Analgesic profile of the nicotinic acetylcholine receptor agonists, (+)-epibatidine and ABT-594 in models of persistent inflammatory and neuropathic pain.

The anti-nociceptive and locomotor effects of the nicotinic acetylcholine receptor (nAChR) agonists (+)-epibatidine and ABT-594 were compared in the rat. Acute thermal nociception was measured using the tail flick test. Mechanical hyperalgesia was measured as paw withdrawal threshold (PWT) in response to a mechanical stimulus in two animal models of persistent pain; (1) 24 h following subplantar injections of Freund's complete adjuvant (FCA) into the left hind paw or (2) 11-15 days following a partial ligation of the left sciatic nerve. Disruption of locomotor function was assessed using an accelerating rotarod device. In all tests, (+)-epibatidine was significantly more potent than ABT-594. Both (+)-epibatidine and ABT-594 dose-dependently increased tail flick latencies but only at doses that also disrupted performance in the rotarod test. On the other hand, (+)-epibatidine and ABT-594 dose-dependently reversed inflammatory and neuropathic hyperalgesia at significantly lower doses than that needed to disrupt performance in the rotarod test. In summary, ABT-594 is less potent than (+)-epibatidine in assays of acute and persistent pain and in the rotarod assay. However, ABT-594 displayed a clearer separation between its motor and anti-hyperalgesic effects. This shows that nicotinic agonists with improved selectivity between the nicotinic receptor subtypes could provide strong analgesic effects with a much improved therapeutic window.

Selective neurokinin-1 receptor antagonists are anti-hyperalgesic in a model of neuropathic pain in the guinea-pig.

Neuropathic pain is poorly managed by conventional analgesic therapy, such as non-steroidal anti-inflammatory drugs and opiates. The development of animal models of peripheral neural damage has aided in our understanding of the pathology and pharmacology of neuropathic pain. This report is the first clear demonstration using selective neurokinin-1 receptor antagonists of a potentially novel therapeutic approach to the treatment of neuropathic pain resulting from peripheral nerve damage in a guinea-pig model. The neurokinin-1 receptor antagonists, SDZ NKT 343 and LY 303,870 significantly reduced mechanical hyperalgesia following oral and intrathecal administration. (R,R)-SDZ NK T343, the enantiomer of SDZ NKT 343 did not show anti-hyperalgesic activity. RPR 100,893 showed significant anti-hyperalgesic activity only following intrathecal administration suggesting poor absorption or low level penetration of the blood-brain barrier. These results imply that neurokinin-1 receptor antagonists offer a new class of anti-hyperalgesic drugs with a largely central site of action in neuropathic pain.

In vitro characterization of a peripheral afferent pathway of the rat after chronic sciatic nerve section.

1. We have studied the characteristics of the abnormal properties of damaged myelinated fibers (conduction velocity > 2.0 m/ s) after peripheral nerve injury in a novel in vitro model of the rat sciatic nerve/dorsal root ganglion/dorsal root (L4-5) preparation removed from control naíve or sham-operated rats and animals that had received sciatic neurectomy 12-24 days before the in vitro study. A total of 122-245 filaments were recorded in each dorsal root. The proportion of A alpha, beta and A delta fibers were not significantly different between control, sham-operated, and axotomized nerves. Spontaneous activity was recorded in 3.4% (A alpha, beta) and 4.6% (A delta) of fibers in comparison with 0.4% (A alpha, beta) and 0.3% (A delta) in naíve controls. 2. A sporadic, irregular, low-frequency (< 1 Hz) firing was seen in 26% of the fibers with spontaneous activity. Periodical (irregular) bursting pattern was observed in 43% of spontaneously active fibers, whereas a relatively stable, ongoing firing pattern (median frequency: 7.1 Hz) was displayed by 31% of active fibers. 3. Mechanosensitivity of the neuroma/peripheral nerve was displayed in preparations from lesioned [axotomized: 18.2% (A alpha, beta) and 14.1% (A delta), sham operated: 2% (A alpha, beta) and 0% (A delta)], but not control naíve animals. There was no correlation between the presence of spontaneous activity and mechanosensitivity in single fibers. 4. The principal site of spontaneous activity generation was the dorsal root ganglion. Transection of the peripheral nerve (or removal of the neuroma), while recording from dorsal root filaments, produced a cessation of firing in 21% of fibers firing with ongoing discharge. The remaining active fibers continued firing until the DRG was removed. A sustained injury discharge was observed in damaged fibers but not control, undamaged fibers from naíve animals after acute peripheral nerve transection. 5. We present an in vitro model for the study of abnormal primary sensory activity in peripheral neuropathy. Although our data are consistent with in vivo electrophysiological findings in published reports, the proportion of damaged afferent fibers displaying spontaneous activity was significantly lower under in vitro conditions. This model may serve as a valuable tool for further physiological and pharmacological studies of peripheral neuropathy.