The role of neuropeptide FF (NPFF) in the expression of sensitization to hyperlocomotor effect of morphine and ethanol

Brain RFamide Neuropeptides in Stress-Related Psychopathologies

The RFamide peptide family is a group of proteins that share a common C-terminal arginine-phenylalanine-amide motif. To date, the family comprises five groups in mammals: neuropeptide FF, LPXRFamides/RFamide-related peptides, prolactin releasing peptide, QRFP, and kisspeptins. Different RFamide peptides have their own cognate receptors and are produced by different cell populations, although they all can also bind to neuropeptide FF receptors with different affinities. RFamide peptides function in the brain as neuropeptides regulating key aspects of homeostasis such as energy balance, reproduction, and cardiovascular function. Furthermore, they are involved in the organization of the stress response including modulation of pain. Considering the interaction between stress and various parameters of homeostasis, the role of RFamide peptides may be critical in the development of stress-related neuropathologies. This review will therefore focus on the role of RFamide peptides as possible key hubs in stress and stress-related psychopathologies. The neurotransmitter coexpression profile of RFamide-producing cells is also discussed, highlighting its potential functional significance. The development of novel pharmaceutical agents for the treatment of stress-related disorders is an ongoing need. Thus, the importance of RFamide research is underlined by the emergence of peptidergic and G-protein coupled receptor-based therapeutic targets in the pharmaceutical industry.

Anorexigenic neuropeptides as anti-obesity and neuroprotective agents: exploring the neuroprotective effects of anorexigenic neuropeptides

Since 1975, the incidence of obesity has increased to epidemic proportions, and the number of patients with obesity has quadrupled. Obesity is a major risk factor for developing other serious diseases, such as type 2 diabetes mellitus, hypertension, and cardiovascular diseases. Recent epidemiologic studies have defined obesity as a risk factor for the development of neurodegenerative diseases, such as Alzheimer's disease (AD) and other types of dementia. Despite all these serious comorbidities associated with obesity, there is still a lack of effective antiobesity treatment. Promising candidates for the treatment of obesity are anorexigenic neuropeptides, which are peptides produced by neurons in brain areas implicated in food intake regulation, such as the hypothalamus or the brainstem. These peptides efficiently reduce food intake and body weight. Moreover, because of the proven interconnection between obesity and the risk of developing AD, the potential neuroprotective effects of these two agents in animal models of neurodegeneration have been examined. The objective of this review was to explore anorexigenic neuropeptides produced and acting within the brain, emphasizing their potential not only for the treatment of obesity but also for the treatment of neurodegenerative disorders.

Kisspeptin-8 Induces Anxiety-Like Behavior and Hypolocomotion by Activating the HPA Axis and Increasing GABA Release in the Nucleus Accumbens in Rats

Kisspeptins (Kp) are RF-amide neuropeptide regulators of the reproductive axis that also influence anxiety, locomotion, and metabolism. We aimed to investigate the effects of intracerebroventricular Kp-8 (an N-terminally truncated octapeptide) treatment in Wistar rats. Elevated plus maze (EPM), computerized open field (OF), and marble burying (MB) tests were performed for the assessment of behavior. Serum LH and corticosterone levels were determined to assess kisspeptin1 receptor (Kiss1r) activation and hypothalamic-pituitary-adrenal axis (HPA) stimulation, respectively. GABA release from the nucleus accumbens (NAc) and dopamine release from the ventral tegmental area (VTA) and NAc were measured via ex vivo superfusion. Kp-8 decreased open arm time and entries in EPM, and also raised corticosterone concentration, pointing to an anxiogenic effect. Moreover, the decrease in arm entries in EPM, the delayed increase in immobility accompanied by reduced ambulatory activity in OF, and the reduction in interactions with marbles show that Kp-8 suppressed exploratory and spontaneous locomotion. The increase in GABA release from the NAc might be in the background of hypolocomotion by inhibiting the VTA-NAc dopaminergic circuitry. As Kp-8 raised LH concentration, it could activate Kiss1r and stimulate the reproductive axis. As Kiss1r is associated with hyperlocomotion, it is more likely that neuropeptide FF receptor activation is involved in the suppression of locomotor activity.

The kisspeptin derivative kissorphin reduces the acquisition, expression, and reinstatement of ethanol-induced conditioned place preference in rats

Research has shown that opioids are involved in the rewarding effects of ethanol. Neuropeptide FF (NPFF) has been described as an anti-opioid peptide because, in many cases, it inhibits opioid and ethanol effects in rodents. Kissorphin (KSO) is a new peptide derived from kisspeptin-10 with structural similarities to NPFF. This peptide possesses NPFF-like biological activity in vitro. The aim of the current study was to investigate whether KSO (Tyr-Asn-Trp-Asn-Ser-Phe-NH2) influences the acquisition, expression, and reinstatement of ethanol-induced conditioned place preference (ethanol-CPP) in rats. The ethanol-CPP was established (conditioning for 5 days) by intraperitoneal (i.p.) administration of ethanol (1 g/kg, 20%, w/v) using an unbiased procedure. After that, one group of rats was used in final post-conditioning testing (expression of CPP) and the other group received a priming injection of ethanol after 10 days of extinction (reinstatement of CPP). Our experiments showed that KSO, given intravenously (i.v.) at the doses of 1, 3, and 10 nmol before every ethanol administration, inhibited the acquisition and, given acutely before the post-conditioning test or before the priming dose of ethanol, inhibited the expression and reinstatement of ethanol-CPP, respectively, in a dose-dependent manner. KSO given by itself neither induced place preference nor aversion and did not alter locomotor activity and coordination of rats. These results suggest that KSO can alter rewarding/motivational effects of ethanol. These data suggest this peptide possesses an anti-opioid character.

The new kisspeptin derivative - kissorphin (KSO) - attenuates acute hyperlocomotion and sensitization induced by ethanol and morphine in mice

Kissorphin (KSO) is a new peptide derived from kisspeptin-10. This peptide possesses neuropeptide FF (NPFF)-like biological activity in vitro; NPFF, in many cases, inhibits opioid and ethanol effects in rodents. Therefore, the current study explored the influence of KSO on acute ethanol- and morphine-induced hyperactivity, and on the development and expression of locomotor sensitization induced by these drugs. In the present study, sensitization to locomotor effects was induced by repeated exposure to ethanol (2.4 g/kg, intraperitoneally [i.p.], 1 × 4 days) or morphine (10 mg/kg, subcutaneously [s.c.], 1 × 7 days). We found that KSO (1-10 nmol/300 μL, intravenously [i.v.]) did not have an impact on locomotor activity of naïve mice. However, it reduced both acute ethanol- (10 nmol/300 μL) and morphine-induced hyperactivity (3 and 10 nmol/300 μL). Pretreatment of animals with KSO (10 nmol/300 μL), before every ethanol or morphine injection during development of sensitization or before the ethanol or morphine challenge, attenuated the development, as well as the expression of locomotor sensitization to both substances. Moreover, prior administration of the NPFF receptor antagonist RF9 (10 nmol/300 μL, i.v.) inhibited the ability of KSO (10 nmol/300 μL) to reduce the expression of ethanol and morphine sensitization. KSO given alone, at all used doses, did not influence the motor coordination measured via the rotarod test. The results from this study show that KSO effectively attenuated acute and repeated effects of ethanol and morphine. Thus, KSO possesses NPFF-like anti-opioid activity in these behavioral studies.

Evaluation of the CART peptide expression in morphine sensitization in male rats

The importance of Cocaine- and amphetamine-regulated transcript (CART) peptide in reinforcing effects of addictive drugs specially alcohol and psychostimulants has been stablished. Involvement of CART peptide in rewarding effects of opioids in brain has recently been reported. Here we have studied the expression of CART mRNA and peptide in the reward pathway in morphine-induced sensitization phenomenon and also evaluated the peptide level fluctuations in CSF and plasma. Male Wistar rats received 7-day morphine injection (20mg/kg) and then after a 7-day washout period, a challenge dose of 10mg/kg morphine was administered and locomotor activity and oral stereotypical behaviors were recorded. Besides, the expression level of CART mRNA and peptide in four important areas of the mesocorticolimbic reward pathway including nucleus accumbens, striatum, prefrontal cortex, and hippocampus were measured by real-time PCR and western blotting, respectively. The level of the peptide in CSF and plasma was measured by Elisa method. The expression level of CART mRNA and protein in brain regions and also the peptide level in CSF and plasma were significantly down-regulated after 7-day morphine administration. These reduced levels returned to nearly normal rates after 7-day wash-out period. Administration of morphine challenge dose led to significant upregulation of CART gene expression (both mRNA and peptide) in the brain, and elevation of peptide level in CSF and plasma in morphine-sensitized rats. It can be concluded that CART is released in the framework of reward pathway and may serve as an important neurotransmitter in the process of morphine dependence and sensitization.

Pain regulation of endokinin A/B or endokinin C/D on chimeric peptide MCRT in mice

The present study focused on the interactive pain regulation of endokinin A/B (EKA/B, the common C-terminal decapeptide in EKA and EKB) or endokinin C/D (EKC/D, the common C-terminal duodecapeptide in EKC and EKD) on chimeric peptide MCRT (YPFPFRTic-NH2, based on YPFP-NH2 and PFRTic-NH2) at the supraspinal level in mice. Results demonstrated that the co-injection of nanomolar EKA/B and MCRT showed moderate regulation, whereas 30 pmol EKA/B had no effect on MCRT. The combination of EKC/D and MCRT produced enhanced antinociception, which was nearly equal to the sum of the mathematical values of single EKC/D and MCRT. Mechanism studies revealed that pre-injected naloxone attenuated the combination significantly compared with the equivalent analgesic effects of EKC/D alone, suggesting that EKC/D and MCRT might act on two totally independent pathways. Moreover, based on the above results and previous reports, we made two reasonable hypotheses to explain the cocktail-induced analgesia, which may potentially pave the way to explore the respective regulatory mechanisms of EKA/B, EKC/D, and MCRT and to better understand the complicated pain regulation of NK1 and μ opioid receptors, as follows: (1) MCRT and endomorphin-1 possibly activated different μ subtypes; and (2) picomolar EKA/B might motivate the endogenous NPFF system after NK1 activation.

Metabolism of cryptic peptides derived from neuropeptide FF precursors: the involvement of insulin-degrading enzyme

The term “cryptome” refers to the subset of cryptic peptides with bioactivities that are often unpredictable and very different from the parent protein. These cryptic peptides are generated by proteolytic cleavage of proteases, whose identification in vivo can be very challenging. In this work, we show that insulin-degrading enzyme (IDE) is able to degrade specific amino acid sequences present in the neuropeptide pro-NPFF_A (NPFF precursor), generating some cryptic peptides that are also observed after incubation with rat brain cortex homogenate. The reported experimental findings support the increasingly accredited hypothesis, according to which, due to its wide substrate selectivity, IDE is involved in a wide variety of physiopathological processes.

Involvement of Mammalian RF-Amide Peptides and Their Receptors in the Modulation of Nociception in Rodents

Mammalian RF-amide peptides, which all share a conserved carboxyl-terminal Arg-Phe-NH2 sequence, constitute a family of five groups of neuropeptides that are encoded by five different genes. They act through five G-protein-coupled receptors and each group of peptide binds to and activates mostly one receptor: RF-amide related peptide group binds to NPFFR1, neuropeptide FF group to NPFFR2, pyroglutamylated RF-amide peptide group to QRFPR, prolactin-releasing peptide group to prolactin-releasing peptide receptor, and kisspeptin group to Kiss1R. These peptides and their receptors have been involved in the modulation of several functions including reproduction, feeding, and cardiovascular regulation. Data from the literature now provide emerging evidence that all RF-amide peptides and their receptors are also involved in the modulation of nociception. This review will present the current knowledge on the involvement in rodents of the different mammalian RF-amide peptides and their receptors in the modulation of nociception in basal and chronic pain conditions as well as their modulatory effects on the analgesic effects of opiates.

Characterization of novel RFamide peptides in the central nervous system of the brown hagfish: isolation, localization, and functional analysis

RFamide (RFa) peptides play various important roles in the central nervous system in both invertebrates and vertebrates. However, there is no evidence of the existence of any RFamide peptide in the brain of hagfish, one of the oldest lineages of vertebrates. In this study, we sought to identify novel RFamide peptides from the brains of hagfish (Paramyxine atami). We identified four novel RFamide peptides, which had the C-terminal Pro-Gln-Arg-Phe-NH2 structure. cDNA cloning revealed that the identified RFamide peptides are encoded in two types of cDNA. Molecular phylogenetic analysis of the two precursors indicated that the hagfish RFamide peptides belong to the PQRFamide peptide group that includes mammalian neuropeptide FF and AF. Based on immunohistochemistry and in situ hybridization, hagfish PQRFamide peptide precursor mRNA and its translated peptides were localized in the infundibular nucleus of the hypothalamus. Immunoreactive fibers were terminated on blood vessels in the infundibular nucleus. Dense immunoreactive fibers were also observed in other brain regions. We further showed that one of the hagfish PQRFamide peptides significantly stimulated the expression of gonadotropin-β mRNA in the cultured hagfish pituitary. These results indicate that the control mechanism of gonadotropin expression by a hypothalamic neuropeptide evolved in the agnathan brain. This is the first evidence describing the identification of RFamide peptides in the hagfish brain. This is also the first report showing the regulation of gonadotropin expression by a homolog of neuropeptide FF that belongs to the PQRFamide peptide group in any vertebrate.

Inhibitory effect of berberine on the motivational effects of ethanol in mice

It is believed that drug-induced rewarding effects play an important role in the development of substance dependence. Recently, berberine was reported to inhibit the rewarding effects of drugs of abuse such as cocaine, morphine, and nicotine. Berberine is also demonstrated to modulate the activity of several neurotransmitter systems like, dopamine, nitric oxide, serotonin, and NMDA, which are implicated in rewarding effects of ethanol. Hence, we hypothesized that berberine may modulate the ethanol-induced rewarding effects. Therefore, we studied the effect of berberine on locomotor sensitization, conditioned place preference (CPP), and ethanol drinking preference in mice. The results revealed that acute administration of berberine (2.5, 5, and 10 mg/kg, i.p.) dose dependently reduced locomotor stimulant effect of acute ethanol and expression of sensitization to locomotor stimulant effect of ethanol. Further, pretreatment with berberine (2.5, 5, and 10 mg/kg, i.p.) prior to each dose of ethanol, blocked the development as well as expression of sensitization to locomotor stimulant effect of ethanol. In another set of experiment, treatment with berberine (5 and 10 mg/kg, i.p.) reduced the induction and expression of ethanol-induced CPP in mice. In addition, berberine in these doses also reduced preference to ethanol drinking over water, but did not alter the general reward. In conclusion, the results of the present study revealed that berberine attenuates ethanol-induced rewarding effects in mice and that could be attributed to its neuro-modulatory action.

Opioid-modulating properties of the neuropeptide FF system

Opioid receptors are involved in the control of pain perception in the central nervous system together with endogenous neuropeptides, termed opioid-modulating peptides, participating in a homeostatic system. Neuropeptide FF (NPFF) and related peptides possess anti-opioid properties, the cellular mechanisms of which are still unclear. The purpose of this review is to detail the phenomenon of cross-talk taking place between opioid and NPFF systems at the in vivo pharmacological level and to propose cellular and molecular models of functioning. A better knowledge of the mechanisms underlying opioid-modulating properties of NPFF has potential therapeutic interest for the control of opioid functions, notably for alleviating pain and/or for the treatment of opioid abuse.

Inhibitory influence of mecamylamine on the development and the expression of ethanol-induced locomotor sensitization in mice

Several evidences have indicated the involvement of neuronal nicotinic acetylcholine receptors (nAChR) in behavioral effects of drugs of abuse, including ethanol. nAChRs are implicated in ethanol-induced behaviors as well as neurochemical responses to ethanol. Recently, it is demonstrated that mecamylamine, a nAChR antagonist blocks cocaine-, d-amphetamine-, ephedrine-, nicotine-, and methylphenidate-induced psychomotor sensitization. However, no reports are available on its role in ethanol-induced psychomotor sensitization. Therefore, an attempt was made to evaluate its effect on ethanol-induced locomotor sensitization using a model previously described by us. The results revealed that acute administration of mecamylamine (1 and 2mg/kg, i.p.) blocked the acute stimulant effect of ethanol (2.0g/kg, i.p.). In addition, treatment with mecamylamine (0.5-2.0mg/kg, i.p.), 30min prior to the challenge dose of ethanol (2.0g/kg, i.p.) dose dependently attenuated expression of sensitization to locomotor stimulant effect of ethanol. Moreover, administration of mecamylamine (1 and 2mg/kg, i.p.) during development (prior to each ethanol injection on days 1, 4, 7, and 10) blocked acquisition as well as expression (day 15) of sensitization to locomotor stimulant effect of ethanol. Mecamylamine per se did not affect locomotor activity. Further, it also did not influence blood ethanol levels and rotarod performance in mice. These results support the hypothesis that neuroadaptive changes in nAChRs may participate in the development and the expression of ethanol-induced locomotor sensitization.

Gene expression profiling following short-term and long-term morphine exposure in mice uncovers genes involved in food intake

Addictive drugs including opioids activate signal transduction pathways that regulate gene expression in the brain. However, changes in CNS gene expression following morphine exposure are poorly understood. We determined changes in gene expression following short- and long-term morphine treatment in the hypothalamus and pituitary using genome-wide DNA microarray analysis and confirmed those alterations in gene expression by real-time reverse transcriptase polymerase chain reaction (RT-PCR) analysis. In the hypothalamus, short-term morphine administration up-regulated (at least twofold) 39 genes and down-regulated six genes. Long-term morphine treatment up-regulated 35 genes and down-regulated 51 genes. In the pituitary, short-term morphine administration up-regulated 110 genes and down-regulated 29 genes. Long-term morphine treatment up-regulated 85 genes and down-regulated 37 pituitary genes. Microarray analysis uncovered several genes involved in food intake (neuropeptide Y, agouti-related protein, and cocaine and amphetamine-regulated transcript) whose expression was strongly altered by morphine exposure in either the hypothalamus or pituitary. Subsequent RT-PCR analysis confirmed similar regulation in expression of these genes in the hypothalamus and pituitary. Finally, we found functional correlation between morphine-induced alterations in food intake and regulation of genes involved in this process. Changes in genes related to food intake may uncover new pathways related to some of the physiological effects of opioids.

Dansyl-PQRamide, a putative antagonist of NPFF receptors, reduces anxiety-like behavior of ethanol withdrawal in a plus-maze test in rats

Much evidence indicates that endogenous opioid peptides are involved in effects caused by ethanol. The aim of the present study was to determine whether dansyl-PQR amide, a putative antagonist of receptors for an anti-opioid peptide-neuropeptide FF (NPFF) could affect anxiety-like behavior measured during withdrawal from acute-, and chronic ethanol administration in the elevated plus maze test in rats. Our study indicated that intracerebroventricular (i.c.v.) administration of dansyl-PQRamide (2.4 and 4.8 nmol) reversed anxiety-like behavior measured as a percent time spent in the open arms, and a percent open arm entries onto the open arms in the elevated plus-maze test in rats. These effects were inhibited by NPFF (10 and/or 20 nmol, i.c.v.) in the experiments performed during withdrawal from acute- and chronic ethanol administration. During withdrawal from acute ethanol, naloxone (1mg/kg, i.p.), a nonselective opioid receptor antagonist, attenuated only an increased percent time spent in the open arms induced by dansyl-PQR amide (4.8 nmol). Dansyl-PQR amide, NPFF and naloxone given alone to naive rats did not have influence on spontaneous locomotor activity of animals. Furthermore, NPFF potentiated anxiety-like behavior during withdrawal from chronic, but not acute, ethanol administration in rats. Our data suggest that NPFF system is involved in regulation of affective symptoms of ethanol withdrawal. It seems that involvement of the NPFF system in ethanol withdrawal anxiety-like behavior is associated with regulation of the opioid system activity.

The 5-HT3 receptor antagonist, ondansetron, blocks the development and expression of ethanol-induced locomotor sensitization in mice

Manipulation of the serotonergic system has been shown to alter ethanol sensitization. Ondansetron is a 5-HT3 receptor antagonist, reported to attenuate cocaine and methamphetamine-induced behavioral sensitization, but no reports are available on its role in ethanol-induced behavioral sensitization. Therefore, an attempt has been made to assess this issue by using an earlier used animal model of ethanol-induced locomotor sensitization. Results indicated that ondansetron (0.25-1.0 mg/kg, subcutaneously) given before the challenge dose of ethanol (2.4 g/kg, intraperitoneally) injection, significantly and dose dependently attenuated the expression of sensitization. In addition, ondansetron (1.0 mg/kg, subcutaneously) given before ethanol injection on days 1, 4, 7, and 10 significantly blocked the development (days 1, 4, 7, and 10), and expression (day 15) of sensitization to the locomotor stimulant effect of ethanol injection. Ondansetron had no effect per se on locomotor activity and did not affect blood ethanol levels. Therefore, the results raise the possibility that ondansetron blocked the development and expression of ethanol-induced locomotor sensitization by acting on 5-HT3 receptors.

Endogenous opiates and behavior: 2007

This paper is the thirtieth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2007 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.

Leuprolide: a luteinizing hormone releasing hormone agonist attenuates ethanol withdrawal syndrome and ethanol-induced locomotor sensitization in mice

Ethanol inhibits the synthesis, content and release of hypothalamic luteinizing hormone releasing hormone (LHRH), and LHRH modulates the activity of several neurotransmitters that experience adaptive changes on chronic exposure to ethanol, and also implicate in ethanol dependence. Hence, it was contemplated that LHRH agonist such as leuprolide may influence the behavioral consequences of withdrawing ethanol in dependent state. In the present study, ethanol dependence was produced in mice by providing ethanol liquid diet for 10 days; and its withdrawal on day 11 led to physical signs of hyperexcitability with its peak at 6th h. Acute treatment with leuprolide (20 ng/mouse, i.c.v.), 10 min prior to peak, significantly attenuated hyperexcitability. Such effect of leuprolide was evident even in castrated mice, and castration significantly increased the hyperexcitability in ethanol withdrawal state. Chronic treatment with leuprolide (10 ng/mouse, twice daily, i.c.v.) till day 10 significantly reduced the signs of hyperexcitability in ethanol withdrawal state. In another set of experiment, ethanol (2.4 g/kg, i.p.) was administered on day 1, 4, 7, 10 and 15, which caused gradual increase in locomotor activity indicating ethanol-induced sensitization. Leuprolide (20 ng/mouse, i.c.v.), 10 min prior to the challenge dose of ethanol (2.4 g/kg, i.p.) on day 15 significantly attenuated the expression of sensitization to hyperlocomotor effect of ethanol. Similarly, administration of leuprolide (20 ng/mouse, i.c.v.), 10 min prior to ethanol on day 1, 4, 7 and 10 not only reduced the gradual increase in locomotor activity but also attenuated the sensitized locomotor response on day 15, indicated attenuation of development of sensitization. Leuprolide per se did not affect physical signs and locomotor activity in control group. In conclusion, the present study demonstrated that leuprolide treatment attenuates expression and development of ethanol dependence and sensitization in mice.

Neuropeptide FF (NPFF) reduces the expression of cocaine-induced conditioned place preference and cocaine-induced sensitization in animals

The endogenous brain opioid system is believed to play an important role in mediating reward mechanisms. Opioid innervation is high in many limbic regions and reinforcing actions of many drugs of abuse, including cocaine, are thought to be mediated via endogenous opioid system. The aim of the present study was to indicate whether the anti-opioid peptide, neuropeptide FF (NPFF; FLFQPQRF-NH2) was able to modify the rewarding effect of cocaine (5 mg/kg) measured in the expression of conditioned place preference (CPP) test in rats and the expression of sensitization to hyperlocomotor effect of cocaine (10 mg/kg) in mice. Our results indicate that NPFF (5, 10, and 20 nmol) given intracerebroventricularly (i.c.v.) inhibited the expression of cocaine-induced CPP at the dose of 10 nmol (P<0.01) and 20 nmol (P<0.001). Moreover, NPFF inhibited the expression of cocaine-induced sensitization to its hyperlocomotor effect at the dose of 20 nmol (P<0.05) and acute hyperlocomotor effect of cocaine at doses of 5 nmol (P<0.01), 10 nmol (P<0.01), and 20 nmol (P<0.05). Our study suggests that NPFF may participate in a rewarding effect of cocaine measured in the CPP paradigm. On the other hand, our experiments indicate that NPFF is involved in the mechanism of expression of sensitization to cocaine hyperlocomotion but this effect seems to be non-specific because NPFF also inhibited the acute hyperlocomotor effect of cocaine.

Morphine-induced behavioral sensitization increased the mRNA expression of NMDA receptor subunits in the rat amygdala

This study was designed to evaluate the effect of repeated morphine treatment on rat behavioral responses. In the genetic section, the mRNA expression of NMDA receptor subunits (NR1 and NR2A) was measured in certain areas of the male rat brain (striatum, prefrontal cortex, hippocampus, hypothalamus and amygdala). In the behavioral section, the effect of repeated morphine treatment on animal models such as locomotion, oral stereotypy, and state-dependent memory in a passive avoidance test was evaluated in the presence or absence of MK801 (NMDA receptor antagonist). Our results showed that chronic morphine treatment, followed by a 7-day (but not 24-hour) washout period, potentiated the effect of test doses of morphine, which is referred to as behavioral sensitization. Meanwhile, pretreatment of animals with MK801 (0.1 and 0.25 mg/kg), 30 min before a test dose of morphine (5 mg/kg), failed to attenuate the locomotion and oral stereotypy in the behavioral sensitization state. Interestingly, a higher dose of MK801 (0.25 mg/kg) decreased memory retrieval induced by morphine (2.5 mg/kg) in state-dependent memory. This effect may be due to the intrinsic motor enhancer property of higher doses of MK801, rather than the blockade of NMDA receptors. It can be concluded that MK801 does not affect morphine-induced behavioral sensitization in the expression phase. In the genetic section of the study, results of quantitative real-time RT-PCR clearly indicated that morphine sensitization increased the expression of NMDA receptor subunits mRNA in the amygdala (NR1 by 104% and NR2A by 85%), while the other areas of the brain were unaffected. Maenwhile, no change in the mRNA levels was observed in non-sensitized animals (chronic morphine treatment followed by a 24-hour washout period). In summary, the present study indicates that repeated morphine treatment followed by long-term (7-day washout) induces behavioral sensitization and causes a delayed increase in mRNA levels of NMDA receptor subunits in the rat amygdala. Meanwhile, it has previously been reported that the amygdala is involved in behavioral sensitization. Thus, it can be concluded that the increase in NMDA receptor expression is associated with morphine-induced behavioral sensitization.

A novel cryptic peptide derived from the rat neuropeptide FF precursor reverses antinociception and conditioned place preference induced by morphine

Neuropeptide FF (NPFF) precursors from different species contain at least three known neuropeptides, i.e. FF (FLFQPQRF-NH(2)), AF (AGEGLSSPFWSLAAPQR-NH(2)) and SF (SLAAPQRF-NH(2)). We demonstrate that the rat NPFF precursor contains another bioactive sequence, NAWGPWSKEQLSPQA, spanning between positions 85 and 99. Synthetic NPFF precursor (85-99) (10 and 20 nmol, i.c.v.) blocked the expression of conditioned place preference induced by morphine (5 mg/kg, s.c.). This peptide alone (10 and 20 nmol, i.c.v.) had no influence on the baseline latency of a nociceptive reaction but reversed the antinociceptive activity of morphine (5 mg/kg, s.c.) in the tail-immersion test in rats. These data suggest the existence of a novel bioactive cryptic peptide within an already known NPFF precursor.