Impaired Tubular Secretion of Organic Solutes in Advanced Chronic Kidney Disease.

BACKGROUND: The clearance of solutes removed by tubular secretion may be altered out of proportion to the GFR in CKD. Recent studies have described considerable variability in the secretory clearance of waste solutes relative to the GFR in patients with CKD. METHODS: To test the hypothesis that secretory clearance relative to GFR is reduced in patients approaching dialysis, we used metabolomic analysis to identify solutes in simultaneous urine and plasma samples from 16 patients with CKD and an eGFR of 7±2 ml/min per 1.73 m2 and 16 control participants. Fractional clearances were calculated as the ratios of urine to plasma levels of each solute relative to those of creatinine and urea in patients with CKD and to those of creatinine in controls. RESULTS: Metabolomic analysis identified 39 secreted solutes with fractional clearance >3.0 in control participants. Fractional clearance values in patients with CKD were reduced on average to 65%±27% of those in controls. These values were significantly lower for 18 of 39 individual solutes and significantly higher for only one. Assays of the secreted anions phenylacetyl glutamine, p-cresol sulfate, indoxyl sulfate, and hippurate confirmed variable impairment of secretory clearances in advanced CKD. Fractional clearances were markedly reduced for phenylacetylglutamine (4.2±0.6 for controls versus 2.3±0.6 for patients with CKD; P<0.001), p-cresol sulfate (8.6±2.6 for controls versus 4.1±1.5 for patients with CKD; P<0.001), and indoxyl sulfate (23.0±7.3 versus 7.5±2.8; P<0.001) but not for hippurate (10.2±3.8 versus 8.4±2.6; P=0.13). CONCLUSIONS: Secretory clearances for many solutes are reduced more than the GFR in advanced CKD. Impaired secretion of these solutes might contribute to uremic symptoms as patients approach dialysis.

Effects of choice on neuronal activity in anterior cingulate, prelimbic, and infralimbic cortices in the rat: Comparison of serial lever pressing with delayed nonmatching to position.

The ability to choose between response alternatives based on their likely consequences depends on distributed neural circuits that involve rodent medial prefrontal cortex (mPFC). To understand the effects of choice on mPFC function, we compared the activity of mPFC neurons in rats performing two tasks: dynamic delayed nonmatching to position (dDNMTP), a task with a prefrontal-dependent conditional choice, and serial lever pressing (SLP), a task lacking a choice but trained in the same apparatus with sequences of actions and reinforcements matched to dDNMTP. More neurons exhibited event-related responses during dDNMTP than SLP. Average firing rate during recording sessions was higher during dDNMTP for neurons with event-related responses, but lower for neurons with activity unrelated to behavioural events. Thus, compared to SLP, dDNMTP appears to enhance the activity of neurons that represent behaviourally relevant information and to suppress the activity of neurons that do not. dDNMTP was associated with responses related to preparation and memory delay that were not observed during SLP as well as enhanced responses related to movement and reinforcement. These results provide evidence that choice in the dDNMTP task is associated with adaptive changes in background firing rates and coding properties of mPFC neurons.

Accumulation of uremic solutes in the cerebrospinal fluid in experimental acute renal failure.

The accumulation of uremic solutes in kidney failure may impair mental function. The present study profiled the accumulation of uremic solutes in the cerebrospinal fluid (CSF) in acute renal failure. CSF and plasma ultrafiltrate were obtained from rats at 48 h after sham operation (control; n = 10) or bilateral nephrectomy (n = 10) and analyzed using an established metabolomic platform. Two hundred forty-eight solutes were identified as uremic based on their accumulation in the plasma ultrafiltrate of nephrectomized compared with control rats. CSF levels of 124 of these solutes were sufficient to allow calculation of CSF-to-plasma ultrafiltrate concentration ratios. Levels of many of the uremic solutes were normally lower in the CSF than in the plasma ultrafiltrate, indicating exclusion of these solutes from the brain. CSF levels of the great majority of the uremic solutes increased in renal failure. The increase in the CSF was, however, relatively less than in the plasma ultrafiltrate for most solutes. In particular, for the 31 uremic solutes with CSF-to-plasma ultrafiltrate ratios of <0.25 in control rats, the average CSF-to-plasma ultrafiltrate ratio decreased from 0.13 ± 0.07 in control rats to 0.09 ± 0.06 in nephrectomized rats, revealing sustained ability to exclude these solutes from the brain. In summary, levels of many uremic solutes are normally kept lower in the CSF than in the plasma ultrafiltrate by the action of the blood-brain and blood-CSF barriers. These barriers remain functional but cannot prevent accumulation of uremic solutes in the CSF when the kidneys fail.

Central thalamic inactivation impairs the expression of action- and outcome-related responses of medial prefrontal cortex neurons in the rat.

The mediodorsal (MD) and adjacent intralaminar (IL) and midline nuclei provide the main thalamic input to the medial prefrontal cortex (mPFC) and are critical for associative learning and decision-making. MD neurons exhibit activity related to actions and outcomes that mirror responses of mPFC neurons in rats during dynamic delayed non-match to position (dDNMTP), a variation of DNMTP where start location is varied randomly within an open octagonal arena to avoid confounding behavioral events with spatial location. To test whether the thalamus affects the expression of these responses in mPFC, we inhibited the central thalamus unilaterally by microinjecting muscimol at doses and sites found to affect decision-making when applied bilaterally. Unilateral inactivation reduced normalized task-related responses in the ipsilateral mPFC without disrupting behavior needed to characterize event-related neuronal activity. Our results extend earlier findings that focused on delay-related activity by showing that central thalamic inactivation interferes with responses related to actions and outcomes that occur outside the period of memory delay. These findings are consistent with the broad effects of central thalamic lesions on behavioral measures of reinforcement-guided responding. Most (7/8) of the prefrontal response types affected by thalamic inactivation have also been observed in MD during dDNMTP. These results support the hypothesis that MD and IL act as transthalamic gates: monitoring prefrontal activity through corticothalamic inputs; integrating this information with signals from motivational and sensorimotor systems that converge in thalamus; and acting through thalamocortical projections to enhance expression of neuronal responses in the PFC that support adaptive goal-directed behavior.

A pathway for spatial memory encoding.

The medial prefrontal cortex has been shown to play a role for rodents in successful completion of tasks that require spatial memory, but the pathways responsible for the transmission of spatial information to the mPFC, and the nature and timing of such information, are unknown. Recently, Spellman, Rigotti, Ahmari, Fusi, Gogos, and Gordon (Nature, 522, 309-314, 2015) addressed these questions in an eloquent and ingenious series of experiments, which we review in the broader context of the neurobiology of spatial memory.

Advances in understanding mechanisms of thalamic relays in cognition and behavior.

The main impetus for a mini-symposium on corticothalamic interrelationships was the recent number of studies highlighting the role of the thalamus in aspects of cognition beyond sensory processing. The thalamus contributes to a range of basic cognitive behaviors that include learning and memory, inhibitory control, decision-making, and the control of visual orienting responses. Its functions are deeply intertwined with those of the better studied cortex, although the principles governing its coordination with the cortex remain opaque, particularly in higher-level aspects of cognition. How should the thalamus be viewed in the context of the rest of the brain? Although its role extends well beyond relaying of sensory information from the periphery, the main function of many of its subdivisions does appear to be that of a relay station, transmitting neural signals primarily to the cerebral cortex from a number of brain areas. In cognition, its main contribution may thus be to coordinate signals between diverse regions of the telencephalon, including the neocortex, hippocampus, amygdala, and striatum. This central coordination is further subject to considerable extrinsic control, for example, inhibition from the basal ganglia, zona incerta, and pretectal regions, and chemical modulation from ascending neurotransmitter systems. What follows is a brief review on the role of the thalamus in aspects of cognition and behavior, focusing on a summary of the topics covered in a mini-symposium held at the Society for Neuroscience meeting, 2014.

Inactivation of ventral midline thalamus produces selective spatial delayed conditional discrimination impairment in the rat.

The reuniens (Re) and rhomboid (Rh) nuclei are organized to influence activity in distributed limbic networks involving hippocampus and medial prefrontal cortex (mPFC). To elucidate the role of these nuclei in spatial memory we inactivated Re and Rh in rats with the GABA(A) agonist muscimol and compared effects on two spatial delayed conditional discriminations: delayed nonmatching to position (DNMTP) and varying choice radial maze delayed nonmatching (VC-DNM). DNMTP is trained in operant chambers and requires rats to choose between the same two levers on all trials. VC-DNM is trained in automated radial mazes and requires rats to choose between two arms, randomly selected from eight alternatives on each trial. DNMTP is affected by hippocampal and mPFC lesions while VC-DNM is affected by hippocampal, but not mPFC lesions (Porter et al. (2000) Behav Brain Res 109:69-81). We found evidence of a localized (low dose) effect of ReRh inactivation on DNMTP but not VC-DNM. This was confirmed by comparison with muscimol injections in an anatomical control site. These results are consistent with evidence that Re and Rh affect measures of spatial working memory that depend on interactions between hippocampus and mPFC, but not measures that depend on hippocampus alone.

Incidence of hepatocellular carcinoma among US patients with cirrhosis of viral or nonviral etiologies.

BACKGROUND & AIMS: We aimed to identify risk factors for hepatocellular carcinoma (HCC) in patients with cirrhosis in the United States. We performed a prospective study to identify associations between etiologies of cirrhosis and ethnicity with HCC incidence. METHODS: We used convenience sampling to select a cohort of 379 patients with cirrhosis who visited the liver clinic at the Stanford University Medical Center from 2001 to 2009 (65% male, 75% white or Hispanic, and 20% Asian). Study end points were HCC diagnosis by histology or noninvasive criteria, liver transplantation, or last screening without HCC. Patients were followed up, with ultrasound or computed tomographic imaging analyses and measurements of serum levels of α-fetoprotein, approximately every 6 months, for a median time of 34 months (range, 6-99 mo). RESULTS: The etiologies of cirrhosis in the cohort were 68% hepatitis C, 7% hepatitis B, and 25% nonviral. Forty-four patients (12%) were diagnosed with HCC during the follow-up period. Patients with cirrhosis related to viral hepatitis had a statistically significantly higher incidence of HCC than those with nonviral diseases in Kaplan-Meier analysis (P = .04). There was no statistically significant difference in HCC incidence between Asian and non-Asian patients. In a multivariate Cox proportional hazards model that included age, sex, ethnicity, etiology, and Child-Pugh-Turcotte score, viral cirrhosis was associated significantly with HCC, compared with nonviral cirrhosis (hazard ratio, 3.6; 95% confidence interval, 1.3-10.1; P = .02) but Asian ethnicity was not. CONCLUSIONS: In a diverse cohort of patients in the United States with cirrhosis, a viral etiology of cirrhosis was associated with increased incidence of HCC, but Asian ethnicity was not. These findings indicate the importance of cirrhosis etiology in determining risk for HCC.

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.

Lesions of reuniens and rhomboid thalamic nuclei impair radial maze win-shift performance.

The reuniens (Re) and rhomboid (Rh) nuclei are major sources of thalamic input to hippocampus and medial prefrontal cortex. We compared effects of lesions in ReRh and other parts of the midline-intralaminar complex on tasks affected by lesions in terminal fields innervated by these nuclei, including: visuospatial reaction time (VSRT), a measure of sensory guided responding; serial VSRT, a measure of action sequence learning; and win/shift radial arm maze (RAM) measures of spatial memory. ReRh lesions affected RAM, but not VSRT or serial VSRT performance. The effects of caudal intralaminar lesions were doubly dissociated from ReRh lesions, affecting VSRT, but not RAM or serial VSRT performance. Rostral intralaminar lesions did not produce significant impairments, other than a subgroup with larger lesions that were impaired performing a delayed RAM task. Combined lesions damaging all three sites produced RAM deficits comparable to ReRh lesions and VSRT deficits comparable to caudal intralaminar lesions. Thus there was no indication that deficits produced by lesions in one site were exacerbated significantly by the cumulative effect of damage in other parts of the midline-intralaminar complex. The effects of ReRh lesions provide evidence that these nuclei affect memory functions of hippocampus and medial prefrontal cortex. The double dissociation observed between the effects of ReRh and caudal intralaminar nuclei provides evidence that different nuclei within the midline-intralaminar complex affect distinct aspects of cognition consistent with the effects of lesions in the terminal fields they innervate.

Central Thalamic-Medial Prefrontal Control of Adaptive Responding in the Rat: Many Players in the Chamber.

The medial prefrontal cortex (mPFC) has robust afferent and efferent connections with multiple nuclei clustered in the central thalamus. These nuclei are elements in large-scale networks linking mPFC with the hippocampus, basal ganglia, amygdala, other cortical areas, and visceral and arousal systems in the brainstem that give rise to adaptive goal-directed behavior. Lesions of the mediodorsal nucleus (MD), the main source of thalamic input to middle layers of PFC, have limited effects on delayed conditional discriminations, like DMTP and DNMTP, that depend on mPFC. Recent evidence suggests that MD sustains and amplifies neuronal responses in mPFC that represent salient task-related information and is important for detecting and encoding contingencies between actions and their consequences. Lesions of rostral intralaminar (rIL) and ventromedial (VM) nuclei produce delay-independent impairments of egocentric DMTP and DNMTP that resemble effects of mPFC lesions on response speed and accuracy: results consistent with projections of rIL to striatum and VM to motor cortices. The ventral midline and anterior thalamic nuclei affect allocentric spatial cognition and memory consistent with their connections to mPFC and hippocampus. The dorsal midline nuclei spare DMTP and DNMTP. They have been implicated in behavioral-state control and response to salient stimuli in associative learning. mPFC functions are served during DNMTP by discrete populations of neurons with responses related to motor preparation, movements, lever press responses, reinforcement anticipation, reinforcement delivery, and memory delay. Population analyses show that different responses are timed so that they effectively tile the temporal interval from when DNMTP trials are initiated until the end. Event-related responses of MD neurons during DNMTP are predominantly related to movement and reinforcement, information important for DNMTP choice. These responses closely mirror the activity of mPFC neurons with similar responses. Pharmacological inactivation of MD and adjacent rIL affects the expression of diverse action- and outcome-related responses of mPFC neurons. Lesions of MD before training are associated with a shift away from movement-related responses in mPFC important for DNMTP choice. These results suggest that MD has short-term effects on the expression of event-related activity in mPFC and long-term effects that tune mPFC neurons to respond to task-specific information.

Metabolomic analysis of uremic pruritus in patients on hemodialysis.

Pruritus is a common debilitating symptom experienced by hemodialysis patients. Treatment is difficult because the cause of uremic pruritus is not known. This study addressed the hypothesis that pruritus is caused by solutes that accumulate in the plasma when the kidneys fail. We sought to identify solutes responsible for uremic pruritus using metabolomic analysis to compare the plasma of hemodialysis patients with severe pruritus versus mild/no pruritus. Pruritus severity in hemodialysis patients was assessed using a 100-mm visual analogue scale (VAS), with severe pruritus defined as >70 mm and mild/no pruritus defined as <10 mm. Twelve patients with severe pruritus (Itch) and 24 patients with mild/no pruritus (No Itch) were included. Pre-treatment plasma and plasma ultrafiltrate were analyzed using an established metabolomic platform (Metabolon, Inc.). To identify solutes associated with pruritus, we compared the average peak area of each solute in the Itch patients to that of the No Itch patients using the false discovery rate (q value) and principal component analysis. Dialysis vintage, Kt/Vurea, and serum levels of calcium, phosphorus, PTH, albumin, ferritin, and hemoglobin were similar in the Itch and No Itch patients. Metabolomic analysis identified 1,548 solutes of which 609 were classified as uremic. No difference in the plasma or plasma ultrafiltrate levels of any solute or group of solutes was found between the Itch and No Itch patients. Metabolomic analysis of hemodialysis patients did not reveal any solutes associated with pruritus. A limitation of metabolomic analysis is that the solute of interest may not be included in the metabolomic platform's chemical library. A role for uremic solutes in pruritus remains to be established.

Impaired Tubular Secretion of Organic Solutes in Acute Kidney Injury.

BACKGROUND: Impairment of kidney function is routinely assessed by measuring the accumulation of creatinine, an organic solute cleared largely by glomerular filtration. We tested whether the clearance of solutes that undergo tubular secretion is reduced in proportion to the clearance of creatinine in humans with AKI. METHODS: Four endogenously produced organic solutes (phenylacetylglutamine [PAG], hippurate [HIPP], indoxyl sulfate [IS], and p-cresol sulfate [PCS]) were measured in spot urine and plasma samples from ten patients with AKI and 17 controls. Fractional clearance relative to creatinine was calculated to assess tubular secretion. Fractional clearance values were calculated in terms of the free, unbound levels of HIPP, IS, and PCS that bind to plasma proteins. RESULTS: Fractional clearance values for PAG, HIPP, IS, and PCS were >1.0 in patients with AKI as well as controls, indicating that these solutes were still secreted by the tubules of the injured kidneys. Fractional clearance values were, however, significantly lower in patients with AKI than controls, indicating that kidney injury reduced tubular secretion more than glomerular filtration (AKI versus control: PAG, 2.1±0.7 versus 4.6±1.4, P<0.001; HIPP, 10±5 versus 15±7, P=0.02; IS, 10±6 versus 28±7, P<0.001; PCS, 3.3±1.8 versus 10±3, P<0.001). Free plasma levels rose out of proportion to total plasma levels for each of the bound solutes in AKI, so that calculating their fractional clearance in terms of their total plasma levels failed to reveal their impaired secretion. CONCLUSIONS: Tubular secretion of organic solutes can be reduced out of proportion to glomerular filtration in AKI. Impaired secretion of protein-bound solutes may be more reliably detected when clearances are expressed in terms of their free, unbound levels in the plasma.

Memory enhancement with event-related stimulation of the rostral intralaminar thalamic nuclei.

The rostral intralaminar thalamic nuclei (ILn) are organized to activate pathways originating in medial prefrontal cortex (mPF) that mediate memory-guided responding during alert, wakeful states. Previous studies have shown that rostral ILn or mPF lesions produce deficits in delayed matching to position (DMTP). Here, we manipulated rostral ILn activity in rats by microinjecting drugs or applying electrical current and examined effects on DMTP. Inhibiting activity with the GABA(A) agonist muscimol impaired DMTP. Decreasing GABA(A) tone with FG-7142 (N-methyl-beta-carboline-3-carboxamide) improved DMTP at low but not high doses. Orexin A, which depolarizes thalamocortical neurons locally within the ILn, improved DMTP, whereas the cholinergic agonist carbachol impaired performance at the highest dose tested. These drug effects were unaffected by partial mPF lesions in a subset of animals. Microinjection results are consistent with an inverted-U relationship between thalamic activity and DMTP. This relationship was confirmed by event-related electrical stimulation, which produced improvement at low stimulation currents and impairment at higher currents. Electrical stimulation affected DMTP when applied at the start of the memory delay or choice response, but not earlier when trials began or the sample lever was presented. Our results are consistent with evidence that the rostral ILn play a role in retrieval, carrying response-related information across brief memory delays and facilitating memory-guided responding. They also provide evidence that treatments stimulating rostral ILn activity may be an effective means to enhance working memory and related cognitive processes and thus to treat disorders that affect these functions.

Representation of actions and outcomes in medial prefrontal cortex during delayed conditional decision-making: Population analyses of single neuron activity.

BACKGROUND: To respond adaptively in a dynamic environment, it is important for organisms to utilise information about recent events to decide between response options. METHODS: To examine the role of medial prefrontal cortex in adaptive decision-making, we recorded single neuron activity in rats performing a dynamic delayed non-matching to position task. RESULTS: We recorded activity from 1335 isolated neurons, 458 (34%) with criterion event-related activity, of which 431 (94%) exhibited 1 of 10 distinct excitatory response types: five at different times relative to delivery (or lack) of reinforcement following sample and choice responses and five correlated with movements or lever press actions that occurred multiple times in each trial. Normalised population averages revealed a precisely timed cascade of population responses representing the temporal organisation behavioural events that constitute delayed non-matching to position trials. Firing field analyses identified a subset of neurons with restricted spatial fields: responding to the conjunction of a behavioural event with a specific location. Anatomical analyses showed considerable overlap in the distribution of different response types in medial prefrontal cortex with a significant trend for dorsal areas to contain more neurons with action-related activity and ventral areas more responses related to action outcomes. CONCLUSION: These results indicate that medial prefrontal cortex contains discrete populations of neurons that represent the temporal organisation of actions and outcomes during delayed non-matching to position trials. They support the hypothesis that medial prefrontal cortex promotes flexible control of complex behaviours by action-outcome contingencies.

Uremic Toxin Clearance and Cardiovascular Toxicities.

Uremic solutes contribute to cardiovascular disease in renal insufficiency. In this review we describe the clearance of selected uremic solutes, which have been associated with cardiovascular disease. These solutes-indoxyl sulfate (IS), p-cresol sulfate (PCS), phenylacetylglutamine (PAG), trimethylamine-n-oxide (TMAO), and kynurenine-exemplify different mechanisms of clearance. IS and PCS are protein-bound solutes efficiently cleared by the native kidney through tubular secretion. PAG and TMAO are not protein-bound but are also cleared by the native kidney through tubular secretion, while kynurenine is not normally cleared by the kidney. Increases in the plasma levels of the normally secreted solutes IS, PCS, TMAO, and PAG in chronic kidney disease (CKD) are attributable to a reduction in their renal clearances. Levels of each of these potential toxins are even higher in patients on dialysis than in those with advanced chronic kidney disease, which can be accounted for in part by a low ratio of dialytic to native kidney clearance. The rise in plasma kynurenine in CKD and dialysis patients, by contrast, remains to be explained. Our ability to detect lower levels of the potential uremic cardiovascular toxins with renal replacement therapy may be limited by the intermittency of treatment, by increases in solute production, and by the presence of non-renal clearance. Reduction in the levels of uremic cardiovascular toxins may in the future be achieved more effectively by inhibiting their production.

Characteristics of Colon-Derived Uremic Solutes.

BACKGROUND AND OBJECTIVES: Colon microbial metabolism produces solutes that are normally excreted in the urine and accumulate in the plasma when the kidneys fail. This study sought to further identify and characterize human colon-derived uremic solutes. DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS: Colon-derived solutes normally excreted in the urine were identified by comparing urine from controls (n=17) and patients with total colectomies (n=12), using an established metabolomic platform. Colon-derived solutes that accumulate in kidney failure were then identified by comparing the plasma of the control patients with that of patients on dialysis (n=14). RESULTS: Ninety-one urinary solutes were classified as colon-derived on the basis of the finding of a urine excretion rate at least four-fold higher in control patients than in patients with total colectomies. Forty-six were solutes with known chemical structure, 35 of which had not previously been identified as colon-derived. Sixty of the colon-derived solutes accumulated in the plasma of patients with ESKD to a degree greater than urea and were therefore classified as uremic. The estimated urinary clearance for 27 out of the 32 colon-derived solutes for which clearance could be calculated exceeded that of creatinine, consistent with tubular secretion. Sulfatase treatment revealed that 42 out of the 91 colon-derived solutes detected were likely conjugates. CONCLUSIONS: Metabolomic analysis identified numerous colon-derived solutes that are normally excreted in human urine. Clearance by tubular secretion limits plasma levels of many colon-derived solutes.

The role of striatum in initiation and execution of learned action sequences in rats.

To understand the role of striatum in motor sequence learning, we trained rats to perform a series of tasks measuring speed and accuracy of responding to luminance cues presented as discriminative stimuli for single nose pokes or for sequences of nose pokes in a serial reaction time task. Habit (stimulus-response) learning was measured by comparing performances when stimuli were repeated (predictable) with when they were selected randomly (unpredictable). Sequences had defined start and end points and were limited to five nose pokes to minimize chunking. When sequences were repeated, response time (RT) increased for nose pokes initiating the sequence and decreased for nose pokes completing it. These effects developed incrementally across sessions, consistent with the time course of habit learning. Medial (mCPu), lateral, and complete (CPu) caudate-putamen lesions affected speed and accuracy of single nose poke responses, confirming the role of these areas in guiding responses with external sensory stimuli. None of these lesions affected the short-term increase in accuracy observed when single nose poke responses were repeated. Both mCPu and CPu lesions increased RTs for initiating sequential responses, effects that were exacerbated across sessions in which specific sequences were repeated. None of the lesions affected the gradual decrease in RT for nose pokes completing repeated sequences. Correlational analyses confirmed the relationship between the extent of dorsal striatal damage and the ability to respond to brief luminance cues and to initiate learned sequences. These results provide evidence implicating dorsal striatum in higher-level organizational aspects of learning reflected in planning that precedes the execution of learned action sequences.

Cholinergic modulation of visuospatial responding in central thalamus.

Central thalamus has extensive connections with basal ganglia and frontal cortex that are thought to play a critical role in sensory-guided goal-directed behavior. Central thalamic activity is influenced by cholinergic projections from mesopontine nuclei. To elucidate this function we trained rats to respond to lights in a reaction time (RT) task and compared effects of muscarinic (2.4, 7.3, 22 nmol scopolamine) and nicotinic (5.4, 16, 49, 98 nmol mecamylamine) antagonists with the GABA(A) agonist muscimol (0.1, 0.3, 1.0 nmol) in central thalamus. We compared this with subcutaneous (systemic) effects of mecamylamine (3.2, 9.7, 29 micromol/kg) and scopolamine (0.03, 0.09, 0.26 micromol/kg). Subcutaneous scopolamine increased omissions (failure to respond within a 3-s response window) at the highest dose tested. Subcutaneous mecamylamine increased omissions at the highest dose tested while impairing RT and per cent correct at lower doses. Intrathalamic injections of muscimol and mecamylamine decreased per cent correct at doses that did not affect omissions or RT. Intrathalamic scopolamine increased omissions and RT at doses that had little effect on per cent correct. Anatomical controls indicated that the effects of mecamylamine were localized in central thalamus and those of scopolamine were not. Drug effects did not interact with attention-demanding manipulations of stimulus duration, proximity of stimulus and response locations, or stimulus array size. These results are consistent with the hypothesis that central thalamus mediates decisional processes linking sensory stimuli with actions, downstream from systems that detect sensory signals. They also provide evidence that this function is specifically influenced by nicotinic cholinergic receptors.

Amphetamine depresses excitatory synaptic transmission at prefrontal cortical layer V synapses.

Dopamine modulates the function of glutamatergic synapses in prefrontal cortex, modifying synaptic strength and influencing synaptic plasticity. Here we have explored the ability of endogenous dopamine, present in slices containing the prefrontal cortex, to influence excitatory synaptic transmission. We found that 10 microM amphetamine, which releases and blocks the reuptake of dopamine from dopaminergic nerve terminals, significantly depressed excitatory field potentials recorded in layer V during stimulation of layer II/III. The depression was reversible, dose dependent and correlated with increased paired pulse facilitation, suggesting that amphetamine inhibits the presynaptic release of glutamate. Pharmacological dissection of this response showed that dopamine D1 receptors are likely to mediate the effects of endogenous dopamine on excitatory synaptic transmission, with little effect of alpha2 adrenergic receptors, serotonin receptors, or D2 dopamine receptors. The time to peak amphetamine effect was longer than expected based on diffusion, suggesting that to raise dopamine levels in brain slices amphetamine may need to be transported into the presynaptic terminals. These results provide evidence that D1/D5 receptors depress glutamate release at this cortical synapse, and suggest that amphetamine will have profound and persistent effects on PFC functioning in vivo. Dysregulation of this mechanism could contribute to the impairment in cognitive performance associated with abnormal PFC dopamine receptor activation.