Lesion of dopamine mesolimbic neurons blocks behavioral effects induced by the endogenous enkephalins but not by a mu-opioid receptor agonist

Contribution of Delta-Opioid Receptors to Pathophysiological Events Explored by Endogenous Enkephalins

Very few discoveries in the neurosciences have triggered clinical speculation and experimentation regarding the etiology of psychiatric illness to the same extent as that following identification of the opiate receptor(s) and subsequent isolation of endogenous morphine-like peptides. There is overwhelming evidence in animals and in human that opioids are involved in behaviorally relevant issues such as the modulation of pain, the response to stress, motivation, addiction, sexuality, food intake, etc., but our knowledge on the possible relation between opioids and mental illness is still very limited.These responses could be explored eitheir by using higlhy selective delta agonist or by emphasizing the effects of phasically secreted endogenous opioid peptides, enkephalin. Both approaches were investigated in particular through protection of enkephalin degradation by dual enkephalinase ihibitors DENKIs such as RB101, PL37 or PL265.

The CCK-B agonist, BC264, increases dopamine in the nucleus accumbens and facilitates motivation and attention after intraperitoneal injection in rats

Although it is known that panic attacks are triggered by the cholecystokinin fragment CCK4, the specific involvement of peripheral or central cholecystokinin CCK receptors in various adaptive processes such as emotion, memory and anxiety has yet to be demonstrated. With this aim, we have investigated the biochemical and pharmacological effects resulting from the administration of BC264, a highly potent and selective CCK-B agonist able to cross the blood-brain barrier. Very low doses of BC264 (microg/kg i.p.), increased the exploration of animals submitted to an unknown territory but were devoid of anxiogenic properties in the elevated plus maze. BC264 increased locomotion and rearings of rats newly placed in an open field and improved their spontaneous alternation in a Y-maze. The use of vagotomized animals showed that the increased alternation induced by BC264 did not require an intact vagus nerve, unlike the locomotor activation. These behavioural effects, prevented by the prior i.p. administration of the CCK-B antagonist L-365,260 but not by the CCK-A antagonist L-364,718, were shown to depend on dopaminergic systems, since they were blocked by D1 (SCH23390, 25 microg/kg i.p.) or D2 (sulpiride, 50 or 100 mg/kg i.p.) antagonists. In addition, bilateral perfusion in freely moving rats of BC264 at pharmacologically active doses, using a newly designed microdialysis system, was found to increase the extracellular levels of DA, DOPAC and HVA in the anterior part of the nucleus accumbens. These results show that activation of CCK-B receptors by BC264 does not produce anxiogenic-like effects but appears to improve motivation and attention, whereas other CCK-B agonists such as BocCCK4 induce anxiogenic responses. Several explanations, including the existence of different sub-sites of the CCK-B receptor, could account for these differential effects.

Persistent increases in basal cerebral metabolic activity induced by morphine sensitization

To characterize the underlying neuroanatomic substrate of morphine (MS) sensitization, changes in the local cerebral metabolic rate for glucose (LCMRglu) were examined in 95 brain regions of male F-344 rats using the 2-deoxy-D-[1-14C]glucose method. The results of these experiments demonstrate that MS-induced sensitization is manifested by increases in basal metabolic activity that last for at least 6 days. Although changes in basal metabolic rate were found to be more extensive in the presence of conditioned cues, the increases in LCMRglu in nonconditioned sensitized rats indicate a basic underlying pharmacologic effect of MS sensitization on basal brain activity. Regions in which MS sensitization had a lasting pharmacologic effect include the shell of the nucleus accumbens, the prelimbic area of the prefrontal cortex, and the dorsolateral prefrontal cortex. Interestingly, the core of the nucleus accumbens and regions of the caudate were found to have an increased LCMRglu only in the presence of conditioned cues, indicating conditioned brain activity without observable changes in behavior. The previous administration of an MS-sensitizing treatment was also found to alter the cerebral metabolic response to a subsequent acute MS challenge (0.5 mg/kg, subcutaneously), most notably in forebrain systems. The more widespread activation of brain structures in the basal state in the presence of conditioned cues suggests that these MS-sensitized rats may model an altered brain state related to craving in the abstinent opiate addict.

Antinociceptive and behavioral effects of synthetic deltorphin analogs

A possible correlation of behavioral, antinociceptive and cataleptic responses with central delta- and mu-opioid receptor stimulation was tested for in the rat by i.c.v. injections of some synthetic deltorphin analogs. At doses ranging from 0.1 to 3.0 nmol/rat, the selective delta-opioid receptor agonist, [D-Ala2,Glu4]deltorphin (Tyr-D-Ala-Phe-Glu-Val-Val-Gly-NH2), induced a dose-dependent stereotyped pattern of locomotor activity, reaching the maximum in the first 30 min; doses higher than 30 nmol induced early and fleeting antinociception. The replacement of Glu4 by Gly, Ala, Val, His or Asn yielded peptides with a lower delta-selectivity because of a gain in mu-affinity. [D-Ala2,Ala4]deltorphin (0.14-4.0 nmol) induced negligible behavioral stimulation but a rapidly appearing and long-lasting analgesia and catalepsy. The other four synthetic peptides induced biphasic effects: low dosages stimulated locomotion whereas higher doses initially suppressed, then increased locomotor activity. At doses ranging from 1 to 70 nmol all the peptides induced analgesia and catalepsy. In experiments examining the locomotor and antinociceptive effects induced by 14 nmol of [D-Ala2,Gly4]deltorphin in rats pretreated with mu and delta antagonists, the non-selective mu-opioid receptor antagonist, naloxone (1 mg/kg i.p.), reduced analgesia and abolished the initial hypolocomotion. The delta-selective antagonist, naltrindole (10 mg/kg i.p.), abolished locomotor activity without affecting analgesia. The mu1 -selective antagonist, naloxonazine (10 mg/kg i.v.), seemed to prolong analgesia and immobility. Hence this peptide appears to activate, in addition to delta-receptors, mainly the opioid receptor mu2-subtype, which mediates catalepsy in the rat. We suggest that the mu2- and delta-opioid receptors of the rat brain modulate locomotor behavior by activating functionally opposed responses. [D-Ala2,Ala4]deltorphin had an antinociceptive and cataleptic potency higher than would have been expected from its mu-affinity. A possible explanation might be a mu/delta-opioid receptor interaction.

The therapeutic role of naltrexone in negative symptom schizophrenia

1. Naltrexone (50 mg bid, p.o.) was administered in a double-blind fashion (with placebo control) to chronic schizophrenic patients who maintained their routine neuroleptic and anxiolytic therapy. 2. Both positive and negative symptom patients who received naltrexone improved with regard to symptoms involving deterioration and social withdrawal. No significant amelioration was recorded in subjects assuming placebo relative to the same psychopathological areas. 3. Favourable results were obtained mainly from patients affected by negative symptom schizophrenia. 4. Naltrexone may have acted by direct or indirect neurochemical mechanisms related to negative symptom schizophrenia.

Intra-VTA injections of the mu-opioid antagonist CTOP enhance locomotor activity

In this paper we report on the effects of microinjections of the mu-opioid antagonist CTOP (D-Pen-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2) into the ventral tegmental area (VTA) on activity and ingestive behavior in the rat. Intra-VTA CTOP (0.015, 0.15, and 1.5 nmol per side) dose-dependently increased activity, whereas it had no effect on feeding and drinking behavior. These results are consistent with previous reports that intra-VTA injections of CTOP enhance extracellular dopamine levels in the nucleus accumbens. Furthermore, we propose a model of VTA mu-opioid mechanisms that might account for these surprising effects of intra-VTA CTOP.

Opposite role of CCKA and CCKB receptors in the modulation of endogenous enkephalin antidepressant-like effects

Systemic administration of RB 101, a complete inhibitor of the enkephalin degrading enzymes, has been reported to induce naltrindole-reversed antidepressant-like effects in the conditioned suppression of motility (CSM) test in mice. The selective CCKB antagonist L-365,260 also elicits the same naltrindole-blocked responses on CSM. The aim of this study was therefore to investigate the possible modulation of RB 101 induced behavioral responses by activation or blockade of CCK receptors. Thus, the effects induced by RB 101 administered alone or associated with an ineffective dose of a selective CCKB agonist (BC 264), a CCKB antagonist (L-365,260) or a CCKA antagonist (L-364,718), were evaluated on the CSM in mice. RB 101 alone decreased the stress-induced loss of motility, as previously reported. The antidepressant-like effect of RB 101 was potentiated by L-365,260, and suppressed by BC 264 and to a lesser extent by L-364,718. The facilitatory effect induced by L-365,260 on RB 101 responses was blocked by the delta selective antagonist naltrindole. All these effects occurred only in shocked animals. The present results suggest that the activation of CCKA and CCKB receptors by endogenous CCK, could play an opposite role in the control of behavioral responses induced by endogenous enkephalins. Delta opioid receptors seem to be selectively involved in this interaction.

Antidepressant-like effects of CCKB antagonists in mice: antagonism by naltrindole

1. The effects of selective CCKB agonists, BC 264 and BC 197 were investigated in the conditioned suppression of motility test in mice, an animal model used to select antidepressant drugs. The results showed that both CCKB agonists at doses of 3 and 30 micrograms kg-1, accentuated the suppression of motility in shocked mice and did not modify the behaviour of non-shocked mice. The effects of BC 264 were suppressed by L-365,260. 2. L-365,260 alone, at doses of 0.2 and 2 mg kg-1 decreased motor inhibition in shocked mice and had no effect in non-shocked mice. 3. The effects of L-365,260 observed in shocked mice were suppressed by naltrindole, a selective antagonist for delta-opioid receptors, suggesting the occurrence of physiological adverse interactions between CCK and opioid systems. 4. Together, these results suggest that CCKB antagonists could block centrally located CCKB receptors to produce antidepressant-like effects which could indirectly involve delta-opioid receptor stimulation.

Ventral mesencephalic delta opioid receptors are involved in modulation of basal mesolimbic dopamine neurotransmission: an anatomical localization study

Microdialysis was used to examine the anatomical localization of mesencephalic delta opioid receptors that participate in modulation of mesolimbic dopamine (DA) neurotransmission. Independent groups of rats were injected with DPDPE into the ventral tegmental area (VTA) or interpeduncular nucleus (IPN). Extracellular nucleus accumbens DA and DOPAC concentrations were elevated after DPDPE injections into either site, but injections into the VTA were effective at lower doses than were injections into the IPN. Thus, it appears that the DA-modulating actions of DPDPE are mediated in the VTA rather than the IPN.

Neurotransmitter regulation of dopamine neurons in the ventral tegmental area

Over the last 10 years there has been important progress towards understanding how neurotransmitters regulate dopaminergic output. Reasonable estimates can be made of the synaptic arrangement of afferents to dopamine and non-dopamine cells in the ventral tegmental area (VTA). These models are derived from correlative findings using a variety of techniques. In addition to improved lesioning and pathway-tracing techniques, the capacity to measure mRNA in situ allows the localization of transmitters and receptors to neurons and/or axon terminals in the VTA. The application of intracellular electrophysiology to VTA tissue slices has permitted great strides towards understanding the influence of transmitters on dopamine cell function, as well as towards elucidating relative synaptic organization. Finally, the advent of in vivo dialysis has verified the effects of transmitters on dopamine and gamma-aminobutyric acid transmission in the VTA. Although reasonable estimates can be made of a single transmitter's actions under largely pharmacological conditions, our knowledge of how transmitters work in concert in the VTA to regulate the functional state of dopamine cells is only just emerging. The fact that individual transmitters can have seemingly opposite effects on dopaminergic function demonstrates that the actions of neurotransmitters in the VTA are, to some extent, state-dependent. Thus, different transmitters perform similar functions or the same transmitter may perform opposing functions when environmental circumstances are altered. Understanding the dynamic range of a transmitter's action and how this couples in concert with other transmitters to modulate dopamine neurons in the VTA is essential to defining the role of dopamine cells in the etiology and maintenance of neuropsychiatric disorders. Further, it will permit a more rational exploration of drugs possessing utility in treating disorders involving dopamine transmission.

Effect of inhibiting enkephalin catabolism in the VTA on motor activity and extracellular dopamine

The mixed inhibitor of enkephalin catabolism, kelatorphan, was microinjected into the ventral tegmental area (VTA) of rats to determine if endogenous enkephalins can modulate dopamine transmission in the mesoaccumbens projection. The concentration of extracellular dopamine content in the nucleus accumbens was monitored using in vivo microdialysis simultaneously with measuring motor behavior. Kelatorphan microinjection into the VTA produced a dose-related increase in motor activity and extracellular dopamine in the nucleus accumbens. While the change in extracellular dopamine was modest as compared to exogenous stimulation by a mu agonist such as DAMGO, there was a marked increase in the extracellular content of dopamine and serotonin metabolites. This suggests that mesoaccumbens dopamine transmission is under tonic control of endogenous enkephalins at the ventral tegmental area level.