Where Actions Meet Outcomes: Medial Prefrontal Cortex, Central Thalamus, and the Basal Ganglia

Medial prefrontal cortex (mPFC) interacts with distributed networks that give rise to goal-directed behavior through afferent and efferent connections with multiple thalamic nuclei and recurrent basal ganglia-thalamocortical circuits. Recent studies have revealed individual roles for different thalamic nuclei: mediodorsal (MD) regulation of signaling properties in mPFC neurons, intralaminar control of cortico-basal ganglia networks, ventral medial facilitation of integrative motor function, and hippocampal functions supported by ventral midline and anterior nuclei. Large scale mapping studies have identified functionally distinct cortico-basal ganglia-thalamocortical subnetworks that provide a structural basis for understanding information processing and functional heterogeneity within the basal ganglia. Behavioral analyses comparing functional deficits produced by lesions or inactivation of specific thalamic nuclei or subregions of mPFC or the basal ganglia have elucidated the interdependent roles of these areas in adaptive goal-directed behavior. Electrophysiological recordings of mPFC neurons in rats performing delayed non-matching-to position (DNMTP) and other complex decision making tasks have revealed populations of neurons with activity related to actions and outcomes that underlie these behaviors. These include responses related to motor preparation, instrumental actions, movement, anticipation and delivery of action outcomes, memory delay, and spatial context. Comparison of results for mPFC, MD, and ventral pallidum (VP) suggest critical roles for mPFC in prospective processes that precede actions, MD for reinforcing task-relevant responses in mPFC, and VP for providing feedback about action outcomes. Synthesis of electrophysiological and behavioral results indicates that different networks connecting mPFC with thalamus and the basal ganglia are organized to support distinct functions that allow organisms to act efficiently to obtain intended outcomes.

The neurobiology of thalamic amnesia: Contributions of medial thalamus and prefrontal cortex to delayed conditional discrimination

Although medial thalamus is well established as a site of pathology associated with global amnesia, there is uncertainty about which structures are critical and how they affect memory function. Evidence from human and animal research suggests that damage to the mammillothalamic tract and the anterior, mediodorsal (MD), midline (M), and intralaminar (IL) nuclei contribute to different signs of thalamic amnesia. Here we focus on MD and the adjacent M and IL nuclei, structures identified in animal studies as critical nodes in prefrontal cortex (PFC)-related pathways that are necessary for delayed conditional discrimination. Recordings of PFC neurons in rats performing a dynamic delayed non-matching-to position (DNMTP) task revealed discrete populations encoding information related to planning, execution, and outcome of DNMTP-related actions and delay-related activity signaling previous reinforcement. Parallel studies recording the activity of MD and IL neurons and examining the effects of unilateral thalamic inactivation on the responses of PFC neurons demonstrated a close coupling of central thalamic and PFC neurons responding to diverse aspects of DNMTP and provide evidence that thalamus interacts with PFC neurons to give rise to complex goal-directed behavior exemplified by the DNMTP task.

Brain and behavioral pathology in an animal model of Wernicke's encephalopathy and Wernicke-Korsakoff Syndrome

Animal models provide the opportunity for in-depth and experimental investigation into the anatomical and physiological underpinnings of human neurological disorders. Rodent models of thiamine deficiency have yielded significant insight into the structural, neurochemical and cognitive deficits associated with thiamine deficiency as well as proven useful toward greater understanding of memory function in the intact brain. In this review, we discuss the anatomical, neurochemical and behavioral changes that occur during the acute and chronic phases of thiamine deficiency and describe how rodent models of Wernicke-Korsakoff Syndrome aid in developing a more detailed picture of brain structures involved in learning and memory.

Blunted hippocampal, but not striatal, acetylcholine efflux parallels learning impairment in diencephalic-lesioned rats

A rodent model of diencephalic amnesia, pyrithiamine-induced thiamine deficiency (PTD), was used to investigate the dynamic role of hippocampal and striatal acetylcholine (ACh) efflux across acquisition of a nonmatching-to-position (NMTP) T-maze task. Changes in ACh efflux were measured in rats at different time points in the acquisition curve of the task (early=day 1, middle=day 5, and late=day 10). Overall, the control group had higher accuracy scores than the PTD group in the latter sessions of NMTP training. During the three microdialysis sampling points, all animals displayed significant increases in ACh efflux in both hippocampus and striatum, while performing the task. However, on day 10, the PTD group showed a significant behavioral impairment that paralleled their blunted hippocampal--but not striatal--ACh efflux during maze training. The results support selective diencephalic-hippocampal dysfunction in the PTD model. This diencephalic-hippocampal interaction appears to be critical for successful episodic and spatial learning/memory.

Effects of excitotoxic thalamic intralaminar nuclei lesions on attention and working memory

In rats, lesions of the thalamic intralaminar nuclei (ILn) impair measures of working memory, but it is unclear whether alterations of attention contribute to the mnemonic deficits. The present experiment tested the effects of ILn lesions on a two-lever attention task that required discrimination of visual signals and non-signals. Rats were trained presurgically in the task and then received sham surgery or infusions of n-methyl-d-aspartate (NMDA) into the ILn to induce excitotoxic lesions. ILn lesions transiently decreased accurate detection of signals. ILn lesions also increased omissions. Compared to sham-lesioned rats, ILn-lesioned animals were not differentially affected when task demands were increased by presenting a visual distracter. Finally, a retention interval was incorporated into the task to assess whether the lesions affected acquisition of a working memory version of this behavioral paradigm. Unlike sham-lesioned animals, ILn-lesioned rats did not demonstrate a significant improvement in signal detection when a retention interval was introduced. The transient lesion-induced deficits in the attention task suggest that, in rats, the ILn may contribute to aspects of attentional processing, but through neural re-organization or activity in other regions, there is compensation for the loss of ILn functioning. The ILn appear to be necessary for maintaining performance when working memory demands are increased.

Lesions of Specific and Nonspecific Thalamic Nuclei Affect Prefrontal Cortex-Dependent Aspects of Spatial Working Memory

Three studies compared lesions of specific mediodorsal (MD) and nonspecific midline/intralaminar (M/IL) and ventromedial (VM) thalamic nuclei placed to spare the anterior nuclei. Lesions of MD, M/IL, or VM impaired delayed matching trained with retractable levers, a measure of spatial memory affected by prefrontal cortical lesions. The effects of the MD lesion increased at longer retention intervals and thus appeared delay dependent. The effects of M/IL and VM lesions were delay independent. Even when combined, these lesions had no effect on varying choice radial maze delayed nonmatching, a task sensitive to hippocampal or anterior thalamic (but not prefrontal) lesions. These results demonstrate effects of MD, M/IL, and VM lesions distinct from the contributions of hippocampus or anterior thalamus to spatial memory.

Effects of intralaminar thalamic lesions on sensory attention and motor intention in the rat: a comparison with lesions involving frontal cortex and hippocampus

A self-paced serial reaction task was developed to differentiate between the effects of intralaminar thalamic lesions on sensory attention and intentional motor function. Results were compared for hippocampal and frontal cortical lesions to test for the possible involvement of pathways involving these parts of the brain in any impairments associated with the thalamic lesion. Lesions of the intralaminar thalamic nuclei affected response latency without affecting accuracy. This increase in latency was unaffected by variations in stimulus duration, even though this manipulation had a substantial effect on response accuracy. Intralaminar lesions did not affect the response to distracting stimuli or to manipulations of stimulus salience. Thus it seems unlikely that the effects of intralaminar lesions on motor function were related to sensory loss or attentional dysfunction. Hippocampal lesions had no significant effect on any measure of performance. Frontal cortical lesions were associated with an increase in latency comparable to the intralaminar group and also affected the accuracy of responding to brief stimuli or under conditions of reduced stimulus salience. These results are discussed in light of evidence that lesions of the intralaminar nuclei affect functions mediated by anatomically related areas of frontal cortex and striatum.

Effects of dorsal and ventral striatal lesions on delayed matching trained with retractable levers

Recent evidence has suggested that thalamic amnesia results from damage to the intralaminar nuclei, an important source of input to striatum. To test the hypothesis that intralaminar damage disrupts functions mediated by striatum, we studied the effects of striatal lesions on a delayed matching task known to be affected by intralaminar lesions. Rats were trained to perform the task and given one of five treatments: sham surgery or a lesion of medial or lateral caudate/putamen, nucleus accumbens, or ventral striatum. Rats with ventral striatal lesions were impaired compared to all other groups. Rats with medial caudate/putamen or nucleus accumbens lesions were impaired compared to controls. The effects of ventral striatal lesions were sufficient to account for impairments in the accuracy and latency of delayed matching responses observed in previous studies of intralaminar and medial frontal cortical lesions. The ventral striatal lesions involved portions of ventral pallidum and thus it seems likely that they affected functions mediated by the nucleus accumbens as well as striatal areas of the tubercle. Serial reversal learning trained in the same apparatus with the same reinforcer was unaffected by all of the lesions. These results are discussed in terms of the roles of midline thalamic nuclei and of thalamo-cortico-striatal circuits in delayed conditional discrimination tasks.

Effects of reversible inactivation of thalamo-striatal circuitry on delayed matching trained with retractable levers

The intralaminar thalamic nuclei are characterized by their prominent projections to striatum. Lesions of the intralaminar nuclei have been found to impair delayed matching trained with retractable levers. Comparable impairments have been observed for rats with lesions of the olfactory tubercle, involving ventral areas of striatum and pallidum. We conducted two experiments to test the functional dependence of thalamic and striatal lesions on the delayed matching task. In experiment 1, we determined the effects of inactivating the intralaminar nuclei with bilateral lidocaine infusions. In experiment 2, we compared the effects of unilateral thalamic inactivations in rats with unilateral olfactory tubercle lesions. We trained rats to perform the delayed matching task to criterion and then implanted dual cannulas aimed at the bilaterally symmetrical areas in the intralaminar nuclei. Rats in experiment 2 were also given a unilateral olfactory tubercle lesion. The results of experiment 1 showed dose-dependent impairments for bilateral infusions that were qualitatively similar, although of lesser severity than delayed matching impairments observed in previous studies for rats with lesions involving extensive areas of the intralaminar nuclei. A comparable impairment was observed in experiment 2 when thalamus was inactivated on the side opposite the olfactory tubercle lesion. Performances were significantly worse when thalamus was inactivated on the contra-lesion than on the ipsi-lesion side of the brain. These results are discussed in terms of the role of ventral striatum and related thalamic nuclei in memory.

A comparison of the effects of frontal cortical and thalamic lesions on measures of spatial learning and memory in the rat

Two experiments were conducted to compare the effects of radiofrequency lesions of thalamus and frontal cortex on three measures of spatial learning and memory: delayed non-matching to sample (DNMTS), radial arm maze with imposed delays, and serial reversal learning. Thalamic lesions were aimed at the lateral internal medullary lamina (L-IML) and cortical lesions at the projection areas of the mediodorsal nucleus along the medial wall (MW) and dorsal to the rhinal sulcus (RS) in frontal cortex. In Experiment 1 rats were trained on DNMTS prior to surgery. After recovery, rats with MW lesions showed persistent deficits on DNMTS that were significantly greater than for RS lesions. The deficits observed in MW lesioned animals were comparable to the effects of L-IML lesions on this task that have been described in previous studies. In Experiment 2 animals were trained to perform the radial arm maze task prior to treatment. After recovery, animals with L-IML lesions were impaired on the radial arm maze and on subsequent acquisition of the serial reversal task. Rats with RS and MW lesions showed transient impairments on the radial arm maze task, but otherwise performed as well as controls on both these tasks.

Thiamine deficiency in rats produces cognitive and memory deficits on spatial tasks that correlate with tissue loss in diencephalon, cortex and white matter

Exploratory activity, spontaneous alternation, learning and memory abilities were examined in the pyrithiamine-induced thiamine deficiency (PTD) rat model of Wernicke-Korsakoff's syndrome and pair-fed controls (CT). PTD and CT animals showed normal retention of a single trial of a passive avoidance task acquired prior to the acute stages of thiamine deficiency. While there were no significant group differences in spontaneous activity, PTD animals with extensive damage to internal medullary lamina (IML-lesioned) of thalamus and mammillary body nuclei demonstrated a significant decrease in spontaneous alternation and were significantly impaired in learning both the initial spatial non-matching-to-position (NMTP) task and the reverse MTP task. PTD animals without IML damage (IML-spared) were only impaired on the acquisition of NMTP. Examination of response patterns suggest that the learning impairment was related to an inability to adopt or shift to the appropriate response rule. Performance of PTD IML-lesioned animals on NMTP mixed-delay sessions (4, 30, 60, 90 s) was similar to controls and PTD IML-spared, but was significantly lower on MTP delay trials. These IML-lesioned rats also had significant reductions in thickness of frontal and parietal cortex, corpus callosum and severe neuronal loss in anterior and reticular thalamic nucleic. Four PTD IML-lesioned animals that were unable to learn the NMTP task had more extensive cortical, white matter and thalamic damage than the PTD IML-lesioned animals that did learn the task. These results demonstrate that thiamine deficiency in the rat produces behavioral changes ranging from mild cognitive deficits to severe learning and memory impairments. Pathologic damage following a bout of thiamine deficiency also varies from neuronal loss in select thalamic nuclei to tissue loss in large regions of thalamus, mammillary bodies and cortex. Learning and memory deficits are closely related to the degree of cortical and diencephalic damage.

Influence of thiamine on the behavioral sensitivity to ethanol

Changes in sensitivity to ethanol's rate-decreasing effects on operant performance were examined in control rats and cohorts that received diet-induced or diet+pyrithiamine-induced thiamine deficiency. Seven groups of male Sprague-Dawley rats (12 rats/group) were trained in a 5-cycle lever-press operant task under a fixed-ratio 30 schedule of food reinforcement. Once trained to maintain consistent operant performance across all 5 cycles, each rat was tested with various doses of ethanol injected at the beginning of each time-out cycle. Each group of rats demonstrated equivalent saline baseline operant performance and ED50 for ethanol's rate-suppressing effects. Training sessions were suspended and rats received either a short- (9 days) or long-term (5-week) exposure to regular rat chow diet or thiamine-deficient diet, and received either saline or pyrithiamine injections in a 2 x 2 design. Three additional control groups were maintained on a regular rat chow diet and received supplemental injections of either thiamine+pyrithiamine injections, thiamine+saline injections, or saline+pyrithiamine injections. The controlled diet phase continued until the development of overt signs of thiamine deficiency, at which time thiamine supplements were administered for 4 days. In phase 3, all rats were retrained in the operant task and a second ethanol dose-effect function was generated. A history of thiamine deficiency and recovery failed to shift the behavioral dose-effect functions significantly for ethanol and their associated blood alcohol curves. Most interestingly, significant behavioral sensitization to ethanol's rate suppressant effects was demonstrated in the two control groups of rats receiving regular rat chow diet in combination with supplemental injections of thiamine and either saline or pyrithiamine.(ABSTRACT TRUNCATED AT 250 WORDS)

On the role of thalamic pathology in diencephalic amnesia

Although it is now accepted that medial diencephalic lesions can produce severe amnesia in humans, the specific nuclei and neural pathways that must be damaged to impair memory have not yet been identified. Recent studies have shown that pyrithiamine-induced thiamine deficiency (PTD) in the rat can produce a consistent pattern of pathology in the thalamus and mammillary bodies and result in permanent impairments on behavioral measures of working memory. Behavioral deficits comparable to the PTD model have been observed in rats with thalamic lesions involving lateral portions of the internal medullary lamina (the L-IML site). Such impairments are not observed following lesions of limbic-related pathways associated with the fornix, mammillary bodies, or midline thalamus. The L-IML lesion affects the mediodorsal nucleus (MDn) and both the intralaminar and paralaminar non-specific thalamic nuclei. The relationship between the non-specific thalamic nuclei and working memory is underscored by the limited behavioral effects of MDn lesions, as compared to either L-IML or PTD-induced lesions, and by anatomical analyses of PTD-related pathology, which seems to destroy the non-specific nuclei while sparing large portions of the MDn. Recent physiological studies of thalamocortical processes suggest that there are several possible mechanisms by which the non-specific nuclei might participate in memory and by which lesions in these pathways might interfere with the consolidation of memories within the cortex.