For example, to distinguish direct from indirect synaptic connections, the uncertain parameter of “time after injection” was often used as a determinant EPZ-6438 order for this critical distinction (Jovanovic et al., 2010,

Rathelot and Strick, 2006 and Ugolini, 2010). A recently introduced modification to this technology now allows for an unambiguous assignment of synaptic connectivity in the central nervous system (CNS) (Callaway, 2008 and Wickersham et al., 2007). In this strategy, genomic deletion of the gene encoding a glycoprotein (Gly) essential for transsynaptic spread renders the rabies virus spreading incompetent. Introduction of Gly expression by genetic or viral tools to selectively complement Gly-deficient rabies in primarily infected neurons reestablishes the ability for transsynaptic spread to label neurons presynaptic to primary infection but prohibits subsequent Thiazovivin mw spread due to absence of Gly in presynaptic neurons (Callaway, 2008 and Wickersham et al., 2007). This monosynaptically restricted transsynaptic rabies virus system was used in two recent studies to map the three-dimensional distribution

of spinal interneurons with direct synaptic connections to motor neurons in mice (Stepien et al., 2010 and Tripodi et al., 2011). Using retrograde motor axonal coinfection strategy from specific muscles, transsynaptic spread is initiated from functionally defined motor neuron Methisazone pools (Figures 5A and 5B). Analysis of the overall distribution patterns of spinal premotor interneuron connectivity to an individual motor neuron pool demonstrates a high degree of reproducibility across animals (Stepien et al., 2010). In contrast, analysis of premotor interneurons

connecting to motor neuron pools with distinct function in motor behavior reveals striking differences in overall distribution (Stepien et al., 2010 and Tripodi et al., 2011). These observations uncover the existence of anatomical or structural engrams at the premotor circuit level that correlate with motor function. The results raise a number of interesting and currently unresolved questions. Premotor neurons encompass a diverse array of neuronal subpopulations, including distinct neurotransmitter phenotypes, synaptic input driving their activation, and additional synaptic partners contacted. It will be interesting to determine the relationship between connectivity-based anatomical maps and functional maps assessing activity patterns in relation to locomotor output. At present, it is unclear which of the many premotor interneurons are required for or involved in the core components of interneuron circuits that give rise to rhythm generation and perpetuation. In addition, motor neuron pools may tap into connections from distinct possible premotor interneuron populations differentially.