Parkhurst A. Shore

Effects of chronic desipramine treatment on rat brain noradrenergic responses to α-adrenergic drugs☆

It has been previously reported that long-term tricyclic antidepressant treatment in the rat causes a subsensitivity of central beta-receptor-stimulated adenylate cyclase along with alterations of brain norepinephrine (NE) content and metabolism. We have confirmed earlier findings that after one week of desipramine treatment (5.0 mg/kg b.i.d.) brain NE levels decline while NE metabolism is similar to control animals, but is above control after 12 days of treatment. Single cell recordings from noradrenergic neurons of the locus coeruleus (LC) show that after one week of desipramine treatment, neuronal firing rate is lower than in control rats but greater than that seen in response to acutely administered drug. Furthermore, desipramine injection in a dose which profoundly altered LC impulse flow in control rats produced little or no effect on impulse flow in chronically treated rats. Of 25 or 250 microgram/kg doses of clonidine, which are equieffective for decreasing brain NE metabolism in control animals, only the larger dose decreased NE metabolism in 12 day desipramine-treated rats. The postsynaptic alpha-antagonist prazosin (5.0 mg/kg) increased NE metabolism in both groups. These results suggest that presynaptic (alpha 2) adrenoreceptors become subsensitive during long-term desipramine treatment, thus allowing recovery of noradrenergic impulse flow in the presence of NE uptake inhibition.

Functional and pharmacological significance of brain dopamine and norepinephrine storage pools

The concept that multiple pools of stored catecholamines exist in catechol~ine-containing neurons has been the subject of research interest for a number of years. The initial observations suggesting the existence of more than one pool came from studies utilizing various methods to determine catecholamine turnover. These methods included determination of decline rates of catecholamines after synthesis inhibition as well as administration of labeled catecholamine or precursor and comparison of the specific activity of released transmitter with that remaining in the tissue. The results of many of these experiments indicated that a non-uniform release of catechoiamines occurs and that the most recently stored amine is the first released. Although such experiments are valuable, they do not yield information on the functional roles of apparent multiple pools of catecholamines. The purpose of this commentary is to examine the evidence for the operational significance of transmitter pools in norepinephrine (NE)and dopamine (DA)-containing neurons of the CNS. The picture that emerges indicates that central DA-containing neurons operate with two distinct transmitter pools, whereas central NE-containing neurons appear to operate with a single functional pool. These considerations appear to dictate pharmacological responses to various dopaminergic and adrenergic drugs. Neff and Costa [l] showed that, after inhibition of catecholamine synthesis with the tyrosine hydroxylase inhibitor ct-methyl-p-tyrosine (a-MT), DA and NE exhibit a log-linear decline for several hours. As expected, receptor antagonists or agonists could enhance or inhibit, respectively, the rates of decline. If the catecholamine stores were labeled by administering either radio-labeled precursor or exogenous amines (thus allowing accurate measurements over a short time frame), radio-labeled DA and NE disappeared rapidly for the first 2CL30 min, after which disappearance curves similar to those of the endogenous transmitter substances were observed [Z-4]. The specific activity of catecholamines released shortly after labeling of the neurons was greater than that remaining in the tissue, but at later times the specific activity of NE or DA released into the medium was the same as that in the tissue [4-61. Glowinski and co-workers showed that endogenous striatal DA concentrations exhibited a two-phase decline after tyrosine hydroxylase inhibition and that the apparent rapidly disappearing * Author to whom all correspondence should be addressed. pool represented about 20 per cent of total DA (71. They obtained similar results with the noradrenergic system using radio-tracer methods [6]. By measuring radio-labeled DA and DA metabolites, Groppetti et al. [X] showed that newly synthesized DA was preferentially catabolized, as DA metabolites had a higher specific activity than DA in rat striatum. Recent studies with superfused striatal synaptosomes indicate that DA is taken up and released by two compartment kinetics [9, lo]. Thus, these data suggest that newly taken up or newly synthesized catecholamines are preferentially released over older amine. Other investigators [ll. 121 questioned the validity of some of these experiments. The question arose as to whether a sufficient blockade of tyrosine hydroxylase with rr-MT occurred in the earlier-mentioned studies and whether the initial rapid rate of [3H]DA decline might be due to possible metabolites of o-MT such as p-hydroxyamphetamine. It was claimed, based on kinetic consideration of the decline rates of newly labeled and endogenous DA. that if two pools of DA exist, then the rapidly turning over pool must be less than 5 per cent of the total striatal DA content (i.e. smaller than experimental error). These reports leave open the significance of the difference in specific activity of released amines that is observed after labeling DA or NE pools.

In vivo monoamine oxidase inhibition by d-amphetamine

Abstract In vitro, d- and l-amphetamine (AMPH) are reversible monoamine oxidase (MAO) type A inhibitors, the d-form being approximately five times more potent. Experiments were conducted in rats to determine whether MAO inhibition occurs in vivo. d-AMPH was more effective than l-AMPH at decreasing striatal 3,4-dihydroxyphenylacetic acid (DOPAC). However, assays of striatal MAO activity following administration of AMPH in vivo failed to show MAO inhibition. In other experiments, rats were treated with d-AMPH (zero time) followed by phcnelzine (1 hr), an irreversible MAO inhibitor, and were killed at 25 hr. MAO activity was determined in vitro for the striatum and the rest of the brain using serotonin (MAO-A) and phenylethylaminc (MAO-B) as substrates. d-AMPH provided significant protection against MAO-A inhibition by phenelzine, whereas l-AMPH and cocaine (used instead of AMPH) were without effect. d-AMPH failed to protect against MAO-B inhibition by phenelzine. Thus, d-AMPH appears to inhibit reversibly MAO type A in vivo. However, using the same ‘protection protocol’, d-AMPH failed to oppose phenelzine-induced lowering of striatal DOPAC. Experiments were undertaken to determine whether the protective effect of d-AMPH on MAO type A would influence striatal dopamine depletion by RO4-1284, a rapidly acting reserpine-like agent. RO4-1284-induced depletion of dopamine was inhibited by phenelzine. Prior treatment with d-AMPH reduced significantly the protective effect of phenelzine, suggesting reversible, intraneuronal MAO inhibition by d-AMPH in vivo. The possible neuronal mechanisms for these events are discussed.

RESERPINE: BASIC AND CLINICAL PHARMACOLOGY

The introduction, somewhat more than two decades ago, of reserpine as a purified alkaloid of Rauwolfia has contributed greatly to the remarkable advances in the emerging field of psychopharmacology. Notwithstanding the disappearance of the use of reserpine in clinical psychiatry, owing largely to its overshadowing by better drugs, reserpine has made its mark in the stimulation of investigative work and in its continuing use as a tool in the understanding of the mode of action of psychoactive drugs and of the functioning of central monoaminergic neuronal systems and the peripheral adrenergic nervous system. Reserpine rightly deserves historical recognition as one of those key drugs, like nicotine or muscarine, which has allowed a “quantum jump” in our knowledge of the nervous system.

Comparative effects of clozapine and α-adrenoceptor blocking drugs on regional noradrenaline metabolism in rat brain☆

Clozapine increased brain noradrenaline (NA) metabolism, as indicated by changes in 3-methoxy-4-hydroxyphenylglycol sulfate content, in brain regions corresponding to the predominance of alpha- over beta-receptors, i.e., hypothalamus, medulla, midbrain and cortex, but not corpus striatum or cerebellum. Phenoxybenzamine had a stronger effect in the hypothalamus than did clozapine, but did not change cortical NA metabolism within a 60 min treatment time; however, cortical NA metabolism was increased 150 min after phenoxybenzamine. The delayed effect of phenoxybenzamine may be due to either a poor affinity for some central receptors or a slow rate of entry into certain brain regions. Thioridazine and the benzodioxane, dibozane, had regional effects similar to clozapine. The similarity between clozapine and dibozane in ther effects on regional brain NA metabolism may reflect a preference for presynaptic alpha-receptors. It is unlikely that the antipsychotic activity of clozapine is related to a specific adrenolytic effect, but may reflect the combined activity of this drug on several transmitter systems.

On a prime role for newly synthesized dopamine in striatal function

Rats were given either the tyrosine hydroxylase inhibitor, alpha-methyltyrosine (alphaMT), in doses of 10 or 250 mg/kg or the neuroleptic, haloperidol (0.25 mg/kg). Other rats received both drugs (haloperidol 30 min after alphaMT). This dose of haloperidol alone caused only a slight, gradually developing catalepsy, while alphaMT alone caused none. The combination quickly caused a strong catalepsy. Striatal dopamine (DA) stores were only minimally depleted at the time of catalepsy potentiation. Th e marked elevation of striatal homovanilluc acid concentration seen after haloperidol administration was greatly inhibited by alphaMT pretreatment. It is concluded that the marked potentiation of haloperidol-induced catalepsy by alpha MT is related to the absence of newly synthesized DA rather than to an exhausted main DA pool and that newly synthesized DA has a greater role in striatal function than does DA of the main striatal storage pool.

Dopaminergic neuronal responses to a non-amphetamine CNS stimulant

The present study compares the effects of d-amphetamine (d-AMP) and the potent non-amphetamine CNS stimulant, amfonelic acid (AFA), on the firing rate of single midbrain dopaminergic (DA) neurons and on neostriatal DA metabolism (dihydroxyphenylacetic acid—DOPAC). The results indicate that AFA, like d-AMP, reduces the firing rate of DA neurons, although unlike d-AMP, AFA does not cause a decrease in neostriatal DOPAC content and, in fact, enhances that produced by haloperidol (HALO). The AFA-induced decrease in firing rate, like d-AMP, is reversed by the DA receptor blocker HALO, but again unlike d-AMP, the decrease in firing rate is not prevented by catecholamine synthesis inhibition withα-methyl-para-tyrosine. Thus, both amphetamine and amfonelic acid have identical electrophysiological effects on DA neurons but act by different mechanisms.

Effects of amphetamine and amfonelic acid on the disposition of striatal newly synthesized dopamine

A comparison of the in vivo biochemical actions of the psychotomimetic central stimulants, d-amphetamine (d-AMPH) and amfonelic acid (AFA), on the metabolism of rat striatal newly synthesized [3H]dopamine (DA) was made by pulse labeling with [3H]tyrosine. No evidence for the formation of the alcoholic DA metabolites [3H]3-methoxy-4-hydroxyphenylethanol (MOPET) or [3H]3,4-dihydroxyphenylethanol (DOPET) was found in control or drug-treated animals. Both [3H]3,4-dihydroxyphenylacetic acid (DOPAC) and [3H]homovanillic acid (HVA) concentrations were increased by AFA in the presence of haloperidol, while [3H]DA content was decreased. In contrast, d-AMPH, in the presence of haloperidol, decreased [3H]DOPAC and increased [3H]DA, even in monoamine oxidase-blocked rats. Thus monoamine oxidase inhibition did not appear to be a major factor in the action of amphetamine to increase [3H]DA, but cannot be excluded as a contributing factor to the lowering of [3H]DOPAC. Similar actions of d-AMPH were seen on preformed DA. Amphetamine may release newly synthesized DA in such a way that some of the released DA enters the neuronal storage system.

Effects of severe dopamine depletion on dopamine neuronal impulse flow and on tyrosine hydroxylase regulation.

Reserpine depletes dopamine (DA) levels and increases tyrosine hydroxylase (TH) activity in the rat corpus striatum. TH is activated not only by enhancement of DA neuronal impulse flow, but also by cessation of impulse flow. To assist in the understanding of the relative contribution of impulse flow to the regulation of TH activity in the DA depleted neuron, we examined the consequences of severe DA depletion on substantia nigra DA neuronal impulse flow and on in vivo TH activity in the rat corpus striatum. One day after reserpine or 30 min after the reversible reserpine-like compound, Ro4-1284, striatal DA levels were severely depleted and in vivo TH activity was enhanced about three-fold. DA depletion was found to significantly increase DA neuronal impulse flow. Although the DA neuron is firing faster than normal in the DA depleted rat, because there is no DA being released it is still not clear whether the elevation in TH activity is due to the enhancement of impulse flow or to the lack of DA at presynaptic receptor sites, or both. gamma-Butyrolactone (GBL), causes a cessation of DA neuronal impulse flow and activates TH by a presynaptic autoreceptor mechanism. GBL inhibited by over 50 percent the elevation in TH activity produced by severe DA depletion. This finding suggests that the enhanced TH activation after DA depletion in in large part due to increased DA impulse flow. Furthermore, the TH activity seen with GBL in DA depleted rats was significantly less than that seen after GBL administration in normal rats. This finding is consistent with the hypothesis that the DA storage granule also plays a role in TH regulation.

Midbrain dopamine neurons: differential responses to amphetamine isomers

Intravenously administered D- and L-amphetamine have different potency ratios in reducing the firing rates of dopamine cells in the substantia nigra and in the ventral tegmental area. While D-amphetamine is considerably more potent than L-amphetamine in reducing ventral substantia nigra dopamine neuronal impulse flow, D- and L-amphetamine are of similar potency in reducing dorsal substantia nigra and ventral tegmental dopamine neuronal impulse flow. These results suggest that all dopamine cell groups are not pharmacologically identical and that different dopamine nuclei may respond differently to psychoactive drugs. The comparable potencies of the D- and L-isomers on dorsal substantia nigra and ventral tegmental area dopamine neurons may explain, by a dopamine mechanism, the finding that comparable doses of the isomers produce schizophrenic-like symptoms.