THE concept of a blood-brain barrier has gradually evolved from a large number of different observations concerning the peculiar impermeability of the central nervous system to a wide range of substances. The original observations (Ehrlich, 1885) and much of the subsequent work (Goldmann, 1913; Spatz, 1933;Broman, 1949; Jeppsson, 1962;Bakay and Haque,' 1964; and many others) dealt with the behaviour of acid dyes, notably trypan blue which has been extensively used to demonstrate the presence or absence of this barrier.Two problems of the blood-brain barrier, its development and its site, have been' the subject of considerable discussion. If the first of these seems to have been settled by the works of Grontoft (1954) and Grazer and Clemente (1957), who demonstrated complete impermeability to trypan blue in brains at various stages of development, the second, and most controversial problem, is still unsolved. Numerous hypotheses for the site of the barrier have been proposed, and to-day there are two main schools of thought: those who believe that the blood-brain barrier is located in the endothelium of the vessels of the central nervous system (
The photoreceptors of dark-adapted skate retinas bathed in a Ringer solution containing horseradish peroxidase (HRP) incorporate the tracer into membranebound compartments within the synaptic terminal of the cell; after 1 or 2 h of incubation, approx. 10-38% of the synaptic vesicles were labeled. The receptors appeared to be functioning normally throughout the incubation period, since electrical potentials of normal amplitude could be elicited in response to dim photic stimuli. However, it was possible to block the uptake of peroxidase by a regimen of light adaptation that effectively suppressed light-induced activity in the electroretinogram. If, during incubation with peroxidase, retinas were exposed at 10-min intervals to an intense 1-ms flash from a xenon discharge tube, the receptor terminals were almost completely devoid of peroxidase; fewer than 2% of the vesicles were labeled. The suppression of HRP uptake could also be achieved in dark-adapted retinas by adding magnesium to the bathing solution, suggesting that calcium is necessary for transmitter release from vesicles in the receptor terminals.These findings are consistent with the view that vertebrate photoreceptors discharge a neurotransmitter in darkness, and that light decreases the release of this substance. It seems likely that the incorporation of peroxidase into vesicles of physiologically active receptor terminals reflects a mechanism for the retrieval of vesicle membrane after exocytosis.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.