Non-competitive N-methyl-D-aspartate antagonists are potent activators of ventral tegmental A10 dopamine neurons

Further characterization of the effects of BMY 14802 on dopamine neuronal activity

Further evaluation of the effects of BMY 14802 on dopamine (DA) neuronal activity in the rat substantia nigra pars compacta (A9) was conducted with single-unit recording and microiontophoresis in anesthetized rats. Microiontophoretic administration of BMY 14802 (sigma, serotonin (5-HT)-1A and alpha-1 adrenoceptor ligand) had no effect on DA neurons. Microiontophoretic administration of (+)-3-PPP (weak D2 agonist with high affinity for sigma receptors) and quinpirole (D2/D3 agonist) inhibited A9 DA neuronal activity. Co-iontophoresis or i.v. pretreatment with BMY 14802 had no effect on the current-response curves for the effects of microiontophoretic (+)-3-PPP or quinpirole on A9 DA neurons. Co-iontophoretic administration of (-)-sulpiride, a selective D2 antagonist, blocked the inhibitory effects of microiontophoretic (+)-3-PPP. The effects of BMY 14802 (0.25-8 mg/kg, i.v.) on DA neurons (increased firing rate, increased burst-firing, reduced regularity of firing pattern) were not altered by acute brain hemitransection, but were blocked by pretreatment with NAN-190, an antagonist of 5-HT-1A and alpha-1 receptors. The alpha-1 receptor antagonist, prazosin, did not block these effects of BMY 14802. In conclusion, the effects of BMY 14802 on DA neuronal firing rate and firing pattern are indirect, perhaps due in part to the occupation of 5-HT-1A receptors.

MK-801 elevates the extracellular concentration of dopamine in the rat prefrontal cortex and increases the density of striatal dopamine D1 receptors

In the present study we found that MK-801 (dizocilpine), given peripherally in doses (0.2 and 0.4 mg/kg) which evoked locomotor activation in rats, enhanced in a dose-dependent manner the extracellular concentration of dopamine (DA) in the rat prefrontal cortex (PFC). MK-801 used in similar range of doses (0.2, 0.4 mg/kg) failed to alter the DA content in superfusates of the rat striatum (STR). It was also found that single doses of MK-801 enhanced the density of D1 receptors, assessed by the [3H]SCH 23390 binding in the rat STR, but not in the limbic forebrain. An increase in the density of D1 receptors was observed at 24, but not 2, h after MK-801 administration. MK-801 failed to alter the density of D2 receptors in the STR and limbic forebrain. The available data indicate that MK-801 may enhance the dopaminergic neurotransmission by at least two separate mechanisms: a fast one, associated with the release of DA in PFC, and a slow one, resulting from the increase in the D1 receptor density.

Morphine-induced potentiation of brain stimulation reward is enhanced by MK-801

Morphine (2.5 mg/kg, i.p.) and MK-801 (0.05 mg/kg, i.p.) each enhanced the rewarding impact of electrical stimulation of the medial forebrain bundle, causing small but reliable parallel leftward shifts of the functions relating response rate to stimulation frequency. Administration of MK-801 and morphine together caused a profound leftward shift in the functions. This effect was not due to sensitization to either drug, and suggests that disruption of glutamatergic function can potentiate the rewarding impact of opiates.

Selective reduction of N-methyl-D-aspartate-evoked responses by 1,3-di(2-tolyl)guanidine in mouse and rat cultured hippocampal pyramidal neurones

1. The effects of 1,3-di(2-tolyl)guanidine (DTG) were examined on the responses of cultured hippocampal neurones to the excitatory amino acid analogues N-methyl-D-aspartate (NMDA), kainate, quisqualate and (RS)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA). 2. In rat hippocampal neurones loaded with the Ca(2+)-sensitive dye Fura-2, DTG (10-100 microM) produced a concentration-dependent depression of the NMDA-evoked rises in intracellular free calcium ([Ca2+]i), an effect that was not modified by changes in the extracellular glycine concentration. DTG (at 50 and 100 microM) also attenuated, although to a lesser extent, the rises in [Ca2+]i evoked by naturally-derived quisqualate. In contrast, 50 and 100 microM DTG did not depress responses evoked by kainate, AMPA and synthetic, glutamate-free (+)-quisqualate although on occasions DTG enhanced kainate- and AMPA-evoked rises in [Ca2+]i. 3. DTG attenuated NMDA-evoked currents recorded from mouse hippocampal neurones under whole-cell voltage-clamp with an IC50 (mean +/- s.e. mean) of 37 +/- 5 microM at a holding potential of -60 mV. The DTG block of NMDA-evoked responses was not competitive in nature and was not dependent on the extracellular glycine or spermine concentration. The block did, however, exhibit both voltage-, and use-, dependency. The steady-state current evoked by naturally-derived quisqualate was also attenuated by DTG whereas those evoked by kainate and AMPA were not. 4. We conclude that DTG, applied at micromolar concentrations, is a selective NMDA antagonist in cultured hippocampal neurones, the block exhibiting both Mg(2+)- and phencyclidine-like characteristics. Given the nanomolar affinity of DTG for sigma binding sites it is unlikely that the antagonism observed here is mediated by sigma-receptors, but the data emphasize the potential danger of ascribing the functional consequences of DTG administration solely to sigma receptor-mediated events.

The role of nucleus accumbens dopamine in the neurochemical and behavioral effects of phencyclidine: a microdialysis and behavioral study

A series of experiments were conducted in order to characterize the role of nucleus accumbens dopamine (DA) in the neurochemical and behavioral effects of phencyclidine (PCP). In the first study, microdialysis probes were implanted in nucleus accumbens to determine the effects of 4.0 and 8.0 mg/kg PCP on extracellular levels of DA and its metabolites, dihydroxyphenyl-acetic acid (DOPAC) and homovanillic acid (HVA) in behaving rats. PCP increased extracellular DA, DOPAC and HVA in the same dose range that produced increases in locomotor activity, stereotypy and ataxia. The increases in extracellular DA that were induced by 4.0 mg/kg PCP were significantly correlated with the increases in locomotor activity. In the second study, rats received bilateral injections of 6-hydroxydopamine in order to deplete DA in nucleus accumbens. DA-depleted and control rats received injections of saline and 4.0 mg/kg PCP and were tested in an 'intruder' paradigm. In this procedure, saline-and PCP-treated rats were placed in a stable colony of three other rats and social behavior was observed for 30 min. PCP reduced the frequencies of various social behaviors, but accumbens DA depletion did not reverse the effects of PCP on social behavior. Subsequently, all rats received 8.0 mg/kg PCP and were assessed for locomotor activity, stereotypy and ataxia. Depletion of DA in nucleus accumbens attenuated PCP-induced locomotion, but did not alter the effects of the drug on stereotypy or ataxia.(ABSTRACT TRUNCATED AT 250 WORDS)

The sigma receptor ligand 1,3-di(2-tolyl)guanidine is anticonvulsant in the rat prepiriform cortex

Unilateral focal injection of 1,3-di(2-tolyl)guanidine (DTG) caused a dose-dependent and potent (ED50 = 5.25 nmol, 95% confidence limits 1.1 to 25.0 nmol) suppression of generalized motor seizures induced by (-)-bicuculline methiodide in the rat prepiriform cortex. These findings indicate that DTG is equipotent to the noncompetitive NMDA receptor antagonist MK-801 ((+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate) as an anticonvulsant. This potent pharmacological effect of DTG distinguishes it from two other prototypic sigma ligands, haloperidol and (+)-pentazocine, which are ineffective as anticonvulsants. Pretreatment of animals with haloperidol failed to block the anticonvulsant effects of DTG. These data therefore document a novel anticonvulsant action of DTG in vivo by a mechanism that does not involve sigma receptors.

The sigma receptor ligand 1,3-di-(2-tolyl)guanidine in animal models of schizophrenia

The behavioral effects of the selective sigma ligand 1,3-di(2-tolyl)guanidine (DTG) were studied in rats. In the radial 8-arm maze, DTG (2, 4 and 8 mg/kg i.p.) reduced the number of arm entries in the spontaneous alternation task. In animals receiving 4 mg/kg DTG, the percentage of 135 degrees angles between consecutive arm entries decreased. In the open field, equipped with a holeboard, DTG (8 mg/kg) reduced the number of line crossings, rearings and head dips. Sniffing, measured in an experimental chamber, was also reduced. DTG prolonged the time that the animals were inactive. In combination with DL-amphetamine (4 mg/kg) or dizocilpine (0.16 mg/kg), DTG (8 mg/kg) decreased--but did not antagonize--the induced enhancement of locomotion and sniffing. These results demonstrate motor depressant effects of DTG on locomotion, rearing and sniffing. Since antagonists of sigma binding sites are known to produce opposite effects, we conclude that DTG--in behavioral terms--acts like an antagonist at sigma binding sites.

Repeated administration of Sigma ligands alters the population activity of rat midbrain dopaminergic neurons

Acute and repeated administration of antipsychotic drugs produce distinctive profiles of electrophysiological effects on the population activity of midbrain dopaminergic (DA) neurons which correlate with their clinical effects. Sigma receptors have been hypothesized to be involved in psychosis and in the efficacy of antipsychotic drugs, but little is known about the effects of repeated treatment with sigma ligands on the activity of midbrain DA neuronal populations. In the present study, the cells-per-track cell-sampling method was used to evaluate the effects of 3 sigma ligands on the numbers of spontaneously active A9 and A10 DA neurons in chloral hydrate-anesthetized rats. One-hour pretreatment with either (+)-pentazocine (10 mg/kg, i.p.), DTG (2 mg/kg, i.p.), or JO 1784 (1 or 10 mg/kg, s.c.) did not alter the number of spontaneously active DA neurons encountered per electrode track. Repeated treatment (21 daily injections) with (+)-pentazocine (1 or 10 mg/kg) or DTG (0.2 or 2 mg/kg) increased the number of A10 DA cells per track; JO 1784 (10 mg/kg but not 1 mg/kg) moderately decreased the number of active A9 DA cells and increased the firing rate of A10 DA neurons. The effect of JO 1784 on A9 DA neurons was not due to depolarization inactivation. The effects of all 3 sigma ligands differ from those of antipsychotic drugs, all of which inactivate A10 DA neurons after repeated treatment. Clinical studies are necessary to determine if selective sigma ligands will provide a novel alternative to DA antagonists in the treatment of psychosis.

Effects of dizocilpine (MK-801) on rat midbrain dopamine cell activity: differential actions on firing pattern related to anatomical localization

The effects of the non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist dizocilpine ((+)-MK-801) on the firing pattern of midbrain dopamine neurons were studied with single cell recording techniques in male albino rats anaesthetized with chloral hydrate. The extracellularly recorded electrical activity of single, identified dopamine neurons was studied with respect to firing rate, burst firing and regularity of firing. MK-801 (0.01-1.0 mg/kg IV) induced different effects in different subgroups of midbrain dopamine neurons. In the substantia nigra, firing rate was increased while the pattern was regularized and burst firing slightly increased. In the ventral tegmental area, firing rate and regularity of firing was also increased while effects on burst firing were bidirectional. Histological inspections revealed that neurons which responded with an increase in burst firing were mainly located in the nucleus paranigralis subdivision of the ventral tegmental area, while cells responding with a decrease were predominantly found in the nucleus parabrachialis pigmentosus subdivision. The effects of MK-801 were similar to previously described effects of phencyclidine, another non-competitive NMDA antagonist. The present effects of MK-801 might shed some light on the mechanisms involved in psychotic symptoms induced by phencyclidine and other non-competitive NMDA antagonists.

Heterogeneous responses of substantia nigra pars reticulata neurons to gamma-hydroxybutyric acid administration

The effect of intravenous administration of gamma-hydroxybutyric acid (GHB) (50-400 mg/kg) on the firing rate of substantia nigra pars reticulata (SN-PR) neurons was studied by making single cell extracellular recordings in unanesthetized rats. For comparison, the effect of intravenous muscimol (0.5-2 mg/kg) and ethanol (0.5-2 g/kg) was also studied. GHB produced variable effects: dose-related inhibition in 7 out of 18 (38.8%) neurons and no significant change in 11 out of 18 (61.2%) neurons tested. In contrast, muscimol and ethanol produced a dose-related inhibition of the SN-PR firing rate. The results indicate that GHB, unlike muscimol and ethanol, has no profound effect on the activity of SN-PR neurons, and thus disinhibition of dopaminergic units, through inhibition of SN-PR neurons, is probably not the mechanism by which GHB stimulates the firing rate of dopaminergic neurons.

Effects of phencyclidine, MK-801 and 1,3-di(2-tolyl)guanidine on non-dopaminergic midbrain neurons

The effects of i.v. administration of the noncompetitive NMDA receptor antagonists, phencyclidine and MK-801, and the sigma receptor ligand, 1,3-di(2-tolyl)guanidine (DTG), on the firing rates of non-dopaminergic mid brain neurons were evaluated in chloral hydrate-anesthetized rats. Phencyclidine and MK-801 inhibited the activity of putative gamma-aminobutyric acid (GABA)-containing interneurons identified by their response to foot-pinch. DTG did not significantly alter neuronal activity. These results suggest that the reported excitatory effects of non-competitive NMDA receptor antagonists on dopamine neuronal activity are due, in part, to disinhibition secondary to the inhibition of interneuron activity.

Development of both conditioning and sensitization of the behavioral activating effects of amphetamine is blocked by the non-competitive NMDA receptor antagonist, MK-801

The non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist, MK-801, has been shown to block the development of sensitization of the behavioral activating effects of amphetamine. Three experiments were designed to determine in rats whether MK-801 had its effects through interference with long-term changes underlying sensitization, per se, or through interference with the development of conditioning of the drug effect to the environment where the drug was given. In experiment 1, conditioning was promoted by explicitly pairing amphetamine (1.5 mg/kg, IP) with the testing environment. In experiment 2, a random-pairing procedure was used to eliminate the possibility of association between the drug and a specific context. Experiment 3 was carried out to assess the duration of the blockade of sensitization by MK-801. The effect of MK-801 (0.25 mg/kg, IP) during amphetamine pre-exposure was studied in tests for conditioning (following saline injections, experiment 1) and in tests for sensitization (following 0.75 mg/kg amphetamine, experiments 1, 2 and 3). It was found in experiment 1 that MK-801 given with amphetamine during the amphetamine pre-exposure phase blocked the development of both conditioning activity and environment-specific sensitization to amphetamine. The results of experiment 2, showing that sensitization to amphetamine was blocked by MK-801 even when conditioning was prevented, suggest that the two effects of MK-801 are independent, and may implicate different sites of action.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of haloperidol and clozapine on the disruption of sensorimotor gating induced by the noncompetitive glutamate antagonist MK-801

The amplitude of the acoustic startle response in rats is decreased if the startle stimulus is preceded by a nonstartle-eliciting auditory stimulus. This sensory gating phenomenon, known as prepulse inhibition, is diminished in schizophrenic individuals. In rats, the noncompetitive glutamate antagonist MK-801 disrupts prepulse inhibition. The present study examined whether the disruption by MK-801 is reversible in rats pretreated with the classical antipsychotic haloperiodol or the atypical antipsychotic clozapine. Male Sprague-Dawley rats were placed into a startle chamber and presented with auditory stimuli consisting of either 95 or 105 dB tones presented alone or preceded by a 70 dB tone. Rats treated with 0.1 mg/kg MK-801 demonstrated a significant disruption of prepulse inhibition. Haloperidol (0.1 and 0.5 mg/kg) and clozapine (1.0 and 5.0 mg/kg) each consistently failed to antagonize the MK-801-induced blockade of prepulse inhibition. The effects of haloperidol and clozapine on prepulse inhibition were also examined in saline-treated rats. Clozapine and, to some extent, haloperidol produced a dose-related facilitation of prepulse inhibition. Although preliminary, this finding raises the possibility that the enhancement of prepulse inhibition by antipsychotics might provide a useful rodent model for screening potential antipsychotic drugs.

Similar ameliorating effects of benzomorphans and 5-HT2 antagonists on drug-induced impairment of passive avoidance response in mice: comparison with acetylcholinesterase inhibitors

Mice were trained to avoid electric shocks by means of step-down type passive avoidance learning tasks, and memory retention was measured 24 h after the training session. Memory impairment (amnesia) was produced by administering either p-chloroamphetamine (PCA), a serotonin (5-HT) releaser or scopolamine (SCOP), a muscarinic cholinoceptor antagonist, 30 min prior to the training session. Benzomorphans, 5-HT2 antagonists and acetylcholinesterase (AChE) inhibitors were administered immediately after the training session. PCA- but not SCOP-induced amnesia was attenuated by the post-training administration of two benzomorphans, (+)N-allylnormetazocine ((+)SKF-10,047) and (+/- )pentazocine ((+/- )PTZ). Similarly, PCA-induced amnesia was reversed by the post-training administration of 5-HT2 antagonists, ritanserin (RIT) and mianserin (MIA), but SCOP-induced amnesia was not. However, the AChE inhibitors, tetrahydroaminoacridine (THA) and physostigmine (PHY) attenuated both PCA- and SCOP-induced amnesia when administered immediately after the training session. These results indicated that benzomorphans and 5-HT2 antagonists have antiamnestic effects in mice, as do AChE inhibitors. In addition, it is interesting that the patterns of ameliorating effect of benzomorphans were similar to those of 5-HT2 antagonists, which differ from those of AChE inhibitors.

The NMDA receptor antagonist MK-801 elicits conditioned place preference in rats

(+)-5-Methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate, (MK-801) a potent noncompetitive antagonist of central NMDA receptors, has been hypothesized to have rewarding properties indicative of abuse potential. To test this hypothesis, the effects of MK-801 on the acquisition of a conditioned place preference and on locomotor activity were assessed and compared with d-amphetamine. Both MK-801 (0.03 and 0.1 mg/kg, SC) and d-amphetamine (1.0 mg/kg, SC) administration resulted in the acquisition of a conditioned place preference. However, while both amphetamine and the higher dose of MK-801 produced a behavioral activation during the training period the lower dose of MK-801 did not. These results suggest that MK-801, at doses that produce behavioral activation and below, is rewarding and therefore may have abuse potential.

Effects of phencyclidine on spontaneous and excitatory amino acid-induced activity of ventral tegmental dopamine neurons: an extracellular in vitro study

Extracellular single-unit recordings in rat midbrain slices were used to assess the effects of phencyclidine (PCP) on the spontaneous and excitatory amino acid-induced firing of ventral tegmental (VTA) dopamine neurons. Perfusion with PCP concentrations from 10 nM to 100 microM resulted in only minimal decreases in firing rate. Neither excitatory effects nor changes in firing pattern were ever observed upon exposure to PCP. In contrast, PCP selectively antagonized the stimulatory effects of N-methyl-D-aspartate (NMDA), but did not alter alpha-amino-3-hydroxy-5-methyl-4-isoxazole proprionic acid (AMPA)- or kainate (KA)-induced excitations. These results are discussed in relation to the low-dose stimulatory effects of PCP on ventral tegmental A10 dopamine neurons in the whole animal preparation.

Differential behavioural and neurochemical effects of competitive and non-competitive NMDA receptor antagonists in rats

The behavioural and biochemical effects of the non-competitive N-methyl-D-aspartate (NMDA) receptor antagonists, dizocilpine and memantine, and the competitive NMDA receptor antagonist, CGP 39551, were investigated in rats. Systemic injections of dizocilpine (0.33 mg/kg) increased locomotion and rearing in an open field, whereas memantine (20 mg/kg) increased only locomotor activity. CGP 39551 (10 and 20 mg/kg) did not change open field activity. Dopamine (DA) metabolism--as measured by the ratio of dihydroxyphenylacetic acid/dopamine (DOPAC/DA)--increased in response to dizocilpine in the prefrontal cortex and the nucleus accumbens. Memantine enhanced DOPAC/DA in the prefrontal cortex, the nucleus accumbens and to a lesser degree in the posterior striatum. In contrast to non-competitive NMDA receptor antagonists, CGP 39551 did not increase DA metabolism of subcortical structures and even decreased DOPAC/DA in the prefrontal cortex. These results indicate that competitive and non-competitive NMDA receptor antagonists affect spontaneous locomotion differentially in rats. The biochemical data imply that the stimulant actions non-competitive NMDA receptor antagonists are at least partially due to activation of ascending dopaminergic systems. Potential mechanisms involved in the differential effects of both types of NMDA receptor antagonists are discussed.

Iontophoretic effects of sigma ligands on rubral neurons in the rat

Iontophoretic application of the sigma ligands, 1,3-di-o-tolylguanidine (DTG), dextrallorphan, and (+)-pentazocine reliably inhibited the firing rate of rubral neurons. Dextrallorphan inhibited 87% of the neurons tested, DTG inhibited 76%, and (+)-pentazocine inhibited 50%. These inhibitions were current dependent and occurred without significant changes in spike amplitude or duration, suggesting that local anesthetic effects were not involved. In contrast to the other sigma ligands, iontophoretic application of (+)-3-PPP in the rat red nucleus resulted in very few inhibitions and tended to elicit weak excitations instead. Only 14% of rubral neurons were inhibited by (+)-3PPP, while 36% were excited. Although unusual, (+)-3-PPP has atypical effects when compared to other sigma ligands in numerous functional assays for sigma receptor activity. (+)-3-PPP, therefore, appears to have complex effects and may act through nonsigma mechanisms or through a different type of sigma binding site than the other compounds. The inhibition of firing rate produced by the more typical sigma ligands may contribute to the postural changes produced by microinjection of sigma ligands into the rat red nucleus.

Marked inhibition of mesolimbic dopamine release: a common feature of ethanol, morphine, cocaine and amphetamine abstinence in rats

Withdrawal of rats from chronic ethanol, morphine, cocaine and amphetamine resulted in a marked reduction in extracellular dopamine (DA) concentration in the ventral striatum as measured by microdialysis. Following ethanol and naloxone-precipitated morphine withdrawal, the time course of DA reduction paralleled that of the withdrawal symptomatology. On the other hand, following discontinuation of chronic cocaine, DA reduction was delayed by over 24 h but persisted for several days. After amphetamine withdrawal the fall in DA occurred more rapidly but the reduction also persisted for several days. The administration of the NMDA receptor antagonist, MK-801, to rats withdrawn from chronic ethanol, morphine or amphetamine, but not from chronic cocaine, readily reversed the fall in DA output. The reduction in extracellular DA during ethanol withdrawal was also reversed by SL 82.0715, another NMDA receptor antagonist.

Electrophysiological effects of MK-801 on rat nigrostriatal and mesoaccumbal dopaminergic neurons

The electrophysiological effects of the non-competitive N-methyl-D-aspartate (NMDA) antagonist (+)-MK-801 (MK-801) on nigrostriatal and mesoaccumbal dopaminergic (DA) neurons were evaluated in chloral hydrate-anesthetized rats. MK-801 (0.05-3.2 mg/kg, i.v.) stimulated the firing rates of 14 (74%) of 19 nigrostriatal DA (NSDA) neurons and all 16 mesoaccumbal DA (MADA) neurons tested. Stimulatory effects of the drug were more prominent on MADA neurons. Interspike interval analysis revealed that MK-801 also regularized DA neuronal firing pattern. Acute brain hemitransection between the midbrain and forebrain attenuated the stimulatory effects of MK-801 on firing rate and blocked the effects on firing pattern. Similar to MK-801, hemitransection itself increased NSDA and MADA cell firing rates and regularized firing pattern. Both i.v. and iontophoretic MK-801 blocked the excitatory effects of iontophoretic NMDA but did not affect excitations caused by the non-NMDA glutamatergic receptor agonists quisqualate and kainate. Iontophoretic MK-801 had no effect alone. These results suggest that the excitatory effects of i.v. MK-801 on DA neuronal activity are not due to direct actions on DA neurons. Glutamatergic projections originating anterior to the hemitransection appear to play a role in the effects of MK-801 on DA neuronal activity.

Lack of involvement of haloperidol-sensitive sigma binding sites in modulation of dopamine neuronal activity and induction of dystonias by antipsychotic drugs

The present studies evaluated previous suggestions that haloperidol-sensitive sigma binding sites are involved in the modulation of dopamine (DA) neuronal activity and in the induction of the dystonic effects of antipsychotic drugs. These issues were addressed by evaluating the effects of compounds that have differing affinities for sigma binding sites, on the firing activity of DA neurons in the substantia nigra in chloral hydrate-anesthetized rats and on the ability to induce extrapyramidal motor dysfunction in squirrel monkeys sensitized to the dystonic effects of haloperidol. The agents studied included haloperidol, DTG (1,3-di-o-tolylguanidine), (+)-pentazocine, (+)-SKF 10,047, BMY 14802, 8-OH-DPAT and sulpiride. There was no relationship between affinity for sigma binding sites and the ability to either alter DA neuronal activity or to induce extrapyramidal motor dysfunction.

Acute effects of sigma ligands on the electrophysiological activity of rat nigrostriatal and mesoaccumbal dopaminergic neurons

The effects of acute i.v. administration of several sigma ligands on the single-unit activity of nigrostriatal and mesoaccumbal dopaminergic (DA) neurons were evaluated in chloral hydrate-anesthetized rats. DTG (1,3-di(o-tolyl)guanidine) did not alter DA neuronal activity at nontoxic doses and JO 1784 [(+)-N-cyclopropylmethyl-N-methyl-1,4-diphenyl-1-ethylbut-3-en-1-+ ++ylamine] was inactive. (+)-Pentazocine was more effective in increasing mesoaccumbal vs. nigrostriatal DA cell firing rates. BMY 14802(alpha-(4-fluorophenyl)-4-(5-fluoro-2-pyrimidinyl)-1-piperazine-but anol) dose-dependently increased DA cell firing rate in both populations. The inhibition of nigrostriatal DA cell firing rate by (+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine [(+)-3-PPP] was reversed by (-)-eticlopride and (+)-but not (-)-butaclamol, which supports previous evidence that (+)-3-PPP-induced inhibition is due to the DA agonist properties of the drug. From what is known of the pharmacological properties of these compounds, it is concluded that acute sigma receptor occupation does not markedly alter the firing rate of DA neurons. The dose-response curve for inhibition of nigrostriatal DA neuronal activity by the D2 DA agonist, quinpirole, was shifted to the right tenfold by BMY 14802 pretreatment (8 mg/kg, i.v.) and twofold by (+)-pentazocine (8 mg/kg, i.v.), but was not changed by DTG (2 mg/kg, i.v.). It is concluded that the marked effects of certain sigma ligands on DA cell electrophysiology are likely due to their non-sigma properties.

Competitive NMDA receptor antagonists attenuate phencyclidine-induced excitations of A10 dopamine neurons

The binding of phencyclidine (PCP) within the channel gated by the NMDA-receptor complex can be positively or negatively modulated by compounds which facilitate or prevent the interaction of glutamate to the NMDA recognition site. In the present study extracellular recordings were used to evaluate the possibility that the negative modulation of NMDA channel function by the competitive NMDA antagonists (+/-)-CPP (3-((+/-)2-carboxypiperazin-4- yl)propyl-1-phosphonate) and CGS 19755 (cis-4-phosphonomethyl-2-piperidine carboxylate) could affect the response of A10 dopamine neurons to PCP. Pretreatment with 40 mg/kg of (+/-)-CPP or CGS 19755 completely blocked the low-dose excitatory effects of PCP, whereas 10 mg/kg of CGS 19755 produced only a partial blockade. However, neither CGS 19755 or (+/-)-CPP affected the amount of attenuation of A10 firing occurring with large doses of PCP. (+/-)-CPP and CGS 19755 pretreatment also failed to alter the morphine-induced stimulation of dopamine activity. These findings not only provide further evidence that the low-dose PCP-induced activation of A10 neurons is mediated through the NMDA-ion channel complex, but suggest that some physiological or behavioral effects evoked by PCP might be prevented by treatment with competitive NMDA receptor blockers.

Opposing tonically active endogenous opioid systems modulate the mesolimbic dopaminergic pathway

The mesolimbic dopaminergic system has been implicated in mediating the motivational effects of opioids and other drugs of abuse. The site of action of opioids within this system and the role of endogenous opioid peptides in modulating dopamine activity therein remain unknown. Employing the technique of in vivo microdialysis and the administration of highly selective opioid ligands, the present study demonstrates the existence of tonically active and functionally opposing mu and kappa opioid systems that regulate dopamine release in the nucleus accumbens, the major terminal area of A10 dopaminergic neurons. Thus, stimulation of mu-type receptors in the ventral tegmental area, the site of origin of A10 dopaminergic neurons, increases dopamine release whereas the selective blockade of this opioid receptor type results in a significant decrease in basal dopamine release. In contrast, stimulation of kappa-type receptors within the nucleus accumbens decreases dopamine release whereas their selective blockade markedly increases basal dopamine release. These data show that tonic activation of mu and kappa receptors is required for the maintenance of basal dopamine release in the nucleus accumbens. In view of the postulated role of the mesolimbic system in the mediation of drug-induced alterations in mood and affect, such findings may have implications for the treatment of opiate dependence and affective disorders.

Increases in extracellular dopamine levels and locomotor activity after direct infusion of phencyclidine into the nucleus accumbens

Three studies were conducted to provide a further characterization of the neurochemical and behavioral effects of phencyclidine (PCP). The first experiment utilized in vivo microdialysis to measure extracellular levels of dopamine (DA), dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the nucleus accumbens. Administration of PCP (4.0 x 10(-4) M) in the dialysis perfusion medium produced large increases in extracellular DA, and biphasic changes in DOPAC. In the second experiment, chronic indwelling cannulae for drug infusion were implanted bilaterally into the nucleus accumbens. Local infusions of PCP (15.0, 30.0 or 60.0 micrograms per side) produced substantial increases in locomotor activity. Little or no headweaving, stereotypy or ataxia was observed after intra-accumbens injections of PCP. In the third experiment, systemic administration of 0.4 mg/kg haloperidol significantly reduced the locomotor activity induced by intra-accumbens injection of 15.0 micrograms PCP. These results indicate that PCP can increase extracellular levels of DA by direct actions in the DA terminal region, and that these effects are related to some of the behavioral stimulant properties of PCP.

Increase of extracellular acetylcholine level in rat frontal cortex induced by (+)N-allylnormetazocine as measured by brain microdialysis

Acute administration of (+)N-allylnormetazocine ((+) SKF-10,047) was found to increase the extracellular acetylcholine (ACh) level in rat frontal cortex, as measured by brain microdialysis in freely-moving rats. The increase elicited by (+)SKF-10,047 was dose-dependent and the increase was significant at a dose of 5.0 mg/kg, compared with the saline-treated group. On the other hand, the extracellular dopamine metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) level in the frontal cortex was not changed by (+)SKF-10,047 at 5.0 mg/kg, whereas extracellular ACh level increased markedly in this area. These results suggest that (+)SKF-10,047 can affect some central nervous system (CNS) functions through the increment of extracellular ACh level in rat frontal cortex at lower doses.

Typical and atypical neuroleptics antagonize MK-801-induced locomotion and stereotypy in rats

The effects of typical and atypical neuroleptics on MK-801-induced locomotor activity and stereotyped sniffing were tested. Pretreatment with the typical neuroleptic haloperidol (0.01, 0.05, 0.1, 0.5 mg/kg SC) and the dopamine D2 receptor selective antagonist eticlopride (0.005, 0.01, 0.05 mg/kg SC) each resulted in significant and dose-dependent reductions of locomotor activity and sniffing. The atypical neuroleptic clozapine (1.0, 5.0, 10.0 mg/kg SC) was somewhat unique in that all doses reduced locomotor activity, but only the highest dose (10.0 mg/kg) significantly reduced sniffing. The data support a functional interaction between glutamate and dopamine systems, and suggest that the behavioral activation associated with MK-801 may represent a valid model for detecting potential therapeutic agents in the treatment of schizophrenia. The data should be viewed as preliminary, however, until neuroleptics are characterized in other glutamate-based models that minimized or exclude the possible influence of nonspecific motor effects.

Differential locomotor interactions between dopamine D1/D2 receptor agonists and the NMDA antagonist dizocilpine in monoamine-depleted mice

Previous work in our laboratory has shown that the non-competitive N-methyl-D-aspartate antagonist dizocilipine (MK-801) interacts synergistically with the mixed dopamine (DA) receptor agonist apomorphine and the DA D 1 agonist SKF 38393 to promote locomotion in monoamine-depleted mice. The purpose of the present study was to compare the roles of DA D 1 and DA D 2 receptors in this interaction. To that end, dizocilpine was given in combination with either the DA D 1 receptor agonist SKF 38393 or the selective DA D 2 receptor agonist quinpirole or the preferential DA D 2 agonist bromocriptine. In general, the locomotor stimulatory effects produced by SKF 38393 were potentiated by dizocilpine, whereas the locomotor stimulation produced by quinpirole and bromocriptine was counteracted. However, baseline activity, which partly depends on how much time is allowed to elapse between administration of the DA agonist and commencement of locomotor recording, and partly on the dose of the DA agonist, seems to be an important factor that determines whether dizocilpine will have a weakening or a potentiating effect. Interestingly, the competitive NMDA antagonist D-CPPene displayed a different pattern of interaction with SKF 38393 and quinpirole in that synergistic effects were observed with both DA agonists, most conspicuously so with the DA D 2 receptor agonist. The results are interpreted in the light of present knowledge of basal ganglia neuroanatomy; they are discussed in relation to the "direct" and "indirect" pathways from the striatum to the thalamus, proposed to form part of positive and negative cortico-striato-thalamo-cortical loops, respectively, as well as to the presumed presynaptic D 2 receptors on corticostriatal glutamatergic neurons.

Effects of competitive N-methyl-D-aspartate antagonists on midbrain dopamine neurons: an electrophysiological and behavioral comparison to phencyclidine

Electrophysiological and behavioral methods were used to evaluate and compare the effects of the competitive N-methyl-D-aspartate (NMDA) receptor blocker, NPC 12626, with the non-competitive NMDA antagonist, phencyclidine (PCP), on the activity of mesolimbic dopamine neurons. NPC 12626 (50 mg/kg, i.p.) produced a degree of locomotor hyperactivity comparable to that seen with PCP (5 mg/kg). However, 6-hydroxydopamine lesions of the nucleus accumbens blocked the PCP-induced hyperactivity but not the behavioral activation evoked by NPC 12626. Single-unit extracellular recordings from ventral tegmental A10 dopamine neurons also found marked differences between the competitive and non-competitive NMDA antagonists. Intravenous injections of NPC 12626 and CGS 19755 in doses up to 60 mg/kg failed to change A10 activity. This was in contrast to the striking bimodal dose-dependent increase-decrease in firing rate elicited by PCP. The absence of an effect of NPC 12626 on A10 neurons was not evidently related to a lack of access to central sites since NPC pretreatment (40 mg/kg, i.v.) completely antagonized the neurotoxicity caused by intrastriatal injection of quinolinic acid, an NMDA agonist, but not that caused by the non-NMDA compound, kainic acid. Thus, competitive NMDA antagonists do not share PCP's properties of activating mesolimbic dopaminergic systems, and as such they may be devoid of the potent psychotomimetic effects or the abuse liability associated with non-competitive NMDA receptor blockers such as PCP.