Increased opioid release in specific brain areas in animals exposed to prenatal morphine and emotional stress later in life.

Prenatal morphine treatment and emotional stress both have been shown to increase sensitivity to reward-related behaviors. It has been postulated that this increased sensitivity to rewarding stimuli may be the result of an enhanced release of endogenous opioids. In the present study, in vivo autoradiography was employed to investigate the endogenous opioid release in specific brain areas in rats. Pregnant animals were exposed to morphine or saline from day 8 of gestation till birth. Development of pups was monitored and play behavior was tested on postnatal day 21. Adult rats were exposed to repeated emotional stress or control treatment for five consecutive days and tested in a small open field 5 days later. [(3)H]-Diprenorphine was injected following this test to investigate endogenous opioid release. Prenatal morphine treatment increased play behavior and endogenous opioid release in a number of cortical and subcortical brain areas after being subjected to an open field challenge later in life. Emotional stress exposure increased locomotor activity in the open field irrespective of the type of prenatal treatment and increased endogenous opioid release in some specific brain areas. It is suggested that the increased release of endogenous opioids in the substantia nigra, the piriform cortex and the septum observed after both types of treatments is related to the increased sensitivity to reward.

Chronic cocaine injections attenuate behavioral response of kappa-opioid receptors to U-50,488H agonist.

Chronic injections of cocaine (20 mg/kg daily for 10 days) increase activity and decrease anxiety in male C57Bl/6j mice in comparison with animals chronically injected with normal saline. U-50,488H (kappa-opioid receptor agonist; 2.5 mg/kg) produced an anxiolytic effect in animals preinjected with normal saline and had no effect in animals chronically injected with cocaine. Presumably, chronic activation of dopaminergic systems caused by cocaine injections is paralleled by desensitization of kappa-opioid receptor system.

Increased gabaergic input to ventral tegmental area dopaminergic neurons associated with decreased cocaine reinforcement in mu-opioid receptor knockout mice.

There is general agreement that dopaminergic neurons projecting from the ventral tegmental area (VTA) to the nucleus accumbens and prefrontal cortex play a key role in drug reinforcement. The activity of these neurons is strongly modulated by the inhibitory and excitatory input they receive. Activation of mu-opioid receptors, located on GABAergic neurons in the VTA, causes hyperpolarization of these GABAergic neurons, thereby causing a disinhibition of VTA dopaminergic neurons. This effect of mu-opioid receptors upon GABA neurotransmission is a likely mechanism for mu-opioid receptor modulation of drug reinforcement. We studied mu-opioid receptor signaling in relation to cocaine reinforcement in wild-type and mu-opioid receptor knockout mice using a cocaine self-administration paradigm and in vitro electrophysiology. Cocaine self-administration was reduced in mu-opioid receptor knockout mice, suggesting a critical role of mu-opioid receptors in cocaine reinforcement. The frequency of spontaneous inhibitory post-synaptic currents onto dopaminergic neurons in the ventral tegmental area was increased in mu-opioid receptor knockout mice compared with wild-type controls, while the frequency of spontaneous excitatory post-synaptic currents was unaltered. The reduced cocaine self-administration and increased GABAergic input to VTA dopaminergic neurons in mu-opioid receptor knockout mice supports the notion that suppression of GABAergic input onto dopaminergic neurons in the VTA contributes to mu-opioid receptor modulation of cocaine reinforcement.

Early amygdala damage disrupts performance on medial prefrontal cortex-related tasks but spares spatial learning and memory in the rat.

Recent studies have demonstrated that the postnatal development of connections between the basolateral amygdala (BLA) and the medial prefrontal cortex (mPFC) mature around postnatal days 13-15 (pd13-15), whereas these between the BLA and other structures such as the nucleus accumbens and the mediodorsal thalamus are completed by pd7. Accordingly, it is hypothesized that mPFC cytoarchitecture and hence its function may be specifically affected by neonatal (i.e. on pd7) but not later induced (i.e. on pd21) damage to the BLA. To test this hypothesis, rats received excitotoxic lesions to the BLA on either pd7 or pd21 and were subjected to two tests putatively sensitive to mPFC dysfunction, namely food hoarding and spontaneous alternation. In addition, rats were tested for spatial learning and memory, to determine any possible effects on hippocampal function. Consistent with the documented effects of mPFC lesions, pd7 damage to the BLA impaired spontaneous alternation and food hoarding performance, an effect that was not found in rats with BLA lesions induced on pd21. Spatial learning and memory, however, were not affected by the (neonatal) lesion procedure. Together, these results indicate that neonatal BLA damage affects species-specific sequential behavior and flexibility, which may be attributed to abnormal functioning of the mPFC.

ERK1/2 activation in rat ventral tegmental area by the mu-opioid agonist fentanyl: an in vitro study.

Opioid receptors in the ventral tegmental area, predominantly the mu-opioid receptors, have been suggested to modulate reinforcement sensitivity for both opioid and non-opioid drugs of abuse. The present study was conducted to study signal transduction proteins, which may mediate the functioning of mu-opioid receptors in the neurons of the ventral tegmental area. Therefore, brain slices of the ventral tegmental area were exposed in vitro to the specific mu-opioid agonist fentanyl and immunohistochemically stained for four different activated proteins using phospho-specific antibodies. Fentanyl dose-dependently activated extracellular signal-regulated protein in brain slices of the ventral tegmental area. This activation was reversible with naloxone. Furthermore, naloxone itself also activated extracellular signal-regulated protein kinase. Under the present conditions fentanyl did not affect extracellular signal-regulated protein kinase 1 and 2, Stat and cyclic AMP-response element-binding protein activity. The direct activation of extracellular signal-regulated protein kinase in ventral tegmental area slices by the mu-opioid agonist fentanyl may suggest a role of extracellular signal-regulated protein kinase in reward processes.

Decrease in basal dopamine levels in the nucleus accumbens shell during daily drug-seeking behaviour in rats.

Accumbal dopamine (DA) is generally accepted to participate in the neural mechanisms underlying drug dependence. Recently the involvement of accumbal DA in drug-seeking behaviour has gained more experimental attention. To study an involvement of accumbal DA in drug-seeking behaviour within and between daily self-administration behaviour, changes in extracellular DA concentration in the nucleus accumbens (NAc) shell were measured during the daily dynamics of intravenous heroin and cocaine self-administration. Groups of drug naive rats were allowed to intravenously self-administer heroin (30 microg/infusion) and cocaine (30 microg/infusion) during five consecutive daily 3 h sessions. Extracellular DA concentrations in the NAc were measured before and after a single 3 h session (acute) and before and after 5 consecutive 3 h sessions (repeated). Following acute and repeated heroin and cocaine self-administration the extracellular DA concentration in the NAc shell was increased by two-fold to three-fold over baseline. These changes in DA concentrations are thought to reflect a direct effect of heroin and cocaine on DA neurotransmission in the NAC shell. Measurement of basal DA concentrations before the self-administration sessions revealed that just before the scheduled 5th self-administration session the (absolute) basal DA levels in the NAc in heroin or cocaine self-administering animals were decreased by approximately halve, as compared to drug-naive animals. It is assumed that just before a scheduled next session the (daily) desire for the drug is high. This decrease in basal DA neurotransmission in the NAc shell may, therefore, reflect an involvement of accumbal DA in drug-seeking behaviour during daily self-administration behaviour. The results demonstrate that initiation of i.v. heroin and cocaine self-administration is linked with changes in extracellular levels of DA in the NAc shell. Moreover, the present data suggest that accumbal DA might be involved in processes underlying the motivational aspects involved in daily drug-seeking behaviour, and that neuroadaptive changes in the mesolimbic DA system due to repeated drug intake lead to an tonic decrease in overall DA activity in the NAc.

Early amygdala damage in the rat as a model for neurodevelopmental psychopathological disorders.

Neurodevelopmental disorders in medial temporal lobe structures may underlie psychopathological diseases such as schizophrenia and autism. To construct an animal model for these developmental disorders, social and non-social behavioural responses were assessed in rats with ibotenic acid lesions of the (baso-)lateral and central amygdala or ventral hippocampus, induced early in life. Lesioning the amygdala on day 7 after birth resulted in a variety of behavioural disturbances later in life, whereas after similar lesions on day 21 after birth no disturbances developed, except for deficits in social behaviours. Lesioning the hippocampus led to much less disturbances. The results show that amygdala and hippocampus damage at a specific point early in life results in enduring behavioural disturbances that become more manifest after puberty. In particular, lesions of the amygdala on day 7 of life may serve as a rat model with face and construct validity for neurodevelopmental disorders in studying psychopathology.

Influence of buprenorphine, butorphanol and nalbuphine on the initiation of intravenous cocaine self-administration in drug naive mice.

The influence of different mixed mu-kappa-opioid receptor agonists-antagonists on cocaine reinforcement was studied using the method of initiation of intravenous cocaine self-administration in naive mice. Self-administration of cocaine was readily initiated according to an inverted U-shaped unit dose-response curve. Buprenorphine, butorphanol and nalbuphine tested against the optimal unit dose of cocaine (0.8 microg per infusion), inhibited initiation of cocaine self-administration in a dose-dependent manner. When tested against a scale of cocaine unit doses (0.2 -1.6 microg per infusion) buprenorphine (0.1 mg/kg, s.c.) and nalbuphine (2 mg/kg, s. c.) produced a shift of the optimal cocaine dose from 0.8 to 0.4 microg/inf, while butorphanol (1 mg/kg, s.c.) shifted the optimal unit dose of cocaine to the right along the cocaine unit doses axis. Co-administration of naloxone (0.1 mg/kg, s.c.) significantly reduced the effect of buprenorphine but failed to influence the effect of nalbuphine and butorphanol on cocaine intake. Taken together, these results suggest that nalbuphine is capable of affecting cocaine's reinforcing properties in the same manner as buprenorphine during the initiation phase of cocaine self-administration behavior, while butorphanol causes the opposite effect. Although the exact opioid profile of action of the mixed opioid receptor agonists-antagonists is as yet not precisely known, the present findings suggest that multiple opioid receptor systems (i.e. mu and kappa) play a role in reinforcing properties of cocaine and that a co-operative interaction between mu- and kappa-opioid systems may be of importance during initiation of cocaine self-administration.

Endogenous opioids and reward.

The discovery of endogenous opioids has markedly influenced the research on the biology of addiction and reward brain processes. Evidence has been presented that these brain substances modulate brain stimulation reward, self-administration of different drugs of abuse, sexual behaviour and social behaviour. There appears to be two different domains in which endogenous opioids, present in separate and distinct brain regions, are involved. One is related to the modulation of incentive motivational processes and the other to the performance of certain behaviours. It is concluded that endogenous opioids may play a role in the vulnerability to certain diseases, such as addiction and autism, but also when the disease is present, such as alcoholism.

Endogenous opioids implicated in the dynamics of experimental drug addiction: an in vivo autoradiographic analysis.

Endogenous opioids have been implicated in the neurobiological mechanisms underlying drug addiction. Although some information is available concerning effects of abused drugs on the endogenous opioid systems, the interpretation of these effects is hampered because data on the actual changes in the endogenous opioids during the dynamics of the drug addiction are lacking. The present report deals with changes in endogenous opioid activity before and after the daily self-administration session in rats offered cocaine or ethanol, using an in vivo autoradiographic receptor occupancy procedure. In separate saline-controlled experiments drug-naive rats were allowed to intravenously self-administer cocaine (30 microg/infusion) and ethanol (0.05%) for five consecutive daily sessions of 6 h. Immediately following the last session on day 5 or just before a scheduled next daily session on day 6, the rats were injected with [3H]diprenorphine and subsequently prepared for autoradiography. Decreased [3H]diprenorphine binding was observed throughout the subcortical brain after the daily session in cocaine, but hardly in animals self-administering ethanol. These changes are thought to reflect a direct or an indirect effect of the drug on endogenous opioid systems. Before the daily session, the [3H]diprenorphine binding was decreased in restricted areas of the mesocorticolimbic system and of the thalamus in both cocaine and ethanol self-administering animals. These data suggest that release of endogenous opioids at the time the desire for cocaine or ethanol is high, which may be pertinent for drug-induced craving and relapse of drug addicts.

Morphine treatment during juvenile isolation increases social activity and opioid peptides release in the adult rat.

The consequences of juvenile isolation and morphine treatment on general activity, social activity and endogenous opioid release during a social interaction test were investigated in the adult rat. Rats were either isolated or socially housed during weeks 4 and 5 of age and treated daily during this isolation period subcutaneously with either saline or morphine. Directly after a social interaction test at 10 weeks of age, rats were injected with [3H]-diprenorphine and subsequently prepared for in vivo autoradiography. The autoradiographic technique was used to visualise neuroanatomical changes in opioid receptor occupancy, probably reflecting changes in opioid peptide release, as a result of social activity. Juvenile isolation increased general activity during the social interaction test, an effect which was accompanied by a reduction of opioid receptor occupancy in many brain areas, suggesting an increased opioid peptide release as a consequence of socially-induced general activity. Morphine treatment in isolated rats caused an increase in adult social activity and enhanced opioid peptide release in some cortical regions and the ventral tegmental area as compared to saline treated rats. Both social activity and opioid receptor occupancy were unaffected by morphine treatment in non-isolated rats. The present study underscores the role of opioid systems in adult social behaviors as a consequence of juvenile isolation. The results suggest a relationship between social activity and opioid peptide release during social contact. Increased social activity seems to be accompanied by elevated opioid peptide release in distinct brain areas after morphine treatment during juvenile isolation.

Naltrexone affects cocaine self-administration in naïve rats through the ventral tegmental area rather than dopaminergic target regions.

Behavioural studies have shown an involvement of central endogenous opioid systems in experimental cocaine addiction. Seeking to further localize the attenuating effect of opioid blockade on the reinforcing effects of cocaine, naltrexone was administered locally to different regions of the mesocorticolimbic system, which are thought to be critically involved in cocaine self-administration behaviour. Both cell body and nerve terminal regions of this system were targeted. Using a model for the initiation of cocaine self-administration behaviour, no effect of naltrexone was found in caudate, amygdaloid or accumbens nuclei, nor in the medial prefrontal cortex. However, blockade of endogenous opioid receptors in the ventral tegmental area region attenuated cocaine self-administration. With the initiation model, this finding reflects an attenuating effect on the reinforcing effects of cocaine. The attenuation of self-administration was dependent on the naltrexone dose. The present findings suggest that endogenous opioid systems in the ventral tegmental area modulate the reinforcing efficacy of cocaine.

Kappa-opioid receptor blockade with nor-binaltorphimine modulates cocaine self-administration in drug-naive rats.

The modulation of the reinforcing effects of cocaine by the kappa-opioid receptor antagonist, nor-binaltorphimine was studied by using the initiation of intravenous self-administration in drug-naive Wistar rats. Treatment with nor-binaltorphimine (3.0 mg/kg s.c.) 48 h before the start of the first of five daily self-administration sessions significantly decreased the intake of cocaine when offered in a threshold unit dose (30 micrograms per infusion), but had no effect on cocaine intake when it was offered in a higher unit dose (60 micrograms per infusion). It is concluded that blockade of the kappa-opioid receptor by nor-binaltorphimine may produce a rightward shift of the unit dose-response relationship of cocaine reward, thus decreasing the sensitivity to cocaine reward. These data suggest an involvement of endogenous kappa-opioid systems in the mechanisms underlying the initiation of cocaine self-administration behaviour.

[Treatment of Frey's syndrome with botulinum A toxin]. / Behandeling van het syndroom van Frey met botuline A toxine.

A 74-year-old woman suffered from severe gustatory sweating and flushing of the preauricular skin following parotidectomy (Frey's syndrome). She was treated with intracutaneous botulinum A toxin injections in the affected skin area. Minor's test was used to determine the extent of the affected area. Within one week, the symptoms disappeared. After three weeks, Minor's test was repeated and showed minimal residual hyperhidrosis. These small areas were treated again. No side effects were noted. At follow-up one year later, the patient was free of symptoms.

Kappa-opioid receptor agonist U50,488H modulates cocaine and morphine self-administration in drug-naive rats and mice.

Modulation of the reinforcing effects of cocaine and morphine by the kappa-opioid receptor agonist U50,488H (trans-3,4-dichloro-N-methyl-N-(2-1-pyrrolidinyl)-cyclohexyl-benzeace tamide) was studied by using the method of intravenous (i.v.) self-administration in drug-naive Wistar rats and DBA/2 mice. Self-administration of cocaine (by rats) and morphine (by mice) was readily initiated and showed an inverted U-shaped unit dose-response curve. Treatment with the kappa-opioid receptor agonist U50,488H dose dependently decreased the intake of both cocaine and morphine when offered in doses that readily initiated and sustained self-administration behavior. Interestingly, treatment with U50,488H induced self-administration behavior with lower sub-threshold doses of cocaine and morphine. With regard to the inverted U-shaped relation between the dose of the drug and the number of self-infusions, it seems that activation of the kappa-opioid receptor with U50,488H produced an almost parallel shift to the left, indicating an increased sensitivity of the animals for the reinforcing effects of cocaine and morphine. These data demonstrate an involvement of kappa-opioid systems in the neurobiological mechanisms underlying drug addiction in general, and sensitivity for drug reward in particular. Furthermore, the dual effect of kappa-opioid receptor agonists on drug self-administration may prompt further research into the mechanisms underlying the role of endogenous opioids in drug self-administration.

Naloxone inhibits the reinforcing and motivational aspects of cocaine addiction in mice.

The opioid receptor antagonist naloxone is known to influence a wide range of behavioral effects of cocaine, including its addictive property. In the present study the effects of different doses of naloxone and naloxone-methyl-iodide, a methylated analogon of naloxone that does not penetrate the blood-brain barrier, on the action of cocaine in the intravenous self-administration and conditioned place preference (CCP) paradigm were assessed. Systemic naloxone, but not naloxone-methyl-iodide, dose-dependently suppressed cocaine intake during self-administration and decreased the preference for the cocaine-associated compartment in the CCP paradigm. A significant blockade of cocaine's effects was only present at a relatively high dose of NLX (1.0 mg/kg, s.c.). In addition, NLX produced a rightward shift in the inverted U-shaped dose-response curve for cocaine reward during self-administration, indicating a decrease in sensitivity for the reinforcing effects of cocaine. These data demonstrate that blockade of opioid receptors in the brain block both the reinforcing and conditioned motivational effects of cocaine. An interaction between endogenous opioid systems and local dopaminergic systems is suggested in mediating the effects of NLX on cocaine.

Effect of nucleus accumbens dopamine depletion on motivational aspects involved in initiation of cocaine and heroin self-administration in rats.

The involvement of mesolimbic dopamine (DA) systems in motivational aspects of drug-taking behavior during initiation of drug self-administration was investigated using a recently developed behavioral paradigm. In separate experiments animals were allowed to self-administer cocaine or heroin (0.16 and 0.32 mg . kg-1 per inf) during 5 consecutive daily 3-h sessions. During a 15-min period preceding the last four self-administration sessions lever-press behavior was measured in absence of the drug as an index of the motivational aspects involved in drug-taking behavior. The effect of 6-hydroxydopamine (6-OHDA) lesion of the nucleus accumbens (NAC) on lever-press behavior before and during self-administration was measured. Destruction of DA terminals in the NAC did not affect initiation of heroin self-administration nor the lever-press behavior during the period preceding the self-administration sessions. In cocaine animals 6-OHDA lesion of the NAC decreased the total intake of cocaine during the self-administration sessions and impaired discriminative lever-responding for the drug, both during cocaine self-administration, and during preceding periods when no cocaine was available. It is concluded that DAergic systems in the NAC might be involved in the reinforcement and/or motivational processes underlying cocaine self-administration. The present findings, however, do not support the notion of a critical role of NAC DA in the motivational aspects of drug-taking behavior in general.

Assessment of motivational aspects involved in initiation of cocaine and heroin self-administration in rats.

A behavioral paradigm was explored to assess the motivational aspects involved in drug-taking behavior during initiation of drug self-administration. In separate saline-controlled experiments, naive animals were allowed to self-administer either cocaine or heroin (0.16 and 0.32 mg/kg per infusion) during five consecutive daily 3-h sessions by pressing one of two levers present in the test cage. During 15 min preceding the last four self-administration sessions, the animals had access to the levers but pressing the reinforcement lever did not result in a drug infusion. The animals properly self-administered both doses of cocaine and heroin, because the amount of self-infusions was higher than their saline control groups. Animals self-administering the high dose of cocaine and either dose of heroin performed lever-press behavior during the preceding period in a similar fashion as during the self-administration sessions, suggesting that this behavior during the preceding period was performed in the absence of the primary reinforcer, this behavior likely reflects the motivational state of animals to obtain the drug reinforcer, and thus may serve as a measure of the motivational aspects involved in the initiation of drug self-administration.