Severe deficits in 5-HT2A -mediated neurotransmission in BDNF conditional mutant mice

BDNF is thought to provide critical trophic support for serotonin neurons. In order to determine postnatal effects of BDNF on the serotonin system, we examined a line of conditional mutant mice that have normal brain content of BDNF during prenatal development but later depletion of this neurotrophin in the postnatal period. These mice show a behavioral phenotype that suggests serotonin dysregulation. However, as shown here, the presynaptic serotonin system in the adult conditional mutant mice appeared surprisingly normal from histological, biochemical, and electrophysiological perspectives. By contrast, a dramatic and unexpected postsynaptic 5-HT2A deficit in the mutant mice was found. Electrophysiologically, serotonin neurons appeared near normal except, most notably, for an almost complete absence of expected 5-HT2A -mediated glutamate and GABA postsynaptic potentials normally displayed by these neurons. Further analysis showed that BDNF mutants had much reduced 5-HT2A receptor protein in dorsal raphe nucleus and a similar deficit in prefrontal cortex, a region that normally shows a high level of 5-HT2A receptor expression. Recordings in prefrontal slice showed a marked deficit in 5-HT2A -mediated excitatory postsynaptic currents, similar to that seen in the dorsal raphe. These findings suggest that postnatal levels of BDNF play a relatively limited role in maintaining presynaptic aspects of the serotonin system and a much greater role in maintaining postsynaptic 5-HT2A and possibly other receptors than previously suspected.