Knock-in mice with the common human BDNF Val66Met polymorphism have impaired trafficking of BDNF mRNA to dendrites. Given evidence that local synapse formation is dependent on dendritic translation of BDNF mRNA, it was hypothesized that loss-of-function Met allele mice would show synaptic deficits both at baseline and in response to ketamine, an NMDA antagonist that stimulates synaptogenesis in prefrontal cortex (PFC).
Whole-cell recordings from layer V medial PFC pyramidal cells in brain slices were combined with 2-photon laser scanning for analysis of WT, Val/Met, and Met/Met mice both at baseline and in response to a low dose of ketamine.
Val/Met and Met/Met mice were found to have constitutive atrophy of distal apical dendrites and decrements in apically-targeted excitatory postsynaptic currents (EPSCs) in layer V pyramidal cells of PFC. In addition, spine density and diameter were decreased, indicative of impaired synaptic formation/maturation (synaptogenesis). In Met/Met mice the synaptogenic effect of ketamine was markedly impaired, consistent with the idea that synaptogenesis is dependent on dendritic translation/release of BDNF. In parallel behavioral studies we found that the antidepressant response to ketamine in the forced swim test was blocked in Met/Met mice.
The results demonstrate that expression of the BDNF Met allele in mice results in basal synaptic deficits and blocks synaptogenic and antidepressant actions of ketamine in PFC, suggesting that the therapeutic response to this drug may be attenuated or blocked in depressed patients who carry the loss of function Met allele.
Keywords: antidepressant, dendritic spines, EPSC, hypocretin, major depression, NMDA, serotonin