Classical aversive conditioning induces increased expression of mature-BDNF in the hippocampus and amygdala of pigeons

Abstract

The expression of brain-derived neurotrophic factor (BDNF), which is found in the pro-BDNF, truncated-BDNF and mature-BDNF isoforms, changes with learning. Mature-BDNF shows a peak of late expression in the hippocampus that is involved in the persistence of aversive memory in rodents. However, the role of BDNF in the hippocampal synaptic mechanisms involved in the classical conditioning aversive memory in birds still needs clarification. This study investigated the late expression of BDNF in the hippocampus and amygdala of pigeons trained with tone-shock conditioning and the effects of intra-hippocampal infusion of anisomycin (Ani) in these changes. Seven days after implantation of intra-hippocampal microcannulae, adult pigeons trained with three tone-shock pairings were assigned to one of three groups: Conditioning and Ani (CondANI), Conditioning and saline vehicle (CondSAL) and Conditioning only (Cond). NAIVE group had no treatment or conditioning. Homogenates of tissues from the hippocampus and amygdala, obtained 12h after training, were used to determine the content of mature-BDNF, truncated-BDNF and pro-BDNF using Western blotting. Higher values for mature-BDNF than for truncated- and pro-BDNF content were seen in the hippocampus of Cond and CondSAL birds, but not in the hippocampus of CondANI or NAIVE birds (p<0.05). The values of mature-BDNF in the amygdala of all the three conditioned groups were higher than those observed for truncated- and pro-BDNF (p<0.05), which indicates that the activation of this protein in the amygdala was not affected by the infusion of Ani in the hippocampus. The data indicate that the tone-shock conditioning induced the activation of molecular pathways of BDNF in the hippocampus and amygdala of the pigeons. The decreases in the content of truncated- and pro-BDNF isoforms found in conditioned pigeons may suggest cleavage mechanisms induced by the training. Our data confirm previous observations of rodent studies and extend these observations to pigeons, revealing that, in spite of the anatomical differences between the hippocampus of rodents and pigeons, there are functional and molecular mechanisms that are conservative between the species.

Keywords: ANOVA; AP; Ani; BDNF; CAU; CS; Careful Exploration; Cond; CondANI; CondSAL; Conditioning and Ani; Conditioning and saline vehicle; Conditioning only; Discrete Movements; EXP; Exploration; FRE; Freezing; L; LOC; Locomotion; MOV; PRN; Preening; RST; Resting; SEM; VIG; Vigilance; amygdala; analysis of variance; anisomycin; anteroposterior; brain-derived neurotrophic factor; classical aversive conditioning; conditioned stimulus; hippocampus; lateral; mature-BDNF; pro-BDNF; standard error of mean; tPA; tissue plasminogen activator; truncated-BDNF.