Discriminative stimulus effects of a centrally administered,delta-opioid peptide (d-Pen2-d-Pen5-enkephalin) in pigeons

The present study assessed the discriminative stimulus effects of the delta-opioid agonist [d-Pen²-d-Pen⁵]enkephalin (DPDPE) in pigeons. Food-restricted pigeons were trained to discriminate between ICV injections of 100 µg [d-Pen²-d-Pen⁵]enkephalin (DPDPE) and saline in a two-key operant procedure; acquisition of discriminative control was rapid (14–28 daily sessions). [d-Ser², Leu⁵, Thr⁶]enkephalin (DSLET) and [d-Ala²]deltorphin II, peptides selective for delta-opioid receptors, produced discriminative stimulus effects similar to DPDPE, and were approximately equipotent to DPDPE. The non-peptidic, delta-opioid agonist BW373U86 (0.032–100 mg/kg, IM) partially generalized to DPDPE. The kappa-opioid agonist U69,593 (0.01–1 mg/kg, IM), and the mu-opioid agonists, DAMGO (0.1–3.2 µg, ICV) and morphine (1–10 mg/kg, IM), did not produce discriminative stimulus effects similar to DPDPE, up to doses that markedly decreased response rates. Naltrindole (0.1 mg/kg, IM), an antagonist selective for deltaopioid receptors, produced approximately a 30-fold reduction in the potency of DPDPE. DPDPE's discriminative stimulus effect in pigeons appears to be mediated through a delta-opioid receptor; this effect may provide a procedure for assessing delta-opioid receptor function in vivo.     

Discriminative stimulus effects of a centrally administered,delta-opioid peptide (d-Pen2-d-Pen5-enkephalin) in pigeons

The present study assessed the discriminative stimulus effects of the delta-opioid agonist [d-Pen²-d-Pen⁵]enkephalin (DPDPE) in pigeons. Food-restricted pigeons were trained to discriminate between ICV injections of 100 μg [d-Pen²-d-Pen⁵]enkephalin (DPDPE) and saline in a two-key operant procedure; acquisition of discriminative control was rapid (14–28 daily sessions). [d-Ser², Leu⁵, Thr⁶]enkephalin (DSLET) and [d-Ala²]deltorphin II, peptides selective for delta-opioid receptors, produced discriminative stimulus effects similar to DPDPE, and were approximately equipotent to DPDPE. The non-peptidic, delta-opioid agonist BW373U86 (9.032–100 mg/kg, IM) partially generalized to DPDPE. The kappa-opioid agonist U69,593 (0.01–1 mg/kg, IM), and the mu-opioid agonists, DAMGO (0.1–3.2 μg, ICV) and morphine (1–10 mg/kg, IM), did not produce discriminative stimulus effects similar to DPDPE, up to doses that markedly decreased response rates. Naltrindole (0.1 mg/kg, IM), an antagonist selective for delta-opioid receptors, produced approximately a 30-fold reduction in the potency of DPDPE. DPDPE’s discriminative stimulus effect in pigeons appears to be mediated through a delta-opioid receptor; this effect may provide a procedure for assessing delta-opioid receptor function in vivo.     

Morphine,d-Pen2,d-Pen5 enkephalin and U50,488H differentially affect the locomotor activity and behaviours induced by quinpirole in guinea-pigs

The effects of morphined-Pen²,d-Pen⁵ enkephalin (DPDPE) and U50,488H on the behavioural syndrome elicited by the dopamine (DA) D-2 agonist quinpirole, were investigated. Morphine (1, 5 and 15 mg/kg SC) and morphine administered intracerebroventricularly (ICV) (2×5 µl, 10^–3 M; total dose = 10 nmol) produced piloerection and sedation. DPDPE-ICV (2×5 µl and 2×10 µl, 10–³ M; total doses = 10 and 20 nmol) produced piloerection and sedation similar to morphine. U50,488H (1 mg/kg SC) induced locomotor activity and some stereotyped behaviour, whereas U50,488H (5 and 10 mg/kg SC) induced muscle rigidity and dystonic-like movements. The locomotor and behavioural response elicited by quinpirole (3 mg/kg IP) was attenuated in guinea-pigs pretreated with morphine (1, 5 and 15 mg/kg SC), morphine-ICV (2 × 5 µl, 10–³ M), and DPDPE-ICV (2 × 5 µl and 2 × 10 µl, 10^–3 M). These effects were reversed by naloxone (15 mg/kg SC). U50,488H (1 mg/kg SC) increased the quinpirole-induced locomotor activity, whereas U50,488H (5 and 10 mg/kg SC) decreased the locomotor activity and stereotyped behaviours produced by quinpirole. These results indicate that the gross behavioural effects of mu, delta and kappa opioids differ in guinea-pigs compared to other rodent species, and suggest differential involvement of these opioid receptor subtypes with DA D-2 receptor-mediated activity.     

Delta opioid receptors: reflexive,defensive and vocal effective responses in female rats

Ultrasonic vocalizations may be an expression of the affective pain response in laboratory animals. The present experiment compares the effects of morphine to the delta agonist, DPDPE (d-Pen²,d-Pen⁵ enkephalin) on a range of reflexive, behavioral and affective responses during an aggressive interaction. In experiment 1, naive female Long-Evans rats received morphine (0, 1, 3, 6, 10 µg ICV), or DPDPE (0, 30, 60, 100 µg ICV). In experiment 2, female rats were treated with naltrindole (1.0 mg/kg IP) 20 min before DPDPE (0, 60, 100 µg ICV). The following endpoints were measured: (1) latency to tail flick in response to heat stimuli; (2) high (33–65 kHz) and low (20–32 kHz) frequency ultrasonic and audible vocalizations; (3) defensive behavior; and (4) motoric activity. Following a brief exposure to attack, rats were threatened by the aggressor but protected from further attack by a large, wire mesh cage, thereby allowing for continued behavioral and vocal measurement without the risk of physical injury; video and audio recordings were made during the attack and then during a portion of the protected encounter (2 min). Morphine suppressed pain reactions varying in complexity from a spinal reflex, to an organized escape reaction, to an affective vocal response. The delta agonist, DPDPE, attenuated high frequency ultrasonic calling and tail flick responding. Defensive behaviors were also modulated by DPDPE at doses that had no effect on walking or rearing, indicating behavioral specificity. By contrast, doses of morphine that decreased defensive upright and escape also decreased motor activity. In female rats, morphine and DPDPE share a common profile of effects on a range of functional end-points, but DPDPE appears to modulate more selectively the reactions related to aversiveness without exerting sedative effects. These data demonstrate a possible physiological role for delta receptors in affective and defensive reactions.     

The effects of dopamine D1 and D2 receptor antagonists on the rewarding effects of δ1 and δ2 opioid receptor agonists in mice

The effects of the dopamin D₁ antagonist SCH23390 and the D₂ antagonist sulpiride on the rewarding effects of opioid receptor agonists were examined in mice. Both [d-Pen², Pen⁵]enkephalin (DPDPE, 1–15 nmol, ICV), a selective ₁ opioid receptor agonist, and [d-Ala²]deltorphin II (DELT, 0.5–5 nmol, ICV), a selective ₂ opioid receptor agonist, produced a dose-dependent place preference in mice. The DPDPE (15 nmol, ICV)-induced place preference was abolished by BNTX (0.5 mg/kg, SC), a ₁ opioid receptor antagonist, but not by NTB (0.5 mg/kg, SC), a ₂ opioid receptor antagonist. In contrast, the DELT (5 nmol, ICV)-induced place preference was antagonized by NTB, but not BNTX. Pretreatment with SCH23390 (3 µg/kg, SC) abolished the DPDPE-induced place preference, but not affect the DELT-induced place preference. Moreover, pretreatment with sulpiride (40 mg/kg, SC) did not modify the place preference induced by DPDPE or DELT. In the present study, we found that the activation of both central ₁ and ₂ opioid receptors produced rewarding effects. Furthermore, these results suggest that the rewarding effects of ₁ opioid receptor agonist may be produced through activation of the central dopaminergic system, especially dopamine D₁ receptors, whereas the rewarding effects of ₂ opioid receptor agonists may be produced by some other mechanism(s).     

Morphine-like discriminative stimulus effects of opioid peptides: possible modulatory role ofd-Ala2-d-Leu5-enkephalin (DADL) and dynorphin A(1-13)

The role of the different opioid receptors was studied in rats trained to discriminate SC injections of 3.0 mg/kg morphine from saline by tests for generalization to graded doses of morphine and receptor-selective peptides administered into the lateral cerebral ventricle. Dose-dependent morphine-like stimulus effects were produced over a wide range of doses (0.001–30 g), depending upon ligand and animal, by morphine, by themu-selective peptides DAGO[d-Ala²-NMePhe⁴-Gly(ol)-enkephalin] and FK33824[d-Ala²,NMePhe⁴-Met(O)⁵-(ol)-enkephalin], and by thedelta-selective peptide, DADL[d-Ala²,d-Leu⁵enkephalin]. The order of relative potency of these substances was: FK33824>DAGO>morphine>DADL. In contrast, DPLPE[d-Pen²,l-Pen⁵)enkephalin], which has much greaterdelta receptor selectivity than does DADL, and dynorphin A(1-13) (0.1–10 g), akappa-receptor agonist, engendered choice responding appropriate for saline. When 1.0 g DADL, a dose lacking morphine-like discriminative effects, was administered concurrently with SC morphine, the stimulus effects of morphine were potentiated. Concurrent administration of 10 g dynorphin A(1-13) and morphine attenuated the stimulus effects of morphine inconsistently. These results support previous findings thatmu-opioid receptors are of primary importance in mediating the morphine-like discriminative effects of opioid peptides. They also suggest that morphine-like discriminative effects can be modulated by other types of opioid receptors.     

Reduction in ATP-sensitive potassium channel-mediated antinociception in diabetic mice

To test our hypothesis that the abnormally low efficacy of -opioid agonists in diabetic mice may be due to functional changes in ATP-sensitive potassium channels, we evaluated the effects of cromakalim on the tail-flick latencies in diabetic and non-diabetic mice. Anti nociceptive effects of morphine (10 µg, ICV) in diabetic mice were significantly less than that in non-diabetic mice. Morphine-induced antinociception in non-diabetic mice was antagonized by pretreatment with glibenclamide (30 µg, ICV), an ATP-sensitive potassium channel blocker. Cromakalim (0.3 and 1 µg, ICV) produced significant, dose-dependent antinociception in non-diabetic mice, which was significantly reduced by pretreatment with glibenclamide. However, cromakalim did not markedly affect the tail-flick latencies in diabetic mice, even at higher doses (3 µg, ICV). On the other hand, [D-Pen^2,5]enkephaline (DPDPE, 5 µg, ICV), a selective -opioid receptor agonist, produced significant antinociception in both diabetic and non-diabetic mice. Since pretreatment with glibenclamide significantly reduced the antinociceptive effect of DPDPE in non-diabetic mice but not in diabetic mice, -opioid receptor-mediated antinociception in diabetic mice may be independent of potassium channels. These results suggest that dysfunction of ATP-sensitive potassium channels may contribute to the demonstrated poor antinociceptive response of diabetic mice to -opioid agonists.     

Different types of opioid receptors mediating analgesia induced by morphine,DAMGO, DPDPE,DADLE and β-endorphin in mice

Summary The effects of intracerebroventricular (i.c.v.) administration of d-Phe-Cys-Tyr-d-Try-Orn-Thr-Pen-Thr-NH₂ (CTOP), a selective mu-opioid receptor antagonist, (Allyl)2-Tyr-Aib-Aib-Phe-Leu-OH (ICI 174864) and (N,N-Bisallyl-Tyr-Gly-Gly--(CH₂S)-Phe-Leu-OH (ICI 154129), selective delta-opioid receptor antagonists on blocking analgesia induced by -endorphin, morphine, d-Ala²-NMePhe⁴-Gly-ol-enkephalin (DAMGO), d-Ala²-d-Leu⁵-enkephalin (DADLE) and d-Pen²-enkephalin (DPDPE) administered i.c.v. were studied in male ICR mice. The analgesia was assessed by the tail-flick and paw-licking (hot-plate) tests. The potencies of opioid agonists injected i.c.v. for producing analgesia were DAMGO > DADLE > -endorphin > morphine > DPDPE. Intracerebroventricular administration of CTOP (0.05 g) selectively antagonized inhibition of the tail-flick and paw-licking response induced by morphine, DAMGO or DADLE but not -endorphin or DPDPE. ICI 174864 (5 g) and ICI 154129 (5 g) injected i.c.v. selectively antagonized analgesia induced by DPDPE or DADLE but not -endorphin, morphine or DAMGO injected i.c.v. These results indicate that analgesia induced by morphine and DAMGO is mediated by the stimulation of mu-opioid receptors while analgesia induced by DPDPE is mediated by the stimulation of delta-opioid receptors. DADLE-induced analgesia is mediated by the stimulation of both mu- and delta-opioid receptors. Analgesia induced by -endorphin is mediated by neither munor delta-opioid receptors.Abbreviations i.c.v. intracerebroventricular - i.t. intrathecal - CTOP d-Phe-Cys-Tyr-d-Try-Orn-Thr-Pen-Thr-NHZ - DAMGO d-Ala²-NMePhe²-Gly-ol-enkephalin - DADLE d-Ala²-d-Leus-enke-phalin - DPDPE dd-Pen²-dd-Pen⁵-enkephalin - ICI 174864 (Allyl)2Tyr-Aib-Aib-Phe-Leu-OH - ICI 154129 (N,N-Bisallyl-Tyr-Gly-Gly-(CH₂S)-Phe-Leu-OH     

Selective potentiations in opioid analgesia following scopolamine pretreatment

The effects of the muscarinic receptor antagonist scopolamine upon analgesia induced by d-ala-d-leu-enkephalin (DADL), beta-endorphin (BEND) and morphine were examined. While scopolamine (10 mg/kg, IP) significantly potentiated the analgesic responses following DADL (40 g, ICV) and morphine (5 mg/kg, SC) on the jump test, it failed to alter significantly BEND (1 g, ICV) analgesia.     

The antiepileptic agent gabapentin (Neurontin) possesses anxiolytic-like and antinociceptive actions that are reversed byd-serine

This report describes the activity of the antiepileptic agent gabapentin (Neurontin) in animal models predictive of anxiolysis and analgesia. Gabapentin displayed anxiolytic-like action in the rat conflict test, the mouse light/dark box and the rat elevated X-maze with respective minimum effective doses (MEDs) of 3, 10 and 30 mg/kg. Furthermore, gabapentin also induced behavioural changes suggestive of anxiolysis in the marmoset human threat test with a MED of 30 mg/kg. In the rat formalin test of tonic nociception, gabapentin dose-dependently (30–300 mg/kg) and selectively blocked the late phase with a MED of 100 mg/kg. However, it failed to block carrageenan-induced paw oedema. The intracerebroventricular (ICV) administration of the glycine/NMDA receptor agonistd-Serine, dose-dependently (10–100 μg/animal) reversed the antinociceptive action of gabapentin (200 mg/kg, SC).d-Serine (30 μg/animal, ICV) also reversed the anxiolytic-like effects (in the light/dark box and the rat elevated X-maze) of gabapentin (30 mg/kg). In contrast,l-Serine (100 μg, ICV) failed to block the antinociceptive action of gabapentin. The antinociceptive action of (+)-HA-966 (25 mg/kg, SC), a partial agonist at the glycine/NMDA receptor, was reversed byd-Serine (100 μg/animal, ICV). However,d-Serine (100 μg/animal, ICV) failed to affect the antinociceptive action of a competitive NMDA receptor antagonist CGS 19755 (3 mg/kg, SC). Gabapentin has negligible affinity for the strychnine insensitive [³H]glycine binding site. This indicates that the interaction between gabapentin andd-Serine may not involve the NMDA receptor complex. Gabapentin may represent a novel type of anxiolytic and analgesic agent.     

Single residue modifications of the delta opioid receptor selective peptide, [d-Pen2,d-Pen5]-enkephalin (DPDPE)

Six analogs of the highly delta opioid receptor selective, conformationally restricted, cyclic peptide [d -Pen²,d -Pen⁵]enkephalin, Tyr-d -Pen-Gly-Phe-d -PenOH (DPDPE), were synthesized and evaluated for opioid activity in rat brain receptor binding and mouse vas deferens (MVD) smooth muscle assays. All analogs were single amino acid modifications of DPDPE and employed amino acid substitutions of known effects in linear enkephalin analogs. The effect on binding affinity and MVD potency of each modification within the DPDPE structural framework was consistent with the previous reports on similarly substituted linear analogs. Conformational features of four of the modified DPDPE analogs were examined by ¹H NMR spectroscopy and compared with DPDPE. From these studies it was concluded that the observed pharmacological differences with DPDPE displayed by diallyltyrosine¹-DPDPE ([DAT¹]DPDPE) and phenylglycine⁴-DPDPE ([Pgl⁴]DPDPE) are due to structural and/or conformational differences localized near the substituted amino acid. The observed enhanced μ receptor binding affinity of the carboxamide terminal DPDPE-NH₂ appears to be founded solely upon electronic differences, the NMR data suggesting indistinguishable conformations. The observation that the α-aminoisobutyric acid substituted analog [Aib³]DPDPE displays similar in vitro opioid behavior as DPDPE while apparently assuming a significantly different solution conformation suggests that further detailed conformational analysis of this analog will aid the elucidation of the key structural and conformational features required for action at the δ opioid receptor.     

Mesoaccumbens dopamine-opiate interactions in the control over behaviour by a conditioned reinforcer

These experiments examined the role of dopamine-opiate interactions in the ventral tegmental area (VTA) and nucleus accumbens in the mediation of reinforcement-related behaviour. It has been shown previously that opiates induce a dopamine-dependent increase in locomotor activity in rats when infused into the VTA, and a dopamine-independent hyperactivity when infused into the nucleus accumbens. The present study investigated the generality and significance of these two findings, by examining dopamine-opiate interactions in the control over behaviour exerted by a conditioned reinforcer (CR), an arbitrary stimulus which gains control by association with primary reinforcement. Rats were trained to associate a light/noise stimulus with sucrose reinforcement, and the efficacy of the CR in controlling behaviour was assessed by measuring its ability to support a new lever pressing response. Responding on one lever (CR lever) produced the CR, responding on the other lever had no programmed consequences. In experiment 1, intra-accumbens infusions ofd-amphetamine (10 µg), the D₁ dopamine receptor agonist SKF-38393 (0.1 µg), the D₂ dopamine receptor agonist LY-171555 (quinpirole; 0.1 µg) or the opiate receptor agonist [d-Ala²]-methionine enkephalinamide (DALA; 1 µg) selectively increased responding on the CR lever. Infusion with DALA intra-VTA had no effect. However, pretreatment with DALA intra-VTA (10 × 1 µg/day) subsequently reduced the selectivity of the response to infusions intra-accumbens withd-amphetamine or SKF-38393, and blocked the response to LY-171555 or DALA. Pretreatment also shifted to the right the dose-response function for DALA intra-accumbens. In experiment 2, intra-accumbens infusions ofd-amphetamine, SKF-38393, LY-171555 or DALA again increased responding on the CR lever only. Pretreatment with intra-accumbensd-amphetamine (5 × 1 µg/day) reduced the selectivity of the response subsequently tod-amphetamine, and blocked the response to SKF-38393, LY-171555 or DALA. In experiment 3, intra-accumbens infusions of the -opiate receptor agonist [d-Ala²,N-Me-Phe⁴,Gly⁵-ol]-enkephalin (0.003–0.1 µg), or the -opiate receptor agonist [d-Pen^2, 5]-enkephalin (0.03–1 µg) enhanced selectively responding on the CR lever. Thus, the dopamine-dependent locomotor-stimulant properties of intra-VTA infusions of opiates are associated with impaired conditioned reinforcer efficacy. Finally, repeated stimulation of the mesoaccumbens dopamine pathway may compromise the dopamine-independence of the opiate system within the nucleus accumbens.     

Effects of μ and δ opioid agonists and antagonists on affective vocal and reflexive pain responses during social stress in rats

The present experiments evaluated the influence of intraventricular μ and δ opioid receptors 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. Defeat stress consisted of: (1) an aggressive confrontation 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 stimulus rat with a wire mesh screen for 10–20 min. Defeat stress was immediately followed by an experimental session with tactile startle (20 psi). The μ opioid receptor agonists morphine (0.1–0.6 μg ICV) and [D-Ala²-N-Me-Phe⁴-Gly⁵-ol]-enkephalin (DAMGO; 0.01–0.3 μg ICV), and the δ opioid receptor agonist [D-Pen^2,5]-enkephalin (DPDPE; 10–100 μg ICV) dose-dependently decreased startle-induced USV and increased tail-flick latencies in socially inexperienced and defeated rats. Of greater interest, morphine, DAMGO and DPDPE increased the occurrence of the submissive crouch posture, and defeated rats were more sensitive than socially inexperienced rats to the startle-induced USV-suppressive and antinociceptive effects of morphine and DPDPE. The antinociceptive effects of DAMGO were likewise obtained at lower doses in defeated rats. Finally, the USV-suppressive effects of morphine and DAMGO were reversed with the μ receptor antagonist naltrexone (0.1 mg/kg IP), but the USV-suppressive effects produced by DPDPE were not reversed with the δ receptor antagonist naltrindole (1 mg/kg IP). These results confirm μ, but not δ opioid receptor activation as significant in affective vocal, passive-submissive behavior, as well as reflexive antinociception. Furthermore, similar to previous studies with restraint and electric shock stress, the facilitation of μ opioid effects on vocal responses and antinociception is consistent with the proposal that defeat stress activated endogenous opioid mechanisms. Received: 23 September 1997/Final version: 15 January 1998     

Comparison of the discriminative and antinociceptive effects of morphine and its glucuronide metabolites after central or systemic administration in the rat

 The potential of centrally (ICV) or systemically (SC) administered M6G to substitute for morphine in a drug discrimination task was characterized in the present study. Rats with a cannula in the lateral cerebral ventricle were trained to discriminate between injections of morphine (3 mg/kg, SC) and saline using a discrete-trial avoidance/escape procedure. Substitution tests were conducted with SC or ICV morphine, morphine-3-β-d-glucuronide (M3G), or morphine-6-β-d-glucuronide (M6G) and response latency in a tail-flick test was measured before each session began. The stimulus effects of morphine (ED₅₀=1.02 mg/kg SC or 2.1 μg/kg ICV) were fully shared by M6G, with potency dependent on route of administration (ED₅₀=3.12 mg/kg SC or 0.34 μg/kg ICV). The stimulus effects of M6G were highly correlated with its antinociceptive activity (r=0.84 SC or 0.46 ICV) and, at equipotent systemic doses, they lasted longer (t1/2=391 min) than those of morphine (t1/2=185 min). M3G was inactive in both procedures by both routes of administration. Naltrexone SC, given 30 min prior to testing, completely attenuated the stimulus effects of ICV M6G (AD₅₀=0.011 mg/kg), indicating that they are mediated by opioid receptors. The results of this study suggest that M6G might contribute to the interoceptive effects of morphine that underlie its potential for abuse. Received: 29 January 1998 / Final version: 21 March 1998     

Roles ofδ andμ opioid receptors in mediating the effects of enkephalins on avoidance conditioning

The effects on one-way active avoidance conditioning of pre-training, systemic administration of the selective -receptor agonist [d-Ala²,N-Me-Phe⁴, Glyol]enkephalin (DAGO), and the selective -receptor antagonist (d-Phe-Cys-Tyr-d-Trp-Orn-Thr-Pen-Thr-NH₂ (CTOP), were determined in Swiss-Webster mice. A low dose of DAGO (0.92 µg/kg) moderately enhanced avoidance acquisition, whereas a 100 µg/kg dose of CTOP more dramatically impaired acquisition. However, the avoidance-enhancing dose of DAGO significantly increased locomotor activity as measured in a separate group of mice in the avoidance chamber, and the avoidance-impairing dose of CTOP significantly decreased activity. Under these same training conditions, earlier studies (Schulteis et al. 1988; Schulteis and Martinez 1990) demonstrated that enkephalins impaired avoidance learning, and selective-receptor antagonists such as ICI 174,864 enhanced learning; in contrast to the present study, both of these effects were dissociated from performance effects such as alterations in locomotor activity. Taken together, the results suggested that the effects of enkephalins were mediated by the-, but not -, class of opioid receptor.     

Low dose of 1,3-di(2-tolyl)guanidine (DTG) attenuates MK-801-induced spatial working memory impairment in mice

MK-801 (30–100 µg/kg, SC) impaired spontaneous alternation behavior of mice, a behavior related to the spatial working memory. 1,3-Di-(2-tolyl)guanidine (DTG),(+)-pentazocine and (+)-SKF 10,047 (100 µg/kg, SC), putative agonists, administered 10 min before MK-801, partially but significantly reversed the impairment, without affecting the concomitant hyperlocomotion. The antagonizing effects by DTG were prevented by BMY-14802 (5 mg/kg, IP), a purported antagonist. These findings suggest that, at low doses, ligands may modulate theN-methyl-d-aspartate dependent memory processes.     

Behavioural effects of selective μ-, κ-, and δ-opioid agonists in neonatal rats

The behavioural effects of selective -, - and -opioid agonists in 5-, 10- and 20-day-old rats were investigated by observational analysis. The predominant response to -agonists was behavioural depression. High doses (10 mg/kg IP) of morphine and DAGO (d-Ala², NMe-Phe⁴, Glyol⁵-enkephalin) produced overt sedation in all the age groups and also induced catalepsy which was particularly apparent in the 5- and 10-day-old animals. These compounds did not produce any signs of behavioural activation in the neonatal rats. In contrast, rat pups treated with the -agonists U50,488H and PD 117,302 (1,10 mg/kg IP) exhibited marked hyperactivity with increases in wall-climbing and locomotion. Sedative effects of the highest dose of the -agonists began to emerge, however, as the animals grew older, resulting in significant decreases in behaviours such as gnawing and grooming at 20 days of age. The -agonist (+)-tifluadom (0.1–10 mg/kg), but not its corresponding (-)-isomer, produced an increase in activity in 5-day-old rats, thus extending the observations made with U50,488H and PD 117,302 and establishing the stereoselective nature of the response. The involvement of -receptors in opioid-induced hyperactivity was further substantiated by using a variety of opioid antagonists. In this context, the increase in activity induced by U50,488H (10 mg/kg) in 5-day-old neonates was attenuated by naltrexone (1 mg/kg IP) but not by larger doses (10 mg/kg) of either M8008 (which has low affinity for -receptors) or the selective -receptor antagonist ICI 174,864. Finally, DPDPE (d-Pen², d-Pen⁵-enkephalin) which acts selectively at -opioid receptors, did not exert any behavioural effects in either the 5-, 10- or 20-day-old rat pups at doses of up to 10 mg/kg. These results demonstrate behavioural effects of - and -but not -agonists in neonatal rats. There is a clear differentiation between - and -receptor effects and both - and -mediated behaviours show dissimilarities from the adult profile.     

Antinociceptive activity of glycosidic enkephalin analogues

The antinociceptive activity of two new enkephalin analogues:N ^1.5-(-d-glucopyranosyl)[d-Met², Pro⁵]enkephalinamide andN ^1.5-(-d-galactopyranosyl)[d-Met², Pro⁵]enkephalinamide was assessed using the tail immersion and paw pressure behavioural tests. Both enkephalin analogues appear to be more active than morphine when injected either into the fourth ventricle or intrathecally; the galactose analogue is more than 5000 times more active than morphine when injected into the fourth ventricle. The analgesic effects produced by the analogues are partially reversed by SC naloxone (0.1 mg/kg) and totally reversed when the dose of naloxone used was 1 mg/kg, suggesting that the analogues act upon more than one type of opiate receptor (/).     

Role of central ATP-sensitive potassium channels in the hyperthermic effect of morphine in mice

Morphine (10 mg/kg, SC) in combination with ICV vehicle induced a significant hyperthermic effect at 120 min (peak time) after injection compared to ICV vehicle plus SC saline (control group). Glibenclamide (50 µg, ICV), a selective adenosine triphosphate-sensitive potassium (K_ATP) channel blocker, in combination with SC saline hardly affected the rectal temperature compared to the control group. ICV glibenclamide antagonized the hyperthermia induced by SC morphine in a dose-dependent manner. From these results, we demonstrated that K_ATP channels play an important role as modulators of the hyperthermic effect of agonists.     

β-endorphin-sensitive opioid receptors in the rat tail artery

Summary Isolated tail arteries of rats were perfused and field-stimulated every 2 min with 2 pulses at 1 Hz. Different opioid peptides depressed the contractile responses to stimulation; their concentration-response curves showed a maximum at about 40% inhibition. The rank order of potency of the peptides was -endorphin (IC₅₀ = 97 nmol/1) BAM-22P > FK-33824 > DAGO > [d-Ala²,d-Leu⁵]-enkephalin metorphamide > dynorphin A-(1-13) [Met⁵]enkephalin. All these substances have in common a certain activity at opioid -receptors, although the enkephalins are preferential -, and the dynorphins preferential -agonists. However, the selective -agonist [d-Pen²,l-Pen⁵]enkephalin was ineffective at up to 10 mol/l, and the -agonists ethylketocyclazocine and U-50488 acted only at concentrations higher than 3 mol/l. Whereas the effects of -endorphin, DAGO and [d-Ala²,d-Leu⁵]enkephalin could be reduced by the -preferential antagonist naloxone, the effects of ethylketocyclazocine and U-50488 were not changed. The -selective antagonist ICI 174864 did not influence the action of [d-Ala²,d-Leu⁵]enkephalin. Naloxone in a concentration (1 mol/l) which nearly abolished the effect of DAGO 3 mol/l, slightly enhanced responses to stimulation. Neither -endorphin nor DAGO influenced vasoconstriction evoked by the application of noradrenaline or adenosine triphosphate; U-50488 reduced it. In arteries preincubated with [³H]noradrenaline DAGO depressed, whereas naloxone enhanced the tritium overflow and vasoconstriction evoked by field stimulation (0.4 Hz, 24 pulses every 14 min). In addition, naloxone antagonized the effect of DAGO. We suggest that the axon terminals of postganglionic sympathetic neurones in the rat tail artery possess -endorphin-sensitive opioid receptors of the -type. The activation of these receptors by exogenous or endogenous opioids inhibits the release of the neuroeffector transmitter.This work was supported by the Deutsche Forschungsgemeinschaft (SFB 325) Send offprint requests to P. Illes at the above address