Lavidis, Nickolas A.

Abstract The total motor neuron (MN) somato‐dendritic surface area is correlated with motor unit type. MNs with smaller surface areas innervate slow (S) and fast fatigue‐resistant (FR) motor units, while MNs with larger surface areas innervate fast fatigue‐intermediate (FInt) and fast fatigable (FF) motor units. Differences in MN surface area (equivalent to membrane capacitance) underpin the intrinsic excitability of MNs and are consistent with the orderly recruitment of motor units (S > FR > FInt > FF) via the Size Principle. In amyotrophic lateral sclerosis (ALS), large MNs controlling FInt and FF motor units exhibit earlier denervation and death, compared to smaller and more resilient MNs of type S and FR motor units that are spared until late in ALS. Abnormal dendritic morphologies in MNs precede neuronal death in human ALS and in rodent models. We employed Golgi‐Cox methods to investigate somal size‐dependent changes in the dendritic morphology of hypoglossal MNs in wildtype and SOD1 G93A mice (a model of ALS), at postnatal (P) day ~30 (pre‐symptomatic), ~P60 (onset), and ~P120 (mid‐disease) stages. In wildtype hypoglossal MNs, increased MN somal size correlated with increased dendritic length and spines in a linear fashion. By contrast, in SOD1 G93A mice, significant deviations from this linear correlation were restricted to the larger vulnerable MNs at pre‐symptomatic (maladaptive) and mid‐disease (degenerative) stages. These findings are consistent with excitability changes observed in ALS patients and in rodent models. Our results suggest that intrinsic or synaptic increases in MN excitability are likely to contribute to ALS pathogenesis, not compensate for it.

ABSTRACT The motor neuron (MN) soma surface area is correlated with motor unit type. Larger MNs innervate fast fatigue‐intermediate (FInt) or fast‐fatiguable (FF) muscle fibers in type FInt and FF motor units, respectively. Smaller MNs innervate slow‐twitch fatigue‐resistant (S) or fast fatigue‐resistant (FR) muscle fibers in type S and FR motor units, respectively. In amyotrophic lateral sclerosis (ALS), FInt and FF motor units are more vulnerable, with denervation and MN death occurring for these units before the more resilient S and FR units. Abnormal MN dendritic arbors have been observed in ALS in humans and rodent models. We used a Golgi‐Cox impregnation protocol to examine soma size‐dependent changes in the dendritic morphology of lumbar MNs in SOD1 G93A mice, a model of ALS, at pre‐symptomatic, onset and mid‐disease stages. In wildtype control mice, the relationship between MN soma surface area and dendritic length or dendritic spine number was highly linear (i.e., increased MN soma size correlated with increased dendritic length and spines). By contrast, in SOD1 G93A mice, this linear relationship was lost and dendritic length reduction and spine loss were observed in larger MNs, from pre‐symptomatic stages onward. These changes correlated with the neuromotor symptoms of ALS in rodent models. At presymptomatic ages, changes were restricted to the larger MNs, likely to comprise vulnerable FInt and FF motor units. Our results suggest morphological changes of MN dendrites and dendritic spines are likely to contribute ALS pathogenesis, not compensate for it. Anat Rec, 303:1455–1471, 2020. © 2019 American Association for Anatomy

Quantitative assessments of neuronal subtypes in numerous brain regions show large variations in dendritic arbor size. A critical experimental factor is the method used to visualize neurons. We chose to investigate quantitative differences in basolateral amygdala (BLA) principal neuron morphology using two of the most common visualization methods: Golgi–Cox staining and neurobiotin (NB) filling. We show in 8-week-old Wistar rats that NB-filling reveals significantly larger dendritic arbors and different spine densities, compared to Golgi–Cox-stained BLA neurons. Our results demonstrate important differences and provide methodological insights into quantitative disparities of BLA principal neuron morphology reported in the literature.