Assessing Monosynaptic Reflex Alterations in Rat Pups

We assessed alterations in the monosynaptic reflex loop using an in vitro whole spinal cord preparation on P5 to P6 to determine the early impact of MIUH on functional reorganization and excitation/inhibition balance in rat pups. The spinal cord below T8 was isolated from neonatal rats between P4 and P6, as previously described^{16 (link),46 (link)}, and transferred to a recording chamber perfused with oxygenated (95% O₂/5% CO₂) CSF composed of the following (in mM): NaCl 130, KCl 4, CaCl₂ 3.75, MgSO₄ 1.3, NaH₂PO₄ 0.58, NaHCO₃ 25, and glucose 10 (pH 7.4; 24–26 °C). Extracellular stimulation and recording were performed at the level of ventral (VR5) and dorsal (DR5) roots of L5 by touching them with stainless steel electrodes insulated with petroleum jelly. AC recordings from the VR5 were amplified (× 2000) and bandpass-filtered between 70 Hz and 3 kHz. Supramaximal stimulation of the DR5 elicited a monosynaptic response in the ipsilateral homonymous VR5 in vitro, corresponding to the earliest component of motor neuron excitation. To determine the level of post-activation depression at different frequencies, we discarded responses to the first three stimulations required for the occurrence of depression. The responses were rectified, and the areas under the curves were measured. The monosynaptic response was expressed in percentage relative to the mean response at 0.1 Hz in the same series of measurements^{16 (link)}.