Modulatory functions of neurotransmitters in the striatum: ACh/dopamine/NMDA interactions

Effect of MK 801 on priming of D1-dependent contralateral turning and its relationship to c-fos expression in the rat caudate-putamen

In rats with a unilateral 6-hydroxydopamine lesion of the ascending dopamine neurons, we investigated the relationship between the expression of Fos-like immunoreactivity in the caudate-putamen and contralateral turning behavior in response to dopamine agonists during the induction and expression of sensitization (priming) to D1-dependent turning behavior. Priming was induced by apomorphine (0.1 mg/kg s.c.) or by SKF 38393 (10 mg/kg s.c.) 14 days after 6-hydroxydopamine lesions and was expressed by challenge with SKF 38393 (3 mg/kg s.c.). In the induction phase of priming, administration of MK 801 (0.1 mg/kg s.c.) potentiated contralateral turning but differentially influenced stimulation of Fos expression in the caudate-putamen by apomorphine and by SKF 38393. Thus, MK 801 reduced in the expression phase of priming the stimulation of Fos expression by apomorphine in the dorsolateral caudate-putamen, but did not affect that by SKF 38393. MK 801, while preventing priming of SKF 38393-induced turning by apomorphine, failed to affect priming by SKF 38393. MK 801, given with apomorphine in the induction phase, reduced the stimulation of Fos expression in the dorsolateral caudate-putamen by SKF 38393. No such inhibitory effect of MK 801 on SKF 38393-stimulated Fos expression was observed in rats primed with SKF 38393. These results are consistent with the possibility that MK 801 disrupts sensitization of D1 transduction by reducing the activation of c-fos by the DA agonist during the induction phase of priming.

Carbachol induces inward current in neostriatal neurons through M1-like muscarinic receptors

The effects of carbachol on rat neostriatal neurons were examined in the slice and the freshly dissociated neuron preparations using intracellular and whole-cell voltage-clamp recording methods. Superfusion of carbachol (30 microM) produced a depolarization concomitant with an increase in the rate of spontaneous action potentials. This depolarization was associated with an increase in the input resistance. The carbachol-induced membrane depolarization was blocked by pirenzepine (1 microM), a selective M1 muscarinic receptor antagonist. In other experiments, we observed that carbachol induced a transient inward current on the freshly dissociated neostriatal neuron at a holding potential of -60 mV in a concentration-dependent manner underlying the whole-cell voltage-clamp mode. The inward current caused by carbachol was not reduced by tetrodotoxin (1 microM), calcium-free recording solution or Cd2+ (100 microM). However, it was blocked by Ba2+ (100 microM). In addition, the carbachol-induced inward current reversed polarity at about the potassium equilibrium potential. The whole-cell membrane inward current in response to voltage-clamp step from -90 to -140 mV was reduced by 30 microM carbachol. With stronger hyperpolarization beyond the potassium equilibrium potential, carbachol produced a progressively greater reduction in membrane current. This inhibitory effect was also abolished by Ba2+ (100 microM). A concentration of 30 microM carbachol-induced inward current could be reversibly antagonized by the M1 muscarinic receptor antagonist pirenzepine (0.1-1 microM), with an estimated IC50 of 0.3 microM. However, other muscarinic receptor subtype (M2 or M3) antagonists could also block the carbachol-induced inward current. The rank order of antagonist potency was: pirenzepine (M1 antagonist) > 4-diphenylacetoxy-N,N-methyl-piperidine methiodide (M3/M1 antagonist) > gallamine (M2 antagonist). Based on these pharmacological data, we concluded that carbachol can act at M1-like muscarinic receptors to reduce the membrane K+ conductances and excite the neostriatal neurons.

A D1 dopamine agonist stimulates acetylcholine release from dissociated striatal cholinergic neurons

We tested the hypothesis that a D1 dopamine agonist could stimulate acetylcholine release directly from striatal cholinergic neurons. A suspension of viable dissociated striatal cells was made enzymatically and mechanically from normal adult male rats. The heterogeneous suspension was incubated in [3H]choline to allow synthesis of [3H]acetylcholine selectively by cholinergic neurons. Fractional [3H]acetylcholine release from the cholinergic cells in the suspension was recorded during continuous dynamic perifusion. The D1 agonist, 50 microM (+/-) SKF 38393, increased the basal rate of release from the cholinergic cells by 50% and the action was inhibited by the D1 antagonist, SKF 83566. Stimulation of [3H]acetylcholine secretion was recorded as low as 500 nM SKF 38393. The (S, -) SKF 38393 stereoisomer was significantly less effective than the (R, +) isomer in stimulating release. The D1-mediated stimulation of acetylcholine secretion was abolished in a low-calcium environment that also inhibited basal release. The data suggest that striatal cholinergic cells express D1 receptors functionally coupled to the regulation of acetylcholine release. These D1 actions in the absence of synaptic circuitry imply that such circuitry is not required in situ. In vivo however, indirectly mediated D1 actions and those of other transmitters may modify the manifestations of this direct cholinergic stimulation.

Blockade of A2a adenosine receptors positively modulates turning behaviour and c-Fos expression induced by D1 agonists in dopamine-denervated rats

In rats with unilateral 6-hydroxydopamine lesions of the dopaminergic nigrostriatal pathway, administration of the A2a adenosine antagonist SCH 58261 alone did not induce any motor asymmetry but strongly potentiated the contralateral turning behaviour induced by the dopamine D1 agonist SKF 38393. SCH 58261 also increased the number of Fos-like positive nuclei induced by SKF 38393 in the 6-hydroxydopamine-lesioned striatum. Intense potentiation of D1-dependent turning behaviour and c-Fos expression was also observed after administration of the A2a/A1 antagonist CGS 15943. Administration of the A1 adenosine receptor antagonist DPCPX induced a small potentiation of D1-mediated contralateral turning while c-Fos expression induced by SKF 38393 was not modified. The results suggest that endogenous adenosine acting on A2a receptors can exert an inhibitory influence on the functional expression of D1-mediated responses in dopamine-denervated rats, and propose new possible therapeutic approaches in the treatment of Parkinson's disease.

Scopolamine augments c-fos and zip/268 messenger RNA expression induced by the full D(1) dopamine receptor agonist SKF-82958 in the intact rat striatum

It is generally accepted that the widely used, partial dopamine D(1) receptor agonist, SKF-38393, does not induce immediate early gene expression in striatal projection neurons unless D(1) receptors are sensitized and uncoupled from D(2) receptors by 6-hydroxydopamine lesions or reserpine treatment. In contrast, this study demonstrates, using quantitative in situ hybridization, that the full D(1) receptor agonist, SKF-82958, induced robust expression of c-fos and zif/268 messenger RNAs in the intact rat striatum, especially in the entire shell and medial and ventral core areas of the nucleus accumbens and olfactory tubercle, and in the cerebral cortex, 45 min after one injection. The induction of the striatal immediate early genes is characterized by (i) induction in only medium-sized spiny neurons, (ii) dose-dependent induction, which correlates well with dose-dependent increases in motor activity, and (iii) blockade by the D(1) receptor antagonist, SCH-23390. The muscarinic cholinergic receptor antagonist, scopolamine, which itself did not alter striatal gene expression, profoundly augmented the behaviors and expression of the two immediate early genes in the ventral and dorsal striatum induced by 0.1, 0.5 and 2.0 mg/kg SKF-82958. However, scopolamine attenuated basal, and SKF-82958-stimulated, expression of c-fos and zif/268 messenger RNAs in the cortex. Scopolamine also enabled SKF-38393 to induce locomotor stimulation and c-fos and zif/268 messenger RNA expression in the normosensitive striatum of the rat when SKF-38393 alone caused no such changes. These data demonstrate an ability of SKF-82958 to induce immediate early gene messenger RNA expression in normosensitive dorsal and ventral striatum. Furthermore, intrinsic muscarinic receptor-mediated cholinergic transmission in the striatum may provide an activity-dependent inhibitory control on striatal D(1) receptor stimulation.

Effects of opioid and dopamine receptor antagonists on relapse induced by stress and re-exposure to heroin in rats

The effects of blockade of opioid and dopamine receptors on relapse to heroin-seeking induced by footshock stress and re-exposure to heroin were examined in a reinstatement procedure. Male rats were trained to self-administer heroin (100 micrograms/kg per infusion, IV; four 3-h sessions/day for 8-11 consecutive days). Extinction sessions were given for 5-7 days during which saline was substituted for heroin. In nine groups, the effects on relapse induced by footshock (10 min, 0.5 mA, 0.5 s on with a mean off period of 40 s), heroin priming (0.25 mg/kg), and saline priming were studied after pretreatment with either naltrexone (1 or 10 mg/kg, SC), the D1-like receptor antagonist SCH 23390 (0.05 or 0.1 mg/kg, IP), the D2-like receptor antagonist raclopride (0.25 or 0.5 mg/kg, IP), the mixed dopamine antagonist flupenthixol decanoate (3 or 6 mg/kg, IM), or IP injection of saline (control condition). Naltrexone, flupenthixol, raclopride, and the highest dose of SCH 23,390 attenuated heroin-induced relapse: only the mixed DA receptor antagonist, flupenthixol, attenuated foot-shock-induced relapse. These results, and those from microdialysis showing that heroin elicits greater locomotor activity and DA release in the nucleus accumbens than footshock, suggest that the neurochemical events underlying stress- and heroin-induced relapse are not identical.

Acetylcholinesterase inhibitors block acetylcholine-evoked release of dopamine in rat striatum, in vivo

In the rat striatum, acetylcholine (ACh) increases dopamine (DA) release. The role of increased cholinergic activity provoked by acetylcholinesterase inhibitors (AChEi) on DA release is currently under revision after recent papers have shown a blockade of nicotinic transmission by AChEi in vitro. To study the effects of AChEi in vivo, Fasciculin2 (FAS), a peptidergic AChEi, and physostigmine (PHY), a classical carbamate AChEi, were applied through push-pull or microdialysis cannulae respectively, to the striatum of rats, alone or with ACh. Extracellular concentrations of DA were assessed by HPLC with electrochemical detection. Alone, the AChEi studied did not provoke changes in basal extracellular levels of DA, in the different doses studied. ACh (100 microM, 1 and 5 mM) applied through the push-pull cannulae in basal conditions provoked a dose-dependent increase of extracellular DA. This effect was not observed with ACh in concentrations of 100 microM and 1 mM if FAS (0.4 and 4.2 microM) was applied first. Higher concentrations of ACh (5 mM) evoked a partial response after FAS 0.42 microM, an effect still blocked by FAS at 4.2 microM. PHY 50 microM applied through microdialysis completely blocked the increase in DA release provoked by ACh 10, 20 mM, while at ACh 30 mM, PHY 50 microM only partially blocked the evoked increase. A partial blockade was also observed with PHY 20 microM, on the three different concentrations of ACh. On the other hand PHY 10 microM did not block any of the ACh doses perfused. These results showed that AChEi like FAS and PHY interfere with the ACh-evoked DA release in the striatum.

Functional neuroanatomy of the nigrostriatal and striatonigral pathways as studied with dual probe microdialysis in the awake rat--II. Evidence for striatal N-methyl-D-aspartate receptor regulation of striatonigral GABAergic transmission and motor function

In the present study we used the dual probe approach to investigate striatal N-methyl-D-aspartate receptor regulation of GABA release from the substantia nigra pars reticulata of the awake, freely moving rat. One microdialysis probe of concentric design was implanted in the dorsolateral striatum and another in the ipsilateral substantia nigra pars reticulata. Perfusion with N-methyl-D-aspartate (100 microM) in the dorsolateral striatum decreased local dopamine release (-25%) and increased both glutamate (+40%) and GABA (+35%) release. Moreover, perfusion with N-methyl-D-aspartate (100 microM) in the dorsolateral striatum increased GABA release (+20%) in the substantia nigra pars reticulata. Perfusion with the lower (10 microM) N-methyl-D-aspartate concentration in the dorsolateral striatum did not affect striatal dopamine, glutamate and GABA release or nigral GABA release. Intrastriatal perfusion with the N-methyl-D-aspartate receptor antagonist dizocilpine maleate (10 microM), at a dose which by itself did not affect basal striatal or nigral neurotransmitter levels, prevented the effects of striatal perfusion with N-methyl-D-aspartate on both striatal and nigral neurotransmitter release. Intrastriatal dizocilpine maleate was also perfused concurrently with intranigral tetrodotoxin (10 microM) (see accompanying paper). Intrastriatal perfusion with dizocilpine maleate prevented the tetrodotoxin-induced rise in both striatal and nigral GABA levels and profoundly reduced the tetrodotoxin-induced contralateral turning. In addition, intrastriatal dizocilpine maleate delayed the increase in striatal glutamate release evoked by intranigral tetrodotoxin without affecting the associated decrease in striatal dopamine release. The present study demonstrates that N-methyl-D-aspartate receptors in the dorsolateral striatum regulate GABA release in the substantia nigra pars reticulata of the awake rat and provides evidence that this regulation plays a key role in motor function.

Activation of corticostriatal pathway leads to similar morphological changes observed following haloperidol treatment

Treatment with haloperidol, a dopamine receptor D-2 antagonist, for one month resulted in an increase in the mean percentage of asymmetric synapses containing a discontinuous, or perforated, postsynaptic density (PSD) [Meshul et al. (1994) Brain Res., 648:181-195] and a change in the density of striatal glutamate immunoreactivity within those presynaptic terminals [Meshul and Tan (1994) Synapse, 18:205-217]. We speculated that this haloperidol-induced change in glutamate density might be due to an activation of the corticostriatal pathway. To determine if activation of this pathway leads to similar morphological changes previously described following haloperidol treatment, GABA (10(-5) M, 0.5 microliters) was injected into the thalamic motor (VL/VM) nuclei daily for 3 weeks. This treatment resulted in an increase in the mean percentage of striatal asymmetric synapses containing a perforated PSD and an increase in the density of glutamate immunoreactivity within nerve terminals of asymmetric synapses containing a perforated or non-perforated PSD. Subchronic injections of GABA into the thalamic somatosensory nuclei (VPM/VPL) had no effect on the mean percentage of synapses with perforated PSDs but resulted in a small, but significant, increase in density of glutamate immunoreactivity. Using in vivo microdialysis, an acute injection of GABA (10(-5) M, 15 microliters) into VL/VM resulted in a prolonged rise in the extracellular level of striatal glutamate. The increase in asymmetric synapses with perforated PSDs and in glutamate immunoreactivity within nerve terminals of the striatum following either subchronic haloperidol treatment or GABA injections into VL/VM suggest that an increase in glutamate release may be a common factor in these two experiments. It is possible that the extrapyramidal side effects associated with haloperidol treatment may be due, in part, to an increase in release of glutamate within the corticostriatal pathway.

Quantitative autoradiography reveals regionally selective changes in dopamine D1 and D2 receptor binding in the genetically dystonic hamster

Dystonia has been proposed to be caused by abnormal input from thalamus to premotor cortex due to altered activity of the striatum projecting by way of the globus pallidus and substantia nigra pars reticulata to the thalamus. However, in the case of idiopathic dystonia, i.e. the most common form of dystonia in humans, there is only limited evidence to support such a neuroanatomic concept. In view of the problems of studying the pathophysiology of idiopathic dystonia in patients, genetically determined animal models of idiopathic dystonia may be used as a practical means of studying brain dysfunctions involved in this movement disorder. The genetically dystonic hamster is an animal model of idiopathic dystonia that displays sustained abnormal movements and postures either spontaneously or in response to mild environmental stimuli. Autoradiographic analysis of dopamine D1 receptor density, using the ligand [3H]SCH 23390, revealed significant decreases of D1 binding in several parts of the striatum and substantia nigra pars reticulata of dystonic hamsters. Binding of the D2 ligand [3H]YM-09151-2 was decreased in the dorsomedial caudate-putamen, but increased in nucleus accumbens. In most other sites studied, no significant changes were found in either [3H]SCH 23390 or [3H]YM-09151-2 binding. By studying groups of dystonic hamsters in the absence and presence of dystonic attacks, it was shown that most changes in D1 and D2 binding were not secondary to abnormal movement but rather due to the dystonic condition of the animals. The study provides evidence of altered dopamine receptor binding in dystonia and confirms the concept that basal ganglia dysfunction may be a primary component of dystonia.

Ultrastructural characterization of the acetylcholine innervation in adult rat neostriatum

The ultrastructural features of acetylcholine axon terminals (varicosities) in adult rat neostriatum were characterized by electron microscopy after immunostaining with a sensitive monoclonal antibody against rat choline acetyltransferase. Several hundred single sections from these varicosities were analysed for shape, size and content, presence of a synaptic membrane specialization, and composition of the microenvironment. An equivalent number of unlabeled varicosities selected at random from the same micrographs were similarly examined. The immunostained varicosity profiles were relatively small and seldom showed a junctional membrane specialization. Stereological extrapolation to the whole volume of these varicosities indicated that less than 10% were synaptic. Far fewer dendritic spines were juxtaposed to these predominantly asynaptic profiles than to their unlabeled counterparts. This difference seemed imputable to the low synaptic incidence of the acetylcholine varicosities and was consistent with the view that these are randomly distributed in relation to surrounding elements. The bulk of the data was suggestive of volume transmission. This raised the possibility that, in such a densely innervated area, a basal level of acetylcholine is permanently maintained around all cellular elements, contributing to the modulatory properties of this transmitter. This basal level of acetylcholine could also serve as a regulatory signal controlling the expression of different receptor subtypes in neurons, glia and blood vessels.

Role of the parafascicular thalamic nucleus and N-methyl-D-aspartate transmission in the D1-dependent control of in vivo acetylcholine release in rat striatum

We investigated the involvement of glutamatergic neurotransmission in the modulation of D1 receptor-mediated stimulation of acetylcholine outflow in dorsal striatum in freely moving rats, and the relative roles of the thalamostriatal and corticostriatal pathways in this regulation using in vivo microdialysis. The selective N-methyl-D-aspartate non-competitive antagonist dizocilpine maleate (0.1 mg/kg i.p.), but not the kainate/quisqualate receptor antagonist, 6,7-dinitroquinoxaline-2,3-dione (3 micrograms per side i.c.v.), completely prevented the rise in striatal extracellular acetylcholine elicited by maximal effective doses of the full D1 agonist SKF 82958 (3 mg/kg s.c.) and of the dopamine releaser d-amphetamine (2 mg/kg s.c.). Acute bilateral electrolytic lesions of the parafascicular nucleus of the thalamus prevented the stimulation of striatal acetylcholine output by SKF 82958 and d-amphetamine but only slightly reduced basal acetylcholine release. In contrast acute interruption of the corticostriatal pathway did not alter the effect of the two dopaminergic drugs although it markedly reduced basal striatal acetylcholine release. Lesions of the parafascicular thalamic nucleus, or a low dose of dizocilpine maleate (0.1 mg/kg i.p.), also prevented the acetylcholine-increasing effect of the neuroleptic remoxipride (10 mg/kg s.c.), an effect known to be D1 receptor dependent. The results suggest that striatal projections arising from the parafascicular thalamic nucleus and utilizing N-methyl-D-aspartate receptors play a critical role in the D1-mediated stimulation of acetylcholine release in dorsal striata.

Promoter region of the rat m4 muscarinic acetylcholine receptor gene contains a cell type-specific silencer element

We describe here the characterization of the rat m4 muscarinic acetylcholine receptor gene and the identification of its regulatory region. Two 5'-noncoding exons are located approximately 5 kilobases upstream from the coding exon, and at least two alternatively spliced variants of m4 mRNA are expressed in the neuronal cell line PC12D. There are two transcription initiation sites. The promoter region is GC-rich, contains no TATA-box, but has two potential CAAT boxes and several putative binding sites for transcription factors Sp1 and AP-2. We assessed the m4 promoter activity functionally in transient expression assays using luciferase as a reporter. The proximal 435-base pair (bp) sequence of the 5'-flanking region produced luciferase activity in both m4-expressing neuronal cell lines (PC12D and NG108-15) and non-neuronal cell lines (L6 and 3Y1B). A longer fragment containing an additional 638-bp sequence produced luciferase activity only in m4-expressing neuronal cell lines. These data suggest that the proximal 435-bp sequence contains a constitutive promoter and that a 638-bp sequence farther upstream contains a cell type-specific silencer element. A consensus sequence for the neural-restrictive silencer element is found within this 638-bp segment.

FS stress induces long-lasting memory facilitation: involvement of cholinergic pathways

We tested in vivo the hypothesis that foot-shock (FS) stress-induced prolongation of latencies in the one-trial step-through passive avoidance learning task in mice occurred through a long-term facilitation process. Whereas behavioral responses in control mice lasted for 24 h, decreasing progressively in the subsequent days, FS-stress exposure for 15 min before training (pretraining), immediately after training (posttraining), or 15 min before the test (pretest) resulted in a profound and sustained enhancement of test latencies that lasted for at least 96 h. These facilitating effects disappeared when FS exposure was delivered with a 2- or 3-h difference with respect to the training trial. Scopolamine (Scop) (1 mg/kg, intraperitoneally) 30 min before the training session caused impairment of test latencies in control and pretest stressed animals, but failed to affect both pre- and posttraining FS stress-induced enhancement. Our working hypothesis is that FS stress may increase the levels of acetylcholine in the presynaptic terminal or the firing rate of cholinergic input. Animals pretreated with FS stress daily for 1 or 4 days followed by the acute schedule described above showed no enhancements of test latencies. Pretraining Scop impaired test latencies in pre- and posttraining and pretest stressed animals, suggesting that unpredictability is a critical factor in activating behavioral long-term facilitation.

The survival of striatal cholinergic neurons cultured from postnatal 2-week-old rats is promoted by neurotrophins

Although the expression of nerve growth factor (NGF) in the rat striatum is the highest at 2 postnatal weeks (P2w), the action of NGF at that age has not been studied in detail. We examined the effects of several neurotrophic factors, including NGF, on striatal cholinergic neurons cultured from P2w rats. We also examined the effects of a cyclic AMP (cAMP) analog and high K(+)-evoked depolarization. NGF specifically promoted the survival of choline acetyltransferase (ChAT)-positive neurons, and consequently increased the ChAT activity per well, whereas it did not induce the ChAT activity per cholinergic neuron. NGF-responsiveness was the highest in striatal cultures from P2w rats, but it was almost lost in cultures from P4w rats. Brain-derived neurotrophic factor (BDNF), neurotrophin-4/5 (NT-4/5), and a cAMP analog had survival-promoting effects on striatal total neurons including cholinergic neurons. On the other hand, high K+ hardly promoted the survival of striatal cholinergic neurons in cultures from P2w rats, although it increased the viable number of total striatal neurons. High K+ did not increase the ChAT activity in any tested cultures from postnatal 3- to 28-day-old rats. These results demonstrated that NGF prevented the death of striatal cholinergic neurons in cultures from P2w rats, but not from P4w rats, and that high K+ could not rescue these deaths. We propose that cholinergic neurons in the striatum are programmed to die at P2w, and that this programmed cell death can be restored by neurotrophins, but not by depolarization.

Progesterone in vitro increases NMDA-evoked [3H] dopamine release from striatal slices in proestrus rats

The dopaminergic nerve terminals in rat striatum appear to be an important target for progesterone (Pg) and the excitatory amino acid glutamate. In the present study the possible interaction between glutamate and Pg upon [3H]DA release in striatal slices from rats in proestrus was examined. [3H]DA release was augmented by NMDA in a concentration-dependent manner. The presence of Pg (400 nM) in the perfusion medium produced an amplification of the responses to NMDA (50 microM) as shown by significant increase in the tritium outflow. The NMDA selective antagonists AP-7 (100 microM) and MK-801 (0.1 microM) prevented the effects of both NMDA and NMDA plus Pg on [3H]DA release. In contrast, the AMPA/kainate receptor antagonist CNQX (10 and 20 microM) was ineffective. Furthermore, AP-7 (100 microM) attenuated the enhancing effect of 400 nM Pg on [3H]DA release evoked by 28 mM K+. The antagonist was unable to alter the effect produced by K+ alone. These results indicate a specific action of Pg on dopaminergic terminals mediated by NMDA receptors and suggest a close interaction between glutamate and dopamine systems in the striatum, apparently modulated by progesterone.

Long-term neuroleptic treatments counteract dopamine D2 agonist inhibition of adenylate cyclase but do not affect pertussis toxin ADP-ribosylation in the rat brain

We have investigated the response of adenylate cyclase to GTP and to dopamine (DA) in striatal membranes of rats treated for 3 weeks with chlorpromazine or haloperidol, and further measured the level of Gi (an inhibitory GTP-binding protein) or Go (a similar GTP-binding protein of unknown function) in 3 areas (cerebral cortex, striatum and hippocampus) utilizing pertussis toxin-catalyzed ADP ribosylation. In saline-treated control membranes, GTP exerted a biphasic effect on basal and DA-stimulated enzyme activity--peak levels of stimulation by DA plus GTP were observed at 1 microM GTP. Conversely, dopaminergic inhibitory effects at 10-100 microM GTP were completely attenuated in chlorpromazine or haloperidol-treated membranes. D2 inhibition of adenylate cyclase by the selective D2 agonist PPHT was also attenuated due to these neuroleptic treatments, while an increase in D2 receptor binding was observed. The pertussis toxin ADP-ribosylation of G-proteins (Gi/Go) did not differ significantly in any area. This indicates that long-term neuroleptic treatments increased D2 receptor binding, but attenuated D2 inhibition of adenylate cyclase, and exercised no influence on pertussis toxin ADP-ribosylation.

Effects of selective adenosine A1 and A2a agonists on amphetamine-induced locomotion and c-Fos in striatum and nucleus accumbens

Low to moderate doses of amphetamine produce locomotion which is dependent on release of dopamine in the anteromedial striatum and nucleus accumbens. The effects of selective adenosine A1 and A2a receptor agonists on locomotion and c-Fos induction following a moderate dose of amphetamine was assessed in rats. Pretreatment with the adenosine A1 receptor agonist N6-cyclohexyladenosine (CHA) or the adenosine A2a receptor agonist 2-[(2-aminoethylamino)carbonylethylphenylethylamino]-5'-N- ethylcarboxamidoadenosine (APEC) inhibited locomotion following an injection of amphetamine (1.5 mg/kg). This dose of amphetamine induced Fos-like immunoreactivity in an antero-dorsomedial distribution in the caudate-putamen and uniformly in the core and shell of the nucleus accumbens. Pretreatment with the adenosine A2a receptor agonist APEC, but not the adenosine A1 receptor agonist CHA, attenuated c-Fos induction in caudate-putamen and nucleus accumbens by amphetamine. These findings indicate that amphetamine-induced behavior is subject to modulation by adenosine receptors through mechanisms which are both related to and independent of c-Fos induction.

The involvement of dopamine in the striatum in passive avoidance training in the chick

Quantitative receptor autoradiography was used to investigate the distribution of binding of [3H]SCH 23390 to dopamine (D1) and [3H]spiroperone to D2 receptors in regions of the forebrain of the one-day-old domestic chick (Gallus domesticus). High levels of specific binding of the D1 and D2 ligands were found in the striatal regions (paleostriatum augmentatum and lobus parolfactorius) of the one-day-old chick, as reported previously in the pigeon, turtle and rat, whilst binding levels were considerably lower in the pallidum (paleostriatum primitivum), hippocampus and hyperstriatum ventrale. The proportions of D1 and D2 receptor binding in the chick were relatively similar in the striatum and pallidum, apart from the paleostriatum augmentatum, where D2 receptors outnumber those of D1 by a factor of two. Binding of the D1 and D2 ligands to forebrain regions was also investigated 30 min after one-trial passive avoidance training of one-day-old chicks in which the aversive stimulus was a bead coated with a bitter tasting substance, methyl anthranilate. These experiments demonstrated a large and highly significant bilateral increase (compared to control birds) in binding to D1 (but not D2) receptors in the lobus parolfactorius. In this striatal region, equivalent to the caudate-putamen of mammals, previous studies have shown that synaptic and dendritic alterations occur following avoidance training. It is concluded that alterations in dopamine binding may be involved in processes that result in modification of the pecking response in chicks after avoidance training.

Changes in extracellular levels of dopamine metabolites in somatosensory cortex after peripheral denervation

This study examined the effects of a nerve transection on monoamine release from primary somatosensory cortex. The technique of microdialysis was employed to sample extracellular levels of norepinephrine (NE), 3,4-dihydroxyphenylacetic acid (DOPAC), 5-hydroxyindole-3-acetic acid (5-HIAA) and homovanillic acid (HVA) in the barrel field of freely moving rats following the surgical transection of the contralateral infraorbital nerve. Microdialysates obtained 3, 4, and 5 days after deafferentation were analyzed using high-performance liquid chromatography with electrochemical detection. We found a significant increase in the release of the dopamine metabolites, DOPAC and HVA from the deafferented cortex. Three days after deafferentation the release of DOPAC was three-fold higher in the deafferented than in the control animals, and remained about 100% higher in the next two days in this group of animals. The release of HVA showed a gradual increase following the deafferentation procedure, since a 92% larger value on day 3 increased to a 338% difference on day 5. On the other hand, the release rate of NE and the levels of the serotonin metabolite 5-HIAA were not significantly affected by the deafferentation procedure. These results are discussed in the context of the possible participation of dopamine in the reorganization of the deafferented somatosensory cortex.

Effect of alpha-MSH upon cyclic AMP levels induced by the glutamatergic agonists NMDA, quisqualic acid, and kainic acid

This study was carried out to investigate possible interactions between some glutamatergic agonists and the peptide alpha-MSH upon the cyclic AMP levels. We used an in vitro tissue slice preparation incubated in the presence of different glutamatergic agonists such as N-methyl-D-aspartic acid (NMDA), quisqualic acid (QUIS), kainic acid (KA), and the peptide alpha-MSH together with each agonist. Slices containing caudate putamen and accumbens were chosen according to neurochemical data indicating that the striatum contains a moderate amount of MSH binding sites and also receives glutamatergic innervation. Exposure of these slices to either MSH or to the agonists NMDA or QUIS resulted in an increase in the cAMP levels in relation to controls. Nevertheless, incubation with KA resulted in no changes in the nucleotide levels. The combination of MSH/NMDA induced a reduction of cAMP levels in relation to those obtained with NMDA alone. The combinations of QUIS/MSH or KA/MSH also induced variations in the values of nucleotide in relation to the those obtained with the peptide alone or with the corresponding agonist; these changes were related to the dose of agonist used in each case. The results obtained in these experiments suggest the existence of some interaction between the peptide and the agonist used.

Glutamate receptor activation induces carrier mediated release of endogenous GABA from rat striatal slices

The regulation of striatonigral and striatopallidal GABAergic neurons by glutamatergic afferents is thought to play a critical role in normal basal ganglia function. Here we report that in striatal slices about 17% of K(+)-induced endogenous GABA release was Ca(2+)-independent and this could be blocked by a GABA transport inhibitor. Activation of N-methyl-D-aspartate (NMDA)- and quisqualate-sensitive receptors induced endogenous GABA efflux only in the presence of a GABA transaminase inhibitor; this efflux was inhibited by 60-80% with a GABA transport inhibitor. NMDA-induced GABA release was blocked by phencyclidine, Mg2+ and CGS 19755. Quisqualate-induced GABA release was blocked completely by a combination of the metabotropic antagonist, L-AP3 and CNQX, a non-NMDA receptor antagonist. These data indicate that excitatory amino acid agonists-induced GABA release is distinct from that induced by high K+ depolarization.

The effects of D1 or D2 dopamine receptor blockade on zif/268 and preprodynorphin gene expression in rat forebrain following a short-term cocaine binge

Selective D1 or D2 dopamine receptor antagonists were used to investigate the transynaptic regulation of mRNAs coding for the opioid peptide, preprodynorphin, and the nuclear transcription factor, zif/268 after an acute cocaine binge. Rats were injected intraperitoneally with the D1 receptor antagonist, SCH 23390, or the D2 receptor antagonist, sulpiride, 30 min prior to 3 hourly injections of saline or 20 mg/kg cocaine and killed 1 h after the final injection. Behavioral ratings indicated that SCH 23390 blocked, whereas sulpiride augmented, cocaine-induced stereotypical behaviors. Striatal sections were hybridized with oligonucleotides coding for zif/268 and preprodynorphin. Quantitative image analysis of autoradiograms revealed that (1) SCH 23390 completely suppressed basal and cocaine binge-induced zif/268 mRNA in the striatal and cerebral cortical areas examined; (2) sulpiride enhanced basal levels of zif/268 mRNA in the medial caudate and dorsomedial shell of the nucleus accumbens; (3) sulpiride partially blocked cocaine binge-induced levels of zif/268 mRNA in the dorsal striatum but had no effect in sensory cortex; (4) SCH 23390, but not sulpiride, significantly reduced the constitutive expression of preprodynorphin mRNA; and (5) SCH 23390 and sulpiride blocked cocaine binge-induced expression of preprodynorphin mRNA in the dorsal striatum.

Involvement of adenosine and glutamate receptors in the induction of c-fos in the striatum by haloperidol

The psychostimulant drugs amphetamine and cocaine induce the expression of immediate early genes, such as c-fos, in the striatum via D1 dopamine receptor activation. This occurs primarily in the striato-nigral neurons. Conversely, neuroleptic drugs, such as haloperidol, which block D2-type dopamine receptors, induce c-fos expression in striatal neurons projecting to the globus pallidus. In order to gain insight into the neurochemical substrates of neuroleptic-induced c-fos expression, we examined the effects of adenosine A2 and N-methyl-D-aspartate (NMDA) receptor antagonists as well as inhibition of nitric oxide synthase, on haloperidol-induced Fos immunoreactivity in the striatum. While blockade of D1 receptors had no effect on haloperidol-induced Fos expression, adenosine A2 receptor antagonists decreased the number of neurons in the striatum expressing haloperidol-induced Fos by half. NMDA receptor antagonists also potently blocked the induction of Fos immunoreactivity by haloperidol, while inhibition of nitric oxide synthase activity had no effect. These results indicate that in the presence of a dopamine D2 antagonist, Fos expression in striato-pallidal neurons is mediated in part through activation of A2 receptors by adenosine, and via NMDA receptor activation by glutamate.

Intermittent morphine treatment causes long-term desensitization of functional dopamine D2 receptors in rat striatum

3 weeks following cessation of intermittent morphine administration (10 mg/kg, s.c., once daily for 14 days), [3H]dopamine and [14C]acetylcholine release induced by 10 microM N-methyl-D-aspartate (NMDA) from superfused rat striatal slices appeared to be significantly higher than the release from striatal slices from saline-treated rats. A similar adaptive increase of the NMDA-evoked release of these neurotransmitters was observed in slices of the nucleus accumbens, whereas that of [3H]noradrenaline from hippocampal slices remained unchanged. Blockade of dopamine D2 receptors by 10 microM (--)-sulpiride enhanced NMDA-induced [3H]dopamine and [14C]acetylcholine release from striatal slices from saline-treated animals, but was found to be ineffective in this respect following intermittent morphine treatment. Moreover, morphine administration appeared to cause a profound decrease in the apparent affinity of the full dopamine D2 receptor agonist LY171555 (quinpirole) for these release-inhibitory dopamine D2 receptors, indicating the occurrence of dopamine D2 receptor desensitization. It is suggested that such a desensitization of dopamine D2 receptors on dopaminergic nerve terminals as well as on cholinergic interneurons may play a pivotal role in the long-lasting nature of behavioural sensitization upon cessation of treatment with morphine and possibly other drugs of abuse.

Permanent alteration of muscarinic acetylcholine receptor binding in rat striatum after circling training during development

We evaluated the effect of circling training (CT) in the expression of muscarinic acetylcholine receptor (mAchR) in developing rat striatum. For this, male and female rats were subjected to CT at 20, 30, 40 and 60 days of age during 7 days. Animals trained at 30 days but not at other ages showed an average decreased binding to mAchR of 33% in males and 24% in females, representing a significant difference with respect to control non-trained animals (males P < 0.001, females P < 0.005), and showing also a differential response between sex (P < 0.01). mAchR drop was found invariably either 2 months or 1 year after training indicating a long term plastic change due to circling training. Scatchard analysis showed that altered binding represents a variation of the total receptor number instead of its binding affinity, with no significant differences found among Kd (P > 0.1). mAchR variation was correlated with the motor performance accomplished in the test. Regarding total distance run, male rats trained for 3 days (300 meters. run), for 5 days (600 meters) and for 7 days (900 meters) showed a drop of 19, 28 and 33% respectively (r2 = 0.91, P < 0.001), while female changes were of 21, 23 and 24% (r2 = 0.78, P < 0.001). Nevertheless, no correlation with running speed was found (r2 = 0.13 male, r2 = 0.02 female; P > 0.1). In summary, these results demonstrate the presence of a limited sensitivity period during striatum development where mAchR expression may be affected by the activity performed during CT, representing a permanent alteration of the receptor levels.

Reversal of levodopa-induced motor fluctuations in experimental parkinsonism by NMDA receptor blockade

Dopaminoceptive system alterations in the basal ganglia have been implicated in the pathogenesis of wearing-off fluctuations that complicate levodopa therapy of Parkinson's disease. To evaluate the contribution of glutamatergic mechanisms to the associated changes in striatal efferent pathway function, we examined the ability of N-methyl-D-aspartate (NMDA) receptor blockade to modify the motor response changes produced by chronic levodopa administration to hemiparkinsonian rats. Unilaterally 6-hydroxydopamine lesioned rats, given levodopa/benserazide (25/6.25 mg/kg) twice daily for 3 weeks, developed a progressive shortening in the duration of their motor response to levodopa similar to that occurring in parkinsonian patients with wearing-off phenomenon. The acute systemic administration of MK-801 (0.1 mg/kg) to these animals completely reversed the decrease in turning duration (P < 0.01). Intrastriatal injection of the NMDA antagonist was even more effective in prolonging the levodopa response (P < 0.01), while intranigrally injected MK-801 produced no statistically significant change in the duration of levodopa-induced rotation. Rotational intensity was unaffected by all routes of MK-801 administration. These results suggest that drugs capable of blocking NMDA receptors, especially in striatum, may help ameliorate motor fluctuations in patients with advanced Parkinson's disease.

Cortical stimulation induces Fos expression in striatal neurons via NMDA glutamate and dopamine receptors

Cortical electrical stimulation has been shown to induce dense and widespread Fos expression throughout the ipsilateral and contralateral striatum. This raises interest for studying the mechanisms underlying the regulation of striatal neuron activity by cortical afferents, and for elucidating the interactions with other systems. However, the receptors mediating cortical-stimulation-induced expression of Fos in striatal neurons have not been identified. This was studied in the work reported here by stimulating the cortex after administration of glutamate or dopamine receptor antagonists, or after 6-hydroxydopamine (6-OHDA) lesion of the nigrostriatal dopaminergic system. Pretreatment with the non-competitive N-methyl-D-aspartate (NMDA) glutamate receptor antagonist MK-801 led to a marked reduction in the stimulation-induced density of Fos-immunoreactive nuclei in both the medial (about 80% reduction) and lateral (about 50-60% reduction) striatum. Preadministration of the D1-selective dopamine antagonist SCH-23390 alone or in combination with the D2-selective dopamine antagonist eticlopride led to a reduction in the stimulation-induced density of Fos-positive nuclei of about 60-65% in the lateral striatum, but no significant change in the medial region. The effects of 6-OHDA lesion were less pronounced, and the stimulation-induced density of Fos-immunoreactive nuclei decreased by only about 25% in the lateral region. These results indicate that both dopamine and NMDA glutamate receptors are involved in the induction of Fos by cortical stimulation, and support the hypothesis that cortex-dopamine interactions in the lateral striatum may be functionally different from those in the medial striatum.

Intermittent morphine treatment causes a protracted increase in cholinergic striatal neurotransmission measured ex vivo

Considering the long-lasting neuroadaptations that occur in the brain after exposure to drugs of abuse, we found that the facilitatory effect of an EC50 concentration (0.1 microM) of the acetylcholinesterase inhibitor physostigmine, unlike that of the muscarinic receptor agonist oxotremorine, on K(+)-induced [3H]dopamine release from rat striatal slices was enhanced about 2-fold 1 month after cessation of intermittent morphine treatment. Similarly, the inhibitory effect of physostigmine on K(+)-induced [14C]acetylcholine release from the slices was enhanced subsequent to morphine treatment, whereas that of oxotremorine appeared to be unchanged. Therefore, intermittent morphine administration may cause a very long-lasting increase of muscarinic receptor activation by released endogenous acetylcholine in rat striatum, which may play a pivotal role in the enduring character of stimulus hyperresponsiveness after exposure to drugs of abuse.

Independent mediation of unconditioned motor behavior by striatal D1 and D2 receptors in rats depleted of dopamine as neonates

The effects of systemic administration of DA receptor antagonists suggest that unconditioned motor behavior in rats depleted of DA as neonates continues to be dependent upon dopaminergic transmission, yet the specific contribution of D1 and D2 receptors to these behaviors has been altered. The purpose of the present study was to determine whether these depletion-induced receptor changes are occurring at the level of striatal DA terminals and their targets. The ability of bilateral intrastriatal injections (0.5 microliter) of DA receptor antagonists to induce motoric deficits was determined in adult rats treated with vehicle or 6-OHDA (100 micrograms, intraventricular) on postnatal day 3. Administration of the D1-like antagonist SCH 23390 (0.5-2.0 micrograms) or the D2-like antagonist clebopride (1.0-4.0 micrograms) induced dose-dependent akinesia, catalepsy, and somatosensory neglect in vehicle-treated controls. In contrast, neither antagonist produced deficits in rats depleted of forebrain DA as neonates. However, combined administration of SCH 23390 + clebopride induced similar akinesia, catalepsy, and somatosensory neglect in both controls and DA depleted animals. Animals depleted of DA were more sensitive than controls to the low doses of this combined D1 + D2 antagonism. These results demonstrate that activation of striatal DA receptors remains necessary for unconditioned motor behavior in rats depleted of DA as neonates. However, the specific contributions of D1- and D2-like receptors to these behaviors differ between intact animals and those depleted of DA as neonates. The ability of endogenous DA acting at either D1 or D2 receptors to support spontaneous motor behavior in rats depleted of DA as neonates may contribute to their relative sparing from parkinsonian deficits.

Intermittent and chronic morphine treatment induces long-lasting changes in delta-opioid receptor-regulated acetylcholine release in rat striatum and nucleus accumbens

Intermittent treatment of rats with morphine (10 mg/kg s.c., once daily) caused an increase (of about 30%) of the electrically evoked release of [14C]acetylcholine from cholinergic interneurons of superfused striatal slices 1-21 days after morphine withdrawal. Similarly, chronic treatment with escalating doses of morphine (5-50 mg/kg s.c., 3 times daily), causing physical dependence (unlike intermittent treatment), resulted in an enduring enhanced response of these neurons towards depolarization. Following chronic morphine treatment this adaptive increase of acetylcholine release was associated with a slight but long-lasting decrease of the (delta-opioid receptor-mediated) maximal inhibitory effect of [Met5]enkephalin, whereas upon intermittent drug treatment delta-opioid receptor desensitization was observed 1 day after opiate withdrawal only. Also in slices of the nucleus accumbens both intermittent as well as chronic morphine administration caused a long-lasting increase of the electrically evoked [14C]acetylcholine release. Therefore, we hypothesize that an enhanced (re)activity of striatal and accumbal cholinergic neurons, which are regulated by dopaminergic neurons of the ventral mesencephalon, may represent a long-lasting neuroadaptive effect of morphine (and possibly other drugs of abuse) playing a crucial role in behavioral sensitization associated with enhanced vulnerability to drugs of abuse.

Social isolation and social interaction alter regional brain opioid receptor binding in rats

Endogenous opioid systems have been implicated in the consequences of social isolation and in the regulation of social behavior, although their precise role is not clear. There is not much information on a possible locus in the brain at which opioids exert their effects on social behavior. In an effort to address this issue we analyzed regional opioidergic activity upon social isolation-induced social interaction using in vivo autoradiography. Animals were either socially isolated for 7 days or group housed, and tested singly or in a dyadic encounter. Subsequently, a tracer dose of [3H]diprenorphine was administered and in vivo autoradiographic analysis was performed. Seven days of social isolation caused changes in both social behavior (dyadic encounters) and non-social behavior (singly tested animals). Opioid receptor binding was increased in the medial prefrontal cortex and the parafascicular area in isolates, suggesting that social isolation may evoke an upregulation of opioid receptors in these areas. Social interaction increased opioid binding in the parafascicular area of non-isolated rats. In substantia nigra para compacta and ventral tegmental area binding was increased upon social isolation, and social interaction decreased opioid binding in isolates, but these changes failed to reach significance. These observed local changes in opioid receptor binding suggest a role for opioid systems in discrete areas in the consequences of social isolation and the regulation of social behavior in rats.

Acute dopamine depletion potentiates independent stimulatory and inhibitory D1 DA receptor-mediated control of striatal acetylcholine release in vitro

Fractional release of [3H]ACh was evaluated under basal and evoked conditions in striatal slices from normal and acutely dopamine-depleted adult rats for the influence of D1- and D2-DA receptor agonists. The D1 ligand had no effect on normal slices but DA depletion unmasked two independent but simultaneous supersensitive responses: augmentation of K(+)-evoked and inhibition of glutamate-evoked release. The D2 ligand inhibited evoked release in normal slices and this effect was not potentiated. This is a new cholinergic model of acute D1 receptor supersensitivity.

Social play alters regional brain opioid receptor binding in juvenile rats

An in vivo autoradiographic procedure was employed to visualize local changes in brain opioid receptor occupancy in juvenile rats. This procedure is based on the assumption that released endogenous ligand will exclude exogenously applied tracer, in this case [3H]diprenorphine, from opioid receptors. Increases in availability of opioid peptides will then result in decreased opioid receptor binding. From behavioral studies there is ample evidence that opioid systems are involved in the regulation of social play behavior in juvenile rats. In the present study, changes in regional brain opioid activity as a result of social isolation-induced social play behavior were monitored. Twenty-one-day-old rats were socially isolated for 0, 3.5 or 24 h prior to testing, and tested alone or in a dyadic encounter. After behavioral testing, [3H]diprenorphine was administered and the brain was prepared for autoradiography. Social isolation caused increases in social behavior (dyadic encounters) but not in non-social behavior (singly tested animals). Modest differences in brain opioid receptor binding due to social isolation, social play behavior, or an interaction of the two, were found in claustrum, nucleus accumbens, globus pallidus, paraventricular and arcuate nuclei of the hypothalamus, and the dorsolateral and paratenial thalamic nuclei. These results support the notion that opioid systems are involved in the regulation of social play behavior. In addition, the observation of changes in opioid binding in areas involved in reward processes, adds evidence to the hypothesis that opioid systems are involved in the regulation of the rewarding aspects of social play in juvenile rats.

The role of dopamine in drug abuse viewed from the perspective of its role in motivation

Drugs of abuse share with conventional reinforcers the activation of specific neural pathways in the CNS that are the substrate of their motivational properties. Dopamine is recognized as the transmitter of one such neural pathway, being involved in at least three major aspects of motivation: modulation of motivational state, acquisition (incentive learning) and expression of incentive properties by motivational stimuli. Drugs of abuse of different pharmacological classes stimulate in the low dose range dopamine transmission particularly in the ventral striatum. Apart from psychostimulants, the evidence that stimulation of dopamine transmission by drugs of abuse provides the primary motivational stimulus for drug self-administration is either unconvincing or negative. However, stimulation of dopamine transmission is essential for the activational properties of drugs of abuse and might be instrumental for the acquisition of responding to drug-related incentive stimuli (incentive learning). Dopamine is involved in the induction and in the expression of behavioural sensitization by repeated exposure to various drugs of abuse. Sensitization to the dopamine-stimulant properties of specific drug classes leading to facilitation of incentive learning of drug-related stimuli might account for the strong control over behaviour exerted by these stimuli in the addiction state. Withdrawal from drugs of abuse results in a reduction in basal dopamine transmission in vivo and in reduced responding for conventional reinforcers. Although these changes are likely to be the expression of a state of dependence of the dopamine system their contribution to the motivational state of drug addiction is unclear.

Glutamate decarboxylase messenger RNA in rat pallidum: comparison of the effects of haloperidol, clozapine and combined haloperidol-scopolamine treatments

We have investigated the effects of neuroleptic treatments which do, or do not, induce catalepsy on the level of expression of glutamate decarboxylase, the rate limiting enzyme in GABA synthesis, in efferent neurons of the pallidum in adult rats. Different regimens of haloperidol (1 mg/kg s.c., three, seven or 14 days; 2 mg/kg, s.c., 10 days) induced catalepsy in a majority of rats and increased glutamate decarboxylase messenger RNA levels in the globus pallidus (external pallidum) in those rats exhibiting catalepsy. Levels of glutamate decarboxylase messenger RNA were also increased in the entopeduncular nucleus (internal pallidum), but only after 14 days of treatment with haloperidol. The atypical antipsychotic clozapine (seven days, 20 mg/kg, s.c.), which did not induce catalepsy, slightly decreased glutamate decarboxylase messenger RNA levels in the globus pallidus. When co-administered with haloperidol (seven days, 1 mg/kg s.c.), the muscarinic antagonist scopolamine (1 mg/kg, s.c.) completely blocked both haloperidol-induced catalepsy and increases in glutamate decarboxylase messenger RNA levels in the globus pallidus. In contrast, scopolamine was not able to block increased glutamate decarboxylase and enkephalin messenger RNA expression induced by haloperidol in the striatum. These results reveal a good correlation between increases in glutamate decarboxylase messenger RNA levels in the globus pallidus and catalepsy after these drug treatments and suggest that anticholinergic blockade of the behavioral and molecular effects of neuroleptics may involve non-striatal mechanisms.

Pharmacology of memory: cholinergic-glutamatergic interactions

Both acetylcholine and glutamate are now thought to play important roles in memory. Recent evidence suggests that the interaction of these two neurotransmitters may be important for some forms of memory, and that acetylcholine, in particular, may function to facilitate glutamate activity by coordinating states of acquisition and recall in the cortex and hippocampus.

Role of vesicular dopamine in the in vivo stimulation of striatal dopamine transmission by amphetamine: evidence from microdialysis and Fos immunohistochemistry

The role of vesicular and newly synthesized dopamine in the action of amphetamine was investigated by studying the effect of reserpine and alpha-methyl-p-tyrosine pretreatment on amphetamine-induced changes in extracellular dopamine and acetylcholine, estimated by brain microdialysis, and on c-fos expression, estimated by quantitative immunohistochemistry of the Fos antigene, in the dorsal caudate-putamen of rats. Blockade of dopamine synthesis by alpha-methyl-p-tyrosine pretreatment (1 or 2 h) only partially prevented the increase in extracellular dopamine concentrations elicited by 0.5 and 2 mg/kg s.c. of amphetamine. Inactivation of vesicular amine uptake by reserpine pretreatment (3 h) reduced the increase in extracellular dopamine by 2 mg/kg but not by 0.5 mg/kg of amphetamine. Combined pretreatment with reserpine (3 h) and alpha-methyl-p-tyrosine (1 h) drastically reduced the increase in extracellular dopamine by both doses of amphetamine (0.5 and 2 mg/kg s.c.). alpha-Methyl-p-tyrosine pretreatment reduced c-fos expression stimulated by amphetamine (2 mg/kg) in the dorsomedial and dorsolateral caudate-putamen while reserpine pretreatment reduced it only in the dorsolateral caudate-putamen. Amphetamine (2 mg/kg s.c.) stimulated acetylcholine release but this effect was not modified by reserpine or alpha-methyl-p-tyrosine pretreatment. The results indicate that blockade of dopamine synthesis, by itself, is insufficient to prevent the stimulation of dopamine transmission by amphetamine and, conversely, that inactivation of vesicular dopamine significantly reduces amphetamine effects at pre- and postsynaptic levels. Therefore, vesicular dopamine appears to contribute to the stimulation of dopamine transmission elicited by amphetamine in the dorsal caudate-putamen.

Modulation of dopamine D1-mediated turning behavior and striatal c-fos expression by the substantia nigra

In order to study the possible contribution of the substantia nigra (SN) in the positive interaction between dopamine D1 receptor agonists and glutamate antagonists in unilaterally 6-hydroxydopamine (6-OHDA) lesioned rats, the effect of the D1 agonist, SKF 38393, was studied in combination with intranigral infusions of glutamate antagonists of the NMDA (MK 801, CPP) or AMPA (NBQX) type of receptor. Local infusion into the SN of the 6-OHDA lesioned side of MK 801, CPP or NBQX at doses inducing no or minimal behavioral effects significantly increased the turning behavior and the expression of c-fos induced, in the lesioned caudate-putamen (CPu), by a parenteral administration of SKF 38393. The same result was obtained after intra-SN infusion of the GABA agonist, muscimol. High doses of MK 801, CPP or muscimol infused into the SN produced intense contralateral turning per se and induced a sparse c-fos expression in the lesioned CPu which was antagonized by parenteral administration of MK 801. The results indicate that a depression of SN pars reticulata efferent neurons potentiates D1-mediated responses and suggest that this area may play a role in the positive interaction between glutamate antagonists and D1 receptor agonists.

Glutamate/dopamine D1/D2 balance in the basal ganglia and its relevance to Parkinson's disease

The recent availability of selective ligands for NMDA and AMPA receptors has enabled neuroscientists to test the hypothesis that Parkinson's disease is a glutamate hyperactivity disorder and hence treatable with glutamate antagonists. This review takes a critical look at the motor characteristics of this new class of drugs in rodent and primate models of parkinsonism and assesses the clinical potential and pitfalls of this radical new approach. Monotherapy of Parkinson's disease with glutamate antagonists appears impractical at the present time, due to their low efficacy and unacceptable side effects, but polypharmacy with L-DOPA and a glutamate antagonist as adjuvant is a more realistic prospect. This review will focus on the ways in which glutamate receptor blockade facilitates motor recovery with L-DOPA and will examine whether the basis for this beneficial effect can be traced to a specific interaction with dopamine at D1 or D2 receptors, and therefore to discrete motor pathways within the basal ganglia.

Neurochemical and some related psychopharmacological aspects of Tourette's syndrome: an update

Neurochemical investigations of Tourette's syndrome (TS) suggest that the symptoms of this disorder may be the result of an imbalance among several neurotransmitter and/or neuromodulator systems. Neurochemicals which have been studied included: catecholamines; acetylcholine; tryptophan and its metabolites; the amino acids γ-aminobutyric acid (GABA), glutamate, phenylalanine and p-tyrosine; trace amines; opioid peptides; cyclic AMP and androgenic hormones. A suitable animal model of TS would do much to advance our understanding of this disorder, and there are some interesting recent developments in this regard.

Differential effect of MK 801 and scopolamine on c-fos expression induced by L-dopa in the striatum of 6-hydroxydopamine lesioned rats

In rats with a unilateral 6-hydroxydopamine lesion of the dopaminergic nigro-striatal pathway, striatal D1-receptor-stimulated c-fos expression and turning behavior are positively modulated by D2 receptor stimulation and by blockade of N-methyl-D-aspartate (NMDA) or muscarinic receptors. Combined D1/D2 receptor stimulation by L-dopa activates c-fos in a manner not additive with muscarinic receptor blockade by scopolamine. On the other hand, blockade of NMDA receptors by MK 801 reduced c-fos expression induced by L-dopa while, depending on the dose of L-dopa, differentially affecting contralateral turning behavior. The results are interpreted to suggest that D2 receptor stimulation amplifies D1-receptor-mediated c-fos expression by two mechanisms differentially related to muscarinic and NMDA receptors.

Dopamine receptors: molecular biology, biochemistry and behavioural aspects

The description of new dopamine (DA) receptor subtypes, D1-(D1 and D5) and D2-like (D2A, D2B, D3, D4), has given an impetus to DA research. While selective agonists and antagonists are not generally available yet, the receptor distribution in the brain suggests that they could be new targets for drug development. Binding characteristics and second messenger coupling has been explored in cell lines expressing the new cloned receptors. The absence of selective ligands has meant that in vivo studies have lagged behind. However, progress has been made in understanding the function of DA-containing discrete brain nuclei and the functional consequence of the DA's interaction with other neurotransmitters. This review explores some of the latest advances in these various areas.