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Effects of naloxone on hippocampal seizure activity
BEHAVIORAL AND NEURAL BIOLOGY48, 159--164 (1987)
BRIEF REPORT Effects of Naloxone on Hippocampal Seizure Activity MARY A N N LINSEMAN AND WILLIAM A . CORRIGALL 1
Neurobiology Section, Addiction Research Foundation, 33 Russell Street, Toronto, Canada, M5S 2SI
Although the presence of several types of opiate receptors and ligands has been demonstrated within the hippocampus, little is known about the circumstances in which endogenous opiates may be released. Previous studies have suggested that opiates may be involved in producing seizure activity within the hippocampus, or alternatively, that they may be released by seizure activity within the limbic system to prolong the period of postictal depression and thereby prevent the recurrence of seizures during this period. In this experiment we examined the effect of opiate receptor blockade produced by 20 mg/kg ip naloxone on the duration of afterdischarge produced by high-frequency hippocampal stimulation and on inhibition of subsequent seizure activity in male Sprague-Dawley rats. Contrary to the stated hypotheses, naloxone had no effect on either measure. @1987AcademicPress, Inc.
Opiates have been shown to have a pronounced excitatory effect on the hippocampus whether they are applied directly to the isolated slice preparation or administered systemically in vivo (see Corrigall, 1983, for a review). These effects are both stereospecific and naloxone reversible. Anatomical studies have demonstrated that several opiate receptor subtypes and various opiate ligands are indeed present in the hippocampus (Corrigall, 1983; Watson, Khachaturian, Akil, Coy, & Goldstein, 1982). These findings together suggest that endogenous opiates probably play a role in hippocampal function, and determining the nature of this role might be a clue to understanding why opiates are addictive drugs. To date, however, there is little evidence showing under what circumstances endogenous opiates are released within the hippocampus of intact animals; that is, We thank Ms. Myrnalee Elliott, Ms. Rose Marie D'Onofrio, and Dr. Dan L6 for their excellent technical assistance, Dr. Hau Lei for statistical analyses, and E. I. Du Pont De Nemours & Co. for their gift of naloxone. Correspondence and requests for reprints should be addressed to: Dr. M. A. Linseman, Biobehavioral Research Department, Addiction Research Foundation, 33 Russell Street, Toronto, Canada M5S 2S1. 159 0163-1047/87 $3.00 Copyright © 1987by AcademicPress, Inc All rightsof reproductionm any form reserved
160
LINSEMAN AND CORRIGALL
operationally, what responses within or behaviors attributable to the hippocampus are blocked by administration of the opiate antagonist naloxone. Since the intracerebroventricular administration of opiates produces widespread seizure activity within the limbic system (Henriksen, Bloom, McCoy, Ling, & Guillemin, 1978) and since opiates applied to the hippocampal slice produce effects similar to known convulsive agents (Corrigall & Linseman, 1980), it has been hypothesized that opiates may mediate seizure activity within the hippocampus (Linseman & Corrigall, 1982). This is supported by the fact that only the highest doses of morphine administered systemically in the intact rat were effective in modifying hippocampal field potentials and that these changes were often accompanied by convulsive activity of cortical and hippocampal EEG. There have also, however, been recent reports that the enkephalin content of whole brain is enhanced following kindled seizures of the amygdala (Vindrola, Briones, Asai, & Fernandez-Guardiola, 1979) and that the enkephalin content of hippocampus specifically is increased following recurrent seizures produced by intrahippocampal administration of kainic acid (Hong, Wood, Gillin, Yang, & Costa, 1980). It has been proposed that endogenous opiates may be released as a result of seizures and may contribute to the postictal depression period during which there is an increased threshold for further seizure elicitation. In this regard, pretreatment with naloxone has been shown to reduce the period of postictal depression following kindled seizures of the amygdala, as measured by resumption of normal behavioral activity (Frenk, Engel, Ackermann, Shavit, & Liebeskind, 1979). Similarly, the postictal depression, measured by the duration of subsequently elicited seizures, was reduced by naloxone following kindled seizures of the amygdala and globus pallidus when measured at 10 min but not 1 h following the initial stimulus train (Kelsey & Belluzzi, 1982). The purpose of the present experiment was to determine whether endogenous opiates might serve one of these functions within the hippocampus, i.e., mediation of seizures themselves or postictal inhibition of seizures. Although the above-mentioned studies concerned themselves with inhibition following fully kindled seizures, such a period of inhibition can also be seen following elicitation of a single focal seizure. Since this is probably more representative of local activity within the hippocampus than kindled seizures, this phenomenon was of potentially even greater interest. In this study, therefore, initial afterdischarge durations and afterdischarge durations to subsequent tetanic stimuli were measured in nonkindled animals following naloxone and saline pretreatment as a test of the two hypotheses. Twenty-six male Sprague-Dawley rats, weighing approximately 350 g, were implanted with chronic electrodes consisting of a twisted bundle of four blunt-cut insulated nichrome wires--two of 250-/~m diameter for
OPIATES AND HIPPOCAMPAL SEIZURES
161
stimulating and two of 150-/~m diameter for recording. In one-half of the animals, electrodes were aimed at the CA1 area (coordinates: AP-3.5/L 1.0/DV3.5, relative and perpendicular to bregma); in the other half, for dentate (coordinates: AP-3.5/L 1.0/DV4.2). After a recovery period of at least 1 week, animals were tested for thresholds of afterdischarge (AD) in the hippocampal EEG. Animals were placed into a 12-in. 2 Plexiglas box and connected to the stimulating/ recording apparatus. Signals were fed through a commutator mounted on a counterbalanced arm which allowed the animal freedom of movement. After a 10-min adaptation period, stimulation, consisting of a 5-s train of bipolar pulses, each phase 0.1 ms in duration, at a frequency of 62.5 Hz, was administered to the animal. Initial amplitude of stimulation was 40 /zA and if this failed to produce an AD the intensity was raised in steps of 20/zA at l-rain intervals until an AD was evoked. Animals which failed to show an AD at or below 300/zA were dropped from the experiment. In all cases histology showed that these electrodes were outside the hippocampus. The mean AD threshold determined in this way was 72 /~A. CA1 thresholds were lower than dentate thresholds, but this difference failed to be significant (CAI, 57 /xA; dentate, 82 ~A; F(1, 14) = 2.5, p = .14). A stimulus intensity of 2 × AD threshold was chosen for the focal seizure test. The day following AD threshold testing, all animals were stimulated at the 2 × AD intensity to ensure that AD would, in fact, be reliably induced. The animals were then divided into two groups matched on the basis of AD threshold and intended electrode location. Beginning the next day, one group of animals received 20mg/kg naloxone hydrochloride ip (concentration: 20 mg/ml) just prior to being placed in the recording apparatus. The high dose of naloxone was used to ensure that the drug would be effective over a significantly long period of time, since it is known that naloxone has a very short half-life, and to ensure that the dose would be sufficiently large to antagonize K-agonist effects, since dynorphin, a putative K-agonist, has been identified as one of the opiates present within the hippocampus (Watson et al., 1982). Ten minutes following the injection, a 5-s train of stimulation was administered followed by a second identical train after an interval of 10 min. EEG was recorded continuously. The other group of animals was given a saline injection (1 ml/kg) and treated similarly. After an interval of at least 3 days, animals previously treated with naloxone were given saline and vice versa, and the same procedure was followed. Since order of treatments was counterbalanced, data from the two naloxone treatments were combined as were data from the two saline treatments for each anatomical group. At the conclusion of the experiment, animals were given an overdose of pentobarbital and perfused with physiological saline followed by a 10% Formalin solution. Brains were removed and later sectioned to permit verification of electrode placements.
162
LINSEMAN AND CORRIGALL
TABLE 1 Afterdischarge, in s, for All Animals following Each of Two High-Frequency Stimulus Trains Applied 10 rain Apart, 10 rain following an ip Injection of either Naloxone or Saline Pretreatment Saline Subject
S~
Naloxone S2
S1
S2
1.4
CA1 2 3 4 5 6 Median (n = 4)
-54.4 17.6 22.2 -16.8
-0.4 5.2 35.2 -3.3
18.6 5.6 37.0 13.8 20.4 15.2 19.5
1.4 6.8 0.4 101.4 3.4 2.8 2.4
Dentate 7 8 9 I0 11 12 13 14 15 16 Median (n = 9)
17.2 14.2 11.8 14.0 37.0 25.0 13.6 47.4 -26.8 17.2
97.4 68.4 2.8 0 0 0 0 0 -0 0
20.6 17.0 19.8 17.8 51.6 15.4 12.6 32.6 12.8 25.6 19.8
0 0 4.8 0 0 0 0 0 29.2 0 0
1
16
Note. Responses to S~ reflect initial excitability of the hippocampus; responses to $2 may reflect postictal depression as a result of the previously elicited afterdischarge to $1. Data from three animals who completed testing following naloxone but not saline pretreatment are included in the table although they were not included in the statistical analysis.
Duration of AD was measured following each of the two stimulus t r a i n s h e r e a f t e r r e f e r r e d to as $1 a n d $2. A D d u r a t i o n w a s d e f i n e d as the time of greater than baseline amplitude EEG immediately following t h e s t i m u l u s t r a i n . A D d u r a t i o n s f o l l o w i n g e a c h s t i m u l u s f o r all r a t s a r e s h o w n in T a b l e 1. A D d u r a t i o n s f o l l o w i n g $1 w e r e c o m p a r e d b y a t w o - w a y a n a l y s i s o f v a r i a n c e , t h e t w o f a c t o r s b e i n g a r e a (CA1 o r d e n t a t e ) a n d p r e t r e a t m e n t (saline or naloxone). There was no significant main effect of treatment o n t h e d u r a t i o n o f t h e initial A D ( i . e . , t o SO (F(1, 11) = 0 . 7 1 , p = .42) n o r o f a r e a (F(1, 11) = 0.05, p = .82), n o r w a s t h e r e a s i g n i f i c a n t t r e a t m e n t x a r e a i n t e r a c t i o n ( F ( 1 , 11) = 1.2, p = .30).
OPIATES AND HIPPOCAMPAL SEIZURES
163
Although initial ADs were comparable across areas and appeared to conform to a normal distribution, the data obtained in response to $2 were no longer suitable for parametric statistical tests. Accordingly, a sign test was used to assess effects of pretreatment and the two areas were considered separately. In CA1, responses to $2 did not differ with pretreatment (1 + , 1 - , 2 ties; p = .75) one-tailed. The results were similar in dentate (1 + , 2 - , 6 ties; p = .50, one-tailed). Although the data obtained following application of exogenous opiates suggest that these substances can lead to the production of seizure activity in the hippocampus, naloxone failed to alter AD produced by highfrequency electrical stimulation, suggesting that endogenous opiates do not produce this activity following tetanic stimuli. The results of this experiment also do not support the idea that endogenous opiates within the hippocampus contribute to a postictal depression because there was no significant difference in amount of seizure activity produced by a second high-frequency train following pretreatment with an opiate antagonist. This is in contrast to previously reported results in kindled animals following stimulation in other areas of the limbic system (CaldecottHazard, Shavit, Ackermann, Engel, Frederickson, & Liebeskind, 1982; Kelsey & Belluzzi, 1982). This discrepancy could be due to the fact that stimulation in previously kindled animals might represent a more global phenomenon than that produced by local stimulation in unkindled animals, and therefore, be more susceptible to disruption by a specific chemical antagonist. Or, since previous positive results generally involved stimulation of the amygdala, there could be differences across various regions of the brain, even within the limbic system; it has been reported (Kelsey & Belluzzi, 1982) that the effect was less marked in amygdala than in globus pallidus. However, a more recently reported study of the phenomenon, even in amygdaloid-kindled rats, found results more comparable to those reported here for the hippocampus, i.e., that the effects of naloxone were not very marked and that there was a great deal of variability in the data (Jarvis & Freeman, 1983). Finally, it should be noted that, for reasons stated above, only a large dose of naloxone was tested in this study. Should the dose/response curve not be a monotonic function in this case, it is possible that other effects might have been seen should a lower dose of antagonist have been used. In summary, we failed to find an effect of naloxone on afterdischarge duration or on postictal inhibition following tetanic stimulation of the hippocampus, the latter being in contrast to previously reported positive effects following stimulation of other areas of the limbic system in kindled animals. It would seem, therefore, that these conditions are insufficient to cause opiate release within the hippocampus and, therefore, the function of hippocampal opiates is still unclear.
164
LINSEMAN AND CORRIGALL
REFERENCES Caldecott-Hazard, S., Shavit, Y., Ackermann, R. F., Engel, J., Frederickson, R. C. A., & Liebeskind, J. C. (1982). Behavioral and electrographic effects of opiates on kindled seizures in rats. Brain Research, 251, 327-333. Corrigall, W. A. (1983). Opiates and the hippocampus: A review of the functional and morphological evidence. Pharmacology, Biochemistry and Behavior, 18, 255-262. Corrigall, W. A., & Linseman, M. A. (1980). A specific effect of morphine on evoked activity in the rat hippocampal slice. Brain Research, 192, 227-238. Frenk, H., Engel, J., Ackermann, R. F., Shavit, Y., & Liebeskind, J. C. (1979). Endogenous opiates may mediate behavioral depression in amygdaloid-kindled rats. Brain Research, 167, 435-440. Henriksen, S. J., Bloom, F. E., McCoy, F., Ling, N., & GuiUemin, R. (1978)./~-endorphin induces nonconvulsive limbic seizures. Proceedings of the National Academy of Sciences, USA, 75, 5221-5225. Hong, J. S., Wood, P. L., Gillin, J. C., Yang, H. Y. T., & Costa, E. (1980). Changes of hippocampal met-enkephalin content after recurrent motor seizures. Nature (London), 285, 231-232. Jarvis, M. F., & Freeman, F. G. (1983). The effects of naloxone and interstimulation interval on postictal depression in kindled seizures. Brain Research, 288, 235-241. Kelsey, J. E., & Belluzzi, J. D. (1982). Endorphin mediation of post-ictal effects of kindled seizures in rats. Brain Research, 253, 337-340. Linseman, M. A., & Corrigall, W. A. (1982). Effects of morphine on CA1 versus dentate hippocampal field potentials following systemic administration in freely-moving rats. Neuropharmacology, 21, 361-366. Vindrola, O., Briones, R., Asai, M., & Fernandez-Guardiola, A. (1979). Amygdaloid kindling enhances the enkephalin content in the rat brain. Neuroscience Letters, 167, 4354440. Watson, S. J., Khachaturian, H., Akil, H., Coy. D. H., & Goldstein, A. (1982). Comparison of the distribution of dynorphin systems and enkephalin systems in brain. Science, 218, 1134-1136.
BRIEF REPORT Effects of Naloxone on Hippocampal Seizure Activity MARY A N N LINSEMAN AND WILLIAM A . CORRIGALL 1
Neurobiology Section, Addiction Research Foundation, 33 Russell Street, Toronto, Canada, M5S 2SI
Although the presence of several types of opiate receptors and ligands has been demonstrated within the hippocampus, little is known about the circumstances in which endogenous opiates may be released. Previous studies have suggested that opiates may be involved in producing seizure activity within the hippocampus, or alternatively, that they may be released by seizure activity within the limbic system to prolong the period of postictal depression and thereby prevent the recurrence of seizures during this period. In this experiment we examined the effect of opiate receptor blockade produced by 20 mg/kg ip naloxone on the duration of afterdischarge produced by high-frequency hippocampal stimulation and on inhibition of subsequent seizure activity in male Sprague-Dawley rats. Contrary to the stated hypotheses, naloxone had no effect on either measure. @1987AcademicPress, Inc.
Opiates have been shown to have a pronounced excitatory effect on the hippocampus whether they are applied directly to the isolated slice preparation or administered systemically in vivo (see Corrigall, 1983, for a review). These effects are both stereospecific and naloxone reversible. Anatomical studies have demonstrated that several opiate receptor subtypes and various opiate ligands are indeed present in the hippocampus (Corrigall, 1983; Watson, Khachaturian, Akil, Coy, & Goldstein, 1982). These findings together suggest that endogenous opiates probably play a role in hippocampal function, and determining the nature of this role might be a clue to understanding why opiates are addictive drugs. To date, however, there is little evidence showing under what circumstances endogenous opiates are released within the hippocampus of intact animals; that is, We thank Ms. Myrnalee Elliott, Ms. Rose Marie D'Onofrio, and Dr. Dan L6 for their excellent technical assistance, Dr. Hau Lei for statistical analyses, and E. I. Du Pont De Nemours & Co. for their gift of naloxone. Correspondence and requests for reprints should be addressed to: Dr. M. A. Linseman, Biobehavioral Research Department, Addiction Research Foundation, 33 Russell Street, Toronto, Canada M5S 2S1. 159 0163-1047/87 $3.00 Copyright © 1987by AcademicPress, Inc All rightsof reproductionm any form reserved
160
LINSEMAN AND CORRIGALL
operationally, what responses within or behaviors attributable to the hippocampus are blocked by administration of the opiate antagonist naloxone. Since the intracerebroventricular administration of opiates produces widespread seizure activity within the limbic system (Henriksen, Bloom, McCoy, Ling, & Guillemin, 1978) and since opiates applied to the hippocampal slice produce effects similar to known convulsive agents (Corrigall & Linseman, 1980), it has been hypothesized that opiates may mediate seizure activity within the hippocampus (Linseman & Corrigall, 1982). This is supported by the fact that only the highest doses of morphine administered systemically in the intact rat were effective in modifying hippocampal field potentials and that these changes were often accompanied by convulsive activity of cortical and hippocampal EEG. There have also, however, been recent reports that the enkephalin content of whole brain is enhanced following kindled seizures of the amygdala (Vindrola, Briones, Asai, & Fernandez-Guardiola, 1979) and that the enkephalin content of hippocampus specifically is increased following recurrent seizures produced by intrahippocampal administration of kainic acid (Hong, Wood, Gillin, Yang, & Costa, 1980). It has been proposed that endogenous opiates may be released as a result of seizures and may contribute to the postictal depression period during which there is an increased threshold for further seizure elicitation. In this regard, pretreatment with naloxone has been shown to reduce the period of postictal depression following kindled seizures of the amygdala, as measured by resumption of normal behavioral activity (Frenk, Engel, Ackermann, Shavit, & Liebeskind, 1979). Similarly, the postictal depression, measured by the duration of subsequently elicited seizures, was reduced by naloxone following kindled seizures of the amygdala and globus pallidus when measured at 10 min but not 1 h following the initial stimulus train (Kelsey & Belluzzi, 1982). The purpose of the present experiment was to determine whether endogenous opiates might serve one of these functions within the hippocampus, i.e., mediation of seizures themselves or postictal inhibition of seizures. Although the above-mentioned studies concerned themselves with inhibition following fully kindled seizures, such a period of inhibition can also be seen following elicitation of a single focal seizure. Since this is probably more representative of local activity within the hippocampus than kindled seizures, this phenomenon was of potentially even greater interest. In this study, therefore, initial afterdischarge durations and afterdischarge durations to subsequent tetanic stimuli were measured in nonkindled animals following naloxone and saline pretreatment as a test of the two hypotheses. Twenty-six male Sprague-Dawley rats, weighing approximately 350 g, were implanted with chronic electrodes consisting of a twisted bundle of four blunt-cut insulated nichrome wires--two of 250-/~m diameter for
OPIATES AND HIPPOCAMPAL SEIZURES
161
stimulating and two of 150-/~m diameter for recording. In one-half of the animals, electrodes were aimed at the CA1 area (coordinates: AP-3.5/L 1.0/DV3.5, relative and perpendicular to bregma); in the other half, for dentate (coordinates: AP-3.5/L 1.0/DV4.2). After a recovery period of at least 1 week, animals were tested for thresholds of afterdischarge (AD) in the hippocampal EEG. Animals were placed into a 12-in. 2 Plexiglas box and connected to the stimulating/ recording apparatus. Signals were fed through a commutator mounted on a counterbalanced arm which allowed the animal freedom of movement. After a 10-min adaptation period, stimulation, consisting of a 5-s train of bipolar pulses, each phase 0.1 ms in duration, at a frequency of 62.5 Hz, was administered to the animal. Initial amplitude of stimulation was 40 /zA and if this failed to produce an AD the intensity was raised in steps of 20/zA at l-rain intervals until an AD was evoked. Animals which failed to show an AD at or below 300/zA were dropped from the experiment. In all cases histology showed that these electrodes were outside the hippocampus. The mean AD threshold determined in this way was 72 /~A. CA1 thresholds were lower than dentate thresholds, but this difference failed to be significant (CAI, 57 /xA; dentate, 82 ~A; F(1, 14) = 2.5, p = .14). A stimulus intensity of 2 × AD threshold was chosen for the focal seizure test. The day following AD threshold testing, all animals were stimulated at the 2 × AD intensity to ensure that AD would, in fact, be reliably induced. The animals were then divided into two groups matched on the basis of AD threshold and intended electrode location. Beginning the next day, one group of animals received 20mg/kg naloxone hydrochloride ip (concentration: 20 mg/ml) just prior to being placed in the recording apparatus. The high dose of naloxone was used to ensure that the drug would be effective over a significantly long period of time, since it is known that naloxone has a very short half-life, and to ensure that the dose would be sufficiently large to antagonize K-agonist effects, since dynorphin, a putative K-agonist, has been identified as one of the opiates present within the hippocampus (Watson et al., 1982). Ten minutes following the injection, a 5-s train of stimulation was administered followed by a second identical train after an interval of 10 min. EEG was recorded continuously. The other group of animals was given a saline injection (1 ml/kg) and treated similarly. After an interval of at least 3 days, animals previously treated with naloxone were given saline and vice versa, and the same procedure was followed. Since order of treatments was counterbalanced, data from the two naloxone treatments were combined as were data from the two saline treatments for each anatomical group. At the conclusion of the experiment, animals were given an overdose of pentobarbital and perfused with physiological saline followed by a 10% Formalin solution. Brains were removed and later sectioned to permit verification of electrode placements.
162
LINSEMAN AND CORRIGALL
TABLE 1 Afterdischarge, in s, for All Animals following Each of Two High-Frequency Stimulus Trains Applied 10 rain Apart, 10 rain following an ip Injection of either Naloxone or Saline Pretreatment Saline Subject
S~
Naloxone S2
S1
S2
1.4
CA1 2 3 4 5 6 Median (n = 4)
-54.4 17.6 22.2 -16.8
-0.4 5.2 35.2 -3.3
18.6 5.6 37.0 13.8 20.4 15.2 19.5
1.4 6.8 0.4 101.4 3.4 2.8 2.4
Dentate 7 8 9 I0 11 12 13 14 15 16 Median (n = 9)
17.2 14.2 11.8 14.0 37.0 25.0 13.6 47.4 -26.8 17.2
97.4 68.4 2.8 0 0 0 0 0 -0 0
20.6 17.0 19.8 17.8 51.6 15.4 12.6 32.6 12.8 25.6 19.8
0 0 4.8 0 0 0 0 0 29.2 0 0
1
16
Note. Responses to S~ reflect initial excitability of the hippocampus; responses to $2 may reflect postictal depression as a result of the previously elicited afterdischarge to $1. Data from three animals who completed testing following naloxone but not saline pretreatment are included in the table although they were not included in the statistical analysis.
Duration of AD was measured following each of the two stimulus t r a i n s h e r e a f t e r r e f e r r e d to as $1 a n d $2. A D d u r a t i o n w a s d e f i n e d as the time of greater than baseline amplitude EEG immediately following t h e s t i m u l u s t r a i n . A D d u r a t i o n s f o l l o w i n g e a c h s t i m u l u s f o r all r a t s a r e s h o w n in T a b l e 1. A D d u r a t i o n s f o l l o w i n g $1 w e r e c o m p a r e d b y a t w o - w a y a n a l y s i s o f v a r i a n c e , t h e t w o f a c t o r s b e i n g a r e a (CA1 o r d e n t a t e ) a n d p r e t r e a t m e n t (saline or naloxone). There was no significant main effect of treatment o n t h e d u r a t i o n o f t h e initial A D ( i . e . , t o SO (F(1, 11) = 0 . 7 1 , p = .42) n o r o f a r e a (F(1, 11) = 0.05, p = .82), n o r w a s t h e r e a s i g n i f i c a n t t r e a t m e n t x a r e a i n t e r a c t i o n ( F ( 1 , 11) = 1.2, p = .30).
OPIATES AND HIPPOCAMPAL SEIZURES
163
Although initial ADs were comparable across areas and appeared to conform to a normal distribution, the data obtained in response to $2 were no longer suitable for parametric statistical tests. Accordingly, a sign test was used to assess effects of pretreatment and the two areas were considered separately. In CA1, responses to $2 did not differ with pretreatment (1 + , 1 - , 2 ties; p = .75) one-tailed. The results were similar in dentate (1 + , 2 - , 6 ties; p = .50, one-tailed). Although the data obtained following application of exogenous opiates suggest that these substances can lead to the production of seizure activity in the hippocampus, naloxone failed to alter AD produced by highfrequency electrical stimulation, suggesting that endogenous opiates do not produce this activity following tetanic stimuli. The results of this experiment also do not support the idea that endogenous opiates within the hippocampus contribute to a postictal depression because there was no significant difference in amount of seizure activity produced by a second high-frequency train following pretreatment with an opiate antagonist. This is in contrast to previously reported results in kindled animals following stimulation in other areas of the limbic system (CaldecottHazard, Shavit, Ackermann, Engel, Frederickson, & Liebeskind, 1982; Kelsey & Belluzzi, 1982). This discrepancy could be due to the fact that stimulation in previously kindled animals might represent a more global phenomenon than that produced by local stimulation in unkindled animals, and therefore, be more susceptible to disruption by a specific chemical antagonist. Or, since previous positive results generally involved stimulation of the amygdala, there could be differences across various regions of the brain, even within the limbic system; it has been reported (Kelsey & Belluzzi, 1982) that the effect was less marked in amygdala than in globus pallidus. However, a more recently reported study of the phenomenon, even in amygdaloid-kindled rats, found results more comparable to those reported here for the hippocampus, i.e., that the effects of naloxone were not very marked and that there was a great deal of variability in the data (Jarvis & Freeman, 1983). Finally, it should be noted that, for reasons stated above, only a large dose of naloxone was tested in this study. Should the dose/response curve not be a monotonic function in this case, it is possible that other effects might have been seen should a lower dose of antagonist have been used. In summary, we failed to find an effect of naloxone on afterdischarge duration or on postictal inhibition following tetanic stimulation of the hippocampus, the latter being in contrast to previously reported positive effects following stimulation of other areas of the limbic system in kindled animals. It would seem, therefore, that these conditions are insufficient to cause opiate release within the hippocampus and, therefore, the function of hippocampal opiates is still unclear.
164
LINSEMAN AND CORRIGALL
REFERENCES Caldecott-Hazard, S., Shavit, Y., Ackermann, R. F., Engel, J., Frederickson, R. C. A., & Liebeskind, J. C. (1982). Behavioral and electrographic effects of opiates on kindled seizures in rats. Brain Research, 251, 327-333. Corrigall, W. A. (1983). Opiates and the hippocampus: A review of the functional and morphological evidence. Pharmacology, Biochemistry and Behavior, 18, 255-262. Corrigall, W. A., & Linseman, M. A. (1980). A specific effect of morphine on evoked activity in the rat hippocampal slice. Brain Research, 192, 227-238. Frenk, H., Engel, J., Ackermann, R. F., Shavit, Y., & Liebeskind, J. C. (1979). Endogenous opiates may mediate behavioral depression in amygdaloid-kindled rats. Brain Research, 167, 435-440. Henriksen, S. J., Bloom, F. E., McCoy, F., Ling, N., & GuiUemin, R. (1978)./~-endorphin induces nonconvulsive limbic seizures. Proceedings of the National Academy of Sciences, USA, 75, 5221-5225. Hong, J. S., Wood, P. L., Gillin, J. C., Yang, H. Y. T., & Costa, E. (1980). Changes of hippocampal met-enkephalin content after recurrent motor seizures. Nature (London), 285, 231-232. Jarvis, M. F., & Freeman, F. G. (1983). The effects of naloxone and interstimulation interval on postictal depression in kindled seizures. Brain Research, 288, 235-241. Kelsey, J. E., & Belluzzi, J. D. (1982). Endorphin mediation of post-ictal effects of kindled seizures in rats. Brain Research, 253, 337-340. Linseman, M. A., & Corrigall, W. A. (1982). Effects of morphine on CA1 versus dentate hippocampal field potentials following systemic administration in freely-moving rats. Neuropharmacology, 21, 361-366. Vindrola, O., Briones, R., Asai, M., & Fernandez-Guardiola, A. (1979). Amygdaloid kindling enhances the enkephalin content in the rat brain. Neuroscience Letters, 167, 4354440. Watson, S. J., Khachaturian, H., Akil, H., Coy. D. H., & Goldstein, A. (1982). Comparison of the distribution of dynorphin systems and enkephalin systems in brain. Science, 218, 1134-1136.