Sensorimotor function is modulated by the serotonin receptor 1d, a novel marker for gamma motor neurons

Gamma motor neurons (MNs), the efferent component of the fusimotor system, regulate muscle spindle sensitivity. Muscle spindle sensory feedback is required for proprioception that includes sensing the relative position of neighboring body parts and appropriately adjust the employed strength in a movement. The lack of a single and specific genetic marker has long hampered functional and developmental studies of gamma MNs. Here we show that the serotonin receptor 1d (5-htld) is specifically expressed by gamma MNs and proprioceptive sensory neurons. Using mice expressing GFP driven by the 5-htld promotor, we performed whole-cell patch-clamp recordings of 5-htld::GFP(+) and 5-htld::GFP(-) motor neurons from young mice. Hierarchal clustering analysis revealed that gamma MNs have distinct electrophysiological properties intermediate to fast-like and slow-like alpha MNs. Moreover, mice lacking 5-htld displayed lower monosynaptic reflex amplitudes suggesting a reduced response to sensory stimulation in motor neurons. Interestingly, adult 5-htld knockout mice also displayed improved coordination skills on a beam-walking task, implying that reduced activation of MNs by la afferents during provoked movement tasks could reduce undesired exaggerated muscle output. In summary, we show that 5-htld is a novel marker for gamma MNs and that the 5-hrld receptor is important for the ability of proprioceptive circuits to receive and relay accurate sensory information in developing and mature spinal cord motor circuits. (C) 2012 Elsevier Inc. All rights reserved.