Inhibition of THIP on morphine-induced hyperactivity, reverse tolerance and postsynaptic dopamine receptor supersensitivity

Conditioned Reward of Opioids, but not Psychostimulants, is Impaired in GABA-A Receptor δ Subunit Knockout Mice

Extrasynaptic δ subunit-containing γ-aminobutyric acid type A receptors (δ-GABA_A Rs) are emerging as targets for a number of neuropsychopharmacological drugs, including the direct GABA site agonist gaboxadol and neuroactive steroids. Among other regions, these δ-GABA_A Rs are functionally expressed in the ventral tegmental area (VTA), the cell body region of mesocorticolimbic dopamine (DA) system important for motivated behaviours, and in the target region, the nucleus accumbens. Gaboxadol and neurosteroids induce VTA DA neuron plasticity ex vivo, by inhibiting the VTA GABA neurons, and aversive place conditioning, which are absent in the δ-GABA_A R knockout mice (δ-KO). It is not known whether δ-GABA_A Rs are important for the effects of other drugs, such as opioids (that also inhibit GABA neurons) and stimulants (that primarily elevate monoamine levels). Here, we used δ-KO mice and conditioned place preference (CPP) test to study the rewarding effects of morphine (20 mg/kg), methamphetamine (1 mg/kg) and mephedrone (5 mg/kg). Morphine-induced nociception was also assessed using tail-flick and hot-plate tests. We found that the δ-KO mice failed to express morphine-induced CPP, but that they were more sensitive to morphine-induced analgesia in the tail-flick test. In contrast, stimulant-induced CPP in the δ-KO mice was similar to that in the wild-type controls. Thus, the conditioned rewarding effect by opioids, but not that of stimulants, was impaired in the absence of δ-GABA_A Rs. Further studies are warranted to assess the potential of δ-GABA_A R antagonists as possible targets for reducing morphine reward and potentiating morphine analgesia.

A GABA(B) agonist reverses the behavioral sensitization to morphine in rats

RATIONALE

In laboratory animals, repeated administration of drugs of abuse causes sensitization to their stimulant and rewarding effects. Neuroadaptations underlying sensitization could be related to those that contribute to addictive behaviors. An increased understanding of the molecular mechanisms of sensitization could lead to improved treatments for addiction.

OBJECTIVES

Since baclofen (BCF) co-administration has been reported to block the development and the expression of motor sensitization to morphine (MOR), the present study examined the hypothesis that a chronic treatment with BCF alone might reverse and/or prevent MOR-induced sensitization.

METHODS

Rats were first sensitized to MOR (saline or 10 mg/kg MOR i.p.; days 1-10) and then chronically treated with BCF (saline or 2 mg/kg BCF i.p.; days 11-20). Finally, the motility effect of MOR (10 mg/kg i.p.) was assessed 3 and 30 days after the end of BCF treatment. The same rats were again challenged with MOR on day 70, after a further period of saline or MOR treatment (days 51-60).

RESULTS

Behavioral sensitization to MOR was observed in control animals but not in rats chronically treated with BCF (days 23 and 50). Thus, BCF completely reversed MOR-induced sensitization, and its effect was long lasting. However, a previous repeated BCF treatment did not prevent the development of sensitization to MOR both in naive and desensitized rats.

CONCLUSIONS

The present results confirm that gamma-aminobutyric acid (GABA)(B) receptors play an important role in the expression of motor sensitization to MOR and suggest that GABA(B) agonists could be useful for reversing the neuroadaptations related to drug addiction.

β-Phenylethylamines and the isoquinoline alkaloids

This review covers beta-phenylethylamines and isoquinoline alkaloids derived from them, including further products of oxidation. condensation with formaldehyde and rearrangement, some of which do not contain an isoquinoline system, together with naphthylisoquinoline alkaloids, which have a different biogenetic origin. The occurrence of the alkaloids, with the structures of new bases, together with their reactions, syntheses and biological activities are reported. The literature from July 2002 to June 2003 is reviewed, with 568 references cited.

Endogenous opiates and behavior: 2002

This paper is the twenty-fifth 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 2002 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 and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).