The µ opioid irreversible antagonist beta-funaltrexamine differentiates the discriminative stimulus effects of opioids with high and low efficacy at the μ opioid receptor

 The purpose of the present study was to determine the relative intrinsic efficacy of various opioids using the irreversible μ opioid antagonist beta-funaltrexamine (βFNA). To this end, pigeons were trained to discriminate 3.0 (n=6) or 1.8 (n=1) mg/kg morphine from distilled water in a two-key, food-reinforced, drug discrimination procedure. The μ opioids fentanyl, l-methadone, buprenorphine, butorphanol, nalorphine, nalbuphine and levallorphan, as well as the δ opioid BW373U86, substituted completely for the morphine stimulus. The stimulus effects of morphine were antagonized (i.e., produced a significant increase in the ED₅₀ value) by a 10 mg/kg but not a 5 mg/kg dose of βFNA. Antagonist effects of βFNA were observed following a 2-h pretreatment, but not following 26-, 50-, 74-, 98- or 146-h pretreatments. The stimulus effects produced by fentanyl, l-methadone and buprenorphine were not antagonized by doses of βFNA as high as 20, 10 and 10 mg/kg, respectively. The lowest dose of βFNA required to antagonize the stimulus effects of butorphanol was 10 mg/kg, whereas the effects of nalorphine, nalbuphine and levallorphan were antagonized by a dose of βFNA as low as 5 mg/kg. The δ opioid BW373U86 substituted for the morphine stimulus, and this effect was not antagonized by 10 mg/kg βFNA. The pkB values for naloxone (1.0 mg/kg) against the stimulus effects of fentanyl (6.70) and morphine (6.52) were considerably higher than that for BW373U86 (4.60), indicating further that the morphine-like stimulus effects produced by BW373U86 were not mediated by activity at the μ opioid receptor. These findings indicate that the strategy of irreversible antagonism can be used effectively to differentiate opioids with varying degrees of intrinsic efficacy at the μ opioid receptor in a pigeon drug discrimination procedure. In particular, the ranking of these drugs by relative intrinsic efficacy at the μ opioid receptor is: l-methadone=fentanyl≥buprenorphine≥morphine≥ butorphanol>nalorphine=nalbuphine=levallorphan. Additionally, the short-acting effect of βFNA in the pigeon suggests that the recovery of μ opioid receptor function varies across species. Received: 19 August 1997 / Final version: 28 March 1998     

The behavioral effects of sertraline, fluoxetine, and paroxetine differ on the differential-reinforcement-of-low-rate 72-second operant schedule in the rat

Rationale: Endogenous opioid systems within the mesencephalic periaqueductal gray matter (PAG) appear to be intricately involved in many affective, defensive, submissive, and reflexive responses, and these systems are activated by aversive stimuli. Objectives: The present experiments evaluated the influence of opioid receptors within the PAG on affective vocal and reflexive responses to aversive stimuli in socially inexperienced, as well as defensive and submissive responses in defeated, adult male Long-Evans rats. Methods: Defeat stress consisted of: (1) an aggressive confrontation with a ”resident” stimulus rat in which the experimental ”intruder” rat exhibited escape, defensive and submissive behaviors [i.e. upright, supine postures and ultrasonic vocalizations (USV)], and subsequently, (2) protection from the resident rat with a wire mesh screen for ca. 25 min. Defeat stress was immediately followed by an experimental session with thermal antinociceptive and tactile startle stimuli (20 psi airpuffs). Results: The μ opioid receptor agonist morphine (0.3, 1, 3 μg IC) attenuated startle-induced USV and the tail-flick reflex in socially inexperienced and defeated rats, with both groups of rats demonstrating equal sensitivity to morphine. Morphine decreased defeat-induced USV and increased the display of the crouch posture in defeated rats; these morphine effects in socially inexperienced and defeated rats were re- versed with the opioid receptor antagonist naltrexone (0.1 mg/kg IP). Conclusions: These results reveal that the ventrolateral PAG is an important site in which μ opioid receptor agonists such as morphine mediate affective vocal and submissive responses, yet this structure is not critical in the display of defeat stress-augmented effects of morphine. Endogenous opioid mechanisms appear to participate in the organization of defensive behavior, namely, to facilitate a shift from active to passive forms of coping. Received: 9 November 1998 / Final version: 29 April 1999     

Local administration of mu or kappa opioid agonists attenuates capsaicin-induced thermal hyperalgesia via peripheral opioid receptors in rats

Rationale: By acting on peripheral opioid receptors, opioid agonists can attenuate nociceptive responses induced by a variety of agents. Objectives: This study was conducted to characterize capsaicin-induced thermal hyperalgesia in rats and to evaluate the hypothesis that local administration of either mu or kappa opioid agonists (fentanyl and U50,488, respectively) can attenuate capsaicin-induced nociception. Methods: Capsaicin was administered s.c. in the tail of rats to evoke a nociceptive response, which was measured by the warm-water tail-withdrawal procedure. Either fentanyl or U50,488 was co-administered with capsaicin in the tail to evaluate local antinociceptive effects. In addition, the local antagonism study was performed to confirm the site of action of both opioid agonists. Results: Capsaicin (0.3–10 μg) dose dependently produced thermal hyperalgesia manifested as reduced tail-withdrawal latencies in 45°C water. Co-administration of either fentanyl (0.32–3.2 μg) or U50,488 (10–100 μg) with capsaicin (3 μg) attenuated capsaicin-induced hyperalgesia in a dose-dependent manner. Furthermore, this local antinociception was antagonized by small doses (10–100 μg) of an opioid antagonist, quadazocine, applied s.c. in the tail. However, the locally effective doses of quadazocine, when applied s.c. in the back (i.e., around the scapular region), did not antagonize either fentanyl or U50,488. Conclusions: In this experimental pain model, activation of peripheral mu or kappa opioid receptors can attenuate capsaicin-induced thermal hyperalgesia in rats. It supports the notion that peripheral antinociception can be achieved by local administration of analgesics into the injured tissue without producing central side effects. Received: 10 May 1999 / Final version: 18 August 1999     

Cocaine-opioid interactions in groups of rats trained to discriminate different doses of cocaine

Rationale: The growing abuse of cocaine combined with morphine-like opiates (”speedballs”) in human addicts has prompted efforts to characterize the roles of different opioid receptor subtypes in mediating their combined effects. Previous drug discrimination studies in rats have been inconsistent in showing significant interactions between cocaine and opioid agonists in subjects trained to discriminate a relatively high dose of cocaine from vehicle. It is known, however, that the training dose of cocaine can play a key role in drug-substitution and drug-interaction profiles and, therefore, training rats to discriminate a relatively low dose of cocaine may influence its interactions with opioid agonists. Objectives: The objectives of this study were to examine the degree to which a relatively high (10 mg/kg) versus a relatively low (3.0 mg/kg) cocaine training dose influenced the interactions between cocaine and either the μ opioid agonist morphine or the κ opioid agonist U50,488. Methods: Substitution tests with cumulative doses of cocaine, morphine and U50,488 were conducted, as were studies in which selected doses of morphine or U50,488 were administered prior to cumulative doses of cocaine. Results: In substitution tests, cocaine was 2.9 times more potent under the low- than the high-dose training condition. Morphine substituted fully for cocaine in the majority of subjects trained to discriminate the low, but not the high, dose of cocaine. U50,488 engendered mainly saline-lever responses under both training conditions. In pretreatment studies, morphine enhanced and U50,488 attenuated the discriminative stimulus effects of cocaine in low-dose, but not high-dose, trained rats. In low-dose trained rats, cocaine was five- to eightfold more potent after morphine and three- to fourfold less potent after U50,488 pretreatments. Conclusions: The results demonstrate that cocaine–opioid interactions are dependent on the training dose of cocaine in rats and suggest an opposing influence of μ and κ opioid receptors in modifying the discriminative stimulus effects of cocaine. Received: 9 December 1998 / Final version: 24 June 1999     

Quadazocine decreases responding reinforced by ethanol, sucrose, and phencyclidine fluid deliveries in rhesus monkeys: comparison to naltrexone’s effects

  Rationale: The endogenous opioid system may mediate the reinforcing effects of ethanol as well as sweet-tasting solutions. For example, opioid antagonists, such as naltrexone, reduce ethanol- and sucrose-reinforced responding in rhesus monkeys. If these effects are due to blockade of the μ-receptor, then an opioid antagonist such as quadazocine with a receptor selectivity profile similar to that of naltrexone should reduce responding at doses correlated with its μ-selectivity. Objectives: To determine whether quadazocine would reduce responding for ethanol and sucrose at μ-selective doses, and whether quadazocine and naltrexone would reduce responding for a bitter-tasting drug solution such as phencyclidine. Methods: Rhesus monkeys were given access to ethanol, sucrose, or phencyclidine concurrently with water. Prior to the drinking sessions, quadazocine (0.032–3.2 mg/kg) or saline was injected intramuscularly. During the phencyclidine experiment, naltrexone (0.1 and 0.32 mg/kg) was also tested. Results: The highest quadazocine doses (1 and 3.2 mg/kg) reduced ethanol and sucrose fluid deliveries without affecting the concurrently available water. Quadazocine reduced the fluid deliveries of both phencyclidine and water when concurrently available. Naltrexone reduced only phencyclidine fluid deliveries. Conclusions: The opioid antagonist effect on oral-reinforced responding is not selective for ethanol or sweet-tasting solutions; responding for phencyclidine was reduced as well. Quadazocine and NTX may reduce responding by blocking the μ-receptor because the relative potency of these antagonists to reduce oral self-administration was similar to their relative potency to produce withdrawal in morphine-dependent monkeys. However, water responding was low in these experiments, and thus we cannot rule out rate-dependent effects of the antagonists. Received: 24 June 1998 / Accepted: 18 February 1999     

Discriminative stimulus and antinociceptive effects of dihydroetorphine in rhesus monkeys

Abstract Rationale. Although dihydroetorphine has μ opioid agonist activity there is evidence to suggest that it is not identical to that of morphine. Objective. This study compared dihydroetorphine to other opioids under behavioral conditions that are sensitive to μ opioid agonism. Methods. The acute effects of dihydroetorphine, etorphine and morphine were evaluated using two procedures. In one procedure, monkeys received 3.2 mg/kg per day of morphine and discriminated naltrexone from saline while responding under a fixed-ratio 5 schedule of stimulus shock termination. In addition, a warm-water, tail-withdrawal procedure was used in untreated monkeys. Results. When acutely deprived of morphine, monkeys responded on the naltrexone lever, and this effect was reversed by dihydroetorphine, etorphine and morphine. Each agonist produced the maximum (20-s latency) antinociceptive effect in 50°C water. Naltrexone antagonized the discriminative stimulus and antinociceptive effects of dihydroetorphine and etorphine, although Schild analyses yielded large variability in slopes and pA₂ values. Naltrexone reversed established effects of dihydroetorphine and morphine in both procedures and pretreatment with dihydroetorphine (2, 6 or 24 h) did not alter the discriminative stimulus effects of morphine. Conclusions. Taken together, these data support the notion that dihydroetorphine is a μ agonist with a short duration of action; however, variability in antagonism of dihydroetorphine and morphine might be a manifestation of differences that have been reported for these drugs at the cellular level. Electronic Publication     

Celiprolol: agonist and antagonist effects at cardiac β1- and vascular β2-adrenoceptors determined under in vivo conditions in the rat

Celiprolol is a β-adrenoceptor antagonist which has desirable ancillary properties since it is relatively cardioselective and can exert direct vasodilator and bronchodilator effects. Here agonist and antagonist effects of celiprolol at cardiac β₁- and vascular β₂-adrenoceptors were determined under in vivo conditions in the rat. All experiments were carried out in catecholamine-depleted, pentobarbital anesthetized and vagotomized rats, placed under artificial respiration. I.v. administrations were madevia the femoral vein. Blood pressure was measured from the cannulated right carotid artery and heart rate was recorded with a cardiotachometer. Celiprolol (10 μg/kg to 1 mg/kg i.v.) produced dose-related increases in heart rate and decreases in mean carotid artery blood pressure which were of longer duration than those mediated by standard agonists of β₁-(isoprenaline) or β₂-(salbutamol) adrenoceptors respectively. Although the maximal increase in heart rate by celiprolol (110±4 beats/min, n=7) was approximately half that of isoprenaline (198±1 beats/min, n=5), isoprenaline acted at doses 200-fold lower than celiprolol. Betaxolol (0.03-0.3 mg/kg i.v.), a β₁-adrenoceptor antagonist, inhibited strongly and with similar potency the tachycardic effects of celiprolol (DR₁₀ = 45 μg/kg i.v.) as well as isoprenaline (DR₁₀ = 45 μg/kg i.v.). On the other hand, the hypotensive effects of celiprolol and salbutamol were antagonized markedly and with similar potency by ICI118,551, a relatively selective β₂-adrenoceptor antagonist (DR₁₀ = 15 and 25 μg/kg i.v. respectively). In rats pretreated with celiprolol (0.03 to 0.3 mg/kg i.v.), the heart rate dose-response curves to isoprenaline were shifted to the right of those determined in matched groups of vehicle-pretreated animals. In this respect, celiprolol was half as potent as betaxolol in blocking cardiac β₁-adrenoceptors. Furthermore, celiprolol also antagonized the hypotensive effects of salbutamol, but, in this respect, celiprolol was 90-fold less potent than ICI 118,551. In conclusion, these results clearly indicate that celiprolol has the ability of stimulating and blocking not only cardiac β₁- but also vascular β₂-adrenoceptors. The effects on cardiac β₁-adrenoceptors as well as the agonism of β₂-adrenoceptors are produced by similar doses of celiprolol. These doses are notably lower than those necessary to block β₂-adrenoceptors. Thus, this pharmacological profile, which has also been demonstrated in humans, indicates that celiprolol is a modulator of cardiac β₁-adrenoceptors with vascular β₂-adrenoceptor agonist properties. Received: 26 June 1996 / Accepted: 3 March 1997     

Methoclocinnamox: time course of changes in alfentanil-reinforced responding in rhesus monkeys

Rationale: Methoclocinnamox (MC-CAM) possesses initial partial μ-opioid agonist activity with subsequent long-lasting μ-antagonist effects. This profile of activity is similar to that of buprenorphine, a compound with proposed use in the treatment of opioid abuse, suggesting a possible therapeutic use for MC-CAM as well. Objective: The current study assessed the time course of the ability of MC-CAM and buprenorphine to antagonize the reinforcing effects of alfentanil and compared this with that of buprenorphine. Methods: Rhesus monkeys self-administered a range of doses of alfentanil (0.03–1 μg/kg per injection) under a fixed-ratio 30, time-out 45 s schedule of i.v. drug delivery. MC-CAM was substituted for alfentanil on occasion, and a dose of 1.0 mg/kg MC-CAM or buprenorphine was given prior to sessions in which alfentanil was available. In the pretreatment studies, a wider range of alfentanil doses was utilized (0.03–30 μg/kg per injection). Results: MC-CAM maintained self-administration behavior and was nearly equipotent with buprenorphine as a reinforcer in this para digm. Both drugs, when given prior to a session in which alfentanil was available, produced a decrease in the reinforcing potency of alfentanil. The antagonist effects of the pretreatments were largest 30 min following administration and decreased over the next several days. The duration of MC-CAM’s antagonism of alfentanil was approximately 4 days; the duration of buprenorphine as an antagonist was approximately 2 days. Conclusion: These data suggest that MC-CAM has a longer duration of antagonist effects than buprenorphine and it may therefore have an advantage in the treatment of opioid abuse. Received: 27 April 1999 / Final version: 29 September 1999     

Response of nicotine self-administration in the rat to manipulations of mu-opioid and γ-aminobutyric acid receptors in the ventral tegmental area

Rationale: The mesolimbic dopamine system has been implicated in the reinforcing effects of nicotine, a drug which appears to act at least in part through the ventral tegmental area (VTA). Other neuronal elements in the VTA are important in drug reward. In particular, mu opioid receptors in the VTA have been shown to influence cocaine reinforcement. Objective: The aim of this study was to test whether the mu opioid receptors in the VTA also regulate the intake of nicotine. Methods: This research was carried out with animals trained to self-administer nicotine or cocaine, or to respond for food. Mu receptors were targeted with the selective agonist [D-Ala2,N-Me-Phe4,Gly-ol5]-enkephalin (DAMGO) and γ-aminobutyric acid (GABA) receptors with the selective agonists baclofen and muscimol; each of these compounds was delivered by microinfusion into the VTA. Results: The mu-selective agonist DAMGO, tested over a dose range of 0.005–0.05 μg, had an effect at the highest dose only, where it produced a reduction in self-administration maintained by doses of either 10 μg/kg or 30 μg/kg per infusion of nicotine. Intra-VTA microinfusions of DAMGO did not reinstate extinguished responding previously established for nicotine, nor did they have prominent effects on operant behavior maintained by food. In contrast to the overall limited effects of DAMGO on nicotine self-administration, the GABA agonists muscimol and baclofen each reduced nicotine self-administration substantially when delivered into the VTA, whereas they were less effective against cocaine self-administration. Conclusions: The lesser effect of DAMGO microinfusions in the VTA on nicotine than cocaine self-administration is associated with the opposite efficacy of GABA agonists. These findings suggest that nicotine and cocaine differentially activate circuitry in which mu receptors are situated, especially GABAergic elements. Received: 20 October 1998 / Final version: 24 November 1999     

Diminution of contractile response by κ-opioid receptor agonists in isolated rat ventricular cardiomyocytes is mediated via a pertussis toxin-sensitive G protein

Opioids directly decrease the contractile response of isolated ventricular cardiomyocytes to electrical stimulation. To investigate whether these effects are mediated via GTP-binding G_i/o proteins we examined the influence of pertussis toxin on the effects of the κ-opioid receptor agonist trans-(±)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]-benzeneacetamide (U-50,488) methanesulphonate and on the as yet undescribed effects of the opioid peptide dynorphin A (1–8) on contraction. In isolated, electrically driven, rat ventricular cardiomyocytes both agents concentration dependently reduced cell shortening within 15 min, decreasing the contractile response by 79±4% (n=5) and 62±2% (n=6) of control values at maximal effective concentrations of 10 μM (U-50,488) and 1 μM [dynorphin A (1–8)], respectively. Pertussis toxin pre-treatment (200 ng/ml; 4.5–5 h) completely abolished the effects of U-50,488 and dynorphin A (1–8) on the contractile response, indicating that these effects are mediated via G_i/o proteins. In addition, the non-selective opioid receptor antagonist (–)-naloxone and the κ-opioid receptor antagonist nor-binaltorphimine antagonized the effects of U-50,488 and dynorphin A (1–8) on the contractile response. Furthermore, the μ- and δ-opioid receptor agonist (D-Ala², D-Leu⁵)-enkephalin (DADLE) had no effects on contraction. These results indicate that the decrease in cell shortening is due to stimulation of κ-opioid receptors. The direct effect of κ-opioid receptor agonists on the contractile response thus represents an additional mechanism for decreasing cardiac contractility, besides the M-cholinoceptor- or adenosine receptor-mediated pathway. It is conceivable that increased release of endogenous dynorphins from the heart during hypoxia may protect the heart in a similar manner to adenosine. Received: 16 September 1997 / Accepted: 15 June 1998     

Adenosinergic modulation of 3α-hydroxy-5α-pregnan-20-one induced catalepsy in mice

  Rationale: Neurosteroid 3α, 5α THP, a positive allosteric modulator of the GABA_A receptor Cl^– ionophore complex, induces catalepsy-like dopamine antagonists, adenosine agonists or GABA agonists. Adenosine and dopamine receptors are co-localized on GABAergic neurons in the striatum and regulate GABA-mediated neurotransmission. Moreover, the antagonistic interactions between specific subtypes of adenosine and dopamine receptors are involved in motor depressant or motor stimulant effects of adenosine receptor agonists or antagonists, respectively. Such interaction may modulate neurosteroid-induced catalepsy. Objective: This study examined the modulation of 3α, 5α THP-induced catalepsy by adenosinergic agents. Methods: Catalepsy induced by 3α, 5α THP (2–8 μg, ICV) was assessed by bar test periodically up to 3 h in mice. Adenosine A₁, A_2A or A₃ receptor agonists or antagonists were given IP or ICV prior to 3α, 5α THP. Some animals received IP dopamine D₂ receptor agonist or antagonist 30 min prior to above combination treatment. Results: Adenosine A₁, A_2A, and A₃ receptor agonists potentiated, whereas adenosine A_2A receptor antagonists, but not A₁ antagonists, reversed 3α, 5α THP-induced catalepsy. These effects of adenosine agonists and antagonists were abolished by prior administration of bromocriptine, the dopamine D₂ receptor agonist and spiperone, the dopamine D₂ receptor antagonist, respectively. Conclusions: These findings suggest specific adenosine-dopamine receptor interaction in the striatum to modulate 3α, 5α THP-induced catalepsy. Received: 1 November 1998 / Final version: 5 January 1999     

Dissociation of multiple effects of acute LSD on exploratory behavior in rats by ritanserin and propranolol

Rats tested for 1 h in the Behavioral Pattern Monitor (BPM) after injection of the mixed serotonergic agonistd-lysergic acid diethylamide (LSD) exhibit a behavioral profile similar to that produced by various hallucinogenic 5HT-2 agonists. The characteristic effects of the hallucinogens include suppression of locomotor and exploratory behavior and a preferential decrease in entries into the center of the BPM during the initial half of the test session. After LSD, the initial suppression of responding is followed by a subsequent increase in locomotor activity that is not observed with other serotonergic agonists. In the present studies, the 5HT-1 andβ-adrenergic antagonistd,1-propranolol and the 5HT-2 antagonist ritanserin were administered individually or in combination prior to the acute administration of LSD to test for the involvement of these receptor subtypes in the mediation of the effects of LSD in the BPM paradigm. Propranolol (20 mg/kg) abolished the initial suppression of activity induced by 60 μg/kg LSD without affecting the subsequent increase in locomotion. Conversely, 2.0 mg/kg ritanserin failed to block the initial suppressive effects of 60 or 120 μg/kg LSD, but attenuated the LSD-induced increases in activity during the second half of the session. The combination of propranolol and ritanserin prevented both these effects of LSD. By contrast, the more selective 5HT-2 agonist 2,5-dimethoxy-4-iodoamphetamine (DOI) (0.27 mg/kg) produced an initial suppression of activity in the BPM that was blocked by 2.0 mg/kg ritanserin and was not followed by a subsequent increase in activity. These findings suggest that the initial suppressive effects of LSD in the BPM paradigm are dissociable from the subsequent increases in locomotion and that the two effects are mediated via different serotonergic orβ-adrenergic receptors.     

Endomorphin-1 and endomorphin-2 activate µ-opioid receptors in myenteric neurons of the guinea-pig small intestine

The novel opioid tetrapeptides, endomorphin-1 and endomorphin-2, recently isolated from bovine and human brain bind with high affinity and selectivity to central μ-opioid receptors. In the digestive tract, a comprehensive pharmacological analysis of the receptors involved in endomorphin action has not been reported. In this study, we analyzed the effects of endomorphin-1 and endomorphin-2 on longitudinal muscle-myenteric plexus preparations (LMMPs) from the guinea-pig ileum. Both peptides (30 pM–1 μM) inhibited (–log EC₅₀ values: 8.61 and 8.59, respectively) the amplitude of electrically-induced twitch contractions in a concentration-dependent fashion, up to its abolition. Conversely, in unstimulated LMMPs, they failed to affect contractions to applied acetylcholine (100 nM). In stimulated LMMPs, the highly selective μ-opioid receptor antagonist, d-Phe-Cys-Tyr-d-Trp-Orn-Thr-Pen-Thr-NH₂ (CTOP), caused a concentration-dependent (30 nM–1 μM), parallel rightward shift of endomorphin-1 and endomorphin-2 inhibitory curves, without depression of their maximum. Following Schild analysis, calculated pA ₂ values were 7.81 and 7.85, respectively, with slopes not different from unity. Concentration-response curves to both peptides were not affected by 30 nM naltrindole (a selective δ-receptor antagonist) or 30 nM nor-binaltorphimine (a selective κ-receptor antagonist). These results demonstrate that endomorphins selectively activate μ-opioid receptors located on excitatory myenteric plexus neurons, and that they act as full agonists. Received: 6 August 1998 / Accepted: 5 October 1998     

Local administration of Δ9-tetrahydrocannabinol attenuates capsaicin-induced thermal nociception in rhesus monkeys: a peripheral cannabinoid action

  Rationale: Cannabinoids can reduce nociceptive responses by acting on peripheral cannabinoid receptors in rodents. Objectives: The study was conducted to evaluate the hypothesis that local administration of Δ⁹-tetrahydrocannabinol (Δ⁹-THC) can attenuate capsaicin-induced nociception in rhesus monkeys. Methods: Capsaicin (100 μg) was applied locally in the tail of rhesus monkeys to evoke a nociceptive response, thermal allodynia, in normally innocuous 46°C water. Δ⁹-THC (10–320 μg) was coadministered with capsaicin in the tail to assess local antinociceptive effects. In addition, a local antagonism study was performed to confirm the selectivity of Δ⁹-THC action. Results:Δ⁹-THC dose-dependently inhibited capsaicin-induced allodynia. This local antinociception was antagonized by small doses (10–100 μg) of the cannabinoid CB₁ antagonist, SR141716A, applied in the tail. However, 100 μg SR141716A injected subcutaneously in the back did not antagonize local Δ⁹-THC. Conclusions: These results indicate that the site of action of locally applied Δ⁹-THC is in the tail. It provides functional evidence that activation of peripheral cannabinoid CB₁ receptors can attenuate capsaicin-induced thermal nociception in non-human primates and suggests a new approach for cannabinoids in pain management. Received: 1 December 1998 / Final version: 13 January 1999     

Inhibition of ATP-induced cAMP formation by 5’-p-fluorosulfonylbenzoyladenosine in NG108-15 cells

ATP is known to increase intracellular cAMP levels in NG108-15 cells via a novel purinoceptor and this response is inhibited by the P₁ purinoceptor antagonist methylxanthine. In the present study, we examined the effects of 5’-p-fluorosulfonylbenzoyladenosine (FSBA), an affinity ligand for ATP-binding proteins, on cAMP formation mediated by activation of adenylate cyclase (AC)-linked purinoceptors in NG108-15 cells. cAMP levels were determined by RIA using an anti-succinyl-cAMP antiserum. FSBA (100 μM) increased intracellular cAMP about 2.6-fold. However, FSBA-induced cAMP formation was abolished by pretreatment with adenosine deaminase, suggesting that adenosine, a breakdown product of FSBA, is involved in FSBA-induced cAMP formation. In contrast, pretreatment of cells with FSBA in the presence of adenosine deaminase inhibited cAMP formation induced by ATP and β,γ-methylene-ATP (β,γ-MeATP), without affecting the prostaglandin E₁ (PGE₁)-induced response. The inhibitory effect of FSBA on ATP-induced cAMP formation was concentration-dependent with a concentration required for half-maximal inhibition (IC₅₀) of around 3 μM. The inhibitory effect of FSBA was not affected by pertussis toxin (PTX)-treatment. Pretreatment with FSBA (10 μM) depressed the maximal response to β,γ-MeATP by 60%, but did not affect the response to 5’-N-ethylcarboxamidoadenosine. The inhibitory effect of FSBA (100 μM) increased time-dependently during pretreatment and partly resisted wash-out. The inhibition by FSBA was protected by simultaneous addition of β,γ-MeATP during the FSBA pretreatment, indicating that both FSBA and the ATP analogue interacted with the same receptor site. The pretreatment with FSBA did not affect the increase in [Ca²⁺]_i induced by ATP, UTP or benzoylbenzoic ATP. These results suggest that FSBA inhibits cAMP accumulation induced in NG108-15 cells by ATP or related agonists by selective modification of an AC-linked purinoceptor. Received: 28 July 1997 / Accepted: 14 April 1998     

Effects of methyllycaconitine (MLA), an α7 nicotinic receptor antagonist, on nicotine- and cocaine-induced potentiation of brain stimulation reward

It has been shown that nicotine facilitates intracranial self-stimulation (ICSS) reward and that nicotinic acetylcholine receptors (nAChRs) in the ventral tegmental area (VTA) are of primary importance for its reinforcing and dependence-producing actions. Recently, we have shown that α₇ nicotinic receptors in the VTA contribute to both the acute effects of nicotine on the mesolimbic dopamine system, as well as to nicotine withdrawal reactions. However, it is not yet known whether the same receptor conformation is directly involved in the reinforcing actions of nicotine. Here, using the curve-shift method we studied the effects of methyllycaconitine (MLA), a selective α₇ receptor antagonist, microinjected (graded doses: 1, 3, 9 μg/μl per side) into the VTA on the rewarding efficacy of lateral hypothalamic self-stimulation and on the systemic nicotine-induced potentiation of brain stimulation reward. MLA did not affect baseline self-stimulation. Nicotine produced a significant reduction in ICSS threshold, without altering maximal rates of responding, while MLA attenuated the effect of nicotine at the two lower doses. Given the reported interaction between nicotine and cocaine at both the neuronal and the behavioral level, we also examined whether α₇ receptor antagonism within the VTA can affect the reinforcing action of cocaine, as measured with ICSS. Interestingly, MLA attenuated the reinforcing effect of cocaine in all doses tested, without altering the maximal rate of responding, i.e. the performance of the animals. These results suggest that α₇ nAChRs in the VTA are involved in mediating the reinforcing actions of drugs of abuse, such as nicotine and cocaine, and provide evidence that α₇ nAChR antagonists may be clinically useful in attenuating the rewarding effects of addictive drugs. Received: 1 September 1999 / Final version: 18 December 1999     

Distribution and elimination of hydroflumethiazide in man

Summary The pharmacokinetics of hydroflumethiazide (HFT) were investigated after intravenous and oral administration to healthy subjects. After intravenous infusion, HFT behaved according to a three-compartment model. Two distribution phases were observed, with mean half-lives of 0.26 and 0.85 h, reflecting distribution to red blood cells and tissues. Mean biological half-life (t_1/2β ) after infusion was 5.2 h. Renal blood and plasma clearance of HFT, as well as the ratio renal blood clearance/renal plasma clearance of HFT, were lower after infusion than during the infusion, due to the distribution characteristics of HFT. After a single oral dose of 2 μmol/kg, t_1/2β was significantly shorter in all subjects than after a single oral dose of 6 μmol/kg, with mean t_1/2β of 8.7 and 17.9 h, respectively. Due to lack of a sufficiently sensitive method for determination of HFT in plasma, it could not be established whether the observed dose-dependent difference in biological half-life of HFT was caused by variation in renal clearance and/or the volume of distribution.     

Characterization of the discriminative stimulus effects of buprenorphine in pigeons

Rationale: The pedunculopontine tegmental nucleus (PPTg) has been implicated in the self-administration of drugs, particularly nicotine, which acts directly through the PPTg in addition to targeting midbrain dopamine neurons. The direct action of nicotine in PPTg may be through GABAergic mechanisms that have been shown to influence nicotine self-administration preferentially compared to cocaine. Objective: The purpose of these experiments was to examine several pharmacological manipulations that alter neuronal activity in the PPTg for their specificity or generality in nicotine versus cocaine reinforcement. Methods and results: Rats trained to self-administer nicotine or cocaine intravenously were prepared with brain microcannulae directed to the PPTg. Intra-PPTg microinfusions of the muscarinic agonist carbachol (0.1–1.0 μg), the μ opioid agonist DAMGO (0.005 and 0.05 μg), tetrodotoxin (5 ng) and neostigmine (0.5 nmol) each reduced the self-administration of nicotine and cocaine maintained on an FR5 schedule of reinforcement. The muscarinic antagonist scopolamine (0.1–1.0 μg) and the opioid antagonist CTOP (1 μg) did not affect self-administration, but reversed the effects of the respective agonist when co-administered with it. Carbachol and DAMGO were also tested in self-administration maintained on a progressive-ratio schedule; each agonist again reduced both nicotine and cocaine self-administration. Conclusions: PPTg manipulations are able to alter established self-administration of nicotine, which acts at the level of the ventral tegmental area and the PPTg itself, and cocaine, which acts through the mesolimbic dopamine system. These data suggest that the PPTg is an important substrate in drug dependence. Electronic Publication     

Synergistic enhancement of the acoustic startle reflex by dopamine D1 and 5-HT1A agonists and corresponding changes in c-Fos expression in the dorsal raphe of rats

Rationale: Although there is evidence that central opioid receptors are involved in immunomodulation, it has been only recently that an endogenous agonist, designated endomorphin-1, possessing high selectivity and affinity for the mu opioid receptor has been identified. Objective: The present study assesses the immunomodulatory effects of endomorphin-1 in the rat and provides further evaluation of the antinociceptive effects of endomorphin-1. Methods: Rats were surgically implanted with cannulae directed at the lateral cerebral ventricle. Animals received vehicle or endomorphin-1 at doses of 31.63 or 56.23 μg (ICV) and were tested for antinociception in two different assays, the warm water tail withdrawal procedure and the hotplate assay. Additional studies assessed the effect of naltrexone on the antinociception produced by endomorphin-1 in both antinociceptive assessments. Assessments of immune status following endomorphin-1 treatment included measurements of splenic natural killer cell activity, production of interferon-γ, and lymphocyte proliferative responses to mitogenic stimulation by Con-A, LPS, and the microbial superantigen, TSST-1. Results: Endomorphin-1 induced significant and naltrexone reversible antinociception 30 and 60 min following drug administration, as measured by the hotplate assay and warm water tail withdrawal procedure. In marked contrast, endomorphin-1 did not produce immunomodulatory effects up to 120 min following ICV administration. Conclusions: Endomorphin-1 produces antinociception but does not induce immunomodulatory effects in the rat. These findings suggest that it is possible to develop therapeutic strategies for separating antinociception and immunomodulatory properties through the mu opioid receptor. Electronic Publication     

Histaprodifen, methylhistaprodifen, and dimethylhistaprodifen are potent H1-receptor agonists in the pithed and in the anaesthetized rat

Selective H₂- and H₃-receptor agonists, exhibiting an at least tenfold higher potency than histamine itself at the respective receptors, have been known for several years. Selective H₁-receptor agonists with a potency exceeding that of histamine have become available only recently; the most potent are methylhistaprodifen and dimethylhistaprodifen [N ^α-methyl- and N ^α,N ^α-dimethyl-2-(3,3-diphenylpropyl)histamine, respectively] with 3.4- and 2.4-fold higher potencies than histamine in vitro (in the guinea-pig ileum). The aim of the present study was to examine whether these compounds and the parent compound histaprodifen are potent H₁-receptor agonists in the pithed and in the anaesthetized rat. In pithed, vagotomized rats diastolic blood pressure was decreased by 2-(2-thiazolyl)ethanamine i.v. (which was used as a reference H₁-receptor agonist) and by histaprodifen, methylhistaprodifen, and dimethylhistaprodifen; the maximum decrease was about 45 mmHg for each compound, and the potencies, expressed as pED₅₀, the negative logarithm of the dose (in mole per kilogram body weight) eliciting a half-maximal response, were 7.23, 7.55, 8.43 and 8.12, respectively. The dose/response curves of the four compounds were shifted to the right to about the same extent by the H₁-receptor antagonist dimetindene (1 μmol/kg i.v.). The vasodepressor response was not affected by combined i.v. administration of the H₂- and H₃-receptor antagonists ranitidine and thioperamide, by combined i.v. administration of the α₁- and α₂-adrenoceptor antagonists prazosin and rauwolscine, and by the β-adrenoceptor antagonist propranolol i.v. but was attenuated by the inhibitor of NO synthase, N ^ω-nitro-l-arginine methyl ester i.v. In anaesthetized rats 2-(2-thiazolyl)ethanamine, histaprodifen, methylhistaprodifen and dimethylhistaprodifen i.v. also decreased diastolic blood pressure in a manner sensitive to dimetindene i.v. Our data show that histaprodifen and, in particular, methyl- and dimethylhistaprodifen are highly potent H₁-receptor agonists in vivo. Received: 3 September 1998 / Accepted: 23 October 1998