Anatomical, electrophysiological and behavioural evidence of pain in rainbow trout. (A) Section of the maxillary branch of the trigeminal nerve of the rainbow trout showing the presence of A-delta and C fibres that may act as nociceptors (×1000; scale bar, 2 μm). Adapted from Sneddon (2002) with kind permission from Elsevier. (B) Electrophysiological recordings from a nociceptive receptive field on the trout face showing responses of nociceptors to heat stimulation. This illustrates sensitization of a mechanothermal receptor to heat following noxious chemical stimulation. The firing response to ramp and hold heat stimulation is shown before (left) and 9 min after (right) subcutaneous injection of 1% formalin, <1 mm from the receptive field. The upper trace shows the heat stimulus, the middle panel plots the instantaneous firing frequency (IFF) as scatter graphs and the lower trace shows an extracellular single unit recording from the trigeminal ganglion. Thermal threshold remains the same but firing frequency is greatly increased following formalin injection. Adapted from Ashley et al. (2007) with kind permission from Elsevier. (C) The percentage change in activity (top) and opercular beat rate (OBR; bottom) in rainbow trout 30 min after they were injected subcutaneously with saline or a noxious substance (0.1% acetic acid), or acid combined with intramuscular injection of the opioid buprenorphine (0.1 mg kg⁻¹; Bup.) or the non-steroidal anti-inflammatory drug (NSAID) carprofen (5 mg kg⁻¹; Car.), or injected at the same site as the acid with the analgesic lidocaine (1 mg; Lid.). The grey line represents the impact of saline (control) treatment whereas the black line represents the impact of pain (acid injection). Adapted from Mettam et al. (2011) with kind permission from Elsevier.