Netrin-1 receptor-deficient mice show age-specific impairment in drug-induced locomotor hyperactivity but still self-administer methamphetamine

Rationale

The mesocorticolimbic dopamine system undergoes significant reorganization of neuronal connectivity and functional refinement during adolescence. Deleted in colorectal cancer (DCC), a receptor for the guidance cue netrin-1, is involved in this reorganization. Previous studies have shown that adult mice with a heterozygous (het) loss-of-function mutation in DCC exhibit impairments in sensitization and conditioned place preference (CPP) to psychostimulants. However, the commonly abused psychostimulant methamphetamine (METH) has not been assessed, and the role of DCC in drug self-administration remains to be established.

Objectives

Using dcc het mice and wildtype (WT) littermates, we extended previous findings on dcc haplodeficiency by examining self-administration of METH in adult mice, including cue-induced drug seeking following abstinence. We also examined hyperactivity, sensitization, and CPP to a METH-paired context in adult and adolescent mice.

Results

While adult dcc het mice expressed largely similar METH self-administration and cue-induced drug seeking as WT littermates, they failed to modulate responding according to dose of METH. Compared to WT, both adult and adolescent dcc het mice expressed impaired locomotor hyperactivity to acute METH but nevertheless showed comparable behavioral sensitization. Conditioned hyperactivity increased with age in WT but not in dcc het mice.

Conclusions

Impaired METH-induced hyperactivity and dose-related responding in adult dcc het mice suggest that reduced DCC alters METH-related behaviors. Adolescence is identified as a vulnerable period during which impairment in hyperactivity due to reduced DCC can be overcome with repeated METH injections. Nevertheless, DCC appears to have a somewhat limited role in METH-consumption and seeking following abstinence.     

7,8-Dihydroxyflavone, a TrkB agonist, attenuates behavioral abnormalities and neurotoxicity in mice after administration of methamphetamine

Rationale

It is widely recognized that methamphetamine (METH) induces behavioral abnormalities and dopaminergic neurotoxicity in the brain. Several lines of evidence suggest a role for brain-derived neurotrophic factor (BDNF) and its specific receptor, tropomyosin-related kinase (TrkB), in METH-induced behavioral abnormalities.

Objective

In this study, we examined whether 7,8-dihydroxyflavone (7,8-DHF), a novel potent TrkB agonist, could attenuate behavioral abnormalities and dopaminergic neurotoxicity in mice after administration of METH.

Results

Pretreatment with 7,8-DHF (3.0, 10, or 30 mg/kg), but not the inactive TrkB compound, 5,7-dihydroxyflavone (5,7-DHF) (30 mg/kg), attenuated hyperlocomotion in mice after a single administration of METH (3.0 mg/kg), in a dose-dependent manner. The development of behavioral sensitization after repeated administration of METH (3.0 mg/kg/day, once daily for 5 days) was significantly attenuated by pretreatment with 7,8-DHF (10 mg/kg). Furthermore, pretreatment and subsequent administration of 7,8-DHF (10 mg/kg) attenuated the reduction of dopamine transporter (DAT) in the striatum after repeated administration of METH (3.0 mg/kg?×?3 at 3-hourly intervals). Treatment with ANA-12 (0.5 mg/kg), a potent TrkB antagonist, blocked the protective effects of 7,8-DHF on the METH-induced reduction of DAT in the striatum. Moreover, 7,8-DHF attenuated microglial activation in the striatum after repeated administration of METH.

Conclusions

These findings suggest that 7,8-DHF can ameliorate behavioral abnormalities as well as dopaminergic neurotoxicity in mice after administration of METH. It is likely, therefore, that TrkB agonists such as 7,8-DHF may prove to be potential therapeutic drugs for several symptoms associated with METH abuse in humans.     

Dissociable role of tumor necrosis factor alpha gene deletion in methamphetamine self-administration and cue-induced relapsing behavior in mice

Rationale

During the development of addiction, addictive drugs induce transient and long-lasting changes in the brain including expression of endogenous molecules and alteration of morphological structure. Of the altered endogenous molecules, some facilitate but others slow the development of drug addiction. Previously, we have reported that tumor necrosis factor alpha (TNF-??) is a critical molecule among endogenous anti-addictive modulators using animal models of drug-conditioned place preference and drug discrimination.

Objectives

Does targeted deletion of the TNF-?? gene in mice affect methamphetamine (METH) self-administration, motivation to self-administer METH, cue-induced reinstatement of METH-seeking behavior, and food reinforcement or seeking behavior?

Methods

Both METH self-administration and reinstatement of drug-seeking behavior and food self-delivery and food-seeking behavior were measured in TNF-?? (?/?) and wild-type mice.

Results

There were an upward shift of dose responses to METH self-administration under a fixed ratio schedule of reinforcement and higher breaking points under a progressive ratio schedule of reinforcement in TNF-?? knockout (TNF-?? (?/?)) mice as compared with wild-type mice. There was no significant difference in cue-induced reinstatement of METH-seeking behavior, food-maintained operant behavior, motivation to natural food, and cue-induced food-seeking behavior between TNF-?? (?/?) and wild-type mice.

Conclusion

TNF-?? affects METH self-administration and motivation to self-administer METH but contributes to neither METH-associated cue-induced relapsing behavior nor food reward and food-seeking behavior. TNF-?? may be explored for use as a diagnostic biomarker for the early stage of drug addiction.     

Role of the D1 receptor for the dopamine agonist-induced one-trial behavioral sensitization of preweanling rats

Rationale

The neural mechanisms mediating the ontogeny of behavioral sensitization are poorly understood.

Objective

The purpose of the present study was to determine the role of the D1 receptor for the induction of dopamine agonist-induced behavioral sensitization during the preweanling period.

Methods

In the first experiment, the early ontogeny of R-propylnorapomorphine (NPA)-induced behavioral sensitization was examined by pretreating male and female rats with saline or NPA (0.5, 1, or 2 mg/kg, intraperitoneally (IP)) before placement in activity chambers on postnatal day (PD) 12, 16, 20, or 24. One day later, rats were tested with lower doses of NPA and the occurrence of locomotor sensitization was determined. In subsequent experiments, rats were injected with saline or the D1 receptor antagonist SCH23390 (0.1, 0.5, 1, or 5 mg/kg, IP) 0, 15, 30, or 60 min before cocaine, methamphetamine (METH), or NPA pretreatment. The next day, rats were tested with the same dopamine agonist again and sensitized responding was assessed.

Results

NPA produced one-trial behavioral sensitization at all ages tested. In preweanling rats, SCH23390, regardless of dose, was ineffective at preventing the induction of cocaine-, METH-, or NPA-induced one-trial behavioral sensitization.

Conclusions

The present results are in partial contrast to adult rodent studies, in which SCH23390 blocks the induction of METH- and apomorphine-induced behavioral sensitization, but not cocaine sensitization. When these findings are considered together, it appears that D1 receptor stimulation is not necessary for the induction of behavioral sensitization during the preweanling period, although D1 receptors may play a more important role in adulthood.     

Development of stereotyped behaviors during prolonged escalation of methamphetamine self-administration in rats

Rationale

Experimental animal studies have shown that repeated administration of psychostimulants, such as methamphetamine (METH), results in an altered behavioral response profile, which includes the sensitization of both locomotor and stereotyped behaviors. Although sensitization of these behaviors has been characterized in detail during bolus, investigator-administered drug administration, little is known about the development or expression of stereotypies during psychostimulant self-administration.

Objective/methods

The present study investigated in rats the expression of focused stereotyped behaviors during an extended access, escalation procedure of METH self-administration. Over several weeks during stepwise-extended daily access to METH (3, 6, and 12?h) followed by exposure to 24-h ??binges,?? rats gradually increased daily drug intake.

Results

During the escalation procedure, the rats' behavioral response evolved from locomotor activation to progressively more focused stereotypies, culminating in continuous oral behaviors (licking, gnawing, and chewing), interrupted only by episodic lever presses. Sensitization of stereotyped behaviors was evident, particularly with regard to oral behaviors that exhibited a more rapid onset and intensification in the apparent absence of greater drug intake.

Conclusions

Our data demonstrate that stepwise-extended daily access to METH (3, 6, 12, and 24?h) self-administration in rats closely approximates motivational, pharmacokinetic, as well as behavioral patterns of human METH abuse. The accompanied appearance of sensitization of intense focused stereotyped behaviors, which is probably a consequence of escalation of drug intake, resembles stereotypies associated with investigator-initiated METH administration and may parallel the development of stimulant-induced psychosis seen in human abusers.     

Altered locomotor and stereotyped responses to acute methamphetamine in adolescent, maternally separated rats

Rationale

Neonatal maternal separation (MS) has been used to model the effects of early life stress in rodents. MS alters behavioral responses to a variety of abused drugs, but few studies have examined its effects on methamphetamine sensitivity.

Objectives

We sought to determine the effects of MS on locomotor and stereotyped responses to low-to-moderate doses of methamphetamine in male and female adolescent rats.

Methods

Male and female rat pups were subjected to 3 h per day of MS on postnatal days (PN) 2–14 or a brief handling control procedure during the same period. During adolescence (approximately PN 40), all rats were tested for locomotor activity and stereotyped behavior in response to acute methamphetamine administration (0, 1.0, or 3.0 mg/kg, s.c.).

Results

MS rats of both sexes exhibited increased locomotor activity in a novel environment, relative to handled controls. MS increased the locomotor response to methamphetamine (METH), and this effect occurred at different doses for male (3.0 mg/kg) and female (1.0 mg/kg) rats. MS also increased stereotyped behavior in response to METH (1.0 mg/kg) in both sexes.

Conclusions

MS enhances the locomotor response to METH in a dose- and sex-dependent manner. These results suggest that individuals with a history of early life stress may be particularly vulnerable to the psychostimulant effects of METH, even at relatively low doses.     

Therapeutic potential of nicotine for methamphetamine-induced impairment of sensorimotor gating: involvement of pallidotegmental neurons

Introduction

We have previously found that a disruption to prepulse inhibiton (PPI) induced by methamphetamine (METH) is associated with impaired functioning of pallidotegmental neurons, which play a crucial role in PPI of the startle reflex, through the activation of gamma-aminobutyric acid type B receptors in pedunculopontine tegmental neurons in mice.

Objectives

Here, we examined the effect of nicotine on METH-induced impairment of PPI of the startle reflex focusing on dysfunctional pallidotegmental neurons and the neural system.

Results

Nicotine (0.15–0.5 mg/kg) ameliorated the deficit in PPI induced by acute METH, and the ameliorating effect of nicotine was antagonized by nicotinic receptor antagonists such as methyllycaconitine and dihydro-β-erythroidine. The acute METH-induced disruption of PPI was accompanied by suppression of c-Fos expression in the lateral globus pallidus (LGP) as well as its induction in the caudal pontine reticular nucleus (PnC) in mice subjected to the PPI test. Nicotine-induced amelioration of PPI deficits in METH-treated mice was accompanied by a reversal of the changes in c-Fos expression in both the LGP and PnC to the basal level.

Conclusions

Nicotine is effective in ameliorating the impairment of PPI caused by METH, which may be associated with normalization of the pallidotegmental neurons.     

Suppression of endogenous PPARγ increases vulnerability to methamphetamine-induced injury in mouse nigrostriatal dopaminergic pathway

Rationale

Methamphetamine is a commonly abused drug and dopaminergic neurotoxin. Repeated administration of high doses of methamphetamine induces programmed cell death, suppression of dopamine release, and reduction in locomotor activity. Previous studies have shown that pretreatment with peroxisome proliferator-activated receptor gamma (PPAR??) agonist reduced methamphetamine-induced neurodegeneration.

Objectives

The purpose of this study was to examine the role of endogenous PPAR?? in protecting against methamphetamine toxicity.

Methods

Adeno-associated virus (AAV) encoding the Cre recombinase gene was unilaterally injected into the left substantia nigra of loxP-PPAR?? or control wild-type mice. Animals were treated with high doses of methamphetamine 1?month after viral injection. Behavioral tests were examined using rotarod and rotometer. In vivo voltammetry was used to examine dopamine release/clearance and at 2?months after methamphetamine injection.

Results

Administration of AAV-Cre selectively removed PPAR?? in left nigra in loxP-PPAR?? mice but not in the wild-type mice. The loxP-PPAR??/AAV-Cre mice that received methamphetamine showed a significant reduction in time on the rotarod and exhibited increased ipsilateral rotation using a rotometer. The peak of dopamine release induced by local application of KCl and the rate of dopamine clearance were significantly attenuated in the left striatum of loxP-PPAR??/AAV-Cre animals. Tyrosine hydroxylase immunoreactivity was reduced in the left, compared to right, nigra, and dorsal striatum in loxP-PPAR??/AAV-Cre mice receiving high doses of methamphetamine.

Conclusion

A deficiency in PPAR?? increases vulnerability to high doses of methamphetamine. Endogenous PPAR?? may play an important role in reducing methamphetamine toxicity in vivo.     

Behavioral inhibition in mice bred for high vs. low levels of methamphetamine consumption or sensitization

Rationale

Research indicates that genetics influence methamphetamine self-administration as well as sensitization to the psychomotor-stimulating effects of methamphetamine (MA). Other studies have suggested that heightened levels of impulsivity, including low levels of behavioral inhibition, are associated with the use of drugs, including MA.

Objectives

The current study examined whether lines of mice selected for traits associated with a heightened risk of developing MA dependence would also exhibit low levels of drug-na?ve inhibition and whether administration of MA would result in different levels of inhibition in animals selected to consume or respond more to MA.

Methods

A go/no-go task was used to assess inhibition in male and female mice selected for low or high levels of MA consumption or selected for high or low levels of locomotor sensitization to repeated injections of MA.

Results

Mice selected for MA sensitization differed in false alarms, precue response rates (measures of behavioral inhibition), and also hits (measure of operant responding). Mice selected for MA consumption did not differ in measures of behavioral inhibition, though hits differed. When MA was administered prior to the task, false alarms, precue response rates, and hits decreased for mice from all selected lines. Female high drinking mice were particularly resistant to MA??s effects on hits, but not precue response rate or false alarms.

Conclusions

These data suggest a shared, but complex, genetic association between inhibition processes, general levels of operant responding, and MA sensitization or consumption.     

Melancholic-Like Behaviors and Circadian Neurobiological Abnormalities in Melatonin MT1 Receptor Knockout Mice

Background:

Melancholic depression, described also as endogenous depression, is a mood disorder with distinctive specific psychopathological features and biological homogeneity, including anhedonia, circadian variation of mood, psychomotor activation, weight loss, diurnal cortisol changes, and sleep disturbances. Although several hypotheses have been proposed, the etiology of this disorder is still unknown.

Methods:

Behavioral, electrophysiological and biochemical approaches were used to characterize the emotional phenotype, serotonergic and noradrenergic electrical activity, and corticosterone in melatonin MT1 receptor knockout mice and their wild type counterparts, during both light and dark phases.

Results:

Melatonin MT₁ receptor knockout mice have decreased mobility in the forced swim and tail suspension tests as well as decreased sucrose consumption, mostly during the dark/inactive phase. These mood variations are reversed by chronic treatment with the tricyclic antidepressant desipramine. In addition, MT₁ receptor knockout mice exhibit psychomotor disturbances, higher serum levels of corticosterone the dark phase, and a blunted circadian variation of corticosterone levels. In vivo electrophysiological recordings show a decreased burst-firing activity of locus coeruleus norepinephrine neurons during the dark phase. The circadian physiological variation in the spontaneous firing activity of high-firing neuronal subpopulations of both norepinephrine neurons and dorsal raphe serotonin neurons are abolished in MT₁ knockout mice.

Conclusions:

These data demonstrate that melatonin MT₁ receptor knockout mice recapitulate several behavioral and neurobiological circadian changes of human melancholic depression and, for the first time, suggest that the MT₁ receptor may be implicated in the pathogenesis of melancholic depression and is a potential pharmacological target for this mental condition.     

Increased amphetamine-induced locomotor activity, sensitization, and accumbal dopamine release in M5 muscarinic receptor knockout mice

Introduction

Muscarinic M₅ receptors are the only muscarinic receptor subtype expressed by dopamine-containing neurons of the ventral tegmental area. These cells play an important role for the reinforcing properties of psychostimulants and M₅ receptors modulate their activity. Previous studies showed that M₅ receptor knockout (M ₅ ^?/? ) mice are less sensitive to the reinforcing properties of addictive drugs.

Materials and methods

Here, we investigate the role of M₅ receptors in the effects of amphetamine and cocaine on locomotor activity, locomotor sensitization, and dopamine release using M ₅ ^?/? mice backcrossed to the C57BL/6NTac strain.

Statistical analyses

Sensitization of the locomotor response is considered a model for chronic adaptations to repeated substance exposure, which might be related to drug craving and relapse. The effects of amphetamine on locomotor activity and locomotor sensitization were enhanced in M ₅ ^?/? mice, while the effects of cocaine were similar in M ₅ ^?/? and wild-type mice.

Results

Consistent with the behavioral results, amphetamine-, but not cocaine, -elicited dopamine release in nucleus accumbens was enhanced in M ₅ ^?/? mice.

Discussion

The different effects of amphetamine and cocaine in M ₅ ^?/? mice may be due to the divergent pharmacological profile of the two drugs, where amphetamine, but not cocaine, is able to release intracellular stores of dopamine. In conclusion, we show here for the first time that amphetamine-induced hyperactivity and dopamine release as well as amphetamine sensitization are enhanced in mice lacking the M₅ receptor. These results support the concept that the M₅ receptor modulates effects of addictive drugs.     

Responses of the rat basal ganglia neurotensin systems to low doses of methamphetamine

Rationale

Administration of high doses of methamphetamine (METH) in a manner mimicking the binging patterns associated with abuse reduces NT release and causes its accumulation and elevated NT levels in extrapyramidal structures by a D1 mechanism. The relevance of these findings to the therapeutic use of METH needs to be studied.

Objectives

The effect of low doses (comparable to that used for therapy) of METH on basal ganglia NT systems was examined and compared to high-dose and self-administration effects previously reported.

Methods

Rats were injected four times (2-h intervals) with either saline or low doses of METH (0.25, 0.50, or 1.00 mg/kg/subcutaneously (s.c.)). For the DA antagonist studies, animals were pretreated with a D1 (SCH23390) or D2 (eticlopride) antagonist 15 min prior to METH or saline treatments. Rats were sacrificed 5–48 h after the last injection.

Results

METH at doses of 0.25 and 0.50, but not 1.00 mg/kg, rapidly and briefly decreased NTLI concentration in all basal ganglia structures studied. In the posterior dorsal striatum, the reduction in NT level after low-dose METH appeared to be caused principally by D2 stimulation, but both D2 and D1 stimulation were required for the NT responses in the other basal ganglia regions.

Conclusions

A novel finding from the present study was that opposite to abuse-mimicking high doses of METH, the therapeutically relevant low-dose METH treatment reduced NT tissue levels likely reflecting an increase in NT release and a short-term depletion of the levels of this neuropeptide in basal ganglia structures. The possible significance is discussed.     

Focus on agomelatine

Abstract

Background:

Agomelatine is a novel antidepressant with agonist activity at melatonin receptors (MT₁ and MT₂), and antagonistic effects at the 5HT_2c serotonin receptor.     

Driving on ice: impaired driving skills in current methamphetamine users

Rationale

Previous research indicates a complex link between methamphetamine (METH) and driving performance. Acute dosing with amphetamines has improved driving-related performance in some laboratory studies, while epidemiological studies suggest an association between METH use, impaired driving, and accident culpability.

Methods

Current METH users were compared to a control group of nonusers on driving simulator performance. Groups were matched for age, gender, and driving experience. Subjects were assessed for current drug use, drug dependence, and drug levels in saliva/blood as well as personality variables, sleepiness, and driving performance.

Results

METH users, most of whom met the criteria for METH dependence, were significantly more likely to speed and to weave from side to side when driving. They also left less distance between their vehicle and oncoming vehicles when making a right-hand turn. This risky driving was not associated with current blood levels of METH or its principal metabolite, amphetamine, which varied widely within the METH group. Other drugs were detected (principally low levels of THC or MDMA) in some METH users, but at levels that were unlikely to impair driving performance. There were higher levels of impulsivity and antisocial personality disorder in the METH-using cohort.

Conclusions

These findings confirm indications from epidemiological studies of an association between METH use and impaired driving ability and provide a platform for future research to further explore the factors contributing to increased accident risk in this population.     

Evidence of long-term expression of behavioral sensitization to both cocaine and ethanol in dopamine transporter knockout mice

Introduction

Locomotor sensitization, defined as the progressive and enduring enhancement of the motor stimulant effects elicited by repeated exposure to drugs of abuse, is the consequence of drug-induced cellular neuroadaptations that likely contribute to addictive behavior. Neuroadaptations within the dopaminergic system have been shown to be involved both in the induction phase and in the long-term expression phase of sensitization upon drug readministration after withdrawal.

Materials and methods

Mice lacking the dopamine transporter (DAT-KO) were used to test the effect of constitutive hyperdopaminergia on the durability of behavioral sensitization to both cocaine and ethanol. The effect of the DAT mutation was simultaneously tested on two inbred genetic backgrounds, C57Bl/6 and DBA/2, chosen for their contrasting addiction-related phenotypes, as well as on the hybrid F₁ offspring of a cross between C57Bl/6 and DBA/2 congenic strains.

Results and discussion

In spite of the absence of the DAT, mutant mice were able to develop long-term expression of sensitization to cocaine. Compared to their wild-type littermates, DAT-KO mice exhibited a markedly increased acute ethanol-evoked locomotor activity and developed stronger behavioral sensitization to ethanol during both induction and long-term expression phases. Interestingly, this increased ethanol-induced sensitization was potentiated by the DBA/2 genetic background.

Conclusion

These findings, showing that DAT deletion facilitates sensitization, suggest a cross-sensitization-like effect between genetic- and pharmacological-induced hyperdopaminergia.     

The effect of VMAT2 inhibitor GZ-793A on the reinstatement of methamphetamine-seeking in rats

Rationale

The vesicular monoamine transporter 2 (VMAT2) has been identified as a potential target for the treatment of methamphetamine (METH) abuse. GZ-793A is a potent and selective VMAT2 inhibitor that has been shown to block the primary and conditioned reinforcing effects of METH, while demonstrating no abuse liability when given alone.

Objectives

The aim of the current study was to determine if GZ-793A attenuates METH- or cue-induced reinstatement of METH-seeking after a period of extinction. The effect of acute GZ-793A on locomotor activity also was assessed.

Methods

After a period of extinction, rats were administered GZ-793A (15?mg/kg, s.c.) 15?min prior to a priming injection of METH or re-exposure to cues associated with METH infusions. GZ-793A also was administered 20?min prior to an injection of METH (0.5?mg/kg, s.c.) or saline to determine its effect on locomotor behavior.

Results

Pretreatment with GZ-793A (15?mg/kg) decreased cue-induced reinstatement, without demonstrating any response suppressive effects when administered in the absence of reinstating stimuli. GZ-793A also decreased methamphetamine-induced reinstatement; however, response suppressant effects of GZ-793A were obtained when the compound was presented alone. In this latter experiment, GZ-793A may have reduced responding for the conditioned reinforcing effects of the contingently available cues rather than having nonspecific effects on baseline responding. GZ-793A had no effect on locomotor activity when administered alone or with METH.

Conclusions

GZ-793A and related VMAT2 inhibitors may be promising leads for reducing the risk of relapse to METH use following exposure to drug-associated cues.     

Melatonin inhibits tachykinin NK₂ receptor-triggered 5-HT release from guinea pig isolated colonic mucosa

BACKGROUND AND PURPOSE

Melatonin is involved in the regulation of colonic motility, and sensation, but little is known about the influence of melatonin on 5-hydroxytryptamine (5-HT) release from colonic mucosa. A tachykinin NK₂ receptor-selective agonist, [β-Ala⁸]-neurokinin A_4-10[βAla-NKA-(4-10)] can induce 5-HT release from guinea pig colonic mucosa via NK₂ receptors on the mucosal layer. The present study was designed to determine the influence of melatonin on 5-HT release from guinea pig colonic mucosa, evoked by the NK₂ receptor agonist, βAla-NKA-(4-10).

EXPERIMENTAL APPROACH

The effect of melatonin was investigated on the outflow of 5-HT and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) from muscle layer-free mucosal preparations of guinea pig colon, using high-performance liquid chromatography with electrochemical detection.

KEY RESULTS

Melatonin caused a sustained decline in the βAla-NKA-(4-10)-evoked 5-HT outflow from the muscle layer-free mucosal preparations, but failed to affect its metabolite 5-HIAA outflow. The specific MT₃ receptor agonist, 5-methoxycarbonylamino-N-acetyltryptamine mimicked the inhibitory effect of melatonin on βAla-NKA-(4-10)-evoked 5-HT outflow. A MT₃ receptor antagonist prazosin shifted the concentration-response curve of melatonin to the right in a concentration-dependent manner and depressed the maximum effect, but neither a combined MT₁/MT₂ receptor antagonist luzindole, nor a MT₂ receptor antagonist N-pentanoyl-2-benzyltryptamine modified the concentration–response curve to melatonin.

CONCLUSIONS AND IMPLICATIONS

Melatonin inhibits NK₂ receptor-triggered 5-HT release from guinea pig colonic mucosa by acting at a MT₃ melatonin receptor located directly on the mucosal layer, without affecting 5-HT degradation processes. Possible contributions of MT₁/MT₂ melatonin receptors to the inhibitory effect of melatonin appear to be negligible. Melatonin may act as a modulator of excess 5-HT release from colonic mucosa.     

Methamphetamine and core temperature in the rat: Ambient temperature, dose, and the effect of a D2 receptor blocker

Rationale

Methamphetamine (METH) induces hyperthermia in warm and hypothermia in cool environments. Our first goal was to further study the role of ambient temperature in METH’s effect on core temperature in rats. Previously, these effects were primarily demonstrated in high doses; we extended this investigation to the low-dose range (1 mg/kg METH). Our second goal was to identify the role of the D2 receptor in METH’s effects in cool ambient temperatures.

Method

Rats received METH (saline, 1, 5, and 10 mg/kg), raclopride (saline, 0.3, 0.6, and 1.2 mg/kg), or a combination (all doses of raclopride combined with 10 mg/kg METH). Treatments occurred in ambient temperatures of 18, 24, or 30 °C.

Results and conclusions

Consistent with prior research, 5 and 10 mg/kg METH caused hyperthermia or hypothermia in a dose- and ambient temperature-dependent manner (60 min after METH). In contrast, 1 mg/kg produced similar levels of hyperthermia at all ambient temperatures. These findings suggest that a threshold METH dose exists; below this dose, METH still changes core temperature, but CNS control over temperature regulation is left intact. In our experiments regarding D2 blockade, raclopride decreased METH-induced core temperature at 30 and 24 °C (60 min after METH), consistent with previous findings. We extended these findings by demonstrating that in a cool ambient temperature (18 °C), raclopride pretreatment also lowered the core temperature response to METH. Although the D2 receptor is known to mediate hypothermia, the combination of METH and D2 blockade suggests a complex mediation of the core temperature response, perhaps involving neurotransmitter interactions.     

Cocaine self-administration behaviors in ClockΔ19 mice

Rationale

A key role has been identified for the circadian locomotor output cycles kaput (Clock) gene in the regulation of drug reward. Mice bearing a dominant negative mutation in the Clock gene (ClockΔ19 mice) exhibit increased cocaine-induced conditioned place preference, reduced anxiety- and depression-like behavior, increased sensitivity to intracranial self-stimulation, and increased dopaminergic cell activity in the ventral tegmental area.

Objectives

We sought to determine if this hyperhedonic phenotype extends to cocaine self-administration and measures of motivation.

Methods

Two separate serial testing procedures were carried out (n?=?7–10/genotype/schedule). Testing began with acquisition of sucrose pellet self-administration, implantation of intravenous catheter, acquisition of cocaine self-administration, and dose–response testing (fixed ratio or progressive ratio). To evaluate diurnal variations in acquisition behavior, these sessions occurred at Zeitgeber 2 (ZT2) or ZT14.

Results

WT and ClockΔ19 mice exhibited similar learning and readily acquired food self-administration at both ZT2 and ZT14. However, only ClockΔ19 mice acquired cocaine self-administration at ZT2. A greater percentage of ClockΔ19 mice reached acquisition criteria at ZT2 and ZT14. ClockΔ19 mice self-administered more cocaine than WT mice. Using fixed ratio and progressive ratio schedules of reinforcement dose–response paradigms, we found that cocaine is a more efficacious reinforcer in ClockΔ19 mice than in WT mice.

Conclusion

Our results demonstrate that the Clock gene plays an important role in cocaine reinforcement and that decreased CLOCK function increases vulnerability for cocaine use.