Effects of acute and long-term treatment with methamphetamine on substance P concentration and receptor numbers in the rat brain

To examine biochemical changes in brain substance P (SP) systems associated with development of behavioral sensitization induced by methamphetamine (MAP), regional substance P-like immunoreactivity (SP-LI) and SP receptor binding in the rat brain were measured after acute and long-term MAP administration. Single administration of 8 mg/kg MAP significantly reduced the striatal SP-LI concentration 1 h after the injection. This reduction was blocked by pretreatment with haloperidol. Although repeated administration of 4 mg/kg MAP for 14 consecutive days did not affect the SP-LI concentration in any brain regions including the striatum, it decreased specific SP receptor binding in the striatum and increased those in the frontal cortex. Scatchard analysis of saturation isotherm of specific SP binding revealed that the decreased specific SP binding in the striatum resulted from a decrease in the maximal number (Bmax) of SP receptors and that increased binding in the frontal cortex resulted from an increase in Bmax. These changes in SP receptor binding lasted for at least 7 days. It is emphasized that the persisting changes induced by long-term MAP administration in the SP receptor may contribute to behavioral sensitization to MAP in rats and may be associated with neuronal mechanisms in MAP psychosis.     

Effect of antioxidant N-acetyl-L-cysteine on behavioral changes and neurotoxicity in rats after administration of methamphetamine

Several lines of evidence suggest that oxidative stress may play a role in the behavioral changes and neurotoxicity in rats after administration of methamphetamine (MAP). N-acetyl-L-cysteine (NAC) is a precursor of glutathione, and it also exerts as an antioxidant. In this study, we investigated the effects of NAC on the behavioral changes (hyperlocomotion and development of sensitization) and neurotoxicity in male Wistar rats after administration of MAP. Pretreatment with NAC (30, 100 or 300 mg/kg, i.p.) attenuated significantly hyperlocomotion in rats induced by a single administration of MAP (2 mg/kg, i.p.), in a dose-dependent manner. Furthermore, pretreatment with NAC (100 mg/kg, i.p., 15 min before MAP injection, once daily for 5 consecutive days) blocked significantly the development of behavioral sensitization in rats after repeated administration of MAP (2 mg/kg, once daily for 5 consecutive days), whereas the behaviors in rats after repeated administration of NAC plus saline groups were not different from those of control (vehicle plus saline) groups. One week after administration of MAP (7.5 mg/kg x 4, 2-h intervals), levels of dopamine (DA) in rat striatum were significantly decreased as compared with control groups. Pretreatment with NAC (1, 3, 10 or 30 mg/kg, i.p., 30 min before each MAP injection) attenuated significantly the MAP-induced reduction of DA in rat striatum, in a dose-dependent manner. These results suggest that NAC could prevent the behavioral changes (acute hyperlocomotion and development of behavioral sensitization) in rats and neurotoxicity in rat striatum after administration of MAP, and that NAC would be a useful drug for treatment of several symptoms associated with MAP abuse.     

Changes in brain thyrotropin-releasing hormone (TRH) of seizure-prone El mice

We examined the anticonvulsant effects of DN-1417 an analog of the thyrotropin-releasing hormone (TRH) in seizure-prone El mice. Changes in both immunoreactive TRH (IR-TRH) and TRH receptor binding activity in discrete brain regions of El mice were also measured before and after sensitization and during the postictal period, and they were compared with those in the ddY mice. Intraperitoneal injection of DN-1417 with 150 and 450 mg/kg significantly increased the El mouse seizure threshold in a dose-dependent manner. IR-TRH in the hippocampus of El mice, which was significantly lower than in ddY mice, significantly increased after sensitization. During the postictal period, however, it slowly decreased again and then gradually recovered to the preconvulsive level without any change in TRH receptor binding. In the striatum of El mice, although TRH receptor binding was significantly higher than in ddY mice, it was not affected by sensitization. These findings indicate that the hippocampal TRH system may play an inhibitory role in El mouse seizures whereas the striatal TRH system may be important for its seizure susceptibility.     

In vivo microdialysis reveals a diminished amphetamine-induced DA response corresponding to behavioral sensitization produced by repeated amphetamine pretreatment.

In vivo microdialysis procedures were used to assess the effects of repeated amphetamine administration on behavior and regional brain DA dynamics in freely moving rats. Pretreatment with amphetamine (2.5 or 3.0 mg/kg) for 4-6 days did not alter baseline DA or its metabolites in caudate or accumbens 48 h or 6 days after the last injection. However, whereas this dosage regimen revealed a profound behavioral sensitization in response to challenge with amphetamine (2.5 mg/kg), including a more rapid onset and intensification of stereotypy, the DA response was significantly diminished in both brain regions. In addition, the ratio of caudate to accumbens DA, either before or after amphetamine challenge, was not altered by the pretreatment regimen. These results are consistent with our previous suggestion that there is a dissociation between the DA and behavioral responses to amphetamine, and therefore that other neurotransmitter systems and/or mechanisms significantly contribute to the amphetamine response profile. Furthermore, DA effects may represent only one, albeit critical, aspect in a time-dependent sequence of changes underlying stimulant sensitization.     

Methylphenidate sensitization is prevented by prefrontal cortex lesion

Methylphenidate (MPD), also known as Ritalin, is a widely used treatment for Attention Deficit Hyperactivity Disorder. Repeated administration of MPD causes dose-dependent sensitization. MPD binds to dopamine (DA) transporters, and DA, therefore, remain in the synaptic cleft for longer time, resulting in an indirect DA agonist effect. MPD affects neurotransmission in brain regions including the prefrontal cortex (PFC). The mechanisms of sensitization to MPD are not clear.

The aim of this study was to investigate the role of prefrontal cortex in effects of acute and chronic MPD administration, using the open field assay and male Sprague–Dawley rats with bilateral electrolytic lesions of PFC. After 1 day of control recording, following saline injection, the animals were divided randomly into three groups, (1) an intact control group, (2) a sham group, and (3) a lesion group. Then, groups 2 and 3 underwent surgery, followed by 5 days of recovery. Recordings were resumed following 1 day of saline injection and following six consecutive daily injections of 2.5 mg/kg MPD, 3 days of washout period, and another day of re-challenge injection of 2.5 mg/kg MPD.

Acute MPD elicited increases in locomotor activity, similar to those observed from intact animals, in both sham and lesion groups. The sham group was behaviorally sensitized while the PFC lesion group failed to exhibit behavioral sensitization.

These results suggest that the PFC does not interfere with the acute effects of MPD on locomotor activity but is required for development of behavioral sensitization to MPD.     

Subchronic cocaine treatment enhances cocaine-induced dopamine efflux, studied by in vivo intracerebral dialysis

Repeated administration of cocaine in animals results in behavioral sensitization. In order to investigate the neurochemical mechanism underlying such behavioral sensitization, we designed the following two experiments. In both experiments, rats were pretreated with cocaine (20 mg/kg i.p.) or saline, once daily for 14 consecutive days. Exp. 1: 7 days after withdrawal from the drug, the stereotyped behavioral response to a challenge of cocaine (20 mg/kg i.p.) was measured. Exp. 2: 7 days after withdrawal from the drug, we measured extracellular dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) after the challenge administration of cocaine using an in vivo intracerebral dialysis technique. The rats pretreated with cocaine (20 mg/kg i.p.) exhibited behavioral augmentation in response to a challenge of cocaine. The challenge administration of cocaine caused an increase in DA and a decrease in DOPAC. The DA level in the striatal perfusates of the cocaine-pretreated rats was significantly greater than that in the saline-pretreated rats. These results suggest that the increased extracellular DA concentration in the striatum plays an important role in the cocaine-induced behavioral sensitization.     

Lasting increase in D1 dopamine receptors in the lateral part of the substantia nigra pars reticulata after subchronic methamphetamine administration

To examine the possible involvement of D₁ dopamine receptors in behavioral sensitization induced by subchronic methamphetamine (MAP) administration, regional D₁ receptors labeled with [³H]SCH 23390 were examined using binding assay and quantitative autoradiography. Rats received 4 mg/kg/day MAP (i.p.) for 14 days, and were decapitated after an abstinence period of 24 h, 7 days or 21 days. In MAP-treated rats, a significant decrease in K_d in the mesolimbic area was observed 24 h but not 7 days after the last injection. Neither K_d nor B_max changed in the striatum or medial prefrontal cortex of MAP-treated rats after any period of abstinence. Autoradiography revealed a significant increase in specific [³H]SCH 22390 binding in the lateral part of the substantia nigra pars reticulata (SNr) of MAP-treated rats. Since this increase lasted up to 21 days after cessation of subchronic MAP administration, it is suggested that lasting increase in the nigral D₁ receptors may be associated with the biological changes underlying MAP-induced behavioral sensitization.     

Subchronic methamphetamine treatment enhances methamphetamine- or cocaine-induced dopamine efflux in vivo

Intracerebral dialysis was used to study the mechanism underlying behavioral sensitization. Rats were divided into two groups: a control group that received intraperitoneal injections of saline and an experimental group that was given methamphetamine (MAP) (4 mg/kg) once a day for 14 days. Seven days after the last injection, dopamine (DA) and its metabolites were measured in striatal dialysates obtained from awake freely moving rats. A challenge injection of MAP (4 mg/kg) caused a marked increase in the extracellular concentrations of DA, and the extent of the increase was significantly greater in MAP-pretreated rats than in the saline-pretreated controls. A challenge injection of cocaine (20 mg/kg) also caused a significantly greater increase in extracellular DA levels in MAP-pretreated rats than in saline-pretreated rats. These results suggest that an enhancement in striatal DA efflux may play an important role in MAP-induced behavioral sensitization and cross-sensitization to cocaine.     

Increased responsiveness of mesolimbic and mesostriatal dopamine neurons to cocaine following repeated administration of a selective κ-opioid receptor agonist

Previous data have shown that the repeated administration of κ-opioid receptor agonists attenuates the acute behavioral effects of cocaine. The site and mechanism by which κ-agonists interact with this psychostimulant, however, are unknown. Accordingly, the present microdialysis study characterized the effects of prior, repeated administration of the selective κ-opioid receptor agonist U69593 on basal and cocaine-evoked DA levels within the nucleus accumbens (NAC) and caudate putamen (CPU). The influence of U69593 treatment on the locomotor-activating effects of an acute cocaine challenge was also assessed. Rats received once daily injections of U69593 (0.16–0.32 mg/kg/day) or vehicle (1.0 ml/kg/day) for 3 days. The behavioral and neurochemical effects produced by an acute cocaine challenge (20 mg/kg i.p.) were assessed 2 days following treatment cessation. Administration of cocaine to control animals increased locomotor activity. This effect was attenuated in animals which had previously received U69593 (0.32 mg/kg/day × 3 days). Prior administration of U69593 failed to modify basal DA levels in either the NAC or CPU. Thus, 2 days following the cessation of U69593 treatment, dialysate DA levels did not differ from that of controls. Administration of cocaine to vehicle-treated animals increased dialysate levels of DA in both brain regions. However, in animals previously exposed to U69593 (0.32 mg/kg/day × 3 days), a significant enhancement in the response of DA neurons to cocaine was seen. These data demonstrate that prior, repeated administration of a selective κ-opioid receptor agonist attenuates the locomotor-activating effects of cocaine and increases cocaine-evoked DA overflow in terminal projection areas of mesostriatal and mesolimbic DA neurons. These findings indicate that the behavioral interactions of κ-agonists with cocaine observed in this and previous studies cannot be attributed to a presynaptic inhibition of DA release. Rather, they suggest that postsynaptic or non-DA mechanisms mediate the interaction of these agents with cocaine. Synapse 30:255–262, 1998. © 1998 Wiley-Liss, Inc.     

Long-Term Increase in Striatal Thyrotropin-Releasing Hormone Receptor Binding Caused by Amygdaloid Kindling

Our previous finding that intracerebroventricular (i.c.v.) administration of both thyrotropin-releasing hormone (TRH) and its analogue, gamma-butyrolactone-gamma-carbonyl-L-histidyl-L-prolinamide citrate (DN-1417), suppressed seizure development of amygdaloid (AM) kindling and kindled AM seizures leads to a new hypothesis that endogenous TRH may be an antiepileptic substance in the brain. In this study, we examined postictal chronological changes in both immunoreactive TRH (IR-TRH) and TRH receptor binding activity in discrete brain regions of AM-kindled rats to study the relationship of the brain TRH system to kindling-induced seizure susceptibility. AM-kindled rats were decapitated 30 min, 24 h, 48 h, 7 days, and 21 days after the last kindled convulsion. IR-TRH increased markedly in the AM/pyriform cortex and hippocampus 24 and 48 h after the last convulsion, and returned to the control (unstimulated, sham-operated) value within 3 weeks after the convulsions ended. In contrast, a significant increase in the striatal TRH binding sites was evident 24 h after the cessation of convulsions which lasted 21 days. A lasting change in the striatal TRH neural system may be related to kindling-induced seizure susceptibility.     

Protective effects of minocycline on behavioral changes and neurotoxicity in mice after administration of methamphetamine

The effects of minocycline on behavioral changes and neurotoxicity in the dopaminergic neurons induced by the administration of methamphetamine (METH) were studied. Pretreatment with minocycline (40 mg/kg) was found to attenuate hyperlocomotion in mice after a single administration of METH (3 mg/kg). The development of behavioral sensitization after repeated administration of METH (3 mg/kg/day, once daily for 5 days) was significantly attenuated by pretreatment with minocycline (40 mg/kg). A reduction in the level of dopamine (DA) and its major metabolite, 3,4-dihydroxyphenyl acetic acid (DOPAC), in the striatum after the repeated administration of METH (3 mg/kg × 3, 3-h interval) was attenuated in a dose-dependent manner by pretreatment with and the subsequent administration of minocycline (10, 20, or 40 mg/kg). Furthermore, minocycline (40 mg/kg) significantly attenuated a reduction in DA transporter (DAT)-immunoreactivity in the striatum after repeated administration of METH. In vivo microdialysis study demonstrated that pretreatment with minocycline (40 mg/kg) significantly attenuated increased extracellular DA levels in the striatum after the administration of METH (3 mg/kg). In addition, minocycline was not found to alter the concentrations of METH in the plasma or the brain after three injections of METH (3 mg/kg), suggesting that minocycline does not alter the pharmacokinetics of METH in mice. Interestingly, METH-induced neurotoxicity in the striatum was significantly attenuated by the post-treatment and subsequent administration of minocycline (40 mg/kg). These findings suggest that minocycline may be able to ameliorate behavioral changes as well as neurotoxicity in dopaminergic terminals after the administration of METH. Therefore, minocycline could be considered as a useful drug for the treatment of several symptoms associated with METH abuse in humans.     

Effects of repeated injections of cocaine on catecholamine receptor binding sites, dopamine transporter binding sites and behavior in rhesus monkey.

In order to determine if repeated injections of cocaine produced long-lasting alterations in catecholaminergic binding sites, rhesus monkeys were treated with saline (1.0 ml/15 kg) or cocaine (3.0-4.0 mg/kg) four times daily for 14 consecutive days and sacrificed two weeks after the last injection. The densities of dopamine D1 receptor binding sites, dopamine transporter binding sites and beta adrenergic receptor binding sites were significantly decreased in caudate nucleus to 51%, 17% and 61% of control, respectively, two weeks after repeated cocaine injections. There were no differences in D2 receptor binding site densities in the caudate, nor were there differences in binding sites between groups in the other brain regions examined: prefrontal cortex (D1, D2, dopamine transporter, beta), nucleus accumbens (D1, D2, dopamine transporter) and substantia nigra (D2). Behavioral observation showed that the cocaine-treated monkeys became sensitized to the repeated injections. Early in the regimen, these animals displayed stereotypic grooming, buccal movements and visual checking after each injection that differed significantly from the saline-treated animals. As the regimen progressed, the frequency of grooming decreased while the frequencies of visual tracking and splayed legs increased in a manner consistent with the development of behavioral sensitization. Together, these findings suggest that the caudate nucleus may be more sensitive than other dopamine-containing brain regions to long-lasting pre- and post-synaptic effects of repeated cocaine administration, and that the changes seen in dopaminergic neurons may be related to behavioral sensitization.     

Effect of thyrotropin-releasing hormone on the locomotor and sensitizing effects of cocaine in rats

The present study was designed to find out whether single and repeated treatment with thyrotropin-releasing hormone (TRH) changed the cocaine-evoked hyperactivation or sensitization, and whether cross-sensitization occurred between TRH and cocaine. Like cocaine (10 mg/kg), TRH (10 mg/kg) increased the basal activation of rats; however, when given in combination with cocaine (10 mg/kg), TRH (5-10 mg/kg) did not change the locomotor effect of cocaine. On day 10, cocaine challenge of rats treated repeatedly with the psychostimulant (days 1-5) significantly enhanced locomotor hyperactivity compared to the effect of acute cocaine injection in saline-treated (days 1-5) animals. When co-administered with cocaine for 5 days during the development of sensitization, TRH (10 mg/kg) enhanced the effect of the challenge dose of cocaine (10 mg/kg) following a 5-day withdrawal. Given acutely with cocaine on day 10 to cocaine-treated animals, TRH (5-10 mg/kg) did not change the expression of cocaine sensitization. Significant enhancement of the locomotor response to TRH (10 mg/kg) challenge was observed in animals treated repeatedly with TRH. The response to TRH (5-10 mg/kg) was stronger in repeated cocaine-treated rats than in saline-injected ones; similarly, the response to cocaine (10 mg/kg) was more potent in TRH-treated animals compared to saline-injected ones (cross-sensitization). In conclusion, our results indicate that exposure to TRH induces sensitization to its locomotor hyperactivity effect. They also show that TRH enhances the development of cocaine sensitization, but affects neither the expression phase of the phenomenon nor the locomotor hyperactivity induced by a single dose of cocaine. Moreover, cross-sensitization between cocaine and TRH has also been demonstrated. These findings also may provide an insight into the relationship between TRH and cocaine in humans exposed to the psychostimulant.     

Loss of nicotine-induced behavioral sensitization in micro-opioid receptor knockout mice

Repeated administration of nicotine produces behavioral sensitization. However, the possible mechanism of behavioral sensitization to nicotine remains unclear. The present study was undertaken in micro-opioid receptor knockout mice, to examine the hypothesis that micro-opioid receptors play a crucial role in behavioral sensitization to nicotine. All mice received saline or nicotine (0.05 mg/kg, s.c) twice a day for 7 consecutive days. The mice remained drug free for 3 days and on day 11 each group was challenged with saline or nicotine (0.05 mg/kg, s.c.). On day 1, it was observed that the single injection of nicotine (0.05 mg/kg, s.c.) did not influence locomotor activity in either micro-opioid receptor knockout or in wildtype mice. On day 7 (24 h after mice had been treated twice daily for 6 consecutive days with an injection of 0.05 mg/kg of nicotine), the mice were challenged with a single injection of nicotine, which produced behavioral sensitization in the wildtype but not in micro-opioid receptor knockout mice. On day 11, following 3 days of withdrawal after the second injection of nicotine on day 7, nicotine-treated mice were challenged with a single injection of nicotine and showed the behavioral sensitization of wildtype. However, nicotine challenge did not induce behavioral sensitization in micro-opioid receptor knockout mice. Our data indicate that a lack of micro-opioid receptors can inhibit the effects of nicotine-induced behavioral sensitization. This result strongly suggests that the micro-opioid receptor plays an important role in behavioral sensitization to nicotine.     

Attenuation of methamphetamine-induced behavioral sensitization in mice by systemic administration of naltrexone

Repeated intermittent exposure to psychostimulants was found to produce behavioral sensitization. The present study was designed to establish a mouse model and by which to investigate whether opioidergic system plays a role in methamphetamine-induced behavioral sensitization. Mice injected with 2.5 mg/kg of methamphetamine once a day for 7 consecutive days showed behavioral sensitization after challenge with 0.3125 mg/kg of the drug on day 11, whereas mice injected with a lower daily dose (1.25 mg/kg) did not. Mice received daily injections with either 1.25 or 2.5 mg/kg of methamphetamine showed behavioral sensitization after challenge with 1.25 mg/kg of the drug on days 11, 21, and 28. To investigate the role of opioidergic system in the induction and expression of behavioral sensitization, long-acting but non-selective opioid antagonist naltrexone was administrated prior to the daily injections of and challenge with methamphetamine, respectively. Our results show that the expressions of behavioral sensitization were attenuated by pretreatment with 10 or 20 mg/kg of naltrexone either during the induction period or before methamphetamine challenge when they were tested on days 11 and 21. These results indicate that repeated injection with methamphetamine dose-dependently induced behavioral sensitization in mice, and suggest the involvement of opioid receptors in the induction and expression of methamphetamine-induced behavioral sensitization.     

Behavioral Hypersensitivity to Apomorphine after Chronic Methamphetamine–Intermittent vs. Continuous Regimen

Abstract: The effects of chronic methamphetamine (MAP) on the behavioral response to apomorphine were examined in rats, treated with saline (control) or MAP for 14 days either intermittently (4 mg/kg/day, once daily) or continuously (subcutaneously implanted osmotic pumps releasing 56mg/kg/14 days). Following a 7-day interval after discontinuing the MAP treatment, a challenge dose (0.5 mg/kg) of apomorphine was administered. The rats treated with MAP intermittently showed a marked augmentation in behavioral response to apomorphine than the continuous MAP group and control. It is presumed that the intermittent MAP administration may produce greater reactivity in the brain dopaminergic systems than the continuous administration.     

Behavioral hypersensitivity to apomorphine after chronic methamphetamine--intermittent vs. continuous regimen

The effects of chronic methamphetamine (MAP) on the behavioral response to apomorphine were examined in rats, treated with saline (control) or MAP for 14 days either intermittently (4 mg/kg/day, once daily) or continuously (subcutaneously implanted osmotic pumps releasing 56 mg/kg/14 days). Following a 7-day interval after discontinuing the MAP treatment, a challenge dose (0.5 mg/kg) of apomorphine was administered. The rats treated with MAP intermittently showed a marked augmentation in behavioral response to apomorphine than the continuous MAP group and control. It is presumed that the intermittent MAP administration may produce greater reactivity in the brain dopaminergic systems than the continuous administration.     

"Nonhedonic" food motivation in humans involves dopamine in the dorsal striatum and methylphenidate amplifies this effect

The effects of a single and repeated administration of cocaine on GABA(B) receptor subunit mRNA was investigated in rat brain by in situ hybridization. Following a single administration of cocaine, no significant change was observed in any brain regions examined, neither 1 h nor 24 h after administration. During repeated administration of cocaine, behavioral sensitization with increased stereotyped behavior was observed. A significant increase in the level of GABA(B(1)) mRNA was observed in the nucleus accumbens (11.4%), CA1 field of the hippocampus (16.8%), and thalamus (16.5%) 1 day after repeated administrations of cocaine for 14 consecutive days. The level of mRNA returned to the basal level 1 week after the final injection of repeated cocaine treatment. The observed changes in the mRNA level after the repeated cocaine may imply changes of GABA(B(1)) subunit in molecular mechanisms which underlie development of behavioral sensitization.     

Enhancement of dopamine release from striatal slices of rats that were subchronically treated with methamphetamine

The effect of dopamine (DA) uptake inhibitors (methamphetamine, nomifensine, and phenylethylamine) on the release of endogenous DA from striatal slices of rats pretreated with methamphetamine (6 mg/kg/day for 9 days) was investigated. The exposure of methamphetamine-pretreated rat striatal slices to a low concentration (10(-7) M, 5 X 10(-7) M) of methamphetamine caused a greater increase in DA efflux than that of saline-treated rat striatal slices. The drug-treated rats displayed an enhanced stereotyped behavioral response to a small dose of methamphetamine (1 mg/kg). Removal of Ca2+ from the superfusion medium did not affect the difference in the rates of methamphetamine (10(-7) M) induced DA release between methamphetamine-treated and saline-treated rat striatal slices. Nomifensine- and phenylethylamine-induced DA release from striatal slices was also enhanced by repeated administration of methamphetamine. On the other hand, there was no difference in K+-induced DA release between the two groups. Moreover, repeated administration of methamphetamine caused a significant increase in 3H-dopamine uptake in rat striatal synaptosomes. These results suggest that the behavioral sensitization produced by the repeated administration of methamphetamine is accompanied by an enhancement in the release of DA induced by methamphetamine, nomifensine, and phenylethylamine in vitro and is also accompanied by increased DA uptake into striatal synaptosomes.