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Effects of 5-HT1 receptor agonists injected into the dorsal Raphé nucleus of the rat
DOPAMINE RELEASE MEASURED BY FAST CYCLIC VOLTAI~TRY IN NUCLEUS ACCUMBENS SLICES
D.R. BULL AND M.J. SHEEHAN, Department of Neuropharmacology, Glaxo Group Research Ltd., Ware, Herts., SGI2 0DP, U.K. We have previously applied the technique of fast cyclic voltammetry to the measurement of electrically evoked dopamine release from striatal brain slices in vitro in order to investigate the presynaptic control of neurotransmitter release in these neurones (Palij et al., 1988). We have now extended this technique to the measurement of dopamine release from slices of nucleus accumbens in order to study the pharmacology of mesolimbie dopamine neurones. Slices of rat nucleus aceumbens were prepared and maintained as previously described (PaliJ e t a l., 1988). Neurotransmitter release was elicited by single, square wave pulses of 20V amplitude delivered once every two minutes and applied by bipolar electrodes placed 200-400pm medial to the recording electrode. This release was blocked by tetrodotoxin (10-6M) or removal of Ca ++ implying that it originated from nerve terminals excited axonally. The electrochemical signal was virtually identical to that of exogenous dopamine, although it is not possible to exclude a contribution from noradrenaline. The selective dopamine uptake blocker GBRI2909 increased the size of the electrochemical signal in a concentration-dependent manner, as it had done in stria'al slices. However, desipramine, an agent which selectively blocks the uptake of noradrenaline and 5-HT, also increased the size of the signal. This effect had not been apparent in striatal slices and may be a consequence of the greater population of noradrenergic neurones in the nucleus accumbens (Fonnum and Walaas, 1981). The fast acting reserpine-llke compound, Ro4-1284 (10-SM), virtually abolished release indicating that the released substance was a catecholamine. Finally, the selective D 2 receptor agonist, quinpirole (I0-8-I0-6M), inhibited release in a concentration- related manner, an effect which could be antagonised by sulpiride, a selective D 2 receptor antagonist. This confirms the presence of presynaptic inhibitory dopamine receptors on the terminals of mesolimbic dopamine neurones. This technique offers the opportunity to compare the mechanisms controlling dopamine release in the nucleus accumbens with those in the nigrostriatal pathway. In principle, it may also be used to investigate the pharmacology of other dopamine-rieh brain areas. References: I. Fonnum, F. and Walaas, I. (1981) In 'Neurobiology of the nucleus accumbens', 259-272. Eds. Chronister, R.B. and De France, J.F. Haer Institute for Electrophysiological Research. 2. PaliJ, P., Bull, D.R., Kruk, Z.L., Sheehan, M.J., Stamford, J.A., Millar, J. and Humphrey, P.P.A. (1989) Br. J. Pharmacol., 94, 347P. EFFECTS OF 5-HT I RECEPTOR AGONISTS
G.A. HIGGINS, A.J. BRADBURY, B.J. Research, Ware, Herts SGI2 0DP).
INJECTED INTO THE DORSAL RAPHE NUCLEUS OF THE RAT
JONES,
M.
SKINGLE
(Dept.
of Neuropharmacology,
Glaxo
Group
The dorsal raphe nucleus (DRN) is a major source of ascending 5-HT pathways innervating forebrain structures e.g. frontal cortex (FC), hypothalamus (HYP), hippocampus (HC). Microiontophoretic application of the 5-HTIA agonist, 8-OH DPAT, into the DRN suppress cell firing rate via an action at somatodendritic 5-HTIA receptors (Sprouse & Aghajanian, 1987). Biochemical (Hjorth & Magnusson, 1988) and behavioural (Higgins et al, 1988) responses consistent with decreased central 5-HT function have been reported following intra-DRN injection of 8-OH DPAT. The present study reports the behavioural and biochemical effects of the putative 5-HTIA partial agonist gepirone (Hamon et al, 1987) and mCPP, which at the 5-HT 1 receptor shows some 5-HTIB/I C agonist selectivity (Hoyer, 1989) following DRN injection. Gepirone (0.4-10pg) caused marked falls in the 5-HIAA:5-HT ratio in FC, HC but not HYP, 20min after discrete injection, mCPP only reduced 5-HIAA:5-HT ratio at a high concentration (12.Spg) in FC and HYP, but not in HC. These results contrast sharply with 8-OH DPAT (0.5-2.5pg) and the 5-HT I receptor agonist, 5-carboxamidotryptamine (5-CT) (50-250ng), which decreased 5-HT turnover to a similar extent in all three regions. In the social-interaction and water-lick conflict tests, gepirone (0.2-1pg) produced responses characteristic of a reduced anxiety state, whilst higher doses (>ipg) produced behavioural hypoactivity and flattened body posture (FBP). mCPP (0.i 2.5pg) failed to significantly modify behaviour in the anxiety models, although a high dose (12.Spg) reduced locomotor activity and induced FBP. In conclusion, these studies suggest that stimulation of 5-HTIA receptors in the DRN cause marked changes in forebrain 5-HT metabolism which may underlie some of the behavioural changes mediated by 5-HTIA receptor selective compounds. The lack of effect with mCPP in this study may suggest that 5-HTIB/I C receptors are not critically involved in DRN cell firing. The failure of gepirone to significantly modify 5-HT turnover in the HYP may be a consequence of its partial 5-HTIA agonist profile. HAMON et al (1988) J. Pharm. Exp. Ther. 246 745-753; HIGGINS, G.A. et al (1988) Neuropharmacol. 993-1001; HJORTH, S. & MAGNUSSON, T. (1988) N-S Arch Pharmacol. 338 463-471; HOYER, D. (1988) 9p; SPROUSE, F.S. & AGHAJANIAN, G.K. (1987) Synapse 1 3-9
OQQ
27 96
D.R. BULL AND M.J. SHEEHAN, Department of Neuropharmacology, Glaxo Group Research Ltd., Ware, Herts., SGI2 0DP, U.K. We have previously applied the technique of fast cyclic voltammetry to the measurement of electrically evoked dopamine release from striatal brain slices in vitro in order to investigate the presynaptic control of neurotransmitter release in these neurones (Palij et al., 1988). We have now extended this technique to the measurement of dopamine release from slices of nucleus accumbens in order to study the pharmacology of mesolimbie dopamine neurones. Slices of rat nucleus aceumbens were prepared and maintained as previously described (PaliJ e t a l., 1988). Neurotransmitter release was elicited by single, square wave pulses of 20V amplitude delivered once every two minutes and applied by bipolar electrodes placed 200-400pm medial to the recording electrode. This release was blocked by tetrodotoxin (10-6M) or removal of Ca ++ implying that it originated from nerve terminals excited axonally. The electrochemical signal was virtually identical to that of exogenous dopamine, although it is not possible to exclude a contribution from noradrenaline. The selective dopamine uptake blocker GBRI2909 increased the size of the electrochemical signal in a concentration-dependent manner, as it had done in stria'al slices. However, desipramine, an agent which selectively blocks the uptake of noradrenaline and 5-HT, also increased the size of the signal. This effect had not been apparent in striatal slices and may be a consequence of the greater population of noradrenergic neurones in the nucleus accumbens (Fonnum and Walaas, 1981). The fast acting reserpine-llke compound, Ro4-1284 (10-SM), virtually abolished release indicating that the released substance was a catecholamine. Finally, the selective D 2 receptor agonist, quinpirole (I0-8-I0-6M), inhibited release in a concentration- related manner, an effect which could be antagonised by sulpiride, a selective D 2 receptor antagonist. This confirms the presence of presynaptic inhibitory dopamine receptors on the terminals of mesolimbic dopamine neurones. This technique offers the opportunity to compare the mechanisms controlling dopamine release in the nucleus accumbens with those in the nigrostriatal pathway. In principle, it may also be used to investigate the pharmacology of other dopamine-rieh brain areas. References: I. Fonnum, F. and Walaas, I. (1981) In 'Neurobiology of the nucleus accumbens', 259-272. Eds. Chronister, R.B. and De France, J.F. Haer Institute for Electrophysiological Research. 2. PaliJ, P., Bull, D.R., Kruk, Z.L., Sheehan, M.J., Stamford, J.A., Millar, J. and Humphrey, P.P.A. (1989) Br. J. Pharmacol., 94, 347P. EFFECTS OF 5-HT I RECEPTOR AGONISTS
G.A. HIGGINS, A.J. BRADBURY, B.J. Research, Ware, Herts SGI2 0DP).
INJECTED INTO THE DORSAL RAPHE NUCLEUS OF THE RAT
JONES,
M.
SKINGLE
(Dept.
of Neuropharmacology,
Glaxo
Group
The dorsal raphe nucleus (DRN) is a major source of ascending 5-HT pathways innervating forebrain structures e.g. frontal cortex (FC), hypothalamus (HYP), hippocampus (HC). Microiontophoretic application of the 5-HTIA agonist, 8-OH DPAT, into the DRN suppress cell firing rate via an action at somatodendritic 5-HTIA receptors (Sprouse & Aghajanian, 1987). Biochemical (Hjorth & Magnusson, 1988) and behavioural (Higgins et al, 1988) responses consistent with decreased central 5-HT function have been reported following intra-DRN injection of 8-OH DPAT. The present study reports the behavioural and biochemical effects of the putative 5-HTIA partial agonist gepirone (Hamon et al, 1987) and mCPP, which at the 5-HT 1 receptor shows some 5-HTIB/I C agonist selectivity (Hoyer, 1989) following DRN injection. Gepirone (0.4-10pg) caused marked falls in the 5-HIAA:5-HT ratio in FC, HC but not HYP, 20min after discrete injection, mCPP only reduced 5-HIAA:5-HT ratio at a high concentration (12.Spg) in FC and HYP, but not in HC. These results contrast sharply with 8-OH DPAT (0.5-2.5pg) and the 5-HT I receptor agonist, 5-carboxamidotryptamine (5-CT) (50-250ng), which decreased 5-HT turnover to a similar extent in all three regions. In the social-interaction and water-lick conflict tests, gepirone (0.2-1pg) produced responses characteristic of a reduced anxiety state, whilst higher doses (>ipg) produced behavioural hypoactivity and flattened body posture (FBP). mCPP (0.i 2.5pg) failed to significantly modify behaviour in the anxiety models, although a high dose (12.Spg) reduced locomotor activity and induced FBP. In conclusion, these studies suggest that stimulation of 5-HTIA receptors in the DRN cause marked changes in forebrain 5-HT metabolism which may underlie some of the behavioural changes mediated by 5-HTIA receptor selective compounds. The lack of effect with mCPP in this study may suggest that 5-HTIB/I C receptors are not critically involved in DRN cell firing. The failure of gepirone to significantly modify 5-HT turnover in the HYP may be a consequence of its partial 5-HTIA agonist profile. HAMON et al (1988) J. Pharm. Exp. Ther. 246 745-753; HIGGINS, G.A. et al (1988) Neuropharmacol. 993-1001; HJORTH, S. & MAGNUSSON, T. (1988) N-S Arch Pharmacol. 338 463-471; HOYER, D. (1988) 9p; SPROUSE, F.S. & AGHAJANIAN, G.K. (1987) Synapse 1 3-9
OQQ
27 96