Autonomic Neuroscience

 

Early History

Extracellular actions of purine nucleotides and nucleosides were first described in a seminal paper by Drury and Szent-Györgyi in 1929 in the cardiovascular

system, and later in uterus (Deuticke 1932) and intestine (Gillepsie 1934).  Studies of the effects of purines on the nervous system followed the early emphasis on their cardiovascular actions. There was early recognition for a physiological role for ATP at the neuromuscular junction. Buchthal and Folkow (1948) found that acetylcholine (ACh)-evoked contraction of skeletal muscle fibres was potentiated by exposure to ATP. Parts of the spinal cord were shown to be sensitive to ATP (Buchthal et al, 1947). Emmelin and Feldberg (1948) found complex effects initiated by iv injection of ATP into cats affecting peripheral, reflex, and central mechanisms. Injection of ATP into the lateral ventricle of the cat produced muscular weakness, ataxia, and a tendency of the animal to sleep (Feldberg and Sherwood, 1954). The application of adenosine or ATP to various regions of the brain produced biochemical or electrophysiological changes (Babskii and Malkiman, 1950; Galindo et al, 1967; Shneour and Hansen 1971). ATP and related nucleotides were shown to have anti-anaesthetic actions (Kuperman et al, 1964). The first hint that ATP might be a neurotransmitter in the peripheral nervous system arose when Holton and Holten (1954) proposed that ATP released from sensory nerves during antidromic nerve stimulation of the great auricular nerve caused vasodilatation in the rabbit ear artery, and it was later shown that rabbit ear vessel dilatation was accompanied by ATP release (Holton, 1959). A discussion of the development of the purinergic nerve hypothesis (Burnstock, 1972) follows in the next section.

 

Autonomic purinergic neuromuscular transmission

There was early recognition of atropine-resistant responses of the astrointestinal tract to parasympathetic nerve stimulation (Langley, 1898; McSwiney and Robson, 1929; Ambache, 1951; Paton and Vane, 1963). However, it was not until the early 1960s that autonomic transmission other than adrenergic and cholinergic were suggested. In 1963, Burnstock, Campbell, Bennett and Holman recorded electrical and mechanical activity of the guinea-pig taenia coli using the sucrose-gap technique (Burnstock et al, 1963, 1964) [CV30, 35].  After stimulation of the intramural nerves in the presence of adrenergic and cholinergic blocking agents, an inhibitory hyperpolarisating potential was observed and this work was extended to an analysis of the mechanical response (Burnstock et al, 1966). NANC responses were blocked by tetrodotoxin (TTX), a neurotoxin that prevents the action potential in nerves without affecting the excitability of smooth muscle cells, indicating the neurogenic nature of the inhibitory junction potentials (IJPs) (Bülbring and Tomita, 1967). A comparable demonstration of NANC mechanical responses was made by Martinson and colleagues in the stomach upon stimulation of the vagus nerve (Martinson and Muren, 1963; Martinson, 1965).

.

The excitatory response of the mammalian urinary bladder to parasympathetic nerve stimulation was also shown in the last century to be only partially antagonized by antimuscarinic agents (Langley and Anderson, 1895). It was postulated that the atropine-resistant response was due to the release of a noncholinergic excitatory transmitter (Henderson and Roepke, 1934; Chesher and James, 1966; Ambache and Zar, 1970). However, it was also postulated that atropine was unable to block the subjunctional receptors at which the endogenous ACh acts (Dale and Gaddum, 1930) or that it was displaced from these receptors by the high local concentration of ACh released on parasympathetic stimulation (Hukovic et al, 1965). Hughes and Vane (1967), 1970) also demonstrated the presence of NANC inhibitory innervation of the rabbit portal vein.


By the end of the 1960s, evidence had accumulated for NANC nerves in the respiratory, cardiovascular, and urinogenital systems as well as in the gastrointestinal tract (Burnstock et al, 1966; Burnstock, 1969). The existence of NANC neurotransmission is now firmly established in a wide range of peripheral and central nerves and fuller accounts of the development of this concept are available (see Burnstock et al, 1979; Burnstock 1981, 1986a).

 

In the late 1970s, systematic studies were undertaken in an attempt to identify the transmitter utilised by the NANC nerves of the gut and urinary bladder. Several criteria, which must be satisfied prior to establishing a substance as a neurotransmitter (Eccles, 1964), were considered (Burnstock et al, 1970; Burnstock 1972). First, a putative transmitter must be synthesized and stored within the nerve terminals from which it is released. Once released it must interact with specific postjunctional receptors and the resultant nerve-mediated response must be mimicked by the exogenous application of the transmitter substance. Also, enzymes that inactivate the transmitter and/or uptake systems for the neurotransmitter or its derivatives must also be present and, finally, drugs that affect the nerve-mediated response must be shown to modify the response to exogenous transmitter in a similar manner.


Many substances were examined as putative transmitters in the NANC nerves of the gastrointestinal tract and bladder, but the substance that best satisfied the above criteria was the purine nucleotide, ATP (Burnstock et al, 1970; Burnstock et al, 1972). Nerves utilizing ATP as their principal transmitter were subsequently named “purinergic” (Burnstock, 1971) and a tentative model of storage, release, and inactivation of ATP for purinergic nerves was proposed (Burnstock, 1972). Since then a great deal of evidence followed in support of the purinergic hypothesis (see Gordon, 1986; Olsson and Pearson, 1990; Dubyak and El-Moatassim, 1993; Zimmerman, 1994; Burnstock, 1997; Burnstock and Knight, 2004), although there was also considerable opposition to this idea in the first decade after it was put forward (see Burnstock, 1975b; Stone, 1981; Gillespie, 1982).



Skeletal neuromuscular transmission


There was early evidence that ATP was released together with ACh in cholinergic nerves in various tissues including the electric organ of elasmobranch fish (Dowdall et al, 1974; Zimmerman, 1978), the phrenic nerve endings in rat diaphragm (Silinsky and Hubbard, 1973; Silinsky,1975), although there is also some release of ATP from muscle (Abood et al, 1962; Smith, 1991; Santos et al, 2003). Further, application of ATP or adenosine was shown to inhibit the release of ACh (Ginsborg and Hirst,1972; Ribeiro and Walker, 1975). The effect of ATP was dependent on hydrolysis to adenosine, which then acted on presynaptic A1 receptors (Silinsky, 1980; Ribeiro and Sebastino, 1987, Redman and Silinsky, 1993; Ribeiro et al, 1996). ATP was also shown to act postsynaptically to facilitate the action of ACh (Ewald 1976; Ribeiro 1977). ATP facilitates both spontaneous and agonist-activated ACh channel opening (Lu and Smith, 1991). Later papers confirmed these findings and also recognised that P2 receptors were involved in postjunctional actions of ATP (see Heilbrunn and Eriksson, 1988; Henning, 1997; Silinsky et al, 1999, 2004).  It was also shown that in early development of the neuromuscular junction, the ATP that was released acted on ion channel P2X receptors as a genuine cotransmitter with ACh acting on nicotinic receptors, while in mature animals, ATP no longer acted as cotransmitter, but rather as a postjunctional modulator enhancing the actions of ACh (Ribeiro and Walker, 1975; Silinsky et al, 1999).

 
Thus, the actions of ATP at skeletal muscle junctions were established early, although recent papers have added some further details of the mechanisms underlying these actions. For example, excitatory (probably A2A) adenosine receptors probably coexist with inhibitory (A1) receptors at the rat neuromuscular junction, modulating the evoked release of ACh (Correia-de-Sá et al, 1991), the balance of inhibition or facilitation depending on the frequency of motor nerve stimulation (Correia-de-Sá et al, 1996) Depression of ACh release via presynaptic A1 receptor is by inhibition of N-type channels (Schwartz et al, 2003), but is not the basis of tetanic fade at rat neuromuscular junctions (Malinowski et al, 1997). It has been claimed recently that tetanic depression is overcome by tonic adenosine A2A receptor facilitation of L-type Ca2+ influx at rat motor nerve terminals (Oliveira et al, 2004).  A presynaptic facilitating effect of P2 receptor activation on rat phrenic nerve endings was later also recognised (Giniatullin and Sokolova, 1998; Salgado et al, 2000; Galkin et al, 2001) and a recent report suggests that there are P2X7-like receptors at the mouse neuromuscular junction (Moores et al, 2005). Evidence has been presented that ATP, but not adenosine, inhibits nonquantal ACh release at the mouse neuromuscular junction (Galkin et al, 2001). 


Much of the evidence for purinergic involvement in skeletal neuromuscular transmission has come from studies of the fish electric organ, frog and chick neuromuscular junctions (see section VII).