Nociceptors in the gills of the dogfishSqualus acanthias

Fish gill chemosensing: knowledge gaps and inconsistencies

In this review, we explore the inconsistencies in the data and gaps in our knowledge that exist in what is currently known regarding gill chemosensors which drive the cardiorespiratory reflexes in fish. Although putative serotonergic neuroepithelial cells (NEC) dominate the literature, it is clear that other neurotransmitters are involved (adrenaline, noradrenaline, acetylcholine, purines, and dopamine). And although we assume that these agents act on neurons synapsing with the NECs or in the afferent or efferent limbs of the paths between chemosensors and central integration sites, this process remains elusive and may explain current discrepancies or species differences in the literature. To date it has been impossible to link the distribution of NECs to species sensitivity to different stimuli or fish lifestyles and while the gills have been shown to be the primary sensing site for respiratory gases, the location (gills, oro-branchial cavity or elsewhere) and orientation (external/water or internal/blood sensing) of the NECs are highly variable between species of water and air breathing fish. Much of what has been described so far comes from studies of hypoxic responses in fish, however, changes in CO2, ammonia and lactate have all been shown to elicit cardio-respiratory responses and all have been suggested to arise from stimulation of gill NECs. Our view of the role of NECs is broadening as we begin to understand the polymodal nature of these cells. We begin by presenting the fundamental picture of gill chemosensing that has developed, followed by some key unanswered questions about gill chemosensing in general.

Disruption of tph1 genes demonstrates the importance of serotonin in regulating ventilation in larval zebrafish (Danio rerio)

Serotonergic neuroepithelial cells (NECs) in larval zebrafish are believed to be O₂ chemoreceptors. Serotonin (5-HT) within these NECs has been implicated as a neurotransmitter mediating the hypoxic ventilatory response (HVR). Here, we use knockout approaches to discern the role of 5-HT in regulating the HVR by targeting the rate limiting enzyme for 5-HT synthesis, tryptophan hydroxylase (Tph). Using transgenic lines, we determined that Tph1a is expressed in skin and pharyngeal arch NECs, as well as in pharyngeal arch Merkel-like cells (MLCs), whereas Tph1b is expressed predominately in MLCs. Knocking out the two tph1 paralogs resulted in similar changes in detectable serotonergic cell density between the two mutants, yet their responses to hypoxia (35 mmHg) were different. Larvae lacking Tph1a (tph1a^-/- mutants) displayed a higher ventilation rate when exposed to hypoxia compared to wild-types, whereas tph1b^-/- mutants exhibited a lower ventilation rate suggesting that 5-HT located in locations other than NECs, may play a dominant role in regulating the HVR.

Neuroendocrine control of breathing in fish

Beginning with the discovery more than 35 years ago that oxygen chemoreceptors of the fish gill are enriched with serotonin, numerous studies have examined the importance of this, and other neuroendocrine factors in piscine chemoreceptor function, and in particular on the chemoreceptor-mediated reflex control of breathing. However, despite these studies, there is continued debate as to the role of neuroendocrine factors in the initiation or modulation of breathing during environmental disturbances or physical activity. In this review, we summarize the state-of-knowledge surrounding the neuroendocrine control of oxygen chemoreception in fish and the associated reflex adjustments to ventilation. We focus on neurohumoral substances that either are present in chemosensory cells or those that are localised elsewhere but have also been implicated in the direct control of breathing. These substances include serotonin, catecholamines (adrenaline and noradrenaline), acetylcholine, purines and gaseous neurotransmitters. Despite the growing indirect evidence for an involvement of these neuroendocrine factors in chemoreception and ventilatory control, direct evidence awaits the incorporation of novel methods currently under development.

Branchial innervation

Inspection of the dorsal end of fish gills reveals an impressive set of nerve trunks, connecting the gills to the brain. These trunks are branches of cranial nerves VII (the facial) and especially IX (the glossopharyngeal) and X (the vagus). The nerve trunks carry a variety of nervous pathways to and from the gills. A substantial fraction of the nerves running in the branchial trunks carry afferent (sensory) information from receptors within the gills. There are also efferent (motor) pathways, which control muscles within the gills, blood flow patterns and possibly secretory functions. Undertaking a more careful survey of the gills, it becomes evident that the arrangement of the microanatomy (particularly the blood vessels) and its innervation are strikingly complex. The complexity not only reflects the many functions of the gills but also illustrates that the control of blood flow patterns in the gills is of crucial importance in modifying the efficiency of its chief functions: gas transfer and salt balance. The "respiratory-osmoregulatory compromise" is maintained by minimizing the blood/water exchange (functional surface area of the gills) to a level where excessive water loss (marine teleosts) or gain (freshwater teleosts) is kept low while ensuring sufficient gas exchange. This review describes the arrangement and mechanisms of known nervous pathways, both afferent and efferent, of fish (notably teleosts) gills. Emphasis is placed primarily on the autonomic nervous system and mechanisms of blood flow control, together with an outline of the afferent (sensory) pathways of the gill arches.

Cardio-ventilatory control in rainbow trout: II. Reflex effects of exogenous neurochemicals

The effects of various neurochemicals were examined in intact, unanesthetized rainbow trout (Oncorhynchus mykiss) to assess the role of branchial O2-sensitive chemoreceptors in the cardio-ventilatory responses to exogenous neurochemicals. cyanide stimulated ventilation and elicited bradycardia when give externally but only stimulated ventilation when injected internally. Norepinephrine increased heart rate, blood pressure and ventilatory rate but opercular pressure was not affect. Dopamine had no effect on either heart or ventilatory rate but increased blood pressure and decreased opercular pressure. Serotonin stimulated heart rate and ventilation but decreased blood pressure. Acetylcholine and nicotine stimulated all cardio-ventilatory variables. Muscarine decreased heart rate and blood pressure and had a biphasic effect on ventilation. These results, combined with the results from the preceding study, suggest that the cardio-ventilatory effects of exogenously administered (1) cyanide are entirely mediated by gill O2 receptors, (2) serotonin, and cholinergic drugs could be partly mediated by O2 receptors and (3) catecholaminergic drugs are not mediated by O2 receptors.

Sensory interaction with central 'generators' during respiration in the dogfish

The activity in sensory and motor nerves of the gills was recorded from selected branches of the vagus nerve in decerebrate dogfish, Scyliorhinus canicula. Vagal motoneuronal activity was observed at the start of the rapid pharyngeal contraction and was followed by sensory nerve activity which preceded the slow expansion phase. Rhythmical vagal motoneuronal activity was still present after all movements had been prevented by curare paralysis although the frequency of the rhythm was higher than in the ventilating fish. Electrical stimulation of vagal sensory fibres had 3 effects on the ventilatory movements. (1) It evoked a reflex contraction of several gill muscles after a latency of about 11 ms. (2) It could reset the respiratory cycle because a stimulus given during expansion delayed the onset of the subsequent contraction. (3) The stimulus could entrain the rhythm if it was given continuously at a frequency close to that of ventilation. The vagal motor rhythm was disrupted by trigeminal nerve stimulation in the paralyzed fish but not if the motor rhythm was being entrained by vagal nerve stimulation. Vagal sensory activity may be important, therefore, in maintaining the stability of the generating circuits.

Mechanoreceptor activity in the gills of the carp. I. Gill filament and gill raker mechanoreceptors

Physiological properties of gill filament and gill raker mechanoreceptors in the gills of spontaneously breathing carp, Cyprinus carpio L., were analysed. Stroking stimuli applied to gill filaments elicited a phasic mechanoreceptive response, which was recorded from neurons in the epibranchial ganglia. Sustained deflection resulted in a short on-off response. The same neurons were also activated by slight movements of lamellae on a gill filament. The receptive field extended over all the lamellae of one filament at most, but generally covered a small part of it, including both dorsal and ventral lamellae. Deflection of gill rakers also elicited a brief response in epibranchial ganglion neurons. The threshold of both filament-related and gill raker mechanoreceptors was relatively high. They did not respond during normal respiration. It was therefore argued that these receptors do not function in normal respiratory control, but rather serve against mechanical damage from excessive pressure or particles in the water.

Respiratory responses to stimulation of branchial vagus nerve ganglia of a teleost fish

The effects of electrical stimulation of epibranchial vagus ganglia upon respiration of the carp were investigated. Single shocks evoked fast twitch responses in a number of respiratory muscles with latencies around 18 msec to the beginning and 30-35 msec to the peak of activity. Shocks given during abduction decreased the respiratory cycle duration by shortening abduction and accelerating adduction. Stimuli given throughout most of adduction also shortened the respiratory cycle, accelerating the adduction only. These responses are similar to vagally mediated lung receptor reflexes of mammals. Stimulation with short trains of pulses produced a rapid expansion-contraction movement. This movement resembles in all respects (shape, time in the respiratory cycle, muscle coordination) the intermediate expansion of a normal coughing movement. Continual stimulation at frequencies close to the normal respiratory rate had a synchronising influence upon respiration, speeding up or slowing down its rate. HRP applied to the third vagal ganglion showed that there is a small projection of this ganglion to the nucleus intermedius facialis, although the majority of sensory fibres terminate in the vagal lobe. The nucleus intermedius facialis is already known to connect directly with the respiratory motor centres and thus might provide a pathway for the fast twitch response. A projection was also found to the nucleus ambiguus; in mammals this nucleus plays an important role in the regulation of respiratory movements.