Characterisation of chemosensory trigeminal receptors in the rainbow trout,Oncorhynchus mykiss: responses to chemical irritants and carbon dioxide

Abstract

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.

Aquatic surface respiration and swimming behaviour in adult and developing zebrafish exposed to hypoxia

“…Nociceptors have been identified on the head of rainbow trout (Oncorhynchus mykiss) and mediate responses to noxious stimuli, including high CO 2 and acidity (Sneddon, 2003;Mettam et al, 2012). Evidence in favour of ASR as a reflex response to hypoxia comes primarily from four studies.…”

“…The authors therefore suggested the orobranchial cavity as a site for O 2 chemoreceptors that produced the response. However, the trigeminal nerve is also an important source of innervation to cutaneous receptors of the somatic sensory system (Nilsson, 1984), and fibres of the mandibular branch innervate nociceptors of the head and lower jaw (Sneddon, 2003;Mettam et al, 2012).…”

Reduction in activity by noxious chemical stimulation is ameliorated by immersion in analgesic drugs in zebrafish

“…To determine the effect of several potentially noxious irritant agents known to typically excite nociceptors in adult fish (Mettam et al, 2012), acetic acid at 0.01%, 0.1% and 0.25% (APC Pure, Manchester, UK) and citric acid at 0.1%, 1% and 5% (citric acid anhydrous, APC Pure) were added to the tank water. To avoid disturbing the larvae, the chemicals were delivered via a 5 mm diameter aquarium tubing attached to the experimental tank, with both ends of the tube hidden from the fish behind the tank.…”

“…It has been demonstrated that fish respond to a wide range of noxious stimuli, including thermal (Malafoglia et al, 2014), electrical (Roques et al, 2012) and chemical (Mettam et al, 2012) stimuli. Specifically, exposure to acidic compounds can cause stress in fish if this falls below normal tolerance levels through gill and epidermal damage.…”

“…Indeed, the acetic acid test has been used as the reference model for pain or nociception in fish (Ashley et al, 2007;Newby and Stevens, 2008) and specifically in zebrafish (Correia et al, 2011;Maximino, 2011;Nordgreen et al, 2014). Most studies in adult fish use injection of dilute acid; however, Mettam et al (2012) demonstrated topical application of this chemical stimulated nociceptors on the face of trout, suggesting that any acidic compounds may excite skin nociceptors without any skin damage (below 2% acetic acid). The effect of different concentrations of acetic acid dissolved in water has recently been investigated in 5 dpf larval zebrafish (Steenbergen and Bardine, 2014) but this immersion approach is yet to be developed for other chemicals.…”