Monosynaptic retrograde tracing starts to close the gaps in our understanding of complex premotor networks (Commentary on Sreenivasan et al.)
2015-02-01

To generate movements, the motor cortex acts on an intricate network of intermediate neurons in the brainstem and spinal cord, termed ‘premotor neurons’. Premotor neurons activate the motoneurons driving the muscles (Chakrabarti & Schwarz, 2014). Specifically, premotor networks are involved in the generation of reflexes, and form central pattern generators (CPGs) that contribute to stereotyped movement patterns. The precise anatomical organization of these premotor networks is poorly understood, because traditional tract tracing methods lack the specificity to trace tangled networks and are unable to reveal multi-synaptic networks. In this issue of EJN, Sreenivasan et al. (2015) describe evidence that extends previous work by Takatoh et al. (2013), and identify key components of the premotor networks in developing mouse pups. The authors used an elegant neuroanatomical technique called monosynaptic retrograde tracing (MRT) (Wickersham et al., 2007). MRT employs a genetically defective rabies virus that requires a glycoprotein complement in order for the virus to cross a synapse retrogradely and infect the presynaptic neuron. An essential component of this method is the use of genetic engineering to express the required glycoprotein exclusively in postsynaptic neurons, thereby allowing the virus to pass exactly one synapse and exclusively label ‘first-order’ presynaptic neurons. Applied in combination with immunohistochemistry to visualize transmitter content, MRT serves as a powerful tool with which to dissect the anatomical organization of premotor networks. The current study by Sreenivasan et al., and the former study by Takotah et al., investigated the rodent vibrissal system. Whisking movements consist of rhythmic protractions and retractions that are generated by the activation of two antagonistic muscle groups, the intrinsic and extrinsic muscles, respectively. Upon injection of the defective rabies virus into the antagonistic sets of whisker muscles, both studies revealed a fairly consistent anatomical organization of clusters of first-order ‘whisking’ premotor neurons. Among them are known CPGs in the reticular formation, reticular formation cell clusters with unknown function, cells in the spinal trigeminal nuclei, cells in the vestibular nuclei, and, interestingly, also motor cortex neurons. These results contribute greatly to our understanding of this specialized motor system in rodents. Moreover, the current work indicated a fundamental distinction between premotor networks driving extrinsic muscle groups and those driving intrinsic muscle groups. Extrinsic motoneurons are targeted predominantly by excitatory spinal trigeminal and vestibular neurons, whereas intrinsic motoneurons receive relatively more inputs from whisking CPGs. Sreenivasan et al. also combined MRT with anterograde tracing from primary sensorimotor whisker-related cortices [motor, whisker motor cortex (wM1); tactile, primary whisker somatosensory cortex (wS1)] to show that the target profiles of wM1 and wS1 complement the innervation pattern seen for the extrinsic vs. intrinsic premotor systems: wM1 preferentially controls the intrinsic muscle group via the whisking CPG, whereas wS1 controls the extrinsic muscle group via the spinal trigeminal nuclei. Furthermore, the authors could elicit distinct evoked whisker movements after optogenetic activation of the two areas: stimulation in wM1 initiated rhythmic movement, whereas in wS1 it evoked whisker retraction. Although this observation clearly points to differences in motor control between the two cortical areas, it is questionable whether these evoked movements reflect specialization in terms of movement types. First, wM1 is subdivided in several functionally distinct areas, at least in rats (Chakrabarti & Schwarz, 2014). Second, the interpretation of results from bulk activation of motor structures requires caution. Movements evoked from wS1 may merely reflect modulatory action on the whisking brainstem reflex arc (Nguyen & Kleinfeld, 2005), or alternatively its involvement in corticobulbar control of active touch (Furuta et al., 2010). Consistent with these assumptions, we used bulk stimulation to identify the rhythmic whisking subarea of the rat wM1 (Haiss & Schwarz, 2005), and found that it codes for both whisker position and whisking frequency. These effects occurred on the scale of seconds, and therefore possibly reflect indirect control of movement through various premotor networks (Gerdjikov et al., 2013). Rather than attributing distinct types of movement to specific cortical areas, it may be more informative to focus on the differential access of the sensorimotor cortex to specific premotor subsystems, such as CPGs and reflex arcs. In summary, the findings of Sreenivasan et al. (2015), together with the recent discovery of the whisking CPG (Moore et al., 2013), constitute considerable progress towards the description of the whisker-related premotor network. More work will be required to classify the premotor neuron clusters identified so far into functional subsystems. For example, some of the premotor neurons in reticular formation and spinal trigeminal nuclei may well be components of the brainstem reflex arc (Nguyen & Kleinfeld, 2005). Disentangling the entire polysynaptic premotor network will probably require a multifaceted strategy, including additional ingenious genetic trickery, tracing polysynaptic networks in adult animals, and old-fashioned monitoring of the activity of premotor neurons during whisking behavior.