Role of spinal 5-HT2 receptor subtypes in quipazine-induced hindlimb movements after a low-thoracic spinal cord transection.

A role of serotonin receptors (5-HTRs) in spinal rhythmogenesis has been proposed several years ago based mainly upon data showing that bath-applied 5-HT could elicit locomotor-like rhythms in in vitro isolated spinal cord preparations. Such a role was partially confirmed in vivo after revealing that systemically administered 5-HTR(2) agonists, such as quipazine, could induce some locomotor-like movements (LM) in completely spinal cord-transected (Tx) rodents. However, given the limited binding selectivity of currently available 5-HTR(2) agonists, it has remained difficult to determine clearly if one receptor subtype is specifically associated with LM induction. In situ hybridization, data using tissues from L1-L2 spinal cord segments, where critical locomotor network elements have been identified in mice, revealed greater 5-HTR(2A) mRNA levels in low-thoracic Tx than non-Tx animals. This expression level remained elevated for several days, specifically in the lateral intermediate zone, where peak values were detected at 1 week post-Tx and returned to normal at 3 weeks post-Tx. Behavioral and kinematic analyses revealed quipazine-induced LM in 1-week Tx mice either non-pretreated or pretreated with selective 5-HTR(2B) and/or 5-HTR(2C) antagonists. In contrast, LM completely failed to be induced by quipazine in animals pretreated with selective 5-HTR(2A) antagonists. Altogether, these results provide strong evidence suggesting that 5-HTR(2A) are specifically associated with spinal locomotor network activation and LM generation induced by quipazine in Tx animals. These findings may contribute to design drug treatments aimed at promoting locomotor function recovery in chronic spinal cord-injured patients.

Differential effects of 5-HT1 and 5-HT2 receptor agonists on hindlimb movements in paraplegic mice.

The effects induced by serotonergic (5-HT) agonists of the 5-HT1 and 5-HT2 subclasses were examined on hindlimb movement generation in adult mice completely spinal cord transected at the low thoracic level. One week postspinalization, intraperitoneal injection (0.5-10 mg/kg) of meta-chlorophenylpiperazine (m-CPP; 5-HT(2B/2C) agonist) or trifluoromethylpiperazine (TFMPP; 5-HT(1B) agonist) failed to induce locomotor-like movements. However, dose-dependent nonlocomotor movements were induced in air-stepping condition or on a motor-driven treadmill. In contrast, hindlimb locomotor-like movements were found after the injection of quipazine (5-HT(2A/2C) agonist; 1-2 mg/kg). Combined with L-DOPA (50 mg/kg, i.p.), low doses of quipazine but not of m-CPP and TFMPP produced locomotor-like and nonlocomotor movements in air-stepping condition or on the treadmill. Subsequent administration of m-CPP or TFMPP significantly reduced and often completely abolished the hindlimb movements induced by quipazine and L-DOPA. Altogether, these results demonstrate that 5-HT(2A/2C) receptor agonists promote locomotion while 5-HT(1B) and 5-HT(2B/2C) receptor agonists interfere with locomotor genesis in the hindlimbs of complete paraplegic mice. These results suggest that only subsets of spinal 5-HT receptors are specific to locomotor rhythmogenesis and should be activated to successfully induce stepping movements after spinal cord injury.

Contribution of spinal 5-HT1A and 5-HT7 receptors to locomotor-like movement induced by 8-OH-DPAT in spinal cord-transected mice.

Growing evidence from in vitro studies suggests that spinal serotonin (5-HT) receptor subtypes 5-HTR(1A) and 5-HTR(7) are associated with an induction of central pattern generator activity. However, the possibility of a specific role for these receptor subtypes in locomotor rhythmogenesis in vivo remains unclear. Here, we studied the effects of a single dose (1 mg/kg, i.p.) of 8-hydroxy-2-(di-N-propylamino)-tetralin (8-OH-DPAT), a potent and selective 5-HTR(1A/7) agonist, in mice spinal cord transected at the low-thoracic level (Th9/10). The results show that 8-OH-DPAT acutely induced, within 15 min, hindlimb movements that share some characteristics with normal locomotion. Paraplegic mice pretreated with the selective 5-HTR(1A) antagonists, WAY100,135 or WAY100,635, displayed significantly less 8-OH-DPAT-induced movement. A similar reduction of 8-OH-DPAT-induced movements was found in animals pretreated with SB269970, a selective 5-HTR(7) antagonist. Moreover, a near complete blockade of 8-OH-DPAT-induced movement was obtained in wild-type mice pretreated with 5-HTR(1A) and 5-HTR(7) antagonists, and in 5-HTR(7)-/- mice pretreated with 5-HTR(1A) antagonists. Overall, these results clearly demonstrate that 8-OH-DPAT potently induces locomotor-like movement in the previously paralysed hindlimbs of low-thoracic-transected mice. The results, with selective antagonists and knockout animals, provide compelling evidence of a specific contribution of both receptor subtypes to spinal locomotor rhythmogenesis in vivo.