5-HT<sub>2</sub>Receptor Activation Facilitates a Persistent Sodium Current and Repetitive Firing in Spinal Motoneurons of Rats With and Without Chronic Spinal Cord Injury
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Bibliographic record
Abstract
We examined the modulation of persistent inward currents (PICs) by serotonin (5-HT) in spinal motoneurons of normal and chronic spinal rats. PICs are composed of both a TTX-sensitive persistent sodium current (Na PIC) and a nimodipine-sensitive persistent calcium current (Ca PIC), and we focused on quantifying the Na PIC (and its action on the total PIC), which is known to be critical in enabling repetitive firing. Intracellular recordings were made from motoneurons of the whole sacrocaudal spinal cord of normal adult rats after the cord was acutely transected at the S2 spinal level (acute spinal rat condition), removed from the animal, and then maintained in vitro. In vitro motoneuron recordings were likewise made from rats that had a sacral spinal transection 2 mo previously (chronic spinal rats). In motoneurons from acute spinal rats, moderately high doses of 5-HT (> or = 10 microM), or the 5-HT2 receptor agonist DOI (> or = 30 microM), significantly increased the total PIC, hyperpolarized the PIC onset voltage, and hyperpolarized the spike threshold, whereas lower doses had no effect. Both 5-HT and DOI specifically increased the Na PIC portion of the total PIC (tested with nimodipine blocking the Ca PIC). Additionally, 5-HT, but not DOI, depolarized the resting membrane potential (Vm) and increased the input resistance (Rm) in a dose-dependent manner. Therefore 5-HT2 receptor activation facilitated the Na PIC, whereas other 5-HT receptors modulated Vm and Rm. Motoneurons of chronic spinal rats responded to 5-HT and DOI in the same way, but with larger responses and at much lower doses (0.3-1 microM), thus exhibiting a 30-fold supersensitivity to 5-HT. Specifically the Na PIC was supersensitive to 5-HT2 receptor activation with DOI. Also, Rm and Vm were supersensitive to 5-HT. Consistent with the known critical role of the Na PIC in repetitive firing, enhancement of the Na PIC by DOI or 5-HT facilitated the repetitive firing evoked by steady current injection and enabled repetitive firing in a subpopulation of motoneurons of acute spinal rats that were initially unable to produce sustained repetitive firing. We suggest that after spinal transection, residual endogenous spinal sources of 5-HT help facilitate the Na PIC and repetitive firing. With chronic injury, the developed 5-HT supersensitivity more than compensates for lost brain stem 5-HT, so that the Na PIC is large and motoneurons are very excitable, thus contributing to spasticity.
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Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from it