Box 15 Bioassay

Demonstration of the presence of suspected transmitters in perfusates was done by bioassay - this was, and often still is, the only way of checking whether a putative transmitter is actually being secreted, since the amount of transmitter released is usually far too small to allow chemical detection. In order to go beyond the detection of a putative transmitter or other naturally produced substance and identify what it is, Gaddum developed the important technique of parallel assay. In this, a sample of perfusate or a tissue extract is prepared and tested on a series of different assay tissues, which respond in different ways to the sample. The responses are carefully compared both qualitatively and quantitatively with the response of the same tissues to the suspected substance.

This has to be done with a sound statistical design, so that the natural variability of the response is taken into account. If the response of each of the assays to the perfusate matches the response to the same concentration of the known sample in each of the assays, i.e. if the relative potency of the sample to the known drug is the same for each assay, then it can be increasingly safely assumed that they are the same. In fact, it is really only necessary for the sample to have the same relative potency for four different assay tissues before an extremely high probability is reached that they are the same. Other obvious tests are equal susceptibility to blocking and potentiating drugs, and identical ability to be denatured by various chemical or physical procedures.

noradrenaline as the transmitter originated. In 1933, Bacq was able to obtain sympathin from the aqueous humour of the eye, where it appears after stimulation of the adrenergic cervical sympathetic nerve. Chemical and spectrographic tests led him to believe it was a catechol derivative with an aminated side chain, and in 1934 he suggested that sympathin might be noradrenaline, but it had to wait until 1946 for von Euler to demonstrate unequivocally that this was, in fact, the case.

All additional complexity in the adrenergic division of the autonomic nervous system was the fact that its stimulation could produce excitation or inhibition of a tissue. Applications of adrenaline could likewise have two actions, although not always in the same proportions as adrenergic nerve stimulation. An early explanation for the dual effects of adrenergic nerve stimulation was proposed by Camion, who suggested that there might be two receptive substances in the target organ that could combine with the released transmitter and produce new compounds (sympathin E and sympathin I) which were responsible for the excitatory and inhibitory effects respectively. An explanation for the different proportions of excitation and inhibition that adrenaline and sympathetic nerve stimulation could induce in a tissue required the demonstration that sympathin and adrenaline were chapter i in fact different substances. This knowledge allowed Ahlquist, an American Historical pharmacologist, to study the relative sensitivity of tissues to noradrenaline Background and adrenaline, and led him in 1948 to the proposal (then controversial) that there were two populations of receptors with differential sensitivities to the two principal catecholamines. The response of the tissue to activation of a receptor was specific to the tissue and could be excitatory or inhibitory. This made sense of many of the earlier results, although it took some time to be accepted. Leading on from this was the synthesis and development of synthetic chemicals, both activating and blocking specific receptors. Worth mentioning here is the work of Sir James Black, who developed drugs specifically blocking Ahlquist's [3-receptors - an advance of considerable clinical significance - and later developed specific blockers of a class of histamine receptors. He was recently awarded the Nobel Prize for this work.

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