Modulation of nociceptive afferent nerve fiber activity in inflammatory and non-inflammatory conditions

Background
Inflammation is a major factor modulating nociceptive processing, yet the activity patterns of slowly conducting C-fibers in joint afferents under different mechanical conditions remain insufficiently characterized.

Aims
This study aimed to determine the activity patterns of nociceptive C-fibers innervating the knee joint during rotational movement and during static torque maintenance (plateau) under non-painful (20 mNm) and painful (40 mNm) conditions in rats with either induced or with pharmacologically suppressed inflammation

Material and methods
Experiments were performed on eight adult rats randomly assigned to a control group with inflammation suppressed by acetylsalicylic acid (ASA, n=4) or to an inflammation group with arthritis induced by intra-articular injection of kaolin and carrageenan (n=4). Animals were anesthetized, and C-fibers of the saphenous nerve were surgically isolated. Fiber activity was recorded during mechanical stimulation—internal and external knee rotation at non-painful and painful torque levels—and analyzed with Spike2 (v.7.20). Responses were compared between dynamic (rotation) and static (plateau) phases, as well as between the first and second halves of the plateau. Statistical analyses were conducted in Statistica (v.13.3) with significance set at p < 0.05.

Results
In the inflamed group, C-fiber activity during non-painful rotation was significantly higher and progressively increased across successive trials. Moreover, fiber responses were consistently higher during the plateau than during rotation, whereas in the control group the opposite trend was observed, with activity decreasing over time.

Conclusions
These findings suggest distinct mechanisms of membrane depolarization in the presence versus absence of inflammation. Further studies are required to clarify the underlying neurophysiological processes.