Recent research has shown the possibility of pain perception in fish; therefore, the use of analgesia or "painkillers" should be considered for invasive procedures. However, there is relatively little information on the effectiveness of analgesic drugs nor on the appropriate dose for fish. This study assessed the efficacy of three types of drug: an opioid, buprenorphine, a non-steroidal anti-inflammatory drug (NSAID), carprofen and a local anaesthetic, lidocaine. Each drug was tested at three doses on rainbow trout that were noxiously stimulated and the most effective dose was also given to fish experiencing no pain to investigate side-effects. Ventilation rate and time to resume feeding were used as pain indicators, together with the amount of activity and plasma cortisol concentrations to gauge any detrimental side effects. Buprenorphine at all three doses had limited impact on the fish's response to a painful stimulus. Carprofen ameliorated effects of noxious stimulation on time to resume feeding but activity was reduced more than by noxious treatment alone. Lidocaine reduced all of the pain indicators measured with the lowest, most effective dose being 1 mg per fish. None of the analgesics led to raised plasma cortisol compared to control groups. This study demonstrates that lidocaine could be recommended for use in rainbow trout to ameliorate possible pain-related responses.

SUMMARYTrigeminally innervated, mechanically sensitive chemoreceptors (M) were previously identified in rainbow trout, Oncorhynchus mykiss, but it is not known whether these receptors are responsive only to noxious, chemical irritants or have a general chemosensory function. This study aimed to characterise the stimulus-response properties of these receptors in comparison with polymodal nociceptors (P). Both P and M gave similar response profiles to acetic acid concentrations. The electrophysiological properties were similar between the two different afferent types. To determine whether the receptors have a nociceptive function, a range of chemical stimulants was applied to these receptors, including non-noxious stimuli such as ammonium chloride, bile, sodium bicarbonate and alarm pheromone, and potentially noxious chemical irritants such as acetic acid, carbon dioxide, low pH, citric acid, citric acid phosphate buffer and sodium chloride. Only irritant stimuli evoked a response, confirming their nociceptive function. All receptor afferents tested responded to carbon dioxide (CO 2 ) in the form of mineral water or soda water. The majority responded to 1% acetic acid, 2% citric acid, citric acid phosphate buffer (pH3) and 5.0moll -1 NaCl. CO 2 receptors have been characterised in the orobranchial cavity and gill arches in fish; however, this is the first time that external CO 2 receptors have been identified on the head of a fish. Because the fish skin is in constant contact with the aqueous environment, contaminants with a low pH or hypercapnia may stimulate the nociceptive system in fish.