Biology subjects

Steele, P. R.

Publications and source records attributed to Steele, P. R..

3 recordsLinked to original sources

bioRxiv2023

Spinal motoneurons respond aberrantly to serotonin in a rabbit model of cerebral palsy

Cerebral palsy (CP) is caused by a variety of factors that damage the developing central nervous system. Impaired motor control, including muscle stiffness and spasticity, is the hallmark of spastic CP. Rabbits that experience hypoxic-ischemic (HI) injury in utero (at 70-80% gestation) are born with muscle stiffness, hyperreflexia, and, as recently discovered, increased serotonin (5-HT) in the spinal cord. To determine whether serotonergic modulation of spinal motoneurons (MNs) contributes to motor deficits, we performed ex vivo whole cell patch clamp in neonatal rabbit spinal cord slices at postnatal day (P) 0-5. HI MNs responded to application of -methyl 5-HT (a 5-HT1/5-HT2 receptor agonist) and citalopram (a selective 5-HT reuptake inhibitor) with hyperpolarization of persistent inward currents and threshold voltage for action potentials, reduced maximum firing rate, and an altered pattern of spike frequency adaptation while control MNs did not exhibit any of these responses. To further explore the differential sensitivity of MNs to 5-HT, we performed immunohistochemistry for inhibitory 5-HT1A receptors in lumbar spinal MNs at P5. Fewer HI MNs expressed the 5-HT1A receptor compared to age-matched controls. This suggests many HI MNs lack a normal mechanism of central fatigue mediated by 5-HT1A receptors. Other 5-HT receptors (including 5-HT2) are likely responsible for the robust increase in HI MN excitability. In summary, by directly exciting MNs, the increased concentration of spinal 5-HT in HI rabbits can cause MN hyperexcitability, muscle stiffness, and spasticity characteristic of CP. Therapeutic strategies that target serotonergic neuromodulation may be beneficial to individuals with CP. Key pointsO_LIAfter prenatal hypoxia-ischemia (HI), neonatal rabbits that show hypertonia are known to have higher levels of spinal serotonin C_LIO_LIWe tested responsivity of spinal motoneurons (MNs) in neonatal control and HI rabbits to serotonin using whole cell patch clamp C_LIO_LIMNs from HI rabbits showed a more robust excitatory response to serotonin than control MNs, including hyperpolarization of the persistent inward current and threshold for action potentials, larger post-inhibitory rebound, and less spike frequency adaptation C_LIO_LIBased on immunohistochemistry of lumbar MNs, fewer HI MNs express inhibitory 5HT1A receptors than control MNs, which could account for the more robust excitatory response of HI MNs. C_LIO_LIThese results suggest that after HI injury, the increased serotonin could trigger a cascade of events leading to muscle stiffness and altered motor unit development C_LI

neuroscience↗

bioRxiv2020

Inhibitory interneurons show early dysfunction in a SOD1 mouse model of ALS

Few studies in amyotrophic lateral sclerosis (ALS) measure effects of the disease on inhibitory interneurons synapsing onto motoneurons (MNs). However, inhibitory interneurons could contribute to dysfunction, particularly if altered before MN neuropathology, and establish a long-term imbalance of inhibition / excitation. We directly assessed excitability and morphology of glycinergic (GlyT2 expressing) ventral lumbar interneurons from SOD1G93AGlyT2eGFP (SOD1) and wildtype GlyT2eGFP (WT) mice on postnatal days 6-10. Patch clamp revealed dampened excitability in SOD1 interneurons, including depolarized persistent inward currents (PICs), increased voltage and current threshold for firing action potentials, along with a marginal decrease in afterhyperpolarization (AHP) duration. Primary neurites of ventral SOD1 inhibitory interneurons were larger in volume and surface area than WT. GlyT2 interneurons were then divided into 3 subgroups based on location: (1) interneurons within 100 m of the ventral white matter, where Renshaw cells (RCs) are located, (2) interneurons interspersed with MNs in lamina IX, and (3) interneurons in the intermediate ventral area including laminae VII and VIII. Ventral interneurons in the RC area were the most profoundly affected, exhibiting more depolarized PICs and larger primary neurites. Interneurons in lamina IX had depolarized PIC onset. In lamina VII-VIII, interneurons were least affected. In summary, inhibitory interneurons show very early region-specific perturbations poised to impact excitatory / inhibitory balance of MNs, modify motor output, and provide early biomarkers of ALS. Therapeutics like riluzole which universally reduce CNS excitability could exacerbate the inhibitory dysfunction described here. Abstract Figure O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=144 SRC="FIGDIR/small/348359v4_ufig1.gif" ALT="Figure 1"> View larger version (25K): org.highwire.dtl.DTLVardef@1dde8beorg.highwire.dtl.DTLVardef@1db13edorg.highwire.dtl.DTLVardef@1f95cbeorg.highwire.dtl.DTLVardef@37ec18_HPS_FORMAT_FIGEXP M_FIG O_FLOATNOAbstract Figure:C_FLOATNO SOD1 glycinergic interneurons in the ventral horn show altered morphology and excitability, including depolarization of PICs, depolarized threshold, shorter AHPs, smaller somata and larger primary neurites. Ventrally located interneurons are the most prominently affected. C_FIG Key Points SummaryO_LISpinal inhibitory interneurons could contribute to amyotrophic lateral sclerosis (ALS) pathology, but their excitability has never been directly measured. C_LIO_LIWe studied the excitability and morphology of glycinergic interneurons in early postnatal transgenic mice (SOD1G93AGlyT2eGFP). C_LIO_LIInterneurons were less excitable and had marginally smaller somas but larger primary neurites in SOD1 mice. C_LIO_LIGlyT2 interneurons were analyzed according to their localization within the ventral spinal cord. Interestingly, the greatest differences were observed in the most ventrally-located interneurons. C_LIO_LIWe conclude that inhibitory interneurons show presymptomatic changes that may contribute to excitatory / inhibitory imbalance in ALS. C_LI

neuroscience↗

bioRxiv2019

Altered motoneuron properties and spinal serotonin contribute to motor deficits in a rabbit hypoxia ischemia model of cerebral palsy

Cerebral palsy (CP) is caused by a variety of factors attributed to early brain damage, resulting in permanently impaired motor control, marked by weakness and muscle stiffness. To find out if altered physiology of spinal motoneurons (MNs) could contribute to movement deficits, we performed whole cell patch clamp in neonatal rabbit spinal cord slices after developmental injury at 79% gestation. After preterm hypoxia-ischemia (HI), rabbits are born with motor deficits consistent with a spastic phenotype including hypertonia and hyperreflexia. There is a range in severity, thus kits are classified as severely affected, mildly affected, or unaffected based on modified Ashworth scores and other behavioral tests. At postnatal day (P)0-5, we recorded electrophysiological parameters of 40 MNs in transverse spinal cord slices using whole cell patch clamp. Using a multivariate analysis of neuronal parameters, we found significant differences between groups (severe, mild, unaffected and sham control MNs), age (P0 to P5) and spinal cord region (cervical to sacral). Severe HI MNs showed more sustained firing patterns, depolarized resting membrane potential, and a higher threshold for action potentials. These properties could contribute to both muscle stiffness and weakness, respectively, hallmarks of spastic CP. Interestingly altered persistent inward currents (PICs) and morphology in severe HI MNs would dampen excitability (reduced normalized PIC amplitude and increased dendritic length). In summary, changes we observed in spinal MN physiology likely contribute to severity of the phenotype including weakness and hypertonia, and therapeutic strategies for CP could target excitability of spinal MNs. Key PointsO_LIPhysiology of neonatal spinal motoneurons is altered after in utero hypoxia-ischemic injury C_LIO_LIIn motoneurons from severely affected animals there is more sustained firing (lower {Delta}I values), a depolarized resting potential, but a higher voltage threshold for action potential firing. C_LIO_LIAltered motoneuron excitability could contribute directly to muscle stiffness and spasticity in cerebral palsy. C_LI

neuroscience↗