Brain-derived-neurotrophic-factor (BDNF) stress response in rats bred for learned helplessness

Molecular Psychiatry (2001) 6, 471–474  2001 Nature Publishing Group All rights reserved 1359-4184/01 $15.00 www.nature.com/mp ORIGINAL RESEARCH ARTICLE Brain-derived-neurotrophic-factor (BDNF) stress response in rats bred for learned helplessness B Vollmayr1, H Faust1, S Lewicka2 and FA Henn1 1 Central Institute of Mental Health, J5, D-68159 Mannheim; 2University of Heidelberg, Department of Pharmacology, Im Neuenheimer Feld 366, D-69120 Heidelberg, Germany Keywords: neuronal plasticity; mRNA; hippocampus; rats; inbred strains; corticosterone; depression Stress-induced elevation of glucocorticoids is accompanied by structural changes and neuronal damage in certain brain areas. This includes reduced expression of brain-derived neurotrophic factor (BDNF) in the hippocampus which can be prevented by chronic electroconvulsive seizures and antidepressant drug treatment. In the last years we have bred two strains of rats, one which reacts with congenital helplessness to stress (cLH), and one which congenitally does not acquire helplessness when stressed (cNLH). After being selectively bred for more than 40 generations these strains have lost their behavioural plasticity including their sensitivity to antidepressant treatment. We show here that in cLH rats, acute immobilization stress does not induce a reduction of BDNF expression in the hippocampus which is observed in Sprague–Dawley and cNLH rats. All animals tested exhibited elevated corticosterone levels when stressed, an indication, that in cLH rats regulation of BDNF expression in the hippocampal formation is uncoupled from corticosterone increase induced through stress. This may explain the lack of adaptive responses in this strain. Molecular Psychiatry (2001) 6, 471–474. Beside its classical function as a neurotrophin during development, BDNF (brain-derived neurotrophic factor) modulates synaptic plasticity1 and has been suggested to be involved in stress-induced hippocampal adaptation and pathogenesis of depression2 in the adult animal. This hypothesis is supported by both the fact that exogenous BDNF has an antidepressant effect,3 and that antidepressant treatment ameliorates stress-induced reduction of BDNF mRNA in the hippocampus.4 We have examined BDNF expression in rats with learned helplessness. Learned helplessness is a model for depression based on the hypothesis that depression is induced by uncontrollable stress in individuals with a predisposition. To enhance genetic predisposition Sprague–Dawley rats were selected for breeding based on their behavior in learned helplessness testing. Two strains evolved: rats with congenital helplessness (cLH) and rats with resistance to stressinduced helplessness (cNLH). cLH rats in particular show a markedly reduced behavioral variability to environmental stimuli: they have a deficit in learning under stressful conditions and furthermore do not respond to treatment with antidepressants.5 We were interested in molecular mechanisms underlying this deficit to adapt to stressful stimuli and, therefore, examined the stress-induced BDNF mRNA reduction of cLH rats with in situ hybridisation and compared it to the stress response of cNLH rats and outbred Sprague–Dawley rats as controls. BDNF mRNA was most highly expressed in hippocampal region CA3 and dentate gyrus (DG); lower expression was found in frontal, parietal and pyriform cortex and amygdala (see Image section, p 358) as reported previously.6 There were no differences of the BDNF mRNA baseline levels between strains. This finding is consistent with the results in Flinders Sensitive Line, a different genetic animal model for depression where baseline expression of BDNF and NGF were similar in the hippocampal formation of Flinders Sensitive Line and their controls Flinders Resistant Line.7 This does not argue against a role of the neurotrophins in the pathogenesis of depression, since neurotrophins in the adult animal are known for their prominent role in plasticity8 and constitutive levels may be unchanged while differences become visible only after stress challenges. In Sprague–Dawley rats acute immobilisation stress led to a significant BDNF mRNA reduction of 26% in DG and to a lesser degree in CA3, confirming previously reported results.4,9 cNLH rats, whose learned helplessness response resembles the parental Sprague–Dawley strain, showed comparable BDNF mRNA reductions after stress: 23% (DG) and 19% (CA3). In contrast, in the cLH strain BDNF mRNA levels were not reduced after immobilisation stress (a slight increase of BDNF mRNA after stress was not significant) (Figure 1). BDNF mRNA reduction after stress is dependent on corticosterone, which is highly elevated after immobilisation.9–11 We therefore determined corticosterone levels to find out if the missing BDNF response in cLH rats was due to a lack of corticosterone response after stress. As previously reported there were no statistically significant differences in baseline cortisol levels: (n = 6) in cLH, (mean ± SD) 13 ± 14 ␮g dl−1 18 ± 25 ␮g dl−1 (n = 6) in cNLH, 57 ± 56 ␮g dl−1 (n = 4) in SD. After 45 min immobilisation, corticosterone was elevated more than 10-fold in all strains: 225 ± 59 ␮g dl−1 (n = 5) in LH, 261 ± 71 ␮l dl−1 (n = 5) in cNLH, 248 ± 130 ␮g dl−1 (n = 4) in SD (Figure 2). Thus, the absence of a decrease of BDNF mRNA in LH rats BDNF stress response in learned helplessness B Vollmayr et al 472 Both possibilities are in agreement with previous findings in the cLH strain reporting an impaired stress response after early stress.14,15 Methods Inbred strains A breeding program was established (Department of Psychiatry, SUNY Stony Brook, NY, USA) with Sprague–Dawley rats being selected on the basis of their susceptibility to develop learned helplessness. In the 29th generation two strains emerged: cLH, which exhibit helpless behavior (⬎95% showed escape deficit) without preshock, and cNLH, almost completely acquiring stress control (⬍5% demonstrated escape deficit). Our colony was derived from founder animals of the 35th Stony Brook generation, animals from the 8th, 9th and 10th subsequent generation were used for the experiments. Figure 1 Effects of stress on BDNF mRNA in outbred Sprague–Dawley, cNLH and cLH strains. Quantification of cpm mm−2 with a microimager in CA3 (a) and dentate gyrus (DG) (b). Results are expressed as the mean of the percentage of the unstressed control group ± SEM. ⴱP ⬍ 0.05. Figure 2 Effects of 45-min immobilisation on plasma corticosterone in outbred Sprague–Dawley, cNLH and cLH strains. ⴱⴱⴱP ⬍ 0.001. was not due to a smaller corticosterone elevation after immobilisation, but rather due to a lack of effect of corticosterone on BDNF mRNA regulation. As high concentrations of corticosterone are required to reduce BDNF mRNA, the effect is thought to be mediated by the glucocorticoid receptor (GR).12 Probably, there is no direct action of GR on BDNF mRNA transcription, since there is no glucocorticoid regulat (...truncated)