High sensitivity measurement of brain catechols and indoles in vivo using electrochemically treated carbon-fiber electrodes

Temporal differentiation of pH-dependent capacitive current from dopamine

Voltammetric recording of dopamine (DA) with fast-scan cyclic voltammetry (FSCV) on carbon fiber microelectrodes have been widely used, because of its high sensitivity to dopamine. However, since an electric double layer on a carbon fiber surface in a physiological ionic solution behaves as a capacitor, fast voltage manipulation in FSCV induces large capacitive current. The faradic current from oxidation/reduction of target chemicals must be extracted from this large background current. It is known that ionic shifts, including H(+), influence this capacitance, and pH shift can cause confounding influences on the FSCV recordings within a wide range of voltage. Besides FSCV with a triangular waveform, we have been using rectangular pulse voltammetry (RPV) for dopamine detection in the brain. In this method, the onset of a single pulse causes a large capacitive current, but unlike FSCV, the capacitive current is restricted to a narrow temporal window of just after pulse onset (<5 ms). In contrast, the peak of faradic current from dopamine oxidation occurs after a delay of more than a few milliseconds. Taking advantage of the temporal difference, we show that RPV could distinguish dopamine from pH shifts clearly and easily. In addition, the early onset current was useful to evaluate pH shifts. The narrow voltage window of our RPV pulse allowed a clear differentiation of dopamine and serotonin (5-HT), as we have shown previously. Additional recording with RPV, alongside FSCV, would improve identification of chemicals such as dopamine, pH, and 5-HT.

Nitric oxide and liver microcirculation during autoregulation and haemorrhagic shock in rabbit model

BACKGROUND

Direct evidence of nitric oxide (NO) involvement in the regulation of hepatic microcirculation is not yet available under physiological conditions nor in haemorrhagic shock.

METHODS

A laser Doppler flowmetry was used to measure liver perfusion index and a specific NO-sensitive electrode was inserted into liver parenchyma of anaesthetized rabbits. Hepatic autoregulation during moderate hypovolaemia {mean arterial pressure at 50 mm Hg without liver perfusion alteration; blood withdrawal 17.7 (4.2) ml [mean (SD)]} or haemorrhagic shock [mean arterial pressure at 20 mm Hg associated with liver perfusion impairment and lactic acidosis; blood withdrawal 56.0 (6.8) ml] were investigated over 60 min and were followed by a rapid infusion of the shed blood. Involvement of NO synthases was evaluated using a non-specific inhibitor, NAPNA (Nomega-nitro-L-arginine P-nitro-anilide).

RESULTS

In the autoregulation group, a decrease [30.0 (4.0) mm Hg] of mean arterial pressure did not alter liver perfusion index, whereas the liver NO concentration increased and reached a plateau [125 (10)%; compared with baseline; P<0.05]. This NO concentration was reduced to zero by the administration of NO synthase inhibitor. Haemorrhagic shock led to a rapid decrease in liver perfusion index [60 (7)%; compared with baseline; P<0.05] before an immediate and continuous increase in NO concentration [250 (50)%; compared with baseline; P<0.05]. Infusion of NO inhibitor before haemorrhagic shock reduced the NO concentration to zero and hepatic perfusion by 60 (8)% (P<0.05) of the baseline. Mean arterial pressure increased simultaneously. In these animals, during haemorrhage, a continuous increase in NO concentration still occurred and liver perfusion slightly increased. In all groups but NAPNA+haemorrhagic shock, blood replacement induced recovery of baseline values.

CONCLUSIONS

NO plays a physiological role in the liver microcirculation during autoregulation. Its production is enzyme-dependent. Conversely, haemorrhagic shock induces a rapid increase in hepatic NO that is at least partially enzyme-independent.

Changes in the kinetics of dopamine release and uptake have differential effects on the spatial distribution of extracellular dopamine concentration in rat striatum

The objective of this study was to examine whether the limited diffusion distance of dopamine in rat striatum produces spatial heterogeneity in the extracellular dopamine concentration on a dimensional scale of a few micrometers. Such heterogeneity would be significant because it would imply that the concentration of dopamine at a given receptor depends on the receptor's ultrastructural location. Spatially resolved measurements of extracellular dopamine were performed in the striatum of chloral hydrate-anesthetized rats with carbon fiber microdisk electrodes. Dopamine was monitored during electrical stimulation of the nigrostriatal pathway before and after administration of drugs that selectively affect the kinetics of evoked dopamine release and dopamine uptake. The effects of nomifensine (20 mg/kg), L-DOPA (250 mg/kg), and alpha-methyl-p-tyrosine (250 mg/kg) on the amplitude of the stimulation responses were examined. The outcome of these experiments was compared with predictions derived from a mathematical model that combines diffusion with the kinetics of release and uptake. The results demonstrate that the extracellular dopamine concentration is spatially heterogeneous on a micrometer scale and that changing the kinetics of dopamine release and uptake has different effects on this spatial distribution. The impact of these results on brain neurochemistry is considered.

Suppression of acute and chronic opioid withdrawal by a selective soluble guanylyl cyclase inhibitor

Previous studies have shown that activation of N-methyl-D-aspartate (NMDA) receptors and formation of nitric oxide (NO) contributes to the hyperactivity of locus coeruleus (LC) noradrenergic neurons and behavioural symptoms seen during opioid withdrawal. However, the role of soluble guanylyl cyclase (sGC), the 'physiological' target of NO, in this phenomenon is unclear. In this study, the effect of 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), a highly selective sGC inhibitor, on the naloxone-precipitated morphine withdrawal was examined using differential normal pulse voltammetry (DNPV) to measure LC activity, in vivo microdialysis to measure glutamate/aspartate release response, and behavioural assessment to evaluate withdrawal symptoms. In halothane-anaesthetized rats, acute intracerebroventricular (i.c.v.) morphine (10 microg) reduced the catecholamine oxidation current (CA.OC) (54.5+/-4.9% of baseline). Naloxone (2 mg/kg, i.v.) reversed this action of morphine and produced a rebound increase in CA.OC (136.1+/-6.0% of baseline), representing acute morphine withdrawal. Administration of ODQ (200 nmol, i.c.v.) blocked this response without affecting acute morphine action. In animals chronically treated with morphine (15 microg/microl/h, i.c.v., 5 days), naloxone significantly increased both the CA.OC signal (270.0+/-19.6% of baseline) and the release of L-glu (193+/-30.4%) and L-asp (221.5+/-28.4%) above baseline. These responses were attenuated in animals pretreated with ODQ. In unanaesthetized chronic morphine dependent rats, ODQ treatment suppressed the signs of withdrawal precipitated by naloxone (10 mg/kg). Taken together, the results of this study suggest that sGC plays an intermediary role in the genesis of LC neuronal hyperactivity and behavioural signs of morphine withdrawal.

Nitric oxide and sleep in the rat: a puzzling relationship

To date, only a few studies indicate that nitric oxide may play a role in the regulation of the sleep-wake cycle. However, data reported are controversial and the part played by nitric oxide in sleep-wake cycle regulation still remains uncertain. In the present report, we studied the effects on sleep amounts of two different nitric oxide synthase inhibitors: N-nitro-L-arginine methyl ester, a non-selective nitric oxide synthase inhibitor, and 7-nitro-indazole, a specific inhibitor of neuronal nitric oxide synthase. The above compounds were administered via two routes, i.e. intraperitoneally or locally in the dorsal raphe nucleus, a structure involved in sleep regulation. In order to evaluate their efficiency to inhibit nitric oxide synthesis in the rat brain, they were first administered intraperitoneally to a group of animals, and the cortical release of nitric oxide was determined by means of voltammetric measurements. N-Nitro-L-arginine methyl ester (100 mg/kg, i.p.) did not affect the cortical release of nitric oxide, whereas it increased both slow-wave sleep and paradoxical sleep durations. On the contrary, 7-nitro-indazole (40 mg/kg, i.p.) significantly decreased the cortical release of nitric oxide (-25%) and paradoxical sleep duration. Furthermore, following microinjection of either N-nitro-L-arginine methyl ester or 7-nitro-indazole at 100 ng/0.20 microl into the nitric oxidergic cell area of the dorsal raphe nucleus, decreases in paradoxical sleep duration were obtained (-32.8% and -25.3%, respectively). The results obtained support the existence of a duality in the sleep regulation modalities exerted by nitric oxide, i.e. a peripheral inhibiting influence and a central facilitating role for the nitric oxide-serotoninergic neurons of the dorsal raphe nucleus.

Methodology for coupling local application of dopamine and other chemicals with rapid in vivo electrochemical recordings in freely-moving rats

Methodology is presented for constructing and using an electrode/microcannulae assembly that allows in vivo electrochemical measurements coupled with local application of dopamine (DA) and other chemicals in the unanesthetized freely-moving rat. Rats were implanted with a voltammetric electrode constructed of a carbon fiber sealed in fused silica tubing attached to a pair of stainless steel guide cannulae, into which fused silica injection cannulae were inserted for local application of DA and other chemicals. Precise delivery of nanoliter volumes was accomplished using a syringe drive combined with a fluid swivel to deliver the solutions to the injection cannulae. A newly-designed miniature potentiostat connected to a commutator via a modular telephone jack assembly allowed for high-speed chronoamperometric electrochemical recordings in freely-moving rats. Initial experiments characterized the in vitro electrochemical recording characteristics of the voltammetric electrode. In vivo studies were also carried out to study clearance of locally-applied DA and of potassium-evoked endogenous DA in the striatum and nucleus accumbens of freely-moving rats. In addition, the effects of chloral hydrate anesthesia on DA clearance signals in the nucleus accumbens were investigated. Moreover, the stability and reproducibility of this recording technique for measuring exogenous DA clearance was verified over a period of 5 days. Finally, the concurrent effects of systemic cocaine injection on DA uptake in nucleus accumbens and locomotor activity were examined. These studies support the conclusion that the methodology described herein allows for rapid chronoamperometric electrochemical recordings in freely-moving rats with precise microapplications of DA and other chemicals combined with concurrent measures of animal behavior.

Regional and temporal differences in real-time dopamine efflux in the nucleus accumbens during free-choice novelty

To assess dopamine efflux during novelty-seeking behavior in rats, fast-scan cyclic voltammetry in the nucleus accumbens was combined with free-choice entry into a novel environment. Cyclic voltammograms, confirmed by in vitro testing, revealed that entry into novel, but not familiar, surroundings increased dopamine efflux in a regionally and temporally distinct pattern. Whereas dopamine failed to change in the core region of the accumbens and overlying neostriatum, an abrupt increase occurred in accumbal shell, a limbic-related area implicated in goal-directed behavior. Although the dopamine response was confined to the brief period of entry into novelty (approximately 8 s duration), a less rapid and more persistent dopamine change (> 20 s duration) occurred in the shell-core transition zone, the so-called shore. These results suggest that novelty mimics other positively reinforcing stimuli in enhancing dopamine transmission in the nucleus accumbens, but the regional and temporal heterogeneity of this effect may represent different aspects of accumbal dopamine function.

Nitric oxide (NO): in vivo electrochemical monitoring in the dorsal horn of the spinal cord of the rat

NO synthase (NOS) is largely distributed in the superficial and deep laminae of the dorsal horn as well as in dorsal root ganglion cells. It has been proposed that nitric oxide (NO) participates in the transmission of sustained, and possibly brief, nociceptive, inputs at the spinal level. The aim of this study was to check the ability of in vivo electrochemical monitoring of NO within the dorsal horn of the lumbar spinal cord (L3-L4 level) of chloral hydrate anesthetized or decerebrated spinalized rats. 30 microm diameter and 450 microm length treated carbon fiber electrodes coated with nickel(II) tetrakis (3-methoxy-4-hydroxy-phenyl) porphyrine and NafionR, and associated with differential normal pulse voltammetry, gave a peak of oxidation current around 650 mV (vs. Ag-AgCl) in vitro in NO solutions between 0.125 and 1.25 microM. In vivo, a 650 mV peak appeared which was stable (recording interval 2 min) for up to 3 h (+/-6%). Comparison between in vitro calibration and in vivo voltammograms gave an estimated in vivo extracellular concentration of 0.50 microM. In vivo, peaks decreased by 95% at 90 min and for up to 3 h after an i.p. injection of 100 mg/kg of the NOS inhibitor (NOSI) L-arginine-p-nitroanilide (L-ANA). At the same dose i.p., N(G)-nitro-L-arginine methyl ester (L-NAME) was almost ineffective after 90 min in animals paralyzed with pancuronium bromate or gallamine trethiodide. However, in non-curarized decerebrated spinalized animals, L-NAME depressed the voltammograms by 36% at 90 min. S-Ethylthiourea (80 mg/kg i.p.), also decreased the voltammograms by 45% at 140 min, and finally, 7-nitroindazole (7-NI, 90 mg/kg i.p), induced a important decrease of the 650 mV peak (23% of control) at 120 min. These results are in agreement with biochemical data showing the decrease of NOS activity within the lumbar spinal cord by L-NAME (45% of control at 90 min) and 7-NI (20% of control at 90 min). The NO donor hydroxylamine (30 mg/kg i.p.) significantly increased the peaks (140% at 90 min), and sodium nitroprusside (SNP, 20 mM) when directly superfused upon the spinal cord (200-300 microl min(-1)) induced a large increase in the peak (300% at 90 min). Moreover, SNP 60 min after L-ANA, or 90 min after L-NAME, rapidly restored the 650 mV peak up to control values. These results demonstrate the validity of electrochemical monitoring of NO within the dorsal horn of the spinal cord. The in vivo electrochemical detection of NO is in progress to study the implication of this messenger in the transmission of nociceptive messages at the spinal level.

Use of differential normal pulse voltammetry for the measurement of locus coeruleus catecholaminergic metabolism in an acute anaesthetized rodent model of allodynia: effect of mexiletine

Neuropathic pain can be triggered by non-painful stimuli (e.g., light touch), a sensory abnormality termed allodynia. The acute blockade of spinal glycine receptors with intrathecal strychnine induces a reversible allodynia-like state in the rat. We describe the application of in vivo differential normal pulse voltammetry with carbon fibre micro-electrodes for monitoring the catechol oxidation current (CAOC) of the locus coeruleus (LC) in the strychnine model of allodynia. In addition, we tested the effect of mexiletine, a drug useful in the management of clinical neuropathic pain in this model. Our results show that somatosensory processing in the spinal cord of urethane-anaesthetized rats is radically altered during glycine receptor blockade such that the normally innocuous stimulus of hair deflection causes the marked activation of the LC as determined using in vivo differential normal pulse voltammetry. Mexiletine suppressed the LC and cardiovascular responses of strychnine induced allodynia. Results of this study indicate that LC CAOC, an index of LC neuronal activity: (a) is a sensitive biochemical index of strychnine-allodynia; (b) is temporally correlated with the cardiovascular and motor responses evoked by hair deflection during glycine receptor blockade; and (c) can be used to quantitate allodynia in the strychnine model.

Determination of NADH in the rat brain during sleep-wake states with an optic fibre sensor and time-resolved fluorescence procedures

The present paper reports a nanosecond time-resolved fluorescence derived from the cortex and the area of the periaqueductal gray including the nucleus raphe dorsalis (PAG-nRD) in unanaesthetized freely moving rats. The measurements were acquired through a single optic fibre transmitting a subnanosecond nitrogen laser pulse (337 nm, 15 Hz) and collecting the brain fluorescence occurring at 460 nm which might depend on mitochondrial NADH (reduced form of nicotinamide adenine dinucleotide). The fluorometric method was combined with polygraphic recordings, and this procedure allowed us to define, for the first time, variations of the 460 nm signal occurring throughout the sleep-wake cycle. In the PAG-nRD, the signal exhibited moderate heterogeneous variation in amplitude during slow-wave as compared to the waking state. Constant increases were observed during paradoxical sleep as compared to the waking state. For this state of sleep the magnitude of the variations depended on the optic fibre location. In the cortex and during either slow-wave sleep or paradoxical sleep, the signal presented moderate increases which were significant during paradoxical sleep. The magnitude of the redox variations observed either in the PAG-nRD or in the cortex might be ascribed to the oxidative energy balance which is related to sleep states.

Voltammetric detection of nitric oxide (NO) in the rat brain: its variations throughout the sleep-wake cycle

A sensor allowing the specific detection of nitric oxide (NO) is reported. Together with differential pulsed voltammetry, it allows the detection of a 650 mV signal either in NO solutions or in the rat frontal cortex. The intraperitoneal (i.p.) administration of a NO donor (S-nitrosoglutathione, 20 mg/kg i.p.) increases the signal height (+30%) while that of a nitric oxide synthase (NOS) inhibitor like L-nitro-arginine-p-nitro-anilide (100 mg/kg i.p.), produces its complete disappearance in the cortex of anesthetized rats. These results suggest that the 650 mV signal might be NO-dependent. Some other NOS inhibitors have been found either inefficient (L-nitro-arginine-methyl-ester) or partially efficient (7-nitro-indazole) on the signal height. In freely moving rats, also equipped with polygraphic electrodes, the signal measured in the frontal cortex exhibits the highest height during waking. It decreases during slow-wave sleep (-6%) and paradoxical sleep (-9%).

Amperometric microsensors for monitoring choline in the extracellular fluid of brain

Selective amperometric enzyme microsensors for monitoring low micromolar concentrations of choline in extracellular fluid of rat brain have been developed. Preparation of the choline microsensors involved the modification of carbon fiber microcylinder electrodes (10 microns diameter, 300-400 microns long) with a cross-linked redox-active gel containing horseradish peroxidase and choline oxidase. Rejection of the noise recorded from the choline microsensors implanted in living brain tissue improved the in vivo detection capabilities of the sensors. The microsensors and a differential detection scheme were used to estimate the basal concentration of choline in striatal tissue at 6.6 +/- 2.9 microM and to measure changes in choline concentrations of 6.1 +/- 2.7 microM in vivo. The microsensors were also used to monitor choline produced following the injections of acetylcholine in vivo. Coinjections of neostigmine and acetylcholine significantly lowered the choline response recorded with the microsensors, confirming that the response following the injections of acetylcholine alone was due to the activity of endogenous acetylcholinesterase. Comparison of the maximal rate of decrease in choline concentration following the injections of 1 mM choline and 1 mM acetylcholine was used to estimate the rate of acetylcholine clearance from extracellular fluid through cholinesterase activity at approx. 2.5 microM/min.

In vivo monitoring of brain neurotransmitter release for the assessment of neuroendocrine interactions

1. The neurotransmitter mechanisms regulating neuroendocrine processes have been traditionally inferred from the effects of drugs purportedly acting through specific transmitter systems. The direct appraisal of changes in endogenous neuromediators had to rely initially on analyses of brain samples obtained post-morten. 2. Currently, a more physiological assessment is available through the monitoring ot the extracellular levels of neurotransmitters and their metabolites in discrete brain areas of living animals. Two methodologies, namely in vivo voltammetry and microdialysis, are being increasingly used for this purpose. This article summarizes their principles, relative merits, and limitations and presents some relevant applications. 3. Thus, microdialysis data show a differential response in the amphetamine-induced dopamine release in the nucleus accumbens in adult male and female rats castrated prepuberally. Given their high time-resolution, in vivo electrochemistry techniques seem especially suited for studying the fast, non-genomic effects of steroid hormones. This is illustrated by the voltammetric detection of a rapid release of dopamine in the corpus striatum induced by progesterone in males. 4. These methodologies should be regarded as complementary tools for the assessment of the neurochemical correlates of neuroendocrine interactions.

Innocuous hair deflection evokes a nociceptive-like activation of catechol oxidation in the rat locus coeruleus following intrathecal strychnine: a biochemical index of allodynia using in vivo voltammetry

Blockade of spinal glycinergic inhibition with intrathecal (i.t.) strychnine induces a reversible allodynia-like state in both conscious and lightly-anaesthetized rats. Since the locus coeruleus (LC) is activated by noxious stimuli, we determined the effect of non-noxious hair deflection (HD) on noradrenergic neuronal activity in the LC of rats treated with i.t. strychnine. Differential normal pulse voltammetry was used to measure the catechol oxidation current (CA.OC), an index of LC activity. Rats were maintained in a light plane of anaesthesia with i.v. urethane and i.t. strychnine (40 micrograms) was injected near the L1-L2 segment. HD, applied to the caudal dermatomes affected by i.t. strychnine, evoked a significant increase (max. 141 +/- 7%, n = 5, P < 0.05) in CA.OC and mean arterial pressure as compared to baseline (no strychnine). In contrast, HD had no significant effect on CA.OC or mean arterial pressure in the saline-treated rats (n = 5). Pre-treatment with i.t. MK801 (30 micrograms) significantly blocked the increase in CA.OC and mean arterial pressure evoked by HD in strychnine-treated rats. The results of this study indicated that HD, in the presence of i.t. strychnine but not saline, can evoke noradrenergic activity in the LC of lightly anaesthetized rats. This effect on the LC is: (1) comparable to that observed with noxious stimulation without i.t. strychnine; (2) segmentally localized, corresponding to the spinal site of strychnine injection; and (3) mediated by spinal NMDA receptors, consistent with the role of excitatory amino acids in sensory transmission. These data provide the first neurochemical evidence that HD, in the presence of i.t. strychnine, is a nociceptive event, supporting the use of this preparation as an experimental model of allodynia.

Hypothalamic monoaminergic activity in obese Zucker rats in response to acute and chronic dietary stimuli

Monoamine neuromodulators link diet-related signals with autonomic responses in the regulation of energy balance. In view of evidence that sympathetically mediated expenditure is blunted in genetically obese (fa/fa) Zucker rats, a central defect in neurochemical activity has been proposed. This study tested the hypothesis that genotypic differences in monoaminergic activity in the hypothalamus underlie a blunted sensitivity of fa/fa rats to acute and chronic dietary stimuli, leading to less effective modulation of energy metabolism. Homozygous lean and obese Zucker rats were fed a protein-restricted (PR, protein = 8% of total dietary energy) or control (CF, 21% protein) diet from the age of 5 wk. At 10 wk of age, postprandial oxygen consumption (thermic effect of food, TEF) and levels of brown adipose tissue (BAT) uncoupling protein were significantly elevated (P < 0.0005) in PR vs. CF lean rats. Serotonin turnover was significantly (P < 0.0014) greater in the ventromedial hypothalamus (VMH) of meal-fed lean PR (vs. CF) rats, consistent with the suggestion that VMH serotonin release stimulates sympathetic outflow and may signal a heightened drive for protein intake. Serotonergic activity, BAT uncoupling protein, and TEF were not elevated in PR obese rats in response to a test meal, supporting the view that activation of sympathetic outflow in response to diet-related stimuli in fa/fa rats is impaired.

Voltammetric and microdialysis monitoring of brain monoamine neurotransmitter release during sociosexual interactions

The monoamine neurotransmitters have long been ascribed important modulatory actions on male sexual behavior by a wealth of pharmacological studies. Methodological developments have now made possible the assessment of the extracellular levels of amine transmitters and their metabolites in discrete brain areas of sexually behaving animals using in vivo voltammetry and microdialysis. Studies in our and other laboratories consistently show increased dopamine release in forebrain structures known to be involved in mating activity, including the nucleus accumbens and the medial preoptic area, during both the appetitive (i.e., non-contact exposure to sexual stimuli) and consummatory phases of this behavior. Serotonin utilization seems to be mainly related to consummatory events. These findings are consistent with the pharmacological evidence as well as previous ex vivo work. The state of sexual inactivity that follows unrestricted mating associates with increased dopamine turnover in the preoptic area. According to the available information, it could reflect some blockade of dopaminergic receptors, possibly involving prolactin. No disturbance of ongoing sexual behavior was observed during the neurochemical monitoring sessions with either methodology. These studies show voltammetry and microdialysis as powerful complementary tools for the assessment of sociosexual interactions.

On the significance of brain extracellular uric acid detected with in-vivo monitoring techniques: a review

The concentration of uric acid [UA] in the extracellular fluid (ECF) estimated with in-vivo voltammetry and microdialysis data is compared for probes of different diameters from the day of implantation (acute) to several days (chronic) or even months after surgery. For small probes (diameter < 160 microns) the acute [UA] of ca. 5 microM decreased significantly to ca. 1 microM under chronic conditions. For larger probes (e.g., 320-microns diameter) the acute [UA] was also ca. 5 microM, but this value significantly increased to ca. 50 microM under chronic conditions. Associated with this difference in [UA], there were parallel differences in the extent of gliosis around the probes. These findings are discussed in terms of possible sources of extracellular UA and their implications for in-vivo monitoring techniques in behaving animals.

Involvement of excitatory amino acid pathways in the expression of precipitated opioid withdrawal in the rostral ventrolateral medulla: an in vivo voltammetric study

Previous studies have shown that catecholaminergic neurons in the rostral ventrolateral medulla (RVLM) become hyperactive during opioid withdrawal. In the present study, the role of excitatory amino acid pathways in the expression of opioid withdrawal in the RVLM was examined by using differential pulse voltammetry (DNPV) to measure changes in the catecholamine oxidation current (CA.OC) following naloxone challenge in rats treated with acute or chronic morphine. Acute morphine (10 micrograms i.c.v.) significantly reduced the CA.OC signal in the RVLM and the mean arterial pressure to 37.1 +/- 6.6% and 21.1 +/- 3.5% below baseline, respectively. Naloxone (1 mg kg-1 i.v.) reversed the morphine effect and produced a significant increase in the CA.OC signal to 25.6 +/- 15.2% above baseline. In animals treated with chronic morphine (10 micrograms h-1 i.c.v., 5 days), naloxone (1 mg kg-1 i.v.) produced a significant increase in the CA.OC signal to 54.2 +/- 16.5% above baseline. Both the nonselective excitatory amino acid antagonist, gamma-D-glutamylglycine (DGG, 200 micrograms i.c.v.) and the selective NMDA antagonist, D(-)-amino-7-phosphonoheptanoic acid (D-APH, 25 micrograms i.c.v.) attenuated the naloxone-induced increase in the CA.OC by 50.7% and 46.0% respectively. In morphine naive animals, DGG and D-APH depressed the CA.OC by 42.8 +/- 8.7% and 17.7 +/- 9.8%, respectively. To the extent that the CA.OC is an index of neuronal activity, these results suggest that RVLM hyperactivity during morphine withdrawal is dependent, in part, upon activation of NMDA receptors.

Neurobiological correlates of masculine sexual behavior

The experimental analysis of the neuroendocrine interactions regulating sexual behavior has traditionally relied on studying the effects of CNS lesions and pharmacological treatments with hormones or drugs purportedly acting through specific neurotransmitter systems. New methodological developments have allowed the assessment of several indices of neural function in experimental animals, particularly the rat, as they relate to behavioral changes. In the field of sexual behavior, ex vivo analyses have been used to measure markers of energy metabolism, such as 2-deoxyglucose uptake and Na,K-ATPase activity, the tissue content of neurotransmitters and metabolites, the levels of steroid receptors and neurosteroids, and immediate-early gene expression products in different areas of the CNS. In vivo studies have monitored brain electrical activity and temperature, as well as the extracellular levels of neurotransmitters and metabolites by cerebrospinal fluid sampling, push-pull perfusion and, especially, electrochemical recordings and microdialysis, in the course of mating and exposure to various relevant stimuli. The findings with the different methodologies are generally consistent and agree with those of previous surgical and pharmacological manipulations. They provide data on temporal relationships between neurobiological and behavioral events and suggest new interpretations for different aspects of the male copulatory pattern.

The role of excitatory amino acids in the expression of precipitated acute and chronic clonidine withdrawal: an in vivo voltammetric study in the rat locus coeruleus

It has been previously shown that activation of excitatory amino acid (EAA) pathways contributes to hyperactivity of the locus coeruleus (LC) in antagonist precipitated opioid withdrawal. In this study, using differential normal pulse voltammetry to monitor catechol oxidation as an index of the activity of the LC, the role of EAA pathways in antagonist precipitated withdrawal after acute and chronic clonidine treatment was examined. Intracerebroventricular clonidine (10 micrograms i.c.v.) significantly reduced LC activity to 54.4 +/- 3.1% of baseline 45 minutes following the injection. Subsequent systemic injection of the selective alpha 2 receptor antagonist atipamezole (0.2 mg/kg i.v.) or yohimbine (0.5 mg/kg i.v.) resulted in a rapid reversal of the depressant effects and a significant increase in LC activity above baseline. Pretreatment with the non-selective EAA receptor antagonist gamma-D-glutamylglycine (DGG) (50 micrograms i.c.v.) attenuated the atipamezole-induced rebound response of the LC but not the reversal of clonidine action. However, both the yohimbine-induced rebound and reversal of clonidine effects were attenuated by DGG treated animals. In chronic clonidine treated animals (2, 5, 7, 10 micrograms/h i.c.v., 5 days), a challenge with atipamezole (0.2 mg/kg i.v.) produced an immediate increase in LC activity, blood pressure and heart rate. The magnitude of these responses was dependent on the dose of clonidine. The atipamezole-induced increase in LC activity and blood pressure was significantly attenuated by pretreatment with DGG (200 micrograms i.c.v.). These findings suggest that LC hyperactivity and blood pressure increases elicited during clonidine withdrawal are mediated in part by activation of EAA receptors. In this regard, the mechanisms underlying clonidine withdrawal closely resembles those underlying opioid withdrawal.

A vitamin as neuromodulator: ascorbate release into the extracellular fluid of the brain regulates dopaminergic and glutamatergic transmission

Ascorbate is an antioxidant vitamin that the brain accumulates from the blood supply and maintains at a relatively high concentration under widely varying conditions. Although neurons are known to use this vitamin in many different chemical and enzymatic reactions, only recently has sufficient evidence emerged to suggest a role for ascorbate in interneuronal communication. Ascorbate is released from glutamatergic neurons as part of the glutamate reuptake process, in which the high-affinity glutamate transporter exchanges ascorbate for glutamate. This heteroexchange process, which also may occur in glial cells, ensures a relatively high level of extracellular ascorbate in many forebrain regions. Ascorbate release is regulated, at least in part, by dopaminergic mechanisms, which appear to involve both the D1 and D2 family of dopamine receptors. Thus, amphetamine, GBR-12909, apomorphine, and the combined administration of D1 and D2 agonists all facilitate ascorbate release from glutamatergic terminals in the neostriatum, and this effect is blocked by dopamine receptor antagonists. Even though the neostriatum itself contains a high concentration of dopamine receptors, the critical site for dopamine-mediated ascorbate release in the neostriatum is the substantia nigra. Intranigral dopamine regulates the activity of nigrothalamic efferents, which in turn regulate thalamocortical fibers and eventually the glutamatergic corticoneostriatal pathway. In addition, neostriatonigral fibers project to nigrothalamic efferents, completing a complex multisynaptic loop that plays a major role in neostriatal ascorbate release. Although extracellular ascorbate appears to modulate the synaptic action of dopamine, the mechanisms underlying this effect are unclear. Evidence from receptor binding studies suggests that ascorbate alters dopamine receptors either as an allosteric inhibitor or as an inducer of iron-dependent lipid peroxidation. The applicability of these studies to dopamine receptor function, however, remains to be established in view of reports that ascorbate can protect against lipid peroxidation in vivo. Nevertheless, ample behavioral evidence supports an antidopaminergic action of ascorbate. Systemic, intraventricular, or intraneostriatal ascorbate administration, for example, attenuates the behavioral effects of amphetamine and potentiates the behavioral response to haloperidol. Some of these behavioral effects, however, may be dose-dependent in that treatment with relatively low doses of ascorbate has been reported to enhance dopamine-mediated behaviors. Ascorbate also appears to modulate glutamatergic transmission in the neostriatum. In fact, by facilitating glutamate release, ascorbate may indirectly oppose the action of dopamine, though the nature of the neostriatal dopaminergic-glutamatergic interaction is far from settled. Ascorbate also may alter the redox state of the NMDA glutamate receptor thus block NMDA-gated channel function.(ABSTRACT TRUNCATED AT 400 WORDS)