Genetic ablation of acid ceramidase in Krabbe disease confirms the psychosine hypothesis and identifies a new therapeutic target

Infantile globoid cell leukodystrophy (GLD, Krabbe disease) is a fatal demyelinating disorder caused by a deficiency in the lysosomal enzyme galactosylceramidase (GALC). GALC deficiency leads to the accumulation of the cytotoxic glycolipid, galactosylsphingosine (psychosine). Complementary evidence suggested that psychosine is synthesized via an anabolic pathway. Here, we show instead that psychosine is generated catabolically through the deacylation of galactosylceramide by acid ceramidase (ACDase). This reaction uncouples GALC deficiency from psychosine accumulation, allowing us to test the long-standing "psychosine hypothesis." We demonstrate that genetic loss of ACDase activity (Farber disease) in the GALC-deficient mouse model of human GLD (twitcher) eliminates psychosine accumulation and cures GLD. These data suggest that ACDase could be a target for substrate reduction therapy (SRT) in Krabbe patients. We show that pharmacological inhibition of ACDase activity with carmofur significantly decreases psychosine accumulation in cells from a Krabbe patient and prolongs the life span of the twitcher (Twi) mouse. Previous SRT experiments in the Twi mouse utilized l-cycloserine, which inhibits an enzyme several steps upstream of psychosine synthesis, thus altering the balance of other important lipids. Drugs that directly inhibit ACDase may have a more acceptable safety profile due to their mechanistic proximity to psychosine biogenesis. In total, these data clarify our understanding of psychosine synthesis, confirm the long-held psychosine hypothesis, and provide the impetus to discover safe and effective inhibitors of ACDase to treat Krabbe disease.

Allopregnanolone is required for prepulse inhibition deficits induced by D1 dopamine receptor activation

INTRODUCTION

The extraction of salient information from the environment is modulated by the activation of dopamine receptors. Using rodent models, we previously reported that gating deficits caused by dopamine receptor activation - as measured by the prepulse inhibition (PPI) of startle - are effectively opposed by inhibitors of the steroidogenic enzyme 5α-reductase (5αR). The specific 5αR isoenzyme and steroids implicated in these effects, however, remain unknown.

METHODS

The effects of the selective D₁ dopamine receptor agonist SKF-82958 (SKF, 0.3 mg/kg, IP) and D₂ receptor agonist quinpirole (QUIN, 0.5 mg/kg, IP) were tested in the startle reflex and PPI of knockout (KO) mice for either 5αR type 1 (5αR1) or type 2 (5αR2). Furthermore, we established whether these effects may be modified by the 5α-reduced steroids dihydroprogesterone (DHP), allopregnanolone (AP), dihydrotestosterone (DHT), 5α-androstane-3α,17β-diol (3α-diol), or androsterone. To test the mechanisms whereby 5αR products may alter the PPI-disrupting properties of D₁ agonists, we studied the involvement of GABA-A and PXR, two receptors targeted by neuroactive steroids. Specifically, we tested the effects of SKF in combination with the GABA-A antagonist bicuculline, as well as in KO mice for the GABA-A δ subunit and PXR.

RESULTS

5αR1, but not 5αR2, knockout (KO) mice were insensitive to the PPI-disrupting effects of SKF. This sensitivity was reinstated by AP (3 mg/kg, IP), but not other 5α-reduced steroids. The PPI deficits induced by SKF were not modified by bicuculline, δ-subunit KO mice and PXR KO mice.

CONCLUSIONS

These results collectively suggest that 5αR1 enables the negative effects of D₁ dopamine receptor activation on information processing via production of AP. The contribution of AP to the PPI-disrupting mechanisms of D₁ receptor agonists, however, do not appear to be mediated by either GABA-A or PXR receptors.

Gene-environment interactions in antisocial behavior are mediated by early-life 5-HT2A receptor activation

The ontogeny of antisocial behavior (ASB) is rooted in complex gene-environment (G×E) interactions. The best-characterized of these interplays occurs between: a) low-activity alleles of the gene encoding monoamine oxidase A (MAOA), the main serotonin-degrading enzyme; and b) child maltreatment. The purpose of this study was to develop the first animal model of this G×E interaction, to help understand the neurobiological mechanisms of ASB and identify novel targets for its therapy. Maoa hypomorphic transgenic mice were exposed to an early-life stress regimen consisting of maternal separation and daily intraperitoneal saline injections and were then compared with their wild-type and non-stressed controls for ASB-related neurobehavioral phenotypes. Maoa hypomorphic mice subjected to stress from postnatal day (PND) 1 through 7 - but not during the second postnatal week - developed overt aggression, social deficits and abnormal stress responses from the fourth week onwards. On PND 8, these mice exhibited low resting heart rate - a well-established premorbid sign of ASB - and a significant and selective up-regulation of serotonin 5-HT_2A receptors in the prefrontal cortex. Notably, both aggression and neonatal bradycardia were rescued by the 5-HT₂ receptor antagonist ketanserin (1-3 mg kg^-1, IP), as well as the selective 5-HT_2A receptor blocker MDL-100,907 (volinanserin, 0.1-0.3 mg kg^-1, IP) throughout the first postnatal week. These findings provide the first evidence of a molecular basis of G×E interactions in ASB and point to early-life 5-HT_2A receptor activation as a key mechanism for the ontogeny of this condition. This article is part of the Special Issue entitled 'The neuropharmacology of social behavior: from bench to bedside'.

Delay discounting as impaired valuation: Delayed rewards in an animal obesity model

Obesity is a major public health problem, which, like many forms of addiction, is associated with an elevated tendency to choose smaller immediate rather than larger delayed rewards, a response pattern often referred to as excessive delay discounting. Although some accounts of delay discounting conceptualize this process as impulsivity (placing the emphasis on overvaluing the smaller immediate reward), others have conceptualized delay discounting as an executive function (placing the emphasis on delayed rewards failing to retain their value). The present experiments used a popular animal model of obesity that has been shown to discount delayed rewards at elevated rates (i.e., obese Zucker rats) to test two predictions that conceptualize delay discounting as executive function. In the first experiment, acquisition of lever pressing with delayed rewards was compared in obese versus lean Zucker rats. Contrary to predictions based on delay discounting as executive function, obese Zucker rats learned to press the lever more quickly than controls. In the second experiment, progressive ratio breakpoints (a measure of reward efficacy) with delayed rewards were compared in obese versus lean Zucker rats. Contrary to the notion that obese rats fail to value delayed rewards, the obese Zucker rats' breakpoints were (at least) as high as those of the lean Zucker rats.

Steroid 5α-reductase 2 deficiency leads to reduced dominance-related and impulse-control behaviors

The enzyme steroid 5α-reductase 2 (5αR2) catalyzes the conversion of testosterone into the potent androgen 5α-dihydrotestosterone. Previous investigations showed that 5αR2 is expressed in key brain areas for emotional and socio-affective reactivity, yet the role of this enzyme in behavioral regulation remains mostly unknown. Here, we profiled the behavioral characteristics of 5αR2 heterozygous (HZ) and knockout (KO) mice, as compared with their wild-type (WT) littermates. While male 5αR2 KO mice displayed no overt alterations in motoric, sensory, information-processing and anxiety-related behaviors, they exhibited deficits in neurobehavioral correlates of dominance (including aggression against intruders, mating, and tube dominance) as well as novelty-seeking and risk-taking responses. Furthermore, male 5αR2 KO mice exhibited reduced D₂-like dopamine receptor binding in the shell of the nucleus accumbens - a well-recognized molecular signature of social dominance. Collectively, these results suggest that 5αR2 is involved in the establishment of social dominance and its behavioral manifestations. Further studies are warranted to understand how the metabolic actions of 5αR2 on steroid profile may be implicated in social ranking, impulse control, and the modulation of dopamine receptor expression in the nucleus accumbens.

Targeting Heat Shock Protein 70 to Ameliorate c-Jun Expression and Improve Demyelinating Neuropathy

Increased expression of the c-jun transcription factor occurs in a variety of human neuropathies and is critical in promoting Schwann cell (SC) dedifferentiation and loss of the myelinated phenotype. Using cell culture models, we previously identified KU-32 as a novobiocin-based C-terminal heat shock protein 90 (Hsp90) inhibitor that decreased c-jun expression and the extent of demyelination. Additional chemical optimization has yielded KU-596 as a neuroprotective novologue whose mechanistic efficacy to improve a metabolic neuropathy requires the expression of Hsp70. The current study examined whether KU-596 therapy could decrease c-jun expression and improve motor function in an inducible transgenic model of a SC-specific demyelinating neuropathy (MPZ-Raf mice). Treating MPZ-Raf mice with tamoxifen activates the MAPK kinase pathway, increases c-jun expression and produces a profound demyelinating neuropathy characterized by a loss of motor function and paraparesis. KU-596 therapy did not interfere with MAPK activation but reduced c-jun expression, significantly improved motor performance, and ameliorated the extent of peripheral nerve demyelination in both prevention and intervention studies. Hsp70 was necessary for the drug's neuroprotective efficacy since MPZ-Raf × Hsp70 knockout mice did not respond to KU-596 therapy. Collectively, our data indicate that modulating Hsp70 may provide a novel therapeutic approach to attenuate SC c-jun expression and ameliorate the onset of certain demyelinating neuropathies in humans.

Effort Expenditure during Proportionally Reinforced Responding

A bar-pressing experiment was carried out in which laboratory rats were reinforced approximately in direct proportion to the level of time integral of force (“effort”) exerted per response. Over the course of training, the subjects increased in efficiency, as measured by amount of food obtained per unit effort exerted. It was not demonstrated, however, that the animals learned to take advantage of the effort-proportional reinforcement contingency.

Loss of the neurodevelopmental gene Zswim6 alters striatal morphology and motor regulation

The zinc-finger SWIM domain-containing protein 6 (ZSWIM6) is a protein of unknown function that has been associated with schizophrenia and limited educational attainment by three independent genome-wide association studies. Additionally, a putatively causal point mutation in ZSWIM6 has been identified in several cases of acromelic frontonasal dysostosis with severe intellectual disability. Despite the growing number of studies implicating ZSWIM6 as an important regulator of brain development, its role in this process has never been examined. Here, we report the generation of Zswim6 knockout mice and provide a detailed anatomical and behavioral characterization of the resulting phenotype. We show that Zswim6 is initially expressed widely during embryonic brain development but becomes restricted to the striatum postnatally. Loss of Zswim6 causes a reduction in striatal volume and changes in medium spiny neuron morphology. These changes are associated with alterations in motor control, including hyperactivity, impaired rotarod performance, repetitive movements, and behavioral hyperresponsiveness to amphetamine. Together, our results show that Zswim6 is indispensable to normal brain function and support the notion that Zswim6 might serve as an important contributor to the pathogenesis of schizophrenia and other neurodevelopmental disorders.

Targeted deletion of GD3 synthase protects against MPTP-induced neurodegeneration

Parkinson's disease is a debilitating neurodegenerative condition for which there is no cure. Converging evidence implicates gangliosides in the pathogenesis of several neurodegenerative diseases, suggesting a potential new class of therapeutic targets. We have shown that interventions that simultaneously increase the neuroprotective GM1 ganglioside and decrease the pro-apoptotic GD3 ganglioside - such as inhibition of GD3 synthase (GD3S) or administration of sialidase - are neuroprotective in vitro and in a number of preclinical models. In this study, we investigated the effects of GD3S deletion on parkinsonism induced by 1-methyl-4phenyl-1,2,3,6-tetrahydropyridine (MPTP). MPTP was administered to GD3S-/- mice or controls using a subchronic regimen consisting of three series of low-dose injections (11 mg/kg/day × 5 days each, 3 weeks apart), and motor function was assessed after each. The typical battery of tests used to assess parkinsonism failed to detect deficits in MPTP-treated mice. More sensitive measures - such as the force-plate actimeter and treadmill gait parameters - detected subtle effects of MPTP, some of which were absent in mice lacking GD3S. In wild-type mice, MPTP destroyed 53% of the tyrosine-hydroxylase (TH)-positive neurons in the substantia nigra pars compacta (SNc) and reduced striatal dopamine 60.7%. In contrast, lesion size was only 22.5% in GD3S-/- mice and striatal dopamine was reduced by 37.2%. Stereological counts of Nissl-positive SNc neurons that did not express TH suggest that neuroprotection was complete but TH expression was suppressed in some cells. These results show that inhibition of GD3S has neuroprotective properties in the MPTP model and may warrant further investigation as a therapeutic target.

Assessment of gait and sensorimotor deficits in the D1CT-7 mouse model of Tourette syndrome

Tourette syndrome (TS) is a neurodevelopmental disorder characterized by multiple motor and phonic tics. While TS patients have been also shown to exhibit subtle abnormalities of sensorimotor integration and gait, animal models of this disorder are seldom tested for these functions. To fill this gap, we assessed gait and sensorimotor integration in the D1CT-7 mouse, one of the best-validated animal models of TS. D1CT-7 mice exhibit spontaneous tic-like manifestations, which, in line with the clinical phenomenology of TS, are markedly exacerbated by environmental stress. Thus, to verify whether stress may affect sensorimotor integration and gait functions in D1CT-7 mice, we subjected these animals to a 20-min session of spatial confinement, an environmental stressor that was recently shown to worsen tic-like manifestations. Immediately following this manipulation (or no confinement, for controls), animals were subjected to either the sticky-tape task, to test for sensorimotor integration; or a 60-min session in an open field (42×42cm) force-plate actometer for gait analysis. Gait analyses included spatial, temporal, and dynamic (force) parameters. D1CT-7 mice displayed a longer latency to remove a sticky tape, indicating marked impairments in sensorimotor integration; furthermore, these mutants exhibited shortened stride length, increased stride rate, nearly equal early-phase velocity, and higher late-phase velocity. D1CT-7 mice also ran with greater force amplitude than wild-type (WT) littermates. None of these phenotypes was worsened by spatial confinement. These results highlight the potential importance of testing sensorimotor integration and gait functions as a phenotypic correlate of cortical connectivity deficits in animal models of TS.

Pramipexole enhances disadvantageous decision-making: Lack of relation to changes in phasic dopamine release

Pramipexole (PPX) is a high-affinity D₂-like dopamine receptor agonist, used in the treatment of Parkinson's disease (PD) and restless leg syndrome. Recent evidence indicates that PPX increases the risk of problem gambling and impulse-control disorders in vulnerable patients. Although the molecular bases of these complications remain unclear, several authors have theorized that PPX may increase risk propensity by activating presynaptic dopamine receptors in the mesolimbic system, resulting in the reduction of dopamine release in the nucleus accumbens (NAcc). To test this possibility, we subjected rats to a probability-discounting task specifically designed to capture the response to disadvantageous options. PPX enhanced disadvantageous decision-making at a dose (0.3 mg/kg/day, SC) that reduced phasic dopamine release in the NAcc. To test whether these modifications in dopamine efflux were responsible for the observed neuroeconomic deficits, PPX was administered in combination with the monoamine-depleting agent reserpine (RES), at a low dose (1 mg/kg/day, SC) that did not affect baseline locomotor and operant responses. Contrary to our predictions, RES surprisingly exacerbated the effects of PPX on disadvantageous decision-making, even though it failed to augment PPX-induced decreases in phasic dopamine release. These results collectively suggest that PPX impairs the discounting of probabilistic losses and that the enhancement in risk-taking behaviors secondary to this drug may be dissociated from dynamic changes in mesolimbic dopamine release.

Within-Response Force Variations Accompanying Differentiation of Duration in Rats

By using a force-sensing isometric manipulandum, response force waveforms were recorded from rats performing a duration differentiation. For 2 rats exteroceptive cues were correlated with response force exceeding the force criterion and with attainment of the 1.60-sec. duration required for reinforcement. The other 3 rats received no exteroceptive support, the water reinforcer being delivered upon response termination. In the last phase of the experiment, reinforcement was contingent upon holding response force within a force “band” for a specified time. Relative frequency distributions of response duration indicated that differentiation of duration was more accurate in the presence of exteroceptive cues than in the absence of such stimuli. Spectral analysis of individual response waveforms indicated that rhythmic force oscillations (2 to 6 hz) predominated, despite the subjects' reliance on exteroceptive stimuli. Introduction of the joint force-band-duration contingency reduced but did not completely abolish the oscillations for both cued and uncued subjects. Over-all the results suggest that the rhythmic force variations observed here may be unselected or elicited components of the response rather than manifestations of a proprioceptive self-cueing process.

Effects of Morphine on Behavioral Task Performance in SIV-Infected Rhesus Macaques

The abuse of opiates such as morphine in synergy with HIV infection not only exacerbates neuropathogenesis but significantly impacts behavioral attributes in HIV infected subjects. Thus, the goal of the current study was to characterize behavioral perturbations in rhesus macaques subjected to chronic morphine and SIV infection. Specifically, we assessed three behavioral tasks: motor skill (MS), forelimb force (FFT) and progressive ratio (PR) tasks. After collecting baseline control data (44 weeks) and data during the morphine-only dependency period (26 weeks), a subset of animals were productively infected with neurovirulent strains of SIVmac (R71/E17) for an additional 33 weeks. A general pattern in the results is that behavioral decline occurred with high CSF viral loads but not necessarily with high plasma viral loads. Compared to saline controls, all treated animals showed significant decreases in performance on all three behavioral tasks during the morphine-only dependency period. During the post infection period, only the morphine plus SIV group showed a significant further decline and this only occurred for the MS task. Taken together, these data demonstrate a clear effect of morphine to produce behavioral deficits and also suggest that morphine can act synergistically with SIV/HIV to exacerbate behavioral deficits.

The D1CT-7 mouse model of Tourette syndrome displays sensorimotor gating deficits in response to spatial confinement

BACKGROUND AND PURPOSE

The D1CT-7 mouse is one of the best known animal models of Tourette syndrome (TS), featuring spontaneous tic-like behaviours sensitive to standard TS therapies; these characteristics ensure a high face and predictive validity of this model, yet its construct validity remains elusive. To address this issue, we studied the responses of D1CT-7 mice to two critical components of TS pathophysiology: the exacerbation of tic-like behaviours in response to stress and the presence of sensorimotor gating deficits, which are thought to reflect the perceptual alterations causing the tics.

EXPERIMENTAL APPROACH

D1CT-7 and wild-type (WT) littermates were subjected to a 20 min session of spatial confinement (SC) within an inescapable, 10 cm wide cylindrical enclosure. Changes in plasma corticosterone levels, tic-like behaviours and other spontaneous responses were measured. SC-exposed mice were also tested for the prepulse inhibition (PPI) of the startle response (a sensorimotor gating index) and other TS-related behaviours, including open-field locomotion, novel object exploration and social interaction and compared with non-confined counterparts.

KEY RESULTS

SC produced a marked increase in corticosterone concentrations in both D1CT-7 and WT mice. In D1CT-7, but not WT mice, SC exacerbated tic-like and digging behaviours, and triggered PPI deficits and aggressive responses. Conversely, SC did not modify locomotor activity or novel object exploration in D1CT-7 mice. Both tic-like behaviours and PPI impairments in SC-exposed D1CT-7 mice were inhibited by standard TS therapies and D1 dopamine receptor antagonism.

CONCLUSIONS AND IMPLICATIONS

These findings collectively support the translational and construct validity of D1CT-7 mice with respect to TS.

LINKED ARTICLES

This article is part of a themed section on Updating Neuropathology and Neuropharmacology of Monoaminergic Systems. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v173.13/issuetoc.

Bone Marrow Transplantation Alters the Tremor Phenotype in the Murine Model of Globoid-Cell Leukodystrophy

Tremor is a prominent phenotype of the twitcher mouse, an authentic genetic model of Globoid-Cell Leukodystrophy (GLD, Krabbe’s disease). In the current study, the tremor was quantified using a force-plate actometer designed to accommodate low-weight mice. The actometer records the force oscillations caused by a mouse’s movements, and the rhythmic structure of the force variations can be revealed. Results showed that twitcher mice had significantly increased power across a broad band of higher frequencies compared to wildtype mice. Bone marrow transplantation (BMT), the only available therapy for GLD, worsened the tremor in the twitcher mice and induced a measureable alteration of movement phenotype in the wildtype mice. These data highlight the damaging effects of conditioning radiation and BMT in the neonatal period. The behavioral methodology used herein provides a quantitative approach for assessing the efficacy of potential therapeutic interventions for Krabbe’s disease.

Use of a force-sensing automated open field apparatus in a longitudinal study of multiple behavioral deficits in CAG140 Huntington's disease model mice

Behavioral testing of mouse models of Huntington's disease (HD) is a key component of preclinical assessment for potential pharmacological intervention. An open field with a force plate floor was used to quantify numerous spontaneous behaviors in a slowly progressing model of HD. CAG140 (+/+, +/-, -/-) male and female mice were compared in a longitudinal study from 6 to 65 weeks of age. Distance traveled, wall rears, wall rear duration, number of low mobility bouts, in-place movements, number of high velocity runs, and gait parameters (stride rate, stride length, and velocity) were extracted from the ground reaction forces recorded in 20-min actometer sessions. Beginning at 11 weeks, HD mice (both +/- and +/+) were consistently hypoactive throughout testing. Robust hypoactivity at 39 weeks of age was not accompanied by gait disturbances. By 52 and 65 weeks of age the duration of wall rears increased and in-place tremor-like movements emerged at 65 weeks of age in the +/+, but not in the +/- HD mice. Taken together, these results suggest that hypoactivity preceding frank motor dysfunction is a characteristic of CAG140 mice that may correspond to low motivation to move seen clinically in the premanifest/prediagnostic stage in human HD. The results also show that the force plate method provides a means for tracking the progression of behavioral dysfunction in HD mice beyond the stage when locomotion is lost while enabling quantification of tremor-like and similar in-place behaviors without a change in instrumentation. Use of force plate actometry also minimizes testing-induced enrichment effects when batteries of different tests are carried out longitudinally.

Hyperactivity in the Gunn rat model of neonatal jaundice: age-related attenuation and emergence of gait deficits

BACKGROUND

Neonatal jaundice resulting from elevated unconjugated bilirubin occurs in 60-80% of newborn infants. Although mild jaundice is generally considered harmless, little is known about its long-term consequences. Recent studies have linked mild bilirubin-induced neurological dysfunction (BIND) with a range of neurological syndromes, including attention-deficit hyperactivity disorder. The goal of this study was to measure BIND across the lifespan in the Gunn rat model of BIND.

METHODS

Using a sensitive force plate actometer, we measured locomotor activity and gait in jaundiced (jj) Gunn rats versus their nonjaundiced (Nj) littermates. Data were analyzed for young adult (3-4 mo), early middle-aged (9-10 mo), and late middle-aged (17-20 mo) male rats.

RESULTS

jj rats exhibited lower body weights at all ages and a hyperactivity that resolved at 17-20 mo of age. Increased propulsive force and gait velocity accompanied hyperactivity during locomotor bouts at 9-10 mo in jj rats. Stride length did not differ between the two groups at this age. Hyperactivity normalized, and gait deficits, including decreased stride length, propulsive force, and gait velocity, emerged in the 17-20-mo-old jj rats.

CONCLUSION

These results demonstrate that, in aging, hyperactivity decreases with the onset of gait deficits in the Gunn rat model of BIND.

Long-term Effects of Multiple Glucocorticoid Exposures in Neonatal Mice

Glucocorticoids (GCs) such as dexamethasone (DEX) or betamethasone are repeatedly administered for up to a month to prematurely born infants as a treatment for chronic lung dysfunction. Results of clinical trials have shown that the use of GCs in these infants induces long-term deficits in neuromotor function and cognition. We have previously shown that a single exposure to clinically relevant doses of DEX or other GCs in the mouse during a period corresponding to the human perinatal period produces a dramatic increase in apoptotic cell death of neural progenitor cells in the developing cerebellum. To provide a model approximating more chronic clinical dosing regimens, we evaluated possible behavioral effects resulting from repeated exposures to DEX and subsequent GC-induced neuronal loss where neonatal mouse pups were injected with 3.0 mg/kg DEX or saline on postnatal days 7, 9, and 11 (DEX3 treatment). Adult, DEX3-treated mice exhibited long-term, possibly permanent, neuromotor deficits on a complex activity wheel task, which requires higher-order motor co-ordination skills. DEX3 mice exhibited impaired performance on this task relative to saline controls in each of two independent studies involving separate cohorts of mice. Histopathology studies utilizing stereological neuronal counts conducted in behaviorally-tested mice showed that the DEX3 treatment resulted in a significant decrease in the number of neurons in the internal granule layer (IGL) of the cerebellum, although the number of neurons in the Purkinje cell layer were unchanged. The results suggest that multiple neonatal DEX exposures can produce chronic deficits in fine motor co-ordination that are associated with cerebellar IGL neuronal loss.

Autism-associated neuroligin-3 mutations commonly impair striatal circuits to boost repetitive behaviors

In humans, neuroligin-3 mutations are associated with autism, whereas in mice, the corresponding mutations produce robust synaptic and behavioral changes. However, different neuroligin-3 mutations cause largely distinct phenotypes in mice, and no causal relationship links a specific synaptic dysfunction to a behavioral change. Using rotarod motor learning as a proxy for acquired repetitive behaviors in mice, we found that different neuroligin-3 mutations uniformly enhanced formation of repetitive motor routines. Surprisingly, neuroligin-3 mutations caused this phenotype not via changes in the cerebellum or dorsal striatum but via a selective synaptic impairment in the nucleus accumbens/ventral striatum. Here, neuroligin-3 mutations increased rotarod learning by specifically impeding synaptic inhibition onto D1-dopamine receptor-expressing but not D2-dopamine receptor-expressing medium spiny neurons. Our data thus suggest that different autism-associated neuroligin-3 mutations cause a common increase in acquired repetitive behaviors by impairing a specific striatal synapse and thereby provide a plausible circuit substrate for autism pathophysiology.

Regulation of motor function and behavior by atypical chemokine receptor 1

Atypical Chemokine Receptor 1 (ACKR1), previously known as Duffy Antigen Receptor for Chemokines, stands out among chemokine receptors for high selective expression on cerebellar Purkinje neurons. Although ACKR1 ligands activate Purkinje cells in vitro, evidence for ACKR1 regulation of brain function in vivo is lacking. Here we demonstrate that Ackr1 (-/-) mice have markedly impaired balance and ataxia on a rotating rod and increased tremor when injected with harmaline, which induces whole-body tremor by activating Purkinje cells. Ackr1 (-/-) mice also exhibited impaired exploratory behavior, increased anxiety-like behavior and frequent episodes of marked hypoactivity under low-stress conditions. Surprisingly, Ackr1 (+/-) had similar behavioral abnormalities, indicating pronounced haploinsufficiency. The behavioral phenotype of Ackr1 (-/-) mice was the opposite of mouse models of cerebellar degeneration, and the defects persisted when Ackr1 was deficient only on non-hematopoietic cells. Together, the results suggest that normal motor function and behavior may partly depend on negative regulation of Purkinje cell activity by Ackr1.

Targeted motor rehabilitation dissociates corticobulbar versus corticospinal dysfunction in an animal model of Parkinson's disease

BACKGROUND

Recent evidence suggests that motor training may be beneficial for slowing the onset of motor impairments in Parkinson's disease (PD).

OBJECTIVE

To examine the impact of targeted rehabilitation on limb motor and cranial motor function and the corresponding corticospinal and corticobulbar circuits in a rodent model of PD.

METHODS

Baseline performance of limb (reaching) and cranial (licking) motor function were established prior to and 6 weeks following unilateral intrastriatal 6-hydroxydopamine (6-OHDA) infusions. Animals then received 6 weeks of limb motor rehabilitation (LMR) or cranial motor rehabilitation (CMR), after which motor performance was reassessed. Intracortical microstimulation (ICMS) was used to generate motor maps of corresponding corticospinal (forelimb) and corticobulbar (tongue) movement representations within the motor cortex ipsilateral to the 6-OHDA infusion. Quantitative tyrosine hydroxylase (TH) immunohistochemistry was performed to determine levels of striatal TH depletion in 6-OHDA animals using near infrared densitometry.

RESULTS

(1) unilateral intrastriatal dopamine depletion impaired both reaching accuracy and lick force; (2) targeted LMR ameliorated impairments in reaching performance; however, CMR did not improve lick force impairments; (3) unilateral dopamine depletion significantly reduced forelimb but not tongue motor map topography; (4) LMR partially restored forelimb motor maps, whereas CMR did not alter tongue motor maps; and (5) significant correlations were observed between skilled reaching accuracy, forelimb motor map area, and TH depletion, but no relationships were revealed for cranial motor function, motor maps, or TH depletion.

CONCLUSIONS

These data demonstrate dissociation between striatal dopamine depletion, limb versus cranial motor function, and targeted motor rehabilitation in a rodent model of PD.

Differential sensitivity of cranial and limb motor function to nigrostriatal dopamine depletion

The present study determined the differential effects of unilateral striatal dopamine depletion on cranial motor versus limb motor function. Forty male Long Evans rats were first trained on a comprehensive motor testing battery that dissociated cranial versus limb motor function and included: cylinder forepaw placement, single pellet reaching, vermicelli pasta handling; sunflower seed opening, pasta biting acoustics, and a licking task. Following baseline testing, animals were randomized to either a 6-hydroxydopamine (6-OHDA) (n=20) or control (n=20) group. Animals in the 6-OHDA group received unilateral intrastriatal 6-OHDA infusions to induce striatal dopamine depletion. Six-weeks following infusion, all animals were re-tested on the same battery of motor tests. Near infrared densitometry was performed on sections taken through the striatum that were immunohistochemically stained for tyrosine hydroxylase (TH). Animals in the 6-OHDA condition showed a mean reduction in TH staining of 88.27%. Although 6-OHDA animals were significantly impaired on all motor tasks, limb motor deficits were more severe than cranial motor impairments. Further, performance on limb motor tasks was correlated with degree of TH depletion while performance on cranial motor impairments showed no significant correlation. These results suggest that limb motor function may be more sensitive to striatal dopaminergic depletion than cranial motor function and is consistent with the clinical observation that therapies targeting the nigrostriatal dopaminergic system in Parkinson's disease are more effective for limb motor symptoms than cranial motor impairments.

Motor function and dopamine release measurements in transgenic Huntington's disease model rats

Huntington's disease (HD) is a fatal, genetic, neurodegenerative disorder characterized by deficits in motor and cognitive function. Here, we have quantitatively characterized motor deficiencies and dopamine release dynamics in transgenic HD model rats. Behavioral analyses were conducted using a newly-developed force-sensing runway and a previously-developed force-plate actometer. Gait disturbances were readily observed in transgenic HD rats at 12 to 15months of age. Additionally, dopamine system challenge by ip injection of amphetamine also revealed that these rats were resistant to the expression of focused stereotypy compared to wild-type controls. Moreover, dopamine release, evoked by the application of single and multiple electrical stimulus pulses applied at different frequencies, and measured using fast-scan cyclic voltammetry at carbon-fiber microelectrodes, was diminished in transgenic HD rats compared to age-matched wild-type control rats. Collectively, these results underscore the potential contribution of dopamine release alterations to the expression of motor impairments in transgenic HD rats.

Sensorimotor behavioral tests for use in a juvenile rat model of traumatic brain injury: assessment of sex differences

Modeling juvenile traumatic brain injury (TBI) in rodents presents several unique challenges compared to adult TBI, one of which is selecting appropriate sensorimotor behavioral tasks that enable the assessment of the extent of injury and recovery over time in developing animals. To address this challenge, we performed a comparison of common sensorimotor tests in Long-Evans rats of various sizes and developmental stages (postnatal days 16-45, 35-190 g). Tests were compared and selected for their developmental appropriateness, scalability for growth, pre-training requirements, and throughput capability. Sex differences in response to TBI were also assessed. Grid walk, automated gait analysis, rotarod, beam walk, spontaneous forelimb elevation test, and measurement of motor activity using the force-plate actometer were evaluated. Grid walk, gait analysis, and rotarod failed to meet one or more of the evaluation criteria. Beam walk, spontaneous forelimb elevation test, and measurement of motor activity using the force-plate actometer satisfied all criteria and were capable of detecting motor abnormalities in rats subjected to controlled cortical impact on postnatal day 17. No sex differences were detected in the acute effects of TBI or functional recovery during the 28 days after injury using these tests. This demonstrates the utility of these tests for the evaluation of sensorimotor function in studies using rat models of pediatric TBI, and suggests that pre-pubertal males and females respond similarly to TBI with respect to sensorimotor outcomes.

Effects of acute pramipexole on preference for gambling-like schedules of reinforcement in rats

RATIONALE

Pramipexole and other direct dopamine agonist medications have been implicated in the development of impulsive behavior such as pathological gambling among those taking the drug to control symptoms of Parkinson's disease or restless leg syndrome. Few laboratory studies examining pramipexole's effects on gambling-like behavior have been conducted.

OBJECTIVES

The present study used a rodent model approximating some aspects of human gambling to examine within-subject effects of acute pramipexole (0.03, 0.1, 0.18, and 0.3 mg/kg) on rat's choices to earn food reinforcement by completing variable-ratio (VR; i.e., gambling-like) or fixed-ratio (FR) response requirements.

RESULTS

In a condition in which the VR alternative was rarely selected, all but the lowest dose of pramipexole significantly increased choice of the VR alternative (an average of 15% above saline). The same doses did not affect choice significantly in a control condition designed to evaluate the involvement of nonspecific drug effects. Pramipexole increased latencies to initiate trials (+9.12 s) and to begin response runs on forced-choice trials (VR = +0.21 s; FR = +0.88 s), but did not affect measures of response perseveration (conditional probabilities of "staying").

CONCLUSIONS

The findings are consistent with clinical reports linking pramipexole to the expression of increased gambling in humans. Results are discussed in the context of neurobehavioral evidence suggesting that dopamine agonists increase sensitivity to reward delay and disrupt appropriate feedback from negative outcomes.

Differences in methylphenidate dose response between periadolescent and adult rats in the familiar arena-novel alcove task

Methylphenidate is a psychostimulant widely used in the treatment of attention deficit hyperactivity disorder. In this study, the effects of two nonstereotypy-inducing doses of methylphenidate (2.5 and 5.0 mg/kg s.c.) were examined in periadolescent [postnatal days (P) 35 and 42] and young adult (P70), male Long-Evans rats using a three-period locomotor activity paradigm that affords inferences about exploration, habituation, and attention to a novel stimulus (an "alcove") in a familiar environment in a single test session. In the first test period, P35 and P42 rats were more active than P70 rats, and methylphenidate increased locomotion in a dose-related manner. The introduction of a novel spatial stimulus in the third test period revealed a significant interaction of dose and age such that P70 rats exhibited dose-related increases in distance traveled, but P35 rats did not. Furthermore, methylphenidate dose-relatedly disrupted the rats' tendency to spend increasing amounts of time in the alcove across the test period at P70 but not at P35. Brain and serum methylphenidate concentrations were significantly lower at P35 than at P70, with intermediate levels at P42. Developmental differences in dopaminergic neurochemistry were also observed, including increased dopamine content in the caudate-putamen, nucleus accumbens, and frontal cortex and decreased densities of D(1)-like receptors in the frontal cortex in P70 than in P42 rats. These results raise the possibility that children and adults may respond differently when treated with this drug, particularly in situations involving response to novelty and that these effects involve developmental differences in pharmacokinetics and dopaminergic neurochemistry.

Dopamine Release and Uptake Impairments and Behavioral Alterations Observed in Mice that Model Fragile X Mental Retardation Syndrome

In this study we evaluated the relationship between amphetamine-induced behavioral alterations and dopamine release and uptake characteristics in Fmr1 knockout (Fmr1 KO) mice, which model fragile X syndrome. The behavioral analyses, obtained at millisecond temporal resolution and 2 mm spatial resolution using a force-plate actometer, revealed that Fmr1 KO mice express a lower degree of focused stereotypy compared to wild type (WT) control mice after injection with 10 mg/kg (ip) amphetamine. To identify potentially related neurochemical mechanisms underlying this phenomenon, we measured electrically-evoked dopamine release and uptake using fast-scan cyclic voltammetry at carbon-fiber microelectrodes in striatal brain slices. At 10 weeks of age, dopamine release per pulse, which is dopamine release corrected for differences in uptake, was unchanged. However, at 15 (the age of behavioral testing) and 20 weeks of age, dopamine per pulse and the maximum rate of dopamine uptake was diminished in Fmr1 KO mice compared to WT mice. Dopamine uptake measurements, obtained at different amphetamine concentrations, indicated that dopamine transporters in both genotypes have equal affinities for amphetamine. Moreover, dopamine release measurements from slices treated with quinpirole, a D2-family receptor agonist, rule out enhanced D2 autoreceptor sensitivity as a mechanism of release inhibition. However, dopamine release, uncorrected for uptake and normalized against the corresponding pre-drug release peaks, increased in Fmr1 KO mice, but not in WT mice. Collectively, these data are consistent with a scenario in which a decrease in extracellular dopamine levels in the striatum result in diminished expression of focused stereotypy in Fmr1 KO mice.

Developmental effects of dietary n-3 fatty acids on activity and response to novelty

Insufficient availability of n-3 polyunsaturated fatty acids (PUFA) during pre- and neonatal development decreases accretion of docosahexaenoic acid (DHA, 22:6n-3) in the developing brain. Low tissue levels of DHA are associated with neurodevelopmental disorders including attention deficit hyperactivity disorder (ADHD). In this study, 1st- and 2nd-litter male Long-Evans rats were raised from conception on a Control diet containing alpha-linolenic acid (4.20 g/kg diet), the dietarily essential fatty acid precursor of DHA, or a diet Deficient in alpha-linolenic acid (0.38 g/kg diet). The Deficient diet resulted in a decrease in brain phospholipid DHA of 48% in 1st-litter pups and 65% in 2nd-litter pups. Activity, habituation, and response to spatial change in a familiar environment were assessed in a single-session behavioral paradigm at postnatal days 28 and 70, inclusive. Activity and habituation varied by age with younger rats exhibiting higher activity, less habituation, and less stimulation of activity induced by spatial novelty. During the first and second exposures to the test chamber, 2nd-litter Deficient pups exhibited higher levels of activity than Control rats or 1st-litter Deficient pups, and less habituation during the first exposure, but were not more active after introduction of a novel spatial stimulus. The higher level of activity in a familiar environment, but not after introduction of a novel stimulus is consistent with clinical observations in ADHD. The observation of this effect only in 2nd-litter rats fed the Deficient diet suggests that brain DHA content, rather than dietary n-3 PUFA content, likely underlies these effects.

Effects of differing response-force requirements on food-maintained responding in C57Bl/6J mice

The effect of force requirements on response effort was examined using inbred C57BL/6J mice trained to press a disk with their snout. Lateral peak forces greater than 2 g were defined as responses (i.e., all responses above the measurement threshold). Different, higher force requirements were used to define criterion responses (a subclass of all responses) that exceeded the requirement and produced a reinforcer. The reinforcer was sweetened, condensed milk, delivered upon response termination. All mice were exposed to two ascending series of criterion force requirements (2, 4, 8, 16, and 32 g). Increasing the force requirement initially decreased criterion response rates, but criterion response rates recovered with continued exposure, except at the 32-g requirement. Response rates for all measured responses initially increased with increasing force requirements, but then decreased with continued exposure. The second exposure series produced more stable response rate changes than the first series. The time-integral of force (area under the force-time curve for individual responses, which is proportional to energy expenditure for each response) increased with the increase in the force requirement. The C57BL/6J inbred strain generated average force output similar to CD-1 outbred stock mice trained on the same force requirements. C57BL/6J inbred strain mice differed from CD-1 mice in initial response rates (for all responses above threshold) and had lower response rates at the 16 and 32 g requirements resulting in lower total force output. These data show for both mice types that increased force requirements resulted in increased overall responding (all measured responses), which contradicts a punishment interpretation of criterion response decrements. C57BL\6 inbred mice showed individual differences comparable to the outbred CD-1 stock. C57BL/6 mice did not maintain responding as well at the higher force requirements, which may be due to their small body size and weight, compared to the larger and heavier CD-1 mice.

Effects of pramipexole on impulsive choice in male wistar rats

Clinical reports, primarily with Parkinson's disease patients, note an association between the prescribed use of pramipexole (and other direct-acting dopamine agonist medications) and impulse control disorders, particularly pathological gambling. Two experiments examined the effects of acute pramipexole on rats' impulsive choices where impulsivity was defined as selecting a smaller-sooner over a larger-later food reward. In Experiment 1, pramipexole (0.1 to 0.3 mg/kg) significantly increased impulsive choices in a condition in which few impulsive choices were made during a stable baseline. In a control condition, in which impulsive choices predominated during baseline, pramipexole did not significantly change the same rats' choices. Experiment 2 explored a wider range of doses (0.01 to 0.3 mg/kg) using a choice procedure in which delays to the larger-later reinforcer delivery increased across trial blocks within each session. At the doses used in Experiment 1, pramipexole shifted choice toward indifference regardless of the operative delay. At lower doses of pramipexole (0.01 and 0.03 mg/kg), a trend toward more impulsive choice was observed at the 0.03 mg/kg dose. The difference in outcomes across experiments may be due to the more complex discriminations required in Experiment 2, that is, multiple discriminations between changing delays within each session.

Dopamine D(2) receptor function is compromised in the brain of the methionine sulfoxide reductase A knockout mouse

Previous research suggests that brain oxidative stress and altered rodent locomotor behavior are linked. We observed bio-behavioral changes in methionine sulfoxide reductase A knockout mice associated with abnormal dopamine signaling. Compromised ability of these knockout mice to reduce methionine sulfoxide enhances accumulation of sulfoxides in proteins. We examined the dopamine D(2)-receptor function and expression, which has an atypical arrangement and quantity of methionine residues. Indeed, protein expression levels of dopamine D(2)-receptor were higher in knockout mice compared with wild-type. However, the binding of dopamine D(2)-receptor agonist was compromised in the same fractions of knockout mice. Coupling efficiency of dopamine D(2)-receptors to G-proteins was also significantly reduced in knockout mice, supporting the compromised agonist binding. Furthermore, pre-synaptic dopamine release in knockout striatal sections was less responsive than control sections to dopamine D(2)-receptor ligands. Behaviorally, the locomotor activity of knockout mice was less responsive to the inhibitory effect of quinpirole than wild-type mice. Involvement of specific methionine residue oxidation in the dopamine D(2)-receptor third intracellular loop is suggested by in vitro studies. We conclude that ablation of methionine sulfoxide reductase can affect dopamine signaling through altering dopamine D(2)-receptor physiology and may be related to symptoms associated with neurological disorders and diseases.

Methylphenidate attenuates rats' preference for a novel spatial stimulus introduced into a familiar environment: assessment using a force-plate actometer

Methylphenidate is a psychostimulant widely used in the treatment of attention deficit hyperactivity disorder (ADHD). Here we report a novel paradigm that affords inferences about habituation and attention to a novel stimulus in a familiar environment in a single test session without prior training of the animals. The paradigm was used to assess the effects of methylphenidate (2.5 and 5.0mg/kg, sc) in young adult, male, Long-Evans rats. Methylphenidate increased locomotor activity during the initial exposure to the test apparatus in a non-dose-related manner. However, upon introduction of a novel spatial stimulus (an alcove) in the familiar environment, methylphenidate-treatment resulted in dose-related increases in distance traveled and inhibition of long dwell times in the alcove, the latter behavior being characteristic of vehicle-treated rats' response to the alcove condition. These results demonstrate the utility of this paradigm in the elucidation of the behavioral effects of a drug commonly used in the treatment of ADHD. Findings also suggest that species-typical response preferences in rats (e.g., refuge-seeking) may emerge in experimental settings that add spatial novelty to otherwise featureless test enclosures commonly used to assess locomotor activity.

Dysregulation of coordinated neuronal firing patterns in striatum of freely behaving transgenic rats that model Huntington's disease

Altered neuronal activity in the striatum appears to be a key component of Huntington's disease (HD), a fatal, neurodegenerative condition. To assess this hypothesis in freely behaving transgenic rats that model HD (tgHDs), we used chronically implanted micro-wires to record the spontaneous activity of striatal neurons. We found that relative to wild-type controls, HD rats suffer from population-level deficits in striatal activity characterized by a loss of correlated firing and fewer episodes of coincident spike bursting between simultaneously recorded neuronal pairs. These results are in line with our previous report of marked alterations in the pattern of striatal firing in mouse models of HD that vary in background strain, genetic construct, and symptom severity. Thus, loss of coordinated spike activity in striatum appears to be a common feature of HD pathophysiology, regardless of HD model variability.

Caloric restriction alleviates abnormal locomotor activity and dopamine levels in the brain of the methionine sulfoxide reductase A knockout mouse

Oxidative stress is associated with the aging process, a risk factor for neurodegenerative diseases, and decreased by reduced energy intake. Oxidative modifications can affect protein function; the sulfur-containing amino acids, including methionine, are particularly susceptible to oxidation. A methionine sulfoxide can be enzymatically reduced by the methionine sulfoxide reductase (Msr) system. Previously, we have shown that MsrA(-/-) mice exhibit altered locomotor activity and brain dopamine levels as function of age. Previous studies have demonstrated that a caloric restriction enhances antioxidant defense and reduces the action of reactive oxygen species. Here we examine locomotor behavior and dopamine levels of MsrA(-/-) mice after caloric restriction starting at eight months of age and ending at 17 months. The MsrA(-/-) mice did not have any significant difference in spontaneous distance traveled when compared to controls at 17 months of age. In contrast, our previous report showed decreased locomotor activity in the MsrA(-/-) mice at 12 months of age and older when fed ad-libitum. After completion of the caloric restriction diet, dopamine levels were comparable to control mice. This differs from the abnormal dopamine levels previously observed in MsrA(-/-) mice fed ad-libitum. Thus, caloric restriction had a neutralization effect on MsrA ablation. In summary, it is suggested that caloric restriction alleviates abnormal locomotor activity and dopamine levels in the brain of the methionine sulfoxide reductase A knockout mouse.

Timing and space usage are disrupted by amphetamine in rats maintained on DRL 24-s and DRL 72-s schedules of reinforcement

RATIONALE

A differential-reinforcement-of-low-rate schedule (DRL) delivers reinforcement only when the interresponse time (IRT) exceeds a fixed time interval, thereby shaping rats to discriminate the timing of their responses. However, little is known about the motor behavior and location of the rats in the chamber during the IRTs that lead to reinforcement. Although amphetamine is known to disrupt DRL timing behavior, the effects of this drug on non-operant motor behavior during DRL performance has not yet been quantified.

OBJECTIVE

The purpose of this research was to measure the motor behavior (movement trajectories in the horizontal plane and spatial location in the plane) during longer IRTs after either vehicle or amphetamine treatment.

MATERIALS AND METHODS

Experimental chambers were constructed with a force-plate actometer as the floor, and while performing the operant task, the rats' motor behaviors were measured continuously with high temporal and spatial resolution. Separate groups of eight male Sprague-Dawley rats were maintained on either DRL 24-s or DRL 72-s schedules of water reinforcement in 4-h recording sessions.

RESULTS

Analyses of IRT distributions showed that the rats' timing behavior conformed to their respective DRL requirements. In the absence of drug, analysis of motor behavior in pre-reinforcement intervals showed that rats located themselves away from the operandum and exhibited very low levels of movement. Rats exhibited a significant temporal diminution of horizontal movement that reached a minimum 4-8 s before the rats moved to the operandum to execute operant responses. Amphetamine treatment increased locomotion, abolished the temporal movement gradient, and brought the rats closer to the operandum compared to vehicle treatment. Movement changes induced by amphetamine were accompanied by degraded timing behavior.

CONCLUSIONS

Taken together, the data show that DRL training induced rats to locate themselves away from the operandum and to remain nearly motionless during longer IRTs and that amphetamine treatment interfered with this complex of behavioral features.

Force-plate quantification of progressive behavioral deficits in the R6/2 mouse model of Huntington's disease

The R6/2 mouse is a popular model of Huntington's disease (HD) because of its rapid progression and measurable behavioral phenotype. Yet current behavioral phenotyping methods are usually univariate (e.g., latency to fall from a rotarod) and labor intensive. We used a force-plate actometer and specialized computer algorithms to partition the data into topographically specific behavioral categories that were sensitive to HD-like abnormalities. Seven R6/2 male mice and 7 wild-type (WT) controls were placed in a 42 cm x 42 cm force-plate actometer for 20-min recording sessions at 6-7, 8-9, 10-11 and 12-13 weeks of age. Distance traveled, number of wall rears, and number of straight runs (traveling 175 mm or more in 1.5s) were reduced in R6/2 relative to WT mice at all ages tested. Low mobility bouts (each defined as remaining continuously in a virtual circle of 15 mm radius for 5s) were increased in R6/2 mice at 6-7 weeks and beyond. Independent of body weight, force off-load during wall rears was reduced in R6/2 mice except at 6-7 weeks. Power spectra of force variation during straight runs indicated an age-related progressive loss of rhythmicity in R6/2 compared to WT, suggesting gait dysrhythmia and dysmetria. Collectively, these data, which extend results obtained with other widely different behavioral phenotyping methods, document a multifaceted syndrome of motor abnormalities in R6/2 mice. We suggest, moreover, that the force-plate actometer offers a high-throughput tool for screening drugs that may affect symptom expression in R6/2 or other HD model mice.

An inexpensive infrared detector to verify the delivery of food pellets

The reproducibility of experimental outcomes depends on consistent control of independent variables. In food-maintained operant performance, it is of utmost importance that the quantity of food delivered is reliable. To that end, some commercial food pellet dispensers have add-on attachments to sense the delivery of pellets. Not all companies, however, offer such add-ons. Aside from availability, cost and temporary reduction in throughput may be a problem for smaller labs. The present paper outlines our recent development of a simple, inexpensive infrared device to detect and confirm the delivery of pellets. The in-line construction of the detector routes the falling pellet through a barrel so that it passes between an infrared emitter and receiver. The circuitry was designed to be compatible with all commercially available behavioral measurement systems, and so may be retrofit to any existing system. Our tests with the detector so far have shown that it is 100% accurate in detecting pellet delivery. The individual unit cost is approximately 25 dollars. The low cost and versatility of the device offer an easy method to ensure the integrity of food delivery in operant settings.

MsrA knockout mouse exhibits abnormal behavior and brain dopamine levels

Oxidative stress can cause methionine oxidation that has been implicated in various proteins malfunctions, if not adequately reduced by the methionine sulfoxide reductase system. Recent evidence has found oxidized methionine residues in neurodegenerative conditions. Previously, we have described elevated levels of brain pathologies and an abnormal walking pattern in the methionine sulfoxide reductase A knockout (MsrA(-/-)) mouse. Here we show that MsrA(-/-) mice have compromised complex task learning capabilities relative to wild-type mice. Likewise, MsrA(-/-) mice exhibit lower locomotor activity and altered gait that exacerbated with age. Furthermore, MsrA(-/-) mice were less responsive to amphetamine treatment. Consequently, brain dopamine levels were determined. Surprisingly, relative to wild-type mice, MsrA(-/-) brains contained significantly higher levels of dopamine up to 12 months of age, while lower levels of dopamine were observed at 16 months of age. Moreover, striatal regions of MsrA(-/-) mice showed an increase of dopamine release parallel to observed dopamine levels. Similarly, the expression pattern of tyrosine hydroxylase activating protein correlated with the age-dependent dopamine levels. Thus, it is suggested that dopamine regulation and signaling pathways are impaired in MsrA(-/-) mice, which may contribute to their abnormal behavior. These observations may be relevant to age-related neurological diseases associated with oxidative stress.

Effects of white and infrared lighting on apomorphine-induced pecking in pigeons

This experiment was concerned with the role of the environment in the production and form of apomorphine-induced pecking of pigeons. Earlier literature has suggested that the pecking occurs even when pigeons are placed in complete darkness, but there are no systematic or quantitative reports of such pecking. Six pigeons were tested with doses of 0.1, 0.3, and 1.0 mg/kg apomorphine. Tests were made in conditions of white and infrared light. The apparatus used novel force transduction measures that provided for both the detection of a peck as well as its peak forcefulness. At the lowest dose tested, apomorphine elicited pecking when the pigeon was placed in white light, but not when the dose was examined under infrared lighting. As the dose increased, however, pecking was observed regardless of lighting condition. No consistent differences were found in forcefulness of pecking as a function of lighting condition or dose. Though response output was seemingly unaffected by the lighting condition at higher doses, videotaped analysis revealed important changes in the formal characteristics of pecking. In white light, apomorphine elicited pecking at stimuli in the chamber (e.g. screw heads or the pigeon's own toes), whereas in infrared light pecking was directed at the floor directly in front of the pigeon. Such differences may be attributable to shifts in control to other stimulus modalities when vision is limited. Additionally, apomorphine may have direct effects on retinal dopamine function modulating the expression of pecking in the dark.

Effects of differing response-force requirements on food-maintained responding in CD-1 mice

The effect of force requirements on response effort was examined using outbred (CD-1) mice trained to press a disk with their snout. Lateral peak forces greater than 2 g were defined as threshold responses (i.e., all measured responses). Different force requirements were used to define criterion responses (a subclass of threshold responses) that exceeded the requirement. The reinforcer was sweetened, condensed milk, and it was delivered upon response termination. All mice were exposed to two ascending series of criterion force requirements (2, 4, 8, 16, and 32 g). Increasing the force requirement decreased criterion response rates, but increased threshold response rates. The time-integral of force (area under the force-time curve for individual responses, which is proportional to energy expenditure for each response) increased with the increase in the force requirement. These results conflict with the hypothesis that higher force requirements have aversive qualities and suggest that increased force requirements are more analogous to intermittent schedules of reinforcement. These data suggest that estimations of effort or energy expenditure should be measured independently of the force requirement. Individual differences in responding were found for the CD-1 outbred stock.

Stereotyped and complex motor routines expressed during cocaine self-administration: results from a 24-h binge of unlimited cocaine access in rats

RATIONALE

Cocaine binges represent the most severe form of cocaine taking due to high levels of cocaine consumed and a potential loss of self-control over cocaine taking. Experimentally, regulation of intravenous cocaine self-administration is observed during binges as revealed by nearly constant sequential inter-infusion intervals, suggesting that pharmacological and unconditioned behavioral effects of cumulative cocaine leave intact its reinforcing effect.

OBJECTIVE

To elucidate factors that contribute to the patterning of cocaine self-administration behavior during binges, the current study quantifies the expression of specific motor routines that emerge from the beginning of a cocaine infusion until the next cocaine reinforcement is received, in six separate rats over 24 h of continuous access.

MATERIALS AND METHODS

During each 24-h cocaine binge, behavior was continuously monitored using a force plate actometer that provides reliable quantitation of rodent behaviors, including rotational behavior, locomotor activity, and focused stereotypy.

RESULTS

Corroborating earlier results, each rat accumulated between 4 and 11 mg/kg/h during periods of active cocaine self-administration. All rats displayed a similar narrow range of unconditioned behavioral responses to cocaine, yet each rat varied with regard to the intensity and specific predominant pattern of behavioral activation. Focused stereotyped behavior, in particular, was apparent in all rats and continued for as long as cocaine self-administration behavior remained active.

CONCLUSIONS

The current results support the hypothesis that individual differences in cocaine's pharmacodynamics may contribute to specific behaviors expressed during cocaine self-administration, but leave unresolved whether or not intense unconditioned behavior (e.g., focused stereotypy) conflicts with, or contributes to, ongoing self-administration behavior.

Clozapine and prazosin slow the rhythm of head movements during focused stereotypy induced by d-amphetamine in rats

RATIONALE

Clozapine is an efficacious, symptom-ameliorating, atypical antipsychotic drug with few extrapyramidal side effects. Clozapine has been reported either not to affect or to increase d-amphetamine-induced stereotypy, a behavior that is blocked by typical antipsychotic drugs.

OBJECTIVES

This work used a high-resolution measurement system to reassess clozapine's effects on d-amphetamine-induced focused stereotypy (FS) in rats.

MATERIALS AND METHODS

A force-plate actometer permitted the quantitation of the rhythm and vigor of movements during FS. Eight rats received a sensitizing series of doses of 5.0 mg/kg d-amphetamine sulfate, and this dosing regimen induced head movements with a rhythm near 10 Hz. Thirty minutes after d-amphetamine treatment, rats received acute clozapine (2.5-10.0 mg/kg), followed by eight, daily clozapine injections (5.0 mg/kg) given with d-amphetamine on days 2, 5, and 8. Effects of acute doses of the alpha1-noradrenergic antagonist prazosin (0.5-2.0 mg/kg) on the d-amphetamine response were also examined.

RESULTS

Clozapine dose-dependently slowed the near 10-Hz rhythm and reduced the vigor of the d-amphetamine-induced FS. Clozapine significantly lengthened the duration of the FS phase, but the rhythm remained slowed. No evidence for tolerance to clozapine's rhythm-slowing effects was seen in the subchronic phase. Prazosin dose-dependently reduced the near 10-Hz rhythm induced by d-amphetamine, but prazosin did not lengthen the FS phase.

CONCLUSIONS

The results show that clozapine diminished the rhythm and vigor of d-amphetamine-induced stereotyped head movements but, at the same time, lengthened the duration of the expression of the stereotypy. alpha1 antagonism is a likely contributor to the rhythm-modulating effects of clozapine.

Measuring forelimb force control and movement in Fischer 344/Brown Norway rats: effects of age and lorazepam

The purpose of this study was to measure forelimb force control and movement kinetics in rats, as they are affected by normal aging and the benzodiazepine lorazepam. Young (6 months), middle-aged (18 months), and aged (24 months) rats were trained to emit discrete forelimb responses on an isometric force disk within a 20-25 g force band for water reinforcement. Dependent variables included number of responses, percentage of reinforced responses, peak response forces, and inter-response times. Inter-response times were divided into two categories: inter-response times <0.5 s (reflecting rapid, discrete forelimb responses) and inter-response times 4-8 s (reflecting movement sequences). Aged rats exhibited no apparent deficits in forelimb force control. Although older rats emitted fewer responses than younger rats, their response accuracy was greater. Peak forces did not differ among the groups. Both categories of inter-response times were slower in the aged group, reflecting slowed discrete movements and movement sequencing. Lorazepam increased the number of responses and peak forces, decreased response accuracy, and lengthened inter-response times within the 4-8 s range (but not the <0.5 s range) in all age groups. The results suggest that movement sequences may be more sensitive to the effects of acute benzodiazepines than rapid discrete movements.

Differential acquisition of lever pressing in inbred and outbred mice: comparison of one-lever and two-lever procedures and correlation with differences in locomotor activity

Recent progress in mouse genetics has led to an increased interest in developing procedures for assessing mouse behavior, but relatively few of the behavioral procedures developed involve positively reinforced operant behavior. When operant methods are used, nose poking, not lever pressing, is the target response. In the current study differential acquisition of milk-reinforced lever pressing was observed in five inbred strains (C57BL/6J, DBA/2J, 129X1/SvJ, C3H/HeJ, and BALB/cJ) and one outbred stock (CD-1) of mice. Regardless of whether one or two levers (an "operative" and "inoperative" lever) were in the operant chamber, a concomitant variable-time fixed-ratio schedule of milk reinforcement established lever pressing in the majority of mice within two 120-min sessions. Substantial differences in lever pressing were observed across mice and between procedures. Adding an inoperative lever retarded acquisition in C57BL/6J, DBA/2J, 129X1/SvJ, and C3H/HeJ mice, but not in CD-1 and BALB/cJ mice. Locomotor activity was positively correlated with number of lever presses in both procedures. Analyses of durations of the subcomponents (e.g., time to move from hopper to lever) of operant behavior revealed further differences among the six types of mice. Together, the data suggest that appetitively reinforced lever pressing can be acquired rapidly in mice and that a combination of procedural, behavioral, and genetic variables contributes to this acquisition.

Improvement in survival and muscle function in an mdx/utrn−/− double mutant mouse using a human retinal dystrophin transgene

Duchenne muscular dystrophy is a progressive muscle disease characterized by increasing muscle weakness and death by the third decade. mdx mice exhibit the underlying muscle disease but appear physically normal with ordinary lifespans, possibly due to compensatory expression of utrophin. In contrast, double mutant mice (mdx/utrn(-/-)), deficient for both dystrophin and utrophin die by approximately 3 months and suffer from severe muscle weakness, growth retardation, and severe spinal curvature. The capacity of human retinal dystrophin (Dp260) to compensate for the missing 427 kDa muscle dystrophin was tested in mdx/utrn(-/-) mice. Functional outcomes were assessed by histology, EMG, MRI, mobility, weight and longevity. MCK-driven transgenic expression of Dp260 in mdx/utrn(-/-) mice converts their disease course from a severe, lethal muscular dystrophy to a viable, mild myopathic phenotype. This finding is relevant to the design of exon-skipping therapeutic strategies since Dp260 lacks dystrophin exons 1-29.

Use of a force-plate actometer for detecting and quantifying vertical leaping induced by amphetamine in BALB/cJ mice, but not in C57BL/6J, DBA/2J, 129X1/SvJ, C3H/HeJ, and CD-1 mice

The force-plate actometer is a relatively new computer-based instrument with high temporal and spatial resolution that has been used to measure the behavioral effects of genetic restriction (e.g., inbred mice) and drugs (e.g., dopaminergic agonists and antagonists) on a variety of behaviors in rodents, including locomotor activity, stereotypies, tremor, and wall rearing. In the present study, the force-plate actometer was used to measure the differential effects of amphetamine-induced (10.0mg/kg) vertical leaping in five inbred mouse strains (BALB/cJ, C57BL/6J, DBA/2J, 129X1/SvJ, and C3H/HeJ) and one outbred stock (CD-1). Across a 13-day, five-injection procedure, mice of the BALB/cJ strain leaped an average of 82 times per 60-min session; the C57BL/6J, DBA/2J, 129X1/SvJ, C3H/HeJ strains and CD-1 stock always showed zero or near zero levels of vertical leaping following amphetamine treatment. The quantitative precision afforded by the force-plate actometer revealed that the mean duration of the leaps by the BALB/cJ strain was 0.18 second, and the corresponding peak force averaged 87.4 gram per leap, which was more than 400% of the average body weight of this strain. Although no evidence of behavioral sensitization was indicated for amphetamine's effects on vertical leaping, sensitization to amphetamine's effects on spatial confinement (i.e., bouts of low mobility) was observed in all mouse types. Results indicate that the force-plate actometer is an instrument well suited for detecting and quantifying both vertical leaping and collateral behaviors induced by amphetamine in mice.