Novel Neurotransmitters and the Chemical Coding of Neurones

Summary

It took many years, from 1904 to the middle 1950’s, firmly to establish chemical transmission as a fact and to demonstrate the involvement of acetylcholine and noradrenaline as transmitter substances. There followed a period of re-evaluation of the pharmacology of transmission using antagonists of cholinergic and noradrenergic transmission. This led to a stage, in the 1960’s, where a number of transmission processes that did not seem to depend on either of these substances were recognized. Then, in the late 1970’s, immunochemical and other methods led to the discovery of numerous potential transmitter substances in central and peripheral neurones. A major further discovery has been that neurones frequently contain two or more substances (neuronal markers) that are, or may be, involved in the transmission process. Furthermore, the patterns of association of neuronal markers indicate that there is a chemical coding of neurones that functionally subdivides similar neurones according to classes and species of animals, and according to the targets they supply. The interactions between substances that are involved together in neurotransmission suggests that neurotransmission is a plurichemical process In which the substances that are released may cause acute changes in excitability, may enhance each other’s effectiveness, or may prolong or curtail events in the target cells.

Du Bois Reymond (1877) suggested that two theories of neurotransmission should be entertained: transmission might be chemical, due to a stimulatory secretion, or it might be electrical in nature. The idea that chemical transmission was the more likely began with the thorough considerations of the actions of adrenaline and of the effects of stimulation of sympathetic nerves made by Elliott. In 1904, he published a short summary in which he concluded that sympathetic nerves acted upon smooth muscle by the liberation of adrenaline. The next year there appeared a complete analysis of the experimental data that lead to this conclusion (Elliott, 1905). A year later, Dixon (1906) proposed that parasympathetic nerves acted by the liberation of a muscarine-like substance, which Dale (1914) suggested to be acetylcholine. Dale (1954) recalls that Loewi had visited Cambridge and had discussions with Elliott in 1903, at the time that Elliott was formulating his ideas on the action of adrenaline. It was then Loewi and his colleagues who finally demonstrated the chemical nature of transmission, in experiments performed on the vagal nerve pathways to the frog heart (Loewi, 1921; Loewi and Navratil, 1926). Loewi demonstrated his classical experiment to the Twelfth International Congress of Physiology in Stockholm in 1926, almost exactly 60 years prior to this Thirtieth Congress (Holmstedt and Liljestrand, 1963). This was followed by experiments demonstrating that an adrenaline-like substance was, as Elliott predicted, a transmitter released by sympathetic nerves supplying the intestine (Finkelman, 1930) and the heart (Cannon and Bacq, 1931). It was later shown that, in mammals, this substance is noradrenaline (von Euler, 1956). A series of elegant experimental investigations showed that acetylcholine was also a transmitter at synapses in sympathetic ganglia (Feldberg and Gaddum, 1934) and at the motor end plate (Dale, Feldberg and Vogt, 1936; Brown, Dale and Feldberg, 1936). The idea that transmission from neurones was chemical did not go unchallenged, and it was not until the 1950’s that it was accepted as true for the majority of neuroeffector junctions (Bacq, 1975), there being a few specialized junctions where electrical transmission occurs.

Following the acceptance of chemical transmission as a fact, there was a period of consolidation during which a number of drugs were developed that could block transmission from noradrenergic neurones. Drugs such as atropine and nicotine that blocked cholinergic transmission had been available since the last century and so the stage was now set to re-examine neuroeffector transmission at various sites. Thus in the 1960’s came a number of important papers that pointed to the existence of transmission that was not explainable by the release of acetylcholine or of noradrenaline. Transmission from a class of inhibitory neurones to gastrointestinal muscle was found to be maintained in the presence of antagonists of noradrenergic and cholinergic transmission (Burnstock, Campbell, Bennett and Holman, 1964; Burnstock, Campbell and Rand, 1966; Martinson, 1965; Campbell, 1966). Other instances of transmission with an unexplained pharmacology also were discovered, for example excitatory transmission to intestinal muscle (Ambache and Freeman, 1968) and slow potentials in sympathetic ganglia (Nishi and Koketsu, 1968; see Dun, 1983). Once some of these instances had been documented it was possible to look back over earlier literature and identify more cases where both acetylcholine and noradrenaline seemed inadequate candidates as neurotransmitters (Campbell, 1970).

A further stage in this history was reached when immunohistochemical methods were used to demonstrate a number of small peptides in neurones (e.g. Hökfelt, Efendic, Johansson, Luft and Arimura, 1974; Hökfelt, Elde, Johansson, Luft and Arimura, 1975a; Hökfelt, Johansson, Efendic, Luft and Arimura, 1975b; Hökfelt, Kellerth, Nilsson and Pernow, 1975c; Hökfelt, Johansson, Ljungdahl, Lundberg and Schultzberg, 1980a). These peptides were known stimulants of various tissues, for example smooth muscle and glands, as well as having actions within the central nervous system. Within a few years, several hundred papers relating to the presence of peptides in neurones and to the possibility of their being transmitters had been published and the number of neuropeptides identified had increased to over 30 (see reviews by Cuello, 1978; Hökfelt et al., 1980a; Snyder, 1980; Furness and Costa, 1982).

Even while scientists were wrestling with the problem of the possible transmitter roles for the many peptides that had been discovered in neurones a further complication came to light: individual neurones contained not one but two, or even three, substances that could be proposed to be neurotransmitters (Lundberg, Hökfelt, Anggard, Terenius, Elde, Markey, Goldstein and Kimmel, 1982a; Lundberg and Hökfelt, 1983).

We are thus faced with dual problems: do the small peptides contained in neurones participate in neurotransmission; and what is the significance of there being several substances with the potential to be neurotransmitters in the one neurone? The two parts to the title of this lecture indicate our contention that many of these substances (and also non-peptides) do act as neurotransmitters and that their patterns of co-localization follow some system suggestive of a chemical coding of neurones.

There has been a huge volume of literature published on neuropeptides that it is not possible to review in these pages. Instead we have drawn on a few examples to highlight some of the ideas that have been developing in the last few years.