The increased densities of VGLUT1+ contacts in both Pcdhgtcko/tcko and Pcdhgdel/del mutants indicate alterations in the stretch reflex circuit, where proprioceptive sensory afferents (Ia primary afferents, IaPA) establish monosynaptic contacts with spinal motor neurons innervating the same muscle (Chen et al., 2003). Centrally projecting IaPA axons (Parvalbumin+) in wild type spinal cords are distributed in an orderly fashion around motor pools, but in both mutants they appear clumped and more densely surround motor neurons, consistent with the observed increase in the density of VGLUT1+ contacts (Figures 2N-N”). The percentage of Parvalbumin+ neurons in mutant dorsal root ganglia (DRG) is similar to those of wild type animals (L2 DRG, 23.5±1.3% in Pcdhgdel/del and 21.8±1.7% in Pcdhg+/+, p>0.05), indicating that the increased density of the central projections of IaPAs is not due to increased numbers of proprioceptive neurons, but rather to a defect in their terminal arborization resulting from the loss of interneurons they interact with (Jankowska, 1992). Taken together, these observations indicate that similar alterations in specific types of synapses on motor neurons occur in both Pcdhgtcko/tcko and Pcdhgdel/del mutants.
