Chemoreception and taste perception of a feeding attractant in the Dover sole, Solea solea
Glycine betaine, a biologically relevant feeding stimulant was investigated in the Dover sole, Solea solea, a chemosensory feeder. Spike activity in response to glycine betaine concentrations from 10-7 to 1M were investigated using a whole animal preparation and an anterior end preparation. Typically the response curve showed an exponential increase from around 10-4 to 10-3 M up to a peak activity at 10-2 to 10-1 M. Concentrations above peak activity gave rise to a rapid inhibition of the spike activity. Spike frequency calculated over the entire stimulus presentation period and the peak spikes activity recorded during stimulus presentation showed similar response curves. Binding of 14C-glycine betaine to the receptor site showed a response curve similar to those found from electrophysiological recordings but the peak activity occurred at 3.55 x 10-5 M glycine betaine. Binding activity satisfied receptor binding criteria indicating true specific binding activity to a physiologically relevant receptor site. Feeding behaviour and growth rates were used as indicators of the importance of glycine betaine. Feeding behaviour was quantified as the g diet eaten/100g fish wet weight/day. Diets containing no glycine betaine were unacceptable to the fish, but an increase in the concentration of 10-9 to 10-5 moles glycine betaine/g moist diet showed an increase in consumption. Peak feeding activity occurred at 10-5 moles/g moist diet, and higher concentrations indicated a slight decrease in the feeding response. Growth curves to diets containing different concentrations of glycine betaine mirrored these tendencies. Response concentration data were analysed using Scatchard and Beidler analyses. Two components were revealed of differing Km and Qmax values suggesting two affinity receptor components. Analogues of glycine betaine were used to determine the structural specificity of the receptor site. Electrophysiological and binding data showed good correlation and the structural requirements of the receptor site were found to be : short, straight chain molecules; one or two -CH2- groups between terminal groups; high polarity of the molecule; the presence of methyl groups on the quaternary nitrogen terminal, suggesting a high receptor specificity. Behavioural responses paralled the binding and afferent spike activity, although analogues eliciting low level responses below 4.5 spikes sec-1 and 155 pmol 14C glycine betaine bound/mg protein failed to elicit a feeding response. Suppression of the response to glycine betaine appeared to occur with chemical mixture. Individual components of a synthetic squid mixture accounted for five times the stimulatory capacity of the complete synthetic mixture at the binding site, and ten times at the sensory afferent level. Glycine betaine plus glycine was the most stimulatory mixture, the activity elicited in response to the mixture being equal to the summed responses of the individual components. Enhancement or synergism were not found. Mixture interaction was observed at the receptor site, the sensory afferents and in the behavioural response. Taste buds were found on internal surfaces only, showing a maximum density of 155.1/mm2 at the pharyngeal pads. taste buds were also present on the gill arches. Large bud structure on the abocular surface resembled taste buds in structure but did not show binding activity. High receptor specificity allowed a hypothetical receptor site and possible method of binding interaction to be proposed, and the importance of mixture interaction was discussed.