Catherine R. McCrohan

Nociception in fish: stimulus-response properties of receptors on the head of trout Oncorhynchus mykiss.

This study examined stimulus-response properties of somatosensory receptors on the head of rainbow trout, Oncorhynchus mykiss, using extracellular recording from single cells in the trigeminal ganglion. Of 121 receptors recorded from 39 fish, 17 were polymodal nociceptors, 22 were mechanothermal nociceptors, 18 were mechanochemical receptors, 33 were fast adapting mechanical receptors and 31 were slowly adapting mechanical receptors. Mechanical thresholds were higher in polymodal nociceptors than in either slowly adapting or fast adapting mechanical receptors, whereas thermal thresholds of mechanothermal nociceptors were higher than those of polymodal nociceptors. Polymodal nociceptors and mechanochemical receptors gave similar responses to topical applications of acid. All receptor types except mechanothermal nociceptors showed an increase in peak firing frequency with increased strength of mechanical stimulation, with evidence of response saturation at higher intensities. Mechanothermal, but not polymodal, nociceptors showed an increase in firing response to increased temperature. None out of 120 receptors tested gave any response to the temperature range +4 degrees C to -7 degrees C, indicating an absence of cold nociceptors. Attempts to evoke sensitization of receptors using chemical or heat stimuli were unsuccessful, with receptors showing either a return to control responses or irreversible damage. Comparisons are made between somatosensory receptors characterized here in a fish and those of higher vertebrates.

Bioaccumulation of aluminium in the freshwater snail Lymnaea stagnalis at neutral pH

This study examined the accumulation of aluminium (Al) by the freshwater snail Lymnaea stagnalis at neutral pH, when most Al would be predicted to be in an insoluble form (Al(OH)(3)). Snails were exposed to a range of Al concentrations (38-285 microg l(-1)) for 30 days, followed by 20 days in clean water. Aluminium was measured using atomic absorption spectroscopy. Significant accumulation of Al occurred in the whole soft tissues, gut, digestive gland and kidney at the latest by day 10. High concentration factors were observed, ranging from 4.5 x 10(3) in the whole soft tissues to 6.3 x 10(4) in the kidney, corresponding to actual concentrations of 800 to 7500 microg g(-1), respectively. Proportionality between environmental (water) and tissue concentrations of Al was observed in the gut but not in the other tissues. Following transfer to clean water, rapid loss of Al from the whole soft tissues and gut was seen over the first 10 days. Loss of Al from the digestive gland was much less as a proportion of the total, with approximately 90% of the Al remaining in the tissue. In contrast, significant loss of Al from the kidney occurred between days 20 and 30, even in the continued presence of Al; little further loss occurred following transfer to clean water. Aluminium is clearly available to the snail at neutral pH, the most likely route of entry being the gut. This could facilitate entry of the metal into the food chain. The possible roles of the digestive gland and kidney in the handling of Al are discussed.

Effect of noxious stimulation upon antipredator responses and dominance status in rainbow trout

A potentially painful experience may modify normal behavioural responses. To gauge the importance of pain relative to predation or social status, we presented competing stimuli, a predator cue or an unfamiliar social group, to two groups of noxiously treated rainbow trout, Oncorhynchus mykiss. In the predator cue experiment, fish were classified as bold or shy. Noxiously stimulated fish did not show antipredator responses, suggesting that pain is the imperative. In the social status experiment, noxiously stimulated fish held individually and undisturbed showed an increase in respiration rate and plasma cortisol. As a comparison, we used the dominant or subordinate fish in a group as the noxiously stimulated fish. After the noxious treatment, we returned this test fish to a familiar or unfamiliar social group. Neither dominants nor subordinates showed a negative change in physiology compared to their controls. However, in a familiar group the dominant was much less aggressive, suggesting a behavioural impairment in response to noxious stimulation. In an unfamiliar group, no reduction of aggression was seen, suggesting that maintaining dominance status took priority over showing signs of pain. These findings may reflect an ability to prioritize motivational drivers in fish, and as such provides evidence for central processing of pain rather than merely showing a nociceptive reflex.

Slug preferences for winter wheat cultivars and common agricultural weeds

1. Slugs are a serious pest of many crops, including winter wheat, but current methods of control are unreliable. As part of research study into the possibility that alternative food sources may reduce slug damage to winter wheat crops, the relative palatabilities to field slugs (Deroceras reticulatum) of different wheat cultivars and common agricultural weeds were examined. 2. Individual slugs were given one of 12 winter wheat cultivars for 72h, either as seeds, germinating seeds or young leaves. Individual slugs differed considerably in the amounts of wheat eaten, but there was no clear correlation between slug weight and amount consumed. The slugs did not exhibit any preferences for cultivars at any stage of development. 3. When individual slugs were offered a choice of the leaves of one of 12 common weeds together with wheat leaves, they manifested a hierarchy of preferences for the weeds, some of which were preferred to the wheat leaves. There was some evidence to suggest that when slugs were exposed to less preferred weeds, they consumed less overall, but they ate more wheat than slugs given palatable weed species. 4. The results indicate that the choice of wheat cultivar would not affect levels of slug damage. Weeds could potentially act as a readily available source of alternative food for slugs in the field as part of an integrated pest management programme for slugs. However, because weeds vary considerably in their palatability to slugs, the degree of protection afforded to a wheat crop would depend on the palatability of the weed species present.

The morphology and electrophysiology of the neurones of the paired pedal ganglia of Lynaea stagnalis (L.)

1. A morphological and electrophysiological map of the identifiable neurones and neuronal clusters of the paired pedal ganglia has been prepared. 2. Neuronal morphology was investigated using the fluorescent dye, Lucifer Yellow CH, whilst electrophysiological properties were studied using conventional intracellular recording techniques and the phase plane technique. 3. The paired pedal ganglia are largely symmetrical and giant neurones usually have contralateral homologues. 4. Neuronal clusters are also paired, but minor asymmetries, both of identifiable neurones and neuronal clusters have been found to exist. 5. These asymmetries are thought to be related to asymmetries of body form. 6. Most of the individually identifiable neurones possess obligatory axon branches which are invariant from one preparation to the next, but variant branches also occur. 7. Within the neuronal clusters, morphology appears to be more variable. 8. Individually identifiable neurones and neuronal clusters were characterized electrophysiologically according to the criteria of action potential shape, spontaneous activity pattern, electrical coupling and common synaptic inputs. 9. Homologous pairs of neurones usually have similar electrophysiological properties, as do those within clusters. 10. A number of wide-acting synaptic inputs have been identified on neurones of the pedal, buccal, visceral and parietal ganglia.

Initiation, maintenance and modification of patterned buccal motor output by the cerebral giant cells of Lymnaea stagnaus

1. 1. Regular, evoked bursting activity in the modulatory cerebral giant cells of Lymnaea stagnatis leads to initiation and maintenance of rhythmic feeding motor output from the buccal ganglia. 2. 2. High frequency, short duration cerebral giant cell bursts, either evoked or spontaneous, are associated with longlasting changes in the pattern of rhythmic activity in buccal neurones. 3. 3. Application of food stimuli to the lips and tentacles in a semi-isolated preparation elicits similar brief high frequency bursts in the cerebral giant cells. 4. 4. It is suggested that phasic activity in the cerebral giant cells is physiologically relevant for controlling the nature of feeding motor output in the whole animal.

Food arousal in the pond snail, Lymnaea stagnalis

Factors affecting food arousal were examined in a behavioral study of the freshwater, browsing pulmonate, Lymnaea stagnalis. Presentation of a food stimulus (sucrose) leads to a progressive decrease in interbite interval over the first four subsequent bites. This is thought to represent an increase in food-induced arousal. The latency to first bite is correlated with total number of bites in the first 5 min following application of food. Latency to bite is significantly reduced for up to 15 min following a brief food stimulus. Previous levels of food consumption do not significantly affect latency to bite (i.e., food responsiveness). However, hunger does lead to an increase in spontaneous biting activity in the absence of food (i.e., food search activity). Latency to bite is negatively correlated with a more general measure of behavioral arousal, the Behavioral State Score. These results are discussed in relation to previous work on other gastropod species. It is concluded that mechanisms of food arousal are adapted to the animals habitat and life-style.

The potential for common weeds to reduce slug damage to winter wheat: laboratory and field studies

1. Slugs are serious pests of winter wheat crops in temperate climates, but current methods of chemical control are often unreliable. This paper investigates the potential for common agricultural broad-leaved weeds to act as an alternative food source for slugs, thereby reducing damage to the crop, as part of an integrated approach to pest slug management in wheat crops. 2. An experiment carried out in the field examined the relative effectiveness of metaldehyde pellets and three weed species in reducing damage to wheat seeds and seedlings. Treatments were carried out in open-topped arenas, each containing eight adult field slugs Deroceras reticulatum. The presence of weeds that were palatable to slugs did limit damage to the crop but, over a 72-h period, metaldehyde provided the most effective level of control. 3. A laboratory experiment was carried out to study the feeding behaviour of the slugs in more detail. A single slug was placed in an arena containing food items attached to electronic probes that could detect bites by a slug. The presence of dandelion Taraxacum officinale leaves, a palatable species, reduced the number of wheat seeds damaged, but chickweed Stellaria media leaves, which are less palatable to slugs, had no effect. Most slugs ate the first food item encountered. When dandelion was eaten first, significantly fewer wheat seeds were damaged, and slugs subsequently took fewer bites on seeds than when either a seed or chickweed was eaten first. Slugs were more likely to ignore wheat seeds after a meal on dandelion. 4. Metaldehyde pellets tend to degrade a few days after application. It is suggested that weeds could provide an on-going degree of protection to the crop after the pellets have degraded and until the wheat plants have developed beyond the vulnerable stages. However, the importance of the palatability of the weeds to slugs, and a high weed density to ensure an early encounter with a weed plant during a foraging session, are highlighted by the laboratory study.

The caudal neurosecretory system of Platichthys flesus: General morphology and responses to altered salinity

Abstract 1. The general morphology of the caudal neurosecretory system of the flounder is described. 2. Immunocytochemical staining and quantification of peptides within the spinal cord, associated with the last eight pre-terminal vertebrae, indicate spatial separation of urotensin II (UII) and corticotropin-releasing factor (CRF)/urotensin I (UI) producing cells. 3. Quantification of urophysial peptides indicates that tissue levels are elevated in seawater (SW) compared with freshwater (FW) adapted fish.

Properties of corneal receptors in a teleost fish

Corneal receptors have not previously been identified in lower vertebrates. The present study describes the properties of trigeminal ganglion corneal receptors in a teleost fish, the rainbow trout (Oncoryhnchus mykiss). Out of 27 receptors, 7 were polymodal nociceptors, 6 were mechanothermal nociceptors, 2 were mechanochemical receptors and the largest group, 12, were only responsive to mechanical stimulation. No cold responsive receptors were found on the trout cornea. Mechanical and thermal thresholds were lower and receptive field diameters smaller than those of cutaneous trigeminal receptors in the trout, demonstrating greater sensitivity in the cornea. The lack of cold sensitive neurons may provide evidence for the evolution of cold nociceptors in vertebrates that is related to the transition from poikilothermy to homeothermy.