Effects of chronic haloperidol on reaction time and errors in a sustained attention task: partial reversal by anticholinergics and by amphetamine

The attentional and motor-disruptive effects of low doses of haloperidol were studied in a sustained attention task performed by rats. Five separate groups (n = 7 or 8) of rats were trained to react to a 0.125-sec visual stimulus by executing a nose-poke response within 3 sec of stimulus presentation. Each group of rats received its own dose (0.0, 0.02, 0.04, 0.08 or 0.12 mg/kg) of haloperidol daily for 3 months, and from the 1st week onward dose-effects on reaction time were quite stable across time. Haloperidol treatment disrupted the sustained attention task performance by decreasing the number of behavior-initiated stimulus presentations, decreasing the number of reinforcers earned, increasing the proportion of errors of omission and increasing reaction time to the target stimulus. Testing of challenge drugs began after 23 days of haloperidol treatment. Scopolamine (0.1 and 0.2 mg/kg), benztropine (1.0, 3.0 and 6.0 mg/kg) and d-amphetamine (0.25, 0.5, 1.0 and 2.0 mg/kg) ameliorated haloperidol-induced reaction time slowing, whereas only benztropine and amphetamine lessened haloperidol-induced errors of omission. The 2.0-mg/kg dose of amphetamine by itself produced a significant increase in errors of omission without affecting reaction time. Haloperidol effectively normalized this amphetamine-induced disruption in attention. The results are consistent with a dopaminergic involvement in the expression of both attention and motor processes.

Kinetic evaluation of the commonality between the site(s) of action of cocaine and some other structurally similar and dissimilar inhibitors of the striatal transporter for dopamine

The inhibition by cocaine of the apparent initial rate of the transport of striatal dopamine was compared with inhibitions produced by cocaethylene, benztropine, GBR-12909, mazindol, and nomifensine. Rotating disk electrode voltammetry was used to measure the kinetically resolved, inwardly directed transport of dopamine in striatal suspensions. Evidence is presented that the primary site of action of cocaine may be at the external face of the transporter. Experiments to determine whether or not the other inhibitors bind to the same site as cocaine were conducted by comparing the inhibitions observed for each of the inhibitors alone with that observed when paired with cocaine. The resulting changes in the velocity of the transport of dopamine induced by the inhibitors were then fit to one of the previously developed models of inhibition by pairs of inhibitors affecting the kinetics of actively transporting systems: a single-site model, a two-site model in which the two binding sites for the inhibitors interact, and a two-site model in which the two binding sites for the two inhibitors act independently. Cocaine inhibited the transport of dopamine competitively with its structural analogues, cocaethylene and benztropine. The structurally dissimilar inhibitor, GBR-12909, was found also to be competitive with cocaine. In contrast, mazindol and nomifensine were found to bind to separate interactive sites when individually paired with cocaine. These results suggest that mazindol and nomifensine may interact with the kinetically active transporter for dopamine in a manner different from that of cocaine. Mazindol was tested and found to inhibit competitively the inward transport of dopamine into striatal suspensions. In contrast, our previous published findings show cocaine to be an uncompetitive inhibitor of the transport of striatal dopamine. These results suggest that cocaine inhibits inward transport of dopamine by reducing the intramembrane turnover of the transporter, whereas mazindol alters the kinetics of the recognition of dopamine by the transporter. Finally, the potential effects of these binding modes of inhibitors on synaptic chemical communication in dopaminergic systems were analyzed. The results of these analyses suggest that different effects on the extracellular concentrations of dopamine can result from the different patterns of inhibition, suggesting that different modulatory influences on pre- and postsynaptic receptor occupation can result from inhibition of the transport of dopamine.

A quantitative comparison on the effects of benztropine, cocaine and nomifensine on electrically evoked dopamine overflow and rate of re-uptake in the caudate putamen and nucleus accumbens in the rat brain slice

Fast cyclic voltammetry has been used to study the relative importance of neuronal dopamine (DA) uptake and D2 autoreceptor stimulation on the control of the extracellular concentration of electrically evoked DA in rat brain slices containing aCPu and NAc. The degree of potentiation of DA overflow by benztropine, cocaine and nomifensine was always greater in the aCPu than in the NAc. Although EC50 values for the uptake blockers in the NAc were lower than in the aCPu, they were only significantly lower for 1 p with benztropine. One microM (-)-sulpiride, significantly potentiated DA overflow evoked by 20 p/20 Hz in both regions. In the presence of either 1 microM cocaine or 1 microM nomifensine, sulpiride further potentiated DA overflow in the NAc but not in the aCPu. The results indicate that in brain slices containing the aCPu, neuronal uptake is the main mechanism of controlling extracellular concentrations of DA due to electrical stimulation. Whilst in the NAc both uptake and D2 autoreceptor activation are important control mechanisms. The rate of removal of DA from the extracellular medium was not significantly different in the two regions but was decreased in the presence of (-)-sulpiride. This indicates that D2 receptors have a modulating effect on the DA neuronal uptake mechanism.

Investigation of the ontogenetic patterns of rat hypothalamic dopaminergic neurone morphology and function in vitro

Using fetal rat hypothalamic cells in primary culture maintained in a serum-free defined medium we have investigated the morphological and functional development of the dopamine (DA)-containing neurones intrinsic to the hypothalamus. Immunocytochemical studies demonstrated the presence of three morphologically distinct subtypes of tyrosine hydroxylase-immunopositive neurones. On day 3 in vitro unipolar, bipolar and multipolar cell types were apparent. The latter two subtypes persisted to later days in culture and increased both in perikarya size and neurite length. All subtypes have been shown to have correlates in vivo. Biochemical studies employing [3H]DA demonstrated a time- and temperature-dependent uptake mechanism within the cultures which was significantly attenuated by the uptake inhibitors benztropine and nomifensine in a dose-dependent manner. [3H]DA was also released under both basal and 56 mmol K+/l-stimulated conditions and the magnitude of the response was reduced by exclusion of calcium from the release medium. The amount of [3H]DA accumulated and released by the cultural cells increased with the age of the culture, suggesting functional maturation of the DA-containing neurones within this preparation. The role of oestradiol-17 beta in regulating hypothalamic dopaminergic function was also investigated both indirectly with the use of [3H]DA and by direct measurement of endogenously synthesized DA using high-performance liquid chromatography coupled with electrochemical detection. Both uptake and release of [3H] and release of endogenous DA were significantly modulated by the concentration of steroid in the defined medium. These results demonstrate that hypothalamic dopaminergic neurones, when maintained in primary culture, undergo morphological and functional maturation which have several correlates in vivo. In addition, we have demonstrated that at least one sub-population of dopaminergic neurones within this preparation is responsive to oestradiol-17 beta. As DA is considered to be a vital component in the regulation of neuroendocrine activity we suggest that this model is valuable for the investigation of the functional development of the DA systems of the hypothalamus and the relationship existing between neurotransmitters, neuropeptides and neuroactive steroids.

Intraventricular administration of histidyl-proline-diketopiperazine [Cyclo(His-Pro)] suppresses prolactin secretion and synthesis: a possible role of Cyclo(His-Pro) as dopamine uptake blocker in rat hypothalamus

Histidyl-proline-diketopiperazine [Cyclo (His-Pro) (CHP)] was discovered to be one of the metabolites of TRH. To understand the specific role of CHP in rat hypothalamic dopamine neurons, we examined the in vivo effects of intraventricular (icv) infusion of CHP on the release and synthesis of PRL in the rat pituitary and the 3,4-dihydroxyphenylacetic acid (DOPAC)/dopamine ratio in the rat hypothalamus. We also examined the in vitro effects of CHP on the release of [3H]dopamine from dispersed tuberoinfundibular dopamine neurons, [3H]dopamine reuptake in hypothalamic membrane fractions, and PRL release from rat pituitary cultured cells. Female rats were treated by icv infusion of 1 microM CHP daily for 1, 3, and 7 days, using Alzet osmotic pumps. After 1 day of treatment, the serum PRL concentration was significantly decreased. Northern blot analysis of the total RNA isolated from the pituitary glands of control animals using 32P-labeled PRL cDNA as a probe indicated the presence of PRL gene transcript, 1.0 kilobase in size, and its amount was decreased by CHP treatment. CHP did not affect [3H]dopamine release from dispersed tuberoinfundibular dopaminergic neurons at any concentration up to 1 microM. CHP did not inhibit PRL release from cultured pituitary cells at low concentrations (1-100 nM), but it stimulated PRL release at high concentrations (1 and 10 microM). We also examined the concentrations of dopamine and DOPAC in the rat hypothalamus when CHP was administered icv for 1 or 7 days. There was a significant decrease in the DOPAC/dopamine ratio after CHP treatment for 1 day. Furthermore, CHP caused dose-dependent inhibition of [3H]dopamine uptake by the rat hypothalamus similar to other dopamine uptake blockers, such as benztropine and GBR12909. These data suggest that icv administration of CHP might decrease both PRL secretion and accumulation of PRL gene transcripts in the pituitary by decreasing the DOPAC/dopamine ratio and inhibiting dopamine reuptake in the rat hypothalamus.

MPTP neurotoxicity to cerebellar Purkinje cells in mice

The pyridine derivative 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is recognized as a crucial neurotoxin which destroys nigrostriatal dopamine cells, thereby inducing neurological signs relevant to idiopathic Parkinson's disease. In the present study, we have revealed MPTP neurotoxicity to cerebellar Purkinje cells in mice. Systemic MPTP injections to mice resulted in a substantial loss of Purkinje cells in a dose-dependent fashion. The MPTP-induced Purkinje cell loss occurred markedly in the crus I and II ansiform lobules and the paraflocculus. Such a neurotoxic effect was largely prevented by the monoamine oxidase B inhibitors pargyline and deprenyl, and the dopamine uptake inhibitors mazindol and benztropine.

Amphetamine and other weak bases act to promote reverse transport of dopamine in ventral midbrain neurons

Amphetamine-like psychostimulants are thought to produce rewarding effects by increasing dopamine levels at mesolimbic synapses. Paradoxically, dopamine uptake blockers, which generally increase extracellular dopamine, inhibit amphetamine-induced dopamine overflow. This effect could be due to either inhibition of amphetamine uptake or inhibition of dopamine efflux through the transporter (reverse transport). We used weak bases and dopamine uptake blockers in ventral midbrain neuron cultures to separate the effects on blockade of amphetamine uptake from reverse transport of dopamine. Amphetamine, ammonium chloride, tributylamine, and monensin, at concentrations that produce similar reductions in acidic pH gradients, increased dopamine release. This effect was inhibited by uptake blockers. Although in the case of amphetamine the inhibition of release could have been due to blockade of amphetamine uptake, inhibition also occurred with weak bases that are not transporter substrates. This suggests that reduction of vesicular pH gradients increases cytoplasmic dopamine which in turn promotes reverse transport. Consistent with this model, extracellular 3,4-dihydroxyphenylacetic acid was increased by ammonium chloride and monensin, as would be expected with elevated cytoplasmic dopamine levels. These findings extend the weak base mechanism of amphetamine action, in which amphetamine reduces vesicular pH gradients resulting in increased cytoplasmic dopamine that promotes reverse transport.

Intravenous benztropine and propranolol challenges in tardive akathisia

We challenged five patients suffering from tardive akathisia (TA) with intravenous benztropine (2 mg), propranolol (1 mg) and placebo (saline) using a random, double-blind cross-over design to examine the effects of the drugs on the subjective, objective and global manifestations of neuroleptic-induced akathisia. Benztropine produced a marginally significant, and propranolol a significant improvement in the overall manifestations of the disorder. The patients demonstrated a considerable placebo effect and marked variation in their responses to the drugs. The implications of these findings for the pathophysiology of TA in relation to acute akathisia and tardive dyskinesia are discussed.

Regional differences in evoked dopamine efflux in brain slices of rat anterior and posterior caudate putamen

Fast cyclic voltammetry using carbon fibre microelectrodes in rat brain slices, was used to investigate regional differences in electrically-evoked dopamine (DA) efflux at 10 different sites in the anterior caudate putamen (aCPu) and 10 sites in the posterior caudate putamen (pCPu). For each site DA overflow was evoked by both single pulse (1P) stimulation and by trains of 25 pulses applied at a frequency of 50 Hz (25P/50 Hz). Peak DA efflux evoked by 1P was about 58% greater in the aCPu (0.19 mumol/l DA) than in the pCPu (0.12 mumol/l DA), but showed no mediolateral variation in either region. Peak DA efflux evoked by 25P/50 Hz relative to 1P efflux also varied between the two regions; the aCPu contained predominantly low ratio (25P/50 Hz: 1P) sites ranging from 1.47 to 3.71, whereas in the pCPu these ratios were higher, ranging from 2.73 to 9.40, and were particularly high in the dorsomedial region of the pCPu. Efflux detected in low ratio sites of the aCPu showed little dependence on the frequency (10 to 500 Hz), or the number of pulses (5 to 20) in a train. By contrast DA efflux evoked in high ratio sites of the pCPu responded in a pulse and frequency dependent manner, the maximum ratio (approximately 8 times 1P) being at 20P/20 Hz. Interestingly the frequency response relationship obtained in the pCPu resembled the profile observed in the nucleus accumbens (NAc). Voltammetric evidence and experiments with selective reuptake blockers indicated that only DA was measured in our studies and 5-HT did not significantly contribute to the frequency dependent pattern of efflux detected in high ratio sites of the pCPu, where striatal 5-HT concentrations are highest. Experiments with the selective D2 receptor antagonists metoclopramide or (-)sulpiride revealed that under our experimental conditions, DA efflux in the aCPu was not modulated by DA autoreceptor activation. By contrast, autoreceptor modulation did occur in high ratio sites of the pCPu at stimulations lasting longer than approximately 1000 ms. These observations support the concept that the caudate putamen is heterogeneously organised with respect to the frequency characteristics of evoked DA release. The factors controlling frequency dependent release under these conditions may be a function of A10 innervation, since high ratio release sites occur in areas where the density of such innervation is greatest, for example, the dorsomedial pCPu. This is supported by the observation that high ratio release sites are also found in the NAc, which receives dopaminergic fibres predominantly from an A10 region.(ABSTRACT TRUNCATED AT 400 WORDS)

High-affinity binding of [125I]RTI-55 to dopamine and serotonin transporters in rat brain

RTI-55 (3 beta-(4-iodophenyl)tropan-2 beta-carboxylic acid methyl ester), one of the most potent inhibitors of dopamine uptake reported to date, was radioiodinated and tested as a probe for the cocaine receptor in Sprague-Dawley rat brain. Saturation and kinetic studies in the striatum revealed that [125I]RTI-55 bound to both a high- and low-affinity site. The Kd for the high-affinity site was 0.2 nM, while the Kd for the low-affinity site was 5.8 nM. The corresponding number of binding sites in the striatum was 37 and 415 pmol/g protein. The pharmacological profile of specific [125I]RTI-55 binding in the striatum was consistent with that of the dopamine transporter. Additionally, [125I]RTI-55 was found to bind with high affinity to the cerebral cortex. Scatchard analysis revealed a single high-affinity component of 0.2 nM with a density of 2.5 pmol/g protein. The pharmacological profile demonstrated by [125I]RTI-55 in the cerebral cortex matched that of the serotonin transporter. Autoradiographic analysis of sagittal brain sections with [125I]RTI-55 binding was consistent with these findings. Specific binding of [125I]RTI-55 was blocked by dopamine uptake inhibitors in areas rich in dopaminergic nerve terminals. Conversely, serotonin uptake inhibitors blocked the binding of [125I]RTI-55 in brain areas rich in serotonergic neurons. These results demonstrate that [125I]RTI-55 may be a very useful ligand for the dopamine and serotonin transporters.

Dopaminergic regulation of cholecystokinin mRNA content in rat striatum

The nigrostriatal dopaminergic activity was pharmacologically changed to assess whether dopamine (DA) regulates cholecystokinin (CCK) mRNA steady state in rat striatum. Cocaine and benztropine, two dopaminergic agonists known to induce DA release and to block its re-uptake, produced a time dependent increase in CCK mRNA content in rat striatum. A significant increase in striatal CCK mRNA was observed 8 h after a single injection of cocaine (15 mg/kg, i.p.) or benztropine (15 mg/kg, i.p.) whereas a two-fold increase was observed after a daily treatment for one week with these two dopaminergic agonists. Cocaine and benztropine failed to change CCK mRNA content in the cerebral cortex. Haloperidol, a dopaminergic receptor blocker, injected at 1 mg/kg, i.p., daily for 7 days, decreased CCK mRNA content in striatum but not in the cerebral cortex. Moreover, haloperidol blocked the effect of cocaine and benztropine, suggesting that the stimulation of striatal dopaminergic receptors is necessary for the induction of CCK biosynthesis. The neurotoxin 6-hydroxydopamine injected into the medial forebrain bundle, elicited a 50% decrease in striatal CCK mRNA, supporting the hypothesis that DA tonically regulates CCK biosynthesis in postsynaptic neurons. To characterize the dopaminergic receptor subtype involved in this regulation, BHT 920, a specific D2 receptor agonist and SKF 38393, a specific D1 receptor agonist were used. While one week treatment with BHT 920 (1 mg/kg, i.p.) increases striatal CCK mRNA content, SKF 38393 (3 mg/kg, i.p.) failed to change this parameter. These data suggest that the increase of striatal CCK mRNA is mediated by the activation of the D2 receptor subtype.

Kinetics of dopamine and noradrenaline transport in synaptosomes from cerebellum, striatum and frontal cortex of normal and reeler mice

Recent evidence indicates that the cerebellum has a dopaminergic system. In order to elucidate further the dopaminergic system in the cerebellum, we investigated the transport of dopamine (DA) in synaptosomal preparations of normal and reeler mice. For comparative purposes we also studied DA transport in synaptosomal preparations from striatum and frontal cortex and compared DA transport to noradrenaline (NA) transport. [3H]-DA transport into cerebellar synaptosomes was found to be a Na(+)-dependent, two component system--a high affinity, low capacity and a low affinity, high capacity. In striatum [3H]-DA is transported by a similar high but different low affinity component. Maximal velocities of both transport components in the striatum were higher than the corresponding ones in the cerebellum. In the frontal cortex we also observed two [3H]-DA transport components with affinities significantly lower than those in cerebellum and striatum. [3H]-NA transport into synaptosomes, prepared from the three brain regions studied, showed two transport components with similar Kt and Vmax values, except for the high affinity component in striatum whose affinity is lower. In reeler mice [3H]-DA transport was different from normal only in the cerebellum where the maximal velocity for both transport components was significantly higher (2x). In contrast, no significant difference was observed in the transport of [3H]-NA. The accumulated [3H]-DA from cerebellar slices was found to be releasable by K+ stimulation, in a Ca(++)-dependent manner, and most of the released radioactivity was in the form of [3H]-DA. These results indicate that in the cerebellum there is a low-density dopaminergic system which is distinct from the corresponding noradrenergic system.

Intrastriatal dialysis evidence for a direct inhibitory effect of benztropine on dopamine re-uptake

A direct inhibitory effect of benztropine on dopamine (DA) reuptake in the striatum was established by in vivo microdialysis of anesthetized rats. Benztropine was infused by means of a microdialysis probe positioned stereotaxically in the striatum. Extracellular DA collected from dialysate was measured by high-performance liquid chromatography using an electrochemical detector. Benztropine was found to cause a dose-dependent inhibition of re-uptake of DA in the nerve terminal area of the dopaminergic system.

Distribution of catecholamine uptake sites in human brain as determined by quantitative [3H] mazindol autoradiography

Because of the importance of the catecholamine system in Parkinson's disease and its relevance to a variety of clinical movement disorders, catecholamine uptake sites were mapped in the human brain using [3H] mazindol autoradiography. Displacement studies with known dopamine (DA) and noradrenaline (NA) uptake blockers showed that binding in the striatum was to dopamine uptake sites; binding in the locus coeruleus was to noradrenergic uptake sites. By using the selective noradrenergic uptake blocker desmethylimipramine (DMI), a comprehensive map of both DA and NA uptake sites was generated. In general, catecholamine uptake sites were better seen in terminals than in cells of origin or axonal projections. In some areas, such as the locus coeruleus, punctate binding could be seen over individual pigmented cells. A variegated pattern of binding was seen in caudate nucleus and putamen and some correspondence of patches of low binding with striosomes was observed in the caudate. The highest levels of binding to DA uptake sites was observed in the striatum, where regional differences in binding occurred. The most dense binding was seen in the ventral striatum, and a rostral-to-caudal decrement in binding levels in caudate nucleus and putamen was evident. Binding was more intense in the putamen compared to the caudate and within the caudate lower values were seen laterally. The highest levels of binding to noradrenergic uptake sites were in the locus coeruleus and dorsal raphé, although these sites may be on terminals from other projections. Whereas uptake sites were more often evident in known catecholamine pathways, [3H] mazindol binding was seen in some areas where catecholamine neurons or terminals had not been identified previously. These maps of the catecholamine uptake system add further information concerning the nature of the distribution of catecholamines in human brain and provide an important baseline for the study of disease and ageing processes.

Dopamine synthesis precedes dopamine uptake in embryonic rat mesencephalic neurons

We have measured [3H]dopamine ([3H]DA) uptake and tyrosine hydroxylase-immunopositive immunostaining in cells acutely dissociated from the embryonic ventral mesencephalon (MSC). DA and its metabolites as well as catechol-O-methyltransferase (COMT) and monoamine oxidase (MAO) activities were determined in homogenates taken from the MSC and striatum (STR). In the embryonic ventral MSC measurable DA and tyrosine hydroxylase (TH) immunostaining were present as early as embryonic day (E) 12.5. At E14 the number of TH+ neurons was about 50% of the values at E18. In the MSC, DA concentration increased sharply at E16 and reached a plateau before birth that was 10-fold lower than adult values. In the STR, DA was first detected at E16, suggesting that DA fibers reach the STR at this embryonic stage. High-affinity DA uptake appeared in the MSC only at E16, concomitantly with the arrival of DA fibers in the STR, increased sharply between E16 and E18, and reached a plateau before birth. This uptake mechanism was not selective for catecholamine uptake inhibitors. Thus, DA synthesis in the MSC preceded the onset of high-affinity uptake mechanism, which could be correlated to the beginning of striatal DA innervation. Measurable MAO and COMT activities were detected as early as E13 (MSC) and E15 (STR), but not DA metabolites, which appeared later. We conclude that the high-affinity DA uptake mechanism in MSC DA neurons develops coincident with the arrival of DA fibers to the STR. The sharp increase of DA uptake between E16 and E18 is due only in part to an increase in the number of TH+ cells. These results support the hypothesis that in vivo the target STR neurons regulate the maturation of MSC DA cells.

Physostigmine (alone and together with adjunct) pretreatment against soman, sarin, tabun and VX intoxication

A pretreatment for organophosphorus (OP) anticholinesterase (e.g., soman) intoxication should prevent lethality and convulsions (CNV) at 2 LD50s and be behavioral-decrement-free when given alone. Behavioral-deficit-free pretreatment regimens (PRGs) for guinea pigs consisted of Physostigmine (0.15 mg/kg, im) and adjunct. Adjuncts [mg/kg, im] tested were akineton [0.25], aprophen [8], trihexyphenidyl [2], atropine [16], azaprophen [5], benactyzine [1.25], cogentin [4], dextromethorphan [7.5], ethopropazine [12], kemadrin [1], memantine [5], promethazine [5], scopolamine [0.08] and vontrol [2]. PRGs were given 30 min before soman (60 micrograms/kg, sc; 2 LD50s) or other OP agents. Animals were then observed and graded for signs of intoxication, including CNV at 7 time points and at 24 hr. Physostigmine alone reduced the incidence of CNV and lethality induced by 2 LD50s of soman by 42 and 60%, respectively. All of the PRGs tested abolished lethality and 12 shortened recovery time to 2 hr or less. Also, PRGs including azaprophen or atropine prevented CNV. When selected PRGs were tested against intoxication by sarin, tabun or VX, the efficacy was generally superior to that for soman. The data show that several PRGs are effective against soman intoxication in guinea pigs.

Lack of increase in dopamine transporter binding or function in rat brain tissue after treatment with blockers of neuronal uptake of dopamine

Rats were pretreated daily for 10 days with a dopamine (DA) uptake blocker ([+]amphetamine, benztropine, cocaine, GBR-12909, mazindol, or nomifensine) or control vehicle and, after 1-4 days of no treatment, striatal tissue was fractionated to provide synaptosomes and membranes for assays of transport of 3H-DA or binding of 3H-GBR-12935. There were no significant increases of apparent maxima for uptake (Vmax) or binding (Bmax) or consistent changes in ligand affinity. Pharmacologic characterization of 3H-GBR-12935 binding extended the impression that this ligand has high affinity and selectivity for many agents which block neuronal uptake of DA uptake and much less for those which interact with DA receptors or other amine transporters. The results suggest that dopamine transporters are not regulated in the same way as receptors, nor influenced similarly toward upregulation and supersensitization by repeated treatment with antagonists.

Effects on renal sympathetic axons in dog of acute 6-hydroxydopamine treatment in combination with selective neuronal uptake inhibitors

1. In anaesthetized dogs, we have investigated the effect on renal responses to sympathetic nerve stimulation of acute treatment with the catecholaminergic neurotoxin 6-hydroxydopamine (2 mg kg-1 i.v.), administered alone or after blockade of neuronal catecholamine uptake pathways for noradrenaline (NA) or dopamine with desmethylimipramine or benztropine, respectively. 2. Under control conditions, renal nerve stimulation caused renal vasoconstriction, reduced glomerular filtration and sodium and water excretion and caused net efflux of NA and dopamine into the renal venous plasma. Two h after administration of 6-hydroxydopamine alone, there was abolition of both functional responses and catecholamine efflux during nerve stimulation. 3. In animals pretreated with desmethylimipramine (1 mg kg-1), 6-hydroxydopamine had no significant effect on functional responses to renal nerve stimulation and nerve-evoked efflux of NA was only moderately reduced. Efflux of dopamine was still markedly reduced by 6-hydroxydopamine, but more variably than occurred without desmethylimipramine treatment. 4. In animals pretreated with benztropine (0.2 mg kg-1), nerve-evoked efflux of dopamine, but not that of NA, was protected against reduction by 6-hydroxydopamine. A higher dose of benztropine (1 mg kg-1) protected efflux of both NA and dopamine against 6-hydroxydopamine. 5. We conclude that acute treatment with a low dose of 6-hydroxydopamine is an effective method of inactivating peripheral sympathetic nerves. The differential effects of desmethylimipramine and benztropine in preserving nerve-evoked efflux of NA and dopamine after 6-hydroxydopamine support the view that these catecholamines originate predominantly from different intrarenal axons. However, neither uptake blocker appears to be completely specific in its actions.

High affinity dopamine reuptake inhibitors as potential cocaine antagonists: a strategy for drug development

The addictive and euphorogenic effects of cocaine are thought to result primarily from inhibition of dopamine reuptake. Although the potency of cocaine-like drugs as inhibitors of DA reuptake is highly correlated with their potency as reinforcers in animals, several potent DA reuptake blockers (bupropion, nomifensine, benztropine, and mazindol) have not been reported to produce addiction or euphoria in humans. Based on these observations in humans, DA reuptake inhibitors are classified into two groups; type 1 blockers, which produce addiction and euphoria, and type 2 blockers, which do not. Given that type 1 and type 2 blockers act at the same site (the DA transporter), the author suggests that type 2 agents may antagonize the effects of cocaine, and might prove useful in the treatment of cocaine addiction.

Dissociation of the actions of uptake blockers upon dopamine overflow and uptake in the rat nucleus accumbens: in vivo voltammetric data

Fast cyclic voltammetry, at carbon fibre microelectrodes, was used to monitor stimulated overflow and uptake of DA in the nucleus accumbens of the rat. Several recognised blockers of neuronal uptake of DA were examined for their actions on each process. Benztropine, cocaine, GBR 12909 and nomifensine elevated the overflow of DA, while only nomifensine and bupropion blocked uptake. The actions of the drugs upon uptake did not correlate with their effects on overflow of DA. It is therefore concluded that the mechanisms by which the drugs influence overflow and uptake are different. It cannot be assumed that the elevation of extracellular DA is solely the result of blockade of uptake by these drugs.

Quantified regional and laminar distribution of the noradrenaline innervation in the anterior half of the adult rat cerebral cortex

The regional and laminar distribution of the noradrenaline (NA) innervation in the adult rat cerebral cortex was quantified in radioautographs of semithin sections from whole hemisphere slices incubated with tritiated catecholamines and a monoamine oxidase inhibitor. Uptake-labeled axonal varicosities (aggregates of silver grains) were counted with the help of a computerized image analyzer in seven cytoarchitectonic areas of the rostral half of the cortex: Cg3, rostral AID, Cg2, Fr1, Par1, caudal AID, and Pir (prepiriform) according to Zilles's nomenclature. Both dopamine (DA) and NA terminals were detected after incubation with [3H]DA and citalopram or with [3H]NA alone. In the presence of desipramine (DMI), DA terminals alone were demonstrated; the number of NA terminals was then obtained by subtraction from counts in adjacent slices incubated with or without DMI. These counts suggested that DA and NA varicosities were fully visualized only after labeling with their respective tritiated amine. Similar numbers of labeled NA varicosities as inferred after [3H]NA incubation with or without DMI were observed after [3H]NA incubation in the presence of benztropine (BZ). This indicated that NA terminals were then maximally detected to the exclusion of the DA ones, and the latter approach was adopted for the acquisition of normative data. Since the average diameter of the labeled NA varicosities was known from earlier measurements in electron microscope radioautographs, the initial counts of labeled sites/mm2 of histological section could be expressed as numbers of varicosities/mm3 of tissue following a double correction for incomplete detection at the chosen duration of radioautographic exposure and section thickness. The overall density of NA innervation was thus estimated at 1.2 million varicosities/mm3 of tissue, with no statistically significant differences between the seven cortical areas examined. In every region, the number of NA terminals was the greatest in the molecular layer (1.5-2 times the density in the rest of cortex) and then progressively decreased in the underlying cortex, with a two- to threefold difference between upper and lower layers. These numerical data allowed an estimation to be made of the possible number of cortical NA varicosities per locus coeruleus nerve cell body of origin (at least 300,000), of their average number per cortical neuron (30-50), their actual incidence among all terminals in the cortex (1/1,000), their mean endogenous amine content per varicosity (0.22 fg), and the mean number of recognition sites for the uptake blocker DMI (4,500/varicosity).(ABSTRACT TRUNCATED AT 400 WORDS)

Immunocytochemical and pharmacological evidence for an intrinsic cholinomimetic system modulating prolactin and growth hormone release in rat pituitary

Pituitary cells were cultured as three-dimensional reaggregates in serum-free chemically defined medium supplemented with different concentrations of dexamethasone. Immunostaining of the cells using a polyclonal antiserum and three monoclonal antibodies raised against choline acetyl transferase (CAT), revealed the presence of CAT immunoreactivity in 4-10% of anterior pituitary cells depending on the antibody used. CAT immunoreactivity was also found in freshly dispersed anterior pituitary cells. CAT-immunoreactive cells could be enriched on BSA and Percoll gradients and codistributed with ACTH-immunoreactive cells in these gradients. Perifusion of the aggregates with the potent muscarinic receptor antagonist atropine (Atr) resulted in a dose-dependent (0.1-100 nM) increase in both basal PRL and GH secretion; the response was dependent on the dexamethasone concentration in the culture medium. A similar response to Atr was observed in organ-cultured pituitaries. The specificity of the Atr effect was supported by the findings that the potent and highly specific muscarinic receptor blocker dexetimide showed a similar action, whereas its inactive enantiomer levetimide and the nicotinic receptor blocker hexamethonium failed to do so. Two other muscarinic antagonists, benzatropine and pirenzepine, showed a dose-dependent hormone-releasing action similar to that of Atr, but were less potent than the latter. Pirenzepine was only effective at high molar concentrations, suggesting that an M2 muscarinic receptor subtype was mediating the present phenomenon. Atr also potentiated GH release stimulated by the beta-adrenergic agonist isoproterenol and PRL release stimulated by vasoactive intestinal peptide, but had no effect on GRF-stimulated GH release. The choline uptake blocker hemicholinium abolished the effect of Atr on GH and PRL release. These data suggest that certain pituitary cells can express CAT activity and that these cells exert a tonic inhibitory activity on GH and PRL release which is mediated by a cholinomimetic substance, possibly acetylcholine, through a muscarinic receptor.

Interactions of benztropine, atropine and ketamine with veratridine-activated sodium channels: effects on membrane depolarization, K+-efflux and neurotransmitter amino acid release

1. The effect of benztropine, atropine and ketamine on veratridine-induced efflux of K+, membrane depolarization and release of amino acid neurotransmitters was investigated in the preparation of rat brain synaptosomes. 2. All three drugs inhibited in a concentration-dependent manner the processes measured: the most effective compound was benztropine which exhibited an approximate Kd of 2 microM. The inhibition was not competitive in nature. 3. The veratridine titration curves in the presence of drugs were sigmoid with Hill coefficients of about 1.4. 4. At higher concentrations, benztropine, atropine and ketamine blocked uptake of amino acid neurotransmitters into synaptosomes. 5. It is postulated that benztropine, atropine and ketamine interfere with the veratridine-activated influx of sodium into synaptosomes through voltage-dependent channels by acting at the same site as local anaesthetics. Interactions at this site alter allosterically binding and action of veratridine. In addition, at higher concentrations the drugs interact with the carrier proteins for amino acid neurotransmitters and block their transport.