In vivo electrochemical demonstration of potassium-evoked monoamine release from rat cerebellum

Experimental Methods for Investigating Uptake 2 Processes In Vivo

Neuromodulators are critical regulators of the brain's signaling processes, and thus they are popular pharmacological targets for psychoactive therapies. It is clear that monoamine uptake mechanisms are complicated and subject to multiple uptake mechanisms. Uptake 1 describes uptake of the monoamine via its designated transporter (SERT for serotonin, NET for norepinephrine, and DAT for dopamine), whereas Uptake 2 details multiple transporter types on neurons and glia taking up different types of modulators, not necessarily specific to the monoamine. While Uptake 1 processes have been well-studied over the past few decades, Uptake 2 mechanisms have remained more difficult to study because of the limitations in methods that have the sensitivity and spatiotemporal resolution to look at the subtleties in uptake profiles. In this chapter we review the different experimental approaches that have yielded important information about Uptake 2 mechanisms in vivo. The techniques (scintillation microspectrophotometry, microdialysis, chronoamperometry, and voltammetry) are described in detail, and pivotal studies associated with each method are highlighted. It is clear from these reviewed works that Uptake 2 processes are critical to consider to advance our understanding of the brain and develop effective neuropsychiatric therapies.

Decreased dopamine D4 receptor expression increases extracellular glutamate and alters its regulation in mouse striatum

To better understand the effect of the dopamine D4 receptor (DRD4) on glutamate (Glu) neurotransmission in the brain, we utilized transgenic mice with partial or complete removal of functional DRD4 plasma membrane expression (DRD4+/- and DRD4-/-, respectively). We measured resting extracellular Glu levels, Glu clearance kinetics, and KCl-evoked release of Glu in the striatum and nucleus accumbens core of these mice using in vivo amperometry coupled to a novel microelectrode array configured for sub-second detection of Glu. Recordings from DRD4-/- and DRD4+/- mice were compared with their wild-type littermates (DRD4+/+). Resting extracellular levels of Glu were increased in the striatum of DRD4-/- mice (p<0.01). Glu clearance kinetics were significantly decreased in the dorsal striatum of DRD4-/- mice (p<0.05). KCl-evoked overflow of Glu was reliably measured but unchanged in the striatum of the three groups. By contrast, no changes in resting Glu, Glu uptake kinetics, or KCl-evoked release of Glu were observed in the nucleus accumbens core among the three genotypes. These data indicate that the DRD4 receptor is involved in modulation of Glu neurotransmission, primarily in the striatum. A better understanding of Glu control by the DRD4 may improve our understanding of the physiological role of the DRD4 in disorders such as attention-deficit/hyperactivity disorder and schizophrenia.

Adderall produces increased striatal dopamine release and a prolonged time course compared to amphetamine isomers

RATIONALE

Adderall is currently used for the treatment of Attention-Deficit Hyperactivity Disorder (ADHD) and is composed of a novel mixture of approximately 24% L-amphetamine and 76% D-amphetamine salts. There are, however, no investigations of the pharmacological effects of this combination in vivo.

OBJECTIVES

The technique of high-speed chronoamperometry using Nafion-coated single carbon-fiber microelectrodes was used to study amphetamine-evoked dopamine (DA) release produced by Adderall, D-amphetamine, or D,L-amphetamine in the striatum of anesthetized male Fischer 344 (F344) rats. The amphetamine solutions were locally applied from micropipettes by pressure ejection.

RESULTS

Local applications of Adderall resulted in significantly greater DA release signal amplitudes with prolonged time course of dopamine release and re-uptake as compared to D-amphetamine and D,L-amphetamine.

CONCLUSIONS

These data support the hypothesis that the combination of amphetamine enantiomers and salts in Adderall has effects on DA release, which result in increased and prolonged DA release, compared to D- and D,L-amphetamine.

Dopamine D4 receptor knockout mice exhibit neurochemical changes consistent with decreased dopamine release

Dopamine D4 receptor (D4R) knockout mice (D4R-/-) provided for unique neurochemical studies designed to understand D4R contributions to dopamine (DA) regulation. In this study, post-mortem brain tissue content of DA did not differ between D4R+/+ and D4R-/- mice in the striatum (Str) or nucleus accumbens core (NAc). However, there was a significant decrease (82%) in the content of 3,4-dihydoxyphenylacetic acid (DOPAC), a major metabolite of DA, in the NAc of D4R-/- mice. Microdialysis studies performed in a region of brain spanning of the dorsal Str and NAc showed lower baseline levels of DA and a significant reduction in KCl-evoked overflow of DA in the D4R-/- mice. Baseline extracellular levels of DOPAC and homovanillic acid were also significantly lower in the D4R-/- mice. In vivo chronoamperometric recordings of KCl-evoked release of DA also showed decreased release of DA in the Str and NAc of the D4R-/- mice. These studies demonstrate a role of D4Rs in presynaptic DA regulation and support the hypothesis that alterations in D4Rs may lead to diminished DA function.

Differential effects of amphetamine isomers on dopamine release in the rat striatum and nucleus accumbens core

RATIONALE

Current medications for attention-deficit/hyperactivity disorder (ADHD) include some single isomer compounds [dextroamphetamine (D: -amphetamine, dexedrine) and dexmethylphenidate (Focalin)] and some racemic compounds [methylphenidate and mixed-salts amphetamine (Adderall)]. Adderall, which contains approximately 25% L: -amphetamine, has been successfully marketed as a first-line medication for ADHD. Although different clinical effects have been observed for D: -amphetamine, Adderall, and benzedrine; potential psychopharmacological differences on the level of neurotransmission between D: -amphetamine and L: -amphetamine have not been well characterized.

OBJECTIVES

To evaluate potential differences in the isomers, we used the technique of high-speed chronoamperometry with Nafion-coated single carbon-fiber microelectrodes to measure amphetamine-induced release of dopamine (DA) in the striatum and nucleus accumbens core of anesthetized male Fischer 344 rats. Amphetamine solutions were locally applied by pressure ejection using micropipettes.

RESULTS

The presence of L: -amphetamine in the D: ,L: -amphetamine solutions did not cause increased release of DA but did change DA release kinetics. The D: ,L: -amphetamine-evoked signals exhibited significantly faster rise times and shorter signal decay times. This difference was also observed in the nucleus accumbens core. When L: -amphetamine was locally applied, DA release was not significantly different in amplitude, and it exhibited the same rapid kinetics of D: ,L: -amphetamine.

CONCLUSIONS

These data support the hypothesis that amphetamine isomers have different effects on release of DA from nerve endings. It is possible that L: -amphetamine may have unique actions on the DA transporter, which is required for the effects of amphetamine on DA release from nerve terminals.

Antennal morphology as a physical filter of olfaction: temporal tuning of the antennae of the honeybee, Apis mellifera

There are many different antennal morphologies for insects, yet they all have the same functional role in olfaction. Chemical signals are dispersed through two physical forces; diffusion and fluid flow. The interaction between antennal morphology and fluid flow generates a region of changing flow velocity called the boundary layer. The boundary layer determines signal dispersion dynamics and therefore influences the signal structure and information that arrives at the receptor cells. To investigate how the boundary layer changes the information in the signals arriving at receptor cells, we measured chemical dynamics within the boundary layer around the bee antennae using microelectrodes. We used two types of chemical signals: pulsed and continuous. The results showed that the boundary layer increased the decay time of the chemical signal for the pulsatile stimuli and increased the peak height for the continuous data. Spectral analysis of continuous signals showed that the temporal aspects of the chemical signal are changed by the boundary layer. Particularly the temporal dynamics of the signal are dampened at the slowest flow speed and amplified at the intermediate and fast flow speeds. By altering the structure of the chemical signal, the morphology will function as a sensory filter.

In vivo chronoamperometric measurements of the clearance of exogenously applied serotonin in the rat dentate gyrus

The present study evaluated high-speed chronoamperometry as a method for measuring the clearance of serotonin (5-HT) from extracellular space in vivo. Male Sprague-Dawley rats were anaesthetized and a Nafion-coated, carbon fiber electrode, attached to a multibarrel pipette, was lowered into the subgranular layer of the dentate gyrus, a region which receives dense serotonergic innervation, or the corpus callosum, a fiber tract relatively devoid of the 5-HT transporter (SERT). Serotonin, pressure ejected into these regions, produced replicable electrochemical signals. The amplitude and time course of the signals were significantly prolonged in the corpus callosum compared to the dentate gyrus. Similarly, signals produced by locally applied 5-HT in the dentate gyrus of rats following destruction of hippocampal serotonergic innervation with 5,7-dihydroxytryptamine (5,7-DHT), were significantly enhanced compared to those observed in control animals. The time course of the 5-HT signal was significantly prolonged by local application of the selective 5-HT reuptake inhibitor, fluvoxamine, into the dentate gyrus. By contrast, fluvoxamine did not modify the clearance of 5-HT when locally applied into the dentate gyrus of 5,7-DHT lesioned rats or into the corpus callosum of intact rats. Taken together, these data demonstrate that in intact rats, the SERT contributes to the clearance of exogenously applied 5-HT from the extracellular space. Under the experimental conditions used in this study, high-speed chronoamperometry proved to be a reliable method for directly measuring extracellular 5-HT and appears to be a valuable tool for the study of 5-HT clearance by the SERT in vivo.

Electrophysiological and electrochemical responses of NMDA in the cerebellum: interactions with nonadrenergic pathway

In the present experiments, we measured N-methyl-D-aspartate (NMDA)-induced norepinephrine (NE) release and extracellular action potentials in the cerebellar cortex of urethane-anesthetized rats. The overflow of NE was measured using a Nafion coated-carbon fiber electrode and in vivo chronoamperometry. We found that both NMDA and quisqualate evoked NE release. Our previous work demonstrated that the electrophysiological activity of cerebellar Purkinje neurons could be either excited or inhibited by local NMDA application. It was reported that bicuculline antagonized NMDA-induced inhibition in Purkinje neurons, suggesting that a GABAergic mechanism was activated during NMDA application. We and others previously found that GABA-mediated electrophysiological depressions were enhanced by NE acting via beta-adrenergic receptors while these responses were decreased by activation of alpha-adrenergic receptors. Since NMDA evokes overflow of endogenous NE, the electrophysiological depression induced by NMDA may contain an NE-mediated modulatory component. In this study, we first examined the interaction of NMDA with beta-adrenergic agonist. We found that local application of isoproterenol facilitated NMDA- or GABA-mediated electrophysiological depressions of the Purkinje neurons. Applications of phenoxybenzamine, which antagonized the alpha-adrenergic response of synaptically released NE, also facilitated NMDA-elicited depression. In contrast, the depression induced by GABA, which did not induce NE overflow, was not potentiated by phenoxybenzamine. The facilitation of NMDA-induced depression by phenoxybenzamine was antagonized by the beta-adrenergic antagonist timolol. Taken together, these data suggested that the nonadrenergic pathway is involved in NMDA-induced electrophysiological responses in the cerebellum. NMDA may induce neuronal depression through modulation of GABAergic inhibition via NMDA-evoked NE release onto cerebellar Purkinje neurons.

Effects of the putative antidepressant, ABT 200, on the clearance of exogenous norepinephrine in rat cerebellum

ABT 200 [(RR,SS)-3-phenyl-1-[1',2',3',4'-tetrahydro-5',6'-methylenedioxy- 1'-naphthalenyl-methyl]-pyrrolidine methanesulfonate] is a potent alpha 2-adrenoceptor antagonist (Ki = 1.2 nM) with modest norepinephrine uptake-blocking activity (IC50 = 841 nM) that is currently under clinical evaluation as an antidepressant. The effects of ABT 200, nomifensine (an inhibitor of catecholamine uptake), and rauwolscine (a selective alpha 2-adrenoceptor antagonist) on the clearance of exogenous norepinephrine in the cerebellum of urethane-anesthetized rats was investigated using a vivo electrochemistry. Chronoamperometric recordings were continuously made at 5 Hz using Nafion-coated, single carbon fiber electrodes. When a calibrated amount of norepinephrine was pressure-ejected at 5-min intervals from a micropipette adjacent (290-330 microM) to the electrode, transient and reproducible norepinephrine signals were detected. In response to systemic ABT 200 (30 mg/kg i.p.) or nomifensine (30 mg/kg i.p.), the signals increased in both amplitude and time course, indicating significant inhibition of the norepinephrine transporter. A lower dose (15 mg/kg i.p.) of either ABT 200 or nomifensine had no effect in this paradigm. Local application of ABT 200 (400 microM) or nomifensine (400 microM) prior to pressure-ejection of norepinephrine also significantly increased the amplitude and time course of the norepinephrine signals. In contrast, systemic administration of rauwolscine (30 mg/kg i.p.) or vehicle solution, and local application of vehicle solution, had no effect on the norepinephrine signals. These data indicate that at the higher dose evaluated, both ABT 200 and nomifensine inhibit cerebellar norepinephrine uptake in vivo.

Direct measurement of glutamate release in the brain using a dual enzyme-based electrochemical sensor

The in vivo measurement of the rapid changes in the extracellular concentrations of L-glutamic acid in the mammalian brain during normal neuronal activity or following excessive release due to episodes of anoxia or ischemia has not been possible to this date. Current techniques for the measurement of the release of endogenous glutamate into the extracellular space of the central nervous system are relatively slow and do not measure the actual concentration of free glutamate in the extracellular space. An enzyme-based electrode with rapid response times (about 1 s) and high degree of sensitivity (less than 2 microM) and selectivity for L-glutamic acid is described in this paper. This electrode has both L-glutamate and ascorbate oxidase immobilized on its surface. The latter enzyme removes almost completely any interferences produced by the high levels of extracellular ascorbate present in brain tissue. The response of the electrode to glutamate and other potentially interfering substances was fully characterized in vitro and its selectivity, sensitivity and rapidity in responding to a rise in extracellular glutamate concentrations was also demonstrated in vivo. Placement of the electrode in the dentate gyrus of the hippocampus led to the detection of both KCl-induced release of L-glutamic acid and the release induced by stimulation of the axons in the perforant pathway. The development of this selective, sensitive and rapidly responding glutamate sensor should make it now possible to measure the dynamic events associated with glutamate neurotransmission in the central nervous system.

Dopamine release in the rat cerebellum and hippocampus: a tissue 3-methoxytyramine study

Multiple lines of evidence indicate dopamine is a neurotransmitter or neuromodulator in the cerebellum and hippocampus. In this study, we explored the utility of 3-methoxytyramine as an index of dopamine release in these regions. We found that: (1) cerebellar and hippocampal 3-methoxytyramine levels can be measured by combined gas chromatography-mass fragmentography with negative chemical ionization; (2) basal 3-methoxytyramine accumulation rates following monoamine oxidase inhibition, but not the steady-state tissue levels, are several times lower in these regions than in the frontal cortex; (3) accumulation of 3-methoxytyramine in the hippocampus and cerebellum can be enhanced following electroconvulsive shock, but not acute haloperidol (0.4 mg/kg) treatment. We conclude that 3-methoxytyramine accumulation may be a useful index of dopamine release in the cerebellum and hippocampus, but dopamine release is regulated differently in these regions than in the frontal cortex and striatum.

Ethanol inhibits the uptake of exogenous norepinephrine from the extracellular space of the rat cerebellum

Rapid chronoamperometric recordings using nafion-coated carbon fiber electrodes coupled with pressure-ejection of drugs were used to investigate the effects of ethanol on norepinephrine (NE)-containing nerve terminals in the urethane-anesthetized Fischer 344 rat. Local application of ethanol from a double-barrel micropipette did not produce detectable changes in extracellular levels of NE in the rat cerebellar cortex. However, when ethanol was applied prior to local application of NE, it was seen to inhibit the uptake of NE from the extracellular space. These results were compared to the effects seen from the local application of a known high-affinity uptake inhibitor, nomifensine. Nomifensine was found to inhibit the extracellular uptake of NE in rat cerebeller cortex similar to ethanol. Our results support the hypothesis that one effect of ethanol on the noradrenergic system of the rat cerebellum is an alteration in the uptake of NE into NE-containing nerve endings. In addition, the present data concerning ethanol-induced inhibition of NE clearance or uptake support our previous electrophysiological studies in which we found that ethanol can potentiate the modulatory effects of beta-agonists on GABA responses of cerebellar Purkinje neurons.

Real-time monitoring of electrically stimulated norepinephrine release in rat thalamus: I. Resolution of transmitter and metabolite signal components

Electrical stimulation of an ascending path of the locus ceruleus-norepinephrine system was used to elicit release of norepinephrine at noradrenergic terminal fields of the rat thalamus. Overflow into the extracellular fluid space was measured by fast in vivo chronoamperometry. At pretreated carbon fibers, the electrochemical signal consists of a sharp peak of approximately 20-30 s duration followed by a slower, plateau-like decay to baseline. The peak, characterized by a variety of pharmacological manipulations and dialysis perfusion, is primarily due to norepinephrine. The plateau was shown to correspond to metabolite efflux of 3,4-dihydroxy-phenylacetic acid. By varying the degree of electrochemical pretreatment, the response time and sensitivity of the fibers can be tuned to follow the entire signal or to select the separate components for detailed evaluation. This approach can be used to provide new information on the spatial and temporal characteristics of stimulated neurotransmitter release.

The pharmacological profile of glutamate-evoked ascorbic acid efflux measured by in vivo electrochemistry

A recently described in vivo voltammetric electrode selectively records rapid changes in extracellular fluid (ECF) levels of ascorbic acid. Using this detector, the nature of glutamate-induced efflux of ascorbate into ECF was investigated using pharmacological tools. Ascorbate signals were shown to be directly related to amounts of microinjected glutamate. Blockers of glutamate reuptake, homocysteic acid and D,L-threo-beta-hydroxy-aspartic acid, virtually eliminate the ascorbate signal. A more specific reuptake blocker (the stilbene isothiocyano derivative (SITS) does not completely inhibit ascorbate efflux, suggesting that the glutamate uptake which is coupled to ascorbic acid exchange is both neuronal and glial in nature. Other pharmacological experiments indicate that excitatory amino acid receptors are not involved in the glutamate-elicited ascorbate efflux; it is primarily a function of the glutamate/ascorbate heteroexchange process as described earlier. The possible role(s) of brain ascorbate in the general functioning of the pervasive glutamate neurotransmitter systems are discussed.

Electrically-evoked release of norepinephrine in the rat cerebellum: an in vivo electrochemical and electrophysiological study

Norepinephrine (NE) release from the locus coeruleus (LC) afferents to the cerebellar cortex of urethane anesthetized rats was achieved by electrical stimulation of the locus coeruleus or by local administration of potassium into the cerebellum. Both methods evoked an overflow of NE-like electroactive species. Electrically-evoked and potassium-induced overflow of NE-like responses were found to be reversible and reproducible. Releases were not observed in cerebellar white matter, an area which is relatively devoid of monoamine containing terminals. Systemic administration of desipramine, a potent and selective norepinephrine re-uptake blocker, significantly augmented the electrically-evoked electrochemical responses. Measurements of evoked release taken using high-speed chronoamperometry support the idea that a predominant contributor to electrically-induced signals was NE. Electrophysiological recordings of single Purkinje cells were performed with the same Nafion-coated single carbon fiber electrodes used for electrochemical recordings. Electrical stimulation of the LC was seen to depress Purkinje cell firing rates; an increase in electroactive species was detected at the same site that paralleled the time course of the electrophysiological response. These studies provide further direct evidence that the LC norepinephrine-containing cells have a direct inhibitory effect on Purkinje cells in the cerebellum, and that both pre- and postsynaptic events can be measured with the same recording sensor.

Age-induced changes in single locus coeruleus brain transplants grown in oculo: an in vivo electrochemical study

Brain stem tissue from fetal Sprague-Dawley rats containing the nucleus locus coeruleus (LC) was transplanted into the anterior chamber of the eye of young adult host rats and was studied at 4-6 months (young control) or 24-28 months after grafting (old). High-speed in vivo electrochemical measurements were used to characterize the potassium-evoked synaptic overflow of norepinephrine (NE) in both young and aged LC brain grafts. The amplitudes of potassium-evoked NE overflow were attenuated in the aged grafts as compared to the young LC grafts. In addition, the rise times of potassium-evoked responses were longer in the old LC grafts than in the young transplants. In contrast, the NE content of aged LC grafts, as determined by high-performance liquid chromatography coupled with electrochemical detection (HPLC-EC), was only slightly diminished and not significantly different from the NE levels seen in young LC grafts. However, light microscopical evaluation using tyrosine-hydroxylase immunocytochemistry revealed pyknotic cell bodies and fluorescent accumulations in aged locus coeruleus transplants which were indicative of degeneration in these grafts. The present data demonstrate a significant age-related decline in the presynaptic function of NE-containing neurons in intraocular locus coeruleus transplants of Sprague-Dawley rats.

Function of intraventricular human mesencephalic xenografts in immunosuppressed rats: an electrophysiological and neurochemical analysis

Solid pieces of human fetal mesencephalic tissue were grafted to the lateral ventricle adjacent to dopamine-depleted striata of rats immunosuppressed with cyclosporin A. Apomorphine-induced rotations were performed before and at monthly intervals after grafting. Reductions in rotations were seen at 2 months post-grafting and these reductions progressively increased. Spontaneously active dopaminergic cells were found within the grafts using extracellular single-unit recording techniques. Recordings of striatal cells ipsilateral to the graft revealed "normal" firing rates compared to those of neurons in the control striatum. In response to the local application of the dopamine antagonist cis-flupenthixol, both the dopaminergic and striatal neurons showed dose-dependent excitations. Potassium-evoked releases of electroactive species ipsilateral to the fetal human graft, measured using high-speed in vivo electrochemistry, revealed response amplitudes that were similar to control striatum when an electrode was placed adjacent to the graft; distal to the graft the responses showed smaller amplitudes but prolonged time courses. Much greater levels of dopamine and serotonin were detected in the grafts, compared to in normal rat substantia nigra, as measured with HPLC coupled to a 16-channel electrochemical array detector. Immunocytochemical studies using antibodies against tyrosine hydroxylase (TH), revealed not only TH-positive cells within the graft, but also a few positive neurons that migrated into the host striatum. Numerous TH-immunoreactive fibers penetrated into striatum and reinnervated its total volume. Taken together, these data suggest that intraventricular graft placement may be a highly efficacious technique for studying fetal brain tissues in terms of maturation, reinnervation, and function.

Combined electrochemical and electrophysiological studies of monoamine overflow in rat hippocampal slices

In vivo electrochemical measurements of chronoamperometric recordings from Nafion-coated electrodes were used to investigate monoamine overflow from selected regions of the rat hippocampal slice. Concurrent electrophysiological measurements of evoked CA1 pyramidal cell population spike responses were used to characterize changes in the electrical activity in the slices that occur during potassium-induced neurotransmitter overflow. Superfusion with elevated K+ (10-50 mM, 5 min) elicited consistent concentration-dependent increases in the electrochemical responses recorded from the dentate gyrus. At the onset of K+ perfusion, there was an initial increase in the population spike response, followed by electrical silence, which usually lasted 5-10 min following the return to normal medium, and required 20-30 min for complete recovery of the response. The potassium-induced electrochemical signal always increased following the decline in the electrophysiological response. Although the electrochemical signal usually returned to baseline much before the electrophysiological response (usually within 5 min), both signals remained refractory for some time. Cocaine pretreatment (10-50 microM) caused a dose-dependent augmentation of the electrochemical responses. Local pressure ejection of K+ via a micropipette elicited dose-dependent increases in the electrochemical signals that were of relativity brief duration as compared to superfusion with K+. Such potassium-evoked responses were highly localized, and were attenuated in amplitude in animals that had been previously treated with the selective noradrenergic neurotoxin, DSP-4. In addition to K+, local applications of methyl-amphetamine, tyramine and veratridine also elicited electrochemical signals, and the time courses of these responses were specific to the releasing agent that was used. Preliminary data obtained using high-speed electrochemical recordings of both oxidation and reduction current suggested that tyramine ejections evoked primarily norepinephrine overflow, while K+ evoked the overflow of both norepinephrine and serotonin. The present experiments demonstrate that simultaneous electrophysiological and electrochemical experiments can be used in an isolated preparation of brain such as the hippocampal slice to characterize the electrophysiological events that occur during stimulated transmitter release.

In vivo voltammetry--prospects for the next decade

In vivo voltammetry has been in existence for more than ten years. The technique now benefits from smaller, more selective sensors-true 'chemical microelectrodes'. The scope of experimentation has widened dramatically both in vivo and in vitro as new applications are reported. The speed of measurements has increased giving high temporal and spatial resolution, approaching 'real time'. This article discusses the progress of the past decade and looks forward to the advances of the next ten years.

In vivo electrochemical studies of monoamine release in the medial prefrontal cortex of the rat

The magnitude and duration of release of monoamines evoked by local applications of potassium were measured in vivo in the medial prefrontal cortex using high-speed chronoamperometry. Typical electrochemical signals reflecting released of electroactive species ranging from 0.5 to 3.0 microM and lasting 90-120 s were detected at a variety of dorsal-ventral and anterior-posterior electrode placements in the medial prefrontal cortex. The magnitude of the reduction current measured following the oxidation reaction suggests a contribution of both serotonin and dopamine to the electrochemical signal, dopamine serving as the predominant monoamine in the medial prefrontal cortex proper and serotonin appearing to predominant in the more posterior regions of the frontal cortex. This conclusion was reinforced by the fact that unilateral 6-hydroxydopamine lesions of ascending dopamine fibers almost completely abolished electrochemical signals in the ipsilateral but not in the contralateral medial prefrontal cortex. The present study provides an in vivo characterization of monoamine release in the mesocortical dopamine terminal field, where it has been suggested that psychomotor stimulants may produce some of their positive reinforcing effects.

Age-related changes in cerebellar noradrenergic pre- and postsynaptic mechanisms: intrinsic vs extrinsic determinants evaluated with brain grafts in oculo

Intrinsic versus extrinsic determinants of changes in cerebellar noradrenergic transmission during senescence in the rat were measured using homologous cerebellar grafts in oculo. Postsynaptic sensitivity of Purkinje neurons to catecholamines was determined by perfusing the anterior eye chamber with known concentrations of norepinephrine (NE) dissolved in a balanced salt solution. NE elicited a dose-dependent slowing of spontaneous Purkinje neuron discharge in both young (3-6 months) and aged (20-22 months) cerebellar grafts. Hill plots demonstrated that the dose-response relationships in both age-groups were linear and parallel to one another. Aged transplant Purkinje neurons manifested a marked and highly significant subsensitivity to NE with an EC50 of 583 microM, as compared with an EC50 of only 15.9 microM in the young grafts. Young grafts in 15-21-month-old hosts manifested an EC50 of 20 microM for the depressant actions of NE. Collaterals of host iris sympathetic fibers innervate the grafts. Activity of these fibers can be reflexly altered by changing illumination of the retina. The dynamics of presynaptic NE release from these fibers was evaluated using in vivo electrochemistry with Nafion-coated graphite epoxy capillary electrodes, which are highly selective for the monoamine neurotransmitters. As illumination of the ipsilateral retina is increased, the release of catecholamine in the cerebellar graft decreases. A mean change in the extracellular electroactive species of 4.2 +/- 0.6 microM was found in young cerebellar grafts. Equivalent stimuli induced a mean change of 2.3 +/- 0.8 microM in aged grafts. However, this diminished release was not statistically significant.(ABSTRACT TRUNCATED AT 250 WORDS)