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The presence of opioid, beta-adrenergic and cholinergic receptors has been demonstrated in ciliated protozoa, but little is known about gamma-aminobutyric acid (GABA) receptors. In this study we have analyzed the distribution of GABA(A)-type receptor subunits in Paramecium. Confocal laser microscopy using antibodies specific for alpha(1)-, alpha(2)-, alpha(3)-, alpha(6)-, beta(2/3)-, gamma(2)-, epsilon-, lambda-, and theta-subunits showed that most receptors are aggregated in clusters and are distributed both on cell surface and in the cytoplasm. The intensity of labelling of the alpha(6)-, beta(2/3)- and gamma(2)-subunits was more intense than the alpha(1)-, epsilon-, and theta-subunits, suggesting that the former are present in higher concentrations than the latter.

Acid phosphatase activity was measured in individual cells by determining their optical densities through a scanning confocal laser microscope. The naphthol AS-TR (3-hydroxy-2-naphtoic acid 4'-chloro-2'-methylanilide) phosphate-hexazotized para-rosanilin method was used to visualise the acid phosphatase content in the light microscope. Evidence was obtained that the amount of enzyme varied in exponential growth phase cells as the fission age increased. By comparing the acid phosphatase activity with the rate of food vacuole formation, it appeared that the amount of enzyme inside the cells decreased in early clonal life, whereas the rate of food uptake increased. It was assumed that the reduction of acid phosphatase content could lead to a more extended life of vacuoles and to a decreased membrane recycling rate. In turn, the reduced supply of membrane available for food vacuole formation could partly be responsible for the decrease of the food uptake rate observed after the initial increase.