Julian Millar

Carbon fibre microelectrodes

A technique for making recording microelectrodes containing an ultrafine carbon fibre is described. This technique can be used for single- and multi-barrel microelectrodes. These microelectrodes not only have a very low signal-to-noise ratio comparable with that found in tungsten microelectrodes but are simpler to make. When used in a multi-barrel array for iontophoretic application of drugs the carbon fibre microelectrode has many advantages over a fluid electrolyte-filled recording barrel, including very high signal-to-noise ratio and relative immunity to spike distortion during application of iontophoretic current.

Electrochemical, pharmacological and electrophysiological evidence of rapid dopamine release and removal in the rat caudate nucleus following electrical stimulation of the median forebrain bundle

Fast cyclic voltammetry was used to monitor the release of electroactive material in the striatum following electrical stimulation of the median forebrain bundle. The released material was shown to be dopamine by electrochemical, pharmacological and neurophysiological means. The material gave a voltammogram identical to that of iontophoretically applied dopamine but not DOPAC. Release was increased by L-DOPA, the metabolic precursor of dopamine. NSD 1015, an inhibitor of dopa decarboxylase, and alpha-methyl-p-tyrosine, a tyrosine hydroxylase inhibitor decreased release. Reserpine, which disrupts vesicular dopamine storage, abolished release. Parachlorophenylalanine, a tryptophan hydroxylase inhibitor, had no effect. Finally, square wave stimulation was only effective when pulses longer than 0.5 ms were used. This indicated stimulation of very fine unmyelinated fibres, consistent with the known morphology of nigrostriatal dopamine fibres.

Regional differences in extracellular ascorbic acid levels in the rat brain determined by high speed cyclic voltammetry

High speed cyclic voltammetry was used in combination with pressure ejection of ascorbate oxidase for the determination of extracellular ascorbic acid within the brain of the anaesthetized rat. Large variations in absolute levels of ascorbate were found between animals although distribution patterns showed a good degree of reproducibility. Ascorbate levels in the white matter of the corpus callosum were found to be higher than in adjacent areas of grey matter (striatum and cortex).

Striatal dopamine uptake in the rat: In vivo analysis by fast cyclic voltammetry

Electrical stimulation of the median forebrain bundle of the chloral hydrate anaesthetised rat evoked dopamine release in the ipsilateral striatum, which was monitored with fast cyclic voltammetry. On cessation of stimulation, the extracellular concentration of dopamine fell to sub-detectable levels over a period of about 15 s. This fall appeared to be due to a saturable, low affinity uptake system that could be inhibited by nomifensine (20 mg/kg i.p.). These experiments constitute the first characterisation of a dopamine uptake mechanism obtained in the intact animal.

Application of fast cyclic voltammetry to measurement of electrically evoked dopamine overflow from brain slices in vitro

Fast cyclic voltammetry at a carbon fibre microelectrode was used to monitor the time course of dopamine overflow in slices of rat corpus striatum incubated in a brain slice chamber. Dopamine release occurred in response to electrical stimulation. Electrochemical, physiological and pharmacological evidence indicates that release of endogenous dopamine can be measured reliably for up to 9 h and that fast cyclic voltammetry can be used in brain slices for quantitative studies of dopamine release in the CNS.

Presynaptic regulation of dopamine release in corpus striatum monitored in vitro in real time by fast cyclic voltammetry

Dopamine release was evoked by single electrical pulses in slices of rat corpus striatum, and measured by fast cyclic voltammetry in real time. The magnitude of the release varied in the expected way to agents which modify dopamine storage, release and re-uptake. The presence of functional dopamine D2 autoreceptors was demonstrated by showing that the release was potently and completely inhibited by the selective agonists quinpirole and N,N-dipropyl-5,6-ADTN. The selective D1 agonist SKF 38393 was ineffective. The inhibition by quinpirole was competitively antagonised by haloperidol and metoclopramide with potencies which correspond closely to published values at postsynaptic striatal D2 receptors. Thus, the D2 autoreceptors on striatal nerve terminals appear to be indistinguishable from those on the postsynaptic neurons.

Diffusion and uptake of dopamine in rat caudate and nucleus accumbens compared using fast cyclic voltammetry

Fast cyclic voltammetry was used in the caudate and nucleus accumbens of anaesthetised rats to study the release and reuptake of dopamine following stimulation of the median forebrain bundle. Dopamine uptake was significantly slower in accumbens than caudate, indicating a lower number of functional uptake sites. This implies that dopamine may be able to diffuse further from its sites of release in nucleus accumbens than in caudate and thus may have a neuromodulator role in this region.

Stimulated limbic and striatal dopamine release measured by fast cyclic voltammetry: anatomical, electrochemical and pharmacological characterisation

Fast cyclic voltammetry at carbon fibre microelectrodes was used to monitor stimulated release of electroactive species simultaneously in both caudate and nucleus accumbens. The released species was identified as dopamine in both nuclei on the basis of anatomical, electrochemical and pharmacological criteria. Dopamine release in the nucleus accumbens was consistently evoked at more ventromedial stimulation sites than for caudate. In both nuclei, release was reduced or abolished by alpha-methyl-p-tyrosine and Ro 4-1284. Pargyline, cocaine and GBR 12909 potentiated release while desipramine had no effect. In conjunction with the electrochemical data, the results indicate that dopamine is the sole species detected in caudate and accumbens following stimulation of the median forebrain bundle.

Quantitative ionophoresis of catecholamines using multibarrel carbon fibre microelectrodes

Carbon fibre microelectrodes can be used to measure the local concentration of ionophoretically ejected catecholamines in vitro or in vivo. The method can also be used to measure the transport number for the materials. The concentration measurement takes about 20 ms and can be repeated at up to about 10 Hz without electrode poisoning or deterioration. This paper describes in detail the methodology of the technique and the apparatus required.

Sub-second striatal dopamine release measured by in vivo voltammetry.

High speed cyclic voltammetry was used to measure electrically stimulated striatal dopamine release in vivo with 25 ms time resolution. Dopamine release after 1 s stimulations was readily detectable and could be manipulated by drugs known to influence dopaminergic neurones. Using data-averaging dopamine release could be detected following stimulus trains as short as 100 ms. The use of carbon fibre microelectrodes gave a spatial resolution better than 5 micron.