Frontal-striatal control of behavioral inhibition in the rat

Dorsal striatum coding for the timely execution of action sequences

The automatic initiation of actions can be highly functional. But occasionally these actions cannot be withheld and are released at inappropriate times, impulsively. Striatal activity has been shown to participate in the timing of action sequence initiation and it has been linked to impulsivity. Using a self-initiated task, we trained adult male rats to withhold a rewarded action sequence until a waiting time interval has elapsed. By analyzing neuronal activity we show that the striatal response preceding the initiation of the learned sequence is strongly modulated by the time subjects wait before eliciting the sequence. Interestingly, the modulation is steeper in adolescent rats, which show a strong prevalence of impulsive responses compared to adults. We hypothesize this anticipatory striatal activity reflects the animals’ subjective reward expectation, based on the elapsed waiting time, while the steeper waiting modulation in adolescence reflects age-related differences in temporal discounting, internal urgency states, or explore–exploit balance.

Characterization of a differential reinforcement of low rates of responding task in non-deprived male and female rats: Role of Sigma-1 receptors

Impulsive action can be defined as the inability to withhold a response and represents one of the dimensions of the broad construct impulsivity. Here, we characterized a modified differential reinforcement of low rates of responding (DRL) task developed in our laboratory, in which impulsive action is measured in ad libitum fed/watered subjects. Specifically, we first determined the effects of both sex and estrous cycle on impulsive action by systematically comparing male and estrous-synchronized female subjects. In addition, we evaluated the convergent validity of this modified DRL task by testing the effects of the D2R/5HT2AR antagonist, aripiprazole, and the noncompetitive NMDAR antagonist, MK-801. Finally, we tested the effects of the selective antagonist BD-1063 and agonist PRE-084 of Sigma-1 receptor (Sig-1R) on impulsive action using this modified DRL task. We found that female rats showed and increased inability to withhold a response when compared to males, and this effect was driven by the metestrus/diestrus phase of the estrous cycle. In addition, aripiprazole and MK-801 fully retained their capability to reduce and increase impulsive action, respectively. Finally, the selective Sig-1R antagonist, BD-1063 dose-dependently reduced the inability to withhold a response in both sexes, though more potently in female rats. In summary, we show that impulsive action, as measured in a modified DRL task which minimizes energy-homeostatic influences, is a function of both sex and estrous cycle. Furthermore, we validate the convergent validity of the task and provide evidence that Sig-1R antagonism may represent a novel pharmacological strategy to reduce impulsive action.

Better living through understanding the insula: Why subregions can make all the difference

Insula function is considered critical for many motivated behaviors, with proposed functions ranging from attention, behavioral control, emotional regulation, goal-directed and aversion-resistant responding. Further, the insula is implicated in many neuropsychiatric conditions including substance abuse. More recently, multiple insula subregions have been distinguished based on anatomy, connectivity, and functional contributions. Generally, posterior insula is thought to encode more somatosensory inputs, which integrate with limbic/emotional information in middle insula, that in turn integrate with cognitive processes in anterior insula. Together, these regions provide rapid interoceptive information about the current or predicted situation, facilitating autonomic recruitment and quick, flexible action. Here, we seek to create a robust foundation from which to understand potential subregion differences, and provide direction for future studies. We address subregion differences across humans and rodents, so that the latter's mechanistic interventions can best mesh with clinical relevance of human conditions. We first consider the insula's suggested roles in humans, then compare subregional studies, and finally describe rodent work. One primary goal is to encourage precision in describing insula subregions, since imprecision (e.g. including both posterior and anterior studies when describing insula work) does a disservice to a larger understanding of insula contributions. Additionally, we note that specific task details can greatly impact recruitment of various subregions, requiring care and nuance in design and interpretation of studies. Nonetheless, the central ethological importance of the insula makes continued research to uncover mechanistic, mood, and behavioral contributions of paramount importance and interest. This article is part of the special Issue on 'Neurocircuitry Modulating Drug and Alcohol Abuse'.

The role of anterior insula-brainstem projections and alpha-1 noradrenergic receptors for compulsion-like and alcohol-only drinking

Compulsion-like alcohol drinking (CLAD), where consumption continues despite negative consequences, is a major obstacle to treating alcohol use disorder. The locus coeruleus area in the brainstem and norepinephrine receptor (NER) signaling in forebrain cortical regions have been implicated in adaptive responding under stress, which is conceptually similar to compulsion-like responding (adaptive responding despite the presence of stress or conflict). Thus, we examined whether anterior insula (aINS)-to-brainstem connections and alpha-1 NERs regulated compulsion-like intake and alcohol-only drinking (AOD). Halorhodopsin inhibition of aINS-brainstem significantly reduced CLAD, with no effect on alcohol-only or saccharin intake, suggesting a specific aINS-brainstem role in aversion-resistant drinking. In contrast, prazosin inhibition of alpha-1 NERs systemically reduced both CLAD and AOD. Similar to systemic inhibition, intra-aINS alpha-1-NER antagonism reduced both CLAD and AOD. Global aINS inhibition with GABAR agonists also strongly reduced both CLAD and AOD, without impacting saccharin intake or locomotion, while aINS inhibition of calcium-permeable AMPARs (with NASPM) reduced CLAD without impacting AOD. Finally, prazosin inhibition of CLAD and AOD was not correlated with each other, systemically or within aINS, suggesting the possibility that different aINS pathways regulate CLAD versus AOD, which will require further study to definitively address. Together, our results provide important new information showing that some aINS pathways (aINS-brainstem and NASPM-sensitive) specifically regulate compulsion-like alcohol consumption, while aINS more generally may contain parallel pathways promoting CLAD versus AOD. These findings also support the importance of the adaptive stress response system for multiple forms of alcohol drinking.

Task-Dependent Differences in Operant Behaviors of Rats With Acute Exposure to High Ambient Temperature: A Potential Role of Hippocampal Dopamine Reuptake Transporters

Behavioral or cognitive functions are known to be influenced by thermal stress from the change in ambient temperature (Ta). However, little is known about how increased Ta (i.e., when the weather becomes warm or hot) may affect operant conditioned behavior and the neural substrates involved. The present study thus investigated the effects of high Ta on operant behaviors maintained on a fixed-ratio 1 (FR1) and a differential reinforcement for low-rate responding 10 s (DRL 10-s) schedule of reinforcement. The rats were randomly assigned to three groups receiving acute exposure to Ta of 23°C, 28°C, and 35°C, respectively, for evaluating the effects of high Ta exposure on four behavioral tests. Behavioral responses in an elevated T-maze and locomotor activity were not affected by Ta treatment. Regarding operant tests, while the total responses of FR1 behavior were decreased only under 35°C when compared with the control group of 23°C, those of DRL 10-s behavior were significantly reduced in both groups of 28°C and 35°C. Distinct patterns of inter-response time (IRT) distribution of DRL behavior appeared among the three groups; between-group differences of behavioral changes produced by high Ta exposure were confirmed by quantitative analyses of IRT data. Western blot assays of dopamine (DA) D1 and D2 receptor, DA transporter (DAT) and brain-derived neurotrophic factor (BDNF) were conducted for the sample tissues collected in six brain areas from all the subjects after acute high Ta exposure. Significant Ta-related effects were only revealed in the dorsal hippocampus (dHIP). In which, the DAT levels were increased in a Ta-dependent fashion that was associated with operant behavior changes under high Ta exposure. And, there as an increased level of D1 receptors in the 28°C group. In summary, these data indicate that the performance of operant behavior affected by the present high Ta exposure is task-dependent, and these changes of operant behaviors cannot be attributed to gross motor function or anxiety being affected. The regulation of dHIP DAT may be involved in this operant behavioral change under high Ta exposure.

A Pause-then-Cancel model of stopping: evidence from basal ganglia neurophysiology

Many studies have implicated the basal ganglia in the suppression of action impulses ('stopping'). Here, we discuss recent neurophysiological evidence that distinct hypothesized processes involved in action preparation and cancellation can be mapped onto distinct basal ganglia cell types and pathways. We examine how movement-related activity in the striatum is related to a 'Go' process and how going may be modulated by brief epochs of beta oscillations. We then describe how, rather than a unitary 'Stop' process, there appear to be separate, complementary 'Pause' and 'Cancel' mechanisms. We discuss the implications of these stopping subprocesses for the interpretation of the stop-signal reaction time-in particular, some activity that seems too slow to causally contribute to stopping when assuming a single Stop processes may actually be fast enough under a Pause-then-Cancel model. Finally, we suggest that combining complementary neural mechanisms that emphasize speed or accuracy respectively may serve more generally to optimize speed-accuracy trade-offs.This article is part of the themed issue 'Movement suppression: brain mechanisms for stopping and stillness'.

Setting the occasion for adolescent inhibitory control

During adolescence, individuals experience a broad range of dynamic environments as they strive to establish independence. Learning to respond appropriately in both new and previously encountered environments requires that an individual identify and learn the meaning of cues indicating that a behavior is appropriate, or alternatively, that it should be altered or inhibited. Although the ability to regulate goal-directed behavior continues to develop across adolescence, the specific circumstances under which adolescents experience difficulty with inhibitory control remain unclear. Here we review recent findings in our laboratory that address how adolescents learn to proactively inhibit a response. Much of our research has utilized a negative occasion setting paradigm, in which one cue (a feature) gates the meaning of a second cue (a target). The feature provides information that resolves the ambiguity of the target and indicates the appropriate behavioral response to the target. As such, we have been able to determine how adolescents learn about ambiguous stimuli, such as those whose meaning changes in accordance with other features of the surrounding environment. We consider why adolescents in particular exhibit difficulty in negative occasion setting compared to either pre-adolescents or adults. In addition, we review findings indicating that a balance in neural activity between orbitofrontal cortex and nucleus accumbens is necessary to support normal negative occasion setting. Finally, we consider aspects of associative learning that may contribute to adolescent inhibitory control, as well as provide insight into adolescent behavior as a whole.

Endogenous Opioid Signaling in the Medial Prefrontal Cortex is Required for the Expression of Hunger-Induced Impulsive Action

Opioid transmission and dysregulated prefrontal cortex (PFC) activity have both been implicated in the inhibitory-control deficits associated with addiction and binge-type eating disorders. What remains unknown, however, is whether endogenous opioid transmission within the PFC modulates inhibitory control. Here, we compared intra-PFC opioid manipulations with a monoamine manipulation (d-amphetamine), in two sucrose-reinforced tasks: progressive ratio (PR), which assays the motivational value of an incentive, and differential reinforcement of low response rates (DRLs), a test of inhibitory control. Intra-PFC methylnaloxonium (M-NX, a limited diffusion opioid antagonist) was given to rats in a 'low-drive' condition (2-h food deprivation), and also after a motivational shift to a 'high-drive' condition (18-h food deprivation). Intra-PFC DAMGO (D-[Ala2,N-MePhe4, Gly-ol]-enkephalin; a μ-opioid agonist) and d-amphetamine were also tested in both tasks, under the low-drive condition. Intra-PFC M-NX nearly eliminated impulsive action in DRL engendered by hunger, at a dose (1 μg) that significantly affected neither hunger-induced PR enhancement nor hyperactivity. At a higher dose (3 μg), M-NX eliminated impulsive action and returned PR breakpoint to low-drive levels. Conversely, intra-PFC DAMGO engendered 'high-drive-like' effects: enhancement of PR and impairment of DRL performance. Intra-PFC d-amphetamine failed to produce effects in either task. These results establish that endogenous PFC opioid transmission is both necessary and sufficient for the expression of impulsive action in a high-arousal, high-drive appetitive state, and that PFC-based opioid systems enact functionally unique effects on food impulsivity and motivation relative to PFC-based monoamine systems. Opioid antagonists may represent effective treatments for a range of psychiatric disorders with impulsivity features.

A comparison of presynaptic and postsynaptic dopaminergic agonists on inhibitory control performance in rats perinatally exposed to PCBs

Polychlorinated Biphenyls (PCBs) are very stable environmental contaminants whose exposure induces a number of health and cognitive concerns. Currently, it is well known that PCB exposure leads to poor performance on inhibitory control tasks. It is also well known that dopamine (DA) depletion within medial prefrontal cortex (mPFC) leads to poor performance on inhibitory control tasks. However, what is not well established is whether or not the inhibitory control problems found following PCB exposure are mediated by DA depletion in mPFC. This study was an investigation into the link between perinatal exposure to PCBs, the effect of this exposure on DA neurotransmission in the mPFC, and inhibitory-control problems during adulthood using a rodent model. The current study served to determine if microinjections of different DA agonists (the presynaptic DA transporter inhibitor and vesicular monoamine transporter agonist bupropion, the postsynaptic DA receptor 2 (DAD2) agonist quinpirole, and the postsynaptic DA receptor 1 (DAD1) agonist SKF81297) directly into the mPFC would differentially improve performance on an inhibitory control task in rats perinatally exposed to an environmentally relevant PCB mixture. Findings suggest several significant sex-based differences on differential reinforcement of low rates (DRL) 15 performance as well as some evidence of differential effectiveness of the DA agonists based on PCB exposure group.

The contribution of medial prefrontal cortical regions to conditioned inhibition

Few studies have considered the process by which individuals learn to omit a response, which is an essential aspect of adaptive behavior. Several lines of evidence indicate that two regions of the medial prefrontal cortex have disparate roles in behavioral flexibility. In particular, the prelimbic cortex (PL) is thought to facilitate the generation of a strategy to inhibit a prepotent response, whereas the infralimbic cortex (IL) appears to be more important for maintaining extensively trained inhibitory behaviors. The present experiments were designed to elucidate the contributions of PL and IL to the acquisition and maintenance of Pavlovian conditioned inhibition. In Experiment 1, damage to PL before training in a compound feature negative discrimination task impaired inhibitory learning. By comparison, lesions of IL had little effect. In Experiment 2, lesions of PL or IL occurred after overtraining, and damage to IL significantly impaired subsequent performance in the task, suggesting that this region is involved in the continued expression of Pavlovian conditioned inhibition after thorough training. PL may also be involved in maintaining inhibition, as evidenced by a marginally significant lesion-induced performance deficit. These data support the notion that PL and IL have distinguishable roles in modulating inhibition, while contributing important information about the specific role for PL in acquisition of an inhibitory response and IL in performance.

Learning-dependent Changes in the Neuronal Correlates of Response Inhibition in the Prefrontal Cortex and Hippocampus

It has been suggested that the hippocampus and the prefrontal cortex (PFC) play key roles in representing contextual memory and utilizing contextual information for flexible response selection. During response selection, a correct response should be facilitated and an incorrect response should be inhibited flexibly in association with a cueing stimulus. However, it is poorly understood how the hippocampal and PFC networks behave during such flexible control of facilitation and inhibition of behavioral responses. To find neural correlates of context-cued flexible response selection, the current study employed an object-place paired-associate (OPPA) task in which object A is only rewarded in place 1 and object B is associated with reward in place 2 while recording single units simultaneously from the hippocampus and PFC. During the task, response inhibition in front of a contextually wrong object is required for successful performance and such inhibitory responses were observed before the rat learned the task. A significant proportion of neurons that fired differentially depending on the existence of inhibitory behavior in the PFC was observed during the pre-learning stage. By contrast, the proportion of such neurons in the hippocampus was significantly greater than chance during post-learning stage. The results suggest that the development of inhibitory behavior is a critical behavioral marker that foretells an upcoming acquisition of the task and the hippocampus and PFC are involved in learning contextual response selection by learning how to control the inhibition of behavior as learning progresses.

Dissecting impulsivity and its relationships to drug addictions

Addictions are often characterized as forms of impulsive behavior. That said, it is often noted that impulsivity is a multidimensional construct, spanning several psychological domains. This review describes the relationship between varieties of impulsivity and addiction-related behaviors, the nature of the causal relationship between the two, and the underlying neurobiological mechanisms that promote impulsive behaviors. We conclude that the available data strongly support the notion that impulsivity is both a risk factor for, and a consequence of, drug and alcohol consumption. While the evidence indicating that subtypes of impulsive behavior are uniquely informative--either biologically or with respect to their relationships to addictions--is convincing, multiple lines of study link distinct subtypes of impulsivity to low dopamine D2 receptor function and perturbed serotonergic transmission, revealing shared mechanisms between the subtypes. Therefore, a common biological framework involving monoaminergic transmitters in key frontostriatal circuits may link multiple forms of impulsivity to drug self-administration and addiction-related behaviors. Further dissection of these relationships is needed before the next phase of genetic and genomic discovery will be able to reveal the biological sources of the vulnerability for addiction indexed by impulsivity.

The ontogeny of learned inhibition

Previous studies have examined the maturation of learning and memory abilities during early stages of development. By comparison, much less is known about the ontogeny of learning and memory during later stages of development, including adolescence. In Experiment 1, we tested the ability of adolescent and adult rats to learn a Pavlovian negative occasion setting task. This procedure involves learning to inhibit a behavioral response when signaled by a cue in the environment. During reinforced trials, a target stimulus (a tone) was presented and immediately followed by a food reward. On nonreinforced trials, a feature stimulus (a light) was presented 5 sec prior to the tone and indicated the absence of reward following presentation of the tone. Both adult and adolescent rats learned to discriminate between two different trial types and withhold responding when the light preceded the tone. However, adolescent rats required more sessions than adults to discriminate between reinforced and nonreinforced trials. The results of Experiment 2 revealed that adolescents could learn the task rules but were specifically impaired in expressing that learning in the form of withholding behavior on nonreinforced trials. In Experiment 3, we found that adolescents were also impaired in learning a different version of the task in which the light and tone were presented simultaneously during the nonreinforced trials. These findings add to existing literature by indicating that impairments in inhibitory behavior during adolescence do not reflect an inability to learn to inhibit a response, but instead reflect a specific deficit in expressing that learning.

The effects of clinically relevant doses of amphetamine and methylphenidate on signal detection and DRL in rats

Low dose amphetamine (AMPH) and methylphenidate (MPH, Ritalin(®)) are the most widely prescribed and most effective pharmacotherapy for attention-deficit/hyperactivity disorder (ADHD). Certain low, clinically relevant doses of MPH improve sustained attention and working memory in normal rats, in contrast to higher doses that impair cognitive ability and induce locomotor activity. However, the effects of AMPH of MPH on sustained attention and behavioral inhibition remain poorly characterized. The present experiments examined the actions of AMPH (0.1 and 0.25 mg/kg) and MPH (0.5 and 1.0 mg/kg) in a rat model of 1) sustained attention, where signal and blank trials were interspersed randomly and occurred at unpredictable times, and 2) behavioral inhibition, using a differential reinforcement of low rate (DRL) schedule. In a signal detection paradigm, both 0.5 mg/kg and 1.0 mg/kg MPH and 0.25 mg/kg AMPH improve sustained attention, however neither AMPH nor MPH improve behavioral inhibition on DRL. Taken together with other recent studies, it appears that clinically-relevant doses of AMPH and MPH may preferentially improve attention-related behavior while having little effect on behavioral inhibition. These observations provide additional insight into the basic behavioral actions of low-dose psychostimulants and further suggest that the use of sustained attention tasks may be important in the development of novel pharmacological treatments for ADHD.

Prefrontal cortical-striatal dopamine receptor mRNA expression predicts distinct forms of impulsivity

Variation in dopamine receptor levels has been associated with different facets of impulsivity. To further delineate the neural substrates underlying impulsive action (inability to withhold a prepotent motor response) and impulsive choice (delay aversion), we characterised rats in the Differential Reinforcement of Low Rates of Responding task and a delay discounting task. We also measured performance on an effort-based discounting task. We then assessed D1 and D2 dopamine receptor mRNA expression in subregions of the prefrontal cortex and nucleus accumbens using in situ hybridisation, and compared these data with behavioral performance. Expression of D1 and D2 receptor mRNA in distinct brain regions was predictive of impulsive action. A dissociation within the nucleus accumbens was observed between subregions and receptor subtypes; higher D1 mRNA expression in the shell predicted greater impulsive action, whereas lower D2 mRNA expression in the core predicted greater impulsive action. We also observed a negative correlation between impulsive action and D2 mRNA expression in the prelimbic cortex. Interestingly, a similar relationship was present between impulsive choice and prelimbic cortex D2 mRNA, despite the fact that behavioral indices of impulsive action and impulsive choice were uncorrelated. Finally, we found that both high D1 mRNA expression in the insular cortex and low D2 mRNA expression in the infralimbic cortex were associated with willingness to exert effort for rewards. Notably, dopamine receptor mRNA in these regions was not associated with either facet of impulsivity. The data presented here provide novel molecular and neuroanatomical distinctions between different forms of impulsivity, as well as effort-based decision-making.

Estradiol impairs response inhibition in young and middle-aged, but not old rats

Estrogens have been shown to have a strong influence on such cognitive domains as spatial memory, response learning, and several tasks of executive function, including both working memory and attention. However, the effects of estrogens on inhibitory control and timing behavior, both important aspects of executive function, have received relatively little attention. We examined the effects of estradiol on inhibitory control and timing behavior using a differential reinforcement of low rates of responding (DRL) task. Ovariectomized young (3 month), middle-aged (12 month), and old (18 month) Long-Evans rats were implanted with Silastic implants containing 0, 5 or 10% 17β-estradiol in cholesterol vehicle and were tested on a DRL task requiring them to wait 15s between lever presses to receive a food reinforcer. The ratio of reinforced to non-reinforced lever presses did not differ across age in the cholesterol vehicle group. Conversely, 17β-estradiol impaired learning of the DRL task in young and middle-aged rats, but the learning of old rats was not impaired relative to vehicle controls following either 5% or 10% 17β-estradiol treatment. Overall, old rats also made fewer lever presses than both the young and middle-aged rats. These results provide new evidence that estrogens impair inhibitory control, an important aspect of self regulation, and add to existing evidence that estrogens differentially affect cognition at different ages.

Amygdala and ventrolateral prefrontal cortex activation to masked angry faces in children and adolescents with generalized anxiety disorder

CONTEXT

Vigilance for threat is a key feature of generalized anxiety disorder (GAD). The amygdala and the ventrolateral prefrontal cortex constitute a neural circuit that is responsible for detection of threats. Disturbed interactions between these structures may underlie pediatric anxiety. To date, no study has selectively examined responses to briefly presented threats in GAD or in pediatric anxiety.

OBJECTIVE

To investigate amygdala and ventrolateral prefrontal cortex activation during processing of briefly presented threats in pediatric GAD.

DESIGN

Case-control study.

SETTING

Government clinical research institute.

PARTICIPANTS

Youth volunteers, 17 with GAD and 12 without a psychiatric diagnosis.

MAIN OUTCOME MEASURES

We used functional magnetic resonance imaging to measure blood oxygenation level-dependent signal. During imaging, subjects performed an attention-orienting task with rapidly presented (17 milliseconds) masked emotional (angry or happy) and neutral faces.

RESULTS

When viewing masked angry faces, youth with GAD relative to comparison subjects showed greater right amygdala activation that positively correlated with anxiety disorder severity. Moreover, in a functional connectivity (psychophysiological interaction) analysis, the right amygdala and the right ventrolateral prefrontal cortex showed strong negative coupling specifically to masked angry faces. This negative coupling tended to be weaker in youth with GAD than in comparison subjects.

CONCLUSIONS

Youth with GAD have hyperactivation of the amygdala to briefly presented masked threats. The presence of threat-related negative connectivity between the right ventrolateral prefrontal cortex and the amygdala suggests that the prefrontal cortex modulates the amygdala response to threat. In pediatric GAD, amygdala hyperresponse occurs in the absence of a compensatory increase in modulation by the ventrolateral prefrontal cortex.

Deficiency in inhibitory cortical interneurons associates with hyperactivity in fibroblast growth factor receptor 1 mutant mice

BACKGROUND

Motor hyperactivity due to hyper-dopaminergic neurotransmission in the basal ganglia is well characterized; much less is known about the role of the neocortex in controlling motor behavior.

METHODS

Locomotor behavior and motor, associative, and spatial learning were examined in mice with conditional null mutations of fibroblast growth factor receptor 1 (Fgfr1) restricted to telencephalic neural precursors (Fgfr1(f/f;hGfapCre)). Locomotor responses to a dopamine agonist (Amphetamine 2 mg/kg and Methylphenidate 10 mg/kg) and antagonists (SCH233390 .025 mg/kg and Haloperidol .2 mg/kg) were assessed. Stereological and morphological characterization of various monoaminergic, excitatory, and inhibitory neuronal subtypes was performed.

RESULTS

Fgfr1(f/f;hGfapCre) mice have spontaneous locomotor hyperactivity characterized by longer bouts of locomotion and fewer resting points that is significantly reduced by the D1 and D2 receptor antagonists. No differences in dopamine transporter, tyrosine hydroxylase, or serotonin immunostaining were observed in Fgfr1(f/f;hGfapCre) mice. There was no change in cortical pyramidal neurons, but parvalbumin+, somatostatin+, and calbindin+ inhibitory interneurons were reduced in number in the cerebral cortex. The decrease in parvalbumin+ interneurons in cortex correlated with the extent of hyperactivity.

CONCLUSIONS

Dysfunction in specific inhibitory cortical circuits might account for deficits in behavioral control, providing insights into the neurobiology of psychiatric disorders.

NMDA lesions in the medial prefrontal cortex impair the ability to inhibit responses during reversal of a simple spatial discrimination

Although lesion studies suggest that the rat medial prefrontal cortex (mPFc) is involved in the process necessary for reversal of a particular set of contingencies, the nature of lesion-induced deficits is unclear. The involvement of rat mPFc in reversal of a simple spatial discrimination was examined in the present study. Our hypothesis was that lesion-induced deficits may reflect a failure to inhibit a learned instrumental response. Lister Hooded rats were trained on a spatial discrimination task (SD), which required a correct barpress matching the cue location, then they were trained on reversal of SD (SDR), which required a correct barpress opposite to the cue location. Rats with mPFc lesions showed a slower learning rate compared to the controls. However, behavior of the lesioned rats during early and later reversal differed. During the initial SDR, the lesioned rats showed a greater number of barpresses during the intertrial interval and a slightly higher percent correct responses than that of the controls. Our data suggest that damage to mPFc may produce a lack of response inhibition, leading to an increase in nondiscriminated bapresses, thereby yielding a 'facilitation' during early reversal. mPFc lesion did not affect either open field activity or prepulse inhibition (PPI), a frequently used measure of sensorimotor gating. Disruption of reversal learning following damage to mPFc is partly due to a failure to inhibit instrumental responses, rather than to disruption of other processes involved in sensorimotor gating or general activity.

Impaired DRL 30 performance during amphetamine withdrawal

Repeated administration of psychomotor stimulants may produce an impulsive state that could contribute to the cycle of drug abstinence and relapse seen in human drug addicts. We have previously reported that the inhibitory effects of dopamine (DA) on the firing rate of medial prefrontal cortex (mPFC) neurons were reduced in rats after repeated amphetamine treatment suggesting impaired mPFC DA function. Here, we used a differential reinforcement of low rates of responding (DRL) operant conditioning task, which is dependent on mPFC DA, to test impulsivity and inhibitory control. Food-restricted rats were trained to inhibit a nose poke response for 30s before a subsequent nose poke would result in a food reward (DRL 30). Once training was completed, rats received 5 days of no treatment, daily i.p. saline injections or daily i.p. injections of 5mg/kg amphetamine. Nine days of DRL 30 test performance began following a 3-day withdrawal from treatment. The percent of training active hole nose pokes was significantly increased and the percent of training efficiency was significantly decreased in rats withdrawn from repeated amphetamine administration as compared to saline or nai;ve rats. This suggests that impulsivity is increased during amphetamine withdrawal, which we hypothesize is associated with disrupted DA function in the mPFC.

Prefrontal cortical manipulations alter the effects of intra-ventral striatal dopamine antagonists on fixed-interval performance in the rat

The nature of the functional relationships between areas of prefrontal cortex and ventral striatum remain undefined. This study was designed to examine functional interactions between activity in two areas of prefrontal cortex, the prelimbic (PL) and agranular insular (AI) areas, and ventral striatal (VS) dopamine (DA) function. Interactions were assessed using a Fixed Interval (FI) schedule of reinforcement shown previously in our laboratory to be modulated by VS DA function. The study compared changes in FI performance following intra-VS DA antagonist injections alone (SCH23390 + eticlopride) to those observed when either saline or saline + lidocaine were injected into prefrontal cortex after the intra-VS DA antagonist injections. The intra-VS DA antagonists alone decreased FI response rates and increased postreinforcement pause times at both dose combinations (1/0.1 and 3/0.3 microg of SCH23390/eticlopride per side). Neither saline nor saline + lidocaine injected into the PL area of prefrontal cortex altered the effects of intra-VS DA antagonists on FI performance. Saline administration into the AI area of prefrontal cortex, however, eliminated the FI rate-decreasing effects of intra-VS DA antagonists. The agent or mechanism of this effect, whether it be saline, the act of inserting the cannulae into the cortical tissue, or the act of injecting fluid into this tissue, is not clear. This effect of AI saline was prevented by coadministration of lidocaine with saline into AI. These results, coupled with those from a previous experiment examining lesion effects in PL and AI on FI performance (Evans SB, Cory-Slechta DA. The effects of temporary lesions of the insular and medial prefrontal cortex on fixed-interval schedule-controlled behavior in the rat, Soc Neurosci Abstr 1996;22(1):159) suggest that PL might exert a tonic influence on VS DA function, since FI response rates gradually increase over a 2-week period following lesions of PL. In contrast, AI, although not normally modulating FI performance, can apparently influence VS DA function, possibly when alterations in activity are invoked in AI.

Increased sensitivity to cocaine place-preference conditioning by septal lesions in rats

Rats bearing electrolytic lesions of medial septum and sham-operated controls were trained on cocaine place-preference in a 3-compartment apparatus. Cocaine was paired with a white or a black compartment. An unbiased design was used, in which cocaine was paired with the preferred side in half the animals and with the unpreferred side in the other half. Two low doses of cocaine HCl were used: 2.5 and 5.0 mg/kg. Only two pairings of drug with environment were used to minimize the influence of drug sensitization. Rats with septal lesions, but not controls, showed preference conditioning to the black side at 2.5 mg/kg; lesioned and control animals showed similar conditioning to the black side at 5.0 mg/kg. Lesioned animals could not be conditioned to the white side at either dose. This was attributed to a drug-induced enhancement of a previously described increased reactivity to brightness following septal lesions. Controls conditioned to either side at 5.0 mg/kg. It was concluded that septal lesions lowered the cocaine dose required for preference conditioning, consistent with reports that such damage enhances some behavioral effects of psychostimulants.

Effects of dopamine depletions in the medial prefrontal cortex on active avoidance and escape in the rat

Dopamine systems have been implicated in the performance of avoidance behavior, and the dopaminergic innervation of medial prefrontal cortex is known to be responsive to stressful stimuli. In the present investigation, injections of 6-hydroxydopamine were used to produce moderate depletions of dopamine in the medial prefrontal cortex of rats trained to perform an active avoidance/escape task. In this task, 0.5 mA shock was presented for 5 s every 30 s, and the rat could escape shock presentation, or avoid the shock for 30 s, by pressing a lever. Depletion of dopamine in the medial prefrontal cortex did not affect total number of responses, and did not impair avoidance responding (i.e. responding when the shock was off), and in fact dopamine-depleted animals tended to make slightly more avoidance responses than control animals. Prefrontal dopamine depletions did result in a significant decrease in the number of escape responses (i.e. responding to terminate shock when the shock was on). Moreover, dopamine depletions significantly decreased response efficiency, which is an index of the reduction of shock time produced per lever pressing response. Previous work has indicated that dopamine antagonists and accumbens dopamine depletions have dramatic effects on avoidance behavior; thus, the present results indicate that prefrontal cortex dopamine depletions do not mimic the effects of interference with subcortical dopamine systems. The selective effects of dopamine depletions on escape behavior in the present study suggest that rats with medial prefrontal dopamine depletions have an impairment in the ability to respond appropriately to the direct presentation of footshock.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of dopamine depletions in the medial prefrontal cortex on DRL performance and motor activity in the rat

Two experiments assessed the behavioral effects of dopamine depletions in the medial prefrontal cortex that were produced by injection of the neurotoxic agent 6-hydroxydopamine. In the first experiment, rats were trained to respond on a differential reinforcement of low rates of responding-30 second (DRL30) schedule. On this schedule, rats were only reinforced if they withheld responding for 30 s. Rats with prefrontal dopamine depletions were found to be impaired in the DRL task. These animals responded more than controls, received fewer reinforcers, and were less efficient in their responses. Moreover, an analysis of interresponse times (IRTs) revealed that rats with medial prefrontal dopamine depletions made more responses with short-duration IRTs, and fewer responses with long-duration IRTs. In the second experiment, rats were tested on open field activity, amphetamine-induced locomotor activity and stereotypy. No increase in spontaneous locomotor activity was found following surgery; however, increases in amphetamine-induced locomotor activity and stereotypy were observed. These results are consistent with hypothesized role for the prefrontal cortex in behavioral inhibition, and indicates that prefrontal cortical dopamine is an integral part of the system.

Differential contribution of motor cortex and caudate nucleus to instrumental tongue-forelimb synchronization in rats: a functional ablation study

The functional ablation technique was used to assess the role of motor cortex and caudate nucleus in a complex skilled movement. Rats were trained to synchronize tongue and forepaw movements in a drinking box equipped with a retractable spout which was automatically withdrawn after every lick but could be returned by pressing and releasing a lever placed 4 cm below the spout. The animals learned to perform short presses synchronized with the lick cycle in such a way as to allow continuous drinking. Neural circuits implementing these lick-associated instrumental movements were blocked by intracranial injection of 10 ng of tetrodotoxin. Bilateral blockade of the motor cortex interfered with lick-synchronized bar pressing, but did not significantly influence licking from a stationary spout. Both licking and bar-pressing were impaired by bilateral injection of tetrodotoxin into the lateral part of the caudate nucleus for the duration of the tetrodotoxin-induced inactivation of the target region. The instrumental tongue-forelimb synchronization recovered considerably later, i.e. after two to three days. Functional blockade of the medial part of the caudate nucleus caused only a partial impairment of lick-synchronized bar pressing lasting less than 7 h and did not significantly influence consummatory licking. It is concluded that the tongue-forepaw synchronization is disrupted by blockade of motor cortex or lateral caudate considerably longer than the performance of the isolated component movements.

Multiple reward systems and the prefrontal cortex

Electrical stimulation of the major divisions of the prefrontal cortex, the mediodorsal and sulcal areas, can serve as a reinforcing stimulus. Studies of self-stimulation of the prefrontal cortex have produced behavioral, anatomical and pharmacological evidence that the substrate of these rewarding effects can be dissociated from that subserving self-stimulation of ventral diencephalic sites such as the lateral hypothalamus. Other studies indicate that within the prefrontal cortex itself, self-stimulation of the medial and sulcal divisions can be attributed to dissociable processes. These observations suggest the existence of multiple, largely autonomous prefrontal subsystems involved in reinforcement. This raises the question of the functional significance of such systems, and of their organization. An approach to this problem is to consider the relationship between the behavioral functions of the prefrontal divisions and the characteristics of stimulation-induced reward obtained at each site. Studies of the effects of restricted prefrontal lesions indicate that the medial and sulcal divisions can be dissociated according to their involvement in the control of distinct types of sensory and motor events. Further experiments indicate that damage to each division causes selective deficits in the learning of stimulus-reinforcer and response-reinforcer relations, depending in part on the nature of the reinforcing event. Conditioning experiments further show that the rewarding effects produced by stimulation of these areas are preferentially associated to sensory events which correspond to the functional specialization of each division. These data are interpreted to suggest that different rewarding events and/or different attributes of rewarding stimuli are processed by distinct systems which are reflected by the organization of dissociable self-stimulation pathways.

Independent GABAergic and cholinergic modulation of apomorphine-induced stereotyped rearing in the rat

The injection of GABA into the caudate nucleus inhibited the stereotyped rearing induced by apomorphine in a dose-related manner. Muscimol, a potent GABAergic agonist shared this effect. The inhibitory effect of GABA was easily counteracted by bicuculline but not by pretreatment with atropine. Injection of carbachol into the caudate nucleus inhibited the stereotyped rearing induced by systemically-applied apomorphine in a dose-related manner. This inhibitory effect was easily abolished by atropine but not bicuculline. Thus, the stereotyped rearing induced by apomorphine, an effect due to an increased excitatory state of the dopaminergic system in the caudate nucleus, could be modified (inhibited) by augmentation of either the GABAergic or of the cholinergic state excitation. The two modulatory systems did not appear to be interlinked; most probably, they influence the dopaminergic effect independently of one another.

Unilateral and bilateral lesions of the anteromedial cortex increase perseverative head movements of the rat

Lesions of the anteromedial cortex were made in rats trained to orient their heads to 9 positions located laterally and centrally in upper, middle and lower regions of space. The water-deprived subjects were tested in sessions of 20 trials in each of which all positions were baited once with a small amount of water. Training continued until no position was consistently missed and the number of returns to positions already selected on that trial (perseverative responses) was low and stable. In Expt. 1, rats with unilateral anteromedial lesions tested with 3 days recovery did not miss any position or significantly change the order in which correct positions were selected but they made more perseverative responses to all positions. Bilateral anteromedial lesions moderately increased misses but markedly increased perseverations. Visual cortex lesions comparable in size produced no changes. In Expt. 2, unilateral anteromedial lesions tested with 7 days' recovery did not increase misses and only slightly increased perseverations. In Expt. 3, unilateral anteromedial lesions tested within 1 day increased perseverations more than lesions tested with 6 days' recovery. Neither group increased misses or changed selection order. The perseverations appeared on a trial after 6 of the 9 correct selections and were directed to both recently and remotely selected positions. Lesions of the anteromedial cortex appear to produce a deficit in the suppressive control of head orienting.

Dissociation of locomotor depression and ChE activity after DFP, soman and sarin

The effect of direct intrastriatal injection of three organophosphate cholinesterase inhibitors, DFP (diisopropylphosphorofluoridate), soman (pinacolyl methylphosphonofluoridate) and sarin (isopropyl methylphosphonofluoridate) has been studied on locomotor activity in the rat. The degree of ChE inhibition has been monitored in the striatum, as well as in surrounding brain areas and blood, in order to verify the selectivity of the treatment and rule out effects attributable to actions in these areas and/or the periphery. It has been determined that while enzyme activity is inhibited in the striatum by all three compounds, only DFP significantly reduces locomotor activity at doses that produce no other observable behavioral deficits, or significant leakage into the periphery. Behavioral recovery occurs before enzyme activity returns to control levels. Possible contributions of DFP's action on other neurotransmitters and on ChE in other brain areas to the inhibition of locomotor activity are discussed.

DRL responding under uncertain reinforcement in rats after medial frontal cortical lesions

Rats were studied for their performance on DRL 10-s schedule with a 50% probability of reinforcement of a correct response with a food pellet. It was found that the performance of a group of 8 rats with medial frontal decortication by aspiration was deficient compared to a group of 7 sham-operated control rats. The deficit manifested in the form of an increase in very short interresponse times (IRTs). This signifies an influence of medial frontal cortex on the control of responding by time-correlated stimuli probably arising from collateral behaviours.

Reexamination of functional subdivisions of the rodent prefrontal cortex

Selective patterns of behavioral deficits were observed on tests of spatial or olfactory learning after different cortical lesions in rats. The results clearly distinguished functional subdivisions of the rodent prefrontal cortex: Rats with lesions of the prefrontal cortex that primarily involve the dorsal bank of the rhinal sulcus were impaired selectively and exhibited increased perseveration of responses in a go, no-go odor discrimination task. In contrast, rats with lesions of the region of prefrontal cortex situated along the medial cortical wall were impaired selectively and exhibited increased perseveration of responses in a spatial delayed alteration task. These behavioral deficits were similar in magnitude and quality to those found in monkeys after discrete ablations of frontal lobe regions that are argued to be homologous prefrontal subdivisions.

Neurotoxic lesions of ventrolateral but not anteromedial neostriatum in rats impair differential reinforcement of low rates (DRL) performance

In two separate experiments, rats received either kainic acid (KA), 6-hydroxydopamine (6-OHDA) or control lesions bilaterally into either the anteromedial or ventrolateral neostriatum. Both ventrolateral neurotoxic lesions disrupted post-operative body weight regulation and the acquisition of a DRL-20 sec operant schedule of reinforcement. The pattern of operant responding suggested that these animals had an impairment in response sequencing or switching. Rats with anteromedial lesions manifested neither body-weight nor DRL impairments.

Theories of the dorsal bundle extinction effect

Selective destruction of the noradrenaline systems in the rat brain using the neurotoxin 6-hydroxydopamine has been found to cause resistance to extinction in a number of behavioural situations. Several theories concerning the behavioural mechanism altered by the lesion, and hence about the role of noradrenaline in normal brain functioning, are proposed and evaluated. Theories suggesting a role for noradrenaline in activity, perseveration, internal inhibition, frustrative non-reward, motivation, or secondary reinforcement, fail to explain all the available evidence and direct tests of each theory fails to support its predictions. A model which suggests that noreadrenaline is involved in attentional behaviour, specifically in filtering out or learning to ignore irrelevant environmental stimuli, is successful in explaining all available data and direct tests of the lesioned rats' attentional capacity serve to confirm many of the predictions of an attentional theory of the dorsal bundle extinction effect.

Convergent prefrontal and nigral projections to the striatum of the rat

Projections to the striatum from the prefrontal cortex, the substantia nigra's pars compacta (SNC), and the ventral tegmental area (AVT) were examined autoradiographically. These projections converge on the striatum in an organized manner such that the prefrontal areas innervated by AVT project to the same part of the striatum as AVT, and prefrontal areas innervated by SNC project to the same part of the striatum as SNC.

Dopaminergic antagonism and catalepsy in the developing rat

The cataleptic effect of the dopaminergic blockers spiroperidol and haloperidol was investigated in developing rats. Both neuroleptics were found to produce less catalepsy in 15 day old rats than in either 10 or 20 day old animals. It is proposed that the decrement in catalepsy occurring between 10 and 15 days of age is related to increased dopaminergic activity in the neostriatum. The reversal of this phenomenon by 20 days may be a consequence of maturation of cholinergic local circuit neurons.

Avoidance, operant and locomotor behavior in rats with neostriatal injections of kainic acid

Compared with saline injected controls, rats with bilateral injections of kainic acid (KA) in the dorsal neostriatum showed increased locomotor response to d-amphetamine, increased resistance to extinction and impaired acquisition and retention of passive avoidance. The KA injection resulted in loss of local neurons in the dorsal neostriatum, with no appreciable damage either to dopaminergic terminals or to extrinsic myelinated axons, thus supporting both the selective neurotoxic action of KA on neuronal perikarya and the proposed similarity of KA-induced neostriatal lesions with those found in the caudate-putamen of patients with Huntington's disease. Although loss of hippocampal neurons was occasionally observed, the behavioral results could not be wholly attributed to hippocampal damage, since rats with no demonstrable extrastriatal lesions were not less impaired than those with hippocampal damage. An altered arousal reaction to stressful situations might account for the learning and memory impairments of the KA neostriatal rats.

Identification of a subregion within rat neostriatum for the dopaminergic modulation of lateral hypothalamic self-stimulation

Experiments were conducted to test the hypothesis that the involvement of neostriatal dopaminergic transmission in lateral hypothalamic self-stimulation might be specific to a striatal subregion. Crystalline application of dopamine or D-amphetamine increased self-stimulation rate only when made to ventral anterior striatum (VAS); more dorsal or posterior applications were ineffective. A comparison of dose-response functions for dopamine using solution injections in VAS and posterior striatum (PS) confirmed that only VAS was responsive. Injections or applications of 6-hydroxydopamine suppressed responding only when made into VAS. Haloperidol injections decreased responding only for VAS and not PS injection sites. Applications or injections of scopolamine often increased responding when made into VAS, but this effect was unreliable. Applications or injections of scopolamine to more posterior sites consistently suppressed responding. It was concluded that dopaminergic transmission in VAS, alone among the striatal sites tested, is facilitatory on hypothalamic self-stimulation. The effects of drug applications to nucleus accumbens were generally similar to VAS, and it was suggested that these areas may be functionally similar. An examination of the known afferents to VAS indicated that this area of neostriatum, like n. accumbens, may be influenced by activity in limbic structures. This anatomy may help provide an understanding of how neostriatum, traditionally considered to have a motor function, might be involved in central reward processes.

Anatomical specificity within rat striatum for the dopaminergic modulation of DRL responding and activity

The direct application of microgram quantities of crystalline dopamine, D-amphetamine, or scopolamine to the ventral anterior region of the neostriatum of rats decreased response efficiency on a 'differential reinforcement of low rate' 10 sec schedule of reinforcement. Similar applications to the dorsal globus pallidus or posterior striatum either did not alter or increased response efficiency. A comparison of dose-response functions for injections of dopamine in solution into ventral anterior, central and posterior striatum confirmed that only injections into ventral anterior striatum (VAS) decreased response efficiency on the DRL schedule. The same striatal map was found for the dopamine-induced increase in spontaneous locomotor activity in tilt boxes. It was concluded that dopaminergic transmission in ventral anterior striatum, in contrast to the other striatal and pallidal sites tested, is involved in the modulation of behavioral arousal.

Behavioral and anatomical consequences of small intrastriatal injections of kainic acid in the rat

The effects of bilateral injections of kainic acid into the anteromedial neostriatal region were examined behaviorally and anatomically in two groups of rats. Behaviorally, kainic acid injections resulted in a severe impairment of delayed alternation retention, while the ability for visual discrimination remained unaffected. Anatomically it was found that axons traversing the injected area remain able to transport horseradish peroxidase. Furthermore, histological examinations of the injected regions revealed a heavy loss of neurons and a decrease of histochemical staining for specific acetylcholinesterase. Silver impregnation showed slightly disorganized, but continuous, axons in bundles of the capsula interna. On the other hand, the axonal network throughout the neuropil of the injected area was markedly diminished. No conspicuous change was found in myelin staining or in the intensity or catecholamine fluorescence. The anatomical results suggest that kainic acid appears to affect only perikarya of the neostriatum and the axons originating from these perikarya, whereas passing axons seem to remain intact. Thus, the observed behavioral impairment must be attributed to changes in the neostriatum itself. It is concluded that the neostriatum has 'complex' or 'cognitive' functions and that some mental symptoms in Huntington's chorea may be attributed to a dysfunction of this part of the brain.