Intracerebroventricular effects of histaminergic agents on morphine-induced anxiolysis in the elevated plus-maze in rats

Development of anxiety-like behaviors during adolescence: Persistent effects of adolescent morphine exposure in male rats

Epidemiological studies show the prevalence of opioid use, misuse and abuse in adolescents, which imposes social and economic accountability worldwide. Chronic opioid exposure, especially in adolescents, may have lasting effects on emotional behaviors that persist into adulthood. The current experiments were therefore designed to study the effects of sustained opioid exposure during adolescence on anxiety-like behaviors. Adolescent male Wistar rats underwent increasing doses of morphine for 10 days (PNDs 31-40). After that the open field test (OFT) and elevated plus maze (EPM) test were performed over a 4-week postmorphine treatment from adolescence to adulthood. Moreover, the weight of the animals was measured at these time points. We found that chronic adolescent morphine exposure reduces the weight gain during the period of morphine treatment and 4 weeks after that. It had no significant effect on the locomotor activity in the animals. Moreover, anxiolytic-like behavior was observed in the rats exposed to morphine during adolescence evaluated by OFT and EPM test. Thus, long-term exposure to morphine during adolescence has the profound potential of altering the anxiety-like behavior profile in the period from adolescence to adulthood. The maturation of the nervous system can be affected by drug abuse during the developmental window of adolescence and these effects may lead to behaviorally stable alterations.

A possible anti-anxiety effect of appetitive aggression and a possible link to the work of Donald Winnicott

Various pleasant sensations that give a particularly intense pleasure are able to improve anxiety. In the present study I consider the possibility that their anti-anxiety action depends on the strong pleasure they provide, and I propose a possible mechanism of this action. According to some studies, also appetitive aggression (an aggression that provokes a strong pleasure and that is performed only for the pleasure it provides) can improve anxiety, and in this article I consider the possibility that the pleasure of appetitive aggression is able to reduce anxiety by the same mechanism I have proposed for other intense pleasurable sensations.

The aggression performed by a child against the mother or against a substitute for the mother in the first period of life (a period in which this aggression is not dangerous) is a recurring theme throughout the work of of Donald Winnicott. Winnicott stresses that this aggression is necessary for the normal development of the child, and that the child must be free to practise it. According to Winnicott, this aggression is highly pleasurable and is not a response to unpleasant or hostile external situations. For these characteristics it seems to correspond to appetitive aggression in the adult that has been found to be able to reduce anxiety. Consequently, aggression performed by the child in the first period of life may also relieve anxiety, in the same way that appetitive aggression helps against anxiety in the adult.

In his writings, Winnicott returns several times to an unthinkable or archaic anxiety that children experience when they feel abandoned by their mother for a period that is too long for them, and all children, according to Winnicott, live on the brink of this anxiety. In this study I propose the hypothesis that aggression in the early period of life may be necessary for children because the intense pleasure it provides may help them against this continuously impending anxiety.

Long-time effects of prenatal morphine, tramadol, methadone, and buprenorphine exposure on seizure and anxiety in immature rats

Purpose: This study aimed to investigate seizures and anxiety-like behaviors in immature rats prenatally exposed to opiate drugs.Materials and methods: Pregnant rats were randomly divided into five groups: saline, morphine, tramadol, methadone, and buprenorphine. Administrations were performed intraperitoneally once a day for the last seven days of pregnancy. Neonatal rats were subdivided into ten groups, split according to sex. Anxiety-like behavior was tested on postnatal day (PD) 19. On PD 20, seizure was induced by PTZ injection.Results: Morphine in male rats had an increased time to onset (p < 0.005), whereas there was a decreased number of tonic-clonic seizures in females (p < 0.05). Tramadol had an increased duration of tonic-clonic seizures compared to morphine and methadone in males (p < 0.005). Moreover, tramadol decreased open arm time and locomotor activity in males more than in females (p < 0.05). Methadone decreased open arm time in males (p < 0.05). Furthermore, buprenorphine and tramadol decreased open arm entrance in male rats (p < 0.05).Conclusions: It was demonstrated that prenatal tramadol significantly increases both the duration of seizures and anxiety-like behaviors in immature male rats, whereas morphine decreases both of them. The effects of tramadol on seizure and anxiety-like behavior may be due to the comorbid occurrence of the symptoms of these two disorders.

Effects of morphine on place conditioning and ERK1/2 phosphorylation in the nucleus accumbens of psychogenetically selected Roman low- and high-avoidance rats

RATIONALE

Extracellular signal-regulated kinase (ERK_1/2) phosphorylation is critical for neuronal and behavioural functions; in particular, phosphorylated ERK_1/2 (pERK_1/2) expression in the nucleus accumbens (Acb) of the rat is stimulated by addictive drugs with the exception of morphine, which decreases accumbal ERK_1/2 phosphorylation in the Sprague-Dawley and Wistar rats. The psychogenetically selected Roman low- (RLA) and high-avoidance (RHA) rats differ behaviourally and neurochemically in many responses to addictive drugs. In particular, morphine elicits a greater increment in locomotor activity and in dopamine transmission in the Acb of RHA vs RLA rats. However, the effects of morphine on place conditioning (conditioned place preference (CPP)) and ERK_1/2 phosphorylation in the Roman lines remain unknown.

OBJECTIVES AND METHODS

To characterize in the Roman lines the reinforcing properties of morphine (i.e. morphine-elicited CPP acquisition) and the relationship between these properties and its effects on ERK_1/2 phosphorylation in the Acb, the behavioural effects of morphine were evaluated in a place-conditioning apparatus and ERK_1/2 phosphorylation was assessed by immunohistochemistry in the shell and core subregions of the Acb of rats both acutely administered with morphine or undergoing conditioning.

RESULTS

Morphine elicited CPP in both Roman lines and decreased pERK_1/2 expression in the Acb of RLA but not RHA rats. Such decrease was prevented by conditioning.

CONCLUSIONS

These findings indicate that the selective breeding of the Roman lines has generated a divergence, in terms of morphine-elicited pERK_1/2 expression but not of morphine-elicited CPP, between RLA and RHA rats and sustain the observation that changes in pERK_1/2 expression in the Acb are not a requisite for the reinforcing effects of morphine.

Study of the effects of controlled morphine administration for treatment of anxiety, depression and cognition impairment in morphine-addicted rats

Background:

Morphine dependency usually results in undesired outcomes such as anxiety, depression, and cognitive alterations. In this study, morphine was used to manage morphine dependence-induced anxiety, depression, and learning and memory disturbances.

Materials and Methods:

Forty rats were divided equally into five groups. Group 1 received saline for 21 days. Groups 2–5 were dependent by increasing administration of morphine (15–45 mg/kg) for 7 days. For the next 14 days, morphine was administered as the following regimen: Group 2: once daily; 45 mg/kg (positive controls), Group 3: the same dose with an increasing interval (6 h longer than the previous intervals each time), Group 4: the same dose with an irregular intervals (12, 24, 36 h intervals interchangeably), and Group 5: decreasing doses once daily (every time 2.5 mg/kg less than the former dosage). On days 22–26, elevated plus maze (EPM), open field test (OFT), forced swim test (FST), and tail suspension test (TST) were performed to investigate anxiety level and depression in animals. Between 17^th and 21^st days, Morris water maze (MWM) was used to evaluate the spatial learning and memory.

Results:

Chronic morphine administration caused depression and anxiety as observed by FST, EPM, and TST and decreased motor activity in OFT and caused impairment in learning and memory performance in MWM. Treatment with our protocol as increasing interval, irregular interval, and decreasing dosage of morphine caused marked reduction in depression, anxiety, and improved cognition performance compared with positive control group; and attenuated motor deficits in morphine-dependent rats, remarkably.

Conclusions:

Change in dosage regimens of morphine can reduce morphine-induced anxiety, depression, and cognitive impairments.

Enkephalin analog, cyclo[N(ε),N(β)-carbonyl-D-Lys(2),Dap(5)] enkephalinamide (cUENK6), inhibits the ethanol withdrawal-induced anxiety-like behavior in rats

An analog of enkephalin, cyclo[N(ε),N(β)-carbonyl-D-Lys(2),Dap(5)] enkephalinamide (cUENK6), is predominantly a functional agonist of μ-opioid receptors (MOPr) and, to a lesser extent, of δ-opioid receptors (DOPr) in vitro. The aim of the present study was to determine whether cUENK6 could affect ethanol withdrawal-induced anxiety-like behavior in the elevated plus maze (EPM) test in rats. An anxiety-like effect of withdrawal was predicted to occur in the EPM test 24 h after the last ethanol administration (2 g/kg, intraperitoneally [i.p.]; 15% w/v once daily for 9 days). Ethanol withdrawal decreased the percent of time spent by rats in the open arms and the percent of open-arms entries. cUENK6 (0.25 nmol), given by intracerebroventricular (i.c.v.) injection, significantly reversed these anxiety-like effects of ethanol withdrawal and elevated the percent of time spent by rats in the open arms and the percent of open-arms entries. These effects of cUENK6 were significantly inhibited by the DOPr antagonist naltrindole (NTI) (5 nmol, i.c.v.), but not by the MOPr antagonist β-funaltrexamine (β-FNA) (5 nmol, i.c.v.). The preferential DOPr agonist [Leu(5)]-enkephalin (LeuEnk) (2.7 and 5.4 nmol, i.c.v.) and the MOPr agonist morphine (6.5 and 13 nmol, i.c.v.) reduced the anxiety-like effects of ethanol withdrawal. cUENK6 at the dose of 0.25 nmol did not disturb locomotor activity in the EPM, in contrast to cUENK6 at the dose of 0.5 nmol, and morphine at 6.5 and 13 nmol. However, similarly to LeuEnk, cUENK6 induced the anxiolytic-like effects in naïve rats. Thus, our study suggests that cUENK6 reduced ethanol withdrawal-induced anxiety-like behavior by activation of δ-opioid receptors rather than μ-opioid receptors.

Anxiolytic-like effects of mitragynine in the open-field and elevated plus-maze tests in rats

The effects of mitragynine on anxiety-related behaviours in the open-field and elevated plus-maze tests were evaluated. Male Sprague-Dawley rats were orally treated with mitragynine (10, 20 and 40 mg/kg) or diazepam (10 mg/kg) 60 min before behavioural testing. Mitragynine doses used in this study were selected on the basis of approximately human equivalent doses with reference to our previous literature reports. Acute administration of mitragynine (10, 20 and 40 mg/kg) or diazepam (10 mg/kg) increased central zone and open arms exploration in the open-field and elevated plus-maze tests respectively. These anxiolytic-like effects of mitragynine were effectively antagonized by intraperitoneal administration of naloxone (2 mg/kg), flumazenil (10 mg/kg), sulpiride (0.5 mg/kg) or SCH 23390 (0.02 mg/kg) 15 min before mitragynine treatments. These findings reveal that the acute administration of mitragynine produces anxiolytic-like effects and this could be possibly attributed to the interactions among opioidergic, GABAergic and dopaminergic systems in brain regions involved in anxiety.

Differential modulatory effects of morphine on acute and chronic stress induced neurobehavioral and cellular markers in rats

The present study evaluated the effects of morphine treatments on elevated plus maze test parameters, oxidative stress markers and Hsp70 expression in normal and stressed rats. Acute and chronic stress caused neurobehavioral suppression, altered prooxidant-antioxidant balance and increased Hsp70 expression in brain homogenates in a differential manner. Morphine (1 and 5mg/kg) attenuated RS induced anxiogenesis, changes in MDA and GSH but further enhanced Hsp70 expression. Similar anxiolytic and Hsp70 enhancing effects were seen after morphine in normal rats (no RS). Exposure to chronic RS did not elicit any appreciable neurobehavioral response in EPM but enhanced MDA, lowered GSH and exaggerated the Hsp70 expression. Pretreatment with morphine did not affect the neurobehavioral response to chronic RS, but reverted the GSH and Hsp70 expression. The results suggest that morphine differentially influences acute and chronic stress induced changes in anxiety behavior and complex interactions between oxidative stress markers and Hsp70 expression which may contribute to these effects.

The effect of opiodergic system and testosterone on anxiety behavior in gonadectomized rats

BACKGROUND AND AIM

Removal of the testes (gonadectomy; GDX), the primary source of androgens, increases anxiety behavior in several tasks. Opioids are known to play a role in mediating the effects of androgen. In the present study, the effect of testosterone and opioidergic system on anxiety behavior was investigated.

METHODS

Adult male Wistar rats were bilaterally castrated. The elevated plus maze which is a useful test to investigate the effects of anxiogenic or anxiolytic drugs in rodents was used.

RESULTS

The data indicated that there is a decrease, 10 days after castration, in the percentage of OAT (the ratio of time spent in the open arms to total times spent in any arms × 100) and OAE (the ratio of entries into open arms to total entries × 100) but not locomotor activity, showing anxiogenic-like effects of gonadectomy. Intraperitoneal injection of testosterone (200, 300 and 450 mg/kg) and morphine (2.5, 5 and 7.5mg/kg), before testing 10 days after castration, showed an increase in OAT and OAE. Furthermore, injection of naloxone (5 and 7.5mg/kg, i.p.), 5 min before testing 10 days after castration, decreased OAT and OAE. Also, injection of a significant dose of testosterone (300 mg/kg, i.p.), 1h before the injection of different doses of morphine (1, 2.5, 5 and 7.5mg/kg, i.p.), 10 days after castration, did not significantly alter OAT, OAE and locomotor activity. While, administration of a significant dose of testosterone (300 mg/kg, i.p.), 1h before the infusion of different doses of naloxone (1, 2.5, 5 and 7.5mg/kg, i.p.), 10 days after castration, decreased OAT and OAE.

CONCLUSION

The results show the involvement of testosterone and opioidergic system in anxiogenic-like behaviors induced by gonadectomy.

D1 and D2 dopaminergic systems in the rat basolateral amygdala are involved in anxiogenic-like effects induced by histamine

Involvement of the dopamine receptors in the basolateral amygdala (BLA) in the effects of histamine on anxiety-like behaviors of the elevated plus maze in male Wistar rats was investigated. The results showed that bilateral intra-BLA injections of histamine (2.5, 5 and 7.5 µg/rat) induced an anxiogenic-like effect, revealed by decreases in percentage of open arm time (%OAT) and open arm entries (%OAE). Intra-BLA administration of dopamine D1 receptor agonist, SKF38393 (0.25 µg/rat), and dopamine D2 receptor agonist, quinpirole (0.03 and 0.05 µg/rat), decreased %OAT but not %OAE. Conversely, intra-BLA administration of dopamine D1 receptor antagonist, SCH23390 (0.5 and 1 µg/rat), and dopamine D2 receptor antagonist, sulpiride (0.3 and 0.5 µg/rat), increased %OAT and %OAE, suggesting an anxiolytic-like effect for both drugs. Interestingly, co-administration of a silent dose of SCH23390 or sulpiride prevented anxiogenic-like effects of SKF38393 and quinpirole, respectively. Conjoint administration of a sub-effective dose of SKF38393 (0.125 µg/rat) or quinpirole (0.01 µg/rat) along with lower doses of histamine (1 and 2.5 µg/rat) induced anxiolytic-like effects. On the other hand, intra-BLA pretreatment with a silent dose of SCH23390 (0.25 µg/rat) or sulpiride (0.1 µg/rat) prevented the anxiogenic-like effect of higher doses of histamine (5 and 7.5 µg/rat). No significant change was observed in total closed arm entries, as an index for motor activity of the animals. It can be concluded that the dopamine D1 and D2 receptors in the BLA may be involved in the anxiogenic-like effects induced by histamine.

Social influences on morphine-conditioned place preference in adolescent BALB/cJ and C57BL/6J mice

RATIONALE

Among human adolescents, drug use is substantially influenced by the attitudes and behaviors of peers. Social factors also affect the drug-seeking behaviors of laboratory animals. Conditioned place preference (CPP) experiments indicate that social context can influence the degree to which rodents derive a rewarding experience from drugs of abuse. However, the precise manner by which social factors alter drug reward in adolescent rodents remains unknown.

OBJECTIVES

We employed the relatively asocial BALB/cJ (BALB) mouse strain and the more prosocial C57BL/6J (B6) strain to explore whether "low" or "high" motivation to be with peers influences the effects of social context on morphine CPP (MCPP).

METHODS

Adolescent mice were conditioned by subcutaneous injections of morphine sulfate (0.25, 1.0, or 5.0 mg/kg). During the MCPP procedure, mice were housed in either isolation (Ih) or within a social group (Sh). Similarly, following injection, mice were conditioned either alone (Ic) or within a social group (Sc).

RESULTS

Adolescent B6 mice expressed a robust MCPP response except when subjected to Ih-Sc, which indicates that, following isolation, mice with high levels of social motivation are less susceptible to the rewarding properties of morphine when they are conditioned in a social group. By contrast, MCPP responses of BALB mice were most sensitive to morphine conditioning when subjects experienced a change in their social environment between housing and conditioning (Ih-Sc or Sh-Ic).

CONCLUSIONS

Our findings demonstrate that susceptibility to morphine-induced reward in adolescent mice is moderated by a complex interaction between social context and heritable differences in social motivation.

Increased anxiety-like behaviors in rats experiencing chronic inflammatory pain

For many patients, chronic pain is often accompanied, and sometimes amplified, by co-morbidities such as anxiety and depression. Although it represents important challenges, the establishment of appropriate preclinical behavioral models contributes to drug development for treating chronic inflammatory pain and associated psychopathologies. In this study, we investigated whether rats experiencing persistent inflammatory pain induced by intraplantar injection of complete Freund's adjuvant (CFA) developed anxiety-like behaviors, and whether clinically used analgesic and anxiolytic drugs were able to reverse CFA-induced anxiety-related phenotypes. These behaviors were evaluated over 28 days in both CFA- and saline-treated groups with a variety of behavioral tests. CFA-induced mechanical allodynia resulted in increased anxiety-like behaviors as evidenced by: (1) a significant decrease in percentage of time spent and number of entries in open arms of the elevated-plus maze (EPM), (2) a decrease in number of central squares visited in the open field (OF), and (3) a reduction in active social interactions in the social interaction test (SI). The number of entries in closed arms in the EPM and the distance traveled in the OF used as indicators of locomotor performance did not differ between treatments. Our results also reveal that in CFA-treated rats, acute administration of morphine (3mg/kg, s.c.) abolished tactile allodynia and anxiety-like behaviors, whereas acute administration of diazepam (1mg/kg, s.c) solely reversed anxiety-like behaviors. Therefore, pharmacological treatment of anxiety-like behaviors induced by chronic inflammatory pain can be objectively evaluated using multiple behavioral tests. Such a model could help identify/validate alternative potential targets that influence pain and cognitive dimensions of anxiety.

Opioids and anxiety

The opioid system plays a crucial role in the neural modulation of anxiety. The involvement of opioid ligands and receptors in physiological and dysfunctional forms of anxiety is supported by findings from a wide range of preclinical and clinical studies, including clinical trials, experimental research, and neuroimaging, genetic, and epidemiological data. In this review we provide a summary of studies from a variety of research disciplines to elucidate the role of the opioid system in the neurobiology of anxiety. First, we report data from preclinical studies using animal models to examine the modulatory role of central opioid system on defensive responses conducive to fear and anxiety. Second, we summarize the human literature providing evidence that clinical and experimental human studies are consistent with preclinical models. The implication of these data is that activation of the opioid system leads to anxiolytic responses both in healthy subjects and in patients suffering from anxiety disorders. The role of opioids in suppressing anxiety may serve as an adaptive mechanism, collocated in the general framework of opioid neurotransmission blunting acute negative and distressing affective responses.

Anxiety profile in morphine-dependent and withdrawn rats: effect of voluntary exercise

Withdrawal from chronic opiates is associated with an increase in anxiogenic-like behaviours, but the anxiety profile in the morphine-dependent animals is not clear. Thus, one of the aims of the present study was to examine whether morphine-dependent rats would increase the expression of anxiogenic-like behaviours in novel and stressful conditions. Additionally, recent studies have shown that voluntary exercise can reduce anxiety levels in rodents. Therefore, another aim of this study was to examine the effect of voluntary exercise on the anxiety profile in both morphine-dependent animals and animals experiencing withdrawal. Rats were injected with bi-daily doses (10 mg/kg, at 12 h intervals) of morphine over a period of 10 days in which they were also allowed voluntary exercise. Following these injections, anxiety-like behaviours were tested in the elevated plus-maze (EPM) model and the light/dark (L/D) box. We found reductions in time spent in, and entries into, the EPM open arms and reductions in time spent in the lit side of the L/D box for both sedentary morphine-dependent and withdrawn rats as compared to the sedentary control groups. The exercising morphine-dependent and withdrawn rats exhibited an increase in EPM open arm time and entries and L/D box lit side time as compared with the sedentary control groups. We conclude that voluntary exercise decreases the severity of the anxiogenic-like behaviours in both morphine-dependent and withdrawn rats. Thus, voluntary exercise could be a potential natural method to ameliorate some of the deleterious behavioural consequences of opiate abuse.

Changes in adaptability following perinatal morphine exposure in juvenile and adult rats

The problem of drug abuse among pregnant women causes a major concern. The aim of the present study was to examine the adaptive consequences of long term maternal morphine exposure in offspring at different postnatal ages, and to see the possibility of compensation, as well. Pregnant rats were treated daily with morphine from the day of mating (on the first two days 5mg/kgs.c. than 10mg/kg) until weaning. Male offspring of dams treated with physiological saline served as control. Behavior in the elevated plus maze (EPM; anxiety) and forced swimming test (FST; depression) as well as adrenocorticotropin and corticosterone hormone levels were measured at postpartum days 23-25 and at adult age. There was only a tendency of spending less time in the open arms of the EPM in morphine treated rats at both ages, thus, the supposed anxiogenic impact of perinatal exposure with morphine needs more focused examination. In response to 5min FST morphine exposed animals spent considerable longer time with floating and shorter time with climbing at both ages which is an expressing sign of depression-like behavior. Perinatal morphine exposure induced a hypoactivity of the stress axis (adrenocorticotropin and corticosterone elevations) to strong stimulus (FST). Our results show that perinatal morphine exposure induces long term depression-like changes. At the same time the reactivity to the stress is failed. These findings on rodents presume that the progenies of morphine users could have lifelong problems in adaptive capability and might be prone to develop psychiatric disorders.

Dorsal hippocampal opioidergic system modulates anxiety-like behaviors in adult male Wistar rats

AIMS

In the present study, we investigated the possible influence of the opioidergic system of the dorsal hippocampus on anxiety-like behaviors.

METHODS

Elevated plus-maze, which is one of the methods used for testing anxiety, was used in the present study. Rats were anesthetized with ketamine and xylazine and special cannulas were inserted stereotaxically into the CA1 region of the dorsal hippocampus. After 1 week of recovery, the effects of intra-CA1 administration of morphine (0.25, 0.5, 1 and 2 µg/rat; 1 µl/rat; 0.5 µl/in each side), naloxone (2, 4, 6 and 8 µg/rat), enkephalin (1, 2, 5 and 10 µg/rat) and naltrindole (0.25, 0.5, 1 and 2 µg/rat) on percentage open arm time (%OAT) and percentage open arm entries (%OAE) were determined.

RESULTS

Bilateral administration of morphine into CA1 decreases %OAT and %OAE, indicating an anxiogenic-like effect. Intra-CA1 injection of naloxone, an opioid receptor antagonist, increased both %OAT and %OAE, parameters of anxiolytic-like behavior. Bilateral administration of δ-opioid receptor agonist, [D-Pen(2,5) ]-enkephalin acetate hydrate into the CA1, induced an anxiolytic-like effect. Furthermore, intra-CA1 injection of δ-opioid receptor antagonist, naltrindole hydrochloride, increased anxiety-related behaviors.

CONCLUSIONS

The results of the present study demonstrate that activation of μ-opioid receptors in this area produce an anxiogenic response while activation of δ-opioid receptors produces an anxiolytic response.

Withdrawal from repeated administration of morphine alters histamine-induced anxiogenic effects produced by intra-ventral hippocampal microinjection

In the present study, the influence of withdrawal from repeated administration of morphine on intra-ventral hippocampal microinjection of histamine-induced anxiety-like behavior was investigated in male Wistar rats. Three days subcutaneous administration of morphine (5-10 mg/kg) followed by five days free of the drug decreased the percentage open arm time and the percentage open arm entries. Intra-ventral hippocampal administration of histamine (2.5-7.5 microg/rat) decreased percentage open arm time and percentage open arm entries. Intra-ventral hippocampal histamine-induced anxiogenic effect was reversed in animals that had previously received the three days morphine (7.5 mg/kg) followed by five days free of the drug. Intra-ventral hippocampal administration of pyrilamine (5-20 microg/rat) or ranitidine (10-40 microg/rat) decreased percentage open arm time and percentage open arm entries. Pyrilamine- or ranitidine-induced anxiogenic effect was not changed in animals that had previously received the three days morphine (7.5 mg/kg) followed by five days free of the drug. Intra-ventral hippocampal injections of clobenpropit increased percentage open arm time. The percentage open arm time and percentage open arm entries were decreased in the morphine-treated animals compared with non-morphine-treated controls. Percentage open arm entries and locomotor activity was reduced with some doses of clobenpropit. It can be concluded that the histamine system is involved in anxiety-like behavior, and repeated injections of morphine followed by five days free of the drugs interact with histamine receptor mechanism.

Effects of morphine on rat behaviour in the elevated plus maze: the role of central amygdala dopamine receptors

The objective of the present study was to evaluate the possible role of dopamine D(1)/D(2) receptors of the central amygdala (CeA) on morphine-induced anxiolytic-like behaviour in adult male Wistar rats. The animals were bilaterally implanted with chronic cannulas the CeA and tested in an elevated plus maze (EPM) task. Intraperitoneal (i.p.) administration of morphine (5 and 6 mg/kg) increased the percentage of open arm time (%OAT) and open arm entries (%OAE), indicating an anxiolytic-like response. Intra-CeA administration of different doses of the dopaminergic agonist apomorphine (0.1-0.3 microg/rat) significantly increased %OAE, but not %OAT. Furthermore, co-administration of the same doses of apomorphine with an ineffective dose of morphine (4 mg/kg; i.p.) significantly increased %OAT and %OAE by the opioid. Single microinjection of the D(1) dopaminergic antagonist SCH23390 (0.5-1.5 microg/rat) or sulpiride (0.5-1.5 microg/rat) into the CeA caused no significant change for %OAT and %OAE. The obtained results also show that intra-CeA microinjection of the same doses of SCH23390 or sulpiride inhibits the anxiolytic-like effect of morphine (6 mg/kg; i.p.). Pre-treatment of animals with SCH23390 (intra-CeA) or sulpiride (intra-CeA) reversed the response induced by apomorphine (0.3 microg/rat) plus morphine (4 mg/kg; i.p.). It should be considered that the drugs also did not show any effect on locomotor activity in all experiments. In conclusion, these findings suggest that the central amygdala dopaminergic mechanisms, probably via D(1)/D(2) receptors, may be involved in the modulation of morphine-induced anxiolytic-like behaviour in rat.

Anxiety-like behaviour in rats with mononeuropathy is reduced by the analgesic drugs morphine and gabapentin

Anxiety has been described as an important comorbidity in patients suffering from chronic pain. However, in animals the connection between persistent pain and anxiety has hardly been investigated. Therefore, in the current study it was assessed whether chronic pain also causes anxiety-like behaviour in animals and if it can be reversed by analgesic or anxiolytic drugs. Neuropathic pain was induced in rats by partial sciatic nerve ligation (PNL) and chronic constriction injury (CCI). Mechanical hypersensitivity was assessed by the "electronic algometer", while anxiety-like behaviour was measured by using the elevated plus maze. In both neuropathic pain models, rats exhibited mechanical hypersensitivity, whereas a significant increase in anxiety-like behaviour was observed only in CCI rats (time spent in open arms decreased significantly from 99+/-15.8s in sham animals to 33.4+/-7.5s in CCI animals). Furthermore, midazolam (0.5mg/kg; i.p.) significantly reduced anxiety-like behaviour in both sham- and CCI-operated animals without influencing mechanical hypersensitivity. Morphine (3mg/kg; i.p.) and gabapentin (30 mg/kg; i.p.) significantly attenuated anxiety-like behaviour in the CCI lesioned rats: morphine increased entries into open arms from 3.0+/-0.4 to 7.7+/-1.4 (P=0.01), gabapentin elevated this value from 4.7+/-1 to 7.5+/-0.9 (P=0.02). These data suggest that rats subjected to neuropathic pain models develop anxiety-like behaviour which can be reversed by appropriate analgesic treatment. Morphine and gabapentin had no anxiolytic-like effect in sham treated animals, thus their effect on anxiety-like behaviour in the neuropathic pain model is likely indirect via their anti-nociceptive properties.

The endomorphin system and its evolving neurophysiological role

Endomorphin-1 (Tyr-Pro-Trp-Phe-NH2) and endomorphin-2 (Tyr-Pro-Phe-Phe-NH2) are two endogenous opioid peptides with high affinity and remarkable selectivity for the mu-opioid receptor. The neuroanatomical distribution of endomorphins reflects their potential endogenous role in many major physiological processes, which include perception of pain, responses related to stress, and complex functions such as reward, arousal, and vigilance, as well as autonomic, cognitive, neuroendocrine, and limbic homeostasis. In this review we discuss the biological effects of endomorphin-1 and endomorphin-2 in relation to their distribution in the central and peripheral nervous systems. We describe the relationship between these two mu-opioid receptor-selective peptides and endogenous neurohormones and neurotransmitters. We also evaluate the role of endomorphins from the physiological point of view and report selectively on the most important findings in their pharmacology.

Effects of intracerebroventricularly-injected morphine on anxiety, memory retrieval and locomotor activity in rats: involvement of vasopressinergic system and nitric oxide pathway

Morphine has been shown to alter several behavioural processes. We aimed to investigate the effects of intracerebroventricular (i.c.v.) morphine on anxiety, memory retrieval and locomotor activity in rats and to elucidate the possible involvement of the vasopressinergic system and the nitric oxide (NO) pathway in these effects. Rats were pretreated with morphine (0.5, 5, 50 microg/5 microl; i.c.v.) or saline (5 microl; i.c.v.) 30 min before the elevated plus maze test, the probe trial of the Morris water maze and the open field test. Morphine (5 microg/5 microl; i.c.v.) induced significant anxiolytic effects in the elevated plus maze. None of the doses of morphine produced any effects in the probe trial of the Morris water maze and the open field. Pretreatment with an arginine vasopressin (AVP) V(1) receptor antagonist (25, 125 ng/5 microl; i.c.v.), an AVP V(2) receptor antagonist (25, 125 ng/5 microl; i.c.v.), or L-NAME, an NO synthase inhibitor (5, 25 microg/5 microl; i.c.v.) 30 min before morphine significantly prevented the anxiolytic effects of morphine. These results suggest that i.c.v. morphine has significant anxiolytic effects, probably mediated by both vasopressinergic system and NO pathway, but has no effect on memory retrieval or locomotor activity, at least at the applied doses.

Endogenous opiates and behavior: 2005

This paper is the 28th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2005 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 (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity, neurophysiology and transmitter release (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); immunological responses (Section 17).

The effects of histaminergic agents in the ventral hippocampus of rats in the plus-maze test of anxiety-like behaviours

It has been suggested that histamine have modulatory influence on anxiety-related behaviours both in animals and humans. Ventral hippocampus (VHC) may also be an important brain site in the modulation of fear or anxiety. In the present study, the effects of histaminergic agents on anxiety-related behaviours in the rats, using plus-maze test has been investigated. Intra-VHC administration of histamine (2.5, 5 and 7.5 microg/rat) decreased %OAT and %OAE but not locomotor activity, showing an anxiogenic response. Pretreatment of animals with either pyrilamine, a H1 receptor antagonist (10 microg/rat), or ranitidine, a H2 receptor antagonist (10 microg/rat) reverse anxiogenic response of histamine (2.5, 5 and 7.5 microg/rat). However, intra-VHC microinjection of higher doses of pyrilamine (40 microg/rat) or ranitidine (20 and 40 microg/rat) alone increased anxiety-like behaviours in rats. Our results showed that histamine may modulate anxiety-like behaviours via H1 and H2 receptors in the ventral hippocampus of the rats.

Modulation of anxiety by μ-opioid receptors of the lateral septal region in mice

Morphine and opiates are known to exert anxiolytic effects, probably by interacting with the GABAergic system. The lateral septum (LS), mainly constituted of GABA neurons, exhibits high densities of mu-opiate receptors and could thus represent one the brain sites where opiates interact with GABAergic transmission to modulate anxiety. We examined the effects of intra-LS morphine injections on measures of anxiety using the elevated plus-maze and hole-board tests. Fos imaging was used to identify neural circuits involved in anxiety modulation. Unilateral intra-LS morphine (100 or 500 ng/100 nl) decreased open-arm exploration in the plus-maze and reduced head-dipping frequency in the hole-board, an anxiogenic-like effect associated with decreased Fos expression in the ventral LS, the dorsal hippocampus and the anterior hypothalamus. Anatomical specificity was assessed by injecting morphine into the medial septum, which failed to produce anxiogenesis. Pre-injection of the mu-opioid receptor antagonist naloxonazine (100 ng/100 nl) into LS reversed morphine-induced anxiogenesis and the associated pattern of Fos expression, indicating a specific recruitment of mu-opioid receptors by morphine. Surprisingly, bilateral morphine injections (20 to 500 ng/100 nl) were not found anxiogenic, perhaps due to their stimulant effect. Taken together, these results suggest that LS mu-opioid receptors participate to the modulation of anxiety.