Octopamine probably coexists with noradrenaline and serotonin vesicles of pineal adrenergic nerves

The effects of p-octopamine on salivary flow rates and protein secretion by rat submandibular glands

The effects of p-octopamine injected i.v. and i.p on salivary flow rates and proteins secreted by the submandibular glands of rats were studied with and without various types of autonomic blockers at different doses, and with two enzyme inhibitors. The salivary flow rates and the amounts of protein secreted progressively increased with increasing doses injected both i.v. and i.p., whereas they were dramatically reduced with almost all autonomic blockers and disulfiram, a dopamine-beta-hydroxylase inhibitor. Salivation was completely abolished in response to p-octopamine in combination with metoprolol or phenoxybenzamine at high doses, and simultaneous injections of prazosin and propranolol. The concentration of protein in submandibular saliva in response to p-octopamine injected i.v. and i.p. was not dose-dependent and significantly increased with all of the alpha-blockers except yohimbine, and with atropine and disulfiram. The protease activity was dose-dependent but was reduced significantly with alpha-blockers except yohimbine and with two enzyme inhibitors. The alpha-type of protein was secreted in response to p-octopamine injected i.v. and i.p. at all doses except with the lowest dose i.p., which caused the beta-type to be secreted. The alpha-type was completely replaced by the beta-type with all alpha-blockers at all doses, except with yohimbine, but no change was observed with various types of beta-blockers, yohimbine, atropine, and two enzyme inhibitors. Thus, p-octopamine could stimulate both the alpha- and beta-adrenoceptors in the submandibular glands of rats.

The effects of m-octopamine on salivary flow rates and protein secretion by rat submandibular glands

1. m-Octopamine given i.v. or i.p. was a potent sialogogue for rat salivary glands. 2. Salivation in response to i.v. m-octopamine was completely abolished by prazosin and phenoxybenzamine. 3. The alpha-type of proteins were secreted in response to all doses of i.v. and i.p. m-octopamine and these were converted into the beta-type with prazosin, but not with yohimbine. 4. m-Octopamine stimulated both alpha- and beta-adrenoceptors and was a much more selective alpha 1-agonist than was the p-isomer.

Pharmacological studies on the regulation of N-acetyltransferase activity and melatonin content of the pineal gland of the Syrian hamster

Thus far, all attempts to stimulate melatonin synthesis by beta-adrenergic receptor agonists in the Syrian hamster pineal gland have failed. Neither a wide range of dosages of isoproterenol (0.5 mg/kg to 24 mg/kg), nor prolonged treatment with norepinephrine, the natural neurotransmitter, increased N-acetyltransferase (NAT) activity or melatonin production. In the present study, the administration of isoproterenol at night was likewise ineffective in advancing or enhancing the normal nightly melatonin peak. Also, we did not find a delayed effect 7 or 8 h after the administration of the drug. Furthermore, we tested the idea of coneurotransmitters such as octopamine or dopamine being possibly necessary for stimulation, but could not find any effect of these substances on melatonin synthesis. In addition, a parasympatholytic agent, atropine, did not increase the responsiveness to sympathomimetic agents. Administration of a phosphodiesterase inhibitor was also ineffective in stimulating NAT activity. On the other hand, isoproterenol did retard the drop in NAT and melatonin after lights-on at night, indicating that beta-receptors are involved in maintaining elevated melatonin levels.

Localization of noradrenaline and serotonin in nerves in the pineal gland of rats and guinea-pigs studied by glyoxylic acid histofluorescence and electron microscopy

The nerves in the pineal gland of the rat and guinea-pig contain both noradrenaline and serotonin and fluorescence intensely after histofluorescence procedures. Vesicle-filled terminals in the perivascular space of the pineal body contain numerous clear and dense-cored vesicles. A 5 mg/kg dose of reserpine causes disappearance of histofluorescence from the pineal nerves and a virtual elimination of dense-cored vesicles from vesicle-filled terminals. A 1 mg/kg dose of reserpine results in loss of fluorescence and virtual depletion of dense cores in nerves in the rat, but the guinea-pig pineal nerves continue to fluoresce lightly and the dense-cored vesicles are still present but reduced to about 1/3 in number. Subsequent treatment of lightly reserpinized guinea-pigs with p-chlorophenylalanine, a specific depletor of serotonin, results in disappearance of fluorescence in nerves in the pineal gland and virtual depletion of the remaining dense cores. A dose of 1 mg/kg reserpine succeeds in depleting noradrenaline from most peripheral nervous structures of the guinea-pig. Hence, the remaining monoamine in guinea-pig pineal nerves after depletion of noradrenaline appears to be serotonin located in the remaining dense-cored vesicles. Since, in lightly reserpinized guinea-pig pineal nerves, a number of dense-cored vesicles containing serotonin are still present after depletion of noradrenaline, it is suggested that noradrenaline and serotonin are not in the same vesicles at the same time.

Blood platelets as models for neurons: uses and limitations

Platelets show similarities with 5-hydroxytryptaminergic neurons with respect to (1) uptake kinetics of 5-hydroxytryptamine (5-HT) at the plasma membrane, (2) inhibitory effects of tricyclic antidepressants and neuroleptics on 5-HT uptake, (3) granular storage of 5-HT and possibly catecholamines, (4) action of drugs interfering with granular and possibly extragranular amine storage and (5) reaction of the 5-HT receptor at the plasma membrane to 5-HT agonists and antagonists. Dissimilarities include (1) the uptake of catecholamines at the plasma membrane, (2) the biosynthesis of biogenic amines (absent in platelets, present in neurons) and (3) the turnover of 5-HT (slow or absent in platelets, fast in neurons). Although the above mentioned similarities are not absolute, platelets may be considered as reasonable models for some functions of 5-hydroxytryptaminergic neurons e.g. 5-HT uptake at the plasma membrane, intracellular storage of monoamines and reactions of 5-HT receptors to drugs. In addition, the shape change reaction of platelets can probably be used to identify those basic proteins and polypeptides which cause neuronal depolarization. The significance of disturbances of the monoamine system of platelets in neuropsychiatric disorders including Parkinson's syndrome is not yet clear in all respects. Therefore, some of the current ideas about the validity of platelets as models for neurons will be briefly reviewed in this article.

Pineal N-acetyltransferase and hydroxyindole-O-methyltransferase: control by the retinohypothalamic tract and the suprachiasmatic nucleus

The visual pathway and central neural structures involved in the photic and endogenous regulation of the activity of pineal N-acetyltransferase and hydroxyindole-O-methyltransferase were investigated. The results indicate that the visual pathway regulating both enzymes is the retinohypothalamic tract, and that the inferior accessory optic tract is clearly not involved in the regulation of hydroxyindole-O-methyltransferase activity, as has been previously thought. In addition, the suprachiasmatic nucleus was found to be necessary for the generation of a rhythm in N-acetyltransferase activity in blinded animals, and to be responsible for the tonic elevation of hydroxyindole-O-methyltransferase activity in blinded animals. Finally, it was concluded that the rapid and large daily changes in N-acetyltransferase activity seen in a normal lighting cycle and the much slower and smaller changes in hydroxyindole-O-methyltransferase activity seen only after weeks in constant lighting conditions are mediated by the same neural tract; the different time courses of the effects of environmental lighting may be explained on the basis of different intracellular regulatory mechanisms.

Content and subcellular localization of catecholamines and 5-hydroxytryptamine in human and animal blood platelets: monoamine distribution between platelets and plasma

1 The content of adrenaline (Ad), noradrenaline (NA) and dopamine was measured in human, guinea-pig, cat, rabbit and rat blood platelets by a highly sensitive and specific radioenzymatic method.2 In all platelet specimens analyzed, the content of the three catecholamines (CA) was several thousand times lower than that of 5-hydroxytryptamine (5-HT).3 In basal conditions, the NA concentration in platelets and plasma always exceeded that of Ad and dopamine.4 In rat and rabbit platelets, Ad, NA and dopamine were present only in the free (unconjugated) form.5 Platelets of rats with storage pool deficiency (Fawn-hooded) contained much less 5-HT and CA than normal rat platelets.6 Following restraint stress, platelets of Fawn-hooded rats, in contrast to normal rat platelets, did not accumulate CA in spite of a dramatic rise in plasma CA.7 Reserpine, a monoamine depletor, released CA as well as 5-HT from rabbit platelets in vivo.8 Subcellular fractionation experiments with rabbit platelets indicate that both CA and 5-HT are most concentrated in the fraction consisting of pure 5-HT organelles.9 Both in humans and rabbits the concentration gradient between platelets and plasma was much lower for CA than for 5-HT, indicating that a high affinity transport mechanism operates in vivo for 5-HT but not for CA.10 In conclusion, the present data show that both human and animal platelets contain Ad, NA and dopamine. The bulk of the CA seems to be stored as unconjugated amines together with 5-HT, histamine and p-octopamine in a multitransmitter storage site, namely the 5-HT organelle.

Octopamine

Octopamine is highly concentrated in neurones of several invertebrate species. Unlike in mammals, octopaminergic neurones in invertebrates are spatially separated from catecholaminergic neurons. In identified nerve cells of Aplysia, however, this amine coexists with other putative neurotransmitters. Octopamine is synthesized in nerves from tyrosine and tyramine and metabolised mainly by monoamine oxidase. When lobster nerves are depolarized, octopamine is liberated by a Ca2+-dependent process. A specific adenylate cyclase is stimulated by octopamine in several invertebrates to activate phosphorylase in the cockroach, induce a light-flash in firefly lattern or inhibit rhythm contractions in locust muscle. All of these observations provide compelling evidence that octopamine is a neurotransmitter in invertebrates. In mammals octopamine is localised in nerves in peripheral tissues and brain where it seems to coexist with noradrenaline, the catecholamine being present in much higher concentrations. Octopamine is released from nerves together with noradrenaline and it may under certain conditions modify the actions of the adrenergic neurotransmitter. Octopamine is present in unusually high concentrations in certain neurological and hepatic diseases and may have a pathophysiological role.

Do some nerve cells release more than one transmitter?

The concept that each nerve cell makes and releases only one nerve transmitter (widely known as Dale's Principle) has been re-examined. Experiments suggesting that some nerve cells store and release more than one transmitter have been reviewed. Developmental and evolutionary factors are considered. Conceptual and experimental difficulties in investigating this problem are discussed. It is suggested that the term 'transmitter' should be applied to any substance that is synthesised and stored in nerve cells, is released during nerve activity and whose interaction with specific receptors on the postsynaptic membrane leads to changes in postsynaptic activity. Expressed in this way, it seems likely that while many nerves do have only one transmitter, others in some species, during development or during hormone-dependent cycles, employ multiple transmitters.

Stimulation of beta-adrenergic receptors in the pineal gland increases the noradrenaline stores of its sympathetic nerves

The administration of isoproterenol decreases the level of serotonin in the rat pineal gland and at the same time it increases pineal noradrenaline. These effects depend on the stimulation of a beta-adrenergic receptor because they are blocked by pretreatment of the animals with propranolol; this drug by itself does not modify either serotonin or noradrenaline levels in the pineal. The elevation of noradrenaline produced by isoproterenol is selective for the pineal because it is not observed in the salivary gland innervated by postganglionic adrenergic fibers from the same origin as pineal nerves. Pineal serotonin is stored in equilibrium in two compartments, i.e., the parenchymal cells and the adrenergic nerves and thus is most probably reduced in both sites. Since noradrenaline and serotonin are detected in pineal nerve vesicles and may coexist in them, the diminution of intravesicular serotonin, by making more storage sites available, probably determines the selective increase of pineal noradrenaline. A similar modification in the ratio of intravesicular amines as a result of the physiological stimulation of pineal beta-adrenergic receptors by the adrenergic neurotransmitter may explain some of the changes observed in the content of pineal amines.