Effect of β-Endorphin and naloxone on acquisition,memory, and retrieval of shuttle avoidance and habituation learning in rats

Naloxone impairs acquisition of shuttle avoidance behavior (0.8 mg/kg IP) and habituation to a rearing response to a tone (1.6 mg/kg IP) in rats. -Endorphin (2 g/kg IP) has no effect on acquisition, but, when given prior to test sessions, facilitates retrieval of the two tasks. Naloxone has no effect of its own upon retrieval. In addition to these effects, the pretraining administration of -endorphin disrupts, and that of naloxone facilitates retention of the two tasks. The results are consistent with the hypothesis that these two forms of learning are state-dependent on the release of -endorphin (and, perpaps, of other opiate peptides as well), that this substance is released during training in a sufficient amount for this purpose, and that, in addition, there is a physiological amnesic mechanism mediated by opiate peptides. Furthermore, the results are also consistent with previous observations that -endorphin is released from the rat brain during training, but not during test sessions of the two tasks (Izquierdo et al., 1980b). The possibility is discussed that state-dependency and the amnesic effect comprise one single, rather than two separate mechanisms.     

Calcium channel modulation by dihydropyridines modifies sufentanil-induced antinociception in acute and tolerant conditions

Summary The study was aimed at elucidating the possible participation of l-type Ca²⁺ channel in the acute analgesic effect of an opiate and the development of tolerance to this action. Sufentanil, a selective p agonist, and two dihydropyridines, the Ca²⁺ antagonist nimodipine and the Ca²⁺ agonist Bay K 8644, were selected. The tail-flick test was used to assess the nociceptive threshold. In naive rats, nimodipine (200 g/kg) potentiated the analgesic effect of sufentanil reducing the ED₅₀ from 0.26 to 0.08 g/kg. Similar results were observed with its (–)-enantiomer Bay N 5248, while the (+) enantiomer Bay N 5247 was ineffective. Tolerance to the opiate was induced by chronic subcutaneous administration of sufentanil with minipumps (2 g/h, 7 days). In these conditions the dose-response curve to sufentanil was displaced to the right and the ED₅₀ was increased to 1.49 g/kg. In tolerant rats, nimodipine preserved its potentiating ability and prevented the displacement to the right of the sufentanil dose response-curve (ED₅₀ = 0.48 g/kg). When nimodipine was pumped (1 g/h, 7 days) concurrently with sufentanil, the development of tolerance to the opioid was not disturbed. However, the expression of tolerance was abolished and even the effect of acutely administered sufentanil was markedly potentiated (ED₅₀ = 0.03 g/kg). Similar experiments were performed with Bay K 8644. In naive rats, Bay K 8644 at a low dose (20 g/kg) that behaves as a calcium agonist, antagonized the analgesic effect of sufentanil (ED₅₀ = 0.58 g/kg), whereas at a high dose (200 g/kg) it potentiated this action (ED₅₀ = 0.15 g/kg). In tolerant rats, Bay K 8644 (20 g/kg) preserved its antagonizing ability inducing a displacement to the right of the sufentanildose-response curve (ED₅₀ = 4.2 g/kg). When Bay K 8644 was pumped (1 g/h, 7 days) concurrently with sufentanil, it enhanced the expression of tolerance to the opiate (ED₅₀ = 3.8 g/kg). These results suggest that the calcium fluxes through the l-type channel in neurones are functionally linked to the activation of the opiate receptor: the blockade of the channel increased the potency of sufentanil, whereas its activation reduced the potency of the opiate. In chronic experiments, DHPs concurrently administered with sufentanil did not affect the development of tolerance to the opiate. However, nimodipine prevented the expression of this phenomenon. Even more, the animals became hypersensitive to the opiate suggesting that the adaptative mechanisms induced by chronic opiate could be affected by chronic nimodipine.This work was supported by grants from Universidad de Cantabria-Caja Cantabria (1988) and Bayer AG, Wuppertal, FRGPredoctoral Fellow: Fondo de Investigaciones Sanitarias de la Seguridad Social.Send offprint requests to: M. A. Hurlé at the above address     

Intra-amygdala injections of corticotropin releasing factor facilitate inhibitory avoidance learning and reduce exploratory behavior in rats

The effects of intra-amygdala injections of corticotropin-releasing factor (CRF) on memory and exploratory behavior in rats were examined in the present study. Rats with chronically implanted cannulae received intra-amygdala injections of vehicle or CRF at a dose of 0.01, 0.1 or 1.0 g, either immediately after the inhibitory avoidance training or prior to the open field activity test. Results indicated that while CRF at low (0.01 g) and high (1.0 g) doses produced no significant effect on retention or exploration, immediate post-training intra-amygdala injections of CRF at the medium dose (0.1 g) significantly improved retention of the inhibitory avoidance response. The same dose of CRF, given shortly prior to the open field activity test, decreased locomotor activity, rearing and hole-poke responses in rats. These results suggest that the amygdala is one of the anatomical loci involved in CRF modulation of memory processing and exploration in rats. The implication of CRF in mediating the influences of stress on behavior is discussed.     

Unlike β-endorphin,dynorphin1–13 does not cause retrograde amnesia for shuttle avoidance or inhibitory avoidance learning in rats

Posttraining administration of the opioid peptides, -endorphin or the enkephalins, is known to cause retrograde amnesia for a variety of tasks in rats. The present paper studies the effect of the posttraining administration of dynorphin_1–13 on retention of a step-down inhibitory avoidance task and of a shuttle avoidance task. For the purpose of comparison, the effect of human -endorphin was also studied. In confirmation of previous results, -endorphin (1.0 or 10.0 g/kg, IP) caused retrograde amnesia for the two tasks. Dynorphin_1–13 had no effect at doses between 0.008–125.0 g/kg IP or 1.25–125.0 ng/rat ICV in the inhibitory avoidance task, or at doses of 5.0, 25.0, or 125.0 g/kg in the shuttle avoidance paradigm. These findings suggest that, in contrast to -endorphin, dynorphin_1–13 may not be involved in memory regulation at the posttraining period in rats.     

Differential mechanisms in the acquisition and expression of heroin-induced place preference

These experiments examined the neurochemical mechanisms involved in the development and expression of place conditioning produced by heroin. Conditioned place preferences (CPP) lasting up to 8 weeks were obtained with doses of 50–1000 g/kg heroin, using a regimen shown not to produce physical dependence. Naloxone pretreatment (50 g/kg) during conditioning prevented the acquisition of heroin-induced CPP, but when given only on the test day, naloxone (50 or 1000 g/kg) did not prevent the expression of heroin CPP. Clonidine disrupted the establishment of heroin CPP at 20 g/kg, but disrupted its expression only at debilitating doses (100 and 200 g/kg). Pimozide attenuated the acquisition (100 /kg) and expression (250 g/kg) of heroin CPP. Together, these results support a role for opioid and catecholamine systems in the acquisition of heroin reinforcement, but they suggest that once heroin CPP is established, its expression in opiate-free subjects is not opiate receptor mediated and is relatively refractory to pharmacological treatments which disrupt acquisition. The data challenge the notion that the conditioned effects of opiates in drug-free animals are related to the release of endogenous opioids, and they also may help to explain why naloxone and clonidine are ineffective in the treatment of opiate addiction.     

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.     

Changes in limb tone produced by regional injections of opiates into rat brain

Summary A mechanical apparatus was used to measure the resistance of the hindlegs to flexion in conscious rats. Systemically-administered morphine (7.5–12.5 mg/kg) caused increased muscle tone in the limbs. In rats with intracerebral cannulae, intrastriatal morphine (5–15 g) had no effect on limb tone, whereas an increase in limb tone was induced by morphine (1–30 g) in the globus pallidus. A strong and sustained increase in limb tone developed when morphine (0.2–2.5 g) and the mu opiate receptor agonist DAGO (1.5–2.5 g) were administered into the periaqueductal gray region of the midbrain, whereas the delta opiate receptor agonist DADLE (2.5–8 g) was ineffective. The increased limb tone induced by opiates may be generated by mu-type receptors in at least two areas of brain. Send offprint requests to P. Slater at the above address     

Negative reinforcing properties of naloxone in the non-dependent rhesus monkey: influence on reinforcing properties of codeine,tilidine, buprenorphine,and pentazocine

Scheduled infusions of naloxone (1–100 g/kg/inf.) and of buprenorphine (250 g/kg/inf.) generated drug avoidance behavior in the non-dependent rhesus monkey under a continuous avoidance-escape paradigm. Pentazocine (1–100 g/kg/inf.) codeine, (1–100 g/kg/inf.) and tilidine (1–250g/kg/inf.) were ineffective. Addition of varying doses of naloxone to scheduled infusions of codeine, tilidine, and pentazocine generated avoidance behavior not present with scheduled infusions of these opioids alone. The naloxone doses necessary for generation of avoidance behavior were low with the agonists codeine and tilidine, higher with the weak antagonist pentazocine, and highest with the strong antagonist buprenorphine. When monkeys were presented with the fixed tilidine-naloxone combination (100+8 parts) and pentazocine-naloxone combination (100+1 part) and the buprenorphine-naloxone combination (100+66 parts) presently in clinical use only the tilidine-naloxone combination generated drug avoidance behavior to an appreciable extent.The author is indebted to Reckitt and Colman, Dansom Lane, Hull HU8 7DS, U.K., for buprenorphine; Sterling Winthrop, Alnwick, Northumberland NE66 2JH, U.K., for pentazocine; and Goedecke AG, Mooswaldallee 1–9, D-7800 Freiburg, for tilidine.     

Intrahypothalamic and intrastriatal dopamine and norepinephrine injections in relation to motor hyperactivity in rats

High doses of dopamine (59 g) and norepinephrine (65 g) injected directly into the striatum and hypothalamus induced motor hyperactivity in rats. The motor activity recorded on the Animex for a period of 60 min after injection of 65 g of norepinephrine into the hypothalamus, showed a significant increase (p<0.005) in comparison with the controls. The increase in motor activity after dopamine (intrahypothalamic) and norepinephrine and dopamine (intrastriatal) was distinctly lower, although there was an initial large increase of motor activity after intrastriatally injected dopamine. Pre-treatment with reserpine or parachlorophenylalanine (intraperitoneal injection) to lower the serotonin level in the brain, followed by intracerebral injection of norepinephrine or dopamine failed to produce fighting or mounting behaviour.     

Vasopressin has general rate-decreasing effects on schedules maintaining either high or low response rates

Male and female Wistar rats were treated with different doses of vasopressin (0.05, 0.25, 1.25, 3.75 and 6.25 g/kg) after responding had stabilized on either a differential reinforcement of low rate 15 s (DRL 15 s) or a differential reinforcement of high rate 0.75 s (DRH 0.75 s) schedule of reinforcement. Low to moderate doses of vasopressin did not affect response rates, response efficiency or the number of reinforcers obtained during vasopressin sessions on both the DRL and DRH schedules. Administration of 6.25 g/kg vasopressin reduced low response rates and the number of reinforcers obtained during vasopressin sessions, but increased response efficiency. High response rates and response efficiency were reduced after administration of 3.75 and 6.25 g/kg vasopressin, while the number of reinforcers obtained during vasopressin sessions was reduced at 6.25 g/kg. Sex differences in the effects of vasopressin were not observed on either schedule.     

Anti-nociceptive effect of morphine,opioid analgesics and haloperidol injected into the caudate nucleus of the rat

Summary The effect of intracaudate (i.c.) microinjections of morphine, opioid analgesics and haloperidol was determined on the tail-flick response evoked by radiant heat in rats. Bilateral injections (0.2 l on each side) into the caudate nuclei of morphine 5 g, pethidine 50 g, levorphanol 4 g, dextrorphan 10 g and haloperidol 5 g significantly increased the reaction time of the tail-flick response. The antinociceptive effect of an i.c. injection of morphine or levorphanol was abolished by an intraperitoneal (i.p.) injection of naloxone 0.2 mg/kg or apomorphine 2 mg/kg. The anti-nociceptive effect of pethidine, dextrorphan and haloperidol was reduced but not abolished by an i.p. injection of naloxone 0.2 mg/kg. An i.p. injection of apomorphine 2 mg/kg abolished the effect of an i.c. injection of haloperidol. A bilateral i.c. injection of naloxone 5 g or apomorphine 10 g reduced the anti-nociceptive effect of an i.p. injection of morphine 2 mg/kg or haloperidol 2 mg/kg, but did not abolish it. It is concluded that (1) an anti-nociceptive effect can be achieved by an action on the caudate nucleus of the drugs tested; (2) the anti-nociceptive effect exerted by morphine and levorphanol in the caudate nucleus is due to a specific action mediated by opiate receptors, whilst that produced by pethidine and dextrorphan is due to a specific and/or unspecific action; (3) the anti-nociceptive effect of haloperidol in the caudate nucleus is due to an impairment of dopaminergic impulse transmission, which is also involved in the effect of morphine and levorphanol.Supported by the Sonderforschungsbereich 38 Membranforschung     

Rats self-administer sufentanil in aerosol form

An ultrasonic nebulizer was used to create a drug vapor to develop an animal model for the self-administration of inhaled nonvolatile psychoactive drugs. An aerosol mist of a sufentanil citrate solution (10, 25, 50, or 75 g/ml) was delivered to rats in response to lever presses on an FR 5 schedule of reinforcement. The speed of acquisition of the operant response and the selectivity of the drug effect were examined. Rats given access to sufentanil vapor (50 or 75 g/ml) in 13–15 h overnight training sessions reached an average of one reinforcement per hour on an FR 5 schedule of reinforcement significantly sooner than did rats given access to water vapor. Responding maintained by sufentanil durign 2-h daily testing sessions was dose dependent at 25, 50, and 75 g/ml. Substituting water vapor for each of the four sufentanil concentrations significantly reduced responding within 5–20 sessions. Naloxone (1 mg/kg, IP) decreased responding for sufentanil to the level attained under water vapor. Presentation of drugs in aerosol form thus provides reasonable means of demonstrating in animals the reinforcing properties of non-volatile drugs by the pulmonary or intranasal route.     

Effects of naloxone,metenkephalin, and morphine on phencyclidine-induced behavior in the rat

The effects of naloxone, metenkephalin, and morphine were tested on phencyclidine(PCP)-induced stereotyped behaviors, ataxia, and hyperactivity in the rat. Naloxone (8 mg/kg) significantly decreased stereotypy, ataxia, and hyperactivity across all PCP doses tested (2.0, 4.0, and 6.0 mg/kg). Metenkephalin (40 g/kg) and morphine (5 and 10 mg/kg) increased ataxia at the 4.0 and 6.0 mg/kg PCP doses. Stereotypy was altered by the opiates in a dose-dependent manner; enhanced by metenkephalin (40 g/kg) at 2.0 mg/kg and inhibited by metenkephalin (40 g/kg) and morphine (10 mg/kg) at 4.0 and 6.0 mg/kg PCP. Locomotor activity was increased by morphine (5 mg/kg) at 2 mg/kg PCP. These results suggest an involvement of central opiate receptor mechanisms in the mediation of PCP-induced behaviors in the rat.     

Developmental Responses of a Terrestrial Insect Detritivore, <Emphasis Type="BoldItalic">Megaselia scalaris</Emphasis> (Loew) to Four Selenium Species

Megaselia scalaris (Loew) (Diptera: Phoridae) is an important and ubiquitous terrestrial detritivore that consumes both animal and plant material. Because both plants and animals convert selenium pollutants into various forms, the relative toxicities of ecologically relevant concentrations of sodium selenate, sodium selenite, seleno-L-methionine, and Se-(methyl) selenocysteine hydrochloride to larvae were assessed in diet bioassays. In addition, ovipositional preferences of adults and developmental effects on the eggs and larvae were measured. With chronic exposure selenocysteine was the most toxic of the selenium species to the larvae (LC₅₀: 83 g/g wet weight), followed by seleno-L-methionine (LC₅₀: 130 g/g), selenate (LC₅₀: 258 g/g), and selenite (LC₅₀: 392 g/g). Ovipositing females did not discriminate between the highest treatment concentrations of any of the pollutants as compared to the controls, indicating a lack of avoidance behavior. Larval development time was significantly increased with exposure to selenate at 100 g/g wet weight and above, selenite at 300 g/g and above, and at 50 g/g and 25 g/g and above for seleno-L-methionine and selenocysteine respectively. Pupal development was not affected by any of the selenium treatments. Significant differences between male and female adult eclosion times were observed, with females eclosing later than males as selenium concentrations increased. Significant decreases in larval survival relative to controls occurred at the lowest treatment tested (100 g/g) for both selenate and selenite and at 100 g/g for seleno-L-methionine, and 50 g/g for selenocysteine. The population level implications of lack of avoidance of contaminated food, and the effects of increased development times, reduced survivorship, and non-synchronized male and female emergence are discussed.     

Luteinizing-hormone-releasing hormone modifies retention of passive and active avoidance responses in rats

The influence of posttraining subcutaneous administration of luteinizing-hormone-releasing hormone (LHRH) was tested on the retention of either active or passive avoidance conditioning in male rats. Injection of LHRH (200/kg) immediately after the acquisition of an active avoidance response (two-way shuttle behavior) enhanced retention of the response, assessed 7 days later. When the neuropeptide was injected immediately after a passive avoidance conditioning training, the effects varied with the intensity of the footshock applied. LHRH enhanced retention of avoidance training with weak footshock (0.20 and 0.35 mA) but impaired retention of training with strong footshock (0.70 and 1.0 mA). The effects of LHRH seem to be unspecific since they are similar to those observed after treatment with several hormones. The results are discussed based on the interactions between peripherally injected hormones and endogenous substances released following footshock. A modulatory effect on the monoaminergic pathway involved in memory storage processes is postulated.     

Memory facilitation with posttrial injection of oxotremorine and physostigmine in mice

The immediate posttrial injection of oxotremorine (0.125, 0.250 and 0.500 Mol/kg i.p.) and equimolecular doses of physostigmine can facilitate the retention of a passive avoidance response in mice. Injections given 10 min after training also significantly facilitate retention, but injections given 30 or 120 min after training do not affect retention. These findings suggest an action of oxotremorine and physostigmine on mechanisms involved in memory storage. The enhanced retention produced by oxotremorine and physostigmine was blocked by pretreatment with atropine (2 Mol/kg, 20 min, i.p.) but was not affected by methylatropine (2 Mol/kg, 20min, i.p.). The retention was not modified by posttrial injection of metoxotremorine (0.250 Mol/kg i.p.) or neostigmine (0.250 Mol/kg i.p., quaternary analogs of oxotremorine and physostigmine, respectively. The results suggest a central action of both cholinergic agents attributable to an activation of muscarinic brain receptors.     

Characterization of the discriminative stimulus effects of centrally administered morphine in the rat

The discriminative stimulus effects of centrally administered morphine were characterized in rats trained to discriminate 3.0 mg/kg SC morphine from saline in a two-choice discrete-trial avoidance paradigm. The intracerebroventricular (ICV) administration of 0.3–10 g morphine engendered morphine-appropriate responding, morphine administered ICV being nearly 1000 times as potent as morphine administered SC. Cannula implantation itself did not affect the sensitivity of the rats to the discriminative effects of morphine. The onset of the discriminative stimulus effects of ICV morphine was not immediate; stimulus generalization comparable to that produced by 3.0 mg/kg morphine occurred 30–60 min after the injection of 1.0 or 10 g ICV morphine and persisted for 90 and 150 min, respectively. Naltrexone blocked the discriminative stimulus effects of 10 g ICV morphine in a dose-related manner. Complete antagonism of the stimulus effects of this dose of morphine was obtained with 0.01–0.03 mg/kg SC naltrexone. When administered centrally, the relatively lipid insoluble naltrexone methobromide completely antagonized the discriminative effects of 3.0 mg/kg morphine at a median effective dose of 0.3 g. In contrast, when injected systemically at a dose of 1.0 mg/kg (approximately 500 g), naltrexone methobromide failed to block the discriminative stimulus effects of either 10 g ICV morphine or the SC training dose. Thus, periventricular brain sites appear to be involved in mediating the discriminative stimulus effects of morphine in the rat.     

Modulation of memory by post-training epinephrine: involvement of cholinergic mechanisms

Extensive evidence indicates that memory storage can be modulated by peripheral epinephrine as well as by drugs affecting the muscarinic cholinergic system. Low doses of epinephrine (Epi) facilitate memory while high doses induce amnesia. Retention is enhanced by post-training administration of cholinergic muscarinic agonists and impaired by antagonists. The present experiments examined the interaction of peripheral Epi and cholinergic drugs in memory modulation. Male CFW mice (60 days old) were trained on an inhibitory avoidance response (IA) or a Y-maze discrimination response (YMD), injected (IP) immediately post-training and tested 24 h later. In the Y-maze task, retention was assessed by discrimination reversal training. Findings obtained in the two tasks were comparable. Epi (IA: 3.0 g/kg; YMD: 30.0 g/kg) potentiated the memory-enhancing effect of low doses of oxotremorine (Otm) (IA: 16; YMD: 5.0 g/kg) and physostigmine (Phy) (6.8 and 22.0 g/kg for both IA and YMD). The memory-facilitating effect of Otm (50.0 g/kg) was not blocked by an amnestic dose of Epi (IAT: 0.3 mg/kg; YMD: 1.0 mg/kg). Retention was not affected when these amnestic doses of Epi were administered together with a memory-facilitating dose of Phy (68.0 g/kg). In contrast, atropine (IA: 10.0 mg/kg; YMD: 3.0 mg/kg) completely blocked the facilitatory effect of Epi (IA: 10.0 g/kg; YMD: 300 g/kg). These findings indicate that, when administered in low doses, Epi interacts with Otm and Phy. However, cholinergic drugs can block both the memory-enhancing and the memory-impairing effects of Epi. The findings suggest that Epi may modulate memory through cholinergic systems.     

Physiological disposition of isoergine [from Argyreia nervosa (Burm. f.) Bojer Convolvulaceae] and its effect on the conditioned avoidance response in rats

Physiological disposition of isoergine (d-isolysergamide, iso-LA) obtained from the seeds of Argyreia nervosa (Burm. f.) Bojer were determined in rat liver, brain and plasma. Method of determination involved the extraction of the drug from biological samples and quantitation of the compound by fluorometric analysis. The injection of 5 mg/kg, i.p., of iso-LA resulted after 5 min in peak levels in the liver (7.2 g/g) and after 15 min in peak levels in the brain (1.2 g/g) and plasma (1.9 g/ml). After 120 min, 90% of the compound had disappeared from the tissues and plasma. The minimal dose required to produce a significant decrease in the conditioned avoidance response (CAR) was somewhat less than 5 mg/kg. The minimal brain level which interfered with the CAR was approximately 1 g/g. Brain levels of iso-LA seem to correlate directly with changes in behavior suggesting that iso-LA, and not a metabolite, is the psychoactive agent.     

Dose-response study of oxitropium bromide inhaled as a nebulised solution

Twelve patients suffering from partially reversible chronic obstructive pulmonary disease (COPD) took past in a single blind, randomised, 4-way cross-over trial to determine the optimal dose and duration of action of the anticholinergic agent oxitropium bromide (OTB) inhaled as a nebulised solution.Single doses of 500, 1000, 1500 and 2000 g nebulised OTB were compared during a 6 hour-observation period. Lung function test results indicated that 500 and 100 g OTB only induced slight bronchodilatation, whereas 1500 and 2000 g OTB produced a significantly greater increase in mean FEV1 compared to 500 g. There was a trend for 2000 g to be superior to 1000 g, but 2000 g and 1500 g were not significantly different. Significant bronchodilatation (>15% rise in FEV1 from baseline) persisted for 6 h after 1500 g. A significant decrease in airway resistance (Raw) was observed following inhalation of 2000 g. The mean decrease in Raw was 33% after 30 min, 20% after 4 h and 12% after 6 h.In this trial, 2000 g OTB administered by an ultrasonic nebuliser was the optimal dose, but a satisfactory result was also obtained with 1500 g.