behavior in fawn-hooded (FH/Wjd) rats, an inbred strain that exhibits a high intake and preference for ethanol, and the alcohol-nonpreferring ACI/N strain. Using quantitative autoradiography, we examined whether there were differences in central 5-HT(3) receptor binding in FH/Wjd versus ACI/N rats. Ten to 14 days prior to being used in the autoradiographic studies, rats were first confirmed to be representative of their strains by subjecting them to a two-bottle choice procedure for 2 weeks. The binding of [3H]LY 278584 to 5-HT(3) receptors was significantly reduced in frontal cortex, CA1 region of hippocampus, and in the medial and lateral nuclei of the amygdala of FH/Wjd versus ACI/N rats. In the anterior cingulate cortex and in the dentate gyrus region of the hippocampus the reduction in [3H]LY 278548 binding in the FH/Wjd versus ACI/N strain (40% and 41%, respectively) did not reach statistical significance. In a separate group of animals, the effects of the 5-HT(3) receptor antagonist MDL 72222 (3 mg/kg ip) on anxiety-related behaviors were assessed in the elevated plus maze. In vehicle-treated rats, the FH/Wjd strain exhibited significantly greater percent of time spent on the open arms and percent open arm entries, an indication of less anxiety. Pretreatment with MDL 72222 did not alter these behaviors in the FH/Wjd rats, but had an anxiolytic-like effect in the ACI/N strain, significantly increasing the percent of time spent on the open arms and percent open arm entries. Further research into 5-HT(3) receptor function in the alcohol-preferring FH/Wjd rats is needed to elucidate the relationship among 5-HT(3) receptors, alcohol drinking, and anxiety.