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Shuttle-Box Avoidance Learning in Mice: Improvement by Glucose Combined with Stimulant Drugs
Neurobiology of Learning and Memory 73, 94–100 (2000) doi:10.1006/nlme.1999.3921, available online at http://www.idealibrary.com on
BRIEF REPORT Shuttle-Box Avoidance Learning in Mice: Improvement by Glucose Combined with Stimulant Drugs Mario Sansone, Mario Battaglia, and Flaminia Pavone Istituto di Psicobiologia e Psicofarmacologia, CNR, Rome, Italy
Glucose was tested alone or in combination with two stimulant drugs, amphetamine and nicotine, in mice of the CD-1 strain subjected to five daily shuttle-box training sessions. Pretraining intraperitoneal administration of glucose (50 or 100 mg/kg) had no effect, while amphetamine and nicotine, given alone, significantly improved avoidance acquisition at a dose of 0.5 mg/kg, but not 0.025 mg/kg. Significant improvement of avoidance learning was also produced by a combination of glucose with the lower dose of amphetamine or nicotine. This enhancing action, produced by a combination of glucose and stimulant drugs, at doses ineffective by themselves, might be due to a concomitant cholinergic and dopaminergic activation, induced by glucose and stimulant drugs, respectively. q 2000 Academic Press Key Words: glucose; amphetamine; nicotine; active avoidance; mice; learning improvement by drug combination.
Experimental evidence indicates that glucose facilitates learning and memory and attenuates cognitive deficits induced by age or by pharmacological agents (see Gold, 1986; Stone, Rudd, & Gold, 1992; Stone, Walser, Gold, & Gold, 1991). In the absence of cognitive impairment, most memory-enhancing effects are produced by posttraining administration of glucose (Kopf & Baratti, 1994, Kopf & Baratti, 1996; Gold, 1986; Gold, Vogt, & Hall, 1986; Messier, 1997), while only a few cases of performance improvement by pretraining glucose have been reported (Means & Fernandez, 1992; Ragozzino, Unick, & Gold, 1996). However, a facilitation of shuttle-box avoidance learning was produced in mice by pretraining administration of glucose combined with tacrine, a cognitive enhancer acting mainly through a central cholinergic activation due to acetylcholinesterase inhibition (Pavone, Capone, Battaglia, & Sansone, 1998). This result, in agreement with previous findings showing memory-improving effects of posttraining glucose combined Address correspondence and reprint requests to Flaminia Pavone, Istituto di Psicobiologia e Psicofarmacologia, CNR, Viale Marx 15-43, 00137 Roma, Italy. E-mail: [email protected]. 1074-7427/00 $35.00 Copyright q 2000 by Academic Press All rights of reproduction in any form reserved.
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with the acetylcholinesterase inhibitor physostigmine (Kopf & Baratti, 1994, 1996), supported the hypothesis that glucose interacts with cholinergic mechanisms (Kopf & Baratti, 1994, 1996; Stone, Cottrill, Walker, & Gold, 1988). The present study demonstrates that glucose may facilitate avoidance learning even if combined with stimulant drugs, such as amphetamine and nicotine, that are able to improve shuttle-box avoidance acquisition in mice, presumably acting through catecholaminergic mechanisms (Sansone, Castellano, Battaglia, & Ammassari-Teule, 1991; Vetulani, Battaglia, Castellano, & Sansone, 1993). Spontaneous locomotor activity was also tested in order to assess whether drug effects on avoidance performance may be related to general behavioral changes. The subjects were naive male mice, 9–10 weeks old, belonging to the randomly bred CD-1 strain (Charles River, Calco-Como, Italy). Upon their arrival in the laboratory (at least 1 week before the experiment) the mice were housed in standard transparent plastic cages (four per cage) under standard animal room conditions (free access to food and water, 12:12 h light:dark cycle, ambient temperature of 238C). The experiments were carried out between 9:00 AM and 4:00 PM, by using different animals for the different tests. Care and handling of the animals were in accordance with NIH ethical regulations. The experimental protocol was approved by the Italian Ministry of Health on November 27, 1995 (Decree No. 285/95-B). D-Glucose (Sigma), D-amphetamine sulfate (K & K Laboratories), and nicotine bitartrate (RBI), dissolved in distilled water, were injected intraperitoneally, in a volume of 10 ml/ kg. The pH of the nicotine solutions was adjusted to 7 with NaOH. Control injections consisted of the administration of saline solution (0.9% NaCl). The same apparatus was used to measure active avoidance and spontaneous locomotor activity, as previously described (Pavone et al., 1998; Sansone et al., 1991). The apparatus was computer-controlled and consisted of eight shuttle-boxes, each divided into two 20 3 10-cm compartments, connected by a 3 3 3-cm opening. In avoidance training, a light (10 W) was switched on alternately in the two ompartments and used as a conditioned stimulus (CS). The CS preceded the onset of the unconditioned stimulus (US) by 5 s and overlapped it for 25 s. The US was an electric shock (0.2 mA) applied continuously to the grid floor. The intertrial interval was 30 s. An avoidance response was recorded when the animal avoided the US by running into the dark compartment within 5 s after the onset of the CS. If animals failed to avoid the shock, they could escape it by crossing during the US. Mice were subjected to five daily 100-trial avoidance sessions, in two experiments in which glucose was tested alone or combined with amphetamine or nicotine. To measure spontaneous locomotor activity, the lamps of the shuttle-boxes were switched off and no electric shock was applied to the floor. For each mouse, the number of crossings from one compartment to the other was recorded for 30 min. In a first avoidance experiment nine groups of eight mice each received glucose (0, 50, or 100 mg/kg) 30 min before each session and amphetamine (0, 0.25, or 0.5 mg/kg) 15 min before testing. In a second experiment another nine groups of eight mice received glucose, as in the first experiment, 30 min before each session, and nicotine (0, 0.25, or 0.5 mg/kg, as base), instead of amphetamine, as second injection, 15 min before testing. Control injections (dose 0) consisted of the administration of saline solution. Doses and administration times were chosen on the basis of previous studies, showing the effects
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exerted by glucose (Pavone et al., 1998), amphetamine (Vetulani et al., 1993), and nicotine (Sansone et al., 1991) on shuttle-box avoidance learning in mice of the CD-1 strain. The results of the two experiments were analyzed separately, by combining the groups receiving the same dose of glucose alone. Figure 1 shows the mean percentage avoidance responses, in each daily session and in the five combined sessions, for the experimental groups receiving glucose and amphetamine. The results of the second experiment, involving glucose and nicotine, are reported in Fig. 2. Escape responses are not reported because escape failure seldom occurred. Control mice exhibited a rather poor avoidance performance, reaching a mean avoidance level of 20.37% at the end of the training (fifth session). A two-factor ANOVA, for avoidance responses exhibited by groups treated with glucose and amphetamine, yielded significant main effects of treatment [F (8, 87) 5 4.64, p , .001] and session [F(4, 348) 5 162.01, p , .001], and a significant treatment 3 session interaction [F(32, 348) 5 3.47, p , .001]. A post hoc analysis (Duncan’s test) for the five combined sessions indicated that glucose had no effect alone, while amphetamine significantly improved avoidance acquisition at a dose of 0.5 mg/kg, but not 0.25 mg/kg. However, mice receiving
FIG. 1. Effect of glucose and amphetamine on shuttle-box avoidance acquisition in mice. Mean percentage avoidance responses on the whole of the five 100-trial daily sessions (columns) and in each session (circles). Vertical lines indicate SEM. Mice received glucose (GLU; 0, 50, or 100 mg/kg/ip), 30 min before each daily session, and amphetamine sulfate (0, 0.25, or 0.5 mg/kg/ip), 15 min before testing. Groups receiving glucose alone consisted of 16 mice; all other groups included 8 subjects. *p , .05 vs glucose alone (dose 0 of amphetamine); 8p , .05 vs amphetamine alone (dose 0 of glucose).
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FIG. 2. Effect of glucose and nicotine on shuttle-box avoidance acquisition in mice. Treatments and explanations as in Fig. 1, except that nicotine bitartrate (doses expressed as base) was given instead of amphetamine. Groups receiving glucose alone are the same as reported in Fig. 1. *p , .05 vs glucose alone (dose 0 of nicotine); 8p , .05 vs nicotine alone (dose 0 of glucose).
0.25 mg/kg amphetamine combined with 100 mg/kg glucose performed better than mice treated with amphetamine or glucose alone, at corresponding doses. Conversely, no advantage in avoidance performance was obtained by combining glucose with 0.5 mg/kg amphetamine. A further analysis, by single sessions, revealed that a combination of glucose with the lower dose of amphetamine significantly improved avoidance performance starting from the second daily session. Also in the case of glucose and nicotine, a two-factor ANOVA showed significant main effects of treatment [F(8, 87) 5 5.11, p , .001] and session [F (4, 348) 5 162.17, p , .001] and a significant treatment 3 session interaction [F(32, 348) 5 2.77, p , .001]. The effects of nicotine, in combination with glucose, were similar to those of amphetamine. A post hoc analysis (Duncan’s test) for the five combined sessions indicated that glucose had no effect alone and that nicotine alone significantly improved avoidance acquisition at a dose of 0.5 mg/kg, but not 0.25 mg/kg. A combination of 0.25 mg/kg nicotine with both 50 and 100 mg/kg glucose improved avoidance performance, with the effect being statistically significant starting from the second or third session. It should be noted that intertrial responses (spontaneous crossings from the dark to the lighted compartment) were present at the beginning of the training, in the first two sessions,
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in 2–6% of the trials. Again, intertrial responses, which were punished by electric shock, gradually disappeared as training proceeded. The locomotor activity test was carried out using 15 groups, each including eight naive mice. The animals received glucose (0, 50, or 100 mg/kg) 30 min before testing and amphetamine (0, 0.25, or 0.5 mg/kg) or nicotine (0, 0.25, or 0.5 mg/kg/ip, as base) 15 min later. The results are reported in Fig. 3, which shows the mean activity crossings for each experimental group. Overall one-way ANOVAs showed no significant main effect of either glucose-amphetamine and glucose-nicotine treatments [F(8, 63) 5 1.75 and .79, respectively, ps . .05]. The present results are in agreement with previous findings showing that glucose, given alone, failed to improve shuttle-box avoidance learning (Pavone et al., 1998), while amphetamine (Vetulani et al., 1993) and nicotine (Sansone et al., 1991) exerted avoidance facilitating effects, at a dose of 0.5 mg/kg, but not 0.25 mg/kg, in mice of the CD-1 strain. However, the ineffective dose (0.25 mg/kg) of amphetamine or nicotine improved shuttlebox avoidance acquisition, when given in combination with glucose. Such avoidance facilitation did not seem to be related to any general psychomotor activation, because drug combinations that increased avoidance responses had no effect on spontaneous locomotor activity. Glucose did not enhance the avoidance facilitating action exerted by 0.5 mg/kg of the two stimulant drugs, but it should be noted that, at least in the case of
FIG. 3. Effect of glucose, amphetamine, and nicotine on spontaneous locomotor activity in mice. Columns represent the mean number of activity crossings during 30 min. Vertical lines indicate SEM. Mice received glucose (GLU; 0, 50, or 100 mg/kg/ip), 30 min before the activity test; amphetamine sulfate (AMPH; 0, 0.25, or 0.5 mg/kg) and nicotine bitartrate (NIC; 0, 0.25, or 0.5 mg/kg, as base) were injected ip 15 min before testing.
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nicotine, a higher dose of the drug (1 mg/kg) produced impairing rather than enhancing effects on shuttle-box avoidance acquisition in CD-1 mice (Sansone et al., 1991). Failure of pretraining administration of glucose alone to improve shuttle-box avoidance learning may appear to contradict previous findings, but most memory-enhancing effects in the absence of cognitive impairments were produced by posttraining administration of glucose (Kopf & Baratti, 1994, 1996; Gold, 1986; Gold et al., 1986; Messier, 1997). However, even if ineffective alone, glucose improved shuttle-box avoidance learning when given pretraining in combination with noneffective doses of the acetylcholinesterase inhibitor tacrine, in a previous study (Pavone et al., 1998), or of the stimulant drugs amphetamine and nicotine, in the present study. Enhancement by glucose of the behavioral effects of the central acting anticholinesterase tacrine supported the hypothesis that glucose acts through cholinergic mechanisms, even if an involvement of other neurotransmitter systems could not be excluded (Pavone et al., 1998). The cholinergic hypothesis postulates that under some conditions, such as the posttraining period following a learning experience, glucose administration may increase cholinergic tone (Kopf & Baratti, 1996; Ragozzino et al., 1996). Both muscarinic (Kopf & Baratti, 1994, 1996; Stone et al., 1991, 1992) and nicotinic (Kopf & Baratti, 1996; Ragozzino, Arankowsky-Sandoval, & Gold, 1994; Ragozzino & Gold, 1991) mechanisms may be involved in the activation of the central cholinergic function and seem to play an important role in the enhancing effects exerted by glucose in learning and memory tasks. An involvement of nicotinic mechanisms in the learning-enhancing effects of lucose was suggested by experimental findings showing that glucose was able to counteract the performance impairment induced by the nicotinic antagonist mecamylamine in spontaneous alternation and inhibitory avoidance tasks (Ragozzino et al., 1994; Ragozzino & Gold, 1991). Glucose was also able to reverse the spontaneous alternation impairment produced by combined muscarinic-nicotinic receptor blockade (Ragozzino et al., 1994). An activation of nicotinic receptors by glucose may have contributed to the avoidance facilitation induced in the present study by a combination of glucose with the ineffective dose of nicotine. However, it must be considered that brain dopaminergic mechanisms could be responsible for the avoidance facilitating effects of both amphetamine (Vetulani et al., 1993) and nicotine (Sansone et al., 1991). These mechanisms are strongly implicated in the acquisition and maintenance of avoidance behavior (Oei & King, 1980), as well as in the stimulatory action of amphetamine (Weiner, 1985), but are also involved in various behavioral and cognitive effects of nicotine (Levin, 1992). On the other hand, several neurochemical and behavioral experiments showed a functional interaction between cholinergic and dopaminergic systems (see Di Chiara, Morelli, & Consolo, 1994, and Levin & Simon, 1998, for reviews). Thus, in view of the role played by cholinergicdopaminergic interactions in learning and memory processes (Levin & Simon, 1998), it seems likely that a concomitant cholinergic and dopaminergic activation induced by glucose and stimulant drugs, respectively, may be responsible for the improvement of shuttle-box avoidance learning produced by combined glucose and amphetamine or nicotine. In conclusion, the present results indicate that the learning-enhancing action exerted by glucose through cholinergic activation may be favored, in active avoidance tasks, by dopaminergic stimulating agents possessing arousing properties. If so, the results provide
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further evidence of an interaction of glucose with the cholinergic system and extend the known pharmacological interactions of glucose to include the dopaminergic system. REFERENCES Di Chiara, G., Morelli, M., & Consolo, S. (1994). Modulatory functions of neurotransmitters in the striatum: ACh/dopamine/NMDA interactions. Trends in Neurosciences, 17, 228–233. Gold, P. E. (1986). Glucose modulation of memory storage processing. Behavioral and Neural Biology, 45, 342– 349. Gold, P. E., Vogt, J., & Hall, J. L. (1986). Glucose effects on memory: Behavioral and pharmacological characteristics. Behavioral and Neural Biology, 46, 145–155. Kopf, S. R., & Baratti, C. M. (1994). Memory-improving actions of glucose: Involvement of a central cholinergic muscarinic mechanism. Behavioral and Neural Biology, 62, 237–243. Kopf, S. R., & Baratti, C. M. (1996). Effects of posttraining administration of glucose on retention of a habituation response in mice: Participation of a central cholinergic mechanism. Neurobiology of Learning and Memory, 65, 253–260. Levin, E. D. (1992). Nicotinic systems and cognitive function. Psychopharmacology, 108, 417–431. Levin, E. D., & Simon, B. B. (1998). Nicotinic acetylcholine involvement in cognitive function in animals. Psychopharmacology, 138, 217–230. Means, L. W., & Fernandez, T. J. (1992). Daily glucose injections facilitate performance of a win-stay waterescape working memory task in mice. Behavioral Neuroscience, 106, 345–350. Messier, C. (1997). Object recognition in mice: Improvement of memory by glucose. Neurobiology of Learning and Memory, 67, 172–175. Oei, T. P., & King, M. G. (1980). Catecholamines and aversive learning: A review. Neuroscience and Biobehavioral Reviews, 4, 161–173. Pavone, F., Capone, F., Battaglia, M., & Sansone, M. (1998). Shuttle-box avoidance learning in mice: Improvement by combined glucose and tacrine. Neurobiology of Learning and Memory,69, 204–210. Ragozzino, M. E., Arankowsky-Sandoval, G., & Gold, P. E. (1994). Glucose attenuates the effect of combined muscarinic-nicotinic receptor blockade on spontaneous alternation. European Journal of Pharmacology, 256, 31–36. Ragozzino, M. E., & Gold, P. E. (1991). Glucose effects on mecamylamine-induced memory deficits and decreases in locomotor activity in mice. Behavioral and Neural Biology, 56, 271–282. Ragozzino, M. E., Unick, K. E., & Gold, P. E. (1996). Hippocampal acetylcholine release during memory testing in rats: Augmentation by glucose. Proceedings of the National Academy of Sciences USA, 93, 4693–4698. Sansone, M., Castellano, C., Battaglia, M., & Ammassari-Teule, M. (1991). Effects of oxiracetam-nicotine combinations on active and passive avoidance learning in mice. Pharmacology Biochemistry and Behavior, 39, 197–200. Stone, W. S., Cottrill, K. L., Walker, D. L., & Gold, P. E. (1988). Blood glucose and brain function: Interactions with CNS cholinergic systems. Behavioral and Neural Biology, 50, 325–334. Stone, W. S., Rudd, R. J., & Gold, P. E. (1992). Glucose attenuation of scopolamine- and age-induced deficits in spontaneous alternation behavior and regional brain [3H]2-deoxyglucose uptake in mice. Psychobiology, 20, 270–279. Stone, W. S., Walser, B., Gold, S. D., & Gold, P. E. (1991). Scopolamine and morphine-induced impairments of spontaneous alternation performance in mice: Reversal with glucose and with cholinergic and adrenergic agonists. Behavioral Neuroscience, 105, 264–271. Vetulani, J., Battaglia, M., Castellano, C., & Sansone, M. (1993). Facilitation of shuttle-box avoidance behaviour in mice treated with nifedipine in combination with amphetamine.Psychopharmacology, 113, 217–221. Weiner, N. (1985). Norepinephrine, epinephrine, and the sympathomimetic amines. In A. Goodman Gilman, L. S. Goodman, T. W. Rall, & F. Murad (Eds.), Goodman and Gilman’s the pharmacological basis of therapeutics (pp. 145–180). New York: Macmillan.
BRIEF REPORT Shuttle-Box Avoidance Learning in Mice: Improvement by Glucose Combined with Stimulant Drugs Mario Sansone, Mario Battaglia, and Flaminia Pavone Istituto di Psicobiologia e Psicofarmacologia, CNR, Rome, Italy
Glucose was tested alone or in combination with two stimulant drugs, amphetamine and nicotine, in mice of the CD-1 strain subjected to five daily shuttle-box training sessions. Pretraining intraperitoneal administration of glucose (50 or 100 mg/kg) had no effect, while amphetamine and nicotine, given alone, significantly improved avoidance acquisition at a dose of 0.5 mg/kg, but not 0.025 mg/kg. Significant improvement of avoidance learning was also produced by a combination of glucose with the lower dose of amphetamine or nicotine. This enhancing action, produced by a combination of glucose and stimulant drugs, at doses ineffective by themselves, might be due to a concomitant cholinergic and dopaminergic activation, induced by glucose and stimulant drugs, respectively. q 2000 Academic Press Key Words: glucose; amphetamine; nicotine; active avoidance; mice; learning improvement by drug combination.
Experimental evidence indicates that glucose facilitates learning and memory and attenuates cognitive deficits induced by age or by pharmacological agents (see Gold, 1986; Stone, Rudd, & Gold, 1992; Stone, Walser, Gold, & Gold, 1991). In the absence of cognitive impairment, most memory-enhancing effects are produced by posttraining administration of glucose (Kopf & Baratti, 1994, Kopf & Baratti, 1996; Gold, 1986; Gold, Vogt, & Hall, 1986; Messier, 1997), while only a few cases of performance improvement by pretraining glucose have been reported (Means & Fernandez, 1992; Ragozzino, Unick, & Gold, 1996). However, a facilitation of shuttle-box avoidance learning was produced in mice by pretraining administration of glucose combined with tacrine, a cognitive enhancer acting mainly through a central cholinergic activation due to acetylcholinesterase inhibition (Pavone, Capone, Battaglia, & Sansone, 1998). This result, in agreement with previous findings showing memory-improving effects of posttraining glucose combined Address correspondence and reprint requests to Flaminia Pavone, Istituto di Psicobiologia e Psicofarmacologia, CNR, Viale Marx 15-43, 00137 Roma, Italy. E-mail: [email protected]. 1074-7427/00 $35.00 Copyright q 2000 by Academic Press All rights of reproduction in any form reserved.
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with the acetylcholinesterase inhibitor physostigmine (Kopf & Baratti, 1994, 1996), supported the hypothesis that glucose interacts with cholinergic mechanisms (Kopf & Baratti, 1994, 1996; Stone, Cottrill, Walker, & Gold, 1988). The present study demonstrates that glucose may facilitate avoidance learning even if combined with stimulant drugs, such as amphetamine and nicotine, that are able to improve shuttle-box avoidance acquisition in mice, presumably acting through catecholaminergic mechanisms (Sansone, Castellano, Battaglia, & Ammassari-Teule, 1991; Vetulani, Battaglia, Castellano, & Sansone, 1993). Spontaneous locomotor activity was also tested in order to assess whether drug effects on avoidance performance may be related to general behavioral changes. The subjects were naive male mice, 9–10 weeks old, belonging to the randomly bred CD-1 strain (Charles River, Calco-Como, Italy). Upon their arrival in the laboratory (at least 1 week before the experiment) the mice were housed in standard transparent plastic cages (four per cage) under standard animal room conditions (free access to food and water, 12:12 h light:dark cycle, ambient temperature of 238C). The experiments were carried out between 9:00 AM and 4:00 PM, by using different animals for the different tests. Care and handling of the animals were in accordance with NIH ethical regulations. The experimental protocol was approved by the Italian Ministry of Health on November 27, 1995 (Decree No. 285/95-B). D-Glucose (Sigma), D-amphetamine sulfate (K & K Laboratories), and nicotine bitartrate (RBI), dissolved in distilled water, were injected intraperitoneally, in a volume of 10 ml/ kg. The pH of the nicotine solutions was adjusted to 7 with NaOH. Control injections consisted of the administration of saline solution (0.9% NaCl). The same apparatus was used to measure active avoidance and spontaneous locomotor activity, as previously described (Pavone et al., 1998; Sansone et al., 1991). The apparatus was computer-controlled and consisted of eight shuttle-boxes, each divided into two 20 3 10-cm compartments, connected by a 3 3 3-cm opening. In avoidance training, a light (10 W) was switched on alternately in the two ompartments and used as a conditioned stimulus (CS). The CS preceded the onset of the unconditioned stimulus (US) by 5 s and overlapped it for 25 s. The US was an electric shock (0.2 mA) applied continuously to the grid floor. The intertrial interval was 30 s. An avoidance response was recorded when the animal avoided the US by running into the dark compartment within 5 s after the onset of the CS. If animals failed to avoid the shock, they could escape it by crossing during the US. Mice were subjected to five daily 100-trial avoidance sessions, in two experiments in which glucose was tested alone or combined with amphetamine or nicotine. To measure spontaneous locomotor activity, the lamps of the shuttle-boxes were switched off and no electric shock was applied to the floor. For each mouse, the number of crossings from one compartment to the other was recorded for 30 min. In a first avoidance experiment nine groups of eight mice each received glucose (0, 50, or 100 mg/kg) 30 min before each session and amphetamine (0, 0.25, or 0.5 mg/kg) 15 min before testing. In a second experiment another nine groups of eight mice received glucose, as in the first experiment, 30 min before each session, and nicotine (0, 0.25, or 0.5 mg/kg, as base), instead of amphetamine, as second injection, 15 min before testing. Control injections (dose 0) consisted of the administration of saline solution. Doses and administration times were chosen on the basis of previous studies, showing the effects
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exerted by glucose (Pavone et al., 1998), amphetamine (Vetulani et al., 1993), and nicotine (Sansone et al., 1991) on shuttle-box avoidance learning in mice of the CD-1 strain. The results of the two experiments were analyzed separately, by combining the groups receiving the same dose of glucose alone. Figure 1 shows the mean percentage avoidance responses, in each daily session and in the five combined sessions, for the experimental groups receiving glucose and amphetamine. The results of the second experiment, involving glucose and nicotine, are reported in Fig. 2. Escape responses are not reported because escape failure seldom occurred. Control mice exhibited a rather poor avoidance performance, reaching a mean avoidance level of 20.37% at the end of the training (fifth session). A two-factor ANOVA, for avoidance responses exhibited by groups treated with glucose and amphetamine, yielded significant main effects of treatment [F (8, 87) 5 4.64, p , .001] and session [F(4, 348) 5 162.01, p , .001], and a significant treatment 3 session interaction [F(32, 348) 5 3.47, p , .001]. A post hoc analysis (Duncan’s test) for the five combined sessions indicated that glucose had no effect alone, while amphetamine significantly improved avoidance acquisition at a dose of 0.5 mg/kg, but not 0.25 mg/kg. However, mice receiving
FIG. 1. Effect of glucose and amphetamine on shuttle-box avoidance acquisition in mice. Mean percentage avoidance responses on the whole of the five 100-trial daily sessions (columns) and in each session (circles). Vertical lines indicate SEM. Mice received glucose (GLU; 0, 50, or 100 mg/kg/ip), 30 min before each daily session, and amphetamine sulfate (0, 0.25, or 0.5 mg/kg/ip), 15 min before testing. Groups receiving glucose alone consisted of 16 mice; all other groups included 8 subjects. *p , .05 vs glucose alone (dose 0 of amphetamine); 8p , .05 vs amphetamine alone (dose 0 of glucose).
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FIG. 2. Effect of glucose and nicotine on shuttle-box avoidance acquisition in mice. Treatments and explanations as in Fig. 1, except that nicotine bitartrate (doses expressed as base) was given instead of amphetamine. Groups receiving glucose alone are the same as reported in Fig. 1. *p , .05 vs glucose alone (dose 0 of nicotine); 8p , .05 vs nicotine alone (dose 0 of glucose).
0.25 mg/kg amphetamine combined with 100 mg/kg glucose performed better than mice treated with amphetamine or glucose alone, at corresponding doses. Conversely, no advantage in avoidance performance was obtained by combining glucose with 0.5 mg/kg amphetamine. A further analysis, by single sessions, revealed that a combination of glucose with the lower dose of amphetamine significantly improved avoidance performance starting from the second daily session. Also in the case of glucose and nicotine, a two-factor ANOVA showed significant main effects of treatment [F(8, 87) 5 5.11, p , .001] and session [F (4, 348) 5 162.17, p , .001] and a significant treatment 3 session interaction [F(32, 348) 5 2.77, p , .001]. The effects of nicotine, in combination with glucose, were similar to those of amphetamine. A post hoc analysis (Duncan’s test) for the five combined sessions indicated that glucose had no effect alone and that nicotine alone significantly improved avoidance acquisition at a dose of 0.5 mg/kg, but not 0.25 mg/kg. A combination of 0.25 mg/kg nicotine with both 50 and 100 mg/kg glucose improved avoidance performance, with the effect being statistically significant starting from the second or third session. It should be noted that intertrial responses (spontaneous crossings from the dark to the lighted compartment) were present at the beginning of the training, in the first two sessions,
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in 2–6% of the trials. Again, intertrial responses, which were punished by electric shock, gradually disappeared as training proceeded. The locomotor activity test was carried out using 15 groups, each including eight naive mice. The animals received glucose (0, 50, or 100 mg/kg) 30 min before testing and amphetamine (0, 0.25, or 0.5 mg/kg) or nicotine (0, 0.25, or 0.5 mg/kg/ip, as base) 15 min later. The results are reported in Fig. 3, which shows the mean activity crossings for each experimental group. Overall one-way ANOVAs showed no significant main effect of either glucose-amphetamine and glucose-nicotine treatments [F(8, 63) 5 1.75 and .79, respectively, ps . .05]. The present results are in agreement with previous findings showing that glucose, given alone, failed to improve shuttle-box avoidance learning (Pavone et al., 1998), while amphetamine (Vetulani et al., 1993) and nicotine (Sansone et al., 1991) exerted avoidance facilitating effects, at a dose of 0.5 mg/kg, but not 0.25 mg/kg, in mice of the CD-1 strain. However, the ineffective dose (0.25 mg/kg) of amphetamine or nicotine improved shuttlebox avoidance acquisition, when given in combination with glucose. Such avoidance facilitation did not seem to be related to any general psychomotor activation, because drug combinations that increased avoidance responses had no effect on spontaneous locomotor activity. Glucose did not enhance the avoidance facilitating action exerted by 0.5 mg/kg of the two stimulant drugs, but it should be noted that, at least in the case of
FIG. 3. Effect of glucose, amphetamine, and nicotine on spontaneous locomotor activity in mice. Columns represent the mean number of activity crossings during 30 min. Vertical lines indicate SEM. Mice received glucose (GLU; 0, 50, or 100 mg/kg/ip), 30 min before the activity test; amphetamine sulfate (AMPH; 0, 0.25, or 0.5 mg/kg) and nicotine bitartrate (NIC; 0, 0.25, or 0.5 mg/kg, as base) were injected ip 15 min before testing.
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nicotine, a higher dose of the drug (1 mg/kg) produced impairing rather than enhancing effects on shuttle-box avoidance acquisition in CD-1 mice (Sansone et al., 1991). Failure of pretraining administration of glucose alone to improve shuttle-box avoidance learning may appear to contradict previous findings, but most memory-enhancing effects in the absence of cognitive impairments were produced by posttraining administration of glucose (Kopf & Baratti, 1994, 1996; Gold, 1986; Gold et al., 1986; Messier, 1997). However, even if ineffective alone, glucose improved shuttle-box avoidance learning when given pretraining in combination with noneffective doses of the acetylcholinesterase inhibitor tacrine, in a previous study (Pavone et al., 1998), or of the stimulant drugs amphetamine and nicotine, in the present study. Enhancement by glucose of the behavioral effects of the central acting anticholinesterase tacrine supported the hypothesis that glucose acts through cholinergic mechanisms, even if an involvement of other neurotransmitter systems could not be excluded (Pavone et al., 1998). The cholinergic hypothesis postulates that under some conditions, such as the posttraining period following a learning experience, glucose administration may increase cholinergic tone (Kopf & Baratti, 1996; Ragozzino et al., 1996). Both muscarinic (Kopf & Baratti, 1994, 1996; Stone et al., 1991, 1992) and nicotinic (Kopf & Baratti, 1996; Ragozzino, Arankowsky-Sandoval, & Gold, 1994; Ragozzino & Gold, 1991) mechanisms may be involved in the activation of the central cholinergic function and seem to play an important role in the enhancing effects exerted by glucose in learning and memory tasks. An involvement of nicotinic mechanisms in the learning-enhancing effects of lucose was suggested by experimental findings showing that glucose was able to counteract the performance impairment induced by the nicotinic antagonist mecamylamine in spontaneous alternation and inhibitory avoidance tasks (Ragozzino et al., 1994; Ragozzino & Gold, 1991). Glucose was also able to reverse the spontaneous alternation impairment produced by combined muscarinic-nicotinic receptor blockade (Ragozzino et al., 1994). An activation of nicotinic receptors by glucose may have contributed to the avoidance facilitation induced in the present study by a combination of glucose with the ineffective dose of nicotine. However, it must be considered that brain dopaminergic mechanisms could be responsible for the avoidance facilitating effects of both amphetamine (Vetulani et al., 1993) and nicotine (Sansone et al., 1991). These mechanisms are strongly implicated in the acquisition and maintenance of avoidance behavior (Oei & King, 1980), as well as in the stimulatory action of amphetamine (Weiner, 1985), but are also involved in various behavioral and cognitive effects of nicotine (Levin, 1992). On the other hand, several neurochemical and behavioral experiments showed a functional interaction between cholinergic and dopaminergic systems (see Di Chiara, Morelli, & Consolo, 1994, and Levin & Simon, 1998, for reviews). Thus, in view of the role played by cholinergicdopaminergic interactions in learning and memory processes (Levin & Simon, 1998), it seems likely that a concomitant cholinergic and dopaminergic activation induced by glucose and stimulant drugs, respectively, may be responsible for the improvement of shuttle-box avoidance learning produced by combined glucose and amphetamine or nicotine. In conclusion, the present results indicate that the learning-enhancing action exerted by glucose through cholinergic activation may be favored, in active avoidance tasks, by dopaminergic stimulating agents possessing arousing properties. If so, the results provide
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