Cocaine: excitatory effects on sensorimotor reactivity measured with acoustic startle

The Impact of Muscarinic Antagonism on Psychosis-Relevant Behaviors and Striatal [11C] Raclopride Binding in Tau Mouse Models of Alzheimer's Disease

Psychosis that occurs over the course of Alzheimer's disease (AD) is associated with increased caregiver burden and a more rapid cognitive and functional decline. To find new treatment targets, studies modeling psychotic conditions traditionally employ agents known to induce psychosis, utilizing outcomes with cross-species relevance, such as locomotive activity and sensorimotor gating, in rodents. In AD, increased burdens of tau pathology (a diagnostic hallmark of the disease) and treatment with anticholinergic medications have, separately, been reported to increase the risk of psychosis. Recent evidence suggests that muscarinic antagonists may increase extracellular tau. Preclinical studies in AD models have not previously utilized muscarinic cholinergic antagonists as psychotomimetic agents. In this report, we utilize a human-mutant-tau model (P301L/COMTKO) and an over-expressed non-mutant human tau model (htau) in order to compare the impact of antimuscarinic (scopolamine 10 mg/kg/day) treatment with dopaminergic (reboxetine 20 mg/kg/day) treatment, for 7 days, on locomotion and sensorimotor gating. Scopolamine increased spontaneous locomotion, while reboxetine reduced it; neither treatment impacted sensorimotor gating. In the P301L/COMTKO, scopolamine treatment was associated with decreased muscarinic M4 receptor expression, as quantified with RNA-seq, as well as increased dopamine receptor D2 signaling, as estimated with Micro-PET [11C] raclopride binding. Scopolamine also increased soluble tau in the striatum, an effect that partially mediated the observed increases in locomotion. Studies of muscarinic agonists in preclinical tau models are warranted to determine the impact of treatment-on both tau and behavior-that may have relevance to AD and other tauopathies.

Cocaine regulates sensory filtering in cortical pyramidal neurons

Exposure to cocaine leads to robust changes in the structure and function of neurons within the mesocorticolimbic pathway. However, little is known about how cocaine influences the processing of information within the sensory cortex. We address this by using patch-clamp and juxtacellular voltage recordings and two-photon Ca2+ imaging in vivo to investigate the influence of acute cocaine exposure on layer 2/3 (L2/3) pyramidal neurons within the primary somatosensory cortex (S1). Here, cocaine dampens membrane potential state transitions and decreases spontaneous somatic action potentials and Ca2+ transients. In contrast to the uniform decrease in background spontaneous activity, cocaine has a heterogeneous influence on sensory encoding, increasing tactile-evoked responses in dendrites that do not typically encode sensory information and decreasing responses in those dendrites that are more reliable sensory encoders. Combined, these findings suggest that cocaine acts as a filter that suppresses background noise to selectively modulate incoming sensory information.

Prepulse Inhibition of the Auditory Startle Reflex Assessment as a Hallmark of Brainstem Sensorimotor Gating Mechanisms

When a low-salience stimulus of any type of sensory modality-auditory, visual, tactile-immediately precedes an unexpected startle-like stimulus, such as the acoustic startle reflex, the startle motor reaction becomes less pronounced or is even abolished. This phenomenon is known as prepulse inhibition (PPI), and it provides a quantitative measure of central processing by filtering out irrelevant stimuli. As PPI implies plasticity of a reflex and is related to automatic or attentional processes, depending on the interstimulus intervals, this behavioral paradigm might be considered a potential marker of short- and long-term plasticity. Assessment of PPI is directly related to the examination of neural sensorimotor gating mechanisms, which are plastic-adaptive operations for preventing overstimulation and helping the brain to focus on a specific stimulus among other distracters. Despite their obvious importance in normal brain activity, little is known about the intimate physiology, circuitry, and neurochemistry of sensorimotor gating mechanisms. In this work, we extensively review the current literature focusing on studies that used state-of-the-art techniques to interrogate the neuroanatomy, connectomics, neurotransmitter-receptor functions, and sex-derived differences in the PPI process, and how we can harness it as biological marker in neurological and psychiatric pathology.

Low startle magnitude may be a behavioral marker of vulnerability to cocaine addiction

Cocaine addicted men have low startle magnitude persisting during prolonged abstinence. Low startle rats show greater cocaine self-administration than high startle rats. Low startle may be a marker of a vulnerability to heightened cocaine-related behaviors in rats and similarly may be a marker of vulnerability to cocaine addiction in humans.

Classic Studies on the Interaction of Cocaine and the Dopamine Transporter

The dopamine transporter is responsible for recycling dopamine after release. Inhibitors of the dopamine transporter, such as cocaine, will stop the reuptake of dopamine and allow it to stay extracellularly, causing prominent changes at the molecular, cellular, and behavioral levels. There is much left to be known about the mechanism and site(s) of binding, as well as the effect that cocaine administration does to dopamine transporter-cocaine binding sites and gene expression which also plays a strong role in cocaine abusers and their behavioral characteristics. Thus, if more light is shed on the dopamine transporter-cocaine interaction, treatments for addiction and even other diseases of the dopaminergic system may not be too far ahead. As today's ongoing research expands on the shoulders of classic research done in the 1990s and 2000s, the foundation of core research done in that time period will be reviewed, which forms the basis of today's work and tomorrow's therapies.

Novel cues reinstate cocaine-seeking behavior and induce Fos protein expression as effectively as conditioned cues

Cue reinstatement of extinguished cocaine-seeking behavior is a widely used model of cue-elicited craving in abstinent human addicts. This study examined Fos protein expression in response to cocaine cues or to novel cues as a control for activation produced by test novelty. Rats were trained to self-administer cocaine paired with either a light or a tone cue, or received yoked saline and cue presentations, and then underwent daily extinction training. They were then tested for reinstatement of extinguished cocaine-seeking behavior elicited by response-contingent presentations of either the cocaine-paired cue or a novel cue (that is, tone for those trained with a light or vice versa). Surprisingly, conditioned and novel cues both reinstated responding and increased Fos similarly in most brain regions. Exceptions included the anterior cingulate, which was sensitive to test cue modality in saline controls and the dorsomedial caudate-putamen, where Fos was correlated with responding in the novel, but not conditioned, cue groups. In subsequent experiments, we observed a similar pattern of reinstatement in rats trained and tested for sucrose-seeking behavior, whereas rats trained and tested with the cues only reinstated to a novel, and not a familiar, light or tone. The results suggest that novel cues reinstate responding to a similar extent as conditioned cues regardless of whether animals have a reinforcement history with cocaine or sucrose, and that both types of cues activate similar brain circuits. Several explanations as to why converging processes may drive drug and novel cue reinforcement and seeking behavior are discussed.

Effects of moderate-dose treatment with varenicline on neurobiological and cognitive biomarkers in smokers and nonsmokers with schizophrenia or schizoaffective disorder

CONTEXT

The administration of nicotine transiently improves many neurobiological and cognitive functions in schizophrenia and schizoaffective disorder. It is not yet clear which nicotinic acetylcholine receptor (nAChR) subtype or subtypes are responsible for these seemingly pervasive nicotinic effects in schizophrenia and schizoaffective disorder.

OBJECTIVE

Because α4β2 is a key nAChR subtype for nicotinic actions, we investigated the effect of varenicline tartrate, a relatively specific α4β2 partial agonist and antagonist, on key biomarkers that are associated with schizophrenia and are previously shown to be responsive to nicotinic challenge in humans.

DESIGN

A double-blind, parallel, randomized, placebo-controlled trial of patients with schizophrenia or schizoaffective disorder to examine the effects of varenicline on biomarkers at 2 weeks (short-term treatment) and 8 weeks (long-term treatment), using a slow titration and moderate dosing strategy for retaining α4β2-specific effects while minimizing adverse effects.

SETTING

Outpatient clinics.

PARTICIPANTS

A total of 69 smoking and nonsmoking patients; 64 patients completed week 2, and 59 patients completed week 8. Intervention Varenicline.

MAIN OUTCOME MEASURES

Prepulse inhibition, sensory gating, antisaccade, spatial working memory, eye tracking, processing speed, and sustained attention.

RESULTS

A moderate dose of varenicline (1) significantly reduced the P50 sensory gating deficit in nonsmokers after long-term treatment (P = .006), (2) reduced startle reactivity (P = .02) regardless of baseline smoking status, and (3) improved executive function by reducing the antisaccadic error rate (P = .03) regardless of smoking status. A moderate dose of varenicline had no significant effect on spatial working memory, predictive and maintenance pursuit measures, processing speed, or sustained attention by Conners' Continuous Performance Test. Clinically, there was no evidence of exacerbation of psychiatric symptoms, psychosis, depression, or suicidality using a gradual titration (1-mg daily dose).

CONCLUSIONS

Moderate-dose treatment with varenicline has a unique treatment profile on core schizophrenia-related biomarkers. Further development is warranted for specific nAChR compounds and dosing and duration strategies to target subgroups of schizophrenic patients with specific biological deficits.

Reserpine differentially affects cocaine-induced behavior in low and high responders to novelty

Individuals are known to differ in their sensitivity to cocaine. Cocaine is known to inhibit the re-uptake of monoamines. The response to cocaine has also been found to depend on monoamines inside reserpine-sensitive storage vesicles. The present study examined the effects of reserpine (1-2 mg/kg) on cocaine-induced behavior (10-15 mg/kg) in Low Responders (LR) and High Responders (HR) to novelty rats. LR displayed less cocaine-induced walking, wall rearing, free rearing and stereotyped behavior than HR did. The dose of 1 mg/kg of reserpine decreased cocaine-induced walking, wall rearing, free rearing and stereotyped behavior in LR, but not in HR. A dose of 2 mg/kg of reserpine was required to inhibit cocaine-induced behavior in HR. Combining these behavioral findings with our previously reported neurochemical finding that a higher dose of reserpine was required to inhibit the accumbal dopamine response to cocaine in HR than in LR (Verheij et al., 2008), suggests that HR are more sensitive to the behavioral effects of cocaine than LR because cocaine can release more monoamines from storage vesicles in HR than in LR. Our behavioral data also demonstrate that the individual differences in sensitivity to reserpine are not only limited to the dopaminergic system of the nucleus accumbens.

Rats that differentially respond to cocaine differ in their dopaminergic storage capacity of the nucleus accumbens

Cocaine (COC) inhibits the re-uptake of dopamine. However, the dopamine response to COC also depends on dopamine inside storage vesicles. The aim of this study was to investigate whether rats that differentially respond to COC differ in their dopaminergic storage capacity of the nucleus accumbens. Total and vesicular levels of accumbal dopamine as well as accumbal vesicular monoamine transporter-2 levels were established in high (HR) and low responders (LR) to novelty rats. Moreover, the effects of reserpine (RES) on the COC-induced increase of extracellular accumbal dopamine were investigated. HR displayed higher accumbal levels of total and vesicular dopamine than LR. Moreover, HR displayed more accumbal vesicular monoamine transporters-2 than LR. COC increased extracellular accumbal dopamine more strongly in HR than in LR. A low dose of RES prevented the COC-induced increase of accumbal dopamine in LR, but not in HR. A higher dose of RES was required to inhibit the COC-induced increase of accumbal dopamine in HR. These data demonstrate that HR were marked by a larger accumbal dopaminergic storage pool than LR. It is hypothesized that HR are more sensitive to COC than LR, because COC can release more dopamine from accumbal storage vesicles in HR than in LR.

J. Neurochem. (2008) 105, 2122–2133.

Dopamine, schizophrenia, mania, and depression: Toward a unified hypothesis of cortico-striatopallido-thalamic function

Considerable evidence from preclinical and clinical investigations implicates disturbances of brain dopamine (DA) function in the pathophysiology of several psychiatric and neurologic disorders. We describe a neural model that may help organize theseindependent experimental observations. Cortical regions classically associated with the limbic system interact with infracortical structures, including the nucleus accumbens, ventral pallidum, and dorsomedial nucleus of the thalamus. In our model, overactivity in forebrain DA systems results in the loss of lateral inhibitory interactions in the nucleus accumbens, causing disinhibition of pallidothalamic efferents; this in turn causes rapid changes and a loss of focused corticothalamic activity in cortical regions controlling cognitive and emotional processes. These effects might be manifested clinically by some symptoms of psychoses. Underactivity of forebrain DA results in excess lateral inhibition in the nucleus accumbens, causing tonic inhibition of pallidothalamic efferents; this perpetuates tonic corticothalamic activity and prevents the initiation of new activity in other critical cortical regions. These effects might be manifested clinically by some symptoms of depression. This model parallels existing explanations for the etiology of several movement disorders, and may lead to testable inferences regarding the neural substrates of specific psychopathologies.

Effects of cocaine self-administration history under limited and extended access conditions on in vivo striatal dopamine neurochemistry and acoustic startle in rhesus monkeys

RATIONALE

The transition from infrequent and controlled cocaine use to dependence may involve enduring changes in neurobiology as a consequence of persistent drug use.

OBJECTIVE

The present study utilized an intravenous drug self-administration protocol of increasing cocaine access to evaluate potential changes in dopamine function in vivo, including changes in sensitivity to psychostimulants.

MATERIALS AND METHODS

Drug-naïve rhesus monkeys were provided limited access (1 h) to cocaine self-administration for 60 days followed by 60 days under an extended access condition (4 h). Basal levels of striatal extracellular dopamine and its metabolites, as well as the effectiveness of cocaine and amphetamine to elevate dopamine, were determined with in vivo microdialysis before the initiation of cocaine self-administration and during limited and extended access. The effect of cocaine and amphetamine on the acoustic startle response was also examined to assess complementary behavioral changes as a function of drug history.

RESULTS

Extended access to cocaine self-administration lead to increased daily intake compared to limited access conditions but did not result in escalated intake over time. However, cocaine- and amphetamine-induced increases in striatal dopamine were diminished as a function of cocaine self-administration history. Surprisingly, there was no effect of drug-taking history on sensitivity to psychostimulant-induced enhancement of startle amplitude.

CONCLUSIONS

The present experiments provide evidence of a hypofunctional dopamine system that is not associated with an escalation in drug intake or reflected in measures of acoustic startle.

Mesolimbic alpha-, but not beta-adrenoceptors control the accumbal release of dopamine that is derived from reserpine-sensitive storage vesicles

Mesolimbic beta-, but not alpha-adrenoceptors control the accumbal release of dopamine that is derived from alpha-methyl-para-tyrosine-sensitive pools of newly synthesized neurotransmitter. The aim of this study was to investigate which of these adrenoceptors control the accumbal release of dopamine that is derived from reserpine-sensitive pools of previously stored neurotransmitter. Rats, that were divided in low-responders and high-responders to novelty, were pretreated with 1 mg/kg of reserpine before the alpha-adrenergic-agent phentolamine or the beta-adrenergic-agent isoproterenol was locally applied into the nucleus accumbens. The original finding that phentolamine and isoproterenol increased accumbal dopamine levels in low-responders and high-responders was replicated. Reserpine reduced the phentolamine-induced increase of accumbal dopamine in both types of rat. However, phentolamine could still increase accumbal dopamine levels in reserpine-treated high-responders, but not anymore in reserpine-treated low-responders. Reserpine did not reduce the isoproterenol-induced increase of accumbal dopamine in any type of rat. This study demonstrates that mesolimbic alpha-, but not beta-adrenoceptors control the accumbal release of dopamine that is derived from reserpine-sensitive storage vesicles. In addition, these data confirm our previous finding that dopamine can still be released from storage vesicles of reserpinized high-responders, but not of reserpinized low-responders. The collected data underline our notion that alpha- and beta-adrenergic drugs may have therapeutic effects in patients suffering from diseases in which accumbal dopamine is involved.

Improvement of prepulse inhibition and executive function by the COMT inhibitor tolcapone depends on COMT Val158Met polymorphism

Recent evidence suggests that prepulse inhibition (PPI) levels relate to executive function possibly by a prefrontal cortex (PFC) dopamine (DA) link. We explored the effects of enhanced PFC DA signaling by the nonstimulant catechol-O-methyltransferase (COMT) inhibitor tolcapone, on PPI and working memory of subjects homozygous for the Val (low PFC DA) and the Met (high PFC DA) alleles of the COMT Val158Met polymorphism. Twelve Val/Val and eleven Met/Met healthy male subjects entered the study. Tolcapone 200 mg was administered in two weekly sessions, according to a balanced, crossover, double-blind, placebo-controlled design. PPI was assessed with 5 dB and 15 dB above background prepulses, at 30-, 60-, and 120 ms prepulse-pulse intervals. Subjects also underwent the n-back and the letter-number sequencing (LNS) tasks. PPI was lower in the Val/Val compared to the Met/Met group in the placebo condition. Tolcapone increased PPI significantly in the Val/Val group and tended to have the opposite effect in the Met/Met group. Baseline startle was not affected by tolcapone in the Val/Val group but it was slightly increased in the Met/Met group. Tolcapone improved performance in the n-back and LNS tasks only in the Val/Val group. Enhancement of PFC DA signaling with tolcapone improves both PPI and working memory in a COMT Val158Met genotype-specific manner. These results suggest that early information processing and working memory may both depend on PFC DA signaling, and that they may both relate to PFC DA levels according to an inverted U-shaped curve function.

Twenty years of dopamine research: individual differences in the response of accumbal dopamine to environmental and pharmacological challenges

Individual differences in the dopaminergic system of the nucleus accumbens of rats have extensively been reported. These individual differences have frequently been used to explain individual differences in response to environmental and pharmacological challenges. Remarkably, only little attention is paid to the factors that underlie these individual differences. This review gives an overview of the studies that have been performed in our institute during the last 20 years to investigate individual differences in accumbal dopamine release. Data are summarised demonstrating that individual differences in accumbal dopamine release are due to individual differences in: the functional reactivity of the noradrenergic system, the accumbal concentration of vesicular monoamine transporters and tyrosine hydroxylase as well as in the quantal size of the presynaptic pools of dopamine. Our data are embedded in the available literature to create a model that illustrates the putative hardware giving rise to the individual-specific release of accumbal dopamine. An important role is contributed to individual differences in the reactivity of the: hypothalamic-pituitary-adrenal axes, the reactivity of second messenger systems as well in the aminergic reactivity of the accumbens shell and core. The consequences of the individual-specific make-up and reactivity of the nucleus accumbens on the regulation of behaviour and the response to drugs of abuse will also be discussed. Apart from agents that interact with dopaminergic receptors, re-uptake or breakdown, noradrenergic agents as well as agents that interact with vesicular monoamine transporters or tyrosine hydroxylase are suggested to have therapeutic effects in subjects that are suffering from diseases in which the dopaminergic system is disturbed.

Differential contribution of storage pools to the extracellular amount of accumbal dopamine in high and low responders to novelty: effects of reserpine

The present study examined the effects of reserpine on the extracellular concentration of accumbal dopamine in high responders (HR) and low responders (LR) to novelty rats. Reserpine reduced the baseline concentration of extracellular accumbal dopamine more in HR than in LR, indicating that the dopamine release is more dependent on reserpine-sensitive storage vesicles in non-challenged HR than in non-challenged LR. In addition, reserpine reduced the novelty-induced increase of the extracellular concentration of accumbal dopamine in LR, but not in HR, indicating that the dopamine release in response to novelty depends on reserpine-sensitive storage vesicles only in LR, not in HR. Our data clearly demonstrate that HR and LR differ in the characteristics of those monoaminergic storage vesicles that mediate accumbal dopamine release.

Differences in the cellular mechanism underlying the effects of amphetamine on prepulse inhibition in apomorphine-susceptible and apomorphine-unsusceptible rats

BACKGROUND

Amphetamine is often used to mimic certain aspects of schizophrenia in laboratory animals, such as a decreased prepulse inhibition.

MATERIALS AND METHODS

Apomorphine-susceptible and apomorphine-unsusceptible rats represent a well-characterized animal model for individual differences in the sensitivity to dopaminergic drugs. Moreover, apomorphine-susceptible rats show a wide variety of schizophrenia-like abnormalities. The differential response to administration of amphetamine (1-4 mg/kg, i.p.) was investigated in these two rat lines using the prepulse inhibition paradigm. Because amphetamine promotes dopamine release, the cellular mechanism underlying the line-specific effects of amphetamine was investigated by administration of alpha-methyl-para-tyrosine (aMpT) and reserpine, substances that are known to deplete the cytosolic dopamine pool and the vesicular dopamine pool, respectively, the former being primarily implicated in mediating the effects of amphetamine.

RESULTS

All doses of amphetamine decreased prepulse inhibition in apomorphine-susceptible rats, whereas only the highest doses (2 and 4 mg/kg, i.p.) of amphetamine decreased prepulse inhibition in apomorphine-unsusceptible rats. Alpha-methyl-para-tyrosine, but not reserpine, blocked the amphetamine-induced disruption in prepulse inhibition in apomorphine-unsusceptible rats, whereas both substances alone had no effect in apomorphine-susceptible rats. However, the combination of alpha-methyl-para-tyrosine and reserpine did block the amphetamine-induced effects in the latter rat line.

DISCUSSION

The present study suggests that apomorphine-susceptible rats are more sensitive to systemic administration of amphetamine than apomorphine-unsusceptible rats. In addition, the data show that the cellular mechanism underlying the effects of amphetamine differs between apomorphine-susceptible and apomorphine-unsusceptible rats. Whereas the effects of amphetamine on prepulse inhibition in apomorphine-unsusceptible rats just require the alpha-methyl-para-tyrosine sensitive dopamine pool, the effects in apomorphine-susceptible rats require both the alpha-methyl-para-tyrosine sensitive and the reserpine sensitive dopamine pool. Because apomorphine-susceptible rats share many features with schizophrenic patients, these data open the perspective that in these patients amphetamine may induce dopamine release from both types of dopamine pool. This might provide an explanation for the increased dopamine release after this psychostimulant drug in patients vs controls.

Double dissociating effects of sensory stimulation and cocaine on serotonin activity in the occipital and temporal cortices

Visual cues that become associated with the consumption of psychostimulant drugs energize craving and the intake of the drug by mechanisms of which little is known. In two experiments using in vivo microdialysis in freely moving rats we compared the effects of visual and auditory stimulation with that of cocaine (0, 5, 10, 20mg/kg; i.p.) on the extracellular serotonin (5-HT) activity in the occipital and temporal cortices in relation to behavior. Visual stimulation increased 5-HT in the occipital, but not temporal cortex, parallel to an increase in locomotion. Auditory stimulation decreased 5-HT in the auditory, but not occipital cortex, thus, showing a double dissociated 5-HT response. These data suggest that a locally restricted 5-HT response to sensory stimulation may gate behavioral activity sense-modality selectively. Cocaine affected 5-HT in the occipital cortex and behavioral activity in the same direction as visual stimulation, but in an amplified and prolonged way. In the temporal cortex cocaine also caused an increase in 5-HT. The findings demonstrate common effects of visual stimulation and cocaine on 5-HT activity in the occipital cortex in relation to locomotor activity. The results suggest that concepts of how neutral visual cues become powerful energizers of addiction-related behaviors should be expanded to incorporate not only an acute enhancement of reward processing mechanisms, but, in parallel, also an amplified processing of visual stimuli in the occipital cortex.

Cocaine increases dopamine release by mobilization of a synapsin-dependent reserve pool

Cocaine primarily exerts its behavioral effects by enhancing dopaminergic neurotransmission, amplifying dopamine-encoded sensorimotor integration. The presumed mechanism for this effect is inhibition of the dopamine transporter, which blocks dopamine uptake and prolongs the duration of dopamine in the extracellular space. However, there is growing evidence that cocaine can also augment dopamine release. Here, we directly monitored the actions of cocaine on dopamine release by using electrochemical detection to measure extracellular dopamine in the striatum of anesthetized mice. Cocaine enhanced the levels of striatal dopamine produced by electrical stimulation of dopaminergic neurons. Even after pretreatment with alpha-methyl-p-tyrosine, which depletes the readily releasable pool of dopamine, cocaine was still capable of elevating dopamine levels. This suggests that cocaine enhances dopamine release by mobilizing a reserve pool of dopamine-containing synaptic vesicles. To test this hypothesis, we examined electrically evoked dopamine release in synapsin I/II/III triple knock-out mice, which have impaired synaptic vesicle reserve pools. Knock-out of synapsins greatly reduced the ability of cocaine to enhance dopamine release with long stimulus trains or after depletion of the newly synthesized pool. We therefore conclude that cocaine enhances dopamine release and does so by mobilizing a synapsin-dependent reserve pool of dopamine-containing synaptic vesicles. This capacity to enhance exocytotic release of dopamine may be important for the psychostimulant actions of cocaine.

Persistent suppression of ethanol self-administration by brief social stress in rats and increased startle response as index of withdrawal

Excessive alcohol drinking is often linked to the experience of stress, but experimental approaches using animal models of alcohol self-administration have had widely varying outcomes. The objective was to determine how daily exposure to brief, predictable social stress would change alcohol self-administration in rats in a daily limited access protocol. Male Long-Evans rats had either access to a 10% ethanol solution for 15 min in the home cage setting (n=20) or were reinforced with 15% ethanol deliveries for every fifth lever press (n=10). Subsequently, all rats were subjected to brief social stress for five consecutive days. Social stress consisted of attacks by an opponent for 5 min followed by exposure to threats while in a protective cage for 30 min. In both the home cage drinking and operant conditioning groups, social stress exposure significantly decreased alcohol intake or rate of alcohol reinforcements, respectively. When alcohol intake was scheduled immediately before social stress (i.e., 24 h after the previous social stress episode), a decrease was observed with a delay of 1 or 2 days. When alcohol intake was scheduled 4 h after stress, no changes in intake or alcohol reinforcements were observed. Animals that consumed a low dose of ethanol displayed less defensive behavior during social stress compared to water-drinking animals, and showed an increased startle reflex at 8 and 56 h after discontinuation of daily ethanol access. The current experimental protocols of social defeat stress reveal a transient suppression rather than a facilitation of alcohol consumption.

Prepulse inhibition of the acoustic startle response in cocaine-withdrawn rats

Prepulse inhibition (PPI) of startle is a sensorimotor gating task in which a low-intensity acoustic stimulus presented prior to a high-intensity, startle-eliciting stimulus can attenuate the acoustic startle response (ASR). Previous studies on startle reactivity in cocaine-withdrawn rats have found minimal changes; the present study extends this work to the gating of ASR. In Experiment 1, rats were injected daily with either saline or cocaine (30 mg/kg i.p.) for 2 weeks. ASR and PPI were measured prior to, and at 3- and 14-day withdrawal from, the chronic treatment. No effect of cocaine treatment was found on either measure. In Experiment 2, treatment was extended to 8 weeks, and an earlier withdrawal time point (1 day) was added. Rats treated with cocaine for 8 weeks exhibited lower startle reactivity during withdrawal compared with saline-treated controls. PPI did not differ between treatment groups. Thus, extended chronic treatment with cocaine rendered significant effects on startle responsivity. Further, this finding mirrors the blunted ASR exhibited in chronic cocaine users [Neuropsychopharmacology 22 (2000) 89.].

Fear-potentiated startle response in mice: genetic analysis of the C57BL/6J and DBA/2J intercross

The role of genetic factors in the fear-potentiated startle (FPS) response was examined in the inbred C57BL/6J (B6) and DBA/2J (D2) mouse strains. Mice in the D2 strain displayed a significant potentiation in the acoustic startle response (ASR) when presented with a visual condition stimulus (CS) previously paired with an aversive unconditioned stimulus (US). The maximal FPS response was observed following 20 conditioning trials but a near maximal response was noted following as few as five trials. Forty conditioning trials produced a significant reduction in the FPS response that may be related to overtraining. The FPS response in the B6 strain was significantly lower than the D2 strain, regardless of the number of conditioning trials. The contrasting FPS responses were not related to differences in auditory sensitivity known to exist between these strains. Analysis of a full Mendelian cross formed from the B6 and D2 strains found that the FPS response was a highly heritable trait, best described by a simple additive model of inheritance and with a broad-sense heritability of 0.46. The distribution of the FPS response in F2 hybrids formed from the intercross of the D2 and B6 strains was continuous which suggests a multigenic substrate. The light + noise and noise-alone trial types were highly correlated, but no association was detected between the baseline ASR amplitude and the FPS response. Mice from the phenotypic extremes of the F2 distribution displayed FPS responses that were more extreme than either of the progenitor strains. However, both baseline startle amplitude and the salience of auditory stimuli did not differ in these groups. The results of this study confirm an early report by Falls et al. (1997), and provide additional quantitative genetics information necessary for the eventual mapping of the chromosomal regions or genes associated with the FPS response in mice.

Withdrawal from oral cocaine in rate: ultrasonic vocalizations and tactile startle

While anxiety appears to characterize humans who administer high doses of cocaine or experience withdrawal from cocaine, it is difficult to capture this aspect of cocaine effects in animals. The present study investigated if acute or protracted withdrawal from prolonged low-dose cocaine that is self-administered via the oral route could be detected in tactile startle and vocal "distress" responses of rats. Adult, male Long-Evans rats had access to cocaine solution (0.1 mg/ml) either for 24 or 4 h/day using the two-bottle choice technique. The amount of solution consumed from each bottle was measured daily for 30 or 60 days. On days 1, 3, 7, 14, 21, 28 of withdrawal, startle and ultrasonic vocal responses (USV, 15-35 kHz) were measured in response to 18 air-puff stimuli (20 psi). Rats drank an average of 5-20 mg/kg per day of the cocaine solution. On average, about half of the daily liquid was consumed from the cocaine solution-containing bottle. USVs were emitted at significantly increased rates on day 3 of withdrawal from 30 or 60 days of cocaine drinking. Startle reactions were slightly, but non-significantly increased on day 1 of withdrawal. Comparable to withdrawal from ethanol, morphine, and diazepam treatments, withdrawal from oral self-administration of low to moderate doses of cocaine increases the rate of ultrasonic vocalizations while increasing minimally the amplitude of startle responses to low-intensity tactile stimuli. Nevertheless, no correlation between the total amount of cocaine self-administered or the duration of treatment with the intensity of the withdrawal manifestations could be detected.

Cocaine's effects on speech sound identification and reaction times in baboons

The effects of cocaine on speech sound discriminations was examined to determine whether cocaine's previously demonstrated effect in reducing speech sound discriminability was dependent upon either the type of stimuli employed (simple tones versus complex speech) or the procedure (stimulus detection versus stimulus discrimination). Because of demonstrated similarities in the way that baboons and humans discriminate speech, and in the way the CNS is thought to encode and process speech sounds in these two species, baboons were trained to perform a choice procedure to identify the occurrence of different synthetic vowel sounds (see text). Animals held down a lever and released the lever only when one of four target vowels sounded, and not when a fifth, standard vowel sounded. Acute IM administration of cocaine (0.0032-1.0 mg/kg) produced dose-dependent decreases in vowel discriminability that were mostly due to elevations in false alarms (i.e., releases to the standard vowel) following cocaine. Cocaine also shortened reaction times to the stimuli in two of three baboons, but to a much lesser extent than observed previously. These results suggest that cocaine may interfere with the ability of the CNS to process the acoustic cues in speech sounds, and that the effects of cocaine on reaction times may depend upon the complexity of the reaction time procedure employed.

Cocaine's effects on speech sound discriminations and reaction times in baboons

Three adult baboons were trained using a psychophysical procedure to discriminate between different synthetic vowel sounds [symbol: see text]. Baboons pressed and held a lever down to produce a pulsed train of a single reference vowel that served as the standard stimulus. Animals were trained to release the lever only when this standard vowel sound changed to one of the four remaining comparison vowels. A lever release within 1.5 s of this change in vowel sounds was defined as a correct detection of the change from the standard vowel to one of the comparison vowels, and was reinforced. All baboons readily learned the vowel discriminations and detected vowel changes at the 90-100% correct performance level. Acute IM administration of cocaine prior to test sessions (0.00032-3.2 mg/kg) produced dose-dependent decrements in vowel discriminability. At the same time, cocaine shortened lever release latencies (reaction times) to the vowel stimuli in two of three baboons. The cocaine-induced decrements in vowel discriminability were correlated with the degree to which frequency differences occurred among the different vowels in that lower vowel discriminability scores were found for those vowels with smaller spectral differences from the standard vowel. Further, false alarm rates were not systematically affected by cocaine, indicating that the cocaine-induced decrements in vowel discrimination accuracy occurred in the absence of systematic changes in the reliability of the baboons' discrimination performances.

Prenatal cocaine exposure revealed minimal postnatal changes in rat striatal dopamine D2 receptor sites and mRNA levels in the offspring

It has been reported from this laboratory that prenatal cocaine exposure results in the postnatal transient alterations of rat striatal dopamine uptake sites examined from postnatal 0-32 wk. The present study aims to examine whether this will result in a direct/indirect stimulation of dopamine D2 receptors. Pregnant rats were dosed orally with cocaine hydrochloride (60 mg/kg/d) from gestational day (GD) 7-21. Control animals received an equivalent volume of water. The striatum from the offspring at postnatal 0-32 wk was examined. The radioligand [3H]sulpiride was used for the Scatchard analysis of the D2 receptors, and the changes in the levels of mRNA for the D2 receptor were studied using Northern blot analysis. Results from the present study revealed that in the control group, there was an age-dependent increase in the number of D2 receptor sites (Bmax: 44.00 +/- 2.12 to 178.00 +/- 45.10 fmol/mg protein) and in the levels of D2 mRNA from PN0-32 wk with the most rapid increase occurring during the first 4 wk of postnatal development. Prenatal cocaine exposure resulting in only a significant decrease (p < 0.001) in the number of D2 receptor sites at PN0 wk and in a 10% increase in mRNA levels at PN3, 4, and 12 wk. It was concluded from this study that prenatal cocaine exposure resulted in minimal postnatal changes in the dopamine D2 receptor.

Changes in the acoustic startle response and prepulse inhibition of acoustic startle in rats after local injection of pertussis toxin into the ventral tegmental area

The effect of local injection of pertussis toxin (PTX) into the ventral tegmental area (VTA) on acoustic startle in rats was investigated. The PTX treatment caused only minor effects of its own on the acoustic startle response (ASR) or prepulse inhibition (PPI) of acoustic startle. However, systemic treatment with the indirect DA receptor agonist, amphetamine (2 mg/kg, SC) caused a significant increase in ASR magnitude and a significant disruption of PPI in PTX-treated rats while no such effects were observed in sham-treated rats. Treatment with the direct DA receptor agonist, apomorphine (2 mg/kg, SC), caused a significant disruption of PPI, an effect that was observed in both PTX- and sham-treated rats. Treatment with the 5-HT1A receptor agonist, 8-OH-DPAT (0.5 mg/kg, SC), did not affect PPI in either group but caused a marked increase in ASR magnitude in sham-treated rats. Interestingly, this effect was blocked in PTX-treated rats. The present results suggest that local injection of PTX into the VTA causes an increased sensitivity to the behavioural effects of psychostimulants on acoustic startle and may also suggest that intact midbrain 5-HT1A receptors are essential for the effect of 5-HT1A agonists on acoustic startle.

Amphetamine, cocaine, and dizocilpine enhance performance on a lever-release, conditioned avoidance response task in rats

A lever-release version of the conditioned avoidance response (CAR) task was used to assess the behavioral effects of several psychomotor stimulants in rats. The indirect dopamine agonists, d-amphetamine (0.1 and 0.25 mg/kg) and cocaine (7.5 and 15 mg/kg), enhanced performance on this task. Both drugs increased percent avoidance responses and decreased avoidance latency. A higher dose of amphetamine (0.5 mg/kg) also decreased avoidance latency but failed to improve percent avoidance. Similar effects were seen at low (0.01 and 0.025 mg/kg) and high (0.05 mg/kg) doses of dizocilpine (MK-801), a stimulant that acts as a noncompetitive antagonist of N-methyl-d-aspartate (NMDA) glutamate receptors. When combined with haloperidol (0.1 mg/kg), a dopamine antagonist, amphetamine (0.25 mg/kg) and dizocilpine (0.025 mg/kg) had differential effects on the lever-release CAR task. Thus, amphetamine-haloperidol was significantly better than haloperidol alone on percent avoidance but not on avoidance latency, whereas dizocilpine-haloperidol had the opposite effect: significantly better than haloperidol alone on avoidance latency but not on percent avoidance. Taken together, these results provide further support for dopaminergic mechanisms in CAR performance but suggest an opposing glutamatergic influence.

Effects of repeated cocaine injections on cochlear function

The effects of repeated cocaine administration on cochlear function were evaluated by measuring amplitude-intensity and latency-intensity functions of the whole-nerve action potential of the auditory nerve. Whole-nerve action potential input/output functions obtained using tone-pips of 0.5, 1, 2, 4 and 8 kHz in a group of cocaine-treated subjects were compared with those obtained in saline-treated animals. All measurements were made 24 h after the last treatment. Amplitudes of whole-nerve action potentials were enhanced in the cocaine-treated animals compared to the control group. No statistically significant differences in latency-intensity functions were seen after cocaine treatment. The effect of chronic cocaine exposure also was examined on catecholamine innervation in the cochlea using immunohistochemical techniques. The density of adrenergic innervation was reduced in the cocaine-treated animals.

Lack of altered startle responding in rats following termination of self-administered or noncontingently infused cocaine

The effect of cocaine on the acoustic startle response was assessed in rats trained to intravenously self-administer cocaine (0.25 mg/injection) and in rats continuously infused through an intraperitoneal catheter with up to 170 mg/kg/day of cocaine. Neither a 12-h self-administration "binge" nor 13 days' continuous infusion produced significant effects on the startle response as compared to controls up to 24 h after termination of drug exposure. These findings suggest that prolonged cocaine administration may result in a dependence syndrome dissimilar to that observed with CNS depressant drugs or alcohol.

The effects of haloperidol and clozapine on PCP- and amphetamine-induced suppression of social behavior in the rat

Previous work has shown that phencyclidine (PCP) and amphetamine decrease social behavior in rats. The purpose of the present study was to determine the effects of the dopamine antagonists clozapine and haloperidol on PCP- and amphetamine-induced changes in rat social behavior. An intruder paradigm was used, in which rats were injected with drug and placed into a stable home colony of three other rats. Social behaviors were recorded for 30 min. Both amphetamine (4.0 mg/kg) and PCP (4.0 mg/kg) substantially reduced social behavior. Haloperidol and clozapine did not produce a general reversal of the effects of amphetamine or PCP on the total number of social behaviors. Nevertheless, 0.025 mg/kg haloperidol did reverse the effects of PCP and amphetamine on some of the specific social behaviors observed (side threats, mounting, crawling under). Clozapine had no effect on reversing the actions of amphetamine on social behavior, but 2.0 mg/kg clozapine did reverse the effect of PCP on side threats and mounting. These results indicate that DA antagonists do not restore normal social behavior in animals treated with PCP or amphetamine, but can reverse some of the effects of PCP or amphetamine on specific social behaviors.

Pharmacotherapy of cocaine abuse: preclinical development

Preclinical models of behavioral and toxic effects of cocaine are reviewed and their potential for predicting compounds with efficacy and safety in the medical management of cocaine abuse and toxicity is assessed. Many of the existing models appear to be good predictors of the effects of compounds against specific behavioral or toxicological actions of cocaine. However, the utility of the models for prediction of the efficacy of new therapeutic entities must await clinical validation as no accepted or standard pharmacotherapy currently exists. Preclinical data generated by these models with drugs currently under clinical investigation for cocaine abuse treatment as well as with other compounds are reviewed. These compounds include buprenorphine, bromocriptine, desmethylimipramine, carbamazepine, dopaminergic agonists, antagonists and partial agonists, dopamine reuptake inhibitors, sigma ligands, serotonin antagonists, and excitatory amino acid antagonists. Preclinical information on several drug classes appears sufficiently promising to warrant further evaluation. These include dopamine agonists and partial agonists, D1 receptor antagonists, selective sigma ligands, and modulators of the N-methyl-D-aspartate subtype glutamate receptor.

Effects of cocaine on simple reaction times and sensory thresholds in baboons

The effects of chronic, daily administration of cocaine on auditory and visual reaction times and thresholds were studied in baboons. Single intramuscular injections of cocaine hydrochloride (0.1 to 5.6 mg/kg) were given once daily for periods of 10 to 25 days, and were followed immediately by psychophysical tests designed to assess cocaine's effects on simple reaction times as on auditory and visual threshold functions. Consistent reductions in reaction times were frequently observed over the cocaine dose range of 0.32 to 1.0 mg/kg; at higher doses, either decreases or increases in reaction times were observed, depending upon the animal. Lowered reaction times generally occurred immediately following the 1st day's cocaine injection, and continued through all subsequent days during the dose administration period, suggesting little development of tolerance or sensitivity to these reaction-time effects. Reaction-time decreases showed a U-shaped dose-effect function. The greatest decreases in reaction times occurred from 0.32 to 1.0 mg/kg, and produced an average reaction-time decrease of 10 to 12%. Concurrently measured auditory and visual thresholds showed no systematic changes as a function of cocaine dose. Pausing was observed during performance of the psychophysical tasks, with the length of total session pause times being directly related to cocaine dose.

Postnatal changes in [3H]mazindol-labelled dopamine uptake sites in the rat striatum following prenatal cocaine exposure

The present study showed that prenatal cocaine exposure (60 mg/kg/day) has a transient effect on the [3H]mazindol-labelled dopamine uptake sites in the striatum of the rat offspring examined from postnatal week 0-32. There is a 39% and 21% decrease in the number of binding sites (Bmax) in the cocaine-exposed group at postnatal weeks 3 and 4, respectively, with a recovery to near normal values by postnatal week 8.

Acute effects of cocaine on cochlear function

The effects of a single administration of cocaine on the cochlea was evaluated by measuring amplitude-intensity functions of the N1 response of the auditory nerve. Amplitude-intensity functions of the N1 response to tone-pips of 500 Hz, 1, 2, 4 and 8 kHz were obtained before and after intraperitoneal injection of either saline, 3 mg/kg or 25 mg/kg of cocaine. N1 amplitudes were decreased after the administration of cocaine and this reduction was found to be dose dependent. The influence of cocaine on cochlear blood flow (CBF) was examined in order to test the possibility that cocaine induced reductions in CBF underlie these electro-physiological changes. Corresponding decreases in cochlear blood flow after cocaine exposure were observed.