BDNF is Regulated by Visual Experience and Controls Synaptic Plasticity in Developing Visual Cortex

The visual system 1s the most studied sensory system in vertebrates. During development there is an active shaping of the circuitry, mostly studied at the cortical level. The shaping of neuronal connections is particularly evident during particular time windows of the postnatal development, but some aspects of cortical organization are also modifiable, by experience, in the adult. There is considerable evidence that neuronal activity influences the organization and function of both developing and adult circuits. These processes can also be studied at cellular and molecular levels. Possible mediators of activity-dependent development of visual cortex are neurotrophins. They were initially identified as proteins able to promote the survival and differentiation of target neurons; later they were discovered to be involved in modulating synaptic plasticity in the CNS. In the mammalian visual system, neurotrophins are good candidates to act as a molecular link between neuronal activity and both physiological and stmctural modification of visual cortical circuitry occurring during postnatal development. Brain Derived Neurotrophic Factor (BDNF), is a neurotrophin that specifically binds to Trk:B receptors expressed on the membrane of target neurons. The first aim of this study was to investigate the role of BDNF in one form of synaptic plasticity, called bidirectional plasticity, which represents the capacity of synapses to reverse the sign of a long-term change, in other words to pass from synaptic potentiation to synaptic depression and vice versa. To address this issue we used electrophysi0logical recording in in vitro rat cortical slices containing primary visual cortex. We chose the ability of thalamocortical synapses to be first depressed and then potentiated (de-depression) as a specific indicator of bi-directional plasticity. Extracellular field potentials in cortical layers IIIII were evoked by stimulation of the white matter in rat primary visual cortical slices prepared at different postnatal ages. Low frequency stimulation (900 pulses at 1 Hz) of the white matter was used to induce long-term depression of field potentials amplitude, whereas long-term potentiation was evoked by high-frequency stimulation consisting. of three trains at 100 Hz. We found that bidirectional plasticity is present soon after eye opening (postnatal day 17, PI 7), but disappears rapidly over one week (P23). This loss of plasticity could be prevented by rearing animals in the dark or by applying BDNF from the recording micropipette. Moreover, blockade of the signal transduction pathway activated by Trk signalling prevents this form of plasticity being induced at P 17, an age at which it is normally present. Our data show that BDNF is able to modulate synaptic plasticity during the development of rat visual cortex, but do not reveal the sites of BDNF action during postnatal development. To answer this question the second part of our work was focused on studying the pattern ofBDNF protein cellular expression during postnatal development of rat visual cortex, comparing it with BDNF mRNA. To address this issue we analyzed BDNF mRNA and/or BDNF protein cellular distribution at different postnatal ages by using immunohistochemistry and highly sensitive in situ hybridization. The total amount of protein was measured by quantitative ELISA. We found that before eye opening (P13), all cort.ical layers have a large number of visual cortical neurons containing BDNF mRNA but no detectable amount of BDNF protein. At later ages (P23 and P90) the number of BDNF immunostained cells increases and rnost neurons are double labeled for both BDNF mRNA and protein. The cellular increase of BDNF immunolabeling is blocked in animals deprived of visual experience from birth (dark rearing). In dark reared animals a large population of neurons. contain BDNF mRNA but not BDNF protein, a situation which is similar to that observed before eye opening. Exposure of dark reared rats to a brief period of light restores a good match between BDNF mRNA and BDNF protein cellular content. We propose that visual experience controls the neuronal content of BDNF mRNA and BDNF protein in developing visual cortex. BDNF exerts its action by binding to TrkB receptors. Therefore, we studied the expression of TrkB receptors, considering both cellular expression and protein level. We used two different antibodies: TrkB+ and TrkB - that recognize·, respectively, the full length and truncated form of TrkB receptor. TrkB expression was investigated at different postnatal ages: P13, P23 and adulthood. To detect the protein level, a western blot was performed on homogenate of rat primary visual cortex, taken at the various ages. Our results indicate that the expression of both forms of the receptor increases during postnatal development and that the .distribution patterns are similar at different postnatal ages. This suggests that the full length and the truncated form of the TrkB are regulated in similar ways during postnatal development. The next question was whether the developmental regulation of TrkB receptors is dependent on visual experience. We found that visual deprivation does not affect the expression of TrkB. Thus, contrary to what was found for BDNF mRNA and BDNF protein, the developrnent of TrkB does not depend on visual experience.