Time course effects of MK-801: the relationship between brain neurochemistry and behavior

Altered responsiveness to glutamatergic modulation by MK-801 and to repeated stress of immune challenge in female dopamine transporter knockout rats

Chronic stress is a key factor in psychiatric and neurological disorders often worsening disease symptoms. In this study, a unique animal model, the dopamine transporter knockout (DAT-KO) rat exhibiting behavioral signs resembling those occurring in mania, schizophrenia, attention deficit hyperactivity disorder, and obsessive-compulsive disorder was used. We have tested the hypothesis that the hyperdopaminergic state in DAT-KO rats (i) modulates behavioral response to the NMDA antagonist MK-801 (dizocilpine) and (ii) leads to abnormal endocrine and immune activation under subchronic stress induced by an immune challenge. Glutamatergic modulation with MK-801 induced a different behavioral pattern. While the WT rats responded to MK-801 injection with a robust rise in their locomotor activity, the hyperactive DAT-KO rats exhibited reduced locomotion. Signs of chronic stress including increased basal corticosterone and aldosterone but blunted anxiety were demonstrated in rats lacking the DAT. Repeated injections of increasing doses of lipopolysaccharide (LPS, 5 days) did not modify plasma prolactin concentrations which were however significantly lower in DAT-KO than in WT rats. Concentrations of plasma high mobility group box 1 (HMGB1) protein were significantly higher in LPS-treated DAT-KO than in WT rats. The gene expression of interleukin-6 in the anterior pituitary increased under the stress induced by the immune challenge in the WT but not the DAT-KO rats. The most evident differences between the genotypes were revealed in the spleen. The splenic gene expression of interleukin-1β, interleukin-6, and HMGB1 was lower and that of ferritin was higher in DAT-KO compared to WT rats. Obtained results emphasize the functional interaction of the endocrine and immune systems with monoamine and glutamatergic neurotransmission in the mechanisms leading to behavioral alterations and psychiatric disorders associated with dopamine dysfunction.

Effect of acetaldehyde on behavioral and neurochemical changes induced by MK-801 in rats

Alterations in motor activity related to dopamine changes in some brain regions have been described as consequences of the modifications produced by systemic administration of MK-801 (a noncompetitive NMDA receptor antagonist) in rats. Acetaldehyde (ACH), the main metabolite of ethanol, has been implicated in different alterations in the central nervous system after ethanol ingestion. ACH might exert some control on dopaminergic transmission through the formation of other compounds with dopamine, which eventually may modify dopamine content and its metabolism. In order to evaluate such a hypothesis, we used Wistar rats in the present study to evaluate the effect of ACH on locomotor alterations and dopamine metabolism changes induced by MK-801. Our results show that the MK-801-treated group had a significant increase in locomotor activity. In contrast, we did not find significant differences in locomotion tests after ACH administration. However, the group to which both drugs were administered showed a significant decrease in locomotor activity compared with those given MK-801 alone. Neurochemical analysis showed an increase in dopamine content in the striatum and frontal cortex after MK-801 administration, however; the increase was reversed by giving 200 mg/kg of ACH. These results indicate that ACH can produce an antagonic-like effect on locomotor alterations and dopamine content changes induced by MK-801, thus modulating the MK-801-induced hyperlocomotion by interfering with dopamine metabolism.

Cocaine conditioned behavior: a cocaine memory trace or an anti-habituation effect

Whether cocaine locomotor conditioning represents a cocaine positive effect; i.e., a Pavlovian cocaine conditioned response; or, a cocaine negative effect; i.e., interference with habituation to the test environment, is a subject of some controversy. Three separate experiments were conducted to compare the behavior (locomotion and grooming) of separate groups of rats given 1, 9 or 14 cocaine (10 mg/kg) treatments paired/unpaired with placement into an open-field arena. The behavior of the cocaine groups on subsequent saline tests were compared with the habituation rates of saline treated rats. After one cocaine pairing with the test environment, the subsequent behavior of the cocaine paired group on saline tests was similar to a non-habituated control group. In the two experiments with repeated cocaine pairings to the test environment, the subsequent behavior of the cocaine treated groups did not parallel that of the non-habituated saline control groups. These results were not explicable in terms of cocaine anti-habituation effects. It is suggested that cocaine contextual cues paired with cocaine treatment can activate cocaine memory traces which with subsequent cocaine treatments are reinforced and strengthened. In this way repeated cocaine use can forge conditioned stimulus connections to the cocaine behavioral response that are highly resistant to extinction.

Vitamin D deficiency during various stages of pregnancy in the rat; its impact on development and behaviour in adult offspring

Developmental vitamin D (DVD) deficiency alters brain development and behaviour in the rat. The aim of this study was to vary levels of vitamin D deficiency during gestation and examine the effects on developmental milestones and behaviour in adult offspring. By manipulating the withdrawal and reintroduction of vitamin D in the diet of female Sprague-Dawley rats, their offspring were subjected to four different prenatal vitamin D conditions: (a) control (normal vitamin D throughout gestation); (b) early-DVD deficiency; (c) late-DVD deficiency; and (d) full-DVD deficiency. We show that the standard measure for vitamin D status, 25(OH)D(3), can be significantly manipulated within 7 days by dietary intervention. We also show that levels of the active form of this vitamin, 1,25(OH)(2)D(3), replete within the same time frame as 25(OH)D(3) but are slower to deplete. Developmental milestones remained normal across all four dietary groups. Concerning the adult behavioural phenotype, both full- and late-DVD deficiency were associated with MK-801-induced hyperlocomotion. Overall, these data suggest that vitamin D deficiency restricted to late gestation only is sufficient to disrupt adult brain functioning in the rat. These findings suggest there may be a therapeutic window for maternal dietary intervention in the DVD model of psychosis.

Chronic lithium chloride administration attenuates brain NMDA receptor-initiated signaling via arachidonic acid in unanesthetized rats

It has been proposed that lithium is effective in bipolar disorder (BD) by inhibiting glutamatergic neurotransmission, particularly via N-methyl-D-aspartate receptors (NMDARs). To test this hypothesis and to see if the neurotransmission could involve the NMDAR-mediated activation of phospholipase A2 (PLA2), to release arachidonic acid (AA) from membrane phospholipid, we administered subconvulsant doses of NMDA to unanesthetized rats fed a chronic control or LiCl diet. We used quantitative autoradiography following the intravenous injection of radiolabeled AA to measure regional brain incorporation coefficients k* for AA, which reflect receptor-mediated activation of PLA2. In control diet rats, NMDA (25 and 50 mg/kg i.p.) compared with i.p. saline increased k* significantly in 49 and 67 regions, respectively, of the 83 brain regions examined. The regions affected were those with reported NMDARs, including the neocortex, hippocampus, caudate-putamen, thalamus, substantia nigra, and nucleus accumbens. The increases could be blocked by pretreatment with the specific noncompetitive NMDA antagonist MK-801 ((5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate) (0.3 mg/kg i.p.), as well by a 6-week LiCl diet sufficient to produce plasma and brain lithium concentrations known to be effective in BD. MK-801 alone reduced baseline values for k* in many brain regions. The results show that it is possible to image NMDA signaling via PLA2 activation and AA release in vivo, and that chronic lithium blocks this signaling, consistent with its suggested mechanism of action in BD.

Pleiotrophin gene transcription in the rat nucleus accumbens is stimulated by an acute dose of amphetamine

Pleiotrophin (PTN) is a heparin-binding protein with diverse functions. For example, it stimulates neurite outgrowth, mitogenesis, repair and differentiation, effects that are similar to those of the neurotrophins. The neurotrophins have, in recent years, been implicated as mediators of structural plasticity, suggested to underlie the development of behavioural sensitisation to many drugs of abuse. Since NMDA receptor antagonists inhibit the underlying morphological changes, the mechanisms are thought to be highly dependent on the activation of the NMDA subtype of glutamate receptors. To investigate if PTN has a possible role in structural plasticity, its responsiveness to an acute dose of amphetamine was studied. Amphetamine is a well-characterised inducer of sensitisation. A group of rats was systemically treated with amphetamine (10 mg/kg) and the effect on the PTN gene transcription was studied 4 h later. A separate group of rats was pretreated with the NMDA receptor antagonist MK-801 (0.25 mg/kg) 30 min prior to the administration of amphetamine. Northern blot analysis revealed a significant increase of the PTN transcript after the administration of amphetamine. However, MK-801 pretreatment did not block this effect; in contrast, it further increased PTN mRNA levels. As the response to the two drugs resembles the one earlier reported on the gene expression of brain-derived neurotrophic factor (BDNF), the present results suggest that PTN may be an attractive protein to study further in the field of synaptic plasticity.

Role of dopaminergic system in core part of nucleus accumbens in hyperlocomotion and rearing induced by MK-801 in rats: a behavioral and in vivo microdialysis study

We investigated modification of the MK-801 effect on motor activity and extracellular amines concentration by 6-hydroxydopamine (6-OHDA)-induced lesion of core nucleus accumbens (cACC) of rats. In vivo microdialysis-HPLC showed that the concentrations (fmol/microl) of dopamine (DA), dihydroxyphenylacetic acid (DOPAC) and serotonin were 0.738 +/- 0.135, 155.34 +/- 41.01 and 0.334 +/- 0.024, respectively, in the cACC of intact rats. The DOPAC/DA ratio was 264.24 +/- 94.01. Unilateral lesion of the cACC with 6-OHDA (8 microg/microl) substantially reduced DA (-93%) and DOPAC (-97%) in desipramine (30 mg/kg, i.p.)-pretreated rats (6-OHDA+DMI rats) as compared to the 65% reduction rate of both amines in saline-pretreated rats (6-OHDA+saline rats). Moreover, DOPAC was reduced by 72% in 6-OHDA+DMI rats. MK-801 increased DOPAC (426-467%) and DOPAC/DA ratio (180-230%) in intact rats. On the other hand, MK-801 increased DA by 154% and 505% in 6-OHDA+saline and 6-OHDA+DMI rats, respectively. 6-OHDA reduced the effect of MK-801 on DOPAC and DOPAC/DA ratio. In the behavioral studies, MK-801 (0.01-0.3 mg/kg, i.p.) increased locomotor activity and rearing of intact rats. Bilateral 6-OHDA+DMI lesion of the cACC caused greater reduction in the effect of MK-801 (0.1 mg/kg) than that of the shell nucleus accumbens. These results suggest that increased extracellular DOPAC concentration (but not DA) and DOPAC/DA ratio in the cACC plays an important role in MK-801-hyperactivity.

The role of MK-801 in sensitization to stimulants

Behavioral responses to stimulants can be progressively and persistently enhanced by their repeated administration. This phenomenon, called behavioral sensitization, may underlie substance abuse, psychosis, recurrence in bipolar disorder, or other psychiatric problems. A growing body of work has implicated excitatory amino acid systems in behavioral sensitization. Most of the evidence for a role of excitatory amino acids has come from experiments demonstrating prevention of sensitization by excitatory amino acid antagonists, especially the noncompetitive NMDA receptor antagonist MK-801. Results of studies with MK-801 have varied, however, leading to conflicting interpretations of its relationship to behavioral sensitization. This paper critically discusses the design of experiments that have used MK-801, and interprets data from these experiments in terms of the two leading explanations for the role of MK-801: 1) that sensitization is an example of the family of plastic events that require excitatory amino acid transmission or 2) that interoceptive cues associated with MK-801 lead to state-dependent learning that modifies sensitization because, in essence, the animal does not recognize the stimulant as the same drug if it is given in close association with MK-801. Based on conflicting reports on effects of MK-801, we propose 1) strategies for distinguishing components of MK-801's effects on responses to stimulants, 2) a model that is a hybrid of the two interpretations of its effects on sensitization, and 3) experimental strategies for testing this model.

Blockade of sensitization to methylphenidate by MK-801: partial dissociation from motor effects

The role of MK-801's locomotor effect in blocking the development of sensitization to methylphenidate was investigated utilizing a computerized locomotor activity monitoring system. After 7 days of acclimation to a 12:12 light-dark cycle (lights on at 07:00), male Sprague-Dawley rats (n=62) were housed in test cages and motor activity was recorded continuously for 16 days. The first 2 days of recording served as a baseline for each rat, and on day 3 each rat received a saline injection. On days 4 to 9 rats were randomly divided into seven groups: Rats received either six daily s.c. injections of methylphenidate (2.5 mg/kg; Group 1), or six daily i.p. injections of 0.30 mg/kg, 0.05 mg/kg MK-801 (Groups 2 and 3, respectively); two MK-801 pre-treatment groups received a single i.p. injection of 0.05, or 0.30 mg/kg MK-801 one hour prior to 2.5 mg/kg methylphenidate (n=8 each) on day 4 followed by five daily injections of 2.5 mg/kg methylphenidate; and finally, two cotreatment groups received a challenge dose of 2.5 mg/kg methylphenidate on day 4 followed by either 0.05 or 0.30 mg/kg MK-801 i.p. one hour prior to 2.5 mg/kg methylphenidate from days 5 to 9. All groups were allowed five days of no treatment before being re-challenged on day 15 with the same treatment they received on day 4. Methylphenidate and 0.30 mg/kg MK-801 sensitized to their own locomotor effects, but 0.05 mg/kg MK-801, which had no acute motor effects, did not. The administration of MK-801 (0.30 mg/kg) prior to methylphenidate either singly on day 4, or coadministered throughout the repeated methylphenidate treatment phase, blocked the development of sensitization to methylphenidate. However, MK-801 at 0.05 mg/kg delayed the development of sensitization when co-administered on days 5 to 9, but a single injection 1 h prior to methylphenidate on day 4 did not prevent sensitization to subsequent methylphenidate administration. In conclusion, MK-801 prevents sensitization to methylphenidate; motor stimulation by MK-801 is not necessary for short-term prevention or delay of sensitization to methylphenidate but may be necessary for a persistent blockade of sensitization.

NMDA receptor antagonist disrupts acute and chronic effects of methylphenidate

Methylphenidate (MPD) is a drug widely used for treating attention-deficit/hyperactivity disorder in children. Because of its extensive consumption and because it has pharmacological stimulant properties similar to amphetamine and cocaine, MPD has the potential of abuse. N-methyl-D-aspartate (NMDA) receptors are suggested to be involved in CNS effects of stimulants, and antagonists of the NMDA receptor can potentially alter the stimulants' effects. Dizocilpine (MK-801), a non-competitive antagonist of the NMDA receptor, has been reported to prevent sensitization elicited by repeated administration of amphetamine and cocaine. The objective of the present study was to use the tail-flick latency assay, rectal temperature, and body weight gain to assess effects of repetitive treatment of MPD and whether MK-801 treatment would alter these effects in Sprague-Dawley rats. It was found that: (Ia) Acute administration of MPD or MK-801 did not alter the tail-flick latency, (Ib) Repeated administration of MPD decreased tail-flick latency, while repeated administration of MK-801 had no significant effect on tail-flick latency, (Ic) MK-801 given prior to or with MPD reversed the chronic effect on tail-flick latency produced by MPD; (IIa) When both drugs were independently given, MPD elicited a decrease in rectal temperature, while MK-801 alone produced an increase in temperature, (IIb) When given together, MK-801 had a transient effect in blocking the sensitization to MPD but failed to reverse the sensitization of MPD once it had developed; and (III) Both MK-801 and MPD caused an unstable pattern of body weight gain. Hence, the results of this study in rats suggest that MK-801 can modulate non-motor effects of MPD.

MK-801 blocks the development of sensitization to the locomotor effects of methylphenidate

Male Sprague-Dawley rats were divided to three groups (each n = 8) and were housed in test cages where motor activity was recorded continuously for 16 days using a computerized motor activity monitoring system to determine whether repeated administration of MK-801 could block the development and/or the expression of sensitization to the locomotor effects of methylphenidate (MPD). One group of rats received six daily injections (days 4-9) of 0.30 mg/kg MK-801, followed by 5 days without injection (days 10-14) and re-challenged (day 15) with 0.30 mg/kg MK-801. The second group received a challenge dose of 2.5 mg/kg MPD (day 4) followed by 5 days of co-treatment with MK-801 (0.30 mg/kg) given 1 h prior to MPD (days 5-9). This group was then re-challenged with MPD (2.5 mg/kg) on day 15. The last group received six daily injections of 2.5 mg/kg MPD (days 4-9). They were then split into two subgroups of rats which received either no treatment (control) or five daily injections of 0.30 mg/kg MK-801 (days 10-14) before being re-challenged on day 15 with 2.5 mg/kg MPD. MK-801 sensitized to its own locomotor effects. MK-801 given after sensitization had developed (i.e., days 10-14) was able to mask the expression of a sensitized response on day 15, but the effect was only transient since the sensitized response was present 3 weeks later. Moreover, MK-801, when coadministered during the repeated treatment phase was able to block the development of a sensitized response, which suggest that NMDA receptors involved in the process of MPD sensitization.

Memantine does not substantially affect brain stimulation reward: comparison with MK-801

The non-competitive N-methyl-D-aspartate (NMDA)-receptor antagonist MK-801 (dizocilpine) has been shown to potentiate brain stimulation reward (BSR). Memantine (1-amino-3,5-dimethyladamantane) also binds to the PCP binding site of the NMDA receptor but with markedly different kinetics and affinity than MK-801. Here, we examined the effects of memantine on BSR and compared its effects to those of MK-801. MK-801 (0.05 mg/kg-0.4 mg/kg) produced clear, dose-dependent decreases in threshold frequency, manifest in clear leftward shifts of the function relating stimulation frequency to response rate. Memantine (1 mg/kg-17.5 mg/kg) had only small effects on threshold frequencies and only at high doses, manifest in only small shifts in the frequency-response function. The highest dose of each drug also produced a decrease in maximum response rate. This study shows that memantine failed to substantially influence BSR at low to intermediate doses, suggesting that this substance is likely to be largely devoid of rewarding effects in a therapeutic dose range.

Effects of the NMDA receptor antagonist D-CPPene on extracellular levels of dopamine and dopamine and serotonin metabolites in striatum of kindled and non-kindled rats

Electrical kindling in rats has previously been shown to cause a hypersensitivity to amphetamine-like behavioral effects of competitive NMDA receptor antagonists such as D,L-(E)-amino-4-methyl-5-phosphono-3-pentenoic acid (CGP 37849), D-(E)-2-amino-4-methyl-5-phosphono-3-pentenoic acid (CGP 40116), or 3-(2-carboxypiperazine-4-yl)propenyl-1-phosphonate (SDZ EAA 494; D-CPPene). From this observation, it was concluded that kindling-induced epileptogenesis enhances the potential of competitive NMDA receptor antagonists to induce such unwanted adverse effects, predicting that such drugs may induce more severe side effects in epileptic patients than in healthy volunteers, which was confirmed in clinical trials. In the present study, we thought to examine the biochemical basis for the enhanced susceptibility of kindled rats to amphetamine-like behavioral effects of NMDA receptor antagonists by measuring extracellular levels of dopamine, the dopamine metabolites dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), and the serotonin (5-hydroxytryptamine, 5-HT) metabolite 5-hydroxyindoleacetic acid (5-HIAA) in the striatum of awake, behaving rats, using in vivo microdialysis. When administered systemically, D-CPPene, 15 mg/kg i.p., caused more intense stereotyped behaviors in kindled than in non-kindled rats. While there was no significant alteration in extracellular dopamine, in both groups of rats HVA and 5-HIAA significantly increased. In kindled rats, basal levels of HVA and the increase in HVA in response to D-CPPene were higher compared to non-kindled animals. When administered intrastriatally via the microdialysis probe, D-CPPene, 10 microM, significantly increased dopamine, HVA and 5-HIAA, which was associated with stereotyped behaviors. Again, these behaviors were more intense in kindled rats. The data indicate that a competitive NMDA receptor antagonist at high, behaviorally active doses induces increases in striatal dopamine and presumably also 5-HT release, which most likely underlie the amphetamine-like behavioral effects of such a drug. Kindling enhances the sensitivity to these behavioral effects, which could be related to a more marked dopamine and 5-HT release. Thus, in order to avoid false predictions for the clinical situation, it is important to study the behavioral and biochemical effects of NMDA receptor antagonists not only in naive, healthy animals but also in animals that mimic the disease for which a drug is developed.

Lorazepam attenuates the behavioral effects of dizocilpine

To characterize the potential interaction between the excitatory and inhibitory neurotransmitter systems, the effects of dizocilpine, CPP, and lorazepam on open-field behavior and pentylenetetrazol-induced seizures were evaluated in mice. Dizocilpine (0.01-0.1 mg/kg), CPP (1-10 mg/kg), or vehicle was administered intraperitoneally 15 min prior to lorazepam (0.2-2 mg/kg) or vehicle. Behavioral monitoring began 25 min after the lorazepam injection. Upon completion of testing, unrestrained mice were infused intravenously with pentylenetetrazole until the onset of a full tonic-clonic seizure. The highest dose of dizocilpine by itself significantly increased the average distance traveled, the number of rears, and the number of stereotypies during the test period. Lorazepam alone dose dependently decreased activity on all behavioral parameters. Lorazepam also completely antagonized the hyperactivity produced by dizocilpine when the two compounds were coadministered. This antagonism is most likely due to an interaction in the regulation of dopaminergic tone which underlies motor activity. Lorazepam exerted a dose-dependent anticonvulsant effect. Dizocilpine alone had no effect on seizure induction and did not potentiate the anticonvulsive effect of lorazepam when coadministered with lorazepam. CPP reduced the number of rears and the number of stereotypies during the test period. CPP did not alter the pentylenetetrazol-induced seizure threshold and did not influence the anticonvulsant effect of lorazepam.

Dose-related effects of MK-801 on acute and chronic methylphenidate administration

The non-competitive N-methyl-d-aspartate (NMDA) receptor antagonist MK-801 has been shown to modulate both the effects of stimulants, such as amphetamine and cocaine, in producing locomotion and the chronic effects of stimulants in producing sensitization. In this study, we examine the interactions between MK-801 and the stimulant methylphenidate. Three different doses of MK-801 were administered 60 min prior to methylphenidate injection (2.5 mg/kg) and the acute response to MK-801 alone and the coadministration with methylphenidate were characterized. MK-801 alone was found to produce dose-dependent locomotor activation. The 0.15 mg/kg dose of MK-801 had no effect on the response to methylphenidate, while the 0. 3 and 0.6 mg/kg doses augmented the methylphenidate response. The effect of pretreatment with MK-801 on subsequent repeated methylphenidate administration was assessed. For all three doses tested, MK-801 pretreatment blocked the progressive locomotor sensitization expected during repeated methylphenidate administration. These findings suggest that MK-801 may exert a long-lasting effect on learning and memory process that result in a blocking of the development of sensitization.

Dizocilpine (MK-801) increases not only dopamine but also serotonin and norepinephrine transmissions in the nucleus accumbens as measured by microdialysis in freely moving rats

The extracellular concentrations of dopamine (DA), norepinephrine (NE), and serotonin (5-HT) in the nucleus accumbens (NACC) of freely moving rats were monitored simultaneously via intracerebral microdialysis. Local infusion of the non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist MK-801 (dizocilpine) (5-250 microM) produced significant increases in extracellular levels of DA, NE and 5-HT in a concentration-dependent fashion. Perfusion with tetrodotoxin (TTX, 1 microM) blocked the ability of focal MK-801 (50 microM) to increase DA, NE and 5-HT in the dialysate. Systemic administration of MK-801 (0.3 mg/kg, i.p.) also produced small, but statistically significant, increases in extracellular concentrations of DA, NE and 5-HT in the NACC. Our microdialysis results are consistent with the hypothesis that, in addition to dopaminergic, serotonergic and noradrenergic neurotransmissions in the NACC are involved in the mechanism by which MK-801 alters behavior in rats. Also, the present study gives further support to the concept that NMDA receptors within the NACC do not regulate DA release through direct excitatory control.

Effects of electroconvulsive stimuli and MK-801 on neuropeptide Y, neurokinin A, and calcitonin gene-related peptide in rat brain

Rats were pretreated with 0.9% NaCl, or 0.1 or 1.0 mg/kg MK-801, an anticonvulsant and a psychotomimetic drug, and 60 minutes later given ECS or sham ECS. After six sessions the animals were sacrificed and neuropeptide Y (NPY-), neurokinin A (NKA-), and calcitonin gene-related peptide (CGRP-) like immunoreactivity (-LI) measured with radioimmunoassays. ECS increased NPY-LI in frontal cortex, striatum, occipital cortex and hippocampus, and NKA-LI in occipital cortex and hippocampus. MK-801 increased CGRP in a dose-response manner in frontal cortex, and NKA-LI in occipital cortex. Although the higher MK-801 dose reduced seizure duration by 50%, the ECS induced NPY-LI increase in striatum, occipital cortex and hippocampus, and NKA-LI in occipital cortex was not diminished. In contrast, there was a parallel decrease in seizures and NPY-LI and NKA-LI changes in frontal cortex and hippocampus, respectively. Investigation of neuropeptides in brain may contribute to understanding of the mechanisms of action of antidepressive and antipsychotic treatments and of psychotomimetic drugs.

Prazosin inhibits MK-801-induced hyperlocomotion and dopamine release in the nucleus accumbens

This study examined the putative inhibitory effect of the alpha 1-adrenoceptor antagonist prazosin (1-(4-amino-6,7-dimethoxy-2-quinazolinyl)-4-(2-furanylcarbonyl)pip erazine) on changes evoked by the psychotomimetic, non-competitive NMDA receptor antagonist, MK-801((+)-5-methyl-10,11-dihydroxy-5H-dibenzo-(a,d)cyclohepten-5, 10-imine), in locomotor activity and extracellular concentrations of dopamine and its metabolites, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), and the serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA) in the nucleus accumbens as assessed by microdialysis in freely moving rats. MK-801 (0.1 and 0.3 mg/kg, s.c.) induced a significant, dose-dependent increase in horizontal locomotor activity but did not affect rearing. Prazosin administration alone (1 mg/kg, s.c.) only slightly reduced horizontal activity during an initial 10 min measurement period, although it consistently reduced rearing. However, pretreatment with prazosin effectively suppressed the locomotor stimulation caused by either dose of MK-801 throughout the whole observation period, i.e. 40 min. Both doses of MK-801 significantly increased extracellular levels of dopamine in the nucleus accumbens up to approximately 90%. In addition, MK-801 dose dependently increased dopamine metabolite concentrations in the nucleus accumbens, but 5-HIAA was significantly increased only by the high dose of MK-801. When given alone, prazosin did not affect either dopamine, DOPAC, HVA or 5-HIAA levels. However, prazosin pretreatment effectively blocked MK-801-evoked increases in dialysate dopamine concentrations. Consequently, the potent and selective alpha 1-adrenoceptor antagonist prazosin was found to specifically suppress MK-801-evoked, but not basal dopamine release in the nucleus accumbens, while effectively blocking MK-801-evoked locomotor stimulation with only negligible effects on basal locomotor activity. Thus, alpha 1-adrenoceptor antagonism may act by reducing the sensitivity of the mesolimbic dopamine system to pharmacological or environmental challenge. Since most antipsychotic drugs exhibit both dopamine D2 receptor and alpha 1-adrenoceptor antagonistic properties, they may alleviate psychosis not only through blockade of postsynaptic dopamine receptors, but also presynaptically on the mesolimbic dopamine system, through their alpha 1-adrenoceptor antagonistic action. This latter action may contribute to reduce evoked dopamine hyperactivity, e.g. in response to stress.

18-Methoxycoronaridine, a non-toxic iboga alkaloid congener: effects on morphine and cocaine self-administration and on mesolimbic dopamine release in rats

Ibogaine, a naturally occurring iboga alkaloid, has been claimed to be effective in treating addiction to opioids and stimulants, and has been reported to inhibit morphine and cocaine self-administration in rats. However, ibogaine also has acute nonspecific side effects (e.g. tremors, decreased motivated behavior in general) as well as neurotoxic effects (Purkinje cell loss) manifested in the vermis of the cerebellum. 18-Methoxycoronaridine (MC) is a novel, synthetic iboga alkaloid congener that mimics ibogaine's effects on drug self-administration without appearing to have ibogaine's other adverse effects. Acutely, in rats, MC decreased morphine and cocaine self-administration but did not affect bar-press responding for water. In some rats, treatment with MC (40 mg/kg) induced prolonged decreases in morphine or cocaine intake lasting several days or weeks. MC had no apparent tremorigenic effect, and there was no evidence of cerebellar toxicity after a high dose (100 mg/kg) of MC. Similar to the effects of ibogaine and other iboga alkaloids that inhibit drug self-administration, MC (40 mg/kg) decreased extracellular levels of dopamine in the nucleus accumbens. MC therefore appears to be a safer, ibogaine-like agent that might be useful in the treatment of addictive disorders.

The NMDA receptor and cocaine: evidence that MK-801 can induce behavioral sensitization effects

Antagonism of the NMDA receptor with MK-801 is considered to be an effective pharmacologic manipulation to prevent the development of sensitization effects to drugs such as cocaine. The present study investigated this issue by comparing the behavioral response of separate groups of rats to three treatment cycles of either saline, 0.1 mg/kg MK-801, 10 mg/kg cocaine, or combined MK-801-cocaine (0.1/10 mg/kg). The treatments were spaced 1 week apart and were preceded by two nondrug baseline tests. In the first test cycle, the four groups had equivalent activity levels in the two nondrug tests. In the first drug test only the MK-801-cocaine group exhibited hyperactivity. By the third drug test, the MK-801-cocaine group exhibited an enhanced hyperactivity and the MK-801 group became hyperactive. Thus, behavioral drug sensitization developed but only with groups treated with MK-801. Antagonism of the NMDA receptor under some circumstances can be a highly effective treatment for the induction of behavioral sensitization effects.