- Email: [email protected]
scopolamine treatment on spatial navigation
Bra#z Research, 585 (1992) 322-326 © 1992 Elsevier Science Publishers B.V. All rights reserved 0006-8993/92/$05.00
322
BRES 25227
Effects of combined methysergide and mecamylamine/scopolamine treatment on spatial navigation M. Riekkinen, P. Riekkinen, J. Sirvi6 and P. R i e k k i n e n Jr. Unit'ersity of Kuopio, Department of Neurology, Kuopio (Finland) (Accepted 10 March 1992)
Key words: Acetylcholine; Serotonin; Nicotinic and muscarinic receptors: Serotonin2 receptor; Interaction; Spatial learning
in the present study, we investigated the effects of a 5-HT 2 receptor antagonist, methysergide (2.5, 7.5 and 20 mg/kg), on spatial learning in saline, mecamylamine (10 mg/kg) and scopolamine (0.8 mg/kg) treated rats. Methysergide had no effect on water-maze (WM) spatial learning in rats subjected to saline or mecamylamine pretreatments. However, scopolamine-induced WM learning deficit was augmented by nte,'hysergide at doses of 7.5 and 20 mg/kg. These results further suggest (A) that cholinergic and serotonergic systems may interact in the regulation of spatial learning, and (B) that the cholinergic component of this interaction with serotonin 2 receptors is mediated by muscarinic receptors, but not by nicotinic receptors.
The basal forebrain cholinergic projection neurons have been shown to regulate performance in tasks used to assess learning and memory functions 14'Is. For example, lesions of the septohippocampal cholinergic projections ts, and administration of muscarinic or nice. tinic antagonists impair place navigation strategies in a water-maze (WM)swimming pool paradigm I~, Anatomical, electrophysiological and behavioral studies suggest that cholinergic and serotonergic systerns functionally interact in the brain '~'tt.t6.t~. Anatomi. cal studies have shown that serotonergic and cholinergic neurons may interact either directly in the basal forebrain or in the hippocampal and cortical target areas ~9. Electrophysiological data have shown that proper functioning of the cholinergic and serotonergic systems is a prerequisite for normal cortical and hippocampai electrical activity t:. The mesencephalic reticular formation, including the serotonergic raphe nuclei, is important in brainstem modulation of the septohippocampal cholinergic system. Electrical stimulation of the midbrain areas induced theta activity and increased release of acetyicholine in the hippocampus, an effect which is mediated via the septum 3,'*. Furthermore, combined cholinergic and serotonergic blockade may suppress all hippocampal theta activity. Recent electro-
physiological studies have shown also that concurrent modulation of thalamic and cortical cholinergic and serotonergic systems abolishes all cortical desynchronized EEG waveformst2 Behavioral studies have shown that a combined cholinergic and serotonergic deficit produces a severe impairment in learning performance '~'ll'14''. For example, generalized serotonin depletion induced by PCPA treatment augments WM performance defect induced by muscarinic (scopolamine) or nicotinic (mecamylamine) antagonists tt't¢~. Serotonin, like acetylcholine, interacts with a heterogeneous population of receptors 2. However, the role of the different serotonin receptor subtypes in the behavioral interactions observed has not been extensively studied. Therefore, the present study investigates the effects of methysergide, a serotonin subtype 2 receptor antagonist, on scopolamine- and mecamylamine-induced WM performance defect. Male Kuo:Wistar rats (n = 180) were used in this study. Scopolamine (0.8 mg/kg), mecamylamine (10 mg/kg), methysergide (2.5, 7.5 and 20 mg/kg) and methylscopolamine (1.0 mg/kg) were diluted in saline and injected i.p. (2 ml/kg) 30 rain before daily behavioral testing. Hexamethonium was diluted in saline (0.75 ml/kg) and injected s.c. 30 rnin before daily
Corre,~xmdence: P, Riekkinen Jr,, Department o[ Neurolo~, University of Kuopio, PO Box 1627, SF-70211, Kuopio. Fax: (358) (71) 173-019.
323 behavioral testing. Drug treatments are explained in figure legends. The computerized WM training system has been 2000
A
described in detail previously. Testing consisted of 5 consecutive days of testing: 3 x 70-s trials/day, 5 s on the platform, 30 s intertrial period. Group 5 was trained 2000
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Fig. 1. Water-maze (WM) escape distance values. Y-axis, escape distance values in arbitrary computer units. X.axis, training days I-V. A: Effects of methysergide (2.5, 7.5 and 20 mg/kg) on spatial navigation. Note the lack of an effect of methysergide on WM navigation performance. Abbreviations: C, control; M2.5, methysergide 2.5 mg/kg; M7.5, methysergide 7.5 mg/kg; M20, methysergide 20 mg/kg. B: effects of methysergide (2.5, 7.5 and 20 mg/kg) and mecamylamine (10 mg/kg) on spatial navigation. Note the lack of any interaction between mecamylamine and methysergide on WM performance. Abbreviations: C, control; ME, mecamylamine. C: effects of methysergide (7.5 mg/kg) and scopolamine (0.8 mg/kg) on spatial navigation. Note the interaction between the scopolamine and methysergide (7.5 mg/kg) treatments on WM performance. Abbreviations: S, scopolamine.
324 using a slightly modified training paradigm: 4 days: 5 x 50-s trials/day, 5 s on the platform, 30 s intertrial interval. The ANOVA test was used to analyse group differences in the total distance swum (escape distance) between different groups. Analysis of the escape distance values of methysergide-treated rats revealed no significant overall group effect (F3.ts5 = 1.2, P > 0.05) (Fig. 1A). Analysis of the escape distance values measured after mecamylamine and methysergide injections revealed a significant group effect (F4,t~4 = 8.3, P < 0.05; data not shown). Rats injected with mecamylamine alone or in combination with methysergide (2.5, 7.5 and 20 mg/kg) were impaired (Ft.77 > 5.0, P < 0.05, for all data) (Fig. I B). Analysis of the escape distance values measured after scopolamine and methysergide injections revealed again a significant group effect (F4.t,>4- 11.3, P < 0.05). All the groups injected with scopolamine alone or in combination with methysergide (2.5, 7.5 and 20 mg/kg) were impaired (Ft.7? > 4.5, P < 0.05, for all data). Interestingly, rats injected with methysergide at 7.5 and 20 mg/kg (total escape distance; scopolamine alone: 7076 vs, scopolamine + methysergide 7,5 and 20:8967 and 8947 pixels, respectively) in combination with scopolamine were more impaired than rats injected with scopolamine alone (Fl,?7 > 4,0, P < 0,05; for all data), A significant group effect was observed in the analysis of escape distance data measured in rats after scopolamine a,d methysergide injections (F~,tss- 8,2, P < 0,05) (Fig, IC), The following groups wer~ impaired: scopolamine and scopolamine + methysergide 7,5 mg/kg (F~m > 5,0, P < 0,05; in both comparisons), Again, methysergide treatment aggravated the scopolamine-induced WM performance defect (Ft, m --3.9, P < 0.05), Analysis of the escape distance data measured after injections of muscarinic and nicotinic antagonists which do not gain entry to the central nervous system and methysergide revealed no significant group effect (F.u.~,l ~ 0,7, P > 0.05) (Fig, 2). Consistent with previous data scopolamine- and mecamylamine.treated rats were impaired in WM acquisition ~3. Interestingly, methysergide, a serotonin.., receptor subtype antagonist, alone had no significant effect on WM spatial learning performance. The present result is consistent with earlier data indicating that decreased activity of central serotonin systems (induced by e.g. serotonin synthesis inhibitors or serotonin neurotoxins) does not markedly affect spatial navigation I~a'~a¢~. However, methysergide dose-depen-
2000
1800
!000
600
0
II /
C
!!! ~
M V.8
IV
V
i--'-1 MS+M 7.8
Fig. 2. Water-maze escape distance values of control experiments. 1"-axis, escape distancevalues in arbitrary computer units. X-axis,
training days I-V. Note the lack of an effect of peripherally acting nicotinic and muscarinicreceptorantagonistdrugson spatial navigation of methysergide-pretreatedand naive rats, H, hexamethonium; MS, methylscopolamine:other abbreviationsas in Fig, I.
dently impaired WM performance in scopolamine-pre. treated rats: groups with combined treatments (scopolamine + methysergide at %5 or 20 mg/kg) had longer escape distance values than rats treated with scopolamine alone. However, mecamylamine-induced WM impairment was not aggravated by methysergide. Furthermore, methysergide- and methyiscopolamine. treated rats were not impaired in acquisition of WM tasks, Therefore, it is reasonable to postulate that blockade of central serotonin 2 and cholinergic muscarinic receptors were responsible for the WM learning defects seen in scopolamine + methysergide treated rats, Previous studies have shown that cholinergic and serotonergic systems may jointly modulate WM spatial learning ~a~a~,Nilsson et el. demonstrated that generalized serotonin depletion induced by intraventricular 5,7-dihydroxytryptamine greatly aggravated the WM deficit occurring after lesioning the medial septal nucleus by radiofrequency. Importantly, grafting of fetal serotonergic mesencephalic raphe tissue and cholinergic septal tissue into the hippocampus did not improve WM performance in rats subjected to combined sere-
325
tonergic and cholinergic lesions ~. However, compared to controls 9, the combined-lesioned rats with mixed serotonergic and cholinergic grafts were not impaired. Therefore, Niisson et al. suggested that normal activity and synaptic connectivity of hippocampal cholinergic and serotonergic systems are necessary for effective WM performance 9. Other studies have also provided support for a cholinergic serotonergic interaction in the mediation of WM performance. Firstly, RichterLevin and Segal showed that 5,7-DHT-lesioned rats were supersensitive to muscarinic cholinergic blockade by atropine, and that this supersensitivity could be restored by intrahippocampal serotonergic grafts I°. Secondly, Riekkinen Jr. et al. demonstrated that 5,7DHT lesions decreased the WM performance improving effect of pilocarpine, a muscarinic agonist, in scopolamine-pretreated rats ~'. Thirdly, mecamylamine produced a greater WM acquisition defect in PCPA-lestoned than in control rats ts. Taken together, the present and previous results support the idea that central cholinergic and serotonergic systems functionally interact in the regulation of spatial navigation as assessed by WM. The qualitative difference in the effects of methysergide on WM performance defect seen after muscarinic and nicotinic receptor blockade may be related to modulation of acetylcholine release. Firstly, blockade of serotonin, receptors may disinhibit the activity of cholinergic nerve terminals and increase the release of acetylcholine (e,g. in the hippocampus) H'~. Secondly, scopolamine may facilitate acetylcholine release by acting on Dresynaptic autoreceptors of the M, subtype c''~'~H and produce amnesia by acting on postsynaptic M~ receptors s. Thirdly, the learning defect induced by nicotinic receptor antagonists may be related to decreased release of acetylcholine, and subsequent hypostimulation of postsynaptic cholinergic receptors ~. Therefore, it could be proposed that the increase in acetylcholine release induced by methysergide is not sufficient to displace scopolamine from postsynaptic muscarinic (possibly M~) receptors. In addition, methysergide may augment the effects of scopolamine in WM by blocking selotonin 2 receptors located on noncholinergic neurons. However, it is logical that methysergide would not aggravate WM defect seen after mecamylamine which may be caused by decreased release of acetylcholine from cholinergic fibers. In further studies the effects of methysergide, scopolamine and mecamylamine on acetylcholine release in those brain areas implicated in WM performance (such as the hippocampus) should be investigated. Furthermore, the site(s) of the muscarinic-serotonin2 interaction should be identified.
Ewen MacDonald Ph.D. is gratefully acknowledged for language checking of the manuscript. 1 Beani, L., Bianchi, C., Ferraro, L., Nilsson, L., Nordberg, A., Romanelli, L., Spalluto, P., Sundwall, A. and Tanganeili, E., Effect of nicotine on the release of acetylcholine and amino acids in the brain. In A. Nordberg, K. Fuxe, B. Holmstedt and A. Sundwali (Eds.), Nicotine Receptors in the CNS: Their Role in Synaptic Transmission, Elsevier, Amsterdam, 1989, pp. 149-156. 2 Bradley, B.P., Engel, G., Feaiuk, W., Fuzard, J.R., Humphrey, P., Middlemiss, D.N., Mylecharane, E.J., Richardson, B.P. and Saxena, P.R., Proposals for the classification and nomenclature of functional receptors for 5-hydroxytryptamine, Neuropharmacology, 25 (1986) 563-576. 3 Dudar, J.D., The effect of septal nuclei stimulation on the release of acetylcholine from the rabbit hippocampus, Brah~ Res., 83 (1975) 123-133. 4 Dudar, J.D., The role of the septal nuclei in the release of acetylcholine from the rabbit cerebral cortex and dorsal hippocampus and the effect of atropine, Bra#l Res., 129 (1977) 237-246. 5 Hagan, J.J., Jansen, J,H.M. and Broekkamp, C.L.E., Blockade of spatial learning by the muscarinic antagonist pirenzepine, Psychopharmacology, 13 (I 987) 470-476. 6 Hoss, W., Messer, Jr., W.S., Monsma, Jr., F.J., Miller, M.D., Ellerbrock, B.R., Scranton, T., Ghodsi-Hovsepian, S., Price, M.A., Balan, S., Mazloum, Z. and Bohnett, M., Biochemical and behavioral evidence for muscarinic autoreceptors in the CNS, Brain Res., 517 (1990) 195-201. 7 Meyer, E.M. and Otero, D.H., Pharmacological and ionic characterizations of the muscarinic receptors modulating [3H]acetylcholine release from rat cortical synaptosomes, J. Neurosci., 5 (1985) 1202-1207. 8 Muramatsu, M., Tamaki-Ohashi, J., Usuki, C., Araki, H. and Aihara, H., Serotonin-2 receptor-mediated regulation of release of acetylcholine by minaprine in cholinergic nerve terminals of hippocampus of rat, Neurophannacology, 27 (1988) 603-6(19. 9 Nilsson, O.G., Brundin, P. and Bjiirklund, A., Amelioration of spatial memory impairment by intrahippocampal grafts of mixed septal and raphe tissu~ il~ rats with combined cholinergic and serotonergic denervation of the forebrain, Brain Res., 515 (19t)0) 193-20(~, 10 Richter-Levin, G. and S~gal, M., Raphe cells grafted into the hippocampus can ameliorate spatial memory deficits in rats with combined serotonergic/cholincrgic deficiencies, Brain Res, 478 (1989) 184-18fl. II Riekkinen, Jr., P., Riekkinen, M,, Riukkinen, P. and Sirviii, J., Effects of combined mecamylamine and PCPA treatment on water-maze and passive avoidance learning, Brain Res., 575 (19t)2) 247-25O. 12 Riekkinen, Jr., P., Sirvi/.i, J., Miettinen, R. and Riekkinen, P., Interaction between raphe dorsalis and nucleus basalis magnocellularis in the regulation of high-voltage spindle activity in rat neocortex, Brain Res., 526 (1990) 31-36. 13 Riekkinen, Jr., P., Sirvi~i, J. and Riekkinen, P., Effects of concurrent manipulations of muscarinie and nicotinic receptors on spatial and passive avoidance learning, Phannacol. Biochem. Behav., 37 (1990) 405-410. 14 Riekkinen, Jr., P., Sirvi~i, J. and Riekkinen, P., Interaction between raphe dorsalis and nucleus basalis magnocellularis in spatial learning, Brain Res., 527 (1990) 342-345. 15 Riekkinen, Jr., P., Sirvi/5, J. and Riekkinen, P., Similar memory impairments found in medial septal-vertical diagonal band of Broca and nucleus basalis lesioned rats: are memory defects induced by nucleus basalis lesions related to the degree of nonspecific subcortical cell loss? Behav. Brahl Res., 37 (1990) 81-88. 16 Riekkinen, Jr., P., Sirvi/b, J., Valjakka, A., Miettinen, R. and Riekkinen, P., Pharmacological consequences of cholinergic plus serotonergic manipulations, Brain Res., 552 (1991) 23-26. 17 Robinson, S.E., Effect of specific serotonergic lesions on cholinergic neurons in the hippocampus, cortex and striatum, Life Sci., 32 (1983) 345-353.
326 18 Schoffelmeer, A.N.M., Van Vliet, B.J., Wardeh, G, and Mulder, A.H., Muscarinic receptor mediated modulation of [-~H]dopa, mine and [mC]acetylcholine release from rat neostriatal slices: selective antagonism by gallamine but not pirenzepine, Eur. ]. PharmacoL, 128 (1986) 291-294. 19 Steinbusch, H.W.M,, serotonin-immunoreactive neurons and their
projections in the CNS. In A. Bj6rklund, T. H6kfelt and M.J. Kuhar (Eds.), Handbook of Chemical Neuroanatomy, Vol. 3, Elsevier, Amsterdam, 1984, pp. 68-125. 20 Vanderwolf, C.H., Cerebral activity and behavior: control by central cholinergic and serotonergic systems, lnt. Rer. Neurobiol., 30 (1988) 225-340.
322
BRES 25227
Effects of combined methysergide and mecamylamine/scopolamine treatment on spatial navigation M. Riekkinen, P. Riekkinen, J. Sirvi6 and P. R i e k k i n e n Jr. Unit'ersity of Kuopio, Department of Neurology, Kuopio (Finland) (Accepted 10 March 1992)
Key words: Acetylcholine; Serotonin; Nicotinic and muscarinic receptors: Serotonin2 receptor; Interaction; Spatial learning
in the present study, we investigated the effects of a 5-HT 2 receptor antagonist, methysergide (2.5, 7.5 and 20 mg/kg), on spatial learning in saline, mecamylamine (10 mg/kg) and scopolamine (0.8 mg/kg) treated rats. Methysergide had no effect on water-maze (WM) spatial learning in rats subjected to saline or mecamylamine pretreatments. However, scopolamine-induced WM learning deficit was augmented by nte,'hysergide at doses of 7.5 and 20 mg/kg. These results further suggest (A) that cholinergic and serotonergic systems may interact in the regulation of spatial learning, and (B) that the cholinergic component of this interaction with serotonin 2 receptors is mediated by muscarinic receptors, but not by nicotinic receptors.
The basal forebrain cholinergic projection neurons have been shown to regulate performance in tasks used to assess learning and memory functions 14'Is. For example, lesions of the septohippocampal cholinergic projections ts, and administration of muscarinic or nice. tinic antagonists impair place navigation strategies in a water-maze (WM)swimming pool paradigm I~, Anatomical, electrophysiological and behavioral studies suggest that cholinergic and serotonergic systerns functionally interact in the brain '~'tt.t6.t~. Anatomi. cal studies have shown that serotonergic and cholinergic neurons may interact either directly in the basal forebrain or in the hippocampal and cortical target areas ~9. Electrophysiological data have shown that proper functioning of the cholinergic and serotonergic systems is a prerequisite for normal cortical and hippocampai electrical activity t:. The mesencephalic reticular formation, including the serotonergic raphe nuclei, is important in brainstem modulation of the septohippocampal cholinergic system. Electrical stimulation of the midbrain areas induced theta activity and increased release of acetyicholine in the hippocampus, an effect which is mediated via the septum 3,'*. Furthermore, combined cholinergic and serotonergic blockade may suppress all hippocampal theta activity. Recent electro-
physiological studies have shown also that concurrent modulation of thalamic and cortical cholinergic and serotonergic systems abolishes all cortical desynchronized EEG waveformst2 Behavioral studies have shown that a combined cholinergic and serotonergic deficit produces a severe impairment in learning performance '~'ll'14''. For example, generalized serotonin depletion induced by PCPA treatment augments WM performance defect induced by muscarinic (scopolamine) or nicotinic (mecamylamine) antagonists tt't¢~. Serotonin, like acetylcholine, interacts with a heterogeneous population of receptors 2. However, the role of the different serotonin receptor subtypes in the behavioral interactions observed has not been extensively studied. Therefore, the present study investigates the effects of methysergide, a serotonin subtype 2 receptor antagonist, on scopolamine- and mecamylamine-induced WM performance defect. Male Kuo:Wistar rats (n = 180) were used in this study. Scopolamine (0.8 mg/kg), mecamylamine (10 mg/kg), methysergide (2.5, 7.5 and 20 mg/kg) and methylscopolamine (1.0 mg/kg) were diluted in saline and injected i.p. (2 ml/kg) 30 rain before daily behavioral testing. Hexamethonium was diluted in saline (0.75 ml/kg) and injected s.c. 30 rnin before daily
Corre,~xmdence: P, Riekkinen Jr,, Department o[ Neurolo~, University of Kuopio, PO Box 1627, SF-70211, Kuopio. Fax: (358) (71) 173-019.
323 behavioral testing. Drug treatments are explained in figure legends. The computerized WM training system has been 2000
A
described in detail previously. Testing consisted of 5 consecutive days of testing: 3 x 70-s trials/day, 5 s on the platform, 30 s intertrial period. Group 5 was trained 2000
B
lsoo
lsoo m[
,oooOO,
,0O0o 2 5
50 0
'
0
~
!
[-'~-,[ ME+M . •
~
I
!
Ic
II
~M7.5
llI
~S
ME+M
2O
IV
~S÷MT.6
Fig. 1. Water-maze (WM) escape distance values. Y-axis, escape distance values in arbitrary computer units. X.axis, training days I-V. A: Effects of methysergide (2.5, 7.5 and 20 mg/kg) on spatial navigation. Note the lack of an effect of methysergide on WM navigation performance. Abbreviations: C, control; M2.5, methysergide 2.5 mg/kg; M7.5, methysergide 7.5 mg/kg; M20, methysergide 20 mg/kg. B: effects of methysergide (2.5, 7.5 and 20 mg/kg) and mecamylamine (10 mg/kg) on spatial navigation. Note the lack of any interaction between mecamylamine and methysergide on WM performance. Abbreviations: C, control; ME, mecamylamine. C: effects of methysergide (7.5 mg/kg) and scopolamine (0.8 mg/kg) on spatial navigation. Note the interaction between the scopolamine and methysergide (7.5 mg/kg) treatments on WM performance. Abbreviations: S, scopolamine.
324 using a slightly modified training paradigm: 4 days: 5 x 50-s trials/day, 5 s on the platform, 30 s intertrial interval. The ANOVA test was used to analyse group differences in the total distance swum (escape distance) between different groups. Analysis of the escape distance values of methysergide-treated rats revealed no significant overall group effect (F3.ts5 = 1.2, P > 0.05) (Fig. 1A). Analysis of the escape distance values measured after mecamylamine and methysergide injections revealed a significant group effect (F4,t~4 = 8.3, P < 0.05; data not shown). Rats injected with mecamylamine alone or in combination with methysergide (2.5, 7.5 and 20 mg/kg) were impaired (Ft.77 > 5.0, P < 0.05, for all data) (Fig. I B). Analysis of the escape distance values measured after scopolamine and methysergide injections revealed again a significant group effect (F4.t,>4- 11.3, P < 0.05). All the groups injected with scopolamine alone or in combination with methysergide (2.5, 7.5 and 20 mg/kg) were impaired (Ft.7? > 4.5, P < 0.05, for all data). Interestingly, rats injected with methysergide at 7.5 and 20 mg/kg (total escape distance; scopolamine alone: 7076 vs, scopolamine + methysergide 7,5 and 20:8967 and 8947 pixels, respectively) in combination with scopolamine were more impaired than rats injected with scopolamine alone (Fl,?7 > 4,0, P < 0,05; for all data), A significant group effect was observed in the analysis of escape distance data measured in rats after scopolamine a,d methysergide injections (F~,tss- 8,2, P < 0,05) (Fig, IC), The following groups wer~ impaired: scopolamine and scopolamine + methysergide 7,5 mg/kg (F~m > 5,0, P < 0,05; in both comparisons), Again, methysergide treatment aggravated the scopolamine-induced WM performance defect (Ft, m --3.9, P < 0.05), Analysis of the escape distance data measured after injections of muscarinic and nicotinic antagonists which do not gain entry to the central nervous system and methysergide revealed no significant group effect (F.u.~,l ~ 0,7, P > 0.05) (Fig, 2). Consistent with previous data scopolamine- and mecamylamine.treated rats were impaired in WM acquisition ~3. Interestingly, methysergide, a serotonin.., receptor subtype antagonist, alone had no significant effect on WM spatial learning performance. The present result is consistent with earlier data indicating that decreased activity of central serotonin systems (induced by e.g. serotonin synthesis inhibitors or serotonin neurotoxins) does not markedly affect spatial navigation I~a'~a¢~. However, methysergide dose-depen-
2000
1800
!000
600
0
II /
C
!!! ~
M V.8
IV
V
i--'-1 MS+M 7.8
Fig. 2. Water-maze escape distance values of control experiments. 1"-axis, escape distancevalues in arbitrary computer units. X-axis,
training days I-V. Note the lack of an effect of peripherally acting nicotinic and muscarinicreceptorantagonistdrugson spatial navigation of methysergide-pretreatedand naive rats, H, hexamethonium; MS, methylscopolamine:other abbreviationsas in Fig, I.
dently impaired WM performance in scopolamine-pre. treated rats: groups with combined treatments (scopolamine + methysergide at %5 or 20 mg/kg) had longer escape distance values than rats treated with scopolamine alone. However, mecamylamine-induced WM impairment was not aggravated by methysergide. Furthermore, methysergide- and methyiscopolamine. treated rats were not impaired in acquisition of WM tasks, Therefore, it is reasonable to postulate that blockade of central serotonin 2 and cholinergic muscarinic receptors were responsible for the WM learning defects seen in scopolamine + methysergide treated rats, Previous studies have shown that cholinergic and serotonergic systems may jointly modulate WM spatial learning ~a~a~,Nilsson et el. demonstrated that generalized serotonin depletion induced by intraventricular 5,7-dihydroxytryptamine greatly aggravated the WM deficit occurring after lesioning the medial septal nucleus by radiofrequency. Importantly, grafting of fetal serotonergic mesencephalic raphe tissue and cholinergic septal tissue into the hippocampus did not improve WM performance in rats subjected to combined sere-
325
tonergic and cholinergic lesions ~. However, compared to controls 9, the combined-lesioned rats with mixed serotonergic and cholinergic grafts were not impaired. Therefore, Niisson et al. suggested that normal activity and synaptic connectivity of hippocampal cholinergic and serotonergic systems are necessary for effective WM performance 9. Other studies have also provided support for a cholinergic serotonergic interaction in the mediation of WM performance. Firstly, RichterLevin and Segal showed that 5,7-DHT-lesioned rats were supersensitive to muscarinic cholinergic blockade by atropine, and that this supersensitivity could be restored by intrahippocampal serotonergic grafts I°. Secondly, Riekkinen Jr. et al. demonstrated that 5,7DHT lesions decreased the WM performance improving effect of pilocarpine, a muscarinic agonist, in scopolamine-pretreated rats ~'. Thirdly, mecamylamine produced a greater WM acquisition defect in PCPA-lestoned than in control rats ts. Taken together, the present and previous results support the idea that central cholinergic and serotonergic systems functionally interact in the regulation of spatial navigation as assessed by WM. The qualitative difference in the effects of methysergide on WM performance defect seen after muscarinic and nicotinic receptor blockade may be related to modulation of acetylcholine release. Firstly, blockade of serotonin, receptors may disinhibit the activity of cholinergic nerve terminals and increase the release of acetylcholine (e,g. in the hippocampus) H'~. Secondly, scopolamine may facilitate acetylcholine release by acting on Dresynaptic autoreceptors of the M, subtype c''~'~H and produce amnesia by acting on postsynaptic M~ receptors s. Thirdly, the learning defect induced by nicotinic receptor antagonists may be related to decreased release of acetylcholine, and subsequent hypostimulation of postsynaptic cholinergic receptors ~. Therefore, it could be proposed that the increase in acetylcholine release induced by methysergide is not sufficient to displace scopolamine from postsynaptic muscarinic (possibly M~) receptors. In addition, methysergide may augment the effects of scopolamine in WM by blocking selotonin 2 receptors located on noncholinergic neurons. However, it is logical that methysergide would not aggravate WM defect seen after mecamylamine which may be caused by decreased release of acetylcholine from cholinergic fibers. In further studies the effects of methysergide, scopolamine and mecamylamine on acetylcholine release in those brain areas implicated in WM performance (such as the hippocampus) should be investigated. Furthermore, the site(s) of the muscarinic-serotonin2 interaction should be identified.
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