Suppression of stretch reflex activity after spinal or systemic treatment with AMPA receptor antagonist NGX424 in rats with developed baclofen tolerance

Abstract

Background and purpose: Baclofen (a GABA(B) receptor agonist) is the most commonly used anti-spasticity agent in clinical practice. While effective when administered spinally or systemically, the development of progressive tolerance represents a serious limitation for its long-term use. The goal of the present study was to characterize the treatment potency after intrathecal or systemic treatment with the selective AMPA receptor antagonist NGX424 on stretch reflex activity (SRA) and background muscle activity (BMA) in rats with developed baclofen tolerance.

Experimental approach: Animals were exposed to 10 min of spinal ischaemia to induce an increase in BMA and SRA. Selected animals were implanted with an intrathecal PE-5 catheter and infused intrathecally with baclofen (1 µg·h⁻¹ ) for 14 days. Before and after baclofen infusion, changes in BMA and SRA were measured at 2 day intervals. After development of baclofen tolerance, the animals were injected intrathecally (1 µg) or subcutaneously (3, 6 or 12 mg·kg⁻¹) with NGX424, and changes in BMA and SRA were measured.

Key results: Intrathecal or systemic delivery of NGX424 significantly suppressed the BMA and SRA in baclofen-tolerant animals. This effect was dose dependent. The magnitude of BMA and SRA suppression seen after 1 µg (intrathecal) or 12 mg·kg ⁻¹ (s.c.) of NGX424 injection was similar to that seen during the first 5 days of baclofen infusion. CONCLUSIONS AND IMPLICATIONS These data demonstrate that the use of NGX424 can represent an effective therapy to modulate chronic spasticity in patients who are refractory or tolerant to baclofen treatment.

Figure 1

Quantification of changes in background EMG and SRA in rats with spinal ischaemic injury. (A,B) At 4–10 days after spinal cord ischaemia, an increase in background EMG measured in the gastrocnemius muscle in the absence of any peripheral stimulus is seen (compare A: control to B: ischaemic; background EMG). (B) In ischaemic animals, changes in SRA are identified by the appearance of burst EMG activity (active EMG) which correlates with increased AR (active AR) measured during computer-controlled ankle dorsiflexion (40°·3 s⁻¹). (C) To identify the mechanical component in measured AR (mechanical AR) during ankle dorsiflexion, animals were anaesthetized with isoflurane at the end of the experiment and the magnitude of active AR suppression measured. A decrease in active AR, active EMG during ankle dorsiflexion and background EMG measured under isoflurane anaesthesia is then used as 100% possible effect and is defined in each animal. All drug treatment data generated after NGX424 treatment are then expressed as % of maximum effect seen under isoflurane anaesthesia.

Figure 2

Development of baclofen tolerance and suppression of background EMG activity and SRA after intrathecal NGX424 treatment. (A,B) At day 5 after initiation of baclofen infusion, a significant decrease in active AR (AAR) and EMG activity (active EMG) can be seen. At 10 days after baclofen infusion, the AAR and active EMG activity returned almost completely to pre-baclofen treatment levels. Intrathecal injection of NGX424 (1 µg) led to a potent and significant suppression of AAR, active EMG and background EMG activity (A,B). For statistical analysis, effect of NGX424 treatment on AAR, active EMG and background EMG activity was compared to baseline data recorded at day 10 just before NGX424 administration, and then expressed as % of maximum possible effect measured after induction of isoflurane anaesthesia (*P < 0.05; Student's t-test). (C) A typical EMG and AR recording pattern in an animal before and after development of baclofen tolerance, and the effect of intrathecal NGX424 treatment followed by induction of isoflurane anaesthesia (ISO) at day 10 after initiation of baclofen infusion (A–C: NGX424 = NGX).

Figure 3

Potent suppression of background EMG activity and SRA after systemic NGX424 treatment. (A,B) Subcutaneous injection of NGX424 (3, 6 or 12 mg·kg⁻¹) provided a dose-dependent suppression in active AR and active EMG measured during computer-controlled ankle dorsiflexion, as well as in background EMG activity. The most potent effect was seen at 12 mg·kg⁻¹ dose. For statistical analysis, the effect of NGX424 treatment was compared to baseline data recorded just before NGX424 (or saline) administration, and then expressed as % of maximum possible effect measured after induction of isoflurane anaesthesia (*P < 0.05; paired Student's t-test). (C) A sample EMG and AR recording pattern in an animal before and after subcutaneous 3, 6 or 12 mg·kg⁻¹ NGX424 injection (A–C: NGX424 = NGX).

Figure 4

Systemic administration of NGX424 in naive non-injured animals led to a transient motor weakness. Subcutaneous injection of NGX424 (12 mg·kg⁻¹) led to significant motor weakness lasting for 100 min after NGX424 administration (*P < 0.05).