Reinstatement of ethanol-seeking behavior by drug cues following single versus multiple ethanol intoxication in the rat: effects of naltrexone

Rationale A positive relationship exists between chronic ethanol intoxication experiences and the severity of neural hyperactivity and withdrawal seizures. An important possibility is that withdrawal reactions also influence the motivation to obtain and consume ethanol. Objectives To test this hypothesis, the effects of ethanol-cues on the recovery of extinguished ethanol-seeking and the reversal of this effect by naltrexone, were determined in non-dependent rats and in rats subjected to single versus repeated ethanol intoxications. Methods Rats were trained to self-administer and discriminate between 10% ethanol and water. Instrumental responding then was extinguished and the effects of exposure to ethanol and water cues were determined. Subsequently, rats were divided into three groups and exposed to control vapor (CTRL), to 12-day ethanol vapor prior to withdrawal (SW), or to three cycles of 3-day intoxication experiences (MW), respectively. Following intoxication, reacquisition and breaking point for ethanol self-administration and cues-induced reinstatement of drug-seeking were investigated. Results Ethanol cues significantly reinstated responding in the pre- and post-dependence test, but no significant differences between groups was observed. However, the ability of naltrexone to attenuate the response-reinstatement was significantly reduced in MW rats. Moreover, in the progressive ratio schedule, the breaking points for ethanol were significantly increased in the MW animals. Conclusions The results suggest that repeated intoxication did not enhance cue-induced reinstatement of ethanol-seeking. However, naltrexone effects on cues-induced "relapse" appear to be attenuated in MW rats.     

Effects of subconvulsive electrical stimulation to the hippocampus on emotionality and spatial learning and memory in rats

Objective To observe the effects of repeated subconvulsive electrical stimuli to the hippocampus on the emotional behavior and spatial learning and memory ability in rats.Methods One hundred and eight male Wistar rats were randomized into 3 groups. Animals in group SE (n=42) were given subconvulsive electrical stimulation to the hippocampus through a constant pulsating current of 100 μA with an intratrain frequency of 25 Hz, pulse duration of 1 millisecond, train duration of 10 seconds and interstimulus interval of 7 minutes, 8 times a day, for 5 days. In the electrode control group or CE group (n=33), animals were implanted with an electrode in the hippocampus, but were not stimulated. Group NC (n=33) animals received no electrode or any stimulation. The emotional behavior of experimental rats was examined by activity in an unfamiliar open field and resistance to capture from the open field, while the spatial learning and memory ability was measured during training in a Morris water maze.Results The stimulated rats tested 1 month after the last round of stimulation displayed substantial decreases in open field activity (scale: 10.4±2.3, P<0.05) and increases in resistance to capture (scale: 2.85±0.56, P<0.01). The amount of time for rats in group SE to find the platform (latency) as a measurement for spatial bias was prolonged (29±7) seconds after 15 trials in the water maze, P<0.05). The experimental rats swam aimlessly in all four pool quadrants during the probe trial in the Morris water maze.Conclusions Following repeated subconvulsive electrical stimuli to the hippocampus, rats displayed long-lasting significant abnormalities in emotional behavior, increased anxiety and defensiveness, enhanced ease to and delayed habituation to startlement, transitory spatial learning and memory disorder, which parallels many of the symptoms in posttraumatic stress disorder patients.     

Sound presentation rate is represented logarithmically in human cortex

The encoding of temporal information is critical to auditory processing. Since the mismatch negativity component of the auditory event-related brain potential is thought to reflect properties of auditory sensory memory, we used it to examine the representation of acoustic time intervals in the human cortex. The mismatch negativity occurs in response to deviations in acoustic regularities, which are stored in sensory memory. We used 16 stimulus conditions, randomly presenting short trains of tones with fixed onset-to-onset intervals of 100, 200, 300 or 400 ms (all tones in the study were identical). The first four intervals between the tones established the acoustic regularity on each train (i.e. the 'standard'). The fifth tone in each train was preceded by an interval that varied randomly among the same four intervals. If this interval was different from the standard for that trial, it violated the acoustic regularity (i.e. it was a 'deviant'). The mismatch response to the fifth tone differed significantly among stimulus conditions and was proportional to the absolute value of the logarithm of the deviant/standard interval ratio. This indicates that short acoustic time intervals are represented with a ratio scale in the human cortex. When the fifth tone occurred 100 ms after the fourth, it elicited a somewhat different, although proportional response, supporting the hypothesis that a special integration mechanism may exist for very short time intervals.     

Medial Olivocochlear Efferent Reflex in Humans: Otoacoustic Emission (OAE) Measurement Issues and the Advantages of Stimulus Frequency OAEs

Otoacoustic emissions (OAEs) are useful for studying medial olivocochlear (MOC) efferents, but several unresolved methodological issues cloud the interpretation of the data they produce. Most efferent assays use a probe stimulus to produce an OAE and an elicitor stimulus to evoke efferent activity and thereby change the OAE. However, little attention has been given to whether the probe stimulus itself elicits efferent activity. In addition, most studies use only contralateral (re the probe) elicitors and do not include measurements to rule out middle-ear muscle (MEM) contractions. Here we describe methods to deal with these problems and present a new efferent assay based on stimulus frequency OAEs (SFOAEs) that incorporates these methods. By using a postelicitor window, we make measurements in individual subjects of efferent effects from contralateral, ipsilateral, and bilateral elicitors. Using our SFOAE assay, we demonstrate that commonly used probe sounds (clicks, tone pips, and tone pairs) elicit efferent activity, by themselves. Thus, results of efferent assays using these probe stimuli can be confounded by unwanted efferent activation. In contrast, the single 40 dB SPL tone used as the probe sound for SFOAE-based measurements evoked little or no efferent activity. Since they evoke efferent activation, clicks, tone pips, and tone pairs can be used in an adaptation efferent assay, but such paradigms are limited in measurement scope compared to paradigms that separate probe and elicitor stimuli. Finally, we describe tests to distinguish middle-ear muscle (MEM) effects from MOC effects for a number of OAE assays and show results from SFOAE-based tests. The SFOAE assay used in this study provides a sensitive, flexible, frequency-specific assay of medial efferent activation that uses a low-level probe sound that elicits little or no efferent activity, and thus provides results that can be interpreted without the confound of unintended efferent activation. Present address (Vered Aharonson): Department of Computer Science, Tel Aviv College, 4 Antokolsky St., POB 16131, Tel-Aviv 61161, Israel.     

Dopamine transporter binding without cocaine-like behavioral effects: synthesis and evaluation of benztropine analogs alone and in combination with cocaine in rodents

Rationale: Previous SAR studies demonstrated that small halogen substitutions on the diphenylether system of benztropine (BZT), such as a para-Cl group, retained high affinity at the cocaine binding site on the dopamine transporter. Despite this high affinity, the compounds generally had behavioral effects different from those of cocaine. However, compounds with meta-Cl substitutions had effects more similar to those of cocaine. Objectives: A series of phenyl-ring analogs of benztropine (BZT) substituted with 3′-, 4′-, 3′,4′′- and 4′,4′′-position Cl-groups were synthesized and their pharmacology was evaluated in order to assess more fully the contributions to pharmacological activity of substituents in these positions. Methods: Compounds were synthesized and their pharmacological activity was assessed by examining radioligand binding and behavioral techniques. Results: All of the compounds displaced [³H]WIN 35,428 binding with affinities ranging from 20 to 32.5 nM. Affinities at norepinephrine ([³H]nisoxetine) and serotonin ([³H]citalopram) transporters, respectively, ranged from 259 to 5120 and 451 to 2980 nM. Each of the compounds also inhibited [³H]pirenzepine binding to muscarinic M₁ receptors, with affinities ranging from 0.98 to 47.9 nM. Cocaine and the BZT analogs produced dose-related increases in locomotor activity in mice. However, maximal effects of the BZT analogs were uniformly less than those produced by cocaine, and were obtained 2–3 h after injection compared to the relatively rapid onset (within 30 min) of cocaine effects. In rats trained to discriminate IP saline from 29 μmol/kg cocaine (10 mg/kg), cocaine produced a dose-related increase in responding on the cocaine lever, reaching 100% at the training dose; however, none of the BZT analogs fully substituted for cocaine, with maximum cocaine responding from 20 to 69%. Despite their reduced efficacy compared to cocaine in cocaine discrimination, none of the analogs antagonized the effects of cocaine. As has been reported previously for 4′-Cl-BZT, the cocaine discriminative-stimulus effects were shifted leftward by co-administration of the present BZT analogs. Conclusions: The present results indicate that although the BZT analogs bind with relatively high affinity and selectivity at the dopamine transporter, their behavioral profile is distinct from that of cocaine. The present results suggest that analogs of BZT may be useful as treatments for cocaine abuse in situations in which an agonist treatment is indicated. These compounds possess features such as reduced efficacy compared to cocaine and a long duration of action that may render them particularly useful leads for the development of therapeutics for cocaine abusers. Electronic Publication     

(R)-methanandamide and Delta 9-THC as discriminative stimuli in rats: tests with the cannabinoid antagonist SR-141716 and the endogenous ligand anandamide

RATIONALE AND OBJECTIVES: (R)-methanandamide (AM-356), a metabolically more stable chiral analog of the endocannabinoid ligand anandamide, was used as a representative of fatty acid ethanolamide CB1 receptor ligands to characterize the discriminative stimulus functions of anandamides. METHODS: Rats discriminated between 10 mg/kg (R)-methanandamide and vehicle administered IP 15 min prior to session onset. Another group of rats was initially trained to discriminate between 3 mg/kg Delta9-THC and vehicle given IP 30 min prior to session onset; for anandamide testing, the animals were retrained with 1.8 and 5.6 mg/kg Delta9-THC. A two lever operant methodology (FR10) was used. RESULTS: Delta9-THC was more potent than (R)-methanandamide at both 15 and 30 min post-injection, irrespective of the training drug used. Additional tests with 10 and 18 mg/kg (R)-methanandamide suggested that the effects were declining by 1 h. The cannabinoid antagonist SR 141716 (0.3 and 1 mg/kg) produced rightward shifts in the Delta9-THC dose-response curve for Delta9-THC-appropriate responding and for (R)-methanandamide-appropriate responding (surmountable antagonism). SR-141716 (0.3 and 1 mg/kg) antagonized the ability of (R)-methanandamide to occasion either Delta9-THC-appropriate responding or (R)-methanandamide-appropriate responding. This antagonism was surmountable only at a dose of 0.3 mg/kg SR-1421716 in the (R)-methanandamide-trained rats. SR-141716 did not antagonize the rate-decreasing effects of (R)-methanandamide in either the Delta9-THC or the (R)-methanandamide trained rats. Response suppression precluded testing doses higher than 30 mg/kg (R)-methanandamide. Tests with SR-141716 (1 and 10 mg/kg) alone resulted in <3% Delta9-THC-appropriate responding. With 10 mg/kg SR-141716, response rate was significantly lower as compared to the rate observed during a vehicle test. Tests with anandamide (10 and 18 mg/kg) resulted in 41% and 85% (R)-methanandamide-appropriate responding at a 3-min pre-treatment time, but in a maximum of 15% (R)-methanandamide-appropriate responding at a longer (15 min) pre-treatment time. In the Delta9-THC (1.8 and 5.6 mg/kg) trained rats, anandamide never produced more than about 20% Delta9-THC-appropriate responding. CONCLUSION: The results add to a growing body of evidence indicating that there are both similarities and dissimilarities between classical cannabinoids such as THC and endogenous fatty acid ethanolamides.     

Similar enhancement of the discriminative stimulus effects of cocaine and GBR 12909 by heroin in squirrel monkeys

RATIONALE AND OBJECTIVES: Heroin previously was shown to engender partial cocaine-like discriminative stimulus (DS) effects in squirrel monkeys. The present study assessed the degree to which heroin modified the DS effects of cocaine and the cocaine-like DS effects of the selective dopamine transport blocker GBR 12909. METHODS AND RESULTS: In squirrel monkeys discriminating cocaine (0.3 mg/kg) from saline, cocaine and GBR 12909 dose-dependently engendered levels of responding on the cocaine-associated lever greater than or equal to 90% (full substitution). Heroin engendered full substitution for cocaine in two monkeys, partial substitution (75%) in a third monkey, and no substitution in the fourth monkey. When administered as a pretreatment, heroin shifted the dose-response function for cocaine to the left in the three monkeys for which heroin engendered cocaine-lever responding, but did not alter the DS effects of cocaine in the fourth monkey. Heroin pretreatment also shifted the dose-response function for the cocaine-like DS effects of GBR 12909 to the left in the former three monkeys, and did not alter the effects of GBR 12909 in the fourth monkey. Isobolographic analysis of the DS effects of cocaine-heroin and GBR 12909-heroin combinations in the former three monkeys revealed that the potencies of the combinations were not different from predicted values based on dose-additive effects. CONCLUSIONS: These findings show that heroin can enhance similarly the DS effects of cocaine and GBR 12909, suggesting that activation of dopaminergic systems underlies enhancement of the interoceptive effects of cocaine by heroin.     

Influence of a conditioned light stimulus on cocaine self-administration in rats

RATIONALE: A number of studies have suggested that the continued presentation of stimuli associated with cocaine may contribute to drug-seeking and drug-taking. The influence of conditioned stimuli on the maintenance of self-administration has not, however, been systematically investigated. OBJECTIVES: This study was designed to determine whether omission of a stimulus that had been paired with self-administered cocaine would influence the maintenance of cocaine self-administration and whether the effect was dependent on cocaine dose or session length. METHODS: During self-administration training, self-administered cocaine infusions were always paired with the illumination of a light. On test days, self-administered cocaine was delivered either with or without the cocaine-associated cue. For one group of rats, responding maintained by cocaine (0.50 mg/kg per infusion) was measured during daily 18-h sessions. For other groups, responding maintained by additional doses of cocaine (0.125, 0.25, or 1.0 mg/kg per infusion) was measured during daily 8-h sessions. For a final group, daily test sessions (4-5 h) produced the dose-effect curve (0.015-1.0 mg/kg per infusion) by repeatedly reducing the cocaine dose from a starting dose of 1.0 mg/kg per infusion. RESULTS: Removal of the light cue decreased cocaine self-administration. The magnitude of this effect was dependent on the dose of self-administered cocaine and on the test session duration. Greater decrements in responding were produced as session length increased or when low doses of cocaine were self-administered. CONCLUSIONS: These findings demonstrate that in the absence of a cocaine-associated stimulus, cocaine self-administration is attenuated and that maintenance of cocaine self-administration is maximally affected by the presence or absence of the conditioned stimulus when the self-administered dose is low and/or when session duration is long.     

Retardation in somatosensory cortex development induced by postnatal BrdU treatment in mice

Cerebral dysgeneses are in the background of several neurological and mental disturbances. The aim of the present study was to investigate structural and activity changes following disturbed postnatal neuronal development in mice. Newborn C57Bl6 mice were exposed to 5-bromo-2′-deoxyuridine (BrdU: daily 50 μg/g body weight) during a period between postnatal days P0–P5 or P0–P11, respectively, and neuronal malformation and malfunctioning of somatosensory (barrel field) cortex was analyzed in adolescent animals. Alterations in histological architecture of interneuronal and glial elements were studied and correlated with electrophysiological modifications. Between P30 and P35 days litters underwent ex vivo electrophysiological experiments to examine the changes in basic excitability and in synaptic efficacy. Parallel immunohistochemistry was performed to detect BrdU, GABA and GFAP.