5% baseline amplitude (p < 0.05 versus nonconditioned) (Figure 4A–4Bi). Next we blocked tPA activity to determine if extracellular cleavage of proBDNF to mBDNF was required for the visually induced facilitation. Bath application of the

inhibitor tPA-stop blocked LTP induction (80% ± 7.1%, p < 0.01 versus conditioned) (Figure 4Bii). As tPA can be involved in cascades other than the cleavage of proBDNF, we tested LTP induction in tectal cells in which BDNF expression had been knocked down by BDNF MO electroporation. Knockdown of BDNF prevented the facilitation induced by conditioning (125% ± 5.3%, p < 0.05 versus conditioned; Figure 4Biii). In contrast, electroporation of a control scrambled MEK inhibitor MO (n = 3) did not interfere with facilitation of LTP by visual conditioning, resulting in selleck compound a potentiation that was indistinguishable from that observed in untreated, conditioned animals (n = 6). These groups were therefore combined. These findings imply that proBDNF synthesized in response to visual conditioning may be cleaved in a tPA-dependent manner in response to the LTP protocol, and that the resulting production of mBDNF facilitates LTP. Activation of the TrkB receptor tyrosine kinase is the main pathway by which mBDNF

initiates downstream signaling. Inhibition of TrkB signaling with the receptor tyrosine kinase inhibitor K252a entirely blocked LTP induction in conditioned animals (97% ± 3.8%, p < 0.05 versus no drug; Figure 4iv) in agreement with previous reports (Du et al., 2009 and Mu and Poo, 2006). Together, our findings demonstrate that the BDNF synthesized in response to 20 min of robust visual Histamine H2 receptor conditioning, can facilitate bidirectional plasticity at the retinotectal synapse hours later. As developmental circuit refinement is thought to rely upon environmentally driven strengthening of appropriate, and weakening of inappropriate, synapses through mechanisms like LTP and LTD (Katz and Shatz, 1996 and Zhang and Poo, 2001), we next tested whether visual conditioning might facilitate the ongoing process of circuit refinement. Visual acuity

is a measure of the ability to resolve spatial details. One method for measuring acuity in humans is the Teller acuity test (Dobson and Teller, 1978), in which preverbal infants will preferentially look at a grating that they can resolve, compared to either a gray screen of comparable luminance or a higher spatial frequency grating that they cannot resolve. Furthermore, cortical responses to gratings of different sizes determined by measuring transcranial visually evoked potentials can be extrapolated to determine a subject’s acuity thresholds, with comparable results to the behavioral tests (Campbell and Maffei, 1970 and Good, 2001). To determine if proBDNF produced by visual conditioning participates in the ongoing process of circuit refinement, we subjected tadpoles to visual conditioning and then returned them to their normal rearing environment for 7–11 hr.