Intrinsic and Functional Differences among Commissural Interneurons during Fictive Locomotion and Serotonergic Modulation in the Neonatal Mouse

Abstract

Commissural interneurons (CINs) send their axons across the midline to innervate contralateral targets and have been implicated in the coordination of left-right limb movements during locomotion. In the neonatal mouse spinal cord, we studied the firing properties and responses to serotonin (5-HT) of two classes of CINs: those whose axons turn caudally after crossing the midline (dCINs) and those whose axons bifurcate after crossing the midline (adCINs). During NMDA and 5-HT-induced locomotor-like activity, a majority of lumbar (L2) dCINs fired rhythmically with ventral root-recorded motor activity, although their firing phase was widely distributed throughout the locomotor cycle. In contrast, none of the adCINs fired rhythmically during fictive locomotion. We studied the baseline firing and membrane properties, and responses to current injection, in dCINs and adCINs that had been partially isolated by blockade of rapid synaptic transmission (with antagonists to glutamate, GABA, and glycine). No significant baseline differences were found between the cell types. In contrast, 5-HT significantly increased the excitability of the isolated dCINs by depolarizing the membrane potential, reducing the postspike afterhyperpolarization amplitude and decreasing the action potential threshold. None of these parameters were affected by 5-HT in adCINs. These results, together with our recent study of a third class of CINs, the aCINs whose axons ascend after crossing the midline (Zhong et al., 2006), suggest that dCINs and aCINs, but not adCINs, are excited by 5-HT and are rhythmically active during fictive locomotion. Thus, they may play important roles in the coordination of left-right movements during fictive locomotion.

Firing and Cellular Properties of V2a Interneurons in the Rodent Spinal Cord

“…3C). Similarly to what has been observed for spike activity in rhythmically active interneurons in the rodent CPG area (Tresch and Kiehn, 1999;Raastad and Kiehn, 2000;Butt et al, 2002;Zhong et al, 2006), significantly rhythmically active Chx10 neurons showed broadly tuned spiking with spikes appearing throughout the cycle but at clearly preferred phase of firing (Fig. 3A-C, circular plots to the left and normalized spike histograms to the right).…”

“…This proportion is similar to what has been shown for all ventral interneuronal populations sampled during drug-induced locomotion Tresch and Kiehn, 1999;Raastad et al, 2000;Butt et al, 2002;Wilson et al, 2005Wilson et al, , 2007Hinckley and Ziskind-Conhaim, 2006;Zhong et al, 2006) with the exception of one report in which almost 90% of Hb9-positive neurons were rhythmically active (Hinckley et al, 2005). It is possible that nonrhythmic Chx10 cells are not engaged directly in rhythmic CPG activity or that they are recruited under different speeds or patterns of locomotor activity.…”

“…The membrane properties of the Chx10 population may therefore play a less significant role in recruiting the neurons into the network than the synaptic inputs that they receive. Moreover, rhythmogenic membrane properties (Butt et al, 2002;Wilson et al, 2005;Hinckley and Ziskind-Conhaim, 2006;Zhong et al, 2006) have been identified in other populations of rodent spinal interneurons, suggesting that they themselves do not provide a distinct electrophysiological signature. Having said this, however, it is not impossible that the distinct firing patterns observed here may correspond to uniquely molecularly defined populations since the V2a population can be further subdivided based on expression on other transcription factors than Chx10 (Dougherty et al, 2009).…”

Strategies for delineating spinal locomotor rhythm-generating networks and the possible role of Hb9 interneurones in rhythmogenesis

“…Nevertheless, there have been significant challenges to progress beyond Getting's second step. In recent years, progress has been made in the identification of neurones involved in some aspects of locomotion (step 3), such as right-left coordination (Butt and Kiehn, 2003;Lanuza et al, 2004;Nakayama et al, 2002;Zhong et al, 2006). However, neither the precise connectivity of locomotor-related spinal interneurones nor their intrinsic properties have been defined.…”

“…However, neither the precise connectivity of locomotor-related spinal interneurones nor their intrinsic properties have been defined. That is, despite the demonstration that many ventral interneurones are rhythmically active during locomotor activity in the cat (Angel et al, 2005;Baev et al, 1979;Gossard et al, 1994;Huang et al, 2000;Jankowska et al, 1967a;Matsuyama et al, 2004;Shefchyk et al, 1990) and rodent (Butt and Kiehn, 2003;Butt et al, 2005;Kiehn et al, 1996;Tresch and Kiehn, 1999;Zhong et al, 2006) little is known about the connectivity of these neurones…”