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Stria Terminalis Lesions Attenuate Memory Enhancement Produced by Intracaudate Nucleus Injections of Oxotremorine
NEUROBIOLOGY OF LEARNING AND MEMORY ARTICLE NO.
65, 278–282 (1996)
0033
BRIEF REPORT Stria Terminalis Lesions Attenuate Memory Enhancement Produced by Intracaudate Nucleus Injections of Oxotremorine MARK G. PACKARD,* INES INTROINI-COLLISON,†
AND
JAMES L. MCGAUGH†,‡,1
*Department of Psychology, University of New Orleans, †Center for the Neurobiology of Learning and Memory, and ‡Department of Psychobiology, University of California, Irvine
ing systemic drug and hormone treatments to modulate memory. In rats trained in a one-trial inhibitory avoidance task, lesions of the ST attenuate the memory enhancement induced by posttraining systemic injections of epinephrine (Liang & McGaugh, 1983a), the opioid antagonist naloxone (McGaugh et al., 1986), the cholinergic agonist oxotremorine (Introini-Collison, Arai, & McGaugh, 1989), and the noradrenergic agonist clenbuterol (Introini-Collison, Miyazaki, & McGaugh, 1991). Lesions of the ST also attenuate the memory-impairing effects of posttraining systemic injections of the opioid agonist bendorphin (McGaugh, Introini-Collison, Juler, & Izquierdo, 1986), and the cholinergic antagonist atropine (Introini-Collison, Arai, & McGaugh, 1989). ST lesions also attenuate the memory enhancement produced by intraamygdala administration of norepinephrine (Liang, McGaugh, & Yao, 1990), as well as the memory impairment produced by posttraining electrical stimulation of the amygdala (Liang, & McGaugh, 1983b). Taken together, these findings suggest that the amygdala and stria terminalis are components of a modulatory system mediating the memory-enhancing and memory-impairing effects of posttraining drug treatments. The use of peripheral posttraining hormone and drugs to investigate the role of the ST in memory does not allow for any conclusions concerning the site of action of these treatments. Anatomical evidence suggests that the amygdala may modulate memory storage via ST innervation of other brain regions. ST fibers innervate several brain regions including cerebral cortex (Krettek & Price, 1977; McDonald, 1991), thalamus (Krettek & Price, 1977), hypothalamus (Krettek & Price, 1978), medial cau-
The present study examined the role of the stria terminalis in modulating the memory enhancement produced by posttraining intracaudate nucleus injection of oxotremorine. Male Sprague–Dawley rats with either sham operations or bilateral lesions of the stria terminalis (ST) were trained on a one-trial inhibitory-avoidance task and received a unilateral posttraining intracaudate injection of either a buffer vehicle or the cholinergic agonist oxotremorine (0.3 mg/0.5 ml) into a medial region of the caudate nucleus innervated by the ST. Intracaudate injection of oxotremorine improved memory in sham-operated rats. Although ST lesions did not affect retention in rats given intracaudate injections of the buffer vehicle, ST lesions attenuated the memory enhancement produced by posttraining intracaudate injection of oxotremorine. In view of anatomical evidence indicating that amygdalostriatal projections are nonreciprocol, the present findings suggest that amygdala output via the ST is essential for memory enhancement produced by intracaudate injection of oxotremorine. q 1996 Academic Press, Inc.
Extensive evidence suggests that the amygdaloid complex is involved in the modulation of memory storage processes (for reviews see Packard, Cahill, Williams, & McGaugh, 1995; McGaugh, IntroiniCollison, Kim, & Liang, 1992). In particular, an intact stria terminalis (ST), a major afferent–efferent pathway of the amygdala, is essential for posttrain1 Research was supported by U.S. Public Health Service National Research Service Award 1 F32 NS08973-01 to M.G.P., and U.S. Public Health Service Grant MH12526 from the National Institute of Mental Health and National Institute on Drug Abuse to J.L.M. Address correspondence and reprint requests to Mark G. Packard, Department of Psychology, University of New Orleans, New Orleans, LA, 70148. Fax: 504-286-6049.
278 1074-7427/96 $18.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
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date nucleus (Kita & Kitai, 1990), and brainstem structures (Krettek & Price, 1978). The present study focused on the ST innervation of the medial caudate nucleus and examined the role of this pathway in the memory enhancement produced by posttraining intracaudate injection of the cholinergic agonist oxotremorine. Two lines of behavioral evidence implicate the caudate nucleus as a candidate structure for amygdala modulation of cholinergic effects on memory. First, lesions of the caudate nucleus impair acquisition of one-trial inhibitory avoidance behavior (e.g., Winocur, 1974), the same task in which lesions of the ST attenuate the effects of systemic posttraining cholinergic treatments on memory (Introini-Collison et al., 1989). Second, posttraining intracaudate injection of the muscarinic cholinergic receptor antagonist atropine impairs memory on onetrial inhibitory avoidance tasks (e.g., Prado-Alcala, Signoret, & Figuera, 1981). Thus, we investigated whether amygdala output via the ST is required for inducing the memory-enhancing effects of intracaudate injections of the cholinergic muscarinic receptor agonist oxotremorine. Rats with either sham operations or bilateral lesions of the ST were trained on an inhibitory avoidance task and received an immediate posttraining intracaudate injection of either saline or the cholinergic agonist oxotremorine into the medial caudate nucleus, a region innervated by the basolateral amygdala via the stria terminalis (Kita & Kitai, 1990). Subjects were 48 male Sprague–Dawley rats (225–250 g) individually housed in a temperaturecontrolled environment on a 12-h light/dark cycle with the lights on from 7:00 AM to 7:00 PM. Prior to surgery rats were anesthetized with 50 mg/kg sodium pentobarbital. Half of the animals received bilateral ST lesions, and half received sham lesions. Bilateral electrolytic lesions of the ST were made by passing 3 mA of current for 5 s through an electrode insulated except for 0.5 mm at the tip. Brain coordinates for the ST lesions were AP Å 01.3 mm from bregma, ML Å {2.2 mm, DV Å 05.0 mm. For sham lesions the electrode was lowered but no current was passed. All of the animals were implanted unilaterally (left side) with a guide cannula (23 gauge) in the medial caudate nucleus. The cannula were anchored to the skull with jewelers screws and dental cement, and a sylet was placed in the guide cannula to insure cannula patency. Unilateral cannula placements were used based on previous evidence indicating that the indirect catecholamine agonist D-amphetamine enhances memory following unilateral injection into the caudate nucleus (Packard & White, 1991; Packard, Cahill, & McGaugh, 1994). Co-
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ordinates for the guide cannula placements were AP Å 00.3 mm from bregma, ML Å /2.4 mm, DV Å 03.2 mm. The rats were trained on a trough-shaped inhibitory-avoidance apparatus (McGaugh et al., 1986) consisting of two compartments separated by a sliding door that opened by retracting into the floor. The starting compartment was illuminated by a tensor lamp, which provided the only light in the testing room. On the training trial, each rat was placed in the lighted compartment, facing the closed door. When the rat turned around, the door leading to the dark compartment was opened, and the response latency to enter the dark compartment was measured. When the rat stepped through the door into the dark compartment the door was closed, and a foot shock (0.35 mA, 60 Hz, 0.7 s duration) was delivered. Immediately following the training trial, the rat was removed from the dark compartment and given an intracaudate injection of either vehicle or oxotremorine. The four experimental groups (n Å 12 per group) were: sham lesion-buffer injection, sham lesion-oxotremorine injection, ST lesion-buffer injection, and ST lesion-oxotremorine injection. Oxotremorine (Sigma Co.) was dissolved in a buffered vehicle solution. Injections (0.5 ml) were administered intracerebrally via guide cannulae using 30-gauge injection needles connected by polyethylene tubing to 10ml Hamilton microsyringes. Vehicle and oxotremorine (0.3 mg) injections were delivered over 37 s with a syringe pump (Sage Instruments), and the injection needles (extending 1 mm from the end of the guide cannula) were left in place for an additional 60 s to allow for diffusion of the solution away from the needle tip. The dose of oxotremorine used was chosen based on evidence from our laboratory indicating that intraamygdala injection of this dose enhances memory in the inhibitory-avoidance task (Introini-Collison, Dalmaz, & McGaugh, 1996). After the injection, the animals were returned to their home cages. On the retention test 48 h later, the animals were placed in the lighted compartment as on the training session, and the step through latency (maximum of 600 s) was recorded. For analysis of the lesion and cannula placements, the animals were anesthetized with a 1-cc injection of 30% chloral hydrate solution and perfused with physiological saline followed by a 10% Formalin solution. The brains were removed and fixed in 10% Formalin solution prior to slicing. Frozen sections were cut at 20 mm and stained with cresyl violet. Lesions and cannula placements were verified using the atlas of Paxinos and Watson (1986). Results are shown in Fig. 1. The bilateral stria terminalis lesions
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FIG. 1. (Left) Minimum (hatched) and maximum (striped) extent of stria terminalis lesions. The bilateral lesions ranged AP Å 01.3 to 01.8 mm from bregma. Lesion: top, 1.3; middle, 1.4 mm; bottom, 1.8 mm. (Right) Location of intracaudate nucleus needle tips, located in the medial caudate ranging AP Å 00.26 to 00.4 mm from bregma. Needle tips: top, 00.26 mm; middle, 00.3 mm; bottom, 00.4 mm.
ranged from AP 01.3 to 01.8 mm from bregma (Fig. 1, left side). Occasional partial damage to the lateral tips of the fimbria fornix was observed; however, such damage did not correlate with the behavioral results. Two rats with extensive fornix damage were excluded from the behavioral analyses (one rat from the ST lesion-buffer group and one rat from the ST lesion-oxotremorine group). Cannula tips were located in the medial caudate nucleus, ranging from AP 00.26 to 00.4 mm from bregma (Fig. 1, right side).
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There were no significant differences among groups in the training day entrance latencies (i.e., prior to posttraining injections), the mean group latencies ranged from 8 to 11 s. The mean entrance latencies for all groups on the 48-h retention test are shown in Fig. 2. Some animals were assigned maximal latencies of 600 s on the retention test and therefore nonparametric Mann–Whitney U tests were used to analyze the retention test data. Posttraining intracaudate injections of oxotremorine enhanced retention. The retention test latencies of rats
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FIG. 2. Effect of stria terminalis lesions on memory enhancement produced by intracaudate nucleus injection of the muscarinic cholinergic agonist oxotremorine. STL, stria terminalis lesion; OTM, oxotremorine (0.3 mg/0.5 ml).
with sham lesions given intracaudate injections of oxotremorine were significantly higher than those of rats with sham lesions given intracaudate injections of buffer (U Å 17.5, p õ .05). Lesions of the ST attenuated the memory-enhancing effect of intracaudate injection of oxotremorine. The retention test latencies of rats with sham lesions given intracaudate injections of oxotremorine were significantly higher than those of rats with ST lesions given intracaudate injections of oxotremorine (U Å 32.5, p õ .05). Oxotremorine did not enhance retention in rats with ST lesions. The retention test escape latencies of rats with ST lesions given intracaudate injections of buffer did not differ significantly from those of rats with ST lesions given intracaudate injections of oxotremorine (U Å 54.0, p Å .28). Finally, lesions of the ST alone did not effect retention. The retention test latencies of rats with sham lesions given intracaudate injections of buffer did not differ from those of rats with ST lesions given intracaudate injections of buffer (U Å 60.5, p Å .73). The results suggest that an intact ST is required for the memory enhancement produced by posttraining intracaudate nucleus injection of oxotremorine. Consistent with previous findings (for review, see McGaugh et al., 1992), ST lesions alone did not impair retention in otherwise untreated rats, indicating that the integrity of this pathway is not essential for acquisition of inhibitory-avoidance behavior. The medial region of the caudate nucleus targeted by the cannula placements was chosen because this area receives projections from the amygdala via the stria terminalis (Kita & Kitai, 1990). It should be noted that the medial caudate is proximal to the lateral ventricles, and thus it is possible that the behavioral effects of oxotremorine were due to
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spread to other brain regions. Evidence indicating a important role for cholinergic mechanisms in the caudate nucleus in memory is consistent with the hypothesis that oxotremorine influenced memory via a caudate-mediated mechanism (e.g., Prado-Alcala et al., 1981). Nonetheless, the possibility that oxotremorine influenced memory via spread to other brain regions cannot be ruled out at present. The ST contains both afferent and efferent fibers, and thus the effects of lesions to this pathway on posttraining memory treatments may be due to loss of either ST-mediated amygdala input or output. Importantly, amygdalostriatal projections of the rat brain have been reported to be nonreciprocal (Kita & Kitai, 1990). Thus, the modulatory role of the amygdala in memory may be mediated by direct ST output to the medial caudate nucleus. On this hypothesis, the synergy of amygdala input to the medial caudate nucleus and the activation of cholinergic muscarinic receptors in the caudate would mediate the memoryenhancing effect of oxotremorine. Alternatively, the modulatory effect of amygdala efferents on the caudate nucleus may be mediated through output pathways which include other brain areas. For example, the rat basolateral amygdala innervates interconnected regions of both the prefrontal cortex and medial caudate nucleus (McDonald, 1991). On this hypothesis, amygdala input to the neocortex may modulate cortical projections to the medial caudate nucleus. Further research is necessary to elucidate the circuitry by which memory processes are modulated in brain regions receiving amygdala efferent projections. REFERENCES Introini-Collison, I. B., Arai, Y., & McGaugh, J. L. (1989). Stria terminalis lesions attenuate the effects of posttraining oxotremorine and atropine on retention. Psychobiology, 17, 397– 401. Introini-Collison, I. B., Miyazaki, B., & McGaugh, J. L. (1991). Involvement of the amygdala on the memory enhancing effects of clenbuterol. Psychopharmacology, 104, 541–544. Introini-Collison, I. B., Dalmaz, C., & McGaugh, J. L. (1996). Amygdala B-adrenergic influences on memory storage involve cholinergic activation, Neurobiology of Learning and Memory, 65, 57–64. Kita, H., & Kitai, S. T. (1990). Amygdaloid projections to the frontal cortex and striatum in the rat. Journal of Comparative Neurology, 298, 40–49. Krettek, J. E., & Price, J. L. (1977). Projections from the amygdaloid complex to the cerebral cortex and thalamus in the rat and cat. Journal of Comparative Neurology, 172, 687–722. Krettek, J. E., & Price, J. L. (1978). Amygdaloid projections to subcortical structures within the basal forebrain and brainstem in the rat and cat. Journal of Comparative Neurology, 178, 225–254.
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Liang, K. C., & McGaugh, J. L. (1983a). Lesions of the stria terminalis attenuate the enhancing effect of posttraining epinephrine on retention of an inhibitory avoidance response. Behavioral Brain Research, 9, 49–58. Liang, K. C., & McGaugh, J. L. (1983b). Lesions of the stria terminalis attenuate the amnestic effect of amygdaloid stimulation on avoidance responses. Brain Research, 274, 309–318. Liang, K. C., McGaugh, J. L., & Yao, H. (1990). Involvement of amygdala pathways in the influence of posttraining amygdala norepinephrine and peripheral epinephrine on memory storage. Brain Research, 508, 225–233. McDonald, A. J. (1991). Organization of amygdaloid projections to the prefrontal cortex and associated striatum in the rat. Neuroscience, 4, 1–14. McGaugh, J. L., Introini-Collison, I. B., Juler, R. G., & Izquierdo, I. (1986). Stria terminalis lesions attenuate the effects of post-training naloxone and beta-endorphin on retention. Behavioral Neuroscience, 100, 839–844. McGaugh, J. L., Introini-Collison, I., Kim, M., & Liang, K. C. (1992). Involvement of the amygdala in neuromodulatory influences on memory storage. In J. Aggleton (Ed.), The amygdala (pp. 431–451). New York: Wiley–Liss.
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Packard, M. G., & White, N. M. (1991). Dissociation of hippocampus and caudate nucleus memory systems by post-training intracerebral injection of dopamine agonists. Behavioral Neuroscience, 105, 73–84. Packard, M. G., Cahill, L., & McGaugh, J. L. (1994). Amygdala modulation of hippocampal-dependent and caudate nucleus dependent memory processes. Proceedings of the National Academy of Sciences, 91, 8477–8481. Packard, M. G., Cahill, L., Williams, C. L., McGaugh, J. L. (1995). The anatomy of a memory modulatory system: From periphery to brain. In N. E. Spear, L. P. Spear, & M. L. Woodruff (Eds.), Neurobehavioral plasticity: Learning, development, and response to brain insults (pp. 149–184). Hillsdale, NJ: Erlbaum. Paxinos, G., & Watson, C. (1986). The rat brain in stereotaxic coordinates, New York: Academic Press. Prado-Alcala, R. A., Signoret, A., & Figueroa, M. (1981). Timedependent retention deficits induced by posttraining injections of atropine into the caudate nucleus. Pharmacology, Biochemistry, and Behavior, 15, 633–636. Winocur, G. (1974). Functional dissociation within the caudate nucleus of rats. Journal of Comparative and Physiological Psychology, 86, 432–439.
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BRIEF REPORT Stria Terminalis Lesions Attenuate Memory Enhancement Produced by Intracaudate Nucleus Injections of Oxotremorine MARK G. PACKARD,* INES INTROINI-COLLISON,†
AND
JAMES L. MCGAUGH†,‡,1
*Department of Psychology, University of New Orleans, †Center for the Neurobiology of Learning and Memory, and ‡Department of Psychobiology, University of California, Irvine
ing systemic drug and hormone treatments to modulate memory. In rats trained in a one-trial inhibitory avoidance task, lesions of the ST attenuate the memory enhancement induced by posttraining systemic injections of epinephrine (Liang & McGaugh, 1983a), the opioid antagonist naloxone (McGaugh et al., 1986), the cholinergic agonist oxotremorine (Introini-Collison, Arai, & McGaugh, 1989), and the noradrenergic agonist clenbuterol (Introini-Collison, Miyazaki, & McGaugh, 1991). Lesions of the ST also attenuate the memory-impairing effects of posttraining systemic injections of the opioid agonist bendorphin (McGaugh, Introini-Collison, Juler, & Izquierdo, 1986), and the cholinergic antagonist atropine (Introini-Collison, Arai, & McGaugh, 1989). ST lesions also attenuate the memory enhancement produced by intraamygdala administration of norepinephrine (Liang, McGaugh, & Yao, 1990), as well as the memory impairment produced by posttraining electrical stimulation of the amygdala (Liang, & McGaugh, 1983b). Taken together, these findings suggest that the amygdala and stria terminalis are components of a modulatory system mediating the memory-enhancing and memory-impairing effects of posttraining drug treatments. The use of peripheral posttraining hormone and drugs to investigate the role of the ST in memory does not allow for any conclusions concerning the site of action of these treatments. Anatomical evidence suggests that the amygdala may modulate memory storage via ST innervation of other brain regions. ST fibers innervate several brain regions including cerebral cortex (Krettek & Price, 1977; McDonald, 1991), thalamus (Krettek & Price, 1977), hypothalamus (Krettek & Price, 1978), medial cau-
The present study examined the role of the stria terminalis in modulating the memory enhancement produced by posttraining intracaudate nucleus injection of oxotremorine. Male Sprague–Dawley rats with either sham operations or bilateral lesions of the stria terminalis (ST) were trained on a one-trial inhibitory-avoidance task and received a unilateral posttraining intracaudate injection of either a buffer vehicle or the cholinergic agonist oxotremorine (0.3 mg/0.5 ml) into a medial region of the caudate nucleus innervated by the ST. Intracaudate injection of oxotremorine improved memory in sham-operated rats. Although ST lesions did not affect retention in rats given intracaudate injections of the buffer vehicle, ST lesions attenuated the memory enhancement produced by posttraining intracaudate injection of oxotremorine. In view of anatomical evidence indicating that amygdalostriatal projections are nonreciprocol, the present findings suggest that amygdala output via the ST is essential for memory enhancement produced by intracaudate injection of oxotremorine. q 1996 Academic Press, Inc.
Extensive evidence suggests that the amygdaloid complex is involved in the modulation of memory storage processes (for reviews see Packard, Cahill, Williams, & McGaugh, 1995; McGaugh, IntroiniCollison, Kim, & Liang, 1992). In particular, an intact stria terminalis (ST), a major afferent–efferent pathway of the amygdala, is essential for posttrain1 Research was supported by U.S. Public Health Service National Research Service Award 1 F32 NS08973-01 to M.G.P., and U.S. Public Health Service Grant MH12526 from the National Institute of Mental Health and National Institute on Drug Abuse to J.L.M. Address correspondence and reprint requests to Mark G. Packard, Department of Psychology, University of New Orleans, New Orleans, LA, 70148. Fax: 504-286-6049.
278 1074-7427/96 $18.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
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date nucleus (Kita & Kitai, 1990), and brainstem structures (Krettek & Price, 1978). The present study focused on the ST innervation of the medial caudate nucleus and examined the role of this pathway in the memory enhancement produced by posttraining intracaudate injection of the cholinergic agonist oxotremorine. Two lines of behavioral evidence implicate the caudate nucleus as a candidate structure for amygdala modulation of cholinergic effects on memory. First, lesions of the caudate nucleus impair acquisition of one-trial inhibitory avoidance behavior (e.g., Winocur, 1974), the same task in which lesions of the ST attenuate the effects of systemic posttraining cholinergic treatments on memory (Introini-Collison et al., 1989). Second, posttraining intracaudate injection of the muscarinic cholinergic receptor antagonist atropine impairs memory on onetrial inhibitory avoidance tasks (e.g., Prado-Alcala, Signoret, & Figuera, 1981). Thus, we investigated whether amygdala output via the ST is required for inducing the memory-enhancing effects of intracaudate injections of the cholinergic muscarinic receptor agonist oxotremorine. Rats with either sham operations or bilateral lesions of the ST were trained on an inhibitory avoidance task and received an immediate posttraining intracaudate injection of either saline or the cholinergic agonist oxotremorine into the medial caudate nucleus, a region innervated by the basolateral amygdala via the stria terminalis (Kita & Kitai, 1990). Subjects were 48 male Sprague–Dawley rats (225–250 g) individually housed in a temperaturecontrolled environment on a 12-h light/dark cycle with the lights on from 7:00 AM to 7:00 PM. Prior to surgery rats were anesthetized with 50 mg/kg sodium pentobarbital. Half of the animals received bilateral ST lesions, and half received sham lesions. Bilateral electrolytic lesions of the ST were made by passing 3 mA of current for 5 s through an electrode insulated except for 0.5 mm at the tip. Brain coordinates for the ST lesions were AP Å 01.3 mm from bregma, ML Å {2.2 mm, DV Å 05.0 mm. For sham lesions the electrode was lowered but no current was passed. All of the animals were implanted unilaterally (left side) with a guide cannula (23 gauge) in the medial caudate nucleus. The cannula were anchored to the skull with jewelers screws and dental cement, and a sylet was placed in the guide cannula to insure cannula patency. Unilateral cannula placements were used based on previous evidence indicating that the indirect catecholamine agonist D-amphetamine enhances memory following unilateral injection into the caudate nucleus (Packard & White, 1991; Packard, Cahill, & McGaugh, 1994). Co-
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ordinates for the guide cannula placements were AP Å 00.3 mm from bregma, ML Å /2.4 mm, DV Å 03.2 mm. The rats were trained on a trough-shaped inhibitory-avoidance apparatus (McGaugh et al., 1986) consisting of two compartments separated by a sliding door that opened by retracting into the floor. The starting compartment was illuminated by a tensor lamp, which provided the only light in the testing room. On the training trial, each rat was placed in the lighted compartment, facing the closed door. When the rat turned around, the door leading to the dark compartment was opened, and the response latency to enter the dark compartment was measured. When the rat stepped through the door into the dark compartment the door was closed, and a foot shock (0.35 mA, 60 Hz, 0.7 s duration) was delivered. Immediately following the training trial, the rat was removed from the dark compartment and given an intracaudate injection of either vehicle or oxotremorine. The four experimental groups (n Å 12 per group) were: sham lesion-buffer injection, sham lesion-oxotremorine injection, ST lesion-buffer injection, and ST lesion-oxotremorine injection. Oxotremorine (Sigma Co.) was dissolved in a buffered vehicle solution. Injections (0.5 ml) were administered intracerebrally via guide cannulae using 30-gauge injection needles connected by polyethylene tubing to 10ml Hamilton microsyringes. Vehicle and oxotremorine (0.3 mg) injections were delivered over 37 s with a syringe pump (Sage Instruments), and the injection needles (extending 1 mm from the end of the guide cannula) were left in place for an additional 60 s to allow for diffusion of the solution away from the needle tip. The dose of oxotremorine used was chosen based on evidence from our laboratory indicating that intraamygdala injection of this dose enhances memory in the inhibitory-avoidance task (Introini-Collison, Dalmaz, & McGaugh, 1996). After the injection, the animals were returned to their home cages. On the retention test 48 h later, the animals were placed in the lighted compartment as on the training session, and the step through latency (maximum of 600 s) was recorded. For analysis of the lesion and cannula placements, the animals were anesthetized with a 1-cc injection of 30% chloral hydrate solution and perfused with physiological saline followed by a 10% Formalin solution. The brains were removed and fixed in 10% Formalin solution prior to slicing. Frozen sections were cut at 20 mm and stained with cresyl violet. Lesions and cannula placements were verified using the atlas of Paxinos and Watson (1986). Results are shown in Fig. 1. The bilateral stria terminalis lesions
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FIG. 1. (Left) Minimum (hatched) and maximum (striped) extent of stria terminalis lesions. The bilateral lesions ranged AP Å 01.3 to 01.8 mm from bregma. Lesion: top, 1.3; middle, 1.4 mm; bottom, 1.8 mm. (Right) Location of intracaudate nucleus needle tips, located in the medial caudate ranging AP Å 00.26 to 00.4 mm from bregma. Needle tips: top, 00.26 mm; middle, 00.3 mm; bottom, 00.4 mm.
ranged from AP 01.3 to 01.8 mm from bregma (Fig. 1, left side). Occasional partial damage to the lateral tips of the fimbria fornix was observed; however, such damage did not correlate with the behavioral results. Two rats with extensive fornix damage were excluded from the behavioral analyses (one rat from the ST lesion-buffer group and one rat from the ST lesion-oxotremorine group). Cannula tips were located in the medial caudate nucleus, ranging from AP 00.26 to 00.4 mm from bregma (Fig. 1, right side).
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There were no significant differences among groups in the training day entrance latencies (i.e., prior to posttraining injections), the mean group latencies ranged from 8 to 11 s. The mean entrance latencies for all groups on the 48-h retention test are shown in Fig. 2. Some animals were assigned maximal latencies of 600 s on the retention test and therefore nonparametric Mann–Whitney U tests were used to analyze the retention test data. Posttraining intracaudate injections of oxotremorine enhanced retention. The retention test latencies of rats
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FIG. 2. Effect of stria terminalis lesions on memory enhancement produced by intracaudate nucleus injection of the muscarinic cholinergic agonist oxotremorine. STL, stria terminalis lesion; OTM, oxotremorine (0.3 mg/0.5 ml).
with sham lesions given intracaudate injections of oxotremorine were significantly higher than those of rats with sham lesions given intracaudate injections of buffer (U Å 17.5, p õ .05). Lesions of the ST attenuated the memory-enhancing effect of intracaudate injection of oxotremorine. The retention test latencies of rats with sham lesions given intracaudate injections of oxotremorine were significantly higher than those of rats with ST lesions given intracaudate injections of oxotremorine (U Å 32.5, p õ .05). Oxotremorine did not enhance retention in rats with ST lesions. The retention test escape latencies of rats with ST lesions given intracaudate injections of buffer did not differ significantly from those of rats with ST lesions given intracaudate injections of oxotremorine (U Å 54.0, p Å .28). Finally, lesions of the ST alone did not effect retention. The retention test latencies of rats with sham lesions given intracaudate injections of buffer did not differ from those of rats with ST lesions given intracaudate injections of buffer (U Å 60.5, p Å .73). The results suggest that an intact ST is required for the memory enhancement produced by posttraining intracaudate nucleus injection of oxotremorine. Consistent with previous findings (for review, see McGaugh et al., 1992), ST lesions alone did not impair retention in otherwise untreated rats, indicating that the integrity of this pathway is not essential for acquisition of inhibitory-avoidance behavior. The medial region of the caudate nucleus targeted by the cannula placements was chosen because this area receives projections from the amygdala via the stria terminalis (Kita & Kitai, 1990). It should be noted that the medial caudate is proximal to the lateral ventricles, and thus it is possible that the behavioral effects of oxotremorine were due to
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spread to other brain regions. Evidence indicating a important role for cholinergic mechanisms in the caudate nucleus in memory is consistent with the hypothesis that oxotremorine influenced memory via a caudate-mediated mechanism (e.g., Prado-Alcala et al., 1981). Nonetheless, the possibility that oxotremorine influenced memory via spread to other brain regions cannot be ruled out at present. The ST contains both afferent and efferent fibers, and thus the effects of lesions to this pathway on posttraining memory treatments may be due to loss of either ST-mediated amygdala input or output. Importantly, amygdalostriatal projections of the rat brain have been reported to be nonreciprocal (Kita & Kitai, 1990). Thus, the modulatory role of the amygdala in memory may be mediated by direct ST output to the medial caudate nucleus. On this hypothesis, the synergy of amygdala input to the medial caudate nucleus and the activation of cholinergic muscarinic receptors in the caudate would mediate the memoryenhancing effect of oxotremorine. Alternatively, the modulatory effect of amygdala efferents on the caudate nucleus may be mediated through output pathways which include other brain areas. For example, the rat basolateral amygdala innervates interconnected regions of both the prefrontal cortex and medial caudate nucleus (McDonald, 1991). On this hypothesis, amygdala input to the neocortex may modulate cortical projections to the medial caudate nucleus. Further research is necessary to elucidate the circuitry by which memory processes are modulated in brain regions receiving amygdala efferent projections. REFERENCES Introini-Collison, I. B., Arai, Y., & McGaugh, J. L. (1989). Stria terminalis lesions attenuate the effects of posttraining oxotremorine and atropine on retention. Psychobiology, 17, 397– 401. Introini-Collison, I. B., Miyazaki, B., & McGaugh, J. L. (1991). Involvement of the amygdala on the memory enhancing effects of clenbuterol. Psychopharmacology, 104, 541–544. Introini-Collison, I. B., Dalmaz, C., & McGaugh, J. L. (1996). Amygdala B-adrenergic influences on memory storage involve cholinergic activation, Neurobiology of Learning and Memory, 65, 57–64. Kita, H., & Kitai, S. T. (1990). Amygdaloid projections to the frontal cortex and striatum in the rat. Journal of Comparative Neurology, 298, 40–49. Krettek, J. E., & Price, J. L. (1977). Projections from the amygdaloid complex to the cerebral cortex and thalamus in the rat and cat. Journal of Comparative Neurology, 172, 687–722. Krettek, J. E., & Price, J. L. (1978). Amygdaloid projections to subcortical structures within the basal forebrain and brainstem in the rat and cat. Journal of Comparative Neurology, 178, 225–254.
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Liang, K. C., & McGaugh, J. L. (1983a). Lesions of the stria terminalis attenuate the enhancing effect of posttraining epinephrine on retention of an inhibitory avoidance response. Behavioral Brain Research, 9, 49–58. Liang, K. C., & McGaugh, J. L. (1983b). Lesions of the stria terminalis attenuate the amnestic effect of amygdaloid stimulation on avoidance responses. Brain Research, 274, 309–318. Liang, K. C., McGaugh, J. L., & Yao, H. (1990). Involvement of amygdala pathways in the influence of posttraining amygdala norepinephrine and peripheral epinephrine on memory storage. Brain Research, 508, 225–233. McDonald, A. J. (1991). Organization of amygdaloid projections to the prefrontal cortex and associated striatum in the rat. Neuroscience, 4, 1–14. McGaugh, J. L., Introini-Collison, I. B., Juler, R. G., & Izquierdo, I. (1986). Stria terminalis lesions attenuate the effects of post-training naloxone and beta-endorphin on retention. Behavioral Neuroscience, 100, 839–844. McGaugh, J. L., Introini-Collison, I., Kim, M., & Liang, K. C. (1992). Involvement of the amygdala in neuromodulatory influences on memory storage. In J. Aggleton (Ed.), The amygdala (pp. 431–451). New York: Wiley–Liss.
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Packard, M. G., & White, N. M. (1991). Dissociation of hippocampus and caudate nucleus memory systems by post-training intracerebral injection of dopamine agonists. Behavioral Neuroscience, 105, 73–84. Packard, M. G., Cahill, L., & McGaugh, J. L. (1994). Amygdala modulation of hippocampal-dependent and caudate nucleus dependent memory processes. Proceedings of the National Academy of Sciences, 91, 8477–8481. Packard, M. G., Cahill, L., Williams, C. L., McGaugh, J. L. (1995). The anatomy of a memory modulatory system: From periphery to brain. In N. E. Spear, L. P. Spear, & M. L. Woodruff (Eds.), Neurobehavioral plasticity: Learning, development, and response to brain insults (pp. 149–184). Hillsdale, NJ: Erlbaum. Paxinos, G., & Watson, C. (1986). The rat brain in stereotaxic coordinates, New York: Academic Press. Prado-Alcala, R. A., Signoret, A., & Figueroa, M. (1981). Timedependent retention deficits induced by posttraining injections of atropine into the caudate nucleus. Pharmacology, Biochemistry, and Behavior, 15, 633–636. Winocur, G. (1974). Functional dissociation within the caudate nucleus of rats. Journal of Comparative and Physiological Psychology, 86, 432–439.
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