Relationship between dopamine release in the rat nucleus accumbens and the discharge activity of dopaminergic neurons during local in vivo application of amino acids in the ventral tegmental area

Amino acids were pressure-ejected in the ventral tegmental area of rats which were anesthetized with chloral hydrate and treated with pargyline. The extracellular dopamine concentration was recorded from the nucleus accumbens with an electrochemically treated carbon fiber electrode combined either with differential normal pulse voltammetry or with differential pulse amperometry. In distinct rats the discharge activity of single dopaminergic neurons was monitored in the ventral tegmental area while amino acids were pressure-injected at a distance of 200-300 microns from the recorded cell. GABA (24 and 50 nl, 1 M) induced a complete and reversible inhibition of the firing rate lasting for 3-6 min and a decrease in the basal extracellular dopamine level (-54% and -66%, respectively). Glutamate (32 nl, 10 mM), N-methyl-D-aspartate and quisqualate (100 microM) stimulated the firing rate and enhanced the dopamine extracellular concentration up to 10-times the basal one (18 nM). These increases subsided within 1-5 min. Their amplitude depended on the ejected volume (from 16 to 65 nl). At the time-resolution of the method (some seconds) all these variations in the dopamine release appeared closely time-correlated with those of the firing rate. When the mean discharge rate is considered, N-methyl-D-aspartate was as potent as quisqualate but the former promoted burst firing while the latter induced a sustained activity. As regards dopamine release, N-methyl-D-aspartate was twice as potent as quisqualate. This further shows that dopaminergic terminals convert physiological impulse flow into dopamine release as a high pass filter which favors bursts of action potentials.

Thyrotropin releasing hormone: enhancement of dopa activity by a hypothalamic hormone

Thyrotropin releasing hormone potentiates the behaviorial effects of dopa plus pargyline in mice. Because the potentiation occurs in hypophysectomized mice, as well as in normal mice, the phenomenon is independent of the release of thyroid stimulating hormone from the pituitary. Possible mechanisms and clinical implications are discussed.

Effects of the noradrenaline neurotoxin DSP4 on the postnatal development of central noradrenaline neurons in the rat

The effect of the noradrenaline neurotoxin DSP4 on the postnatal development of central noradrenergic neurons in the rat has been investigated using neurochemical techniques. The results demonstrated a preferential effect of DSP4 on the locus coeruleus noradrenergic neuron system without any notable effects on the dopamine and adrenaline neurons and only a minor neurotoxic effect on the serotonin neurons. The effect of DSP4 on the serotonin neurons could be completely prevented by pretreatment with the uptake blocker zimelidine, without affecting the action of DSP4 on noradrenergic neurons. Neonatal DSP4 treatment systemically led to permanent depletions of noradrenaline in the cerebral cortex and spinal cord and marked increases of noradrenaline in the cerebellum and pons-medulla. These effects of DSP4 were dose-dependent and could be blocked by pretreatment with the noradrenaline uptake blocker desipramine. The alterations in endogenous noradrenaline levels were quantitatively similar to changes observed in [3H]noradrenaline uptake in slices in vitro. There were no significant changes of these noradrenergic parameters when analysing the whole CNS after neonatal DSP4 treatment, in spite of marked regional changes in both directions. Administration of DSP4 to rats of different ages produced acutely marked depletions of noradrenaline in all regions including the pons-medulla and the cerebellum at all developmental stages. Marked and permanent depletions of noradrenaline were found in the distant noradrenergic nerve terminal projections after treatment at all ages, whereas increases in noradrenaline levels in the pons-medulla and cerebellum were only observed in rats treated with DSP4 up to the age of 3-5 days, whereas a DSP4 administration in older rats led to substantial and permanent depletions of noradrenaline in both these regions. The results indicate that the alteration of the postnatal development of noradrenergic neurons after treatment of rats up to the age of 3-5 days is mainly related to a 'pruning effect' of DSP4, in which prevention of the development of distant nerve terminal projections causes an increased outgrowth of nerves in collateral systems spared by the neurotoxin. The results indicate that DSP4 may be a useful denervation tool for studying various aspects of noradrenergic neurotransmission of developing locus coeruleus neurons.

Intra- and extraneuronal formation of the two major noradrenaline metabolites in the cns of rats

The two major noradrenaline metabolites in the rat brain, total 3-methoxy-4-hydroxyphenylglycol (MOPEG) and total 3,4-dihydroxyphenylglycol (DOPEG), were measured by a new analytical procedure after intraventricular (i.v.t.) injection of (3-H) noradrenaline or (3-H) dopamine. I.v.t. injection of (3-H) noradrenaline to rats with a 6-hydroxydopamine-induced destruction of nerve terminals in the central nervous system, resulted in an increased accumulation of 3-H-MOPEG compared to 3-H-DOPEG. On the contrary, reserpine induced a greater increase in 3-H-DOPEG than in 3-H-MOPEG accumulation when these two metabolites were formed from (3-H) noradrenaline synthesized in vivo by i.v.t. injection of (3-H) dopamine. These results indicate that the formation of DOPEG and MOPEG occur with some preference intra- and extraneuronally, respectively. The results, however, also show that these two formations sites should not be considered as specific.

Enhanced dopamine receptor activation in accumbens and frontal cortex has opposite effects on medial forebrain bundle self-stimulation

This study was undertaken to investigate the effects of activating dopamine receptors in accumbens and prefrontal cortex on self-stimulation behavior in the medial forebrain bundle. The experiments were carried out in rats chronically implanted with one stimulating electrode in medial forebrain bundle and two bilaterally-placed cannulas for giving injections into accumbens or prefrontal cortex. After completion of training, animals classified as responders and non-responders were given drug tests. The non-responders were tested to determine the effects of the treatment on motor activity. The self-stimulation task involved the depression of a lever to obtain a stimulus of 0.25 s duration, 60 Hz sine waves applied to the medial forebrain bundle. Dopamine receptor activation in accumbens or prefrontal cortex was induced with bilateral injections in these structures of a mixture containing 5 mg dopamine, 10 mg d-amphetamine sulfate and 5 mg pargyline mixed in 0.5 ml saline containing 0.1% ascorbic acid (dopamine + d-amphetamine sulfate + pargyline, the cocktail). Each injection was of 2 microliters/side, yielding a concentration of 20 micrograms of dopamine, 40 micrograms of d-amphetamine sulfate and 20 micrograms of pargyline/injection. The bilateral injections were given immediately before the self-stimulation session which lasted 12 h, starting in late afternoon. The effects of saline containing the ascorbate were determined in control sessions. Saline injected bilaterally in accumbens or prefrontal cortex of self-stimulators or non-self-stimulators had no effects on the response-rate of self-stimulators or on the gross motor activity of non-responders. In contrast, the cocktail of dopamine + d-amphetamine sulfate + pargyline injected in accumbens of self-stimulators induced a complex response which included first a facilitation, then a prolonged suppression and then again one or two episodes of facilitation interspersed with periods of suppression of self-stimulation and then a return to baseline rats. The same cocktail of dopamine + d-amphetamine sulfate + pargyline injected bilaterally in accumbens of non-self-stimulators resulted also in a complex response including as a first component a facilitation of responding, but the complex effect was of shorter duration and lower magnitude, never raising the rate of lever-pressing to levels meeting self-stimulation criteria. The same cocktail of dopamine + d-amphetamine sulfate + pargyline injected in prefrontal cortex of self-stimulators simply attenuated or suppressed responding, and the effect lasted for most of the session. The same effect was seen in non-self-stimulators indicating a decrease in gross motor activity.(ABSTRACT TRUNCATED AT 400 WORDS)

Pressure reversed extracellular striatal dopamine decrease produced by D1 receptor agonist SKF 38393, and D2 receptor agonist LY 171555, but failed to change the effect of the activation of both D1 and D2 receptors

When human divers or experimental animals are exposed to high pressure, they develop the high-pressure neurological syndrome which is characterized by electroencephalographic changes, and behavioral disturbances. Recently, neurochemical disorders such as a pressure-induced increase in dopamine release have been demonstrated. In the present study, pharmacological experiments, using dopamine receptor agonists such as D1 receptor agonist SKF 38393, D2 receptor agonist LY 171555, and D1/D2 receptor agonist apomorphine, were performed to investigate dopamine receptor function at the neurochemical level. Only apomorphine and mixed SKF 38393 + LY 171555 prevented the pressure-induced increase in dopamine release while SKF 38393 or LY 171555 administered alone failed to do so. The results suggest that the D1-D2 link would be reduced under high pressure because of an abnormal function of D1 receptors which would allow high-affinity D2 states for dopamine. If so, such a preponderance of high-affinity states in D2 postsynaptic receptors could be associated with hyperbaric hyperlocomotor activity. Elsewhere, results also suggested that the pressure-induced disorders in dopamine receptor function could be involved in the pressure-induced elevation in dopamine release.

Central serotonin level-dependent changes in body temperature following administration of tryptophan to pargyline- and harmaline-pretreated rats

1. The effect of tryptophan on body temperature was studied in rats pretreated with pargyline, an irreversible monoamine oxidase inhibitor (MAOI), and harmaline, a reversible MAOI. 2. Tryptophan (100 mg/kg IP) produced hypothermia followed by hyperthermia in pargyline-pretreated rats, and hypothermia in harmaline-pretreated rats, but tryptophan did not cause body temperature changes by itself. 3. The tryptophan-induced hypo- and hyperthermic effects, which peaked at about 1 and 6 hr after tryptophan administration, respectively, were accompanied by a significant increase in serotonin (5-HT) levels in the pargyline-pretreated rat brain (75%-138.7% and 207%-240.9% increase, respectively), and the 5-HT levels in the hyperthermic state were significantly higher than those in the hypothermic state. 4. In harmaline-pretreated rats, tryptophan also increased the central 5-HT levels (80.5%-95.5% increase) in the hypothermic state, and the effect peaked at about 1 hr after tryptophan administration. The central 5-HT levels in harmaline-pretreated rats slightly decreased at 6 hr after tryptophan administration and were significantly lower than those in the hyperthermic state in the pargyline-pretreated rats. 5. Tryptophan (100 mg/kg IP) administration decreased 5-hydroxy indole acetic acid (5-HIAA) levels, 5-HT turnover, and dopamine (DA) turnover in the brain of pargyline-pretreated rats, but these parameters were not significantly different between the hypothermic and hyperthermic states (i.e., at 1 and 6 hr after tryptophan administration, respectively). 6. These results suggest that the tryptophan-induced body temperature change depends on the different 5-HT levels in the brain and that the 5-HT level needed to induce hyperthermia is higher than that needed to induce hypothermia.

Central mediation of the antihypertensive effect of pargyline in spontaneously hypertensive rats

The monoamine oxidase (MAO) inhibitor pargyline induced a moderate (about 20 mm Hg) but persistent (48 h) decrease of systolic blood pressure in unanesthetized adult spontaneously hypertensive rats (SHR) but not in normotensive rats. The fall of blood pressure correlated with the blockade of norepinephrine (NE) deamination by brain homogenates. After an intracerebroventricular (icv) injection of 6-hydroxydopamine, which lowered brain NE content by about 70%, pargyline was unable to diminish arterial pressure. Blockade of central alpha-adrenoceptors by treatment with phentolamine (100 microgram icv) could either prevent or reverse the fall of blood pressure in SHR induced by pargylline. Moreover, a low dose of pargyline injected directly into the brain lowered arterial pressure. We conclude that the hypotensive action of pargylline in SHR appears to be the consequence of NE accumulating at an inhibitory alpha-adrenoceptor in brain.

Inhibition of rat spermatogenesis and seminiferous tubule growth after short-term and long-term administration of a monoamine oxidase inhibitor

Seminiferous tubules from rats killed 24 hours after injection of pargyline, a monoamine oxidase inhibitor, did not grow well in tissue culture when compared to control tubules. Treated tubules showed severe tubular degeneration and loss of cellular detail after nine days in culture. Animals injected with pargyline for ten days had varying degrees of semi-niferous tubule degeneration with depletion of the spermatogenic elements. It is suggested that pargyline has a detrimental effect on spermatogenesis. Pargyline possibly acts by decreasing MAO levels which, in turn, may increase potentially damaging amines which may be responsible for the testicular damage.

Pargyline-induced increase in serotonin levels: correlation with inhibition of lordosis in rats

The effect of intrahypothalamic infusion of the monoamine oxidase inhibitor pargyline on lordosis behavior and monamine levels in the preoptic area and hypothalamus was examined. Progesterone-facilitated lordosis was blocked by pargyline in half the treated rats. The inhibition of lordosis was correlated with increases in serotonin and dopamine levels in the ventromedial nucleus of the hypothalamus and serotonin levels in the arcuate nucleus-median eminence when compared to controls or pargyline-treated rats with high levels of lordosis responding. Changes in norepinephrine levels were not correlated with changes in behavior. The results provide further evidence for an inhibitory role of basomedial hypothalamic serotonin in the control of female sexual behavior.

The stimulating effects of ethanol consumption on synthesis of rat brain monoamine oxidases and their sensitivity to the irreversible inhibitor, pargyline

Administration of a large dose of pargyline (60mg/kg) caused total irreversible inhibition of brain monoamine oxidases (MAOs) in both control and alcoholised rats. During the first 50h the recovery of brain MAO-A (but not MAO-B) activity occurred faster in the alcoholised rats. A low dose of pargyline (10mg/kg) produced significantly higher inhibition of MAO-A in the alcoholised rats, whereas the degree of MAO-B inhibition was the same in both groups. Brain MAOs of control and alcoholised rats exhibited similar sensitivity to pargyline in vitro. Since chronic ethanol feeding reduced the content of reversible endogenous MAO inhibitor, tribulin, higher pargyline-induced inhibition of MAO-A in alcoholised rats may stem from a tribulin deficit. The data obtained suggest that chronic ethanol consumption increases turnover of MAO-A molecules in the brain and reduces the content of endogenous MAO(A) inhibitors.

Mechanisms of catecholamine inactivation by the endocrine pancreas

The present study was designed to determine if mechanisms are present in the islets of Langerhans to inactivate catecholamines. The dopamine content of the pancreatic beta cells was increased by incubating rabbit pancreas segments with L-3, 4-dihydroxyphenylalanine (L-dopa) for forty-five minutes prior to stimulation with glucose, 16.5 mM. Monoamine oxidase (MAO) inhibitors (pargyline, tranylcypromine and harmine) potentiate the inhibition of insulin secretion by increased beta cell dopamine, but not by dopamine added to the incubation media. Catechol-Omethyltransferase (COMT) inhibitors (pyrogallol and tropolone) did not potentiate the inhibition of insulin secretion from rabbit pancreas by beta cell dopamine. Isolated islets obtained from rabbits, rats, and golden hamsters contained MAO; only golden hamster, a species with a rich intrainsular adrenergic plexus, had COMT in their islets. Compounds that inhibit cellular uptake of catecholamines (cocaine, nortryptyline, and imipramine) potentiate the inhibition of insulin secretion resulting from the presence of norepinephrine in the incubation media but not the inhibition resulting from increased beta cell dopamine. The present study suggests that MAO and beta cell catecholamine uptake may play a role in the inactivation of catecholamines by the islets of Langerhans.

Interaction of pargyline with rat hepatic microsomes

When administered by intraperitoneal injection daily for 3, 7 or 14 days, pargyline (75 mg/kg) significantly reduced rat hepatic microsomal ethylmorphine N-demethylase activity and cytochrome P-450 content. Injection of a lower dose of pargyline (15 mg/kg) failed to alter significantly either ethylmorphine N-demethylase activity or cytochrome P-450 content. Studies performed in vitro reveal that pargyline is metabolized to at least three compounds by rat hepatic microsomes. Thin-layer chromatography and other analyses suggest that one metabolite is an N-demethylated form, norpargyline.

The involvement of noradrenaline, 5-hydroxytryptamine and acetylcholine in imipramine-induced seizures in mice

The influence of some noradrenergic, 5-hydroxytryptaminergic and cholinergic agents on imipramine-induced seizures were investigated in mice. DL-threo-3,4-dihydroxyphenylserine (DOPS) and pargyline significantly potentiated imipramine-induced seizures. Phentolamine and prazosin significantly attenuated seizures elicited by imipramine and significantly attenuated the seizure-enhancing effect of DOPs. alpha-Methyl-p-tyrosine and reserpine significantly attenuated seizures induced by imipramine. Disulfiram significantly protected mice against imipramine-induced seizures. However, DOPS significantly potentiated seizures induced by imipramine in disulfiram-pretreated animals. Clonidine effectively protected mice against imipramine-induced seizures. Idazoxan, on the other hand, significantly potentiated seizures induced by imipramine and significantly antagonised the protective effect of clonidine against the seizures. 5-HTP, PCPA, cyproheptadine, mianserin, ketanserin and trazodone did not affect imipramine-induced seizures to any significant extent. Physostigmine antagonised seizures induced by imipramine while atropine significantly potentiated the seizures, and significantly attenuated the protective effect of physostigmine against the seizures. These data suggest that enhancement and attenuation of central noradrenergic and cholinergic neurotransmissions respectively, and not 5-HT mechanisms, may underlie imipramine-induced seizures in mice.

Characterization of acute N-ethyl-3,4-methylenedioxyamphetamine (MDE) action on the central serotonergic system

The effect of N-ethyl-3,4-methylenedioxyamphetamine (MDE) on the central serotonergic system was studied. Within 1 hr after administration of MDE (10 mg/kg), the concentration of 5-hydroxytryptamine (5-HT) and the activity of tryptophan hydroxylase (TPH) had declined significantly in the hippocampus but returned to control within 12 hr. Hippocampal 5-hydroxyindoleacetic acid (5-HIAA) content decreased within 2 hr, rebounded to 22% above control by 12 hr, and returned to control by 24 hr. Blockade of the 5-HT uptake carrier with fluoxetine (10 mg/kg) prevented or attenuated the MDE-induced changes in 5-HT content and TPH activity, except for neostriatal TPH activity which remained unresponsive to the fluoxetine treatment. The MDE-induced decline in TPH activity could be reversed by incubating the TPH preparation with dithiothreitol and Fe2+ under nitrogen for 24 hr. This suggests that the loss in TPH activity induced by MDE results from an alteration of the oxidation-reduction state of a sulfhydryl group located on the enzyme. The inhibition of monoamine oxidase (MAO) by the administration of pargyline (75 mg/kg) failed to protect the neostriatal TPH activity from the MDE-induced decline while potentiating the MDE-induced decrease in cortical TPH activity. This suggests that H2O2 generated by MAO in vivo is not responsible for oxidation of the sulfhydryl site located on TPH during the MDE treatment.

Human trophoblast xenografts in athymic mice: a model for peripheral aromatization

A novel procedure was developed for evaluating aromatase inhibitors using human enzyme in a rodent model. Human choriocarcinoma trophoblast (JAr line) cells injected subcutaneously into athymic nude mice develop into tumor xenografts in 7-14 days which represent sites for peripheral aromatization of androgens. The rapid growth of these trophoblast tumors is estrogen independent. The tumors provide a source of nonovarian human tissue which has relatively high levels of enzyme activity (248 +/- 12 pmol estrogen/g/h) for biochemical determination of in vivo aromatase inhibition. These are major advantages for pharmacological evaluations in comparison to the slow tumor growth response of most carcinogen-induced rodent mammary cancers, which are usually devoid of aromatase activity. In addition, the hormonal dependent components of rodent mammary tumors require several weeks to regress as a result of the indirect effects of estrogen deprivation on tumor growth via inhibition of prolactin dependency, a minor component relative to the role estrogen occupies in hormonally-dependent breast cancer in humans. This model of peripheral aromatization was utilized to evaluate in vivo pharmacological parameters of MDL 18,962 (10-(2-propynyl)estr-4-ene-3,17-dione) such as bioavailability of several formulations, time course and dose responses following different routes of drug administration, pharmacokinetics and tissue distribution of [14C]MDL 18,962. Tumor aromatase activities of trophoblast xenografts were significantly (P less than or equal to 0.05) inhibited when MDL 18,962 was administered intravenously, orally, subcutaneously, or via subcutaneous silastic implants. The ED50 of MDL 18,962 for tumor aromatase inhibition at 6 h after a single treatment was 1.4 mg/kg, s.c. and 3.0 mg/kg, orally. MDL 18,962 blocked aromatase activity more effectively in human trophoblast than in mouse ovarian tissue. Human trophoblast aromatase activity was inhibited by 70% following a single oral dose of 100 mg/kg of MDL 18,962, while the host's ovarian aromatase activity exhibited only marginal inhibition. In vitro, the addition of 10 microM MDL 18,962 to trophoblast tumor cytosol or mouse ovarian cytosol resulted in 99.6 and 91.4% inhibition of aromatase activity, respectively. Tissue distribution of [14C]MDL 18,962 was predominantly associated with endocrine tissues with aromatase activity and organ systems involved in steroid metabolism and excretion. These in vivo data show that MDL 18,962 an enzyme-activated aromatase inhibitor, causes prolonged aromatase inhibition in the absence of saturating levels of inhibitor.