Amylin modulates the formalin-induced tonic pain behaviours in rats

Amylin is a peptide from the calcitonin gene-related peptides (CGRP) family that is expressed by nociceptors. Amylin may modulate pain via a spinal action.

Overall, data suggested that amylin modulates pain with an inflammatory component and the autoanalgesic/inhibitory mechanisms occurring in the interphase of the formalin test. Amylin might have affected the nociceptive system at different levels (spinal cord and brain), explaining the different effects observed according to the time of amylin injection. WHAT DOES THIS STUDY ADD?: Amylin modulated formalin interphase and tonic pain behaviours probably by targeting spinal neurons and affecting supraspinal areas involved in affective and modulatory components of pain. Activation of spinal amylin-receptors may contribute to the initiation of inflammatory pain mechanisms.

The effect of amylin's administration on the formalin test of acute and tonic pain was evaluated. Amylin's ability to modulate neuronal activity was analysed by c-Fos expression at the spinal cord lumbar 4-5 region (L4-5) and brain.

Amylin subcutaneous administration 20 min prior, but not immediately before formalin, shortened the interphase and anticipated the beginning of the tonic pain phase. Amylin reduced the number of activated spinal cord neurons. Blockade of spinal amylin-receptors by prior L4-5 intrathecal administration of an amylin-receptor antagonist (AC187) attenuated these effects, whereas intrathecal BIBN4096 (CGRP-receptor antagonist) did not, proving that part of amylin's effects were spinally mediated via amylin-receptors. The locus coeruleus and other areas involved in descending modulation and affective responses to pain showed an increased number of activated neurons upon amylin subcutaneous administration, suggesting a role for supraspinal areas in some observed effects. L4-5 intrathecal injection of amylin or AC187 showed that both ligands attenuated tonic pain, but blockade of the amylin-receptor action by AC187 decreased further the number of paw jerks in this period. Conclusions: Overall, data suggested that amylin modulates pain with an inflammatory component and the autoanalgesic/inhibitory mechanisms occurring in the interphase of the formalin test. Amylin might have affected the nociceptive system at different levels (spinal cord and brain), explaining the different effects observed according to the time of amylin injection. WHAT DOES THIS STUDY ADD?: Amylin modulated formalin interphase and tonic pain behaviours probably by targeting spinal neurons and affecting supraspinal areas involved in affective and modulatory components of pain. Activation of spinal amylin-receptors may contribute to the initiation of inflammatory pain mechanisms.