Effects of spinally delivered N- and P-type voltage-dependent calcium channel antagonists on dorsal horn neuronal responses in a rat model of neuropathy

Neuropathic pain, due to peripheral nerve damage, can include allodynia (perception of innocuous stimuli as being painful), hyperalgesia (increased sensitivity to noxious stimuli) and spontaneous pain, often accompanied by sensory deficits. Plasticity in transmission and modulatory systems are implicated in the underlying mechanisms. The Kim and Chung rodent model of neuropathy (Kim and Chung, Pain 50 (1992) 355) employed here involves unilateral tight ligation of two (L5 and L6) of the three (L4, L5, and L6) spinal nerves of the sciatic nerve and reproducibly induced mechanical and cold allodynia in the ipsilateral hindpaw over the 14 day post-operative period. In vivo electrophysiological techniques have then been used to record the response of dorsal horn neurones to innocuous and noxious electrical and natural (mechanical and thermal) stimuli after spinal nerve ligation (SNL). Activation of voltage-dependent calcium channels (VDCCs) is critical for neurotransmitter release and neuronal excitability, and antagonists can be antinociceptive. Here, for the first time, the effect of N- and P-type VDCC antagonists (ω-conotoxin-GVIA and ω-agatoxin-IVA, respectively) on the evoked dorsal horn neuronal responses after neuropathy have been investigated. Spinal ω-conotoxin-GVIA (0.1–3.2 μg) produced prolonged inhibitions of both the electrically- and low- and high-intensity naturally-evoked neuronal responses in SNL and control rats. Spinal ω-agatoxin-IVA (0.1–3.2 μg) also had an inhibitory effect but to a lesser extent. After neuropathy the potency of ω-conotoxin-GVIA was increased at lower doses in comparison to control. This indicates an altered role for N-type but not P-type VDCCs in sensory transmission after neuropathy and selective plasticity in these channels after nerve injury. Both pre- and post-synaptic VDCCs appear to be important.