Abstract
Gap junctions (GJs) between neurons are present in both the newborn and the adult nervous system, and although important roles have been suggested or demonstrated in a number of instances, in many other cases a full understanding of their physiological role is still missing. GJs are expressed in the rodent lumbar cord at birth and mediate both dye and electrical coupling between motor neurons. This expression has been proposed to mediate: (i) fast synchronization of motoneuronal spike activity, in turn linked to the process of refinement of neuromuscular connections, and (ii) slow synchronization of locomotor-like oscillatory activity. Soon after birth this coupling disappears. Since in the adult rat regeneration of motor fibers after peripheral nerve injury leads to a recapitulation of synaptic refinement at the target muscles, we tested whether GJs between motor neurons are transiently re-expressed. We found that in conditions of maximal responsiveness of lumbar motor neurons (such as no depression by anesthetics, decerebrate release of activity of subsets of motor neurons, use of temporal and spatial summation by antidromic and orthodromic stimulations, testing of large ensembles of motor neurons) no firing is observed in ventral root axons in response to antidromic spike invasion of nearby counterparts. We conclude that junctional coupling between motor neurons is not required for the refinement of neuromuscular innervation in the adult.
Highlights
Gap junctions (GJs) establish communication between proximate cells through channels called connexons, in turn resulting from the assembly of 12 subunits called connexins, that let the passage of electrical currents, small molecules and dyes
The main result of this study is that no overt signs of electrical coupling, namely firing of lumbar motor neurons in response to antidromic spike invasion of adjacent motor neurons, are detected during adult axonal regeneration
Following the demonstration of the transient expression of GJs and electrical coupling in newborn rodents [3,4,5], we asked whether this is recapitulated in adults after nerve injury and repair, as do many other developmental features of the neuromuscular system
Summary
Gap junctions (GJs) establish communication between proximate cells through channels called connexons, in turn resulting from the assembly of 12 subunits called connexins, that let the passage of electrical currents, small molecules and dyes (for a recent review see [1]). GJs have been shown to be expressed in motor neurons of newborn mammals, transiently mediating both dye and electrical coupling [3,4,5]. Some gap junctional coupling has been shown to be re-expressed among axotomized adult motor neurons [6]. These events are of considerable interest for the development of muscle innervation perinatally and of reinnervation in the adult, for the process of synapse competition and elimination, the link being the timing of action potential firing in motor neurons and in their motor endings on muscle fibers [7,8]. We undertook an investigation of the possible electrical coupling between motor neurons after axotomy in adult rats, in non-anesthetized decerebrate preparations with large motoneuronal populations examined, and special attention to the firing activity of these neurons in response to electrical coupling
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