Striatal efferent fibers play a role in maintaining rotational behavior in the rat.

Rats in which ascending dopamine-containing neurons have been unilaterally destroyed by injections of 6-hydroxydopamine are known to rotate after being injected with apomorphine or L-dopa. The rotation is markedly reduced by either (i) ipsilateral electrocoagulations of the caudate-putamen or internal capsule or (ii) ipsilateral coronal knife cuts immediately rostral to the substantia nigra. Neostriatal efferent fibers, in particular the strionigral projection, appear to be required for the expression of this dopamine-dependent behavior.

Noradrenaline nerve terminals in human cerebral cortices: first histochemical evidence.

The cerebral and cerebellar cortices of man are richly provided with varicose noradrenaline nerve terminals, which are visualized by fluorescence histochemistry of brain smears obtained by a new technique. The density of such nerves in human cortices equals that of the rat. The method permits simple and rapid analysis of noradrenergic nerves of the human cortex during routine neurosurgical operations.

Intravenous glutamine supplementation to head trauma patients leaves cerebral glutamate concentration unaffected.

OBJECTIVE: There is reluctance to use glutamine-containing i.v. nutrition for neurosurgical patients, as this may result in elevated intracerebral glutamate levels, which are thought to be associated with neuronal injury and cell swelling, causing an increase in ICP and an unfavourable outcome. As general ICU patients benefit from i.v. glutamine supplementation in terms of reduced mortality and morbidity, neurosurgical patients might also be candidates for such treatment, if the possible relation between i.v. glutamine supplementation and a possible increase in cerebral glutamate could be sorted out. DESIGN AND SETTING: The study protocol had a crossover design with a 24h treatment period and a 24h placebo period in random order. Treatment was a glutamine containing dipeptide, L-alanyl-L-glutamine 200mg/ml, for 20h; placebo was saline. The rate of infusion was 0.125ml/kg/h, which is equal to 0.34g/kg of glutamine over the 20h period. Microdialysate was collected for analysis in 120min portions. The flow through the microdialysis catheter was 0.3microl/min. SUBJECTS: Patients with severe head trauma (GCS

Regulation of skeletal muscle blood flow during acute insulin-induced hypoglycemia in the rat.

The mechanism behind hyperemia in skeletal muscle during insulin-induced hypoglycemia was investigated in 42 anesthetized male Wistar rats using the microdialysis ethanol technique of monitoring nutritive blood flow. Microdialysis probes were inserted bilaterally into the gastrocnemius muscle and perfused with a modified Krebs-Henseleit buffer containing 20 mmol/l ethanol and one or more of the following compounds: propranolol (10(-6) mol/l), phentolamine (10(-6) mol/l), and calyculin A (1.0 mumol/l). Muscle blood flow increased, as indicated by a decrease in the ethanol outflow:inflow ratio (P < 0.001, n = 6), during hypoglycemia induced by a bolus intravenous infusion of insulin (680 mU/kg body wt). This increase was not present during normoglycemia or during hypoglycemia and local beta-adrenergic blockade via propranolol. However, the hyperemic response was potentiated during hypoglycemia and local alpha-adrenergic blockade via phentolamine. A normal hyperemic response to hypoglycemia was detected during simultaneous alpha- and beta-adrenergic blockade. This response was eliminated on further supplementation of the microdialysis perfusion medium with calyculin A. Therefore, although stimulation of the alpha- and beta-adrenergic receptors does occur during insulin-induced hypoglycemia, it is not essential for the induction of hyperemia in this state. It may be concluded that hyperinsulinemia results in vasodilatation during hypoglycemia, although hyperinsulinemia does not have an effect on skeletal muscle blood flow under normoglycemic conditions.

Effects of insulin deprivation and replacement on in vivo subcutaneous adipose tissue substrate metabolism in humans.

The effects of insulin deprivation and replacement on adipose tissue metabolism were investigated in vivo with microdialysis in nine insulin-dependent diabetic patients with no residual insulin secretion. Dialysis probes, implanted in abdominal subcutaneous fat, were continuously perfused, and tissue dialysate concentrations of glycerol (lipolysis index), glucose, lactate, and pyruvate were determined. Comparisons were made with respective metabolite levels in venous plasma. After termination of intravenous insulin infusion, free insulin in plasma fell from 130 to 70 pM. At the same time, glucose levels in plasma and adipose tissue rose in parallel. However, the relative increase in glucose levels was greater in adipose tissue than in blood. On the other hand, the increase in glycerol concentration in adipose tissue (35%) was markedly less than that in venous plasma (250%). Lactate and pyruvate levels in adipose tissue and blood remained unchanged. After the resumption of intravenous insulin, free insulin in plasma rose to approximately 600 pM. At the same time, the glucose levels in blood and adipose tissue decreased rapidly, and the glycerol concentration in these tissues decreased to 50% of the baseline levels. The lactate and pyruvate levels in subcutaneous tissue increased briefly after insulin replacement, whereas the lactate but not pyruvate levels in blood showed a similar increase. The alpha- or beta-blocking agents phentolamine and propranolol in the ingoing tissue perfusate did not influence tissue glycerol at any time during the experiment. We concluded that insulin-induced changes in circulating metabolites only partly reflect variations in adipose tissue substrate kinetics. During insulin deprivation, glucose is accumulated in the adipose tissue extracellular compartment, probably because of reduced utilization by the adipocytes.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of physiological hyperinsulinemia on blood flow and interstitial glucose concentration in human skeletal muscle and adipose tissue studied by microdialysis.

The effect of an euglycemic-hyperinsulinemic glucose clamp (94 +/- 5 microU/ml) on blood flow and glucose extraction fraction in human skeletal muscle and adipose tissue was investigated. Limb blood flow was measured by venous occlusion pletysmography and tissue blood flow by the microdialysis ethanol technique. Insulin infusion resulted in an increased blood flow in the calf and forearm (64 and 36%, respectively; P < 0.01) but not in the studied muscles of these limbs (ethanol outflow-to-inflow ratio: m. gastrocnemius 0.144 +/- 0.009 to 0.140 +/- 0.011, NS; m. brachioradialis 0.159 +/- 0.025 to 0.168 +/- 0.027, NS). This was accompanied by an increased extraction fraction of glucose, as measured by an increased arteriovenous difference over the forearm (0.16 +/- 0.04 to 0.70 +/- 0.10 mmol/l; P < 0.001) and by an increase in the estimated arterial-interstitial glucose difference in the gastrocnemius (0.82-1.42 mmol/l) and brachioradialis muscle (0.82-1.97 mmol/l). The blood flow in adipose tissue was significantly increased during insulin infusion, as evidenced by a decreased ethanol outflow-to-inflow ratio (0.369 +/- 0.048 to 0.325 +/- 0.046; P < 0.01). This was accompanied by an unchanged concentration of glucose in the dialysate (-2.6%, NS). In summary, during physiological hyperinsulinemia 1) a blood flow increase was detected in the calf and forearm, but not in the studied muscles of these limbs; 2) the blood flow increased in the subcutaneous adipose tissue; and 3) the estimated arterial-interstitial glucose difference increased in both muscles studied and was larger in the forearm muscle than the arteriovenous glucose difference over the forearm. The present study shows that microdialysis is a useful tool to obtain tissue-specific information about the effect of insulin on blood flow and glucose extraction in human skeletal muscle and adipose tissue.

Measurement of glucose and metabolites in subcutaneous adipose tissue during hyperglycemia with microdialysis at various perfusion flow rates.

BACKGROUND: Microdialysis-based glucose sensors have recently been introduced for monitoring glucose levels in diabetic patients. The flow rate by which the fluid sample is pumped through the microdialysis catheter varies in different studies. AIM: To study the effects of various flow rates on glucose and its metabolites sampled by microdialysis during an oral glucose tolerance test. MATERIAL, METHODS: Glucose, lactate, pyruvate and glycerol were measured with microdialysis in interstitial fluid of subcutaneous adipose tissue in twelve healthy young subjects before and during an oral glucose tolerance test using four different flow rates (0.3, 1, 2 and 5 microL/min) and a 30 mm dialysis membrane. RESULTS: At the basal fasting state the dialysate glucose obtained by 0.3 microL/min was equal to capillary glucose concentration. A decrease in dialysate glucose levels during the basal state was observed for higher flow rates but not for 0.3 microL/min, which indicates a depleting effect. The relative increase after OGTT was similar for capillary glucose and flow rate 0.3 microL/min but not for higher flow rates. CONCLUSION: The low microdialysis flow rate (0.3 microL/min) facilitates the capture of true interstitial glucose concentrations during glucose fluctuations. Thus this low flow rate is preferred in studies of local tissue metabolism.

Self-monitoring of blood glucose in type I diabetic patients: comparison with continuous microdialysis measurements of glucose in subcutaneous adipose tissue during ordinary life conditions.

OBJECTIVE: To evaluate whether frequent self-monitoring of blood glucose (SMBG) sufficiently reflects the true diurnal glucose control during ordinary daily life in type I diabetic patients. RESEARCH DESIGN AND METHODS: By using a microdialysis technique, continuous monitoring of adipose tissue glucose was performed in 24 type I diabetic patients during ambulatory conditions. A microdialysis probe was implanted subcutaneously and perfused by a portable microinfusion pump. Dialysate fractions were collected in 1- to 2-h samples during 3 consecutive days. The diurnal microdialysis glucose profiles were compared with those obtained by SMBG recordings performed seven times a day. RESULTS: In seven patients, the SMBG profiles showed marked aberrations as compared to the continuous microdialysis glucose recordings; during the 3-day study period, 5-6 inconsistencies were registered. In only 4 patients (17%) did SMBG provide a valid reflection (0-2 inconsistencies) of the diurnal glucose profile, whereas in 13 patients the SMBG recordings paralleled the diurnal adipose tissue glucose profiles in an intermediate way (3-4 major inconsistencies). The inaccuracy of the SMBG data was due more often to the fact that wide glucose swings remained unrecognized, rather than to erroneous testing techniques (P < 0.05), and it was more evident during the night (P < 0.05). CONCLUSIONS: In many type I diabetic patients, the true diurnal variability in glycemia is too great to be accurately reflected even by frequent self-monitoring of blood glucose.

On the release of glutamate and aspartate in the basal ganglia of the rat: interactions with monoamines and neuropeptides.

Using highly sensitive analytical procedures, glutamate (Glu), aspartate (Asp) and several putative neurotransmitters and metabolites can be monitored simultaneously in the extracellular space of neostriatum, substantia nigra and cerebral cortex of the rat by in vivo microdialysis. Glu and Asp are found at sub-micromolar concentrations in all investigated brain regions. In order to ascertain their neuronal origin, we have extensively studied the sensitivity of extracellular Glu and Asp levels to: (i) K(+)-depolarization, (ii) Na(+)-channel blockade, (iii) removal of extracellular Ca2+, (iv) depletion of presynaptic vesicles, and (v) integrity of neuronal pathways. The relevance of these criteria for several neurotransmitters monitored simultaneously or in parallel experiments has also been examined. The functional interactions among different neuronal pathways in the basal ganglia are studied by using selective pharmacological treatments, administered systemically, or locally via intracerebral injections or the microdialysis perfusion medium. Immunohistochemical evidence for the existence of Glu and/or Asp neuronal pathways in the basal ganglia of the rat is presented, discussing especially new findings indicating the existence of a Glu-independent Asp system, intrinsic to the neostriatum of the rat. The clinical relevance of these interactions is discussed, focusing on the implications for the treatment of neurodegenerative disorders affecting the basal ganglia.

Variability of glutathione levels in normal breast tissue and subcutaneous fat during the menstrual cycle: an in vivo study with microdialysis technique.

A small increase in the risk of breast cancer has been reported after long term use of combined estrogen-progestagen treatment. Free oxygen radicals and antioxidants such as glutathione are involved in the regulation of proliferation and apoptosis and thereby in carcinogenesis. To study whether the glutathione levels are sex hormone dependent, we used the microdialysis technique to measure the in vivo concentrations of glutathione in breast tissue and sc fat during the menstrual cycle. Six healthy women (23-32 yr old) were investigated early in the follicular phase and the midluteal phase. Two 60-min fractions each were collected by microdialysis of periumbilical fat and breast tissue, respectively. The samples were stored at -70 C and analyzed by high performance liquid chromatography. Glutathione concentrations increased in the midluteal phase compared to those in the follicular phase in both adipose tissue and breast tissue (P < 0.05). The variability of glutathione levels during the menstrual cycle, with higher levels late in the menstrual cycle, indicates that the antioxidant system could be sex hormone dependent. This may be of importance in breast cancer development.

Adrenergic regulation of human adipose tissue metabolism in situ during mental stress.

The adrenergic regulation of adipose tissue lipolysis and glucose metabolism was investigated in situ during a standardized mental stress test in 11 nonobese, healthy subjects, using microdialysis of the extracellular water space in sc adipose tissue. Microdialysis probes were inserted in the abdominal sc fat, and were perfused using solvents with or without adrenoceptor blocking agents. The tissue dialysate concentrations of glycerol (lipolysis index) glucose, lactate, and pyruvate were determined. The glycerol concentration in adipose tissue increased markedly during the stress test and decreased in the poststress period. A similar kinetic pattern was observed in blood. In situ administration of the nonselective beta-adrenoceptor blocking agent propranolol almost completely prevented the stress-induced increase in adipose tissue glycerol levels, whereas a nonselective alpha-adrenoceptor blocking agent (phentolamine) was ineffective in this respect. Plasma levels of glucose and lactate remained unaltered during and after the stress test; at the same time plasma pyruvate decreased moderately. By contrast, glucose, lactate, and pyruvate in adipose tissue increased by 25-30% during or after the stress (P < 0.05). The increase in lactate and pyruvate in adipose tissue after the stress was completely off-set by alpha-adrenoceptor blockade in situ, whereas beta-adrenoceptor blockade in situ did not influence the kinetic pattern of these metabolites. It is concluded that the lipolytic activity in human adipose tissue is markedly enhanced during mental stress, owing to adrenergic mechanisms that are mediated via beta-adrenoceptors. After mental stress, adipose tissue glucose utilization is decreased and routed toward nonoxidative pathways. The latter seems to involve adrenergic effects that are mediated via alpha-adrenoceptors.

Changes in cortical extracellular levels of energy-related metabolites and amino acids following concussive brain injury in rats.

The aim of this study was to measure extracellular chemical changes in the cerebral cortex in response to compression contusion trauma in rats. Energy-related metabolites (i.e., lactate, pyruvate, adenosine, inosine, and hypoxanthine) and amino acids were harvested from the extracellular fluid (ECF) using microdialysis and analyzed by high-performance liquid chromatography. The measurements were performed in cortical tissue, where neuronal injury occurs in this model. The severity of the trauma was varied by using different depths of impact: mild trauma, 1.5 mm; severe trauma, 2.5 mm. The trauma induced a dramatic increase in the ECF levels of energy-related metabolites that was conditioned by the severity of the insult. The ECF level of taurine, glutamate, aspartate, and gamma-aminobutyric acid (GABA) also rose markedly, while other amino acids did not change significantly. The results suggest that the trauma induced a transient, profound focal disturbance of energy metabolism in the cortical tissue, probably as a result of mechanically induced disruption of ion homeostasis and reduced blood flow in combination. The data support the potential role of glutamate and aspartate as mediators of traumatic brain injury. However, the concomitantly released adenosine, GABA, and taurine may be protective and ameliorate excitotoxicity. In analogy with the reported cumulative damaging effects of repeated ischemic insults, the observed ECF changes may help explain the vulnerability of traumatized brain tissue to secondary ischemia.

Dynamics of extracellular metabolites in the striatum after middle cerebral artery occlusion in the rat monitored by intracerebral microdialysis.

The aim of this study was to measure changes in the extracellular fluid (ECF) concentration of lactate, pyruvate, purines, amino acids, dopamine, and dopamine metabolites in the striatum of rats subjected to focal cerebral ischemia, using intracerebral microdialysis as the sampling technique. Microdialysis probes were inserted into the lateral part of the caudate-putamen bilaterally 2 h before the experiment. Ischemia was induced by permanent middle cerebral artery occlusion (MCAO) on the left side. Microdialysis samples were analyzed by high performance liquid chromatography. Following MCAO, the concentration of lactate, adenosine, inosine, and hypoxanthine rose markedly in the ECF on the occluded side, while there was no significant change in pyruvate. These changes were accompanied by dramatically elevated levels of aspartate, glutamate, taurine, gamma-aminobutyric acid, and dopamine. There was also a marked increase in alanine/tyrosine, while minor or no changes occurred with other amino acids. Concomitantly, the ECF level of the dopamine metabolites 3,4-dihydroxyphenylacetate and homovanillic acid decreased. There was no significant increase in any of the metabolites measured on the right, nonoccluded side. In relation to the concept of excitotoxicity in brain ischemia, it is concluded that during the acute stage of focal cerebral ischemia, the ECF is flooded with both potentially harmful (e.g., aspartate, glutamate, and DA) and protective (e.g., taurine, GABA, and adenosine) agents. The relative importance of these events for the development of cell death in the ischemic penumbra needs to be elucidated. In addition, lactate, inosine, and hypoxanthine, measured in the ECF by intracerebral microdialysis, may prove to have diagnostic and/or prognostic value in neurometabolic monitoring of the ischemic brain.

Intracerebral microdialysis of extracellular amino acids in the human epileptic focus.

Extracellular levels of aspartate (ASP), glutamate (GLU), serine (SER), asparagine (ASN), glycine (GLY), threonine (THR), arginine (ARG), alanine (ALA), taurine (TAU), tyrosine (TYR), phenylalanine (PHE), isoleucine (ILEU), and leucine (LEU) were monitored by using intracerebral microdialysis in seven patients with medically intractable epilepsy, undergoing epilepsy surgery. In association with focal seizures, dramatic increases of the extracellular ASP, GLU, GLY, and SER concentrations were observed. The other amino acids analyzed, including TAU, showed small changes. The results support the hypothesis that ASP, GLU, GLY, and possibly SER, play an important role in the mechanism of seizure activity and seizure-related brain damage in the human epileptic focus.

Spinal cord stimulation attenuates augmented dorsal horn release of excitatory amino acids in mononeuropathy via a GABAergic mechanism.

Neuropathic pain may be effectively relieved by electric stimulation of the spinal cord (SCS). However, the underlying mechanisms for the ensuing pain relief are poorly understood. In a rat model of neuropathy displaying hypersensitivity to innocuous tactile stimuli, (allodynia), we have earlier demonstrated that SCS may normalise withdrawal response thresholds. In the present study, using microdialysis, it is shown that SCS induces a decreased release of the dorsal horn excitatory amino acids (EAA), glutamate and aspartate, concomitant with an increase of the GABA release. Local perfusion with a GABA(B)-receptor antagonist in the dorsal horn transiently abolishes the SCS-induced suppression of the EAA release. Thus, the effect of SCS on neuropathic pain and allodynia may be due to an activation of local GABAergic mechanisms inhibiting the EAA release which is chronically elevated in such conditions.

Acetylcholine release in vivo: effects of chronic treatment with monosialoganglioside GM1.

Rats with unilateral cortical devascularizing lesions were treated with the monosialoganglioside GM1 in two different ways. One group of animals received GM1 (5 mg/kg/day, for 7 days), through a permanent cannula implanted intracerebroventricularly (i.c.v.) and connected to an osmotic minipump. The other group was treated with microencapsulated GM1 placed directly onto the surface of the lesioned cortex. The effect of GM1, administered into the lateral ventricle and supracortically, on the release of ACh in vivo was studied, using a microdialysis system combined with sensitive high performance liquid chromatography (HPLC). The release of acetylcholine and choline was studied in the cortex and striatum of the rat under nonstimulated (basal) and KCl (100 mM)-stimulating conditions. The non-stimulated release of acetylcholine was only measurable in the presence of neostigmine and was found to be about 30 microM in the cortex and approximately 10 times greater in the striatum. A large concentration of KCl led to a remarkable increase of acetylcholine in the control (C) and vehicle-treated lesioned groups (V i.c.v., V cap: 11-13 fold), but was greater in the GM1-treated groups (GM1 i.c.v., GM1 cap: 20-25 fold). In contrast, KCl-stimulated release of ACh in striata from GM1-treated lesioned groups was significantly less (5-10 fold), compared to the unlesioned controls (C: 16 fold) and lesioned vehicle-treated rats (V i.c.v. and V cap: 16-18 fold). The release of choline was not increased significantly by large concentrations of KCl present in the perfusion medium. In all experimental groups, and in both structures of the brain, ratios between stimulated and non-stimulated release of choline was between 0.7-1.2.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of intranigral substance P and neurokinin A on striatal dopamine release--II. Interactions with bicuculline and naloxone.

The functional roles of striatonigral neurokinins were studied by analysing the effects of intranigral injections of substance P and neurokinin A on the extracellular levels of dopamine and dihydroxyphenylacetic acid in the striatum, as measured by in vivo microdialysis in rats. An opioid antagonist, naloxone, and a GABAergic antagonist, bicuculline, were tested and analysed for their ability to modify the neurokinin effects. Unilateral injections of substance P (0.07 nmol) or neurokinin A (0.09 nmol) into the substantia nigra, pars reticulata of halothane anaesthetized rats produced long-lasting increases in ipsilateral striatal dopamine and dihydroxyphenylacetic acid levels. Intranigral injections of naloxone (30 and 300 nmol) produced short-lasting decreases in striatal dopamine, concomitant with an increase in dihydroxyphenylacetic acid. Intranigral injections of 7.0 nmol bicuculline produced an increase, while 70 nmol produced a decrease in striatal dopamine, however, both doses produced an increase in dihydroxyphenylacetic acid. When co-administered intranigrally, the high dose of naloxone (300 nmol) completely blocked the dopamine stimulation of substance P (0.07 nmol), but only moderately inhibited that of neurokinin A (0.09 nmol). The high dose of bicuculline (70 nmol) completely blocked the dopamine stimulation of neurokinin A, but only moderately inhibited that of substance P. Naloxone (30 and 300 nmol) enhanced the dihydroxyphenylacetic acid response to substance P, while bicuculline (70 nmol) inhibited the dihydroxyphenylacetic acid response to neurokinin A. These findings complement and extend the findings in the preceding paper, demonstrating that intranigral substance P and neurokinin A stimulate striatal dopamine via different neuronal mechanisms. We suggest that opioid drugs have a greater influence over substance P while GABAergic drugs have a greater influence over neurokinin A.

Autoregulation of dopamine release and metabolism by intrastriatal nigral grafts as revealed by intracerebral dialysis.

The autoregulation of dopamine release and metabolism by intrastriatal grafts of mesencephalic dopamine neurons was examined in vivo using an intracerebral dialysis technique. Dopamine-rich cell suspension grafts were implanted into the head of the caudate putamen in rats with complete unilateral 6-hydroxydopamine lesion of the nigrostriatal dopamine pathway. Six months later behavioural tests indicated that the grafts had reversed the lesion-induced rotational behaviour. Extracellular levels of striatal dopamine and its metabolites 3,4-dihydroxyphenylacetic acid and homovanillic acid were monitored bilaterally in the halothane-anaesthetized grafted rat, both under basal conditions, and also following low (0.05 mg/kg) and high (0.5 mg/kg) doses of the dopamine receptor agonist apomorphine. The perfusate from the grafted striatum showed levels of dopamine which were not statistically different from those of the intact contralateral striatum, indicating that the baseline release of dopamine from the graft was close to normal. Similarly, 3-4-dihydroxyphenylacetic acid and homovanillic acid levels were well recovered on the grafted side (67% and 52%, respectively, of control values). Consistent with previous observations, levels of the serotonin metabolite 5-hydroxyindoleacetic acid measured in perfusate collected from the grafted side was elevated significantly above normal. Subsequent histological analysis revealed large grafts, rich in dopamine-containing neurons (mean +/- SEM number equalled 3138 +/- 630), giving rise to an approximately normal density of dopamine-containing fibres in the area of the host caudate putamen surrounding the probe. Treatment with 0.05 mg/kg (subcutaneous) apomorphine did not affect extracellular dopamine recovered from the grafted striatum, while extracellular DA decreased by a maximum of 30% on the intact side. However, a subsequent injection of 0.5 mg/kg apomorphine produced a large decrease of the dopamine recovered from both the grafted (maximum 40% decrease) and intact striata (maximum 80% decrease). Both the low and the high dose of apomorphine reduced extracellular dopamine metabolite levels, a response which was essentially similar for both the intact and grafted sides. Finally, the dopamine reuptake blocker nomifensine (10(-5) M) added to the perfusion medium produced similar large increases in dopamine in perfusates collected from both grafted and intact striata, while 3,4-dihydroxyphenylacetic acid and homovanillic acid did not change.(ABSTRACT TRUNCATED AT 400 WORDS)

Amphetamine regulation of acetylcholine and gamma-aminobutyric acid in nucleus accumbens.

In situ hybridization histochemistry and in vivo microdialysis were combined to study the effect of amphetamine on the expression of choline acetyltransferase and glutamate decarboxylase67 mRNA and in vivo release of acetylcholine and GABA in rat medial nucleus accumbens. Differential effects on acetylcholine and GABA neurons by a single challenge injection of amphetamine (1.5 mg/kg, s.c.) were apparent in saline-pretreated and amphetamine-pretreated (same dose, twice daily for the previous seven days) rats. Extracellular acetylcholine levels were increased up to 50% over a prolonged period following both single and repeated amphetamine. In contrast, extracellular concentrations of GABA were gradually decreased to half the control values, but only in rats receiving repeated amphetamine. Although the increase of acetylcholine release was not associated with any change in choline acetyltransferase mRNA levels, the number of neurons expressing high levels of glutamate decarboxylase67 mRNA was decreased (28%) following repeated injections. Thus we suggest that amphetamine decreases extracellular GABA levels by a slow mechanism, associated with the decreased expression of glutamate decarboxylase67 mRNA in a subpopulation of densely labeled neurons in the medial nucleus accumbens. The delayed response by GABA to amphetamine may reflect supersensitivity in the activity of postsynaptic gamma-aminobutyric acid-containing neurons in nucleus accumbens as a consequence of the repeated amphetamine treatment.

Antagonistic interaction between adenosine A2A receptors and dopamine D2 receptors in the ventral striopallidal system. Implications for the treatment of schizophrenia.

Recent studies have shown the existence of a specific antagonistic interaction between adenosine A2a receptors and dopamine D2 receptors in the brain. This A2a-D2 interaction seems to be essential for the behavioural effects of adenosine agonists and antagonists, like caffeine. In the present study quantitative receptor autoradiography and brain microdialysis were combined to demonstrate a powerful antagonistic A2a-D2 interaction in the ventral striopallidal system. In the presence of the A2a agonist (2-p-carboxyethyl)phenylamino-5'-N carboxamidoadenosine, dopamine exhibited a lower efficacy in displacing the radiolabelled D2 receptor antagonist [125I]iodosulpiride from the rat ventral striatum, specially in the nucleus accumbens. A tonic dopaminergic modulation of the striopallidal neurons from the ventral striopallidal system was demonstrated by a dual-probe approach, by infusing selective dopamine agonists and antagonists in the nucleus and by measuring dopamine extracellular levels in the nucleus accumbens and GABA extracellular levels in the nucleus accumbens and in the ipsilateral ventral pallidum. The infusion of (2-p-carboxyethyl)phenylamino-5'-N-carboxamidoadenosine in the nucleus accumbens induced the same postsynaptic changes as the D2 antagonist raclopride, i.e. an increase in pallidal GABA extracellular levels, without changing those levels in the nucleus accumbens. Furthermore, the coinfusion in the nucleus accumbens of low concentrations of (2-p-carboxyethyl) phenylamino-5'-N-carboxamido-adenosine and raclopride, which were ineffective when administered alone, induced a significant increase in pallidal gamma-aminobutyric acids extracellular levels. These results suggest that A2a agonists, alone or in combination with D2 antagonists, could be advantageous antischizophrenic drugs, as blockage of D2 receptors in the ventral striopallidal system appears to be associated with the antipsychotic activity of neuroleptics but not with their extrapyramidal motor-side effects.