Neuropeptides may interfere with voltammetric recording of catechols using carbon fibre micro-electrodes

NEUROPEPTIDES MAY INTERFERE WITH VOLTAMMETRIC RECORDING OF CATECHOLS USING CARBON FIBRE MICRO-ELECTRODES 1R. RIVEST., 1F.B. JOLICOEUR AND 2C.A. MARSDEN. 1Department of Psychiatry, Faculty of Medecine, University of Sherbrooke, PQ, Canada, JIH 5N4 and 2 Department of Physiology and Pharmacology, Queen's Medical Centre, Medical School, Nottingham, UK, NG7 2UH. Differential pulse voltammetry (DPV) coupled with electrically-pretreated carbon fibres electrodes allows the selective detection of amines or their metabolites. In vivo these electrodes measure the amine metabolites due to their high extracellular concentration (~M) compared to the amines (nM). We are currently using the technique to study the effects of neurotensin (NT) on extracellular DOPAC in vivo. The work involves both ICV and intracerebral injection of NT including administration into the same region as the DPV measurements are made. We report here results suggesting that while neuropeptide administration away from the electrode does not interfer with the DPV recording; injection close to the electrode can lead to misinterpretation of the results. Male Wistar rats (250280g) were anaethetised with chloral hydrate (400 mg/kg i.p.) and stereotaxically implanted with a 12~m carbon fibre electrode in the nucleus accumbens. Administration of peptide or saline via a 30 ga injection needle was made into the lateral ventricle (NT 1, 10 ~g/lO t~1), the ventral tegmental area (NT 1, 3 ~g/~l) and using the same dose beside the electrode implanted in the nucleus accumbens. Extracellular DOPAC was recorded every 2.5 minutes after the peak had stabilized and NT was injected following a 30 minutes base line period. The administration of NT into either the ventricle and the ventral tegmentum induce an increase in extracellular DOPAC in the nucleus accumbens while administration into the nucleus accumbens close to the electrode decreased the peak. Because the peptide is known to stick to various surfaces, we were concerned that this decrease might be caused by a nonspecific alteration to the working electrode by the peptide. To investigate this possibility we studied the effects of neurotensin on the peak produced by dopamine or DOPAC in vitro. The electrode was immersed in a solution containing DOPAC or dopamine (10 -5 M in artificial CSF). The addition of NT 10 -5 M to either DOPAC or dopamine induced a rapid and prolonged decrease in the size of their oxidation peaks. However, when the electrode was removed from the mixture for 30 minutes and then re-immersed, the reduction in the catechol peak was again observed but only following the first scan. The absence of a reduced peak with the first measurement clearly indicate that no chemical interaction had occured between either dopamine or DOPAC and NT during the 30 min prior to making the DPV scan. Similar results were obtained using TRH, substance P and somatostatin. Thus, the decrease of DOPAC in the nucleus accumbens observed after NT administration in this region is likely to be due to the peptide altering the properties of the carbon fibre electrode rather than a chemical interaction between the catechols and NT (Adachi. Soc. Neurosc., Abstrc. 14:114, 1988). Our result suggest that high concentrations of a neuropeptide injected close to a carbon fibre electrode may result in effects that are artifactual. We thank the Wellcome Trust, FCAR (Quebec) and the Canadian MRC for financial support. COMPARISON O F T H E A C T I O N S V O L T A M M E T R I C DATA.

OF

UPTAKE

INHIBITORS

UPON

LIMBIC AND STRIATAL DOPAMINE EFFLUX:

J O N A T H A N A. STAMFORD, ZYGMUNT L. K R U K & JULIAN MILLAR, Department of Pharmacology The London Hospital Medical College, Turner Street, London El 2AD, U.K.

IN V I V O

(Physiology:JM),

The dopamine (DA) systems of the nucleus accumbens (Acb) and striatum (CPu) arise principally in the At0 and A9 cell groups respectively. In a previous report we have shown that there are differences in the uptake and diffusion of DA in the two regions (I). The p r e s e n t study sought to ascertain whether these differences in DA uptake were also reflected in different responses to p u t a t i v e DA uptake inhibitors. All experiments were p e r f o r m e d in male Sprague Dawley rats anaesthetised with chloral hydrate. Carbon fibre microelectrodes were implanted into the Acb and CPu while a stimulating electrode was p o s i t i o n e d in the median forebrain bundle (MFB) to stimulate the mesostriatal axons. DA efflux and subsequent uptake in Acb and cPU was monitored using fast cyclic voltammetry (FCV) with measurements every 50 ms. The experiment consisted of 18 MFB stimulations (50 Hz, i00-ii0 ~A r.m.s 2s train) 5 minutes apart. Drugs were given i.p. after stimulation 6. GBR 12909 (i0 mg/kg) elevated DA efflux to the same degree (by 150%) in both Acb and CPu while benztropine (20 mg/kg), cocaine (i0 mg/kg) and bupropion (50 mg/kg) showed differential effects. Benztropine increased efflux in CPu by 200% while the peak Acb response (+ 40%) was significantly (P < 0.01) lower. Cocaine showed the same initial response (+ 70%) in both nuclei. However, while the CPu effect abated on subsequent stimulations, the Acb response increased to + 150% by stimulation 18. Bupropion showed a similar CPu response to cocaine with significant enhancement of efflux only seen for 25 minutes after administration. The Acb response was more constant. Bupropion was the only drug to show direct evidence of DA uptake blockade, and only in Acb, The absence of correlation between the actions of the drugs in Acb and CPu in response magnitude and duration supports the previous report of differences in limbic and striatal DA uptake. The present data indicates that, in addition to different kinetic properties the uptake systems differ in their sensitivity to drugs. It is also p o s s i b l e that the drugs elevate DA efflux by means other than uptake blockade, a p o s s i b i l i t y supported by an absence of clear actions on uptake with most of the drugs. We thank the Wellcome Trust for funding. (i)

J.A. STAMFORD, Z.L. KRUK, P. PALIJ & J. MILLAR

(1988).

Brain Research 448, 381-5.