Constitutively active 5-HT2/α1 receptors facilitate muscle spasms after human spinal cord injury

Abstract

In animals, the recovery of motoneuron excitability in the months following a complete spinal cord injury is mediated, in part, by increases in constitutive serotonin (5-HT2) and norepinephrine (α1) receptor activity, which facilitates the reactivation of calcium-mediated persistent inward currents (CaPICs) without the ligands serotonin and norepinephrine below the injury. In this study we sought evidence for a similar role of constitutive monoamine receptor activity in the development of spasticity in human spinal cord injury. In chronically injured participants with partially preserved sensory and motor function, the serotonin reuptake inhibitor citalopram facilitated long-lasting reflex responses (spasms) previously shown to be mediated by CaPICs, suggesting that in incomplete spinal cord injury, functional descending sources of monoamines are present to activate monoamine receptors below the lesion. However, in participants with motor or motor/sensory complete injuries, the inverse agonist cyproheptadine, which blocks both ligand and constitutive 5-HT2/α1 receptor activity, decreased long-lasting reflexes, whereas the neutral antagonist chlorpromazine, which only blocks ligand activation of these receptors, had no effect. When tested in noninjured control participants having functional descending sources of monoamines, chlorpromazine was effective in reducing CaPIC-mediated motor unit activity. On the basis of these combined results, it appears that in severe spinal cord injury, facilitation of persistent inward currents and muscle spasms is mainly mediated by the activation of constitutive 5-HT2 and α1 receptor activity. Drugs that more selectively block these constitutively active monoamine receptors may provide better oral control of spasticity, especially in motor complete spinal cord injury where reducing motoneuron excitability is the primary goal.

Fig. 1.

Cyproheptadine in incomplete spinal cord injury (iSCI). A: rectified surface electromyography (EMG) from tibialis anterior (TA) during dorsiflexion contractions (C) that trigger involuntary muscle spasms (S) in a T2–T4 iSCI participant before (Pre) and 60 min after an 8-mg dose of cyproheptadine (subject 3F, Table 1). B: for same participant in A, unrectified surface EMG during long-duration reflex evoked in the TA from medial arch stimulation (22 mA, 300 Hz, 14 pulses, 0.5 ms wide) before (top) and ∼60 min after (bottom) cyproheptadine. The long polysynaptic reflex (LPR; from start of reflex to 300 ms poststimulation) is marked by the gray bar, and the long-lasting reflex (LLR; from 500 ms after stimulation to 3,500 ms) is indicated by the black bar. Insets at right show short polysynaptic reflex (SPR) on expanded time scale; scale bars, 20 ms/0.2 mV. Bottom trace shifted to the right to reveal SPR. Averaged SPR (C), LPR (D), and LLR (E) are expressed as a percentage of predrug values at different time points across the 8 iSCI participants tested (subjects 1M–8M, Table 1). Some of the data from 6 of the 8 iSCI participants have been published previously (Murray et al. 2010). Error bars represent means ± SE. *P < 0.05; **P < 0.005.

Fig. 2.

Effects of cyproheptadine on estimated persistent inward current (PIC). A: instantaneous firing rate of lower threshold control (bottom) and higher threshold test motor units (top) during isometric dorsiflexion before (left) and 60 min after (right) oral intake of 8 mg of cyproheptadine in an iSCI participant (imp/s, impulses per second). The thick black line represents a 5th-order polynomial fit through the firing rate points. Dashed vertical lines mark the time of occurrence of recruitment and derecruitment of the higher threshold test unit. Solid horizontal lines indicate the firing frequency of the control unit when the test unit was recruited and derecruited, with the magnitude of difference in the firing rates between the 2 time points (ΔF) marked by the arrow. B: average ΔF expressed as a percentage of the predrug values at different time points across the 6 iSCI participants tested (subjects 3F–7M and 9M, Table 1). Error bars represent means ± SE. *P < 0.05.

Fig. 3.

Citalopram in noninjured (NI) controls and iSCI. A: paired motor unit recordings as shown in Fig. 2A before (left) and 90 min after 20-mg citalopram administration (right) in a single NI control participant. B: long-duration reflex recorded in TA (medial arch stimulation: 15.7 mA, 300 Hz, 14 pulses, 0.5-ms pulse width) in a single iSCI participant (subject 3F, Table 1) before and ∼90 min after 20 mg of citalopram. Averaged LPR (C) and LLR (D) are expressed as a percentage of predrug values at different time points across the 4 iSCI participants tested (subjects 2M–5M, Table 1). Error bars represent means ± SE.

Fig. 4.

Cyproheptadine in complete SCI (cSCI). A: long-duration reflexes recorded in TA (medial arch stimulation: 60 mA, 300 Hz, 14 pulses, 0.5-ms pulse width) in a single cSCI participant (subject 16M, Table 1) before and 30, 90, and 120 min after 12 mg of cyproheptadine. Averaged LPR (B) and LLR (C) expressed as a percentage of predrug values at different time points across the 9 cSCI participants tested (subjects 10M–18M, Table 1). Error bars represent means ± SE. *P < 0.05; **P < 0.005.

Fig. 5.

Chlorpromazine in rat cSCI and human NI controls and cSCI. A, top: amplitude of LLR measured in ventral root evoked from single-pulse dorsal root stimulation (0.1 ms, 0.02 mA, 3 times reflex threshold) from a single completely spinalized rat before and after 5 μM chlorpromazine (time of drug application marked by gray horizontal line). Bottom, average of LLR expressed as a percentage of predrug value (open bar) at 30 min after (filled bar) application of chlorpromazine to bath solution (n = 6 roots). B: average ΔF expressed as a percentage of predrug values at different time points across 4 NI control participants. Averaged LPR (C) and LLR (D) are expressed as a percentage of predrug values at different time points across 7 of the 9 cSCI participants tested (subjects 10M–16M, Table 1). Error bars represent means ± SE. *P < 0.05.