Judith R. Walters

Central dopaminergic neurons: effects of alterations in impulse flow on the accumulation of dihydroxyphenylacetic acid.

Stimulation of the nigro-neostriatal or mesolimbic dopamine pathway results in a stimulus dependent increase in the accumulation of dihydroxyphenylacetic acid (DOPAC) in the neostriatum and olfactory tubercles, respectively. A block of impulse flow induced pharamacologically by administration of gamma-butyrolactone or by placement of a lesion in the dopamine pathway results in a decrease in the steady state levels of DOPAC. Drugs which have previously been shown to alter impulse flow in central dopaminergic neurons also produce a predictable change in the brain levels of DOPAC. Drugs which increase impulse flow in nigro-neostriatal or mesolimbic dopamine neurons increase DOPAC levels in the striatum and olfactory tubercles and drugs which reduce impulse flow cause a reduction in DOPAC. Pargyline, a monoamine oxidase inhibitor, causes a rapid depletion of striatal DOPAC suggesting that this metabolite is rapidly cleared from the brain. Administration of benztropine, a potent inhibitor of dopamine reuptake, causes a significant decrease in striatal DOPAC and partially prevents the stimulus-induced increase in the accumulation of DOPAC. These observations together with the finding that about 85% of the DOPAC in the striatum disappears when the dopamine neurons in the nigro-neostriatal pathway are destroyed suggests that the majority of striatal DOPAC is formed within the dopaminergic neurons and may reflect the metabolism of dopamine which has been released and recaptured. We conclude that short-term changes in brain levels of DOPAC appear to provide a useful index of alterations in the functional activity of central dopaminergic neurons.

Dopaminergic neurons: An in vivo system for measuring drug interactions with presynaptic receptors

SummaryAn in vivo system has been used to investigate the ability of dopamine agonists and antagonists to alter dopamine synthesis by acting at what appear to be presynaptic dopamine receptors. In order to eliminate postsynaptically induced changes in dopamine synthesis caused by the effects of these drugs on the firing rate of dopamine neurons, gammabutyrolactone was administered to block impulse flow in the nigro-neostriatal pathway. The accumulation of Dopa in the rat striatum after administration of Dopa decarboxylase inhibitor was used as an index of striatal tyrosine hydroxylase activity. It was found that administration of the dopamine agonists, apomorphine or ET-495 [1-(2′-pyrimidyl)-piperonyl-piperazine], modified the apparent activity of striatal tyrosine hydroxylase when impulse flow was blocked in dopamine neurons. This presynaptic effect of apomorphine could be prevented by low doses of loxapine haloperidol and spiroperidol. Chlorpromazine, fluphenazine, and thioridizine were much less effective than the butyrophenones in blocking the effects of apomorphine. Molindone and (+) butaclamol, but not (-) butaclamol, reversed the presynaptic agonist effects, pimozide was a weak blocker and clozapine had no effect at all. All these neuroleptics except (-) butaclamol caused a significant increase in Dopa accumulation when impulse flow was intact. Compared with haloperidol the phenothiazines and pimozide appeared less potent in reversing the presynaptic effects of apomorphine than in blocking the behavioral effects of this agonist. Possible functional significance of the presynaptic dopamine receptors are considered.

Tricyclic antidepressants: Effects on the firing rate of brain noradrenergic neurons

The spontaneous activity of the norepinephrine-containing cells of the locus coeruleus was recorded in chloral hydrate-anesthetized rats. The effect of seven tricyclic antidepressants on the firing rate of single cells in the locus coeruleus was studied. All the drugs tested, except iprindole markedly decreased the rate of firing of the noradrenergic cells. Antidepressants having a secondary amine in the side chain, desipramine, nortriptyline and chlordesipramine, were more potent than their respective tertiary amine analogues, imipramine, amitriptyline and chlorimipramine. Alteration of the rate of drug metabolism by pretreatment with SKF-525A or phenobarbital did not change the doses of tertiary antidepressnats required to decrease norepinephrine cell firing. Depletion of the norepinephrine stores by pretreatment with alpha-methyl-p-tyrosine and reserpine markedly increased the dose of desipramine required to depress the norepinephrine cells. The results are in good agreement with previous studies showing that secondary amine antidepressants are more potent than their tertiary amine homologues in blocking the uptake of norepinephrine into brain and peripheral tissues. Despite their lower potency it is concluded that tertiary antidepressants act on noradrenergic neurons in their unchanged form and not via secondary amine metabolites formed during the recording experiments since alterations in liver metabolism did not influence the response. The findings are consistent with the suggestion made from studies on transmitter turnover that antidepressants by inhibiting reuptake of norepinephrine cause a stimulation of postsynaptic receptors which decreases the activity of the presynaptic neurons by a feed-back mechanism. This view is further supported by the finding of an inverse relation between the norepinephrine content of the brain and the dose of desipramine required to decrease the firing rate of the noradrenergic neurons.

Effect of impulse flow on the release and synthesis of dopamine in the striatum

Publisher Summary This chapter presents a study investigating effect of impulse flow on the release and synthesis of dopamine (DA) in the rat striatum. The effect of γ-hydroxybutyrate (GHB) on the firing of dopaminergic neurons in the zona compacta of the substantia nigra was investigated by means of single unit recording techniques. The identification of DA neurons based on both, histochemical and neurophysiological characteristics made it possible to make extracellular recordings from these neurons. These unit recording experiments revealed that GHB or GBL administered intravenously or intraperitoneally in doses of 100–200 mg/kg routinely, decrease the firing of units localized to the DA containing cells present in the zona compacta of the substantia nigra. Serotonin and norepinephrine (NE) containing neurons in the midbrain raphe and locus coeruleus respectively were much less responsive to the inhibitory properties of GHB. If impulse flow is interrupted in the DA system by placement of a high frequency electrothermic lesion in the nigro-neostriatal pathway or by direct injection of local anesthetics into this pathway an increase in neostriatal DA similar to that produced by GHB treatment is also observed.

Effects of muscimol and picrotoxin on single unit activity of substantia nigra neurons

Intravenous administration of the GABA agonist, muscimol, caused dose-dependent increases in the unit activity of substantia nigra pars compacta (dopamine) neurons and an inhibition of nigral pars reticulata cells. The depressant effects of the drug upon reticulata neurons were reversible by subsequent administration of the GABA antagonists, picrotoxin and bicuculline HCl. However, the stimulatory effects of i.v. muscimol upon dopamine neurons were not abolished by these agents. Intravenous administration of picrotoxin alone caused only moderate increases in the activity of dopamine neurons (31% over baseline at 7.0 mg/kg), but markedly stimulated the firing of pars reticulata cells (154% over baseline at 7.0 mg/kg). In spite of the stimulation of dopamine neurons after i.v. muscimol, microiontophoresis of GABA and muscimol could inhibit the firing of both pars compacta and pars reticulata cells, although the reticulata neurons were much more sensitive to the inhibitory actions of these agents than the dopamine neurons. Considered together, these studies suggest that a population of neurons in the substantia nigra pars reticulata have the capacity to be more affected by a major GABA input to the nigra than the pars compacta dopamine neurons. The results further suggest that if the dopamine cells are regulated by GABAergic neurons of the striatonigral pathway, their regulation must be indirect and could involve a second inhibitory neuron within the nigra.

Effect of gamma-hydroxybutyrate on dopamine and dopamine metabolites in the rat striatum☆

Abstract Gamma-butyrolactone (GBL), a precursor for the naturally occurring central nervous system depressant, gamma-hydroxybutyrate (GHB), administered in anesthetic doses, produces an increase in rat corpus striatum dopamine levels without affecting norepinephrine or serotonin levels. The rise and fall of the dopamine levels coincide with the changes of brain GHB levels and the behavioral effects of the drug. The specific activity of striatal dopamine was found to be greater in rats injected with 3 H-tyrosine shortly before or shortly after GBL, as compared with controls, which were not treated with GBL. The specific activity of cortical norepinephrine in GBL-treated rats was not significantly different from that observed in untreated controls. No significant difference was observed in blood or striatal tyrosine specific activity of GBL-treated rats. Levels of dopamine metabolites, dihydroxyphenylacetic acid and homovanillic acid, also increased in the corpus stratium after GBL, but the increase did not occur until after brain levels of GHB began to fall. These results suggest that the drug either increases dopamine synthesis and or blocks the release of dopamine from a rapidly turning over functional compartment within the neurons, or both. Perhaps as a result of the ability of GHB to block the release of dopamine, this drug also interferes with the metabolism of dopamine for a certain period of time after administration.

Dopaminergic neurons—Alteration in the kinetic properties of tyrosine hydroxylase after cessation of impulse flow

Abstract Studies in vitro conducted on striatal tyrosine hydroxylase have demonstrated that addition of calcium chelators, such as EGTA, results in an alteration in the kinetic properties of tyrosine hydroxylase which can be completely reversed by addition of calcium to the incubation medium. Inhibition of impulse flow in the nigro-neostriatal pathway by either pharmacological or mechanical techniques causes the isolated striatal tyrosine hydroxylase to behave kinetically like the enzyme treated in vitro with EGTA. Tyrosine hydroxylase, isolated from the striatum of rats in which impulse flow has been interrupted in the dopamine pathways, has an increased affinity for both substrate and pterin cofactor and a reduced affinity for the end-product inhibitor, dopamine. The latter change is most dramatic; the K i of the enzyme for dopamine. increases more than 700-fold. These alterations in the kinetic properties of tyrosine hydroxylase can also be completely reversed by addition of calcium to the incubation medium. In rats with a pharmacological blockade of impulse flow induced by administration of γ-hydroxybutyrate, subsequent administration of dopamine receptor stimulants causes the isolated tyrosine hydroxylase to have properties similar to those observed for the enzyme prepared from untreated rats. Administration of dopamine receptor blockers prevents the dopamine receptor stimulants from altering the kinetic changes in tyrosine hydroxylase induced by a cessation of impulse flow. These data are discussed in terms of the possible role presynaptic receptors and calcium fluxes may play in the short-term regulation of tyrosine hydroxylase.

Dopaminergic neurons: effect of acute and chronic morphine administration on single cell activity and transmitter metabolism.

At various time points following acute and chronic administration of morphine to rats, dopamine transmitter metabolism and neuronal activity were determined. Following acute injection of morphine (20 mg/kg intraperitoneally), dopamine cell firing rates increased slowly and steadily. This slow increase was accompanied by a similar slow increase in the accumulation of the dopamine metabolite, dihydroxyphenylacetic acid (DOPAC). Apparentin vivo tyrosine hydroxylase activity, measured by dopa accumulation following inhibition of dopa decarboxylase, also increased. In chronically treated animals the average firing rate of dopamine cells was measured two hours after the last injection of morphine. The distribution of dopamine cell firing rates was significantly higher than in controls. DOPAC levels andin vivo tyrosine hydroxylase activity were also increased at this time. When morphine (100 mg/kg intraperitoneally) was administered to chronically treated animals 12 hours after the last injection a slow increase of firing rates was observed similar to that seen in naive animals after an acute morphine injection. In chronically morphine treated animals naloxone caused a rapid dose-dependent decrease in firing rates and DOPAC levels.In vivo tyrosine hydroxylase activity was not changed.

Dopaminergic neurons—Alteration in the sensitivity of tyrosine hydroxylase to inhibition by endogenous dopamine after cessation of impulse flow☆

Abstract Inhibition of firing in the nigro-neostriatal dopamine system causes a rapid increase in the steady state levels of dopamine and an apparent activation in vivo of tyrosine hydroxylase. Although the apparent activity of tyrosine hydroxylase returns to normal after dopamine levels have been increased, the elevated dopamine levels do not appear to inhibit tyrosine hydroxylase below control levels. In contrast, when dopamine levels are increased by administration of a monoamine oxidase inhibitor, there is a significant reduction in striatal tyrosine hydroxylase activity as measured by the accumulation of dihydroxyphenylalanine (DOPA) after administration of a DOPA decarboxylase inhibitor. These results suggest that a blockade of impulse flow in the dopaminergic neurons causes a decrease in the sensitivity of striatal tyrosine hydroxylase to inhibition by elevated levels of dopamine. Administration of dopamine agonists reverses this effect and restores to normal the sensitivity of tyrosine hydroxylase to inhibition by endogenous dopamine. This effect of the dopamine agonists is prevented or reversed by administration of drugs which block dopamine receptors. Thus, the dopamine agonists appear to alter dopamine synthesis in the absence of impulse flow by interacting at a site, possibly on the presynaptic side of the dopamine terminal, which is similar to the post-synaptic dopa-minergic receptor and which appears capable of altering the sensitivity of tyrosine hydroxylase to inhibition by endogenous dopamine.

Intravenous GABA agonist administration stimulates firing of A10 dopaminergic neurons

Increasing intravenous doses of two gamma-aminobutyric acid (GABA) agonists, muscimol and 4,5,6,7-tetrahydroisoxazolo-[5,4-c]-pyridin-3-ol (THIP), caused dose-related increases in extracellular, single unit activity of A10 dopaminergic neurons of the ventral tegmental area. Muscimol, the more potent of the two compounds, stimulated firing at doses approximately 15-20 times lower than the doses of THIP required to elicit equivalent excitatory effects. The maximum stimulation achieved was approximately 150 and 140% of the baseline firing rate for muscimol and THIP, respectively.