Effects of amphetamine and apomorphine on locomotor activity after 6-OHDA and electrolytic lesions of the nucleus accumbens septi

Cannabidiol has therapeutic potential for myofascial pain in female and male parkinsonian rats

The musculoskeletal orofacial pain is a complex symptom of Parkinson's disease (PD) resulting in stomatognathic system dysfunctions aggravated by the disease rigidity and postural instability. We tested the effect of cannabidiol (CBD), a non-psychotomimetic constituent of Cannabis sativa, in PD-related myofascial pain. Wistar adult female and male rats orofacial allodynic and hyperalgesic responses were tested by Von Frey and formalin tests, before and 21 days past 6-OHDA lesion. Algesic response was tested after masseter muscle injection of CBD (10, 50, 100 μg in 10 μL) or vehicle. Males compared to females in all estrous cycles' phases presented reduced orofacial allodynia and hyperalgesia. According to the estrous cycle's phases, females presented distinct orofacial nociceptive responses, being the estrus phase well-chosen for nociceptive analysis after 6-OHDA lesion (phase with fewer hormone alterations and adequate length). Dopaminergic neuron lesion decreased mechanical and inflammatory nociceptive thresholds in females and males in a higher proportion in females. CBD local treatment reduced the increased orofacial allodynia and hyperalgesia, in males and females. The female rats were more sensitive to CBD effect considering allodynia, responding to the lowest dose. Although females and males respond to the effect of three doses of CBD in the formalin test, males showed a superior reduction in the hyperalgesic response. These results indicate that hemiparkinsonian female in the estrus phase and male answer differently to the different doses of CBD therapy and nociceptive tests. CBD therapy is effective for parkinsonism-induced orofacial nociception.

Amelioration of the nigrostriatal pathway facilitated by ultrasound-mediated neurotrophic delivery in early Parkinson's disease

The blood-brain barrier (BBB) prevents most drugs from gaining access to the brain parenchyma, which is a recognized impediment to the treatment of neurodegenerative disorders like Parkinson's disease (PD). Focused ultrasound (FUS), in conjunction with systemically administered microbubbles, opens the BBB locally, reversibly and non-invasively. Herein, we show that neither FUS applied over both the striatum and the ventral midbrain, without neurotrophic factors, nor intravenous administration of neurotrophic factors (either through protein or gene delivery) without FUS, ameliorates the damage to the nigrostriatal dopaminergic pathway in the sub-acute MPTP mouse model of early-stage PD. Conversely, the combination of FUS and intravenous neurotrophic (protein or gene) delivery attenuates the damage to the nigrostriatal dopaminergic pathway, by allowing the entry of these agents into the brain parenchyma. Our findings provide evidence that the application of FUS at the early stages of PD facilitates critical neurotrophic delivery that can curb the rapid progression of neurodegeneration while improving the neuronal function, seemingly opening new therapeutic avenues for the early treatment of diseases of the central nervous system.

Acute and chronic methylphenidate administration in intact and VTA-specific and nonspecific lesioned rats

Methylphenidate (MPD) is a psychostimulant used for the treatment of ADHD and works by increasing the bioavailability of dopamine (DA) in the brain. As a major source of DA, the ventral tegmental area (VTA) served as the principal target in this study as we aimed to understand its role in modulating the acute and chronic MPD effect. Forty-eight male Sprague-Dawley rats were divided into control, sham, electrical lesion, and 6-OHDA lesion groups. Given the VTA's implication in the locomotive circuit, three locomotor indices-horizontal activity, number of stereotypy, and total distance-were used to measure the animals' behavioral response to the drug. Baseline recording was obtained on experimental day 1 (ED 1) followed by surgery on ED 2. After recovery, the behavioral recordings were resumed on ED 8. All groups received daily intraperitoneal injections of 2.5 mg/kg MPD for six days after which the animals received no treatment for 3 days. On ED 18, 2.5 mg/kg MPD was re-administered to assess for the chronic effect of the psychostimulant. Except for one index, there was an increase in locomotive activity in all experimental groups after surgery (in comparison to baseline activity), acute MPD exposure, induction with six daily doses, and after MPD re-challenge. Furthermore, the increase was greatest in the electrical VTA lesion group and lowest in the 6-OHDA VTA lesion group. In conclusion, the results of this study suggest that the VTA may not be the primary nucleus of MPD action, and the VTA plays an inhibitory role in the locomotive circuit.

Nucleus accumbens core medium spiny neuron electrophysiological properties and partner preference behavior in the adult male prairie vole, Microtus ochrogaster

Medium spiny neurons (MSNs) in the nucleus accumbens have long been implicated in the neurobiological mechanisms that underlie numerous social and motivated behaviors as studied in rodents such as rats. Recently, the prairie vole has emerged as an important model animal for studying social behaviors, particularly regarding monogamy because of its ability to form pair bonds. However, to our knowledge, no study has assessed intrinsic vole MSN electrophysiological properties or tested how these properties vary with the strength of the pair bond between partnered voles. Here we performed whole cell patch-clamp recordings of MSNs in acute brain slices of the nucleus accumbens core (NAc) of adult male voles exhibiting strong and weak preferences for their respective partnered females. We first document vole MSN electrophysiological properties and provide comparison to rat MSNs. Vole MSNs demonstrated many canonical electrophysiological attributes shared across species but exhibited notable differences in excitability compared with rat MSNs. Second, we assessed male vole partner preference behavior and tested whether MSN electrophysiological properties varied with partner preference strength. Male vole partner preference showed extensive variability. We found that decreases in miniature excitatory postsynaptic current amplitude and the slope of the evoked action potential firing rate to depolarizing current injection weakly associated with increased preference for the partnered female. This suggests that excitatory synaptic strength and neuronal excitability may be decreased in MSNs in males exhibiting stronger preference for a partnered female. Overall, these data provide extensive documentation of MSN electrophysiological characteristics and their relationship to social behavior in the prairie vole. NEW & NOTEWORTHY This research represents the first assessment of prairie vole nucleus accumbens core medium spiny neuron intrinsic electrophysiological properties and probes the relationship between cellular excitability and social behavior.

Electrolytic lesions of the nucleus accumbens enhance locomotor sensitization to nicotine in rats

Electrolytic lesions of the medial core of the nucleus accumbens (NAc) in male Long-Evans rats increased spontaneous locomotion, enhanced the locomotor stimulating effect of acute 5.0 mg/kg cocaine, enhanced the development and subsequent expression of locomotor sensitization produced by repeated injections of 0.4 mg/kg nicotine but not 7.5 mg/kg cocaine, and enhanced the expression of conditioned locomotion. Given that 6-hydroxydopamine lesions of the NAc typically have effects on locomotor-related processes that are opposite of those produced by electrolytic and excitotoxic lesions, these data are consistent with a hypothesis that the NAc output, especially from the core, inhibits a variety of such processes and that the DA input to the NAc enhances these processes by inhibiting this inhibitory output.

Descending glutamatergic pathways of PFC are involved in acute and chronic action of methylphenidate

Progressive augmentation of behavioral response following repeated psychostimulant administrations is known as behavioral sensitization, and is an indicator of a drug's liability for abuse. It is known that methylphenidate (MPD) (also known as Ritalin), a drug used to treat attention-deficit hyperactivity disorder (ADHD), induces sensitization in animals following repeated injections. It was recently reported that bilateral electric (non-specific) lesion of prefrontal cortex (PFC) prevented MPD elicited behavioral sensitization. Since PFC sends glutamatergic afferents to both ventral tegmental area (VTA) and nucleus accumbens (NAc), sites that are involved in induction and expression of behavioral sensitization respectively and glutamate from PFC is known to modulate dopamine cell activity in VTA and NAc, this study investigated the role of descending glutamate from PFC in MPD elicited behavioral sensitization. Locomotor activity of three groups of rats-control, sham operated and group with specific chemical lesion of glutamate neurons of PFC-was recorded using an open-field assay. On experimental day (ED) 1, the locomotor activity was recorded post a saline injection. The sham and lesion groups underwent respective surgeries on ED 2, and were allowed to recover for 5 days (from ED 3 to ED 7). The post-surgery baseline was recorded on ED 8 following a saline injection. On ED's 9 through 14, 2.5 mg/kg MPD was given, followed by a 4-day washout period (ED 15 -18). All three groups received a rechallenge injection of 2.5 mg/kg on ED 19 and their locomotor activity on various days was analyzed. It was found that ibotenic acid lesion modulated the acute and chronic effects of MPD and hence suggests that PFC glutamatergic afferents are involved in the acute effect of MPD as well as in its chronic effects such as behavioral sensitization to MPD.

High-frequency stimulation of the nucleus accumbens core and shell reduces quinpirole-induced compulsive checking in rats

Electrical deep brain stimulation (DBS) is currently studied in the treatment of therapy-refractory obsessive compulsive disorders (OCDs). The variety of targeted brain areas and the inconsistency in demonstrating anti-compulsive effects, however, highlight the need for better mapping of brain regions in which stimulation may produce beneficial effects in OCD. Such a goal may be advanced by the assessment of DBS in appropriate animal models of OCD. Currently available data on DBS of the nucleus accumbens (NAc) on OCD-like behavior in rat models of OCD are contradictory and partly in contrast to clinical data and theoretical hypotheses about how the NAc might be pathophysiologically involved in the manifestation of OCD. Consequently, the present study investigates the effects of DBS of the NAc core and shell in a quinpirole rat model of OCD. The study demonstrates that electrical modulation of NAc core and shell activity via DBS reduces quinpirole-induced compulsive checking behavior in rats. We therefore conclude that both, the NAc core and shell constitute potential target structures in the treatment of OCD.

The antimanic-like effect of tamoxifen: Behavioural comparison with other PKC-inhibiting and antiestrogenic drugs

Protein kinase C (PKC) is an important cellular target for mood stabilizers such as lithium and valproate, and tamoxifen, an antiestrogenic drug with PKC inhibition activity, also demonstrates an antimanic effect. Thus, the aim of the present study was to evaluate whether the antimanic effect of tamoxifen is mediated through the PKC inhibitory and/or the antiestrogenic action(s) of the drug. In the present study, the effects of tamoxifen, chelerythrine (a PKC inhibitor) and medroxyprogesterone (an antiestrogenic drug) were investigated in amphetamine-induced hyperlocomotion of mice, an animal model of a manic state. Lithium carbonate (100 and 150 mg/kg, i.p.), tamoxifen (1.0 mg/kg, i.p.) and chelerythrine (1 microg/site, i.c.v.) completely blocked the amphetamine-induced hyperlocomotion. However, while the intermediate medroxyprogesterone dose (3.0 mg/kg, i.p.) partially reduced the amphetamine-induced hyperlocomotion, lower (1.0 mg/g) and higher (6.0 mg/kg) doses produced no effect. Our results indicate a major role for PKC inhibition in the antimanic-like effect of tamoxifen, although its antiestrogenic action may also contribute to this effect.

Lesions of the nucleus accumbens shell can reduce activity in the elevated plus-maze

Across different behavioural tasks, nucleus accumbens (n.acc) lesions have generated conflicting effects on locomotor activity and in particular, the relative roles of the n.acc shell and core subfields in this have been controversial. To date there is only one study examining effects of lesions to the medial n.acc on elevated plus-maze (EPM) behaviour; these lesions were shown to increase both locomotor and exploratory activity. Given the well-documented distinction between shell and core, the present study sought to extend previous research by testing lesions selective to each n.acc subfield in the EPM. Results showed no statistical differences between core lesioned and sham-operated animals on any measure. In contrast, shell lesions consistently reduced locomotion and exploratory activity. This direction of effects is opposite to that previously observed after medial n.acc. lesions. In conclusion, locomotion and exploratory activity were clearly reduced by shell but not core lesions, consistent with other evidence for the functional heterogeneity of n.acc shell and core.

Sexual-incentive motivation and paced sexual behavior in female rats after treatment with drugs modifying dopaminergic neurotransmission

The effects of the dopamine receptor agonist apomorphine, the dopamine releaser amphetamine, and the dopamine receptor antagonist cis(Z)-flupenthixol on sexual-incentive motivation and on paced-mating behavior were studied in female rats. Apomorphine, in the doses of 0.125 and 0.5 mg/kg, showed a tendency to reduce incentive motivation. Ambulatory activity was inhibited, evidenced both by diminished distance moved and reduced velocity of movement. Amphetamine (0.25 and 1 mg/kg) and flupenthixol (0.25 and 0.5 mg/kg) failed to modify incentive motivation while stimulating and reducing ambulatory activity, respectively. In the mating test, apomorphine enhanced the latency to enter the male's half and reduced the number of proceptive behaviors. However, these effects were associated with the appearance of stereotyped sniffing. Amphetamine increased the propensity to escape from the male after a mount without having other effects. Flupenthixol augmented the duration of the lordosis posture. Neither amphetamine nor flupenthixol affected sniffing. These data show that facilitated dopaminergic neurotransmission stimulates neither paced female sexual behavior nor sexual-incentive motivation. Dopamine receptor blockade has slight consequences. It is concluded that dopamine is not a transmitter of major importance for unconditioned female sexual motivation and behavior.

The nucleus accumbens shell is critical for normal expression of pup-retrieval in postpartum female rats

The nucleus accumbens (NA) plays an important modulatory role in the control of normal expression of maternal behavior (termed maternal performance). The present study investigated the relative functions of two subregions of the NA (the shell and core) in maternal performance. Electrolytic lesions of the shell or core were performed either before parturition or immediately after a varying amount of maternal experience (none, 1 or 24h) during the immediate postpartum period. Maternal performance was tested on Day 1 postpartum and re-tested 9 days later using a pup sensitization technique. Results show that lesions of the shell, but not the core, significantly disrupted pup-retrieval: the shell-lesioned rats took significantly longer to finish retrieving all test pups, but their retrieval latency for the first pup was not affected. Neither lesion affected other components of maternal behavior (pup licking, nest building and nursing). These findings suggest that the shell, but not the core, is critical for the normal expression of pup-retrieval behavior possibly through its role in maintaining maternal motivation or attention.

Nucleus accumbens dopamine and learned fear revisited: a review and some new findings

A role for the nucleus accumbens (NAcc) and its dopamine (DA) innervation in fear and fear learning is supported by a large body of evidence, which has challenged the view that the NAcc is solely involved in mediating appetitive processes. Unfortunately, due to conflicting findings in the aversive conditioning literature the role of the NAcc in aversive conditioning remains unclear. This review focuses on the results of recent in vivo microdialysis studies that have examined the release of NAcc DA during Pavlovian aversive conditioning. In addition, we present additional new findings, which re-examine the involvement of NAcc DA in aversive conditioning. DA release was measured in the NAcc core using in vivo microdialysis during discrete cue Pavlovian aversive conditioning in four experiments. In all cases no change in DA levels was observed either during training or in response to the CS presentations despite robust behavioural evidence of discrete cue Pavlovian aversive conditioning. These findings contrast with some previous studies that show that primary and conditioned aversive stimuli increase DA release in the NAcc. We suggest that the inconsistencies in the literature might be due to procedural differences in the measurement of aversive conditioning, and the precise location of the probe in the NAcc region. Hence, rather than discount an involvement of NAcc DA in affective processes, we propose that functionally dissociable sub-regions of the NAcc may contribute to different aspects of Pavlovian aversive learning.

Tonic opioid inhibition of the subiculo-accumbens pathway

There is evidence to suggest that medium spiny neurons (MSNs) in the nucleus accumbens (NAS) should be sensitive to opiate compounds. However, neuronal responses in the NAS evoked by fimbria stimulation (F-D) are insensitive to systemically or iontophoretically administered morphine. The hypothesis of this study was that fimbria-evoked NAS responses may fail to demonstrate sensitivity to morphine because they are under tonic opioid inhibition and can't be further inhibited by opiates. If correct, then pharmacological inhibition of opioid actions on these NAS neuronal responses should result in an increase of response to fimbria stimulation. The effects of systemic and iontophoretic administrations of naloxone on NAS responses evoked by fimbria stimulation were observed. Systemically and locally administered naloxone selectively increased the excitability of accumbens single-unit responses to fimbria stimulation. Conversely, systemic or iontophoretic administration of morphine was without effect on the same types of NAS responses. These observations are consistent with the hypothesis that a tonic opioid inhibition may regulate this pathway. In contrast, naloxone and morphine effect other NAS circuit responses differently than F-D NAS responses. In some cases naloxone and morphine tests have been conducted on different evoked responses from the same neuron. Those results have shown that different responses from the same cell may be differentially affected. Consequently, opioid modulation of activity in the NAS is probably pathway-specific rather than neuron-specific.

An integrative neuroanatomical perspective on some subcortical substrates of adaptive responding with emphasis on the nucleus accumbens

Neuroanatomical substrates associated in the literature with adaptive responding are discussed, with a focus on the nucleus accumbens. While it is emphasized that the accumbens exhibits multiple levels of complex organization, a fairly complete list of brief descriptions of recent studies devoted specifically to the accumbens shell and core subterritories is presented in tabular format. The distinct patterns of connectivity of the accumbens core and shell and structures related to them by connections are described. Multiple inputs, outputs and abundant reciprocity of connections within the ventral parts of the basal ganglia are emphasized and the implications for "through-put" of impulses is considered. It is noted, at least on neuroanatomical grounds, that there is ample reason to expect feed forward processing from shell and structures with which it is associated to core and structures with which it is associated. Furthermore, the potential for additional feed forward processing involving several forebrain functional anatomical systems, inlcuding the ventral striatopallidum, extended amygdala and magnocellular basal forebrain complex is considered. It is intended that from the considerations recorded here a conceptual framework will begin to emerge that is amenable to further experimental substantiation as regards how multiple basal forebrain systems and the cortices to which they are related by connections work together to fashion a unitary object--the adaptive response.

Dissociation in effects of lesions of the nucleus accumbens core and shell on appetitive pavlovian approach behavior and the potentiation of conditioned reinforcement and locomotor activity by D-amphetamine

Dopamine release within the nucleus accumbens (NAcc) has been associated with both the rewarding and locomotor-stimulant effects of abused drugs. The functions of the NAcc core and shell were investigated in mediating amphetamine-potentiated conditioned reinforcement and locomotion. Rats were initially trained to associate a neutral stimulus (Pavlovian CS) with food reinforcement (US). After excitotoxic lesions that selectively destroyed either the NAcc core or shell, animals underwent additional CS-US training sessions and then were tested for the acquisition of a new instrumental response that produced the CS acting as a conditioned reinforcer (CR). Animals were infused intra-NAcc with D-amphetamine (0, 1, 3, 10, or 20 microg) before each session. Shell lesions affected neither Pavlovian nor instrumental conditioning but completely abolished the potentiative effect of intra-NAcc amphetamine on responding with CR. Core-lesioned animals were impaired during the Pavlovian retraining sessions but showed no deficit in the acquisition of responding with CR. However, the selectivity in stimulant-induced potentiation of the CR lever was reduced, as intra-NAcc amphetamine infusions dose-dependently increased responding on both the CR lever and a nonreinforced (control) lever. Shell lesions produced hypoactivity and attenuated amphetamine-induced activity. In contrast, core lesions resulted in hyperactivity and enhanced the locomotor-stimulating effect of amphetamine. These results indicate a functional dissociation of subregions of the NAcc; the shell is a critical site for stimulant effects underlying the enhancement of responding with CR and locomotion after intra-NAcc injections of amphetamine, whereas the core is implicated in mechanisms underlying the expression of CS-US associations.

Direct comparison of projections from the central amygdaloid region and nucleus accumbens shell

Certain neurochemical and connectional characteristics common to extended amygdala and the nucleus accumbens shell suggest that the two represent a single functional-anatomical continuum. If this is so, it follows that the outputs of the two structures should be substantially similar. To address this, projections from the caudomedial shell and central nucleus of the amygdala, a key extended amygdala structure, were demonstrated in Sprague-Dawley rats with different anterograde axonal tracers processed separately to exhibit distinguishable brown and blue-black precipitates. The caudomedial shell projection is strong in the ventral pallidum and along the medial forebrain bundle through the lateral preopticohypothalamic continuum into the ventral tegmental area, distal to which it thins abruptly. The central nucleus projects strongly to the bed nucleus of the stria terminalis and the sublenticular extended amygdala, but substantially to the lateral hypothalamus only at levels behind the rostral part of the entopeduncular nucleus. Innervation of the ventral tegmental area by the central amygdala is minimal, but the lateral one-third of the substantia nigra, pars compacta and an adjacent lateral part of the retrorubral field receive substantial central amygdala input. Central amygdaloid projections are robust in caudal brainstem sites, such as the reticular formation, parabrachial nucleus, nucleus of the solitary tract and dorsal vagal complex, all of which receive little input from the accumbens. The substantial differences in the output systems of the caudomedial shell of accumbens and central amygdala suggest that the two represent distinct functional-anatomical systems.

Injections of N-methyl-D-aspartate into the ventral hippocampus increase extracellular dopamine in the ventral tegmental area and nucleus accumbens

The nucleus accumbens septi receives inputs from dopaminergic neurons of the ventral tegmental area (VTA) and glutamatergic neurons of the ventral subiculum (VS). The convergence of these inputs in the NAS is important for the normal expression of exploratory locomotion; stimulation of the VS by injection of the glutamate receptor agonist N-methyl-D-aspartate (NMDA) causes dopamine-dependent increases in locomotion. In the present study, in vivo microdialysis in conjunction with high-performance liquid chromatography and electrochemical detection (HPLC-EC) was used to estimate changes in extracellular dopamine in the VTA and NAS in response to intra-VS injections of NMDA (0.074, 0.28, 0.74 microg). NMDA injections caused dose-dependent elevations in extracellular dopamine in each region. Each dose of NMDA clearly increased extracellular dopamine in the NAS, whereas only the two higher doses increased dopamine significantly in the VTA. The highest dose of NMDA elevated extracellular dopamine to approximately 180% of baseline in each region. Whereas elevations in NAS dopamine might be induced by impulse-independent local mechanisms, elevations of dopamine in the VTA are presumed to reflect increased somatodendritic release associated with increased impulse flow through dopamine neurons. Thus, the present study suggests that the modulation of dopaminergic neurotransmission by the ventral subiculum results from a trans-synaptic activation of dopamine cell bodies in the VTA.

Motor activation by amphetamine infusion into nucleus accumbens core and shell subregions of rats differentially sensitive to dopaminergic drugs

Selective breeding based on activity in a swim test has been used to produce lines of rats that show a high level of activity in the swim test (Swim High-active (SwHi) rats) and a low level of activity in the swim test (Swim Low-active (SwLo) rats). Previous studies have indicated that dopamine (DA) function is enhanced in SwHi rats and reduced in SwLo rats; a principal finding was that SwLo rats showed much smaller increases in ambulatory activity after systemic administration of amphetamine than did SwHi or non-selected rats. In light of the importance of the nucleus accumbens (NAC) in amphetamine-induced activity, the present study investigated whether DA function in NAC differs in SwHi and SwLo rats. Amphetamine was infused bilaterally into either the core or shell subregion of NAC, and ambulation or swim test activity was then measured. In SwLo rats, infusion of amphetamine (0.2-2.0 microg) into either NAC core or shell produced moderate increases in ambulation. In SwHi rats, infusion of amphetamine into NAC shell produced similar moderate increases in ambulation, but infusion into the core produced markedly larger dose-related increases in ambulation. In the swim test, infusion of amphetamine (1.0 microg) increased activity by affecting the dominant behavior of each line; i.e. struggling increased in SwHi rats and floating decreased in SwLo rats, with large effects seen in both lines with infusion into either NAC core or shell. These results support the idea that the distinct behavioral characteristics of SwHi and SwLo rats are mediated in part by differences in NAC-DA function.

Phasic firing time locked to cocaine self-infusion and locomotion: dissociable firing patterns of single nucleus accumbens neurons in the rat

The activity of single nucleus accumbens (NAcc) neurons of rats was extracellularly recorded during intravenous cocaine self-administration sessions (0.7 mg/kg per infusion, fixed ratio 1). We reported previously that NAcc neurons showed a change, usually a decrease, in firing rate during the first 1 min after the cocaine-reinforced lever press. This postpress change was followed by a progressive reversal of that change, which began within the first 2 min after the press and was not complete until the last 1 min before the next lever press (termed the change + progressive reversal firing pattern). In the present study we documented a regular pattern of locomotion that occurred in parallel with the change + progressive reversal firing pattern. This observation suggested that discharges time locked to locomotion may determine the change + progressive reversal firing pattern. However, 55% of the neurons failed to show firing time locked to locomotion that could have contributed to the change + progressive reversal firing pattern. Moreover, for all neurons, the change + progressive reversal firing pattern was apparent even if the calculation of firing rate excluded all periods of locomotion. The present data showed that the change + progressive reversal firing pattern is not solely attributable to phasic changes in firing time locked to the execution of locomotion. The change + progressive reversal firing pattern closely mirrors changes in drug level and dopamine overflow observed by previous researchers and may thus be a component of the neurophysiological mechanism by which drug level regulates drug-taking behavior during an ongoing self-administration session.

Effects of bilateral electrolytic lesions of the medial nucleus accumbens on exploration and spatial learning

Rats with electrolytic lesions of the medial part of the nucleus accumbens, comprising the shell region, were compared to sham-operated rats in tests of exploration in a T-maze, in a hole-board, and in an elevated (+)-maze and in a test of water maze spatial learning. Rats with medial nucleus accumbens lesions had higher choice latencies than sham-operated controls during the beginning of the spontaneous alternation test. A higher number of hole pokes was found in the lesioned group, but only during the beginning of the second day of testing. In the elevated (+)-maze, lesioned rats had a higher number of closed and total arm entries and spent more time in the center region. The lesioned group did not differ from the control group for the number of alternations in the T-maze, for horizontal and vertical motor activity in the hole-board, and for acquisition or reversal of spatial learning in the Morris water maze. These results indicate that lesions of the medial nucleus accumbens slowed down decision time during spontaneous alternation testing and increased exploration in a time and test-specific manner without altering acquisition of a reference memory task.

Comparison of the effects of dopamine agonists on self-stimulation of the hypothalamus with lesioning of mesolimbic brain structures in rats reared in conditions of social isolation

Initial and phenamine-stimulated frequencies of self-stimulation of the lateral hypothalamus were not significantly different in rats reared in communities and in conditions of social isolation. Unilateral lesioning of the ventral tegmental area and the medial prefrontal cortex in early ontogenesis increased phenamine sensitivity only in isolated rats. The dopamine receptor agonist apomorphine, at a dose of 0.05 mg/kg, which affects presynaptic receptors, inhibited the self-stimulation response in intact group-reared animals and in rats reared in isolation, by 21-23%. Stimulation of the ventral tegmental area did not change, while stimulation of the medial prefrontal cortex doubled the sensitivity of rats to apomorphine in animals reared in isolation (at doses of 0.05 and 0.5 mg/kg). Conditions of partial sensory and complete species isolation resulted in the development of a state of presynaptic receptor hypersensitivity of dopamine receptors in mesocorticolimbic brain systems in rats.

Study of the behavioral effects of bilateral nucleus accumbens lesions on amphetamine and apomorphine in adult cats

The aim of the present work was to study the effects of three different types of bilateral lesions performed on the nucleus accumbens, upon the behaviors elicited in adult cats by parenteral administration of amphetamine and apomorphine, and to obtain an understanding of the functional role played by the cited structure. To this end, 10 cats received bilateral injections of 6-OHDA, 18 microg in each accumbens; 8 cats received a similar treatment with ibotenic acid (20 microg), and 11 cats were submitted to bilateral electrolytic damage. Before and after performing these lesions, in separate sessions, amphetamine (2.5 mg/kg SC) and apomorphine (2.0 mg/kg SC) were administered and their respective behaviors were compared. Besides, in a group of 10 cats, 6 of them were bilaterally injected with the above cited dose of 6-OHDA into the accumbens to determine dopamine concentration and the other four served as control. In three cats, ibotenic acid (20 microg) was unilaterally injected into the accumbens for histological analysis. The contralateral structure served as control. Finally, four cats were sham operated. The results obtained show that the accumbens in cats participates in locomotion, in stereotyped motor behaviors, and in emotional fear-like behavior. Its role in the production of motor behaviors apparently is not as important as has been reported in rodents.

Mapping of locomotor behavioral arousal induced by microinjections of dopamine within nucleus accumbens septi of rat forebrain

Dopamine (DA) at ca. ED50 (16 microg) or saline was stereotaxically microinjected unilaterally 2 h after pretreatment with an MAO inhibitor into left or right nucleus accumbens septi of 697 freely moving rats (1394 injections) to define subregions involved in DA-induced behavioral arousal throughout the anatomical extent of the accumbens. Locomotion was quantified electronically and behavioral responses were assigned to histologically verified injection sites; postural or stereotyped behaviors characteristic of DA injections in caudate-putamen did not occur. Screening with 60 injections across mid-accumbens (2.2-3.2 mm rostral to bregma) indicated that locomotion was elicited non-homogeneously, and was particularly intense dorsomedially. Sites yielding intense arousal and their inactive surround were mapped along the rostrocaudal axis (1.4-4.2 mm anterior to bregma) in coronal sections. Responses to DA showed lateral symmetry and were similar across rostrocaudal levels, with intense responses in dorsomedial accumbens along its border with the caudate-putamen. This functional localization does not coincide closely with reported distributions of DA or its receptors, nor with histologically or histochemically defined core-shell regions of this limbic structure. Nucleus accumbens in rat brain thus appears to be organized functionally into distinct subregions differing markedly in ability to produce locomotor hyperactivity in response to exogenous DA.

The effects of limbic lesions on locomotion and stereotypy elicited by dopamine agonists in the rat

The purpose of this experiment was to investigate the functional contributions of various limbic structures to locomotion and stereotypy induced by dopaminergic drugs. Female rats were randomly assigned to one of 5 groups (n = 10-14 rats/group) that received either a lesion of the hippocampus (colchicine + kainic acid), basolateral amygdala (quinolinic acid), frontal cortex (aspiration), nucleus accumbens (ibotenic acid), or served as unoperated controls. Beginning at least 2 weeks following surgery locomotion (measured as photocell beam breaks) elicited by D-amphetamine (0.0, 0.32, 1.0 and 3.2 mg/kg), SKF 82958 (0.0, 0.04, 0.08 and 0.16 mg/kg) or quinpirole (0.0, 0.25, 0.1 and 0.5 mg/kg) was determined. In agreement with previous results rats with hippocampal lesions were hyperactive in response to amphetamine. In comparison to these changes in drug-induced locomotion, lesions of the basolateral amygdala, and frontal cortex had only minor effects on drug-induced locomotion. Lesions of the nucleus accumbens produced consistent hyperactivity that was suppressed by doses of amphetamine or quinpirole that elicited behavioral stereotypy. These results provide evidence suggesting that, in comparison to other limbic structures that have substantial inputs to the nucleus accumbens, the hippocampus play a relatively prominent role in the modulation of drug-induced locomotion.

Differential involvement of the shell and core subterritories of the nucleus accumbens in latent inhibition and amphetamine-induced activity

Latent inhibition (LI) consists of retardation in conditioning to a stimulus as a consequence of its prior non-reinforced pre-exposure. In view of findings that LI is disrupted in acute schizophrenic patients and evidence from animal experiments pointing to the involvement of the mesolimbic dopamine (DA) system in this phenomenon, the present study investigated the effects of electrolytic lesions to the shell and core subterritories of the nucleus accumbens on LI in rats (Expt. 1). LI was indexed by the amount of suppression of drinking in the presence of a tone that was either pre-exposed or not prior to its pairing with reinforcement (a foot shock). Expt.2 tested the effects of the DA antagonist, haloperidol, on LI in shell- and core-lesioned animals. Expt. 3 tested the effects of shell and core lesions on spontaneous and amphetamine-induced locomotion. In Expt. 1, LI, i.e., lower suppression of drinking in the pre-exposed as compared to the non-pre-exposed animals, was obtained in the sham-operated condition. Core and shell lesions produced distinct effects on LI. Animals with core lesions developed LI, but exhibited an overall lower suppression of drinking in comparison to the sham-operated animals. In contrast, shell lesions led to a disappearance of LI. Expt. 2 replicated the differential effects of shell and core lesions on LI, although in this experiment, core lesion did not attenuate suppression of drinking. Haloperidol prevented shell-induced abolition of LI. In Expt. 3, shell- but not core-lesioned animals were more active than sham controls following amphetamine administration. These results provide evidence for functional differences between the shell and core subregions, as well as for the involvement of the mesolimbic DA system in LI.

Neonatal hippocampal lesions induced hyperresponsiveness to amphetamine: behavioral and in vivo microdialysis studies

The effect of neonatal hippocampal lesions on behavioral sensitivity to amphetamine (AMPH) and dopamine (DA) release in the nucleus accumbens (NAc) were examined. The ventral hippocampus was damaged bilaterally by ibotenic acid on postnatal day 7 (PD7). Spontaneous exploration and AMPH-stimulated locomotor activity were examined on postnatal day 35 (PD35) and day 56 (PD56). Extracellular DA, dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and 5-hydroxyindoleacetic acid (5-HIAA) were sampled using in vivo microdialysis while simultaneously AMPH-stimulated locomotion was examined in freely moving rats on PD56. Spontaneous exploration increased in rats with hippocampal lesions relative to controls on PD56 but not PD35. AMPH (0, 0.187, 0.375, 0.75, 1.5, and 3 mg/kg) enhanced locomotion dose-dependently in both control and lesioned groups. Locomotor activity was higher in lesioned rats than controls following AMPH at the dose of 0.75 mg/kg on PD35 and at the doses of 1.5 and 3.0 mg/kg on PD56. The basal level of DA in the NAc was not different between the hippocampal and control groups. AMPH (1.5 mg/kg) induced hyperlocomotion in lesioned rats relative to controls. DA release in the NAc for both groups was enhanced following injections of AMPH. However, neonatal hippocampal lesions had no further enhancement on AMPH-stimulated release of DA as compared to the control group. The levels of DOPAC and HVA in the NAc were altered by AMPH but not lesions. The level of 5-HIAA was not influenced by either lesions or AMPH. The results of neonatal lesion-induced hyperlocomotion suggest that an emergence of behavioral hyperresponsiveness to AMPH was dependent on an interaction of lesions, age of examination, and dose of the drug. A dissociation between the effect of AMPH on lesion-enhanced hyperlocomotion and a lack of a lesion-enhanced DA release in the NAc suggest that presynaptic release of DA had no major contribution to lesion-enhanced DA transmission in the mesolimbic DA system.

Differential effects of ventral striatal lesions on the conditioned place preference induced by morphine or amphetamine

The present experiment examined the role of the ventral striatum in the rewarding effect of morphine and amphetamine by testing whether lesions of cell bodies within this region disrupt the development of a conditioned place preference to either drug. Bilateral, N-methyl-D-aspartate- or kainic acid-induced lesions of the ventral striatum block a conditioned place preference to amphetamine (1.5 mg/kg x 3 pairings) but not to morphine (2 mg/kg x 3 pairings). Because both lesions spared anterior portions of the ventral striatum, we examined the effect of larger or more selective ventral striatal lesions on a conditioned place preference induced by morphine. Destruction of the entire ventral striatum reduced, but did not eliminate, a conditioned place preference to morphine, whereas selective lesions of the anterior ventral striatum were ineffective. These results indicate that the ventral striatum is not critically involved in morphine's rewarding effect and support the suggestion that the rewarding effects of opiates and stimulants do not involve identical neural substrates.

The nucleus accumbens in monkeys (Macaca fascicularis): II. Emotion and motivation

Changes in incentive and emotion have been demonstrated in monkeys with amygdala lesions and monkeys with cingulate and medial frontal lesions. The nucleus accumbens (NA) receives inputs from the amygdala, hippocampus and anterior cingulate cortex. In order to better understand the role of the NA and anterior cingulate cortex in processing emotional and motivational stimuli, studies were undertaken which compared the emotional and motivational behaviour of monkeys with NA lesions or anterior cingulate lesions with previous studies on amygdala-lesioned monkeys. A food preference task, a food vs. non-food discrimination task, and a approach-avoidance task were used with monkeys which received lesions of the NA or lesion of the anterior cingulate and medial frontal cortex. These tasks had previously been used to examine the emotional response of monkeys with amygdala lesions. In addition, the lesioned monkeys were tested on a frustration tasks and a button press acquisition-extinction task. Unlike amygdala-lesioned monkeys (Aggleton, J.P. and Passingham, R.E., J. Comp. Physiol. Psychol., 96 (1981) 961-977 and 96 (1982) 71-77), the NA-lesioned monkeys maintained normal food preferences, did not show signs of hyperorality in the food vs. non-food task, and performed normally on the approach-avoidance tasks. The NA-lesioned monkeys did, however, show an increase in activity, and violent and aggressive behaviour in response to stress in both the frustration task and the button press extinction task. In addition, the NA-lesioned monkeys performed normally during a button press acquisition task, but extinguished faster on a button press extinction task than the control monkeys. The anterior cingulate-lesioned monkeys were also found to exhibit an increased responsiveness to frustration. Results of the food preference, food vs. non-food discrimination, and approach-avoidance tasks were similar to those obtained with NA lesioned. These studies suggest that lesions of the NA or the anterior cingulate cortex result in substantial changes in emotional behavior, however, these changes do not mimic those found following lesions of the amygdala.

Excitotoxic lesions of the core and shell subregions of the nucleus accumbens differentially disrupt body weight regulation and motor activity in rat

The behavioral effects of bilateral N-methyl-D-aspartate (NMDA) lesions of the core and medial shell subregions of the nucleus accumbens were evaluated in rats. Body weight was monitored for 2 weeks following surgery. Locomotor activity and open field behavior were recorded 1 week after surgery. The core-lesion group had difficulty recovering from the lesion and had significantly lower weights throughout the experiment. The shell-lesion group had normal recovery and weighed significantly more than controls over the course of the experiment. In the activity cage test, the core-lesion group was hyperactive when compared to controls and to the shell-lesion group. Activity of the shell-lesion group was similar to that of their sham-controls. Three weeks postlesion, the core-lesion group was still significantly more active. In the open field test, peripheral locomotion scores were significantly higher in the core-lesion group when compared to their controls, whereas the scores of the shell-lesion group were similar to controls. In the other open field measures, both lesion groups were hyperactive; however, the scores of core-lesion group were significantly higher than those of the shell-lesion group on all measures. Histological analysis indicated small, discrete areas of damage within the core or medial shell accumbens regions. These preliminary results suggest that these two subregions can be behaviorally differentiated.

Disturbance of meal patterning following nucleus accumbens lesions in the rat

In this study, we examined the effects of electrolytic lesions of the nucleus accumbens which had no obvious effect on body weight, or on the short term intake of solid food, sucrose and salt solutions. However in 24 h records of feeding and drinking lesioned animals took many more meals of shorter duration. Challenge with the dopamine D2 antagonist YM-09151-2 resulted in a decrease in feeding rate, together with a facilitation of meal size in drinking, in both control and lesioned subjects.

Dopamine receptors: molecular biology, biochemistry and behavioural aspects

The description of new dopamine (DA) receptor subtypes, D1-(D1 and D5) and D2-like (D2A, D2B, D3, D4), has given an impetus to DA research. While selective agonists and antagonists are not generally available yet, the receptor distribution in the brain suggests that they could be new targets for drug development. Binding characteristics and second messenger coupling has been explored in cell lines expressing the new cloned receptors. The absence of selective ligands has meant that in vivo studies have lagged behind. However, progress has been made in understanding the function of DA-containing discrete brain nuclei and the functional consequence of the DA's interaction with other neurotransmitters. This review explores some of the latest advances in these various areas.

Motoric sensitization and levodopa accumulation after chronic levodopa treatment in an animal model of Parkinson's disease

One month after rats were subjected to unilateral injections of either 6-hydroxydopamine (6-OHDA) or vehicle into the midbrain tegmentum, they were given daily injections of either saline or levodopa (10 mg/kg and 1 mg/kg carbidopa) for 30 days. On the first and last day of treatment the spontaneous behavior of the rats was evaluated with a video image analysis system that detected directional movement asymmetries. Although vehicle-injected rats exhibited very little movement asymmetry, the 6-OHDA rats were strongly asymmetric. On day 1, both saline and levodopa-treated 6-OHDA rats exhibited rotational movement directed toward the dopamine-deficient hemisphere. On day 30 of treatment, however, the chronic levodopa group displayed a complete reversal and exaggeration of the rotational bias, and all asymmetric movement was directed toward the dopamine-intact hemisphere. Thus chronic levodopa treatment shifted behavioral dominance from the intact to the dopamine-denervated hemisphere. Subsequent biochemical measurement of dopamine and levodopa in striatal and limbic tissue samples indicated that chronic levodopa treatment did not alter dopamine tissue concentrations but did substantially increase levodopa concentrations, both in the dopamine-denervated striatum and in limbic tissue. This increased levodopa loading in brain with chronic levodopa treatment occurring in 6-OHDA rats but not in vehicle-injected rats that were given the same levodopa regimen. This selectivity in the effect of chronic levodopa treatment to the 6-OHDA rats appeared to rule out the possibility of peripheral metabolic factors for the levodopa accumulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of striatal or accumbens lesions on the amphetamine-induced abolition of latent inhibition

In this study, we tested the effects of nucleus accumbens or corpus striatum lesions on the abolition of latent inhibition induced by d-amphetamine. In the latent inhibition paradigm, animals learn to ignore a repeatedly presented nonreinforced stimulus. In this paradigm, the repeated nonreinforced preexposure to a stimulus retards subsequent conditioning to that stimulus. Pharmacological manipulations that enhance the dopaminergic function (e.g., d-amphetamine) abolish this ability to ignore an irrelevant stimulus. Previous studies have revealed a major role of the nucleus accumbens in the d-amphetamine-induced abolition of latent inhibition because intraacumbens injections of the drug mimic its systemic effects. The results of this study, however, revealed a significant increase in the disruption of latent inhibition by d-amphetamine between corpus striatum-lesioned and sham-operated rats, but a marginal difference between nucleus accumbens lesioned and sham-operated rats, which had been preexposed to the stimulus. These findings indicate that the corpus striatum plays also a major role in the disruption of latent inhibition by d-amphetamine. It seems, therefore, that the nucleus accumbens and corpus striatum may represent a functionally common system regarding the expression of latent inhibition, although different experimental manipulation can favor the one structure over the other, reflecting probably their complex function.

Regulatory behaviour, exploration and locomotion following NMDA or 6-OHDA lesions in the rat nucleus accumbens

The effects of bilateral, N-methyl-D-aspartate (NMDA)-induced lesions of the nucleus accumbens (N.Acc.) on regulatory and behavioural responding were studied in rats and compared with the effects of bilateral 6-hydroxydopamine (6-OHDA) lesions. After postoperative body weight, food and water intake had been monitored for a period of 4 weeks, rats were tested in an exploration-choice box. Spontaneous locomotion and the locomotor and stereotypy responses to different doses of dopaminergic agonists were measured subsequently. Detailed assessment of NMDA-induced lesion volumes showed that on average 81.53% of total N.Acc. area was damaged, depending on excitotoxin dose. Tyrosine hydroxylase immunohistochemistry was used to confirm loss of mesolimbic dopamine neurones following 6-OHDA. Analysis of the behavioural data showed that NMDA N.Acc. lesions significantly enhanced exploratory behaviour, spontaneous locomotor activity and the locomotor response to a low dose of D-amphetamine. By comparison, 6-OHDA lesions did not affect exploration and spontaneous locomotion but significantly attenuated the locomotor response to a low dose of D-amphetamine. Regulatory responses were unaffected 28 days after surgery, although NMDA-lesioned rats took longer to recover from postoperative hypodipsia. The results suggest that NMDA N.Acc. lesions induce a deficit in the control of general locomotor output and are consistent with the hypothesis that the N.Acc. functions as an interface between sensory input and locomotor output and that it is needed to channel activity levels appropriately.

Behavioral effects of amphetamine and apomorphine after striatal lesions in the rat

It is well established that denervation of the dorsal striatum by its dopaminergic afferents attenuates the stereotyped response to d-amphetamine, which can be considered as an extreme form of motor activation. However, it is difficult to sustain the view that this structure serves primary motor control function because the role of dopamine in the striatum remains difficult to understand. In this study, we compared the effects of two dopaminergic agonists, d-amphetamine and apomorphine, after dorsal striatal lesions with ibotenic acid using a computerized scoring of the behavior. Although d-amphetamine- and apomorphine-induced locomotor activity was no different between lesioned and nonlesioned rats in photobeam activity cages, the structure of their behavioral pattern was quite different. Freezing, a usual response after d-amphetamine, was blocked by the lesion. Lesioned rats exhibited less standing than nonlesioned after d-amphetamine, apomorphine, or saline treatment. Moving was increased in lesioned rats after a low dose of d-amphetamine (0.5 mg/kg) or apomorphine (0.5 mg/kg), while d-amphetamine induced in the same rats an increase of rearing. Stereotyped behavior after both drugs at high doses was not affected by striatal lesion. These results indicate that the dorsal striatum is not involved only in the control of stereotypy, as has been suggested using 6-hydroxydopamine lesions, but also plays a major role in the mediation of behavioral activation in response to stimulant drugs.

Locomotor bias produced by intra-accumbens injection of dopamine agonists and antagonists

Several experiments have shown that the dopamine (DA) receptors in the nucleus accumbens control the intensity of locomotor activity; however, there are several contradictory results concerning the role of the accumbens in the regulation of the direction of locomotion. To further evaluate the contribution of dopaminergic function in the accumbens to the direction of locomotion, we first compared the effect on the direction of locomotor activity of unilateral intra-accumbens injections of the nonspecific DA antagonist haloperidol, the specific D-1 antagonist SCH-23390, the specific D-2 antagonist metoclopramide. In the second part of the experiment, we examined the effect on the direction of locomotor activity of unilateral intra-accumbens injections of the non-specific DA agonist apomorphine, the specific D-1 agonist SKF-38393, the specific D-2 agonist LY-171555, and the combination of SKF-38393 and LY-171555. Haloperidol, metoclopramide and to a lesser extent, SCH-23393 together with peripheral amphetamine injections produced a locomotor bias that resulted in ipsilateral turning. Apomorphine, LY-171555 or the combination of SKF-38393 and LY-171555 (but not SKF-38393 alone) produced a locomotor bias that resulted in contralateral turning. No significant locomotor bias was produced by intra-accumbens injection of the various vehicles. These results suggest that the bilateral DA organization thought to exist in the nigro-striatal pathway for the control of locomotion may also be true for the mesolimbic dopamine system.

The effects of excitotoxic lesions of the nucleus accumbens on a matching to position task

The effects of bilateral ibotenic acid lesions of the nucleus accumbens in the rat were examined on a delayed matching to position task. The lesion induced a stable delay-dependent performance deficit suggestive of a short-term memory problem. Following analysis of the impaired performance on trials with long delays, using measures akin to signal detection indices, the deficit was interpreted as being largely due to the intrusion of a side-dependent response bias. Low-dose amphetamine (0.75 mg/kg) produced a similar disruption in the sham-operated rats, both in terms of accuracy impairment and degree of strategical bias. As well as mimicking the effects of low-dose amphetamine in sham-operated rats, the lesion also protected against the disruptive effects of the drug at low doses. Whilst exhibiting no performance deficit when the matching schedule lacked a delay component, the lesioned rats were very significantly impaired in switching their response strategies when exposed to a series of reversals. In addition, the lesioned rats were remarkably resistant to an extinction procedure. Both these findings indicate that the lesioned rats were unable to exhibit flexibility in their response patterns. These results, taken together with those of the delayed matching procedure, imply that one of the functions of the intact nucleus accumbens is to inhibit habitual responding under conditions of non-reward and low stimulus control, possibly via its connections to the dorsal striatal system.

Dissociable roles of the ventral, medial and lateral striatum on the acquisition and performance of a complex visual stimulus-response habit

The effects of discrete bilateral ibotenic acid lesions to 3 areas of striatum were examined on a conditional visual discrimination task involving temporal frequency (SLOW vs FAST flashes) that had previously been shown to be sensitive to the effects of dorsal striatal dopamine depletion. Two of the groups, namely, those with nucleus accumbens (ACC) and lateral caudate-putamen (LCP) lesions, were very disrupted in the acquisition of the task. The nature of the respective impairments of the 2 groups was dissociable, however. The performance of the ACC group could be improved either by manipulations of stimulus duration or inter-stimulus interval, implying an attentional deficit. In contrast, the rats with lesions of the LCP were not significantly improved by any of the behavioural challenges. Their performance was characterised by a bias to respond to the SLOW discriminandum. Under conditions of non-reward, the LCP group extinguished their responding at a similar rate to control rats whereas the ACC group were very much more persistent. Lesions of the medial caudate-putamen failed to affect any index of performance significantly. These data suggest that the LCP is necessary for the acquisition of arbitrary stimulus-response rules and that damage to an equivalent area in humans, such as in Huntington's disease, may explain deficits of procedural memory. The second part of the experiment investigated the effects of ACC lesions on established performance of the schedule. The lesioned group behaved identically to the ACC group that had been lesioned prior to acquisition, both in terms of accuracy and degree of persistence in extinction, further implying the role of attentional factors and inflexibility in the lesion-induced deficit.

Hippocampal modulation of nucleus accumbens: behavioral evidence from amphetamine-induced activity profiles

The experiments examined amphetamine-induced locomotion and stereotyped behavior in hippocampal-ablated and control rats for 30 days following surgery. Locomotor counts, stereotypy ratings, and locomotor-time profiles showed that d-amphetamine sulfate produced a selective enhancement of locomotion (cage crosses) at the expense of stereotyped behavior in hippocampal rats relative to normal control rats. This enhancement emerged over the first 2 weeks postsurgery. To examine the role of the striatum in this amphetamine-induced effect, combined hippocampal damage and 6-hydroxydopamine-induced damage of the nucleus accumbens or caudate-putamen were used. These results suggested that amphetamine-enhanced locomotion of hippocampal rats is dependent upon the integrity of the nucleus accumbens and may reflect a change of nucleus accumbens activity relative to caudate-putamen activity. Together these findings suggest that the hippocampus may participate in the control of locomotion by projections that modulate the activity of the nucleus accumbens.

A comparative study of the behavioral effects of d-amphetamine and apomorphine in the rat

A wide range of doses of d-amphetamine and apomorphine were injected into rats, in order to better characterize and compare dopaminergic agonist-induced behavioral effects. The study was carried out using a computerized technique for the quantification and analysis of various behavioral elements. Although both drugs increased motor activity and provided dose-dependent stereotyped responses, the whole pattern of behavior in the open field showed a different structure. d-Amphetamine in doses that did not produce stereotyped responses induced a wide range of varied behavioral elements with increased frequency but decreased mean duration, while apomorphine induced a more restricted behavioral profile. Furthermore, a higher frequency of freezing reaction was observed after d-amphetamine treatment in low doses but not after apomorphine treatment. Both drugs in high doses elicited a similar stereotyped syndrome characterized by repetitive movements of great duration, but at low doses the behavioral pattern was completely different. The apomorphine-induced syndrome was characterized mainly by moving and sniffing, interrupted by rearing, while the amphetamine-induced syndrome by sniffing and moving, interrupted by standing and freezing.

Role of dopamine and GABA in the control of motor activity elicited from the rat nucleus accumbens

The application of 1.2 and 12.0 micrograms/side of the GABAA receptor agonist 3-aminopropane sulphonic acid bilaterally into the nucleus accumbens (Acb) of rats nonsignificantly depressed locomotor activity as assessed in automated Animex activity cages, while the highest dose (60 micrograms/side) significantly stimulated activity. The GABAA receptor antagonists picrotoxinin (0.0625 and 0.125 micrograms/saide) and bicuculline (0.895 micrograms/side) produced forward locomotion around the cage accompanied by a number of other behaviours. The GABAB agonist baclofen (0.023 and 0.092 micrograms/side) induced a short-lasting (18 min) locomotor depression. None of the GABAB antagonists tested (2-hydroxysaclofen 2.6 micrograms/side, two novel beta-(benzo[b]furan) analogues of baclofen 9G or 9H each 6.8 micrograms/side, 4-aminobutylphosphonic acid 1.32 micrograms/side and phaclofen 0.535 and 2 micrograms/side) significantly affected locomotor activity. In rats pretreated with reserpine and alpha-methyl-p-tyrosine, picrotoxinin (0.0625 and 0.125 micrograms/side) did not significantly alter locomotor activity. Furthermore, when picrotoxinin (0.0625 micrograms/side) was combined with either the selective dopamine (DA) D1 agonist SKF38393 or the selective D2 agonist quinpirole, no significant alteration in locomotor function occurred. When SKF38393 and quinpirole were coadministered, significant stimulation occurred which was further enhanced by the addition of picrotoxinin. It is concluded that GABAA receptors, together with D1 and D2 receptors, play a major role in modulating the control of motor function by the Acb of rats.

Bidirectional potentiation between D1 and D2 dopamine agonists: effects of unilateral intra-accumbens injections on locomotor activity in mice

We tested the effect of a single unilateral injection of a specific D1 agonist into the nucleus accumbens on the behavioral response to a subsequent unilateral intra-accumbens injection of a selective D2 agonist ten days later. The effect of the inverse order of presentation (D2 agonist followed ten days later by a D1 agonist) was also tested. No significant differences between the locomotor effects of the intra-accumbens injection of either SKF-38393 (3.5 micrograms) or LY-171555 (10 micrograms) were observed during the first test. Ten days later, during the second test, intra-accumbens injection of either the LY-171555 and SKF-38393 increased the percentage of contralateral rotations relative to the first test while LY-171555 also increased the total number of rotations. Control injections showed that these effects of LY-171555 and SKF-38393 were not due to a conditioning process. Rather, the results suggested that the locomotor changes observed during the second test were the result of behavioral sensitization due to the initial acute injection of the agonists.

Hyperactivity following intradentate injection of colchicine: a role for dopamine systems in the nucleus accumbens

The role of dopamine systems in the nucleus accumbens (NA) in mediating the hyperactivity following colchicine-induced granule cell damage was investigated. In the first experiment adult Sprague-Dawley rats were bilaterally injected with 6-hydroxydopamine (6-OHDA; 8 micrograms/2 microliters) or 0.5% ascorbate into the NA. Eight days later, rats received intradentate COLCH or CSF and were tested for locomotor activity 2, 4, 6, 8, and 10 days later. Intradentate COLCH produced a significant hyperactivity which was prevented by prior injection of 6-OHDA into the NA. Neurochemical analysis revealed that 6-OHDA decreased dopamine (80%) but not norepinephrine in the NA without altering either catecholamine in the striatum. In the second experiment animals were injected with COLCH or CSF in the dentate gyrus and immediately implanted with bilateral cannulae in the NA. Animals received intraaccumbens injections of CSF or d-amphetamine (20 micrograms/l microliters) and were tested for locomotor activity. Amphetamine produced a significant increase in locomotor activity in both CSF- and COLCH-treated animals. However, COLCH produced an exaggerated response to the motor stimulating effects of amphetamine. These results suggest that the destruction of dentate granule cells following colchicine results in a "functional" hyperactivity of the mesolimbic dopamine input to the NA which might disinhibit locomotor activity.

Actions of cocaine on rat nucleus accumbens neurones in vitro

1. Intracellular recordings were made from 103 neurones of the rat nucleus accumbens in vitro. 2. Dopamine (3-100 microM; in sulpiride, 1 microM) hyperpolarized neurones (79%) by acting at D1 receptors: dopamine (3-100 microM; in SCH23390, 1 microM) depolarized neurones (55%) by acting at D2 receptors. 5-Hydroxytryptamine (1-100 microM) depolarized 86% neurones. 3. Both actions of dopamine as well as the effect of 5-hydroxytryptamine were potentiated by cocaine (0.3-30 microM), which had no effect of its own on membrane potential. 4. Dose-ratio was computed as [(concentration of agonist causing a 4 mV potential change in cocaine)/(concentration of agonist causing a 4 mV potential change without cocaine)]. Cocaine (1-30 microM) caused the same dose-ratio whether dopamine depolarizations (D2) or hyperpolarizations (D1) were measured; the dose-ratio ranged from 2 (1 microM) to 50 (30 microM). 5. Responses to 5-hydroxytryptamine were increased more than responses to dopamine; cocaine 1 microM gave a dose-ratio of 13.4 and at 30 microM gave a dose-ratio of 118. 6. It is concluded that cocaine acts to inhibit the uptake of dopamine and 5-hydroxytryptamine in slices of rat nucleus accumbens; lower concentrations of cocaine (0.3 to 1 microM) are particularly effective in potentiating the action of 5-hydroxytryptamine.

Effects of repeated apomorphine and haloperidol treatments on subsequent behavioral sensitivity to apomorphine

In a 2 x 2 factorial design, four groups of rats (n = 10 each) were injected daily with haloperidol (0.5 mg/kg IP) or its injection vehicle and apomorphine (1.0 mg/kg SC) or its vehicle for 21 consecutive days. Then, following a six-day drug-free rest interval, all rats were tested for locomotor activity in photocell arenas after an apomorphine injection on four additional days. Major findings were as follows: (a) rats pretreated with apomorphine were significantly more active following an apomorphine injection than rats pretreated with vehicle; (b) the development of sensitization to apomorphine was completely blocked by the concurrent administration of haloperidol during the pretreatment phase; and (c) pretreatment of rats with haloperidol alone did not affect subsequent sensitivity to apomorphine. These results suggest that the development of behavioral sensitization to apomorphine is related specifically to the stimulation of dopamine receptors.

Enhancement of dopamine actions on rat nucleus accumbens neurones in vitro after methamphetamine pre-treatment

1. Intracellular recordings were made from the nucleus accumbens neurons in brain slices from rats previously treated with saline or methamphetamine. 2. In neurones from both methamphetamine- and saline (control)-treated rats, dopamine (0.1 mM) produced three types of responses: a biphasic response consisting of an initial hyperpolarization followed by a depolarization, a monophasic hyperpolarization and a simple depolarization. 3. Haloperidol (1 microM) reversibly suppressed both responses to dopamine; (-)-sulpiride (1 microM) selectively abolished the depolarization and prolonged the hyperpolarization. Forskolin (10 microM) and dibutyryl adenosine 3',5'-cyclic monophosphate (1 mM) mimicked the hyperpolarization. Both of the latter two substances were more effective in neurones from methamphetamine-treated rats than in neurones from control rats. 4. In slices from methamphetamine-treated rats, the dose-response curve for the dopamine hyperpolarization was shifted to the left of that seen in neurones from control rats by a factor of approximately 100. The dose-response curve for the dopamine depolarization was shifted to the right about 10-fold in neurones from rats treated with methamphetamine. 5. In slices from control rats, dopamine (less than or equal to 0.1 mM) and methamphetamine (less than or equal to 1 microM) had no effect on the EPSPs evoked by focal electrical stimulation of the periaccumbens regions: dopamine (greater than or equal to 10 nM) and methamphetamine (1 microM) markedly depressed the EPSPs in slices from methamphetamine-treated rats. Depolarizations evoked by application of exogenous glutamate were unaffected by dopamine (less than 5 microM). 6. In slices from methamphetamine-treated rats, dopamine (greater than or equal to 10 nM), forskolin (greater than or equal to 1 microM) and dibutyryl adenosine 3',5'-cyclic monophosphate (1 mM) depressed Ca2+-dependent spikes as well as the EPSPs. Haloperidol (1 microM) completely reversed the depressions of the EPSPs and Ca2+-dependent spikes by dopamine, while (-)-sulpiride (1 microM) was only partially effective. 7. These results indicate that chronic methamphetamine administration leads to enhancement of the actions of dopamine at D1 receptors located on glutamate and/or aspartate nerve terminals and of the dopamine hyperpolarization of principal neurones, which is also mediated by D1 receptors.

The effects of ibotenic acid lesions of the nucleus accumbens on spatial learning and extinction in the rat

Rats with ibotenic acid lesions of the nucleus accumbens (N. Acc) were studied in two spatial learning paradigms: a T-maze and a Morris water maze. Learning of a spatial discrimination task and its reversal in the T-maze were disrupted by the N. Acc lesions. As both original and reversal learning were impaired, there was no evidence of a specific lesion effect on reversal learning. The lesioned rats did not perseverate excessively in their choice of the previously reinforced arm. There was evidence of behavioural inflexibility during extinction when the lesioned rats failed to slow the pace at which they ran the maze in the absence of reward. Spontaneous alternation was not significantly affected by the lesion. Acquisition of the second spatial task, locating the hidden platform in the Morris water maze, was also impaired. The lesioned rats did eventually learn the task and successfully reached the platform with similar latencies and heading errors to controls. Thus, the N. Acc lesion impaired but did not abolish spatial learning in the T-maze and the water maze. The deficits observed in this study may reflect a role for the N. Acc in the reorganisation of behaviour in response to external change.

CCK-8 injected into the nucleus accumbens attenuates the supersensitive locomotor response to apomorphine in 6-OHDA and chronic-neuroleptic treated rats

Postsynaptic dopamine-cholecystokinin (CCK) interactions in the nucleus accumbens were studied in two behavioral preparations of DA receptor supersensitivity: chronic-neuroleptic treated and 6-hydroxydopamine (6-OHDA) denervated rats. Subcutaneous (SC) injections of apomorphine (APO; 0.15 mg/kg) in experiment 1 produced marked hyperlocomotion in rats following 12 days of pretreatment with cis-[Z]-flupenthixol (2 mg/kg; twice per day). Bilateral intra-accumbens (N.Acc.) microinjections of CCK-8 (2 ng and 2 micrograms) reliably reduced APO-stimulated hyperlocomotion. An intermediate CCK dose (20 ng) was without effect. No change in APO responsivity following chronic vehicle treatment was observed and the baseline APO response was not altered by CCK at any dose. Denervation of mesolimbic dopamine (DA) terminals by intra-N.Acc. injections of 6-hydroxydopamine (6-OHDA; 8 micrograms/side) in experiment 2 similarly resulted in intense locomotor hyperactivity after APO stimulation (0.1 mg/kg; SC). Bilateral intra-N.Acc. injections of CCK-8 (1, 10, 100 ng, and 1 micrograms) significantly attenuated the supersensitive locomotor response to APO. As in experiment 1, CCK produced "biphasic" dose-response effects with strong attenuation that persisted throughout the entire 60-min test at both high (1 microgram) and low (1 ng) doses. Intermediate CCK doses (10 and 100 ng) produced only short-term reductions in activity. Hypomotility induced by APO in SHAM-lesioned rats was not effectively reversed by CCK treatments. CCK had no effect on unstimulated baseline locomotor activity in either 6-OHDA or SHAM-lesioned rats. These results provide further evidence that CCK-8 modulates mesolimbic DA activity by functionally opposing the postsynaptic effects of DA in the region of the nucleus accumbens.