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Oxotremorine attenuates retrograde amnesia induced by post-training administration of the GABAergic agonists muscimol and baclofen
BEHAVIORALAND NEURAL BIOLOGY56, 25--31 (1991)
Oxotremorine Attenuates Retrograde Amnesia Induced by Post-training Administrationof the GABAergicAgonists Muscimol and Baclofen CLAUDIO CASTELLANO
lstituto di Psicobiologia e Psicofarmacologia del CNR, Via Reno 1, 00198 Rome, Italy AND JAMES L . M C G A U G H 1
Center for the Neurobiology of Learning and Memory and Department of Psychobiology, University of California, Irvine, California 92717 These experiments examined the involvement of cholinergic influences in the effects of GABAergic drugs on 24-h retention of an inhibitory avoidance response by mice. A first set of experiments confirmed previous findings indicating that post-training injections (ip) of the GABAergic agonists muscimol (1.0 and 2.0 mg/kg) and baclofen (10.0 and 20.0 mg/kg) impaired retention, as well as previous findings indicating that injections of the cholinergic agonist oxotremorine (5.0 and 10.0 /~g/kg) enhanced retention. The findings of a second set of experiments indicated that the memory-impairing effects of muscimol and baclofen were attenuated by concurrent injections of a low, and otherwise ineffective, dose of oxotremorine (2.5/~g/kg). These findings are interpreted as suggesting that GABAergic drugs affect memory storage through influences on cholinergic systems. © 1991AcademicPress, Inc.
Experiments using a variety of learning tasks have shown that, in rats and mice, retention is enhanced by post-training injections of GABAergic antagonists, including picrotoxin and bicuculline, and impaired by the GABAergic agonists muscimol (GABA-A) and baclofen (GABA-B) (Castellano, Brioni, & McGaugh, 1990). Evidence indicating that the effects of post-training injections of GABAergic drugs are time-dependi Supported in part by USPHS Grant MH12526 from NIMH and NIDA and Office of Naval Research Contract N000014-90-J-1626. We thank Ines Introini-Collison and Nancy Collett for their assistance in the preparation of the manuscript. Address all correspondence and reprint requests to Dr. James L. McGaugh, Center for the Neurobiology of Learning and Memory, University of California, Irvine, CA 92717. 25 0163-1047/91 $3.00 Copyright © 199l by AcademicPress. Inc. All rightsof reproductionin any form reserved.
26
CASTELLANO AND MCGAUGH
ent, and are not state-dependent (Castellano & McGaugh, 1990), provides support for the view that GABAergic drugs affect retention through influences on brain systems involved in regulating memory storage (Castellano et al., 1990; McGaugh, 1989). The memory-enhancing effects of GABAergic antagonists are highly comparable to those produced by the opiate antagonists naloxone and naltrexone (McGaugh, 1989). In addition, naloxone and naltrexone potentiate the effects of GABAergic antagonists on memory (Castellano, Introini-Collison, Pavone, & McGaugh, 1989). Findings indicating that the retention-enhancing effects of opiate as well as GABAergic antagonists are blocked by the fl-adrenergic antagonist propranolol (Izquierdo & Graudenz, 1980; McGaugh, Introini-Collison, & Nagahara, 1988; unpublished findings) suggest that opiate and GABAergic influences on memory are mediated by activation of adrenergic systems. There is also evidence indicating that cholinergic drugs override the effects, on memory, of drugs that influence adrenergic receptors (Introini & Baratti, 1984; IntroiniCollison, & McGaugh, 1988). Such findings suggest that a cholinergic agonist should also attenuate the memory-impairing effects of GABAergic agonists. To investigate this implication, the present experiments examined the effects, on memory, of post-training injections of the cholinergic agonist oxotremorine administered together with amnestic doses of muscimol and baclofen. Male CD1 mice (River Laboratories, Como, Italy) weighing approximately 25 g were caged in groups of eight with food and water available ad libitum and maintained on a 12-h light-dark cycle (lights on at 07:00) at a constant temperature of 21°C for 2 weeks prior to the experiments. The mice were trained and tested in an inhibitory avoidance apparatus using procedures described previously (Castellano & McGaugh, 1990). The apparatus consisted of a 20 × 20 × 20-cm Lucite box with black walls and a grid floor. A platform (12 × 7.5 cm) extended from a small door (4 × 3 cm) in the front of the box. The box was placed at the edge of a table with the platform extending out from the table. The inside of the box was dark. A 40-W lamp was positioned 50 cm above the platform. On the training trial the mouse was placed on the platform facing away from the box. When the animal entered the box with all four feet the step-through latency was recorded, the entry was closed with a sliding door, and a footshock (0.7 mA, 1.0 s, 50 Hz) was delivered. The mouse was then immediately returned to its home cage. For the retention test 24 h later, the mouse was placed on the platform as in the training session and the step-through latency (maximum of 240 s) was recorded. Footshock was not administered on the retention test trial. In all experiments each group consisted of eight mice. To determine dose-response effects, mice in different groups were injected ip immediately after training with saline, oxotremorine (2.5, 5.0,
OXOTREMORINE ATFENUATES AMNESIA IN MICE
27
or 10.0 ~g/kg), muscimol (0.5, 1.0, or 2.0 mg/kg), or baclofen (5.0, 10.0, or 20.0 mg/kg). To examine the attenuating effect of oxotremorine on memory impairment induced by the GABAergic agonists, different groups received either saline or oxotremorine (2.5 /xg/kg) + muscimol (1.0 or 2.0 mg/kg) or baclofen (10.0 or 20.0 mg/kg). Controls received only saline or oxotremorine (2.5 ~g/kg). Oxotremorine sesquifumarate (Sigma), muscimol (Sigma), and baclofen (Ciba-Geigy) were dissolved in saline (0.9% NaCI). Control groups received saline. The drug and control solutions were injected ip in a volume of 10.0 ml/kg. The step-through retention latencies were evaluated by ANOVA (oneand two-way). Further analyses for individual between-group comparisons were carried out with post hoc tests (Duncan multiple range test). As is shown in Fig. 1A, oxotremorine produced dose-dependent enhancement of retention. An ANOVA (one-way) indicated that the groups differed significantly: F(3, 28) = 21.05, p < .001. Individual betweengroup comparisons indicated that the retention latencies of the groups given the two highest doses of oxotremorine were significantly longer (p < .01) than those of the controls. Muscimol (Fig. IB) induced dose-dependent impairment of retention performance. An ANOVA (one-way) indicated that the groups differed significantly: F(3, 28) = 28.57, p < .001. Individual between-group comparisons indicated that the latencies of the groups given the two highest doses of muscimol were significantly lower than those of the saline controls (2 < .01). The dose-dependent effects of baclofen (Fig. 1C) were highly comparable to those obtained with muscimol. An ANOVA (one-way) of the retention latencies indicated that the groups differed significantly: F(3, 28) = 23.28, p < .001. The retention latencies of the groups given the two highest doses of baclofen were significantly lower than those of the saline controls (2 < .01). As is shown in Fig. 2, the low dose of oxotremorine (2.5 t~g/kg), which did not affect retention when administered alone, attenuated the retentionimpairing effects of muscimol (1.0 and 2.0 mg/kg). An ANOVA (twoway) indicated that there were significant main effects for both oxotremorine and muscimol treatments (F(1, 42) = 17.79 and F(2, 42) = 36.67, respectively, p < .001) as well as a significant interaction (F(2, 42) = 7.21, p < .001). Individual between-treatment comparisons indicated that the retention latencies of the two groups given oxotremorine + muscimol were significantly higher than those of the groups given saline + muscimol (2 < .01 for both comparisons). The low dose of oxotremorine (2.5/~g/kg) also attenuated the retentionimpairing effects of baclofen (10.0 and 20.0 mg/kg) (see Fig. 3). An ANOVA (two-way) indicated that there were significant main effects for both oxotremorine and baclofen treatments (F(1, 42) = 14.84 and F(2,
28
CASTELLANO AND MCGAUGH
300 ' Z
_o 240 '
180
z<,
,¢¢ u.i =Z
120
60
0
SALINE
2.5
B 120 100 4
m_. co z
z3 ILl
10.0 .......
(,ug/kg)
o
u.I (/)
5.0
OXOTREMORINE
7-
80, 60'
I 40 20 .I
I
0'
SALINE
0.5
C
1.0
m 2.0
MUSCIMOL, (mg/kg)
120
Z
o
~00
-1uJ¢,~ rc'uj
-H I.-
40
z~ uJ
2o, 0
;ALINE
5.0
10.0
20.0
BACLOFEN (mg/kg)
FIG. 1. Effects of immediate post-training ip injections of oxotremorine (A), muscimol (B), and baclofen (C) on retention tested 24 h after a single training trial in an inhibitory avoidance task. ** p < .01 versus saline-injected controls. N = eight mice per group.
OXOTREMORINE ATTENUATES AMNESIA IN MICE
29
O CONTROL I
120 UJ
ii
lOO
Z g,I .-1
OXOTREMORINE (2.5 Ixg/kg) +
80
+4,
g
pZ
6O,
¢~
40
+&
2O
Z ~u
O,
CONTROL
1.0
2.0
MUSCIMOL(mg/kg) FI6. 2. Effects of immediate post-training injections of saline or a low dose of oxotremorine (2.5/zg/kg) administered together with memory-impairing doses of muscimol (1.0 and 2.0 mg/kg) on retention tested 24 h after a single training trial in an inhibitory avoidance task. ** p < .01 versus saline and oxotremorine (2.5 ~g/kg) groups. + and ++ p < .01 versus saline plus muscimol groups (1.0 and 2.0 mg/kg groups, respectively). N = eight mice per group.
CONTROL "G
[]
120. r.D [J.I
100.
Z M.I ..J
o
-I-
'!' ++
80,
F-
60,
I,,i~l
40.
(/)
20,
+&
OXOTREMORINE (2.5 ~g/kg)
-I-
Z
CONTROL
10,0 20.0 BACLOFEN(mg/kg)
FIG. 3. Effects of immediate post-training injections of saline or a low dose of oxotremorine (2.5 /zg/kg) administered together with memory-impairing doses of baclofen (10.0 and 20.0 mg/kg) on retention tested 24 h after a single training trial in an inhibitory avoidance task. ** p < .01 versus saline and oxotremorine (2.5 /zg/kg) groups. ÷ and ++ p < .01 versus saline plus muscimol groups (1.0 and 2.0 mg/kg groups, respectively). N = eight mice per group.
30
CASTELLANO AND MCGAUGH
42) = 24.32, respectively, p < .001) as well as a significant interaction (F(2, 42) = 4.61, p < .001). Individual between-treatment comparisons indicated that the retention latencies of the two groups given oxotremorine + baclofen were significantly higfier than those of the groups given saline + baclofen (p < .01 for both comparisons). The findings of these experiments agree with those of previous studies reporting that retention of inhibitory avoidance training is impaired by post-training administration of GABAergic agonists and, thus, provide additional evidence supporting the view that memory storage is regulated by GABAergic influences (Castellano et al., 1990). The finding that retention was enhanced by post-training injections of oxotremorine is consistent with previous evidence indicating that retention is enhanced by muscarinic cholinergic agonists (Flood, Smith, & Cherkin, 1983). The findings also clearly indicate that the amnestic effects of both muscimol and baclofen were attenuated by a low dose of oxotremorine (2.5 /xg/kg) which was otherwise ineffective in influencing memory: Oxotremorine significantly attenuated the memory-impairing effects of even the highest doses of muscimol (2.0 mg/kg) and baclofen (20.0 mg/kg). In the present experiment oxotremorine only partially blocked the memory-impairing effects of the highest doses of muscimol and baclofen. The lack of complete attenuation of the effects of muscimol and bac!ofen was no doubt due to the use of a low dose of oxotremorine which, as shown in Fig. 1, did not affect retention when administered alone. The evidence indicating that cholinergic drugs markedly antagonize the memory-modulating effects of drugs affecting several systems, including adrenergic and opiate systems (Introini & Baratti, 1984; Introini-Collison & McGaugh, 1988) as well as GABAergic systems, suggests that these systems may modulate memory through influences on cholinergic systems. There is evidence, for example, indicating that opiate and GABAergic influences in the medial septum regulate the release of hippocampal ACh (Botticelli & Wurtman, 1982; Costa, Panula, Thompson, & Cheney, 1983). Alternatively, regulation of memory storage by these systems may require concurrent activation of cholinergic mechanisms. In either case, cholinergic systems appear to be critically involved. However, in view of the evidence indicating that administration of cholinergic drugs can override the effects of drugs affecting adrenergic and opiate as well as GABAergic systems, cholinergic influences on memory storage do not appear to require concurrent activation of these other neuromodulatory systems. REFERENCES Botticelli, L. J., & Wurtman, R. J. (1982). Septohippocampal cholinergic neurons are regulated trans-synaptically by endorphin and corticotropin neuropeptides. Journal of Neuroscience, 2, 1316-1321. Castellano, C., Brioni, J. D. & McGaugh, J. L. (1990). GABAergic modulation of memory.
OXOTREMORINE ATTENUATES AMNESIA IN MICE
31
In L. Squire & E. Lindenlaub (Eds.), Biology of memory (pp. 361-378). Schattauer: Verlag. Castellano, C., Introini-Collison, I. B., Pavone, F., & McGaugh, J. L. (1989). Effects of naloxone and naltrexone on memory consolidation in CD1 mice: Involvement of GABAergic mechanisms. Pharmacology, Biochemistry and Behavior, 32, 563-567. Castellano, C., & McGaugh, J. L. (1990). Effects of post-training bicuculline and muscimol on retention: Lack of state dependency. Behavioral and Neural Biology, 54, 156-164. Costa, E., Panula, P., Thompson, H. K., & Cheney, D. L. (1983). The transsynaptic regulation of the septal-hippocampal cholinergic neurons. Life Sciences, 32, 165-179. Flood, J. F., Smith, G. E., & Cherkin, A. (1983). Memory retention: Potentiation of cholinergic drug combinations in mice. Neurobiology of Aging, 4, 37-43. Introini, I. B., & Baratti, C. M. (1984). The impairment of retention induced by/3 endorphin in mice may be mediated by reduction of central cholinergic activity. Behavioral and Neural Biology, 41, 152-163. Introini-Collison, I. B., & McGaugh, J. L. (1988). Modulation of memory by post-training epinephrine: Involvement of cholinergic mechanisms. Psychopharmacology, 94, 379385. Izquierdo, I., & Graudenz, M. (1980). Memory facilitation by naloxone is due to release of dopaminergic and fl-adrenergic systems from tonic inhibition. Psychopharmacology, 67, 265-268. McGaugh, J. L. (1989). Involvement of hormonal and neuromodulatory systems in the regulation of memory storage. Annual Review of Neuroscience, 12, 255-287. McGaugh, J. L., Introini-Collison, I. B., & Nagahara, A. H. (1988). Memory-enhancing effects of post-training naloxone: Involvement of fl-noradrenergic influences in the amygdaloid complex. Brain Research, 95, 132-134.
Oxotremorine Attenuates Retrograde Amnesia Induced by Post-training Administrationof the GABAergicAgonists Muscimol and Baclofen CLAUDIO CASTELLANO
lstituto di Psicobiologia e Psicofarmacologia del CNR, Via Reno 1, 00198 Rome, Italy AND JAMES L . M C G A U G H 1
Center for the Neurobiology of Learning and Memory and Department of Psychobiology, University of California, Irvine, California 92717 These experiments examined the involvement of cholinergic influences in the effects of GABAergic drugs on 24-h retention of an inhibitory avoidance response by mice. A first set of experiments confirmed previous findings indicating that post-training injections (ip) of the GABAergic agonists muscimol (1.0 and 2.0 mg/kg) and baclofen (10.0 and 20.0 mg/kg) impaired retention, as well as previous findings indicating that injections of the cholinergic agonist oxotremorine (5.0 and 10.0 /~g/kg) enhanced retention. The findings of a second set of experiments indicated that the memory-impairing effects of muscimol and baclofen were attenuated by concurrent injections of a low, and otherwise ineffective, dose of oxotremorine (2.5/~g/kg). These findings are interpreted as suggesting that GABAergic drugs affect memory storage through influences on cholinergic systems. © 1991AcademicPress, Inc.
Experiments using a variety of learning tasks have shown that, in rats and mice, retention is enhanced by post-training injections of GABAergic antagonists, including picrotoxin and bicuculline, and impaired by the GABAergic agonists muscimol (GABA-A) and baclofen (GABA-B) (Castellano, Brioni, & McGaugh, 1990). Evidence indicating that the effects of post-training injections of GABAergic drugs are time-dependi Supported in part by USPHS Grant MH12526 from NIMH and NIDA and Office of Naval Research Contract N000014-90-J-1626. We thank Ines Introini-Collison and Nancy Collett for their assistance in the preparation of the manuscript. Address all correspondence and reprint requests to Dr. James L. McGaugh, Center for the Neurobiology of Learning and Memory, University of California, Irvine, CA 92717. 25 0163-1047/91 $3.00 Copyright © 199l by AcademicPress. Inc. All rightsof reproductionin any form reserved.
26
CASTELLANO AND MCGAUGH
ent, and are not state-dependent (Castellano & McGaugh, 1990), provides support for the view that GABAergic drugs affect retention through influences on brain systems involved in regulating memory storage (Castellano et al., 1990; McGaugh, 1989). The memory-enhancing effects of GABAergic antagonists are highly comparable to those produced by the opiate antagonists naloxone and naltrexone (McGaugh, 1989). In addition, naloxone and naltrexone potentiate the effects of GABAergic antagonists on memory (Castellano, Introini-Collison, Pavone, & McGaugh, 1989). Findings indicating that the retention-enhancing effects of opiate as well as GABAergic antagonists are blocked by the fl-adrenergic antagonist propranolol (Izquierdo & Graudenz, 1980; McGaugh, Introini-Collison, & Nagahara, 1988; unpublished findings) suggest that opiate and GABAergic influences on memory are mediated by activation of adrenergic systems. There is also evidence indicating that cholinergic drugs override the effects, on memory, of drugs that influence adrenergic receptors (Introini & Baratti, 1984; IntroiniCollison, & McGaugh, 1988). Such findings suggest that a cholinergic agonist should also attenuate the memory-impairing effects of GABAergic agonists. To investigate this implication, the present experiments examined the effects, on memory, of post-training injections of the cholinergic agonist oxotremorine administered together with amnestic doses of muscimol and baclofen. Male CD1 mice (River Laboratories, Como, Italy) weighing approximately 25 g were caged in groups of eight with food and water available ad libitum and maintained on a 12-h light-dark cycle (lights on at 07:00) at a constant temperature of 21°C for 2 weeks prior to the experiments. The mice were trained and tested in an inhibitory avoidance apparatus using procedures described previously (Castellano & McGaugh, 1990). The apparatus consisted of a 20 × 20 × 20-cm Lucite box with black walls and a grid floor. A platform (12 × 7.5 cm) extended from a small door (4 × 3 cm) in the front of the box. The box was placed at the edge of a table with the platform extending out from the table. The inside of the box was dark. A 40-W lamp was positioned 50 cm above the platform. On the training trial the mouse was placed on the platform facing away from the box. When the animal entered the box with all four feet the step-through latency was recorded, the entry was closed with a sliding door, and a footshock (0.7 mA, 1.0 s, 50 Hz) was delivered. The mouse was then immediately returned to its home cage. For the retention test 24 h later, the mouse was placed on the platform as in the training session and the step-through latency (maximum of 240 s) was recorded. Footshock was not administered on the retention test trial. In all experiments each group consisted of eight mice. To determine dose-response effects, mice in different groups were injected ip immediately after training with saline, oxotremorine (2.5, 5.0,
OXOTREMORINE ATFENUATES AMNESIA IN MICE
27
or 10.0 ~g/kg), muscimol (0.5, 1.0, or 2.0 mg/kg), or baclofen (5.0, 10.0, or 20.0 mg/kg). To examine the attenuating effect of oxotremorine on memory impairment induced by the GABAergic agonists, different groups received either saline or oxotremorine (2.5 /xg/kg) + muscimol (1.0 or 2.0 mg/kg) or baclofen (10.0 or 20.0 mg/kg). Controls received only saline or oxotremorine (2.5 ~g/kg). Oxotremorine sesquifumarate (Sigma), muscimol (Sigma), and baclofen (Ciba-Geigy) were dissolved in saline (0.9% NaCI). Control groups received saline. The drug and control solutions were injected ip in a volume of 10.0 ml/kg. The step-through retention latencies were evaluated by ANOVA (oneand two-way). Further analyses for individual between-group comparisons were carried out with post hoc tests (Duncan multiple range test). As is shown in Fig. 1A, oxotremorine produced dose-dependent enhancement of retention. An ANOVA (one-way) indicated that the groups differed significantly: F(3, 28) = 21.05, p < .001. Individual betweengroup comparisons indicated that the retention latencies of the groups given the two highest doses of oxotremorine were significantly longer (p < .01) than those of the controls. Muscimol (Fig. IB) induced dose-dependent impairment of retention performance. An ANOVA (one-way) indicated that the groups differed significantly: F(3, 28) = 28.57, p < .001. Individual between-group comparisons indicated that the latencies of the groups given the two highest doses of muscimol were significantly lower than those of the saline controls (2 < .01). The dose-dependent effects of baclofen (Fig. 1C) were highly comparable to those obtained with muscimol. An ANOVA (one-way) of the retention latencies indicated that the groups differed significantly: F(3, 28) = 23.28, p < .001. The retention latencies of the groups given the two highest doses of baclofen were significantly lower than those of the saline controls (2 < .01). As is shown in Fig. 2, the low dose of oxotremorine (2.5 t~g/kg), which did not affect retention when administered alone, attenuated the retentionimpairing effects of muscimol (1.0 and 2.0 mg/kg). An ANOVA (twoway) indicated that there were significant main effects for both oxotremorine and muscimol treatments (F(1, 42) = 17.79 and F(2, 42) = 36.67, respectively, p < .001) as well as a significant interaction (F(2, 42) = 7.21, p < .001). Individual between-treatment comparisons indicated that the retention latencies of the two groups given oxotremorine + muscimol were significantly higher than those of the groups given saline + muscimol (2 < .01 for both comparisons). The low dose of oxotremorine (2.5/~g/kg) also attenuated the retentionimpairing effects of baclofen (10.0 and 20.0 mg/kg) (see Fig. 3). An ANOVA (two-way) indicated that there were significant main effects for both oxotremorine and baclofen treatments (F(1, 42) = 14.84 and F(2,
28
CASTELLANO AND MCGAUGH
300 ' Z
_o 240 '
180
z<,
,¢¢ u.i =Z
120
60
0
SALINE
2.5
B 120 100 4
m_. co z
z3 ILl
10.0 .......
(,ug/kg)
o
u.I (/)
5.0
OXOTREMORINE
7-
80, 60'
I 40 20 .I
I
0'
SALINE
0.5
C
1.0
m 2.0
MUSCIMOL, (mg/kg)
120
Z
o
~00
-1uJ¢,~ rc'uj
-H I.-
40
z~ uJ
2o, 0
;ALINE
5.0
10.0
20.0
BACLOFEN (mg/kg)
FIG. 1. Effects of immediate post-training ip injections of oxotremorine (A), muscimol (B), and baclofen (C) on retention tested 24 h after a single training trial in an inhibitory avoidance task. ** p < .01 versus saline-injected controls. N = eight mice per group.
OXOTREMORINE ATTENUATES AMNESIA IN MICE
29
O CONTROL I
120 UJ
ii
lOO
Z g,I .-1
OXOTREMORINE (2.5 Ixg/kg) +
80
+4,
g
pZ
6O,
¢~
40
+&
2O
Z ~u
O,
CONTROL
1.0
2.0
MUSCIMOL(mg/kg) FI6. 2. Effects of immediate post-training injections of saline or a low dose of oxotremorine (2.5/zg/kg) administered together with memory-impairing doses of muscimol (1.0 and 2.0 mg/kg) on retention tested 24 h after a single training trial in an inhibitory avoidance task. ** p < .01 versus saline and oxotremorine (2.5 ~g/kg) groups. + and ++ p < .01 versus saline plus muscimol groups (1.0 and 2.0 mg/kg groups, respectively). N = eight mice per group.
CONTROL "G
[]
120. r.D [J.I
100.
Z M.I ..J
o
-I-
'!' ++
80,
F-
60,
I,,i~l
40.
(/)
20,
+&
OXOTREMORINE (2.5 ~g/kg)
-I-
Z
CONTROL
10,0 20.0 BACLOFEN(mg/kg)
FIG. 3. Effects of immediate post-training injections of saline or a low dose of oxotremorine (2.5 /zg/kg) administered together with memory-impairing doses of baclofen (10.0 and 20.0 mg/kg) on retention tested 24 h after a single training trial in an inhibitory avoidance task. ** p < .01 versus saline and oxotremorine (2.5 /zg/kg) groups. ÷ and ++ p < .01 versus saline plus muscimol groups (1.0 and 2.0 mg/kg groups, respectively). N = eight mice per group.
30
CASTELLANO AND MCGAUGH
42) = 24.32, respectively, p < .001) as well as a significant interaction (F(2, 42) = 4.61, p < .001). Individual between-treatment comparisons indicated that the retention latencies of the two groups given oxotremorine + baclofen were significantly higfier than those of the groups given saline + baclofen (p < .01 for both comparisons). The findings of these experiments agree with those of previous studies reporting that retention of inhibitory avoidance training is impaired by post-training administration of GABAergic agonists and, thus, provide additional evidence supporting the view that memory storage is regulated by GABAergic influences (Castellano et al., 1990). The finding that retention was enhanced by post-training injections of oxotremorine is consistent with previous evidence indicating that retention is enhanced by muscarinic cholinergic agonists (Flood, Smith, & Cherkin, 1983). The findings also clearly indicate that the amnestic effects of both muscimol and baclofen were attenuated by a low dose of oxotremorine (2.5 /xg/kg) which was otherwise ineffective in influencing memory: Oxotremorine significantly attenuated the memory-impairing effects of even the highest doses of muscimol (2.0 mg/kg) and baclofen (20.0 mg/kg). In the present experiment oxotremorine only partially blocked the memory-impairing effects of the highest doses of muscimol and baclofen. The lack of complete attenuation of the effects of muscimol and bac!ofen was no doubt due to the use of a low dose of oxotremorine which, as shown in Fig. 1, did not affect retention when administered alone. The evidence indicating that cholinergic drugs markedly antagonize the memory-modulating effects of drugs affecting several systems, including adrenergic and opiate systems (Introini & Baratti, 1984; Introini-Collison & McGaugh, 1988) as well as GABAergic systems, suggests that these systems may modulate memory through influences on cholinergic systems. There is evidence, for example, indicating that opiate and GABAergic influences in the medial septum regulate the release of hippocampal ACh (Botticelli & Wurtman, 1982; Costa, Panula, Thompson, & Cheney, 1983). Alternatively, regulation of memory storage by these systems may require concurrent activation of cholinergic mechanisms. In either case, cholinergic systems appear to be critically involved. However, in view of the evidence indicating that administration of cholinergic drugs can override the effects of drugs affecting adrenergic and opiate as well as GABAergic systems, cholinergic influences on memory storage do not appear to require concurrent activation of these other neuromodulatory systems. REFERENCES Botticelli, L. J., & Wurtman, R. J. (1982). Septohippocampal cholinergic neurons are regulated trans-synaptically by endorphin and corticotropin neuropeptides. Journal of Neuroscience, 2, 1316-1321. Castellano, C., Brioni, J. D. & McGaugh, J. L. (1990). GABAergic modulation of memory.
OXOTREMORINE ATTENUATES AMNESIA IN MICE
31
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