The effects of pargyline on overflow of transmitter and uptake of noradrenaline in the cat spleen

Orphan neurones and amine excess: the functional neuropathology of Parkinsonism and neuropsychiatric disease

The aetiology and treatment of Parkinsonism is currently conceptualised within a dopamine (DA) deficiency-repletion framework. Loss of striatal DA is thought to cause motor impairment of which tremor, bradykinaesia and rigidity are prominent features. Repletion of deficient DA should at least minimise parkinsonian signs and symptoms. In Section 2, based on extensive pre-clinical and clinical findings, the instability of this approach to Parkinsonism is scrutinised as the existing negative findings challenging the DA deficiency hypothesis are reviewed and reinterpreted. In Section 3 it is suggested that Parkinsonism is due to a DA excess far from the striatum in the area of the posterior lateral hypothalamus (PLH) and the substantia nigra (SN). This unique area, around the diencephalon/mesencephalon border (DCMCB), is packed with many ascending and descending fibres which undergo functional transformation during degeneration, collectively labelled 'orphan neurones'. These malformed cells remain functional resulting in pathological release of transmitter and perpetual neurotoxicity. Orphan neurone formation is commonly observed in the PLH of animals and in man exhibiting Parkinsonism. The mechanism by which orphan neurones impair motor function is analogous to that seen in the diseased human heart. From this perspective, to conceptualise orphan neurones at the DCMCB as 'Time bombs in the brain' is neither fanciful nor unrealistic [E.M. Stricker, M.J. Zigmond, Comments on effects of nigro-striatal dopamine lesions, Appetite 5 (1984) 266-267] as the DA excess phenomenon demands a different therapeutic approach for the management of Parkinsonism. In Section 4 the focus is on this novel concept of treatment strategies by concentrating on non-invasive, pharmacological and surgical modification of functional orphan neurones as they affect adjacent systems. The Orphan neurone/DA excess hypothesis permits a more comprehensive and defendable interpretation of the interrelationship between Parkinsonism and schizophrenia and other related disorders.

alpha-Adrenoceptor-mediated inhibition of noradrenaline release in rabbit brain cortex slices. Receptor properties and role of the biophase concentration of noradrenaline

Brain cortex slices from rabbits were preincubated with [3H]noradrenaline and then superfused and stimulated electrically at 3Hz. In the presence of cocaine 30 microM, unlabelled noradrenaline, alpha-methylnoradrenaline, clonidine, oxymetazoline, xylazine and guanabenz decreased, whereas yohimbine, corynanthine, phentolamine, tolazoline and azapetine increased the stimulation-evoked overflow of tritium. Phenylephrine and prazosin had no effect on the evoked overflow except at concentrations that greatly accelerated the basal outflow of tritium. The results indicate that the noradrenergic axons of rabbit brain cortex are endowed with presynaptic alpha-adrenoceptors which are exclusively of the alpha 2-type. Addition of various concentrations of cocaine, addition of pargyline, or stimulation at different current strengths was used to obtain either a high or low stimulation-evoked overflow of tritium. Independently of the method used, a low evoked overflow coincided with a large percentage inhibition produced by 0.1 micro M clonidine, whereas a high evoked overflow coincided with a smaller percentage inhibition produced by clonidine. The results indicate that drugs which block the re-uptake of noradrenaline diminish the presynaptic inhibitory effect of alpha-adrenergic agonists by increasing the biophase concentration of released noradrenaline.

Deamination of released 3H-noradrenaline in the canine saphenous vein

Experiments were designed to determine the effect of monoamine oxidase (MAO) inhibitors on the release and the metabolism of noradrenaline in the canine saphenous vein. Helical strips were incubated with 3H-noradrenaline and mounted for superfusion and measurement of the efflux of labelled transmitter and its metabolites; in certain experiments the tissue content of 3H-noradrenaline and its metabolites was also determined. The MAO-A inhibitor clorgyline, and the non-specific inhibitor pargyline, but not the MAO-B inhibitor deprenyl decreased the appearance of deaminated and O-methylated deaminated metabolites under basal conditions and during electrical stimulation. The MAO-A and the non-specific MAO inhibitor did not decrease the efflux of VMA to the same extent as that of the other deaminated metabolites. During superfusion with etidocaine, an agent causing increased leakage of stored transmitter, clorgyline abolished the appearance of DOPEG. Addition of semicarbazide in preparations treated with pargyline did not affect the efflux of deaminated and O-methylated deaminated metabolites. From the measurement of tissue VMA, it appeared that the efflux of VMA poorly reflects quick changes in the rate of its formation but that formation is abolished by pretreatment with pargyline. These experiments indicate that in the canine saphenous vein: (1) DOPEG is formed mainly in intraneuronal sites, while DOMA, MOPEG and VMA are formed extraneuronally; (2) VMA is retained in the tissue after its formation; and (3) the only subtype of MAO involved in the metabolism of 3H-noradrenaline released from adrenergic nerve endings can be classified as MAO-A.

Inhibition of neuronal uptake reduces the presynaptic effects of clonidine but not of alpha-methylnoradrenaline on the stimulation-evoked release of 3H-noradrenaline from rat occipital cortex slices

The iminoimidazolidine clonidine reduced concentration-dependently the release of 3H-noradrenaline evoked by electrical stimulation from the rate cerebral cortex. Exposure to the neuronal uptake inhibitors cocaine (10 micro M), desipramine (0.1 to 1 micro M) and amphetamine (1 micro M) significantly increased the stimulation-evoked overflow of tritium. These uptake inhibitors antagonized the effects of clonidine on stimulation evoked 3H-noradrenaline release but failed to modify the inhibition induced by the catecholamine alpha-methylnoradrenaline. Inhibition of monoamine oxidase by preincubation of cerebral cortex slices with 0.5 mM pargyline significantly increased the stimulation-evoked overflow of tritium, but clonidine was as effective as in the controls in inhibiting 3H-noradrenaline overflow. The antagonism by desipramine of the clonidine-induced inhibition of neurotransmission could not be attributed to a blockade of presynaptic alpha-adrenoceptors because: (1) the facilitating effect of phentolamine on 3H-noradrenaline overflow was not modified in the presence of desipramine; (2) the magnitude of the inhibition of the stimulation-evoked 3H-noradrenaline release elicited by alpha-methylnoradrenaline was the same in the presence of cocaine or desipramine; (3) exposure to desipramine in the presence of cocaine did not further increase the stimulation-evoked release of 3H-transmitter. Since the catecholamine alpha-methylnoradrenaline inhibited neurotransmission in the presence of desipramine or cocaine, we can conclude that inhibition of neuronal uptake of noradrenaline antagonized selectively the presynaptic inhibitory effects of imidazolines on alpha 2-adrenoceptors. The influence of the inhibition of neuronal uptake on the presynaptic effects of imidazolines and catecholamines should be taken into account when the relative order of potencies of various alpha 2-adrenoceptors agonists is determined.

The effects of piperoxan on uptake of noradrenaline and overflow of transmitter in the isolated blood perfused spleen of the cat

1 The competitive alpha-adrenoceptor blocking agent, piperoxan, in concentrations up to 2 x 10(-4) M, produced large dose-dependent increases in transmitter overflow from the isolated blood perfused spleen of the cat following nerve stimulation at 10 hertz. 2 At concentrations greater than 2 x 10(-4) M, piperoxan produced a rise in perfusion pressure, a contraction of the splenic capsule, and a marked dose-dependent decrease in transmitter overflow. 3 Phenoxybenzamine (10(-4) M) and desmethylimipramine (3 x 10(-5) M) produced further increases in transmitter overflow when added after piperoxan. 4 Piperoxan (5.8 to 6.6 x 10(-6) M) had no effect on the recovery of 3H in the venous blood following the close arterial infusion or injection of (3H)-(--)-noradrenaline, indicating that the drug does not inhibit uptake of the amine. 5 Piperoxan produced dose-dependent inhibition of responses of the splenic vasculature to close arterial injection of 1 microgram of (--)-noradrenaline but was much less effective at inhibiting responses to nerve stimulation. At 2 x 10(-6) M piperoxan produced a considerable reduction of the response to injected noradrenaline but potentiated the response to nerve stimulation. 6 In isolated strips of cat splenic capsule, piperoxan produced a shift to the right of the dose-response curve to noradrenaline with no change of the maximum response. There was no evidence of a postsynaptic sensitizing effect of the type observed in the rat vas deferens.

The uptake kinetics and metabolism of extraneuronal noradrenaline in guinea-pig trachea as studied with quantitative fluorescence microphotometry

1 Extraneuronal uptake of noradrenaline and alpha-methylnoradrenaline into single cells of guinea-pig tracheal smooth muscle have been studied by means of quantitative fluorescence microphotometry. 2 Fluorescence brightness due to accumulation of alpha-methylnoradrenaline was dose-dependent and was increased by the catechol-O-methyltransferase inhibitor drugs, tropolone or beta-thujaplicin (200 micrometer) but not by 3,4-dimethoxy-5-hydroxybenzoic acid (200 microgram). 3 Fluorescence brightness due to accumulation of noradrenaline was increased if animals were pretreated with the monoamine oxidase inhibitor drug, nialamide. 4 The study suggests that metabolism of amines by catechol-O-methyltransferase and monoamine oxidase can occur subsequent to extraneuronal uptake is guinea-pig tracheal smooth muscle. 5 The uptake of noradrenaline into tracheal smooth muscle was concentration-dependent, saturable, and had a Km of 156 micrometer.

The effects of labetalol (AH 5158) on adrenergic transmission in the cat spleen

1. The competitive alpha- and beta-adrenoceptor blocking agent labetalol, in concentrations up to 10(-4) M, produced dose-dependent increases in transmitter overflow from the isolated blood perfused spleen of the cat following nerve stimulation at 10 and 30 Hz. 2. At concentrations above 10(-4) M labetol produced a pronounced decrease in transmitter overflow. 3. Labetalol (1.5 X 10(-4) M) increased the recovery of 3H label in the venous blood following the close-arterial infusion of [3H]-(-)-noradrenaline indicating that the drug inhibits uptake of the amine. 4. Both labetalol (3.8 X 10(-5) M) and piperoxan (7.4 X 10(-6) M) produced parallel shifts to the right of the dose-response curves to noradrenaline and oxymetazoline in isolated strips of cat splenic capsule. In this preparation both drugs acted as competitive postsynaptic alpha-adrenoceptor blocking agents. 5. Labetalol (3.3 X 10(-5) M) increased the transmitter overflow following stimulation of the splenic nerves with 200 impulses at 10 Hz. The overflow could be further increased by subsequent addition of piperoxan (7.2 X 10(-6 M). Piperoxan (5.7 X 10(-6) M) alone produced a marked increase in transmitter overflow which could be further increased by subsequent addition of desmethylimipramine (DMI; 3.2 X 10(-5) M). Cocaine (1.5 X 10(-5) M) or DMI (5.4 X 10(-5 M) produced a small increase in transmitter overflow which was not further increased by addition of labetalol (2.8 X 10(-5) M). 6. Labetalol produced a biphasic effect on the responses of the isolated blood perfused spleen of the cat to nerve stimulation. With low doses (up to 10(-4) M) vascular responses were potentiated and with high doses (greater than 10(-4) M) inhibited. The potentiation was related to uptake blockade and the inhibition to decreased transmitter overflow and postsynaptic alpha-adrenoceptor blockade. 7. Labetalol appears to act as a postsynaptic alpha-adrenoceptor antagonist in the isolated blood perfused spleen of the cat with little effect on presynaptic alpha-adrenoceptors. The moderate elevation of transmitter overflow by the drug is related to the inhibitory effect of the drug on neuronal uptake rather than on presynaptic alpha-adrenoceptors.

Stimulus-secretion coupling processes in brain: analysis of noradrenaline and gamma-aminobutyric acid release

1. Brain synaptosomal fractions released both endogenous and exogenously loaded noradrenaline and gamma-aminobutyric acid (GABA) in response to calcium. Elevation of magnesium concentrations in the release media decreased the calcium-dependent release. 2. The release of noradrenaline and GABA occurred within 250 msec following the application of calcium. Following the initial response to calcium, release progressively decreased with continued application of calcium. GABA release declined more rapidly than noradrenaline release, consistent with a noradrenaline distribution having greater accessibility to the release process. 3. Sodium was required for the loading of noradrenaline and GABA into pools released by calcium. On the other hand, the presence of sodoium was not required for release from previously loaded pools. 4. Microsomal fractions did not exhibit calcium-dependent release of noradrenaline or GABA. Furthermore, exogenously loaded lysine was not released from synaptosomal fractions in response to calcium. 5. Barium and strontium, but not magnesium, stimulated noradrenaline and GABA release in the absence of calcium. The ordering of alkaline earth efficacies was barium greater than strontium greater than calcium. 6. Manganese inhibited calcium-dependent release of noradrenaline and GABA to a greater extent than magnesium. 7. Release, in response to 1 mM calcium, increased linearly with the log. [K+]0, suggesting that a voltage-dependent calcium inophore limits release. The slope of release vs. log. [K+]0 was greater for noradrenaline than for GABA. 8. For a given [K+]0 less than 55 mM, increases in external calcium concentration above 1 mM increased noradrenaline release but decreased GABA release. These data suggest that calcium can decrease its own permeation and that differences in the release process may exist for different neurotransmitters. 9. In the presence of the artificial calcium ionophore, A23187, both noradrenaline and GABA release increased linearly with the log. [Ca2+]0. The slope for noradrenaline release was greater than that for GABA release. 10. Stimulus-secretion coupling in brain is suggested to be regulated at the level of a voltage dependent calcium permeation mechanism. However, basic differences in the interaction of calcium with the release process may exist between the noradrenaline and GABA systems.

An uptake mechanism for L-noradrenaline in the cat spleen, associated with the nerves but distinct from uptake

1. The characteristics of the uptake of [(3)H]L-noradrenaline from 1 mug pulses injected close arterially to the isolated blood perfused cat spleen are described.2. The spleen took up 355.0+/-16.0 ng from a first pulse and 321.1+/-43.6 ng from a second.3. Uptake from a second pulse was 254.0+/-30.3 ng in the presence of the Uptake(1) inhibitor desmethylimipramine (DMI) (3.3 x 10(-5)M) and the Uptake(2) inhibitor 17-beta-oestradiol (17beta0) (1.8 x 10(-4)M).4. Uptake from small 10 ng pulses was also insensitive to DMI and 17beta0.5. Uptake from pulses was abolished by surgical denervation of the spleen, or pre-treatment with phenoxybenzamine (PBA) (8.8 x 10(-5)M).6. The pulse uptake process was impaired by omission of red cells from the perfusate.7. Stimulation of the splenic nerves at 3Hz halved the pulse uptake.8. Uptake from infusion in the presence of DMI and 17beta0 occurred only during the first few minutes of the infusion.9. It is concluded that an uptake process for L-noradrenaline distinct from Uptakes(1) and (2) is present in the spleen and this might be important physiologically.

Proceedings: Connections between the olfactory bulb and the habenula and dorsomedial thalamic nuclei

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The accumulation and metabolism of (-)-noradrenaline by cells in culture

1. Cultured bovine embryonic tracheal cells, EbTr (NBL-4) possess a process for the intracellular accumulation of (-)-noradrenaline with the characteristics generally ascribed to extraneuronal uptake by cardiac and smooth muscle cells in the body. It has a K(m) of 2.6 x 10(-4)M.2. The accumulation process is inhibited competitively by normetanephrine, but only at relatively high concentrations, IC50=2.1 x 10(-4)M. Inhibition also occurs with 17-beta-oestradiol, IC50=1.3 x 10(-5)M.3. The noradrenaline metabolites, 3,4-dihydroxy-mandelic acid and 3,4-dihydroxy-phenylglycol potentiate accumulation and reduce intracellular levels of normetanephrine in a similar manner to known inhibitors of catechol-O-methyl transferase.4. It is suggested that intracellular, rather than extracellular, normetanephrine may exert feedback inhibition upon noradrenaline accumulation by combining with the transport process at the inner surface of the cell membrane.