Regulation of the activity of striatal cholinergic neurons by dopamine

Effect of toluene inhalation on extracellular striatal acetylcholine release studied with microdialysis

Intracerebral microdialysis was performed on awake, freely moving rats in order to record effect of toluene exposure on acetylcholine release in striatum. Acetylcholine release decreased during (about 20%) and after (about 60%) toluene exposure (2 hr, 2000 p.p.m.) Striatal acetylcholine release is thought to be mediated by dopamine. In a previous work we found that extracellular dopamine levels increase during toluene exposure. A dopamine uptake inhibitor (LU 19-005, 2 mg/kg) was therefore injected subcutaneously and the effect of increased extracellular dopamine on acetylcholine release within the striatum was monitored in the absence of toluene exposure. LU 19-005 increased striatal dopamine levels six times and the acetylcholine levels increased to about 145% of basal value. The present study shows that toluene exposure decrease acetylcholine release while an injection of a dopamine uptake inhibitor fails to decrease acetylcholine release. Indicating that acute exposure of toluene decreases striatal acetylcholine release by a mechanism that is not mediated by increased extracellular dopamine levels. Our data suggest that toluene decrease acetylcholine release within the striatum and that this effect not is mediated by increased extracellular dopamine levels.

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

The striatum is viewed as a structure performing fast neurotransmitter-mediated operations through somatotopically organized projections to medium-size spiny neurons. This view is contrasted with another view that depicts the striatum as a site of diffuse modulatory influences mediated by cholinergic interneurons and by dopamine and N-methyl-D-aspartate receptors. These two operational and organizational modes both contribute, through their mutual interaction, to the function of basal ganglia. Detailed knowledge of the neural mechanisms by which such interactions take place and are expressed into behaviour, can provide new insight into the physiopathology and new clues for therapy of disorders of basal ganglia.

Supersensitization of intrastriatal dopamine receptors involved in opposite regulation of acetylcholine release in Parkinson's model rats

Using striatal microdialysis, we studied effects of SKF 38393, a D1 agonist, and quinpirole, a D2 agonist, on the acetylcholine (ACh) release from the striatum of 6-hydroxydopamine (6-OHDA)-lesioned rats by local infusion into the striatum. The present experiments clearly demonstrated evidence for the existence of intrastriatal D1 and D2 receptors regulating ACh release and for supersensitization of the striatal stimulatory D1 and inhibitory D2 receptor mechanisms in 6-OHDA lesioned rats.

Muscarinic modulation of synaptic transmission in slices of the rat ventral striatum is dependent on the frequency of afferent stimulation

Extracellular, intracellular and tight-seal patch-clamp recordings in ventral striatal slices were used to investigate whether the effectiveness of muscarinic neuromodulation of fast synaptic transmission may be dependent on the frequency of afferent stimulation. In all neurons tested, EPSPs were reversibly attenuated by muscarine or carbachol. This action was completely antagonized by atropine or pirenzepine. Several observations indicated a presynaptic site of action. In extracellular recordings, carbachol reduced the monosynaptic population spike but not the non-synaptic compound action potential. The acetylcholinesterase inhibitors eserine and pyridostigmine also induced an atropine-sensitive reduction of the EPSP. When the rate of afferent stimulation was increased, control EPSPs or EPSCs exhibited a decline in peak amplitude until reaching a steady-state value. Muscarinic modulation of steady-state EPSPs/EPSCs was significantly stronger in the range of lower frequencies (0.25-4 Hz) than at higher frequencies (8 and 12 Hz). The GABAA and GABAB-receptor/channel antagonists picrotoxin and 2-hydroxy-saclofen, the opiate receptor antagonist naloxone and atropine failed to alter the shape of the frequency-response curve. These results show that both exogenous and endogenous muscarinic receptor agonists are capable of activating a presynaptic mechanism by which fast excitatory inputs to the ventral striatum are depressed. The depressive effect is clearly stronger at lower rates of afferent stimulation than at high rates. This frequency-dependent attenuation of excitatory synaptic inputs exemplifies a new type of activity-dependent neuromodulation in central neural circuits.

Different effects of scopolamine on extracellular acetylcholine levels in neostriatum and nucleus accumbens measured in vivo: possible interaction with aversive stimulation

The in vivo microdialysis technique was used to measure extracellular concentrations of acetylcholine (ACh) in the neostriatum (NS) and nucleus accumbens (NAc) of freely moving rats after intraperitoneal administration of the muscarinic receptor antagonist scopolamine (0.5 mg/kg) or vehicle. Simultaneously, behavior was monitored. The administration of scopolamine induced an increase in extracellular ACh levels in the NS, which reached a maximum of about 185% within one hour after injection and returned to baseline values about three hours after injection. In the NAc, an increase of similar time-course was observed; however, this increase reached a maximum of 250%, which was significantly higher than the one observed in NS. These changes in ACh levels were accompanied by enhanced locomotion, rearing and grooming; however, the behavioral changes were of shorter time-course than those of extracellular ACh. The injection of vehicle did not affect ACh levels in NS, but induced a significant increase (60%) in the NAc. The levels of behavioral activity after vehicle injection did not differ from pre-injection levels. These results suggest, that the cholinergic systems in the NAc and NS are differently affected by peripheral administration of both scopolamine and vehicle. The differential effects of scopolamine in NS and NAc could reflect pharmacodynamic differences between these two striatal brain areas, perhaps due to a higher density of cholinergic interneurons or muscarinic autoreceptors in the NAc in comparison to the NS. However, the increase of extracellular ACh observed after vehicle injection suggests that factors such as aversive stimulation through the injection procedure can increase ACh release in the NAc and that such a mechanism can interact within the action of scopolamine. Thus, the stronger action of scopolamine on extracellular ACh in the NAc might be an additive effect of the drug with that of the injection procedure.

Effects of trihexyphenidyl and L-dopa on brain muscarinic cholinergic receptor binding measured by positron emission tomography

The effects of pharmacological intervention on brain muscarinic cholinergic receptor (mAChR) binding were assessed in seven patients with Parkinson's disease by positron emission tomography and carbon-11 labelled N-methyl-4-piperidyl benzilate ([11C]NMPB). [11C]NMPB was injected twice, approximately 2 hours apart, in each patient, to assess the effect of single doses of 4 mg of trihexyphenidyl (n = 5) or 400 mg of L-dopa with 57 mg of benserazide (n = 2) on the binding parameter of mAChRs (K3). There was a mean 28% inhibition of K3 values in the brain in the presence of trihexyphenidyl, which was assumed to reflect mAChR occupancy. No significant change in K3 was observed in the presence of L-dopa. This study demonstrates the feasibility of measuring mAChR occupancy by an anticholinergic medication with PET.

Tonic D2-mediated attenuation of cortical excitation in nucleus accumbens neurons recorded in vitro

The effects of dopamine D1 and D2 selective drugs on the responses evoked in accumbens neurons by stimulation of cortical afferents were studied in an in vitro brain slice preparation. The D2-specific antagonist sulpiride (1-10 microM) increased, whereas the D2 agonist quinpirole (1-20 microM) occasionally attenuated the amplitude of stimulation-evoked EPSPs recorded in accumbens neurons. Administration of the D1 agonist SKF 38393 (3-10 microM) or the D1 antagonist SCH 23390 (10 microM) did not alter the EPSP amplitude, although an apparent change in the time course of the EPSP was often observed. In slices obtained from dopamine (DA)-depleted animals, sulpiride failed to induce changes in the amplitude of the EPSPs, whereas quinpirole produced a highly significant suppression of EPSP amplitude that was only occasionally observed in control slices. These results indicate that DA modulates the response of accumbens neurons to cortico-accumbens fiber stimulation via D2 receptors. Furthermore, these D2 receptors appear to be located presynaptically on the cortical afferent terminals, since this action of DA was not accompanied by changes in membrane potential, input resistance, or time constant, and was not modified by changes in the membrane potential. These data provide evidence for a tonic basal level of D2 receptor stimulation in the accumbens slice preparation.

Does dopamine exert a tonic inhibitory control on the release of striatal acetylcholine in vivo?

The role of dopamine transmission on striatal acetylcholine release was investigated by using brain microdialysis. Blockade of dopamine D2 receptors with (-)-sulpiride or haloperidol increased acetylcholine release to a maximum of 80% (after 50 and 0.5 mg/kg, respectively). This effect was prevented by blockade of dopamine D1 receptors with 0.5 mg/kg SCH 39166 or 0.1 mg/kg SCH 23390, or by depletion of dopamine stores after 5 mg/kg reserpine + 150 mg/kg alpha-methyltyrosine. Treatment with SCH 39166, SCH 23390 or reserpine + alpha-methyltyrosine reduced acetylcholine release by about a maximum of 30%. Stimulation of dopamine D2 receptors with LY 171555 (quinpirole) at a low, sedative dose (0.05 mg/kg) reduced acetylcholine release by about 30% with no further reduction at higher doses up to 1 mg/kg. Moreover, LY 171555 (0.1 mg/kg) given to SCH 39166 (0.5 mg/kg)- or SKF 38393 (20 mg/kg)-pretreated rats did not decrease acetylcholine release, suggesting that its effect is through a dopamine D1 receptor-mediated mechanism. In contrast, in dopamine-depleted rats, LY 171555 0.1 mg/kg became more effective in decreasing acetylcholine release (about 70%) also after SCH 39166 (0.5 mg/kg) pretreatment (about 80%), thus acting independently of dopamine D1 receptor mechanisms. These results indicate that, in normal circumstances, endogenous dopamine facilitates striatal acetylcholine release through dopamine D1 receptors. The results argue against the commonly accepted view that dopamine D2 receptors exert a tonic inhibitory control on acetylcholine release. Moreover, they suggest that dopamine D2 receptors, in circumstances of dopamine depletion, may exert an inhibitory control on acetylcholine release independent of dopamine D1 receptor mechanisms.

GABAergic modulation of striatal cholinergic interneurons: an in vivo microdialysis study

Striatal cholinergic interneurons have been shown to receive input from striatal gamma-aminobutyric acid (GABA)-containing cell elements. GABA is known to act on two different types of receptors, the GABAA and the GABAB receptor. Using in vivo microdialysis, we have studied the effect of intrastriatal application of the GABAA-selective compounds muscimol and bicuculline and the GABAB-selective compounds baclofen and 2-hydroxysaclofen, agonists and antagonists, respectively, at GABA receptors, on the output of striatal acetylcholine (ACh). Intrastriatal infusion of 1 and 10 mumol/L concentrations of the GABAA antagonist bicuculline resulted in a significant increase in striatal ACh output, whereas infusion of 1 and 10 mumol/L concentrations of the GABAA agonist muscimol significantly decreased the output of striatal ACh. Both compounds were ineffective in changing the output of striatal ACh at lower concentrations. Infusion of concentrations up to 100 mumol/L of the GABAB-selective antagonist 2-hydroxy-saclofen failed to affect striatal ACh output, whereas infusion of 10 and 100 mumol/L baclofen, but not 0.1 and 1 mumol/L baclofen, significantly decreased the output of striatal ACh. Thus, agonist-stimulation of GABAA and GABAB receptors decreases the output of striatal ACh in a dose-dependent fashion, whereas the GABAAergic system appears to inhibit tonically the output of striatal ACh via GABAA receptors, but not via GABAB receptors. We hypothesize that although GABAA mediated regulation of striatal ACh occurs via GABA receptors on the cholinergic neuron, the GABAB mediated effects may be explained by presynaptic inhibition of the glutamatergic input of the striatal cholinergic neuron.

Effect of M1- and M2-muscarinic drugs on striatal dopamine release and metabolism: an in vivo microdialysis study comparing normal and 6-hydroxydopamine-lesioned rats

Microdialysis was used to study the effect of M1 and M2 selective agonists and antagonists on striatal dopamine release and metabolism. Microdialysis probes were implanted, under anesthesia, in the left and the right striatum of the normal rats and in the normal and denervated striatum of the nigral 6-hydroxydopamine-lesioned rats. Dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were determined by liquid chromatography and electrochemical detection. The different drugs were infused through the dialysis probe during 40 min. Pirenzepine (5 microM), a selective M1 antagonist, produced a significant decrease in DA release in the normal and the 6-hydroxydopamine-lesioned rats, with no significant difference between both groups. Methoctramine, a selective M2 antagonist, produced a dose-dependent increase in DA release between 20 and 200 microM in the normal rats, with no significant effect on DOPAC and HVA. Infusing 75 microM methoctramine produced a significant increase in DA release with a more pronounced effect in the intact animals compared to the 6-hydroxydopamine-lesioned animals. The non-selective agonist carbachol produced a decrease in dopamine release after infusion of 50 microM (M2 effect) and an increase in dopamine release after infusion of 50 mM (M1 effect) in the normal rats. Infusing 50 microM carbachol in the denervated striatum, produced a slight increase in DA release. Our data suggest that presynaptic M1-muscarinic receptors enhance and M2-muscarinic receptors inhibit DA release in the striatum of the rat; and that 3 weeks after 6-hydroxydopamine lesioning there may be a normalisation of the number of M1-receptors with a loss of M2-receptors at the denervated side.

Postnatal development of functional dopamine, opioid and tachykinin receptors that regulate acetylcholine release from rat neostriatal slices. Effect of 6-hydroxydopamine lesion

In the present work we have studied the postnatal development of functional dopamine, opioid and tachykinin receptors, which regulate cholinergic activity in the neostriatum. The release of endogenous acetylcholine from rat striatal slices was measured using a chemiluminescent method. We have observed that the inhibition mediated by dopamine through D2 receptors was not detectable until postnatal day 10, whereas the inhibition mediated by opioid receptors was detectable at postnatal day 15 for delta-receptors ([D-Pen2,D-Pen5]-enkephalin) and at postnatal day 21 for mu-receptors ([D-Ala2,Gly(ol)5]-enkephalin). Excitatory effect mediated by tachykinins through NK1 ([Sar9,Met(O2)11]- Substance P), NK2 ([Nle10]-Neurokinin A4-10), or NK3 (senktide) receptors was already detectable at postnatal day 5. In order to examine the influence of dopamine in the development of tachykinin and opioid systems in the neostriatum, we induced dopamine deficiency by intraventricular injection of 6-hydroxydopamine at postnatal day 3. We observed an increase in senktide-evoked acetylcholine release at postnatal day 30. The effect produced by [Sar9,Met(O2)11]-Substance P and [Nle10]-Neurokinin A4-10 was not modified. Furthermore, at postnatal day 35, we could observed that the two opioid receptor agonists have no effect. Our results show that dopamine, tachykinins and opioids are already able to mediate the modulation of acetylcholine release in early stages of development with a different pattern of postnatal development. Furthermore, the integrity of a dopaminergic system plays an important role in the functional development of the neostriatal cholinergic neurons which are differentially modulated by opioids or tachykinins.

Effect of substance P on acetylcholine and dopamine release in the rat striatum: a microdialysis study

In vivo microdialysis in urethane anaesthetised rats was used to investigate the effects of substance P (SP) on acetylcholine (ACh) and dopamine (DA) release in the rat striatum. Results showed that SP elicited a dose-dependent increase in ACh release between 1 and 50 pmol/l. The rise in ACh release occurred both during SP administration and for up to 60 min after it. Dose-response curves either based on the initial rise in ACh release, or the total duration of increased release, showed a bell shape with 100 fmol/l and 5 nmol/l doses failing to significantly alter release and a 500 pM dose being less effective than 50 pmol/l. In contrast to this, SP did not significantly alter DA release at doses ranging between 100 fmol/l and 5 nmol/l. There was evidence for a strong desensitisation effect of SP administration since after initial treatment with SP subsequent doses of the peptide, even at very high doses, failed to provoke further changes in ACh still showed the expected increase in release in response to a potassium challenge. Physalaemin and neurokinin A increased ACh release with a similar potency to SP at a 50 pmol/l dose whereas neurokinin B and neuropeptide gamma, while increasing ACh release at a 50 pmol/l dose, were less potent than SP. The effect of SP on ACh release is probably mediated via NK-1 receptors since ACh release in response to SP was reduced in a dose dependent manner by the NK-1 receptor antagonists CP-96,345 and RP-67580.

Fos immunoreactivity after stimulation or inhibition of muscarinic receptors indicates anatomical specificity for cholinergic control of striatal efferent neurons and cortical neurons in the rat

Cholinergic neurons play a major role in the control of striatal activity via muscarinic receptors. The action of acetylcholine also appears to be dependent on the striosome-matrix compartmentalization of the striatum. This study was designed to find out whether modification of acetylcholine tone activates neurons in the striatum and forebrain of the rat. We looked for the appearance of immunoreactivity to Fos, a regulatory protein that is thought to convert synaptic signals into changes in gene expression. Pharmacological manipulation of muscarinic receptors was found to induce specific patterns of Fos immunoreactivity in distinct neuronal populations of the forebrain, including the striatum. Oxotremorine, a non-selective muscarinic agonist, induced Fos immunoreactivity in the striatum with a large predominance in striosomes (mostly in enkephalinergic neurons), in layers 4 and 6 of the cortex, and also in the piriform cortex and septum. The muscarinic agonist pilocarpine had an identical effect in the cortex, but the striosomal prevalence was less clear-cut than that observed after oxotremorine. Treatment with dopamine-depleting agents (6-hydroxydopamine or reserpine) and inhibitors of glutamate and opiate receptor (MK-801 and naloxone respectively) had no effect on the action of oxotremorine. This suggests that the induction of Fos provoked by oxotremorine does not involve dopamine, glutamate or opiates. Atropine, a non-specific muscarinic antagonist, also induced Fos immunoreactivity in the striatum but with matrix predominance (mostly in substance P neurons), as well as in the cingulate cortex, and the olfactory tubercle. Scopolamine, a muscarinic antagonist, induced Fos in both striosomal and matrix compartments in the striatum.(ABSTRACT TRUNCATED AT 250 WORDS)

Regional differences in the dopaminergic regulation of cyclic AMP formation within the rat nucleus accumbens: comparison with the striatal complex of the lizard Gekko gecko

Effects of dopamine D2 receptor activation on the forskolin or D1 receptor stimulated formation of cyclic adenosine monophosphate (cyclic AMP) were investigated in tissue slices of two subregions of the rat nucleus accumbens and in the striatal complex of a lizard. Cyclic AMP production in the tissue was estimated by measuring the conversion of [3H]adenine in a superfusion system. Activation of a D2 receptor appeared to inhibit the D1 receptor agonists or forskolin stimulated formation of cAMP in the rostrolateral but not in the caudomedial part of the rat nucleus accumbens. In the striatal complex of the lizard the formation of cAMP was dramatically stimulated by forskolin, but only marginally by dopamine. Neither the forskolin, nor the dopamine stimulated cyclic AMP formation could be inhibited by activation of D2 receptors. These findings are compatible with previously obtained functional data indicating that especially the caudomedial part of the rat nucleus accumbens has much in common with the striatal complex of the lizard.

Spatial distributions of chemically identified intrinsic neurons in relation to patch and matrix compartments of rat neostriatum

The spatial distributions and dendritic branching patterns of chemically identified subpopulations of striatal intrinsic neurons, defined by immunoreactivity for choline acetyltransferase (ChAT), neuropeptide Y or parvalbumin, were studied in relation to patch and matrix compartments of rat neostriatum. ChAT-immunoreactive cells and fibers showed an uneven pattern of distribution in the striatum. ChAT immunoreactivity was higher in the dorsolateral part and lower in the ventromedial part of the striatum. This regional gradient pattern is the inverse of the overall pattern of calbindin D28k immunoreactivity. However, in small regions close to the lateral ventricle and globus pallidus, areas containing fewer ChAT-immunoreactive cells and fibers coincided with those containing low calbindin D28k immunoreactivity. Neuropeptide Y immunoreactivity was uniform in the neostriatum. Certain neuropeptide Y cells (about 20%) were also immunoreactive for calbindin D28k, indicating that at least a small population of calbindin D28k-immunoreactive cells are medium aspiny cells. Parvalbumin immunoreactivity was not uniform in the striatum. A higher density of parvalbumin immunoreactivity was found in the neuropil in lateral and caudal parts than in the medial part. Small regions with weaker parvalbumin-immunoreactive neuropil partially corresponded to calbindin D28k poor patches. Larger cells immunoreactive for parvalbumin were preferentially located in lateral and caudal parts of the striatum. Cells immunoreactive for ChAT, neuropeptide Y or parvalbumin showed basically similar distribution patterns in relation to the patch and matrix compartments. Most stained cells were located in the matrix, but some were located at the borders of patches and a few were inside patches. Most primary dendrites of stained cells in the matrix or patches remained confined to these compartments, but cells on the borders invariably extended dendrites into both compartments. The striatal intrinsic neurons form chemically differentiated neuronal circuits within the matrix, and the patches and those whose dendrites cross the borders may contribute to associational interconnections between the two compartments, unlike the spiny projection neurons whose dendrites are confined to one or the other compartment.

Dopaminergic regulation of cortical acetylcholine release: effects of dopamine receptor agonists

The regulation of the basal forebrain cholinergic system by D1 and D2 dopamine receptors was assessed in the rat using in vivo microdialysis of cortical acetylcholine. The D1 agonist CY 208-243 significantly increased cortical acetylcholine release; in contrast, the D2 agonists quinpirole and (+)-4-propyl-9-hydroxynaphthoxazine were without significant effects. Moreover, when administered in combination with CY 208-243, quinpirole failed to potentiate the D1 agonist-induced increases in cortical acetylcholine release. The non-selective dopamine receptor agonist apomorphine also increased cortical acetylcholine release, and this was completely blocked by the selective D1 receptor antagonist SCH 23390 and slightly, but not significantly attenuated by the D2 antagonist raclopride. The present results indicate that stimulation of D1 receptors activates cortically-projecting cholinergic neurons; however, a minor contribution of D2 receptors cannot be excluded.

Long-lasting reciprocal changes in striatal dopamine and acetylcholine release upon morphine withdrawal

In superfused rat striatal slices, the electrically evoked release of [3H]dopamine was reduced by about 30% and that of [14C]acetylcholine was enhanced by about 25% no less than 3 weeks after morphine withdrawal in rats chronically treated with the opiate for 6 days. The lasting nature of the reduced dopamine release at the level of dopaminergic nerve terminals and the concomittant enhanced excitability of neurons tonically inhibited by released dopamine, such as cholinergic interneurons, could play an important role in the maintenance of opiate dependence.

Differential effect of systemic administration of bromocriptine and L-dopa on the release of acetylcholine from striatum of intact and 6-OHDA-treated rats

A presumed balance between striatal dopaminergic and cholinergic systems forms a major theoretical framework for the development of new agents for the treatment of Parkinson's disease. We therefore studied the effect of two drugs currently used as anti-parkinsonian agents, bromocriptine (BROMO) and L-beta-3,4-dihydroxyphenylalanine (L-DOPA), on the release of striatal acetylcholine (ACh) in intact and 6-hydroxy-dopamine-treated rats using in vivo microdialysis. Lesioned rats with a > 90% tissue depletion of striatal dopamine (DA) had a significantly higher output of striatal ACh than unlesioned rats (88 fmol/min vs. 52 fmol/min; 0.3 mumol/l neostigmine in perfusate). BROMO (4 mg/kg) inhibited the output of striatal ACh in both groups. Whereas the lowest dose of L-DOPA (50 mg/kg) potently stimulated ACh output in lesioned rats, unlesioned rats were significantly less responsive. A higher dose of L-DOPA (100 mg/kg) stimulated ACh output to the same extent in both groups. At the highest dose tested, L-DOPA (200 mg/kg) given to intact rats did not further increase striatal ACh output. Thus, BROMO decreases whereas L-DOPA increases striatal ACh release after systemic application. Therapeutic as well as side effects of L-DOPA may therefore be mediated by neurochemical alterations that are more complex than previously thought.

Cocaine releases limbic acetylcholine through endogenous dopamine action on D1 receptors

Cocaine (10 and 20 mg/kg i.p.) enhanced the extracellular concentration of acetylcholine (ACh) in the ventral striatum of freely moving rats. The enhancement was prevented both by dopamine (DA) D1 receptor blockade with SCH 23390 (0.1 mg/kg s.c.) and by depletion of endogenous DA after coadministration of reserpine (5 mg/kg i.p.) and alpha-methyltyrosine (alpha-MT) (150 mg/kg i.p.). In contrast, blockade of DA D2 receptors with (-)-sulpiride (20 mg/kg i.p.) did not prevent the cocaine-induced increase in ACh release. These results indicate that the cocaine-induced stimulation of ACh release is mediated by an action of DA on D1 receptors, and suggest that the enhancement of ACh release might play a functional role in the central effects of cocaine. Moreover, DA depletion after reserpine + alpha-MT or D1 receptor blockade with SCH 23390 led to a comparable decrease of baseline ACh release, suggesting that striatal cholinergic interneurons are under D1 receptor-mediated facilitatory dopaminergic control.

Sequence-specific effects of neurokinin substance P on memory, reinforcement, and brain dopamine activity

There is ample evidence that the neurokinin substance P (SP) can have neurotrophic as well as memory-promoting effects. This paper outlines a recent series of experiments dealing with the effects of SP and its N- and C-terminal fragments on memory, reinforcement, and brain monoamine metabolism. It was shown that SP, when applied peripherally (IP), promotes memory (inhibitory avoidance learning) and is reinforcing (place preference task) at the same dose of 37 nmol/kg. Most important, however, is the finding that these effects seemed to be encoded by different SP sequences, since the N-terminal SP1-7 (185 nmol/kg) enhanced memory, whereas C-terminal hepta- and hexapeptide sequences of SP proved to be reinforcing in a dose equimolar to SP. These differential behavioral effects were paralleled by selective and site-specific changes in dopamine (DA) activity, as both SP and its C-, but not N-terminus, increased extracellular DA in the nucleus accumbens (NAc), but not in the neostriatum. The neurochemical changes lasted at least 2 h after injection. These results show that the reinforcing action of peripheral administered SP may be mediated by its C-terminal sequence, and that this effect could be related to DA activity in the NAc. Direct application of SP (0.74 pmol) into the region of the nucleus basalis magnocellularis (NBM) was also memory-promoting and reinforcing, and again, these effects were differentially produced by the N-terminus and C-terminus, supporting the proposed structure-activity relationship for SP's effects on memory and reinfrocement. These results may provide a hypothetical link between the memory-modulating and reinforcing effects of SP and the impairment in associative functioning accompanying certain neurodegenerative processes.

The anti-parkinsonian drug amantadine inhibits the N-methyl-D-aspartic acid-evoked release of acetylcholine from rat neostriatum in a non-competitive way

The anti-viral drug amantadine is used in the treatment of Parkinson's disease without the drug having a well established mechanism of action. Amantadine is reported to displace the non-competitive NMDA receptor antagonist MK-801 from its binding site in the central nervous system. We show that amantadine inhibits, in a non-competitive way, the NMDA receptor-mediated stimulation of acetylcholine release from rat neostriatum in vitro in 'therapeutic' (i.e. low micromolar range) concentrations. Moreover, contrary to previous reports, amantadine, in this concentration range, did not affect the in vitro release of dopamine from neostriatal tissue.