Tropomyosin-related kinase (Trk) receptors modulate neuronal structure and function both during development and in the mature nervous system. Interestingly, TrkB and TrkC are expressed as full-length and as truncated splice variants. The cellular function of the kinase-lacking isoforms remains so far unclear. We investigated the role of the truncated receptor TrkB.T1 in the hippocampus of transgenic mice overexpressing this splice variant by analyzing both neuronal structure and function. We observed an impairment in activity-dependent synaptic plasticity as indicated by deficits in long-term potentiation and long-term depression in acute hippocampal slices of transgenic TrkB.T1 mice. In addition, dendritic complexity and spine density were significantly altered in TrkB.T1-overexpressing CA1 neurons. We found that the effect of TrkB.T1 overexpression differs between subgroups of CA1 neurons. Remarkably, overexpression of p75(NTR) and its activation by chemical induction of long-term depression in slice cultures rescued the TrkB.T1-dependent morphological alterations specifically in one of the two subgroups observed. These findings suggest that the TrkB.T1 and p75(NTR) receptor signaling systems might be cross-linked. Our findings demonstrate that TrkB.T1 regulates the function and the structure of mature pyramidal neurons. In addition, we showed that the ratio of expression levels of p75(NTR) and TrkB.T1 plays an important role in modulating dendritic architecture and synaptic plasticity in the adult rodent hippocampus, and, indeed, that the endogenous expression patterns of both receptors change reciprocally over time. We therefore propose a new function of TrkB.T1 as being dominant-negative to p75(NTR).