Derecho a la salud

to elucidate the function of DUM neurones in the central nervous system of the insect. These authors found that regardless of which DUM neurone they recorded from in the locust metathoracic ganglion, action potentials were seen just before the start of any leg movements. This occurred whether the movements were spontaneous or evoked (Sombati and Hoyle, 1 9 8 4 a ) . It had previously been shown that stimulation of the locust fast extensor tibiae motoneurone

(FETi) produces EPSPs in the antagonistic flexor

motoneurones (Hoyle and Burrows, 1973). During repetitive stimulation this FETi-evoked response progressively declines (Sombati and Hoyle, 1 9 8 4 a ) . Sombati and Hoyle (1984a) found, however, that this habituation was reversed by ionoto- phoretic application of octopamine into the neuropilar

region of presumed synaptic action at the flexor

motoneurones, or by concurrent stimulation of a DUM neurone (presumed to be o c t o p a m i n e r g i c ) . When larger amounts of octopamine were iontophoresed the EPSP size recorded in the flexor motoneurones increased and in some instances action

potentials were initiated. In a further series of

experiments Sombati and Hoyle (1984b) showed that ionto­ phoretic application of octopamine at a few specific regions

in the neuropile of the metathoracic ganglion caused

repetitive bouts of either stepping movements or flight motor activity. Hence from these results it appears that octopaminergic neurones may have at least two roles in the insect central nervous system. They may be involved in the

behaviours and also in the po t e n t i a t i o n of synaptic transmission and hence help to m a i ntain certain motor outputs. In the light of these results Sombati and Hoyle

(1984b) suggested the 'Orchestration Hypothesis' which

proposes that the excitation of the appropriate modualtory neurones is a prerequisite to the selection, production and maintenance of specific behaviours.

One other study has recently investigated the m orphology and ultrastructure of DUM neurones in the locust metathoracic ganglion at the light and electron microscopic level (Watson, 1984). This work concentrated on the D UM neurones which send axons to the muscles; no examination was carried out on the DUM neurones that are interneurones. At the light level it is seen that a single primary neurite arises from the soma and then splits into two lateral neurites wh i c h pass into peripheral nerves on each side of the body. Each of these three m ain neurites gives off secondary branches. At the electron m i c r oscopic level Watson (1984) found presynaptic inputs of several types onto the spines of the lateral neurites and onto the secondary branches of D U M neurones. Very few output synapses were observed and they were found

only on lateral neurite spines. Watson (1984) therefore concluded that the D U M neurones examined di d not play a major role in the CNS.

Hence there appears to be a conflict b e t ween the results of Watson (1984) and those of Sombati and Hoyle (1984a and b ) . It may be that although the number of central output synapses a DU M neurone makes fs small, each one is very

page 41c General Introduction - Biogenic Amines potent, or that it is the interneuronal D U M cells w h i c h have the m ajor central actions. Only further experiments will clear up this problem

have been isolated from insect

nervous tissue.

metabolic pathways for handling the biogenic amines

are, however,

understood

studies on the

insect

results

of labelled

precursor

mammalian pathways and thereby deducing the presence or

absence of certain enzymes. Care must be taken with

this,

however, due to the non-neural function of amines in

tanning. It is known that

N-acetyl dopamine acts

protein cross-linking agent in the formation of

sclerotin, and that cuticle formation occurs in the

haemolymph (Whitehead, 1969), but the location of the N-

acetylation step is still uncertain.

What

is

demonstration of

various

specific locations.

From the work so far it

does

synthesized from tyrosine, and 5-HT from tryptophan,

probably by the same pathways as in mammals (Hoyle and Barker, 1975; Maxwell Tait and Hildebrand, 1978; Osborne and Neuhoff, 1 9 7 4 b ) . As yet no synthesis of N A has been demonstrated, but it is present in much smaller amounts. It is also uncertain whether or not the first step in the

conversion

decarboxylation

and Neuhoff, 1976; Mir and Vaughan, 1981) rather than

hydroxylation (as is found in m a m m a l s ) . Finally, it is

still not known if the decarboxylation of the catecholamine and 5-HT

precursors

not,

The

postsynaptic

General Introduction - Biogenic amines page 43 reused, if taken up into the presynaptic terminals, or enzymatically inactivated, if taken up into other neuronal or non-neuronal elements. In 1978b Evans described a three component uptake mechanism for OA in the ventral nerve cord

of the cockroach, Periplaneta a m e r i c a n a . The system

consisted of high and low affinity sodium sensitive

components and a sodium insensitive component. Experiments with agonists and antagonists showed that the high affinity sodium sensitive component is similar to the one found in mammalian heart (Iversen, 1965; Evans, 1 9 7 8 b ) . This does suggest that amines in insects are dealt with in a similar way to mammals, although it must be noted the tissue locations of the octopamine uptake components were unknown. The major inactivating enzymes in vertebrates are monoamine oxidase (MAO) and catechol-O-methyltransferase (COMT). The activities of both these enzymes have been shown to be low in the nervous tissue of locust (Vaughan and Neuhoff, 197 6; Hayashi, Murdock and Florey, 1977), Drosophila (Dewhurst, Croker, Ikeda and McCaman, 1972), and honeybee brain (Evans and Fox, 1972). Although Richter and Rutschke (1977) have shown MAO activity in specific areas of the cockroach brain. In general, however, N-acetylation seems to be the major mechanism of inactivation, at least for DA, OA and 5-HT (Dewhurst et al., 1972; Vaughan and Neuhoff, 197 6; Evans and Fox, 1972; Hayashi et al., 1977; Maranda and Hodgetts, 1977).

The Role of Cyclic Nucleotides

It is becoming apparent that several neurotransmitters and

cyclic nucleotides. These act as intracellular second

messengers, bringing about the appropriate cellular

response by altering the activity of specific enzymes.

Several criteria need to be fulfilled, however, before

the involvement of the cyclic nucleotides can be concluded.

This proof is not complete for any of the insect CNS

responses, and in general amounts simply to the fact that

biogenic amines can alter cyclic nucleotide levels in a nervous tissue homogenate.

Bodnaryk (1979, 1980) reported the presence of both a DA and OA-sensitive adenylate cyclase in the brain of the moth, Mamestra c o n f i g u r a t a . No evidence for these, however, was found in the nerve cord of the larva of the sphinx moth, Manduca se x t a , where only 5-HT-sensitive adenylate cyclase activity was seen (Taylor, Dyer and Newburgh, 1976; Taylor and Newburgh, 1978, 1979). Taylor and Newburgh (1979) also found that several other putative neurotransmitters had no effect on cyclic AMP levels: NA, ACh, GABA, glutamate, glycine, glutamine, aspartate and adrenaline. Instead they found that several of these, namely: ACh, GABA, aspartate, glutamate, and glycine, all increased the level of cyclic G M P . OA, DA and 5-HT-sensitve adenylate cyclase activity has been located in the thoracic ganglia of the cockroach, Periplaneta americana (Nathanson and Greengard, 1973, 1974). NA stimulated activity as well, but only at much higher concentrations. By carrying out experiments in which they

simultaneously added two of the amines, each at a

concentraton which maximally stimulated adenylate cyclase, Nathanson and Greengard (1973) were able to show that DA, OA

General Introduction - B i ogenic amines page 45

and 5-HT acted at different receptors, whereas the N A had no