Anaesthesia abolishes the effect of valproate on extracellular 5-HIAA, DOPAC and ascorbate as measured in rat striatum by differential pulse voltammetry

Ascorbic acid antagonizes the inhibitory effect of acute ethanol on nitrite levels in the striatum of freely moving mice

The effect of exogenous ascorbic acid (AA) on acute ethanol-induced decrease in extracellular nitrites contents in striatum of freely moving mice was investigated by using microdialysis coupled to high performance liquid chromatography with electrochemical detection. The results showed that exogenous AA had no effect on basal extracellular levels of nitrites, but significantly increased striatal extracellular contents of AA in a dose-depended manner. Co-administration of AA with ethanol significantly antagonized the ethanol-induced decrease in extracellular nitrites contents, but did not affect ethanol-induced release of AA. These data provided the first evidence of an antagonistic action of AA on ethanol-induced decrease in extracellular nitrites contents in striatum of freely moving mice.

Extracellular single-unit recordings of piriform cortex neurons in rats: influence of different types of anesthesia and characterization of neurons by pharmacological manipulation of serotonin receptors

In epilepsy research, there is a growing interest in the role of the piriform cortex (PC) in the development and maintenance of limbic kindling and other types of limbic epileptogenesis leading to complex partial seizures. Neurophysiological studies on PC or amygdala-PC slice preparations from kindled rats showed that kindling of the amygdala induces long-lasting changes in synaptic efficacy in the ipsilateral PC, including spontaneous discharges and enhanced susceptibility of PC neurons to evoked burst responses. These long-lasting electrophysiological changes in the PC during kindling appear to be due, at least in part, to impaired function of gamma-aminobutyric acid (GABA)ergic interneurons. The aim of the present study was to develop an anesthetic protocol allowing electrophysiological single-unit recordings from inhibitory, presumably GABAergic PC interneurons in vivo. In addition to recording of spontaneously active PC neurons, microiontophoretic application of glutamate was used to activate silent neurons. Anesthesia of rats with ketamine/xylazine was not suited for single-unit recordings in the PC because of marked cardiovascular depression. Anesthesia with chloral hydrate allowed recording of spontaneous or glutamate-driven single-unit activity in approximately 40% of all animals. A similar percentage was obtained when recordings were done with the narcotic opioid fentanyl (plus gallamine), after all surgical preparations were performed under anesthesia with repeated administration of the barbiturate methohexital. To avoid brain accumulation of methohexital by repeated applications, we modified the anesthetic protocol in that methohexital was only injected once for initiation of surgical anesthesia, followed by the short-acting anesthetic propofol which does not accumulate upon repeated application. Again, after surgical preparation, electrophysiological recordings were done under fentanyl (plus gallamine). By this procedure, spontaneous or glutamate-driven single-unit activity could be measured in all rats in either layer II or III of the PC. Based on shape and frequency of action potentials, two types of neurons were recorded. The predominant type was similar in its firing characteristics to GABAergic neurons in other brain regions, was mainly located in layer III, and could be suppressed by the serotonin2A receptor antagonist MDL 100,907, suggesting that this type of PC neuron represents inhibitory, putative GABAergic interneurons. This new in vivo preparation may be useful for evaluation of PC neurons in kindled rats.

Reduction in firing rate of substantia nigra pars reticulata neurons by valproate: influence of different types of anesthesia in rats

Nondopaminergic, presumably GABAergic neurons in the substantia nigra pars reticulata (SNR) are thought to function as a gating mechanism for seizure propagation. Systemic administration of anticonvulsant doses of the antiepileptic drug valproate (VPA) has previously been reported to inhibit the firing of nondopaminergic SNR neurons in anesthetized but not in awake, paralyzed and locally anesthetized rats, suggesting that the findings in anesthetized rats were due to an interaction between VPA and the general anesthetic used. In the present study, we determined the influence of different anesthetic measures on the effect of an anticonvulsant dose of VPA (100 mg/kg) on extracellularly recorded spontaneous single unit activity of nondopaminergic SNR neurons in rats. Rats were anesthetized by continuous infusion of the general anesthetic chloral hydrate, the dissociative anesthetic ketamine or the narcotic opioid fentanyl, or were only locally anesthetized and paralyzed. VPA significantly reduced SNR firing in all groups with a time course that matched its anticonvulsant time course in rodents. However, VPA's inhibitory effect on SNR firing was significantly less marked under anesthesia with chloral hydrate than in any of the other groups, indicating that this anesthetic suppresses the action of VPA, which may be related to an interaction with GABA-related processes in the SNR. The closest approximation to the effect of VPA in awake rats was obtained under anesthesia with ketamine, while VPA's inhibitory action on SNR neuronal firing seemed to be enhanced in the fentanyl group, which exhibited the highest baseline firing rates of all groups. Determination of VPA in the SN showed that the difference in VPA's inhibitory effect on SNR neurons was not secondary to differences in local drug concentrations. The data demonstrate that VPA is capable of significantly slowing the spontaneous activity of nondopaminergic SNR neurons, but that the magnitude of this effect depends on the anesthetic measures used. In view of the presumed role of SNR neurons in seizure propagation and the finding that VPA consistently inhibits these neurons at an anticonvulsant dose, the present data suggest that suppression of spontaneous SNR neuronal firing may be an important mechanism through which VPA exerts its anticonvulsant properties.

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.

Effects of selective serotonergic ligands on posthypoxic audiogenic myoclonus

Male Sprague-Dawley rats underwent cardiac arrest and resuscitation, subsequently exhibiting posthypoxic myoclonus. The audiogenic posthypoxic myoclonus in these animals could be attenuated with the following drugs: 5-hydroxytryptophan (5-HTP, serotonin [5-HT] precursor), N-(3-trifluoro-methylphenyl)piperazine hydrochloride (TFMPP, 5-HT1B/1C/2 agonist), (+/-)-2,5-dimethoxy-4-iodoamphetamine hydrobromide (DOI, 5-HT2 agonist), and 1-(m-chlorophenyl)-biguanide hydrochloride (m-CPBG, 5-HT3 agonist). In contrast, the following drugs were ineffective: (+/-)-8-hydroxy-dipropylaminotetralin hydrobromide (8-OH-DPAT, 5-HT1A agonist), buspirone hydrochloride (5-HT1A agonist), 7-trifluoromethyl-4(4-methyl-l-piperazinyl)-pyrrolo[1,2- a]quinoxaline maleate (CGS 12066B, 5-HT1B agonist), ketanserin tartrate (5-HT2 antagonist), methysergide maleate (5-HT2 antagonist), fluoxetine (5-HT uptake blocker), and saline (vehicle). The data suggest that enhancement of serotonergic activity, particularly through 5-HT2 and 5-HT3 receptors, have therapeutic potential for the treatment of posthypoxic myoclonus.

Association between brain indole levels and severity of posthypoxic myoclonus in rats

We have previously reported the presence of posthypoxic, audiogenic myoclonus in rats after cardiac arrest and the ability of the 5-HT precursor, 5-HTP, to attenuate these muscle jerks. In addition, we have recently shown that 5-HT2 and 5-HT3 agonists can reduce the severity of myoclonus in these animals, suggesting a deficiency in serotonergic neurotransmission. In the present study, the levels of 5-HTP, 5-HT, and 5-HIAA were measured in seven regions of the brain in myoclonic and normal rats to identify the areas of the brain in which a serotonergic dysfunction resides. Similar to previous studies, we observed pronounced posthypoxic, audiogenic myoclonus 3 and 14 days after resuscitation from cardiac arrest, with a resolution of the abnormal movements by 45 days postarrest. HPLC measurements revealed significant changes in indole levels in the following areas of the brain: cortical 5-HIAA, striatal 5-HT, striatal 5-HIAA, hippocampal 5-HT, mesencephalic 5-HIAA, myelencephalic 5-HT, myelencephalic 5-HIAA, cerebellar 5-HTP, and cerebellar 5-HT. The changes in striatal 5-HT, cortical 5-HIAA, and mesencephalic 5-HIAA appear most relevant to the pathophysiology of posthypoxic myoclonus because regression analyses showed significant correlations between the myoclonus scores of the animals and the levels of these indoles. Based on the observed pattern of results, we postulate a dysfunction in serotonergic lateral (cortical) and far lateral (extrapyramidal) ascending pathways in posthypoxic myoclonus.

Cholinergic stimulation of rostral and caudal substantia nigra pars compacta produces opposite effects on circling behavior and striatal dopamine release measured by brain microdialysis

Turning in circles is among the behaviors elicited by unilateral cholinergic stimulation of the substantia nigra. Recent studies have shown that microinjection of cholinergic agonists into the substantia nigra pars compacta increases dopamine release and turnover in the striatum of anesthetized rats [Hernández-López et al. (1992) Brain. Res. 598, 114-120; Blaha and Winn (1993) J. Neurosci, 13, 1035-1044]. In this study, the relationship between circling behavior and striatal dopamine release following cholinergic stimulation of the substantia nigra pars compacta neurons was assessed by brain microdialysis in awake rats. The results indicate that cholinergic stimulation of the substantia nigra pars compacta with the mixed nicotinic-muscarinic cholinergic agonist carbachol modulates striatal dopamine release, and this effect is accompanied by circling behavior and stereotypies. Microinjection of carbachol (109 nmol) in the caudal portions of the substantia nigra pars compacta induced contralateral circling associated with an increase of dopamine release in neostriatum. On the contrary, ipsilateral circling and reduction of striatal dopamine release was elicited when the same dose of the drug was applied in the rostral portions of the substantia nigra pars compacta. The above findings are in accordance with recent electrophysiological studies suggesting the existence of sub-populations of nigrostriatal dopaminergic neurons, and indicate that the substantia nigra pars compacta is functionally compartmentalized. We conclude that the cholinergic input to the substantia nigra pars compacta could modulate the motor behavior through regulating the firing rate of nigrostriatal dopaminergic neurons and dopamine release in the neostriatum.

Evidence for involvement of amino acid neurotransmitters in anesthesia and naloxone induced reversal of respiratory paralysis

General anesthetics render a person unconscious and may produce respiratory paralysis at therapeutic doses. No pharmacological agent is available to restore respiration and the mechanism/s of anesthesia or apnea is not clearly understood. In this report, we present evidence to show that naloxone reversed respiratory failure induced by thiopental, ketamine, halothane but not that induced by phenobarbital. Furthermore, 25 mg/kg, i.v. thiopental, 140 mg/kg, i.v. ketamine, and 3% halothane produced anesthesia without significantly altering respiratory rate, increased GABA and decreased glutamate (except ketamine and phenobarbital) levels in rat brain stem and cortex, but not in caudate and cerebellum. Aspartate, glycine and alanine levels were not affected in four brain regions studied. Pretreatment with TSC for 30 minutes did not change GABA or glutamate contents, but abolished the anesthetic as well as the respiratory depressant actions of the anesthetics. Increasing the doses of anesthetics produced respiratory failure with further rise in GABA and fall in glutamate in brain stem and cortex. Naloxone reversed respiratory paralysis and restored GABA close to control values in rat brain stem and cortex with no changes in caudate or cerebellum. Data presented here suggest that GABA may be necessary to produce loss of consciousness and naloxone reverses anesthetic induced respiratory failure.

Striatal extracellular dopamine in conscious vs. anesthetized rats: effects of chloral hydrate anesthetic on responses to drugs of different classes

Many investigations using the microdialysis technique have been performed in anesthetized animals, both in this laboratory and elsewhere. Concern arises with this preparation that the anesthetic may compromise neuronal function, or that it may interact with test drugs affecting neurotransmitter overflow. In addition, in these studies the microdialysis probe typically is introduced into the brain on the day of testing, and data collection commences within an hour or two following probe insertion. It has been suggested that transmitter recovered in the perfusate probably represents leakage due to tissue damage as well as exocytotic release, and may not accurately reflect neuronal responses to the manipulations of interest. Such potential confounds present important implications for the interpretation of data from these studies. The present investigation examined the effects of chloral hydrate anesthetic on (1) basal dopamine (DA) overflow in the anterior striatum, and (2) DA responses to systemically delivered drugs of two different classes known to influence DA activity. Three putative indices of impulse-dependent release were measured: (a) the time course and stability of basal DA overflow over several hours; (b) sodium channel involvement by adding tetrodotoxin (TTX) to the artificial CSF; and (c) calcium channel involvement using magnesium (Mg) in a calcium-free perfusate. Basal DA levels became stable in both conscious and anesthetized preparations by the second hour after probe insertion. Levels of recovered DA overflow in the anterior striata of conscious rats were approximately double those in chloral hydrate-anesthetized rats. Consistent with other findings, this suggests a general depression of CNS function by chloral hydrate.(ABSTRACT TRUNCATED AT 250 WORDS)

Rapid changes in striatal ascorbate in response to tail-pinch monitored by constant potential voltammetry

The first peak in the voltammogram recorded with linear sweep and a carbon paste electrode implanted in the rat striatum is due to the oxidation of ascorbic acid. When the potential is held at a level slightly positive to this peak a current is recorded which is abolished by the microinjection of ascorbic acid oxidase in the vicinity of the electrode; this suggests that it is due to the oxidation of ascorbate. This current shows the same diurnal variation as the size of the ascorbate peak and its rise and fall coincides with the onset and offset of motor activity. A tail-pinch applied through a paper clip causes an immediate rise in the ascorbate current which begins to fall as soon as the paper clip is removed. Measurement of the ascorbate current at constant potential provides a technique for monitoring rapid changes in extracellular brain ascorbate in response to physiological stimuli.

In vivo electrochemical detection of 5-hydroxyindole within the trigeminal nucleus caudalis of freely moving rats: the effect of morphine

The trigeminal nucleus caudalis is considered the equivalent of the orofacial nociceptive system of the dorsal horn of the spinal cord. At the level of this trigeminal area (i.e. medullary dorsal horn) the transmission of noxious inputs is strongly modulated by a descending, serotonergic system mainly originating from the nucleus raphe magnus (NRM). The present study in freely moving animals reports the effect of morphine on the 5-hydroxyindole oxidation current recorded in the medullary dorsal horn. Complementary data from recordings in spinal dorsal horn in acutely anesthetized rats are also presented. A current recorded at 270-290 mV (peak '3'), characteristic of 5-hydroxyindoleacetic acid (5-HIAA), was measured with treated multi-fiber carbon electrodes, using differential pulse voltammetry (DPV) or differential normal pulse voltammetry (DNPV). In control rats, the amplitude of the peak remained constant for many hours. Morphine (10 mg/kg i.p.) caused a significant increase which plateaued between 35 and 80 min (mean increase: 127 +/- 5% of control values); recovery was complete by about 3 h. Simultaneous injection of naloxone (1 mg/kg i.p.) totally abolished the effect of morphine. By contrast, morphine was without effect on peak 3 recorded in the spinal dorsal horn of chloral hydrate (450 mg/kg i.p.) anesthetized rats. It is concluded that in non-anesthetized freely moving animals morphine clearly increases the metabolism of serotonin (5-HT) in the medullary dorsal horn. This finding confirms previous neurochemical data showing an increased synthesis or release of 5-HT in the spinal cord after systemic morphine or its microinjection into either the periaqueductal gray matter or the NRM, and underlines the value of in vivo electrochemistry in monitoring changes in 5-HT metabolism directly and continuously during various physiological and pharmacological procedures.

Morphine increases 5-HT metabolism in the nucleus raphe magnus: an in vivo study in freely moving rats using 5-hydroxyindole electrochemical detection

The purpose of this study was to evaluate in freely moving animals the effect of morphine on the 5-hydroxyindole oxidation current recorded in the nucleus raphe magnus (NRM) which is the origin of serotonergic control systems modulating the transmission of noxious inputs at the spinal level. A current recorded at 270-290 mV (peak 3), characteristic of 5-hydroxyindoleacetic acid (5-HIAA), was measured with treated multi-fiber carbon electrodes, using differential pulse (DPV) or differential normal pulse (DNPV) voltammetry. In control rats the amplitude of the peak remains constant for many hours. Morphine (10 mg/kg i.p.) caused a very significant increase which plateaued between 60 and 80 min (mean increase: 142 +/- 7% of control values); recovery was complete by about 3 h. Simultaneous injection of naloxone (1 mg/kg i.p.) completely abolished the effect of morphine. The peak 3 augmentation was still observed (151 +/- 5%) in rats pretreated with the xanthine oxidase inhibitor, allopurinol (12 mg/kg i.p.), but did not occur when animals were given an anaesthetic dose (450 mg/kg i.p.) of chloral hydrate. It is concluded that morphine clearly increases the metabolism of serotonin (5-HT) in the NRM, and one could speculate that the increase in 5-HIAA results from 5-HT release. Such a release could be due either to 5-HT terminals originating in the periaqueductal gray, or to somato-dendritic mechanisms. Thus the question remains as to the relationship between the activation of 5-HT metabolism in the NRM and previous neurochemical evidence for morphine-induced augmentation of 5-HT metabolism within the terminal area of serotonergic raphe-spinal pathways.

Influence of anaesthetics on rat striatal dopamine metabolism in vivo

The effects of 4 anaesthetics, halothane, alpha-chloralose, chloral hydrate and pentobarbitone, on rat striatal extracellular dihydroxyphenylacetic acid (DOPAC) levels were determined using in vivo voltammetry. Stable baseline levels of DOPAC were maintained under halothane/N2O and alpha-chloralose while the extracellular levels of DOPAC gradually declined under chloral hydrate or pentobarbitone anaesthesia. Administration of haloperidol (0.3 mg/kg i.p.) significantly increased DOPAC using halothane/N2O, alpha-chloralose or chloral hydrate but not under pentobarbitone anaesthesia. The greatest increase in DOPAC was seen in rats anaesthetized with alpha-chloralose greater than halothane/N2O greater than chloral hydrate greater than pentobarbitone. Apomorphine (0.5 mg/kg s.c.) given 2 h after haloperidol partially reversed the increase in DOPAC produced by haloperidol. The results suggest care needs to be exercised in the choice of anaesthetic used for voltammetric studies with pentobarbitone being the least recommended.