Spinal ERK2 activation through δ2-opioid receptors contributes to nociceptive behavior induced by intrathecal injection of leucine-enkephalin.

•Nociceptive behavior was not observed in mice treated with i.t. Leu-ENK alone.•Leu-ENK co-injected with peptidase inhibitors evoked nociceptive behaviors.•Leu-ENK co-injected with peptidase inhibitors evoked activation of spinal ERK2.•The nociceptive behavioral response was evoked through spinal δ2-opioid receptor.