Rodolfo Silveira

Monoamine Oxidase Inhibitory Properties of Some Methoxylated and Alkylthio Amphetamine Derivatives: Structure–Activity Relationships

The monoamine oxidase (MAO) inhibitory properties of a series of amphetamine derivatives with different substituents at or around the para position of the aromatic ring were evaluated. In in vitro studies in which a crude rat brain mitochondrial suspension was used as the source of MAO, several compounds showed a strong (IC50 in the submicromolar range), selective, reversible, time-independent, and concentration-related inhibition of MAO-A. After i.p. injection, the compounds induced an increase of serotonin and a decrease of 5-hydroxyindoleacetic acid in the raphe nuclei and hippocampus, confirming the in vitro results. The analysis of structure-activity relationships indicates that: molecules with amphetamine-like structure and different substitutions on the aromatic ring are potentially MAO-A inhibitors; substituents at different positions of the aromatic ring modify the potency but have little influence on the selectivity; substituents at the para position such as amino, alkoxyl, halogens, or alkylthio produce a significant increase in the activity; the para-substituent must be an electron donor; bulky groups next to the para substituent lead to a decrease in the activity; substituents located at positions more distant on the aromatic ring have less influence and, even when the substituent is a halogen (Cl, Br), an increase in the activity of the compound is obtained. Finally, the MAO-A inhibitory properties of some of the compounds evaluated are discussed in relation to: (a) potential antidepressant activity, and (b) their reported hallucinogenic, neurotoxic, or anxiolytic effects.

Effects of the D3 preferring dopamine agonist pramipexole on sleep and waking, locomotor activity and striatal dopamine release in rats.

Abstract Quantitation of 2 h sessions after administration of the D 3 preferring dopamine (DA) agonist pramipexole (10–500 μg/kg) showed dose-related effects on wakefulness (W), slow wave sleep (SWS) and REM sleep in rats. The 30 μg/kg dose of the DA agonist increased SWS and REM sleep and reduced W during the first recording hour, while the 500 μg/kg dose augmented W. On the other hand, W was increased while SWS and REMS were decreased after the 500 μg/kg dose during the second recording hour. The mixed D 2 - and D 3 receptor antagonist YM-09151-2 (30–500 μg/kg), which per se affected sleep variables prevented the increase of REMS induced by pramipexole. Furthermore, the highest doses (500–1000 μg/kg) of the DA antagonist effectively antagonized the increase of W and reduction of SWS induced by the 500 μg/kg dose of the DA agonist. Pramipexole (30–100 μg/kg) induced a decrease of locomotor activity during the 2 h recording period. In addition, the 500 μg/kg dose gave rise to an initial reduction of motor behavior which was reverted 2 h later. Pramipexole (30 and 500 μg/kg) did not significantly affect striatal DA release during the first two hours following drug administration, as measured by microdialysis. It is tentatively suggested that D 3 receptor could be involved in the pramipexole-induced increase of sleep and reduction of locomotor activity. On the other hand, the increase of W and of motor behavior after relatively high doses could be related to activation of postsynaptic D 2 receptor.

Long-term effects of perinatal asphyxia on basal ganglia neurotransmitter systems studied with microdialysis in rat ☆

Asphyxia was induced in pups delivered by caesarean section on pregnant Sprague-Dawley rats. Rats within the last day of gestation were anaesthetised and hysterectomized. The uterus horns including the foetuses were placed in a water bath for various periods of time. Following asphyxia the uterus horns were opened. The pups were removed, stimulated to breathe, left to recover and given to surrogate mothers. Control and asphyctic pups were obtained from each mother. Rats surviving asphyctic periods longer than 20 min at 37 degrees C showed chronic deficits in the release of neurotransmitters monitored with microdialysis in the basal ganglia. The main change observed in 6-month-old male rats that underwent severe perinatal asphyxia was a marked decrease in striatal dopamine release, monitored under basal and D-amphetamine stimulated conditions, as compared with control (normal- or caesarean-delivered) rats. Striatal glutamate and aspartate levels were also decreased following asphyxia. In the substantia nigra, the main effect of asphyxia was a decrease of both gamma-aminobutyric acid (GABA) and aspartate levels. Thus, this study provides evidence that perinatal asphyxia leads to chronic deficits in neurotransmission in the basal ganglia.

On the origin of striatal cholecystokinin release : studies with in vivo microdialysis

Abstract: In the present study, extracellular levels of the neuropeptide cholecystokinin (CCK), of the monoamine dopamine and its metabolites 3, 4‐dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), and of the excitatory amino acids glutamate and aspartate were simultaneously monitored by microdialysis in the neostriatum of halothane‐anesthetized rats under basal and K+‐depolarizing conditions. Extracellular CCK and dopamine levels, but not glutamate and aspartate levels, were decreased by perfusion with a Ca2+‐free medium, under both basal and K+‐depolarizing conditions. HPLC revealed that the majority of the CCK‐like immunoreactivity in the perfusates coeluted with CCK octapeptide. Striatal extracellular CCK levels were decreased by decortication plus callosotomy, with a parallel decrease in glutamate levels. Striatal extracellular levels of dopamine, DOPAC., and HVA were significantly decreased in animals treated previously with a unilateral 6‐hydroxydopamine injection into the medial forebrain bundle. In these animals, however, the effect of decortication plus callosotomy on CCK and glutamate levels was not further augmented. Thus, this study supports the hypothesis of a neuronal origin of extracellular CCK and dopamine monitored with microdialysis in the striatum of the rat, and also supports the idea of a partly contralateral origin of corticostriatal CCK and glutamate inputs.

Recovery of central noradrenergic neurons one year after the administration of the neurotoxin DSP4

The long-term effects of the systemic administration of DSP4 (N-(2-chloroethyl)N-ethyl-2-bromobenzylamine hydrochloride), a selective noradrenergic neurotoxin, on the endogenous levels of monoamines and their metabolites and on alpha- and beta-adrenoceptors in selected brain regions of the rat were examined. After 7 days, DSP4 caused a marked reduction (about 80%) of endogenous noradrenaline levels in locus coeruleus-innervated regions. At 90, 240 and 300 days after DSP4 injection, a partial and gradual recovery (50%, 41% and 25% of control values, respectively) of the noradrenaline cortical levels was evident. One year after DSP4 administration, brain regional noradrenaline stores were almost completely recovered. No changes in 5-hydroxytryptamine levels were observed in the three time intervals, but a mild decrease in cortical and hippocampal 5-hydroxyindolacetic acid levels was found 7 days after DSP4 injection. Following the profound noradrenaline depletion seen at 7 days, the cerebral cortical density of alpha 1-, alpha 2- and beta-adrenoceptors was significantly increased. Assessment of adrenergic receptors in cerebral cortex at 365 days after DSP4 injection, indicated that alpha 1- and alpha 2-adrenoceptor densities did not differ from control values; however, the density of beta-adrenoceptors remained increased. No changes were observed in the affinities of the three types of adrenoceptors studied. These results indicate that after a selective noradrenergic denervation induced by DSP4, there is a slow and gradual recovery of noradrenaline stores and of alpha 1- and alpha 2-adrenoceptor populations, suggesting a possible regrowth and/or collateral sprouting of noradrenergic terminals.

Modulation of catecholamine release by α2-adrenoceptors and I1-imidazoline receptors in rat brain

The physiological and pharmacological effects of imidazoli(di)ne derivatives, such as clonidine, have been related not only to the interaction with alpha 2-adrenoceptors but also to their activity on non-adrenoceptor sites termed imidazoline receptors. The modulation of catecholamine release by imidazoline drugs was studied by monitoring extracellular levels of norepinephrine (NE), dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) with microdialysis in cingulate cortex of rats, with or without irreversible alpha 2-adrenoceptor blockade. NE and DA levels were in the 1 nM range whereas DOPAC and HIVA levels were approximately equal to 100 nM. NE and DA levels were increased when the uptake blocker desipramine (1 microM) or KCl (100 mM) were added to the perfusion medium. Clonidine induced a dose-dependent (0.3-1.2 mg/kg i.p.) decrease in NE (max 61%) and DA (max 40+) levels that was reversed by the alpha 2-adrenoceptor antagonist RX821002. After alpha 2-adrenoceptor irreversible blockade with the alkylating agent N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ), [3H]clonidine binding to alpha 2-adrenoceptors was reduced by 94 +/- 1%. Under such conditions, clonidine elicited a paradoxical dose-dependent (0.6-2.4 mg/kg i.p.) increase of NE (max 56%) without modifications in DA, DOPAC and HVA levels. The stimulatory effect of clonidine was prevented by the imidazoline receptor antagonist idazoxan (10 mg/kg i.p.) but not by RX821002 (5 mg/kg i.p.). In rats pretreated with EEDQ, cirazoline (I1/I2-imidazoline receptor agonist), moxonidine (I1-imidazoline receptor agonist), but not guanabenz (I2-imidazoline receptor agonist) (1.2-2.4 mg/kg i.p.) elicited an increase of NE levels in a similar manner to clonidine (11-82%). Idazoxan also abolished these responses to cirazoline or moxonidine. In contrast to systemic administration, local perfusion of clonidine (10-100 microM) through the microdialysis probe under alpha 2-adrenoceptor alkylating conditions, did not modify extracellular levels of NE and DA suggesting an indirect mechanism. The results demonstrate that clonidine and related imidazoli(di)ne drugs are able not only to inhibit NE release in rat cerebral cortex involving an alpha 2-adrenoceptor mechanism, but also to induce a paradoxical NE release through an indirect extracortical mechanism. The findings evidence that the indirect modulation of NE levels by imidazoline drugs is mainly due to a functional activity on I1-imidazoline receptors.

Neurocircuitry of the basal ganglia studied by monitoring neurotransmitter release

The neurocircuitries of the basal ganglia are studied with in vivo microdialysis, with special consideration to dopamine transmission and its interaction with other neurotransmitter systems. The aim is to develop experimental models to study the pathophysiology and therapy of neurodegenerative disorders of the basal ganglia, as well as to develop models to study the short- and long-term consequences of perinatal asphyctic lesions. A main goal of these studies is to find and to characterize new treatments for these disorders.

Sleep and waking in 5,7-DHT-lesioned or (−)-pindolol-pretreated rats after administration of buspirone, ipsapirone, or gepirone

The effects of partial 5-HT1A receptor agonists buspirone (0.010–4.0 mg/kg), ipsapirone (0.010–6.0 mg/kg), and gepirone (0.025–4.0 mg/kg) on sleep and waking were studied in vehicle-treated and 5,7-dihydroxytryptamine (5,7-DHT)-injected rats. 5,7-DHT-treated animals showed a marked and significant serotonin and 5-HIAA depletion in the raphe regions of the pons and upper brain stem, cerebral cortex, hippocampus, and striatum. Subcutaneous administration of the partial agonists to both the vehicle-infused and the 5,7-DHT-treated animals significantly increased waking (W) and reduced light sleep (LS), slow-wave sleep (SWS), and REM sleep (REMS). Pretreatment with (−)-pindolol (2.0 mg/kg) reversed the effects of buspirone and gepirone on W and non-REM sleep (LS + SWS) whereas REMS remained suppressed. (−)-Pindolol failed to reverse the effects of ipsapirone on sleep and W. The present results tend to indicate that increased W after acute administration of buspirone, ipsapirone, or gepirone depends upon the activation of postsynaptic 5-HT1A receptors. The well-known anxiolytic action observed after chronic administration of the azapirones seems to be related to mechanisms other that these involved in their stimulant effect.

Effects of accumbens m-chlorophenylbiguanide microinjections on sleep and waking in intact and 6-hydroxydopamine-treated rats

Effects of the 5-HT3 receptor agonist, m-chlorophenylbiguanide (10.0-40.0 microg), on sleep and waking were studied in control, vehicle-treated and 6-hydroxydopamine-injected rats. Bilateral injections of m-chlorophenylbiguanide into the nucleus accumbens of the control and the vehicle-infused animals significantly increased waking and reduced slow wave sleep. Rapid eye movement sleep (REM sleep) remained unchanged. Pretreatment with the selective 5-HT3 receptor antagonist, MDL 72222 (1aH,3a,5a, H-tropan-3-yl-3,5-dichloro-benzoate) (0.5 mg/kg, s.c.), reversed the effects of m-chlorophenylbiguanide (10.0-20.0 microg) on sleep and waking in the control group. Administration of the 5-HT3 receptor agonist to the 6-hydroxydopamine-treated animals modified only slightly the time spent in wakefulness and slow wave sleep, while REM sleep was significantly and dose dependently reduced. Our findings further support the proposal that increase of wakefulness and reduction of slow wave sleep after activation of 5-HT3 receptors, is partly related to the release of endogenous dopamine.

Differential cholinergic and non-cholinergic actions of acetylcholinesterase in the substantia nigra revealed by fasciculin-induced inhibition

The effects of the peptide fasciculin (FAS), a potent inhibitor of acetylcholinesterase (AChE) have been examined, following unilateral microinfusion, on tissue levels of monoamines in the rat substantia nigra and concomitant circling behaviour. Although FAS inhibited 87% of total AChE, the levels of dopamine and its metabolites remained unchanged. Furthermore, the treatment induced modest contraversive rotation which was markedly enhanced in the presence of a systemic challenge with apomorphine. This behavioural effect of FAS was partially reversed by systemically administered atropine. Any possible interaction of FAS with nigral dopamine systems was further investigated by testing the peptide in animals that five days earlier had undergone a 6-hydroxydopamine (6-OHDA) lesion of the SN such that dopamine and AChE were significantly but not completely reduced. In a majority of these animals, FAS treatment caused a reversal of the lesion induced ipsiversive rotation, ie restored contraversive rotation. It is concluded that in the SN, FAS can have biochemical and behavioural actions independent of local dopamine systems and linked to cholinergic transmission. In addition, treatment with FAS in the substantia nigra also reveals the possible existence of at least two distinct pools of AChE with, respectively, non-cholinergic and cholinergic actions.