2023 RESEARCH SHOWCASE

Abstracts

Postdoctoral fellows

Primary Afferent Depolarization (PAD) in a rabbit model of CP

Elvia Mena Avila, Emily J. Reedich, Landon T. Genry, K. A. Quinlan

Cerebral palsy (CP) is the most common form of physical disability. CP arises from a variety of perinatal insults and results in permanent, lifelong motor deficits, most often spasticity (Rosenbaum et al., 2007). Spastic CP is thought to arise from damage to the cortex and corticospinal tracts (CSTs) which results in downstream disinhibition of spinal motoneurons (MNs) and hyperreflexia, although the specific mechanism has never been directly tested (Oskoui et al., 2013, Volpe, 2017).

Perinatal hypoxia-ischemia (HI) is a factor in many CP-causing injuries. Rabbits subjected to prenatal HI in late gestation show motor deficits similar to spastic CP including hypertonia and hyperreflexia (Derrick et al., 2004, Synowiec et al., 2019), thus allowing a deeper investigation into mechanisms of spasticity. In the spinal cord, peptidergic nociceptors and CST projections converge directly onto GAD2+ GABAergic interneurons (GABApre) that mediate primary afferent depolarization (PAD), however changes in spinal circuits after perinatal insults are poorly understood.

We hypothesize that loss of CST projections in CP allows expansion of nociceptive innervation of GABApre neurons, increasing conduction of Ia afferent spikes causing reflex irradiation.

We performed extracellular recordings in the in vitro isolated spinal cord of control and HI neonatal rabbits (P 1-5). Stimulation of ventral or dorsal roots were performed to evoked monosynaptic reflex (MSR) and dorsal root potentials (DRPs) respectively. We observed a larger spread of the MSR and DRPs in the HI rabbits compared to control rabbits, suggesting physiopathological changes in spinal circuits in the HI kits. Pharmacological dissection of DRPs (an indirect way to measure PAD) indicates that they origin of this is GABAergic. We can conclude that

1) PAD is GABAergic.

2) PAD circuit is augmented HI rabbits (larger DRPs).

3) Reflex irradiation is increased in HI rabbits (MSR). All of this shows that in the HI rabbits spinal neuronal circuits and sensory afferents are dysfunctional (not just the brain!).

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

Emily Reedich, Landon Genry, Preston Steele, Elvia Mena Avila, Lisa Dowaliby, Alexander Drobyshevsky, Marin Manuel, Katharina Quinlan

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 (5HT) 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 α-methyl 5HT (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.