Single and binge methamphetamine administrations have different effects on the levels of dopamine D2 autoreceptor and dopamine transporter in rat striatum

Transient receptor potential ankyrin 1 channel modulates the abuse-related mechanisms of methamphetamine through interaction with dopamine transporter

BACKGROUND AND PURPOSE

Methamphetamine (METH) use disorder has risen dramatically over the past decade, and there are currently no FDA-approved medications due, in part, to gaps in our understanding of the pharmacological mechanisms related to METH action in the brain.

EXPERIMENTAL APPROACH

Here, we investigated whether transient receptor potential ankyrin 1 (TRPA1) mediates each of several METH abuse-related behaviours in rodents: self-administration, drug-primed reinstatement, acquisition of conditioned place preference, and hyperlocomotion. Additionally, METH-induced molecular (i.e., neurotransmitter and protein) changes in the brain were compared between wild-type and TRPA1 knock-out mice. Finally, the relationship between TRPA1 and the dopamine transporter was investigated through immunoprecipitation and dopamine reuptake assays.

KEY RESULTS

TRPA1 antagonism blunted METH self-administration and drug-primed reinstatement of METH-seeking behaviour. Further, development of METH-induced conditioned place preference and hyperlocomotion were inhibited by TRPA1 antagonist treatment, effects that were not observed in TRPA1 knock-out mice. Similarly, molecular studies revealed METH-induced increases in dopamine levels and expression of dopamine system-related proteins in wild-type, but not in TRPA1 knock-out mice. Furthermore, pharmacological blockade of TRPA1 receptors reduced the interaction between TRPA1 and the dopamine transporter, thereby increasing dopamine reuptake activity by the transporter.

CONCLUSION AND IMPLICATIONS

This study demonstrates that TRPA1 is involved in the abuse-related behavioural effects of METH, potentially through its modulatory role in METH-induced activation of dopaminergic neurotransmission. Taken together, these data suggest that TRPA1 may be a novel therapeutic target for treating METH use disorder.

The influence of dopamine autoreceptors on temperament and addiction risk

As a major regulator of dopamine (DA), DA autoreceptors (DAARs) exert substantial influence over DA-mediated behaviors. This paper reviews the physiological and behavioral impact of DAARs. Individual differences in DAAR functioning influences temperamental traits such as novelty responsivity and impulsivity, both of which are associated with vulnerability to addictive behavior in animal models and a broad array of externalizing behaviors in humans. DAARs additionally impact the response to psychostimulants and other drugs of abuse. Human PET studies of D2-like receptors in the midbrain provide evidence for parallels to the animal literature. These data lead to the proposal that weak DAAR regulation is a risk factor for addiction and externalizing problems. The review highlights the potential to build translational models of the functional role of DAARs in behavior. It also draws attention to key limitations in the current literature that would need to be addressed to further advance a weak DAAR regulation model of addiction and externalizing risk.

Exploring new histone deacetylase 6 inhibitors and their effects on reversing the α-tubulin deacetylation and cell morphology changes caused by methamphetamine

Indazole-based HDAC6 inhibitors with novel zinc-binding modifications were synthesized and evaluated to determine their potential to inhibit HDAC6. The analogs were subjected to a histone deacetylase (HDAC) enzyme assay, which led to identification of compounds 3a and 3b. Both compounds demonstrated higher potency and selectivity as HDAC6 inhibitors with IC50 values of 9.1 nM and 9.0 nM, respectively, and highlighted the importance of the hydroxamic acid moiety for binding to Zn2+ inside the catalytic pocket of HDAC enzymes. In the neuroblastoma SH-SY5Y cell line, both compounds efficiently acetylated α-tubulin but not histone H3 at a low concentration of 0.5 µM. Moreover, compounds 3a and 3b effectively reversed the deacetylation of α-tubulin caused by methamphetamine in the SH-SY5Y cell line, suggesting the potential usefulness of HDAC6 selective inhibition in restoring blood brain barrier integrity by reversing methamphetamine-induced deacetylation.

Methamphetamine-induced region-specific transcriptomic and epigenetic changes in the brain of male rats

Psychostimulant methamphetamine (METH) is neurotoxic to the brain and, therefore, its misuse leads to neurological and psychiatric disorders. The gene regulatory network (GRN) response to neurotoxic METH binge remains unclear in most brain regions. Here we examined the effects of binge METH on the GRN in the nucleus accumbens, dentate gyrus, Ammon's horn, and subventricular zone in male rats. At 24 h after METH, ~16% of genes displayed altered expression and over a quarter of previously open chromatin regions - parts of the genome where genes are typically active - showed shifts in their accessibility. Intriguingly, most changes were unique to each area studied, and independent regulation between transcriptome and chromatin accessibility was observed. Unexpectedly, METH differentially impacted gene activity and chromatin accessibility within the dentate gyrus and Ammon's horn. Around 70% of the affected chromatin-accessible regions in the rat brain have conserved DNA sequences in the human genome. These regions frequently act as enhancers, ramping up the activity of nearby genes, and contain mutations linked to various neurological conditions. By sketching out the gene regulatory networks associated with binge METH in specific brain regions, our study offers fresh insights into how METH can trigger profound, region-specific molecular shifts.

Assessing the Location, Relative Expression and Subclass of Dopamine Receptors in the Cerebellum of Hemi-Parkinsonian Rats

Parkinson's Disease (PD) is a neurodegenerative disease with loss of dopaminergic neurons in the nigrostriatal pathway resulting in basal ganglia (BG) dysfunction. This is largely why much of the preclinical and clinical research has focused on pathophysiological changes in these brain areas in PD. The cerebellum is another motor area of the brain. Yet, if and how this brain area responds to PD therapy and contributes to maintaining motor function fidelity in the face of diminished BG function remains largely unanswered. Limited research suggests that dopaminergic signaling exists in the cerebellum with functional dopamine receptors, tyrosine hydroxylase (TH) and dopamine transporters (DATs); however, much of this information is largely derived from healthy animals and humans. Here, we identified the location and relative expression of dopamine 1 receptors (D1R) and dopamine 2 receptors (D2R) in the cerebellum of a hemi-parkinsonian male rat model of PD. D1R expression was higher in PD animals compared to sham animals in both hemispheres in the purkinje cell layer (PCL) and granule cell layer (GCL) of the cerebellar cortex. Interestingly, D2R expression was higher in PD animals than sham animals mostly in the posterior lobe of the PCL, but no discernible pattern of D2R expression was seen in the GCL between PD and sham animals. To our knowledge, we are the first to report these findings, which may lay the foundation for further interrogation of the role of the cerebellum in PD therapy and/or pathophysiology.

Sexual satiety modifies methamphetamine-induced locomotor and rewarding effects and dopamine-related protein levels in the striatum of male rats

RATIONALE

Drug and natural rewarding stimuli activate the mesolimbic dopaminergic system. Both methamphetamine (Meth) and copulation to satiety importantly increase dopamine (DA) release in the nucleus accumbens (NAc), but with differences in magnitude. This paper analyzes the interaction between Meth administration and the intense sexual activity associated with sexual satiety.

OBJECTIVES

To evaluate possible changes in Meth-induced behavioral effects and striatal DA-related protein expression due to sexual satiety.

METHODS

Meth-induced locomotor activity and conditioned place preference (CPP) were tested in sexually experienced male rats that copulated to satiety (S-S) or ejaculated once (1E) the day before or displayed no sexual activity (control group; C). DA receptors and DA transporter expression were determined by western blot in the striatum of animals of all sexual conditions treated with specific Meth doses.

RESULTS

Meth's locomotor and rewarding effects were exacerbated in S-S animals, while in 1E rats, only locomotor effects were enhanced. Sexual activity, by itself, modified DA-related protein expression in the NAc core and in the caudate-putamen (CPu), while Meth treatment alone changed their expression only in the NAc shell. Meth-induced changes in the NAc shell turned in the opposite direction when animals had sexual activity, and additional changes appeared in the NAc core and CPu of S-S rats.

CONCLUSION

Sexual satiety sensitizes rats to Meth's behavioral effects and the Meth-induced striatal DA-related protein adaptations are modified by sexual activity, evidencing cross-sensitization between both stimuli.

Molecular mechanisms of programmed cell death in methamphetamine-induced neuronal damage

Methamphetamine, commonly referred to as METH, is a highly addictive psychostimulant and one of the most commonly misused drugs on the planet. Using METH continuously can increase your risk for drug addiction, along with other health complications like attention deficit disorder, memory loss, and cognitive decline. Neurotoxicity caused by METH is thought to play a significant role in the onset of these neurological complications. The molecular mechanisms responsible for METH-caused neuronal damage are discussed in this review. According to our analysis, METH is closely associated with programmed cell death (PCD) in the process that causes neuronal impairment, such as apoptosis, autophagy, necroptosis, pyroptosis, and ferroptosis. In reviewing this article, some insights are gained into how METH addiction is accompanied by cell death and may help to identify potential therapeutic targets for the neurological impairment caused by METH abuse.

The Role of HSP90α in Methamphetamine/Hyperthermia-Induced Necroptosis in Rat Striatal Neurons

Methamphetamine (METH) is one of the most widely abused synthetic drugs in the world. The users generally present hyperthermia (HT) and psychiatric symptoms. However, the mechanisms involved in METH/HT-induced neurotoxicity remain elusive. Here, we investigated the role of heat shock protein 90 alpha (HSP90α) in METH/HT (39.5°C)-induced necroptosis in rat striatal neurons and an in vivo rat model. METH treatment increased core body temperature and up-regulated LDH activity and the molecular expression of canonical necroptotic factors in the striatum of rats. METH and HT can induce necroptosis in primary cultures of striatal neurons. The expression of HSP90α increased following METH/HT injuries. The specific inhibitor of HSP90α, geldanamycin (GA), and HSP90α shRNA attenuated the METH/HT-induced upregulation of receptor-interacting protein 3 (RIP3), phosphorylated RIP3, mixed lineage kinase domain-like protein (MLKL), and phosphorylated MLKL. The inhibition of HSP90α protected the primary cultures of striatal neurons from METH/HT-induced necroptosis. In conclusion, HSP90α plays an important role in METH/HT-induced neuronal necroptosis and the HSP90α-RIP3 pathway is a promising therapeutic target for METH/HT-induced neurotoxicity in the striatum.

New designer phenethylamines 2C-C and 2C-P have abuse potential and induce neurotoxicity in rodents

2C (2C-x) is the general name for the family of phenethylamines containing two methoxy groups at the 2 and 5 positions of the benzene ring. The abuse of 2C family drugs has grown rapidly, although the abuse potential and neurotoxic properties of 2C drugs have not yet been fully investigated. In this study, we investigated the abuse potential and neurotoxicity of 4-chloro-2,5-dimethoxyphenethylamine (2C-C) and 2,5-dimethoxy-4-propylphenethylamine (2C-P). We found that 2C-C and 2C-P produced conditioned place preference in a dose-dependent manner in mice, and increased self-administration in rats, suggesting that 2C-C and 2C-P have abuse potential. To investigate the neurotoxicity of 2C-C and 2C-P, we examined motor performance and memory impairment after high doses of 2C-C and 2C-P. High doses of 2C-C and 2C-P decreased locomotor activity, rota-rod performance, and lower Y-maze test, novel objective recognition test, and passive avoidance test scores. We also observed that 2C-C and 2C-P affected expression levels of the D1 dopamine receptor, D2 dopamine receptor, dopamine transporter, and phospho-dopamine transporter in the nucleus accumbens and the medial prefrontal cortex, and increased c-Fos immuno-positive cells in the nucleus accumbens. Moreover, high doses of 2C-C and 2C-P induced microglial activation, which is involved in the inflammatory reaction in the striatum. These results suggest that 2C-C and 2C-P have abuse potential by affecting dopaminergic signaling and induce neurotoxicity via initiating neuroinflammation at high doses.

Effects of chronic methamphetamine abuse on the retinal nerve fiber layer, ganglion cell layer and Bruch's membrane opening minimum rim width

BACKGROUND

Methamphetamine (Meth) is a highly addictive and hallucinogenic agent which is used as the second most common illicit drug globally. Meth could affect the retina and optic nerve by inducing the release of vasoconstrictive agents such as endothelin 1 and induction of severe oxidative stress with accumulation of reactive oxygen species.

AIM

To evaluate the effects of chronic Meth abuse on the retinal nerve fiber layer (RNFL), ganglion cell layer (GCL) and the Bruch's membrane opening minimum rim width (MRW).

METHOD

In this case-control study, we recruited 55 Meth abusers and 49 healthy individuals with mean age of 44.63 ± 0.97 and 43.08 ± 0.91 years, respectively. RNFL thickness, GCL thickness and MRW were evaluated using optical coherence tomography.

RESULTS

We found statistically significant decrease in RNFL, MRW thickness in Meth abusers (P: 0.002 and P: 0.006, respectively). We did not detect statistically significant difference regarding GCL thickness between the groups (P = 0.320). Our results showed a weak but statistically significant correlation of Meth dose increment and decrement of RNFL thickness ((P: 0.005, r = -0.193) and MRW (P: 0.013, r = -0.174). We found no correlation between duration of Meth consumption with RNFL and MRW thickness (P: 0.205, r= -0.124; P: 0.771, r= -0.029, respectively).

CONCLUSION

We found a statistically significant adverse association in meth abusers with RNFL thickness and MRW. These two parameters were also statistically associated with the meth dose as measured by daily dose of Meth. Although we found a decrease in the GCL thickness, it did not reach statistical significance.

Development of Thiazolidinedione-Based HDAC6 Inhibitors to Overcome Methamphetamine Addiction

Thiazolidinedione is a five-membered heterocycle that is widely used in drug discovery endeavors. In this study, we report the design, synthesis, and biological evaluation of a series of thiazolidinedione-based HDAC6 inhibitors. In particular, compound 6b exerts an excellent inhibitory activity against HDAC6 with an IC₅₀ value of 21 nM, displaying a good HDAC6 selectivity over HDAC1. Compound 6b dose-dependently induces the acetylation level of α-tubulin via inhibition of HDAC6 in human neuroblastoma SH-SY5Y cell line. Moreover, compound 6b efficiently reverses methamphetamine-induced morphology changes of SH-SY5Y cells via regulating acetylation landscape of α-tubulin. Collectively, compound 6b represents a novel HDAC6-isoform selective inhibitor and demonstrates promising therapeutic potential for the treatment of methamphetamine addiction.

Long-Term Systemic Treatment With Methamphetamine Causes Retinal Damage in CD1 Mice

As a powerful psychostimulant with high potential for abuse, methamphetamine (Meth) could cause long-lasting abnormalities in retinas. The purpose of this study was to investigate the effects of systemic administration of Meth at low dose on retinal damage and understand the underlying mechanisms of pathology. CD1 mice were treated with 0.5 mg/kg or 1 mg/kg Meth by intraperitoneal injection daily for 2 months, mice treated with saline were used as negative control. Electroretinography (ERG) reflects the mass response of photoreceptor cells and was used to test the outer retinal function after Meth treatment. Toluidine blue staining was used to show the retinal morphology and evaluate the photoreceptor cell loss. Inflammatory factors were measured by enzyme-linked immunosorbent assay to show the inflammatory response. Terminal deoxynucleotidyl transferase dUTP Nick end labeling assay was used to detect the apoptosis-positive cells. Real-time polymerase chain reaction and Western blot were applied to measure the gene and protein change to explore the underlying mechanisms. Results demonstrated that retinal damage was caused by Meth treatment after 2 months, evidenced by loss of rod photoreceptor cells; decreased ERG amplitude; increased apoptotic photoreceptor cells, cytochrome-c release, caspase-3 activity, caspase-9 activity, and apoptosis-related protein expression; increased malondialdehyde level as well as nicotinamide adenine dinucleotide phosphate oxidase 4 protein expression; decreased anti-oxidative agents glutathione as well as superoxide dismutase levels; and increased production and gene expression of inflammatory factors. Our study indicated that systemic administration of Meth caused neurotoxic effects on CD1 mouse retinas, providing the potential mechanisms for the retina damage caused by Meth abuse.

Development and psychometric properties of the methamphetamine decisional balance scale (METH-DBS) for young adults

BACKGROUND

Drug misuse is a major problem that has an extreme negative effect on people's health. Methamphetamine (MA) is frequently used by young adults, despite its harmful consequences. The Transtheoretical Model (TTM) has been known to be very effective in explaining both the achievement and cessation of several health-related behaviors. Therefore, in this study, the TTM was used toward the domain of immoderate MA use among young adults. This study aimed to test the validity and reliability of a decisional balance scale for MA use in young adults.

METHODS

A multi-phase scale development approach was used to develop the scale. First, 41 university students enrolled in a qualitative study that generated content for a primary set of a 40-item instrument. In order to produce a pre-final version of the instrument, face and content validity were calculated in the next step. The instrument validation was assessed with a sample of 250 university students. Then, the construct validity (exploratory and confirmatory), convergent validity, discriminate validity, internal consistency applying test-retest reliability, and Cronbach's alpha of the scale were measured.

RESULTS

Forty items were initially generated from the qualitative data. After content validity, this amount was reduced to 25 items. The exploratory factor analysis revealed four factors (self and other cons, coping and social pros) containing 21 items that jointly accounted for 55.24% of the observed variance. The confirmatory factor analysis indicated a model with appropriate fitness for the data. Cronbach's alpha coefficient for the dimensions ranged from .74 to .87, and the Intraclass Correlation Coefficient (ICC) ranged from .83 to .91, which is within acceptable ranges.

CONCLUSION

The findings showed that the Methamphetamine Decisional Balance Scale is a valid and reliable scale that increases our ability to study motivational factors related to MA use among young adult. Consequently, the instrument could be applied in both practice and future studies.

Characterization of Dopaminergic System in the Striatum of Young Adult Park2-/- Knockout Rats

Mutations in parkin gene (Park2) are linked to early-onset autosomal recessive Parkinson's disease (PD) and young-onset sporadic PD. Park2 knockout (PKO) rodents; however, do not display neurodegeneration of the nigrostriatal pathway, suggesting age-dependent compensatory changes. Our goal was to examine dopaminergic (DAergic) system in the striatum of 2 month-old PKO rats in order to characterize compensatory mechanisms that may have occurred within the system. The striata form wild type (WT) and PKO Long Evans male rats were assessed for the levels of DAergic markers, for monoamine oxidase (MAO) A and B activities and levels, and for the levels of their respective preferred substrates, serotonin (5-HT) and ß-phenylethylamine (ß-PEA). The PKO rats displayed lower activities of MAOs and higher levels of ß-PEA in the striatum than their WT counterparts. Decreased levels of ß-PEA receptor, trace amine-associated receptor 1 (TAAR-1), and postsynaptic DA D2 (D2L) receptor accompanied these alterations. Drug-naive PKO rats displayed normal locomotor activity; however, they displayed decreased locomotor response to a low dose of psychostimulant methamphetamine, suggesting altered DAergic neurotransmission in the striatum when challenged with an indirect agonist. Altogether, our findings suggest that 2 month-old PKO male rats have altered DAergic and trace aminergic signaling.

Phentermine induces conditioned rewarding effects via activation of the PI3K/Akt signaling pathway in the nucleus accumbens

RATIONALE

Phentermine is structurally similar to methamphetamine and is widely used as an anti-obesity drug in the USA and many other countries. The potential for reward of phentermine has been noted; however, the mechanisms of phentermine dependence have not been established.

OBJECTIVES

Here, we investigated the rewarding and dopaminergic behavioral responses to phentermine in mice and found that phentermine produced conditioned rewarding effects through the phosphoinositide 3-kinase (PI3K)/Akt signaling pathway in the nucleus accumbens (NAc).

METHODS

The impact of phentermine was assessed using conditioned place preference (CPP) test, climbing behavior test, and western blot analysis.

RESULTS

Phentermine 1 and 3 mg/kg (i.p.) significantly increased CPP. Phentermine, a known dopamine releaser, boosted apomorphine-induced climbing behavior in mice, and methamphetamine (i.p.) also increased apomorphine-induced dopaminergic behavior. Phentermine and methamphetamine increased the level of expression of the dopamine transporter (DAT) and phospho-Akt proteins to a similar degree in the NAc of CPP mice. To determine whether the conditioned rewarding effects of phentermine were mediated through the PI3K/Akt pathway, we assessed the effects of the Akt inhibitor LY294002 on phentermine-induced place preference and climbing behavior. LY294002 (1 and 3 μg/site, i.c.v.) reduced phentermine-induced CPP and phentermine-increased climbing behavior. However, LY294002 did not change CPP and climbing behavior itself and also did not decrease apomorphine-induced climbing behavior in mice. Further, LY294002 decreased the phentermine-increased levels of DAT protein and phosphorylation of Akt in the NAc of CPP mice.

CONCLUSIONS

Thus, these findings suggest that phentermine induces conditioned rewarding effects via activation of the PI3K/Akt signaling pathway in the NAc.

Sex-Dependent Changes in Striatal Dopamine Transport in Preadolescent Rats Exposed Prenatally and/or Postnatally to Methamphetamine

Methamphetamine (MA) is the most commonly used psychostimulant drug, the chronic abuse of which leads to neurodegenerative changes in the brain. The global use of MA is increasing, including in pregnant women. Since MA can cross both placental and haematoencephalic barriers and is also present in maternal milk, children of chronically abused mothers are exposed prenatally as well as postnatally. Women seem to be more vulnerable to some aspects of MA abuse than men. MA is thought to exert its effects among others via direct interactions with dopamine transporters (DATs) in the brain tissue. Sexual dimorphism of the DAT system could be a base of sex-dependent actions of MA observed in behavioural and neurochemical studies. Possible sex differences in the DATs of preadolescent offspring exposed to MA prenatally and/or postnatally have not yet been evaluated. We examined the striatal synaptosomal DATs (the activity and density of surface expressed DATs and total DAT expression) in preadolescent male and female Wistar rats (31-35-day old animals) exposed prenatally and/or postnatally to MA (daily 5 mg/kg, s.c. to mothers during pregnancy and lactation). To distinguish between specific and nonspecific effects of MA on DATs, we also evaluated the in vitro effects of lipophilic MA on the fluidity of striatal membranes isolated from preadolescent and young adult rats of both sexes. We observed similar changes in the DATs of preadolescent rats exposed prenatally or postnatally (MA-mediated drop in the reserve pool but no alterations in surface-expressed DATs). However, prenatal exposure evoked significant changes in males and postnatal exposure in females. A significant decrease in the activity of surface-expressed DATs was found only in postnatally exposed females sensitized to MA via prenatal exposure. MA applied in vitro increased the fluidity of striatal membranes of preadolescent female but not male rats. In summary, DATs of preadolescent males are more sensitive to prenatal MA exposure via changes in the reserve pool and those of preadolescent females to postnatal MA exposure via the same mechanism. The combination of prenatal and postnatal MA exposure increases the risk of dopaminergic deficits via alterations in the activity of surface-expressed DATs especially in preadolescent females. MA-mediated changes in DATs of preadolescent females could be still enhanced via nonspecific disordering actions of MA on striatal membranes.

An acute, epitope-specific modification in the dopamine transporter associated with methamphetamine-induced neurotoxicity

Preclinical studies demonstrate that repeated, high-dose methamphetamine administrations rapidly decrease plasmalemmal dopamine uptake, which may contribute to aberrant dopamine accumulation, reactive species generation, and long-term dopaminergic deficits. The present study extends these findings by demonstrating a heretofore unreported, epitope-specific modification in the dopamine transporter caused by a methamphetamine regimen that induces these deficits. Specifically, repeated, high-dose methamphetamine injections (4 × 10 mg/kg/injection, 2-h intervals) rapidly decreased immunohistochemical detection of striatal dopamine transporter as assessed 1 h after the final methamphetamine exposure. In contrast, neither a single high dose (1 × 10 mg/kg) nor repeated injections of a lower dose (4 × 2 mg/kg/injection) induced this change. The high-dose regimen-induced alteration was only detected using antibodies directed against the N-terminus. Immunohistochemical staining using antibodies directed against the C-terminus did not reveal any changes. The high-dose regimen also did not alter dopamine transporter expression as assessed using [(125) I]RTI-55 autoradiography. These data suggest that the repeated, high-dose methamphetamine regimen alters the N-terminus of the dopamine transporter. Further, these data may be predictive of persistent dopamine deficits caused by the stimulant. Future studies of the signaling cascades involved should provide novel insight into potential mechanisms underlying the physiological and pathophysiological regulation of the dopamine transporter.

Evidence of D2 receptor expression in the nucleus incertus of the rat

The nucleus incertus (NI), located in the caudal brainstem, mainly consists of GABAergic neurons with widespread projections across the brain. It is the chief source of relaxin-3 in the mammalian brain and densely expresses corticotropin-releasing factor type 1 (CRF1) receptors. Several other neurotransmitters, peptides and receptors are reportedly expressed in the NI. In the present investigation, we show the expression of dopamine type-2 (D2) receptors in the NI by reverse transcriptase-polymerase chain reaction (RT-PCR), western blotting (WB) and immunofluorescence (IF). RT-PCR did not show expression of D3 receptors. D2 receptor short isoform (D2S)-like, relaxin-3, CRF1/2 receptor and NeuN immunoreactivity were co-expressed in the cells of the NI. Behavioural effects of D2 receptor activation by intra-NI infusion of quinpirole (a D2/D3 agonist) were evaluated. Hypolocomotion was observed in home cage monitoring system (LABORAS) and novel environment-induced suppression of feeding behavioural paradigms. Thus the D2 receptors expressed in the NI are likely to play a role in locomotion. Based on its strong bidirectional connections to the median raphe and interpeduncular nuclei, the NI was predicted to play a role in modulating behavioural activity and the present results lend support to this hypothesis. This is the first evidence of expression of a catecholamine receptor, D2-like immunoreactivity, in the NI.