The effect of LSD and 2-bromo LSD on the striatal DOPA accumulation after decarboxylase inhibition in rats.

LSD and BOL (0.125-0.5 mg/kg) were equipotent in increasing the in vivo tyrosine hydroxylation in the striatum as measured by the accumulation of DOPA after inhibition of neuronal decarboxylase. However, with 2--4 mg/kg doses, the maximum effect of BOL was larger than that of LSD. LSD and BOL antagonized the apomorphine-induced decrease of DOPA accumulation, without affecting the haloperidol-induced increase. LSD like apomorphine inhibited the increase of DOPA accumulation seen after reserpine, cerebral hemisection and after gamma-butyrolactone (GBL). The effect of apomorphine in rats given GBL was blocked by haloperidol, but not by BOL and promethazine, whereas that of LSD was inhibited by haloperidol, BOL, and promethazine. These findings suggest that LSD and BOL directly affect nigro-neostriatal dopamine neurons. LSD therefore appears to be a partial agonist and BOL a pure antagonist at dopamine autoreceptors. It is proposed in addition that LSD activates and BOL blocks 5-HT receptors that control DOPA formation.     

The effect of LSD and 2-bromo LSD on the dopa accumulation after central and peripheral decarboxylase inhibition

LSD and BOL were found to be equally potent in increasing the rat brain DOPA accumulation after decarboxylase inhibition. However, at the doses selected, the DOPA levels after haloperidol and apomorphine were increased by LSD but not BOL. It is suggested that this difference is due to LSD's activating effect on central serotoninergic receptors.     

Inhibition of the turnover of the brain dopamine after treatment with the gammaaminobutyrate: 2-oxyglutarate transaminase inhibitor aminooxyacetic acid

Summary Treatment with aminooxyacetic acid (AOAA, 50 mg/kg i.p.) did not significantly change the concentrations of dopamine and noradrenaline in the rat brain. It markedly and similarly decelerated the disappearance of dopamine in the corpus striatum and the limbic system after treatment with dl--methytyrosine methylester (-MT, 250 mg/kg i.p., 4 h) and accelerated the disappearance of noradrenaline. It also blocked the acceleration of the -MT-induced dopamine, but not noradrenaline, disappearance caused by haloperidol. Administration of gamma-hydroxybutyric acid (GHBA, 1.5+1.0 g/kg i.p.) resulted in a larger retardation of the -MT-induced dopamine disappearance which was about the same in the corpus striatum and the limbic system. Treatment with GHBA also increased the endogenous dopamine and this effect was significantly larger in the corpus striatum than in the limbic system like after an axotomy. The effect of AOAA may be due to accumulation of GABA inhibiting the cell bodies of the dopaminergic neurones.     

Complex stimulus properties of LSD: a drug discrimination study with α2-adrenoceptor agonists and antagonists

The influence of several ₂-adrenergic agents on the discriminative stimulus (DS) properties of lysergic acid diethylamide (LSD) was studied in rats trained to discriminate 0.08 mg/kg (186 nmol/kg) of LSD from saline in a two-lever operant paradigm. Only yohimbine fully mimicked LSD with an ED₅₀ of 2.05 mg/kg (5.24 µmol/kg). Yohimbine's 5-HT_1A agonist properties may be responsible for this substitution. Other ₂-adrenoceptor antagonists, idazoxan with an agonist/antagonist profile at 5-HT_1A receptors and RS 26026-197, a highly selective ₂-adrenoceptor antagonist, failed to produce substitution. Clonidine, an ₂-adrenoceptor agonist, did not substitute for LSD but the response rate was dose-dependently reduced. None of the ₂-adrenergic agents used for pretreatment before LSD inhibited the response to the LSD training dose. Coadministration of clonidine with LSD produced a leftward shift of the dose-response relationship of LSD without a significant change in the slope of the dose-response line. Simultaneous administration of ₂-adrenergic agents with LSD shifted the dose-response curve to the left only when the adrenergic agent also possessed at least moderate affinity for the 5-HT_1A receptor. In addition, radioligand competition experiments were performed that showed LSD to have relatively high affinity (K_i=37 nM) for [³H]clonidine-labeled sites in rat cortex with lower affinity for [³H]yohimbine labeled sites. While previous studies have suggested that the nature of the LSD cue may be essentially expressed by 5-HT₂ receptor activation, the present data show that this cue can be modulated by effects of LSD at 5-HT_1A and at other monoamine neurotransmitter receptors.     

NSD 1034: An amino acid decarboxylase inhibitor with a stimulatory action on dopamine synthesis not mediated by classical dopamine receptors

Summary The accumulation rates of 3,4-dihydroxyphenyl-alanine (DOPA) and 5-hydroxytrypthophan (5-HTP) after inhibition of aromatic amino acid decarboxylase (AADC) by 3-hydroxybenzylhydrazine (NSD 1015) or 1-(dl-seryl)-2(2,3,4-trihydroxybenzyl)hydrazine (Ro 4-4602) have widely been used as measurements of the in vivo synthesis rates of monoamines. However, the values of dopamine (DA) turnover in rat striatum obtained using these drugs are much lower than values obtained by other methods. This discrepancy prompted us to further investigate the AADC inhibitor 1-(3-hydroxybenzyl)-1-methylhydrazine (NSD 1034) which earlier has been shown to give a DOPA accumulation rate in the striatum of the same magnitude as other measures of DA turnover. NSD 1034 was found to give a more than twofold higher DOPA accumulation rate than NSD 1015, NSD 1024, NSD 1039, NSD 1055 and Ro 4-4602 in the striatum. Also, in the limbic region and the hemispheres, but not in the substantia nigra, the DOPA accumulation was higher after NSD 1034 than after NSD 1015, but the difference was less pronounced. There was, however, no difference in 5-HTP accumulation between the drugs in any of the brain parts investigated. Although the DOPA accumulation rates are higher after NSD 1034 than after NSD 1015, the NSD 1015-induced DOPA accumulation seems to be more sensitive to changes in dopamine receptor occupancy. The different DOPA accumulation rates obtained with NSD 1015 and NSD 1034 are not due to differences in MAO inhibition, to interference with classical DA receptors, or to different degrees of AADC inhibition, but to an ability of NSD 1034 to stimulate DA synthesis. In addition, under certain conditions NSD 1034 also has a DA releasing action, like amphetamine. It is proposed that NSD 1034 and amphetamine stimulate DA synthesis and release by a common mechanism. The low value of DA synthesis rate, obtained when measured as DOPA accumulation after NSD 1015, is due to a substantial efflux of DOPA from the brain. The efflux of DOPA is equally large after NSD 1034 but the loss is compensated for by an increase in DOPA synthesis. Send offprint request to Hans Nissbrandt at the above address     

Supersensitivity of 5-HT1A-autoreceptors and α2-adrenoceptors regulating monoamine synthesis in the brain of morphine-dependent rats

The sensitivity of 5-HT1A serotonin receptors and alpha2-adrenoceptors (autoreceptors and heteroreceptors) modulating brain monoamine synthesis was investigated in rats during morphine treatment and after naloxone-precipitated withdrawal. The accumulation of 5-hydroxytryptophan (5-HTP) and 3,4-dihydroxyphenylalanine (DOPA) after decarboxylase inhibition was used as a measure of the rate of tryptophan and tyrosine hydroxylation in vivo. Acute morphine (3-100 mg/kg, 1 h) increased the synthesis of 5-HTP/5-HT in various brain regions (15%-35%) and that of DOPA/dopamine (DA) in striatum (28%-63%), but decreased the synthesis of DOPA/noradrenaline (NA) in hippocampus and cortex (20%-33%). Naloxone (2-60 mg/kg, 1 h) did not alter the synthesis of 5-HTP or DOPA in brain. Tolerance to the inhibitory effect of morphine on DOPA/NA synthesis and a sensitization to its stimulatory effects on DOPA/DA and 5-HTP/5-HT synthesis were observed after chronic morphine and/or in morphine-withdrawn rats. In morphine-dependent rats (tolerant and withdrawn states) the inhibitory effects of the 5-HT1A agonists 8-OH-DPAT and buspirone (0.1 mg/kg, 1 h), and that of the alpha2-adrenoceptor agonist clonidine (0.1 mg/kg, 1 h), on the synthesis of 5-HTP/5-HT were potentiated (25%-50%). Moreover, the effect of 8-OH-DPAT was antagonized by WAY 100135, a selective 5-HT1A antagonist. In morphine-dependent rats (tolerant state), the inhibitory effects of clonidine on the synthesis of DOPA/NA (hippocampus, hypothalamus) and DOPA/DA (striatum) also were potentiated (35%-55%). In summary, we conclude that morphine addiction is associated with supersensitivity of 5-HT1A serotonin receptors and alpha2-adrenoceptors (autoreceptors and heteroreceptors) that modulate the synthesis of monoamines in brain.     

α2 Autoreceptor-mediated modulation of tyrosine hydroxylase activity in noradrenergic regions of the rat brain in vivo

Summary The physiological importance of brain ₂-adrenoceptors in controlling the activity of tyrosine hydroxylase in noradrenergic regions was investigated using the accumulation of 3,4-dihydroxyphenylalanine (DOPA) after decarboxylase inhibition as a measure of the rate of tyrosine hydroxylation (and synthesis of noradrenaline) in vivo. In the hypothalamus and cerebral cortex, clonidine (0.025–1 mg/kg, i.p.) decreased (18%–43%) and idazoxan (0.1–80 mg/kg, i.p.) increased (20%–73%) the synthesis of DOPA in a dose-dependent manner. Moreover, pretreatment with idazoxan (0.1 mg/kg) antagonized the effect of clonidine (0.1 mg/kg) in the hypothalamus. After treatment with reserpine (5 mg/kg, s.c., 18 h before decapitation) and depletion of noradrenaline, clonidine (0.5 mg/kg) continued to decrease (50%–55%) but idazoxan (20 mg/kg) failed to increase the synthesis of DOPA, which suggested the involvement of an -autoreceptor mechanism. Acute treatments of rats (not exposed to reserpine) with a wide variety of adrenoceptor agonists such as guanfacine 6, B-HT 920, xylazine, bromoxidine (1 mg/kg) and antagonists such as yohimbine, phentolamine, prazosin (10 or 20 mg/kg) resulted in significant decreases (15%–55%) or increases (21%–99%) in the synthesis of DOPA in both brain regions. However, other agonists (oxymetazoline, azepexole, tramazoline, methoxamine) and antagonists (tolazoline, dihydroergotamine, phenoxybenzamine, propranolol) did not modify the synthesis of DOPA. In the hypothalamus and cerebral cortex the effects of the drugs were consistent with the selectivity of -adrenoceptor agonists and antagonists (except prazosin) for an ₂-adrenoceptor. The results also suggest that the ₂-autoreceptor that modulates the synthesis of noradrenaline in the rat brain appears to belong to the prazosin-sensitive _2B-subtype.Dedicated to Nils-Erik Andén, in memoriamSend offprint requests to J. A. Garcia-Sevilla at the above address at Palma de Mallorca     

Acceleration by stress of dopamine synthesis and metabolism in prefrontal cortex: Antagonism by diazepam

Summary Using liquid chromatography and electrochemical detection (LCEC), we have measured the accumulation of 3,4-dihydroxyphenylalanine (DOPA) (after L-aromatic amino acid decarboxylase inhibition), dopamine (DA) and 3,4-dihydroxyphenylacetic acid (DOPAC) in the frontal cortex and in the corpus striatum of the rat. Mild-footschock stress increased frontal cortex DOPA accumulation, as well as DA and DOPAC, without changing the concentration of these substances in the corpus striatum. The increases in cortical DA synthesis and metabolism were antagonized by diazepam which, given alone, tended to decrease DOPA accumulation to a small degree. In addition, we have measured the indoles serotonin (5-HT) and its metabolite 5-hydroxyindoleacetic acid (5-HIAA), and the noradrenergic metabolite MHPG, none of which were altered by stress. The accumulation of 5-hydroxytryptophan (5-HTP) was also unaffected by stress but, like DOPA accumulation, was reduced to a small degree by diazepam.This study directly demonstrates a selective activation of frontal cortex catechol synthesis (in vivo tyrosine hydroxylation) by a mild stress, which did not significantly alter cortical noradrenergic or serotonergic metabolism.     

Possible involvement of the central dopaminergic system in the antireserpine effect of LSD

Both LSD and apomorphine produced hyperactivity in reserpine-treated mice, LSD being more potent and longer-acting than apomorphine. 2-Brom LSD and the serotonin (5-HT) receptor agonist quipazine were ineffective in reversing reserpine sedation. Treatments with phenoxybenzamine and methysergide failed to block the stimulant effects of either LSD or apomorphine, demonstrating the noninvolvement of 5-HT or norepinephrine (NE) receptors in their action. The ineffectiveness of -methyl-p-tyrosine in modifying the stimulant effects of LSD and apomorphine indicated a probable direct stimulant effect of these two drugs on the dopamine (DA) receptors. Low doses of chlorpromazine, haloperidol, or pimozide blocked the effects of apomorphine in reserpinized mice. Although these neuroleptics significantly reduced the effects of LSD, they failed to block completely the LSD effects even at higher doses. Apomorphine reduced the -methyl-p-tyrosine-induced depletion of DA in the whole brain of mice, but LSD failed to do so. From these findings it is postulated that apomorphine acts as a direct DA receptor agonist, and that LSD may act directly on a site structurally closely related to DA receptor, but not necessarily identical with it. Repeated treatments with LSD did not lead to development of tolerance to its locomotor effects in reserpinized mice. Moreover, mescaline and psilocybin, which are known to exhibit cross-tolerance to LSD, failed to produce LSD-like effects in reserpine-treated mice. The effects of LSD on the DA or a related site probably are not solely responsible for its psychotomimetic effects.     

Modification of the effect of some central stimulants in mice pretreated with α-methyl-l-tyrosine

Summary In mice treated with -methyltyrosine (AMT) so as to deplete the brain norepinephrine by 65 percent of the normal, the central stimulant effects of d-amphetamine, morphine, mescaline and cocaine were significantly reduced. The effects were evaluated by observing the spontaneous motor activity, the degree of eye closure and rectal temperature of mice. The hypermotility and hyperthermia caused by lysergic acid diethylamide remained unaffected by AMT treatment. Desmethylimipramine failed to antagonize the sedation produced by repeated doses of AMT which lowered brain norepinephrine by 95 percent. These results showed that except that of lysergic acid diethylamide, the central stimulant effects of other drugs and the antidepressant effect of desmethylimipramine were mediated through norepinephrine.     

Effect of morphine on the accumulation of DOPA after decarboxylase inhibition in the rat.

Acute systemic administration of morphine (10 mg/kg s.c.) to rats increased in vivo tyrosine hydroxylation in the striatum measured as the accumulation of DOPA after decarboxylase inhibition. DOPA accumulation reached a maximum 30-60 min after morphine. The morphine antagonist naloxone (1, 10 or 100 mg/kg s.c.) did not significantly after DOPA accumulation. However, naloxone completely antagonized the effect of morphine. The DA agonist apomorphine decreased and the DA antagonist haloperidol increased DOPA accumulation. The effect of apomorphine (0.05 mg/kg) was counteracted by morphine. Naloxone did not significantly change the accumulation of DOPA after apomorphine or after haloperidol. In rats treated with gamma-butyrolactone (GBL) or with reserpine DOPA accumulation was not altered by treatment with morphine or naloxone. However, the inhibiting effect of apomorphine (0.5 mg/kg) on the accumulation of DOPA in rats treated with reserpine was weakly counteracted by morphine (0.5 mg/kg) on the accumulation of DOPA in rats treated with reserpine was weakly counteracted by morphine (10 mg/kg s.c.). Since the effects of morphine on the apomorphine-induced inhibition of DOPA accumulation were antagonized by naloxone, we suggest that the effects on striatal DOPA accumulation produced by morphine were mediated via opioid receptors and not directly via DA receptors.     

Effect of lisuride and LSD on monoamine synthesis after axotomy or reserpine treatment in rat brain

Summary The effect of d-lysergic acid diethylamide (LSD) and lisuride on the synthesis of monoamines was evaluated in rat brain regions using an in vivo method in which the accumulation of dopa and 5-hydroxytryptophan (5-HTP) is measured after inhibition of the aromatic amino acid decarboxylase by means of 3-hydroxybenzylhydrazine.LSD (50–1000 g/kg i.p.) caused a dose-dependent increase in dopa formation and a slight elevation of tyrosine and tryptophan concentrations in the intact rat forebrain; moreover, it reduced the accumulation of 5-HTP.The increase in dopa formation in the terminal system of the rat forebrain following an axotomy of the ascending monoaminergic fiber system was antagonized by LSD at a dose of 0.5 mg/kg i.p. Haloperidol (2 mg/kg i.p.) counteracted the effect of LSD.The increase in dopa formation in c. striatum and the dopamine-rich part of the limbic system following treatment with reserpine was antagonized by lisuride as well as by LSD. However, lisuride was at least 10 times as potent as LSD. In reserpinized animals LSD counteracted the inhibitory effect on dopa accumulation of the direct dopamine receptor stimulant apomorphine while lisuride did not. The data suggest that LSD is a weak agonist at dopamine receptors in vivo with partial receptor blocking properties at higher doses, while the action of lisuride on dopamine receptors is a potent, purely agonistic one.with technical assistance of Ruth NeumeisterPart of this paper has been presented at the Spring Meeting 1977 of the German Pharmacological Society at Mainz (Kehr, 1977b)     

Effects of dopaminergic and cholinergic drugs,naloxone and l-prolyl-leucyl-glycinamide on LSD-induced catalepsy

Summary In an attempt to elucidate the mechanism of d-lysergic acid diethylamide (LSD) induced catalepsy, the effects of cholinergic and dopaminergic agents, naloxone and l-propyl-leucyl-glycinamide (PLG) were studied in rats. The dose-dependent (50–500 g kg^–1 s.c.) and time-related cataleptic response elecited by LSD was preceded by a phase of hyperexcitability. The non-hallucinogenic analogue, 2-bromo-LSD (BOL), was without effect. Both apomorphine, the dopamine agonist, and l-DOPA antagonized LSD-induced catalepsy whereas the dopamine depleting agent -methyl-p-tyrosine (-MPT) slightly prolonged the cataleptic effect. Cholinergic muscarinic receptor stimulation with pilocarpine antagonized LSD-induced catalepsy. The muscarinic antagonists, atropine and scopolamine, intensified the hyperexcitable phase and potentiated the cataleptic effects of LSD. Nicotine slightly potentiated LSD action but mecamylamine antagonized it. While pre-treatment with naloxone, the narcotic antagonist and PLG prolonged the cataleptic response, post-treatment with naloxone effectively attenuated LSD-induced catalepsy. The behavioural data are interpreted to suggest that LSD-induced catalepsy may be mediated through diminished dopaminergic and cholinergic neuronal activity and under enkephalinergic modulation. The neuroanatomical foci and exact mechanism of action remain to be delineated.     

The effects of GBR 12909, a dopamine re-uptake inhibitor,on monoaminergic neurotransmission in rat striatum,limbic forebrain,cortical hemispheres and substantia nigra

Summary In order to investigate the physiological importance of the membrane pump in eliminating released dopamine (DA) we have studied the effects of the putative selective dopamine re-uptake inhibitor, GBR 12909, on synthesis and metabolism of monoamines in the rat striatum, limbic forebrain, cortical hemispheres and substantia nigra (SN). The effects of the drug on the firing rate of catecholamine containing neurons in the SN and locus coerulus (LC) were also investigated. For comparison we have investigated the effects of desipramine and maprotiline. As a measure of the synthesis of noradrenaline (NA), DA and 5-hydroxytryptamine (5-HT) we determined the 3,4-dihydroxyphenylalanine (DOPA) and 5-hydroxytryptophan (5-HTP) accumulation after inhibition of aromatic l-amino acid decarboxylase by 3-hydroxy-benzylhydrazine (NSD 1015). As indirect measurements of DA and NA release in vivo, we have assessed pargyline-induced 3-methoxytyramine (3-MT) and normetanephrine (NM) accumulation and disappearance rates of DA and NA after inhibition of their synthesis by -methyl-p-tyrosine (-MT). Administration of GBR 12909 (2.5, 5, 10, 20 or 40 mg/kg) decreased the NSD 1015-induced DOPA accumulation in the striatum and in the limbic forebrain. In contrast, only minor effects of the drug were seen on the DOPA accumulation in the cortical hemisphere and on the cerebral 5-HTP accumulation. GBR 12909 increased the 3-MT accumulation in the striatum, limbic forebrain and the cortical hemispheres, an effect that was even more pronounced in haloperidol-pretreated animals. However, GBR 12909 did not alter the 3-MT accumulation in the SN either when given alone or when given to haloperidol-pretreated rats. In haloperidol-pretreated rats GBR 12909 markedly enhanced the DA disappearance in the striatum and in the limbic forebrain, but not in the SN. Furthermore, GBR 12909 did not significantly affect the firing rate of dopaminergic neurons in the SN or that of noradrenergic neurons in the LC. Taken together, our results support the notion that GBR 12909 is a specific DA uptake inhibitor without a transmitter releasing action. In addition, our findings indicate that DA re-uptake is of physiological importance in the elimination of DA from the synaptic cleft in the striatum, limbic forebrain and cortical hemispheres, but not in the SN. Furthermore, a large part of the DA taken up by the dopaminergic terminals in the striatum and in the limbic forebrain seems to be re-incorporated into the storage vesicles. Send offprint requests to H. Nissbrandt at the above address     

Simultaneous measurement of tyrosine and tryptophan hydroxylase activities in brain in Vivo using an inhibitor of the aromatic amino acid decarboxylase

Summary DOPA and 5-HTP accumulated in vivo in rat brain after decarboxylase inhibition with NSD 1015 (3-hydroxybenzylhydrazine). This accumulation was linear for the first 30 min and occurred in several brain regions over a wide range of NSD 1015 doses. After a peripheral decarboxylase inhibitor much less, if any, DOPA or 5-HTP accumulated in the brain. The accumulation of DOPA was prevented by H 44/68 (methylester of -methyl para-tyrosine), a tyrosine hydroxylase inhibitor. DOPA, which accumulated before H 44/68 was given, appeared stable for at least 20 min. There were no significant changes in the levels of NA, DA, 5-HT or tryptophan shortly after NSD 1015 administration, but a rise in tyrosine was noted. Increased brain tyrosine after l-tyrosine administration did not alter the DOPA accumulation, however. These data as well as the distribution of the accumulated amino acids suggest that the accumulation of DOPA and 5-HTP after decarboxylase inhibition occurs intraneuronally, that the decarboxylase enzyme is completely inhibited, and that the accumulated products are not appreciably metabolized or transported from the region studied. Amine synthesis rates and rate constants were calculated from the data and compare well with similar values determined by other methods. Thus this accumulation appears to be a reliable measure of the in vivo hydroxylation of tyrosine and tryptophan.     

Antagonism of 5-hydroxytryptamine by LSD 25 in the central nervous system: A possible neuronal basis for the actions of LSD 25

1. 5-Hydroxytryptamine (5-HT), acetylcholine (ACh), noradrenaline (NA), glutamate, D,L-homocysteic acid (DLH), glycine and gamma-aminobutyric acid (GABA) were applied to single neurones in the brain stem of decerebrate cats by microiontophoresis. The abilities of D-lysergic acid diethylamide tartrate (LSD 25), methysergide maleate (UML 491) and 2-bromo-lysergic acid diethylamide (BOL 148) to antagonize the actions of these compounds were studied.2. LSD 25 antagonized 5-HT excitation of single neurones when applied iontophoretically or administered intravenously. LSD 25 also antagonized glutamate excitation of neurones which could be excited by 5-HT. Inhibitory effects of 5-HT, the action of glutamate on neurones which could be inhibited by 5-HT and the actions of all the other compounds tested were unaffected by LSD 25.3. Iontophoretically applied UML 491 was also a specific antagonist to 5-HT and glutamate excitation but was less potent than LSD 25, and BOL 148 rarely exhibited antagonism.4. It is suggested that antagonism to 5-HT and glutamate excitation of brain stem neurones may be the basis of the psychotomimetic action of LSD 25. It is also suggested that there may be similarities in the mechanisms by which 5-HT and glutamate produce excitation where they act on the same neurone.     

Effects of iminodibenzyl antipsychotic drugs on cerebral dopamine and alpha-adrenergic receptors

The iminodibenzyl antipsychotic drugs, clocapramine, carpipramine and Y-516 were studied in order to elucidate their mechanisms of action. They all accelerated the accumulation of the dopamine (DA) metabolites, homovanillic acid (HVA) and 3,4-dihydroxyphenylacetic acid (DOPAC), in the striatum and nucleus accumbens of the rat brain. Only Y-516 antagonized in vivo the apomorphine-induced inhibition of DA synthesis as estimated from the accumulation of 3,4-dihydroxyphenylalanine (DOPA) in the decarboxylase-inhibited rat striatum after cessation of nerve impulse flow. All three drugs showed high affinity for DA receptors labelled by [3H]haloperidol and [3H]ADTN in the rat striatum in vitro, with the order of potency Y-516 greater than clocapramine greater than carpipramine. All accelerated the accumulation of the norepinephrine metabolite, 3-methoxy-4-hydroxyphenylglycol (MHPG), in mouse brain. They showed high affinity for alpha 1-adrenoceptors labelled by [3H]WB 4101 and for alpha 2-adrenoceptors labelled by [3H]clonidine in the rat cerebral cortex in vitro. Although they all had the same level of affinity for the alpha 1-adrenoceptors, Y-516 had less affinity for the alpha 2-adrenoceptors than did clocapramine and carpipramine. The above results indicate that these drugs are potent DA antagonists which block alpha 1- and alpha 2-adrenoceptors in the brain.     

Chronic nicotine treatment increases dopamine levels and reduces dopamine utilization in substantia nigra and in surviving forebrain dopamine nerve terminal systems after a partial di-mesencephalic hemitransection

Summary In order to further study the previously demonstrated protective action of chronic nicotine treatment on lesioned meso-striatal dopamine (DA) pathways, the following study was carried out on DA utilization in these lesioned neurons. Male Sprague-Dawley rats were partially hemitransected at the meso-diencephalic junction and treated with nicotine (0.125 mg · kg^–1 · h^–1) by means of Alzet minipumps implanted subcutaneously for 2 weeks. The overall serum nicotine level obtained was 64.6 ± 2.7 ng · ml^–1.The results demonstrated that partial di-mesencephalic hemitransections produced a marked reduction of DA fluorescence (quantitative histofluorimetry) on the lesioned side in the nucleus caudatus putamen, anterior nucleus accumbens and posterior lateral tuberculum olfactorium. No significant effects were observed on the intact side. Furthermore, studies on changes in DA utilization as evaluated 2 h after tyrosine hydroxylase inhibition showed an augmentation in the -methyl-(±)-p-tyrosine methyl ester (-MT)-induced depletion of the DA stores on the hemitransected side in comparison with the operated side of the sham-operated animals. On the hemitransected side chronic nicotine treatment increased DA stores in the DA nerve terminals of the nucleus caudatus putamen and the posterior lateral tuberculum olfactorium. No significant effects were observed on the intact side. Following chronic nicotine treatment a marked and preferential attenuation of the -MT-induced depletion of DA stores was seen in the various DA nerve terminal systems of the forebrain on the hemitransected side. In the substantia nigra reduced DA levels (HPLC) were demonstrated on the hemitransected side, while no effects on the non-operated side were observed. Also an increase of the -MT-induced depletion of the DA stores was seen on the hemitransected side in comparison with the operated side of the sham-operated animals. In contrast, on the non-operated side an attenuation of the a MT-induced depletion of the DA stores was found. Following chronic nicotine treatment the lesion induced reduction of the nigral DA stores on the hemitransected side was counteracted, as was the lesion induced increase in the -MT-induced depletion of DA stores, which was replaced by a reduction of the -MT-induced depletion of the nigral DA stores. However, on the non-operated side an increased DA depletion was observed after -MT treatment in rats treated chronically with nicotine. Chronic nicotine treatment under the present conditions did not significantly alter serum levels of corticosterone and reduced prolactin serum levels in sham-operated rats.The present results indicate that a partial hemitransection produces marked increases in DA utilization of the forebrain and of the substantia nigra on the lesioned side; whereas on the non-operated side a reduction in nigral DA utilization was found. On the hemitransected side chronic nicotine treatment increases DA stores of the nucleus caudatus putamen, tuberculum olfactorium and substantia nigra, suggesting a protective action of nicotine. Chronic nicotine treatment preferentially and substantially reduces striatal, accumbens and nigral DA utilization on the lesioned side. On the non-operated side chronic nicotine treatment abolished the lesion-induced reduction of DA utilization in the substantia nigra. These results are in support of the hypothesis that a protective action of chronic nicotine treatment on ascending DA systems may be produced via a desensitization of excitatory nicotine cholinoceptors regulating the ascending DA pathways, leading to reduced firing rates and thus to reduced energy demands. The endocrine system does not seem to be involved in these effects.Abbreviations ACC dif anterior part of nucleus accumbens - ACC dot posterior part of nucleus accumbens - LAUD cent central part of nucleus caudatus - CAUD marg marginal zone of nucleus caudatus - CAUD med medial part of nucleus caudatus - DA dopamine - HPLC high performance liquid chromatography - -M -methyl-(±)-p-tyrosine methyl ester - TUB dif posterior lateral part of the olfactory tubercle - TUB dot posterior medial part of the olfactory tubercle Send offprint requests to A. M. Janson at the above address     

Unsurmountable antagonism of brain 5-hydroxytryptamine2 receptors by (+)-lysergic acid diethylamide and bromo-lysergic acid diethylamide.

Lysergic acid diethylamide (LSD) and its structural analogue 2-bromo-lysergic acid diethylamide (BOL) act as unsurmountable antagonists of serotonin-elicited contractions in smooth muscle preparations. Two different models, allosteric and kinetic, have been invoked to explain these findings. The present studies investigate the mechanism of antagonism of brain 5-hydroxytryptamine (5HT)2 receptors, utilizing cells transfected with 5HT2 receptor cDNA cloned from rat brain. A proximal cellular response, phosphoinositide hydrolysis, was examined in order to minimize possible postreceptor effects. Even though LSD behaved as a partial agonist and BOL as a pure antagonist, both drugs blocked the effect of serotonin in an unsurmountable manner, i.e., increasing concentrations of serotonin could not overcome the blocking effect of LSD or BOL. Radioligand binding studies showed that preincubation of membranes with either LSD or BOL reduced the density of [3H]ketanserin binding sites, suggesting that the drugs bind tightly to the 5HT2 receptor and are not displaced during the binding assay. Two additional experiments supported this hypothesis. First, the off-rate of [3H] LSD was slow (20 min), relative to that of [3H]ketanserin (approximately 4 min). Second, when the length of incubation with [3H]ketanserin was increased to 60 min, the LSD-induced decrease in Bmax was essentially eliminated. The possibility that LSD and BOL decrease [3H]ketanserin binding by interacting with an allosteric site was rejected, because neither drug altered the rate of dissociation of [3H]ketanserin. The most parsimonious interpretation of these results is that unsurmountable antagonism reflects prolonged occupancy of the receptor by slowly reversible antagonists.     

Daily LSD administration selectively decreases serotonin2 receptor binding in rat brain

The effect of ten daily injections of saline or d-lysergic acid diethylamide (LSD) (260 micrograms/kg i.p.) on serotonin1 (5-hydroxytryptamine1, 5-HT1) and 5-HT2 receptor binding was determined in brain membranes from rats killed 24 h after the last injection. [3H]LSD (3.0 nM) was used with either 30.0 nM 5-HT or 70.0 nM cinanserin to estimate 5-HT1 and 5-HT2 receptors, respectively. LSD administration decreased 5-HT2 binding in cortex, striatum, hippocampus, and diencephalon/midbrain without altering 5-HT1 or total specific binding.