HIGH SENSITIVITY OF THE PALATAL CHEMORECEPTORS OF THE CARP TO CARBON DIOXIDE

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The sensitivities of the palatal, gill and lip chemoreceptors of the carp, the lip chemoreceptors of the sea catfish and the tongue chemoreceptors of the frog to carbon dioxide, nitrogen, oxygen and air were studied, by recording the electrical responses from the afferent nerves innervating these chemoreceptors. The palatal chemoreceptors were found to be highly sensitive to carbon dioxide. No detectable responses to oxygen, nitrogen or air were obtained from the above chemoreceptors. Carbon dioxide diluted in nitrogen had less stimulatory effect on the palatal chemoreceptors than it was diluted in air or in oxygen.
The effects of carbon dioxide dissolved in water, acetic acid and hydrochloric acid were studied on the carp palatal and frog tongue chemoreceptors. In the carp the effect of carbon dioxide was distinguishable from those of the other two stimulus solutions of the same pH, whilst in the frog it was nearly the same as that of hydrochloric acid, over the pH range investigated.
The stimulatory effects of mixtures of the above acid stimulants in phosphate buffer upon the palatal chemoreceptors were studied. Addition of carbon dioxide and acetic acid to 0.025M phosphate buffer (pH 7.1) increased the stimulating effectiveness of the phosphate buffer, while addition of hydrochloric acid tended to depress the response, over the pH range 7.0 to 6.3. The enhancing effect of the carbon dioxide was greater than that of acetic acid at the same pHs.
The effects of prior stimulation of the receptor by one acid on the response of the receptor to another acid solution were studied on the carp palatal and frog tongue chemoreceptors. In the carp the cross adaptation of the receptors appeared to be slight between carbon dioxide and the other two, whilst in the frog it was pronounced between carbon dioxide and hydrochloric acid.
The effects of stimulation of the chemoreceptors by respiratory gases upon the opercular movements of the carp were studied. Application of carbon dioxide to the palatal organ exposed in air caused immediate decrease of the opercular movements of the animal in amplitude and occasionally in frequency. Neither air nor oxygen had any apparent effect on the respiratory movements of the animal.