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Naloxone facilitates amygdaloid kindling in rats
Brain Research, 194 (1980) 293-297 © Elsevier/North-Holland Biomedical Press
293
Naloxone facilitates amygdaloid kindling in rats
C H E R Y L HARDY, J A A K PANKSEPP, J O H N ROSSI III and A. J. ZOLOVICK
Department of Psychology, Bowling Green State University, Bowling Green, Ohio 43403 (U.S.A.) (Accepted March 13th, 1980)
Key words: amygdala - - naloxone - - kindling --epilepsy
The aim of the present investigation was to assess the possible contribution of the brain opioid system to the development of epileptic seizures following amygdaloid stimulation in rats treated with the narcotic antagonist naloxone. Since both narcotic alkaloids and endorphins have been found to induce EEG seizure activity, brain opioids have been implicated in the etiology of epileptic brain activity7,1°,13,17,21. Kindled seizures, produced by repeated low intensity electrical stimulations of certain brain sites, have been used as one potential experimental model of epilepsy 9,22. In the kindling paradigm repeated daily administration of brain stimulation, which is initially unable to evoke a behavioral response, leads to a progressive development of brain seizure activity accompanied by bilateral clonic convulsions 15. In the present study the amygdala was chosen as the site for stimulation because it has been demonstrated to be the most easily kindled brain area 2,9,15,22 as well as a site of high opiate receptor concentrationl,a,al,la, 20. Bipolar stainless-steel electrodes, with a tip separation of 1 mm, were unilaterally implanted into the right amygdala of 10 female Wistar rats. Coordinates were anterior 6.5 mm, lateral 4.4 mm, and 7.9 mm below the dural surfacOL Following a one-week post-operative period for recovery and handling, 6 subjects received injections of naloxone (i.p., 1 mg/kg) and 4 subjects received an equivalent amount of water vehicle 15-20 min prior to each daily stimulation (1 sec train of 235 /zA 1 msec bipolar rectangular pulses at 62.5 Hz). Amygdaloid EEG was monitored on an oscilloscope and recorded using a Narco-Biosystems Physiograph (filter bandpass: 0.3-100 Hz). Behavioral manifestations were simultaneously noted. All rats were kindled at least until a Class 5 (C5) seizure, as defined by Racine 14, was elicited. After the 12-day kindling period all injections were terminated. Following 6 additional baseline days when the animals received kindling stimulations only, a 6-day testing phase was conducted. Half of each original group received injections of naloxone (i.p., 5 mg/kg) and the remaining animals received injections of morphine (i.p., 5 mg/kg) 15-20 min prior to the daily kindling stimulations. A seventh test day was conducted in which only the kindling stimulations were given. Data were analyzed for mean afterdischarge (AD) duration, number of stimula-
294 TABLE I Percentage o f animals with Class 5 seizures during the kindling period and mean ( :L S.D.) afterdischarge duration (see) Kindling day
Naloxone with Class 5 seizures AD duration Control with Class 5 seizures AD duration
6
7
8
9
10
II
12
0~ 28.5 (±28.5)
50~/oo 41.7 (±26.1)
67~o 55.3 (±33.1)
83~o 45.3 (±21.2)
lOOK 44.8 (~ 13.8)
10OK, 54.5 (~ 18.4)
100~ 59.0 (±19.1)
0~o 19.8 (±12.9)
0~ 26.8 (±13.3)
0~o 35.5 (±22.9)
0~ 56.3 (±24.0)
75~o 65.3 (±28.1)
75 'k,~ 58.5 (±10.9)
100~ 49.3 (±12.6)
tions to a C5 seizure, and postictal behavioral depression (defined as the number of seconds taken by the rat to initiate a stepping response following cessation of the AD). Standard histological techniques verified that all electrode placements were in the amygdala. All animals exhibited a systematic development of post-stimulation afterdischarges. Although the variability between AD durations among animals was large (Table I), each animal exhibited a systematic lengthening of AD durations across successive test days. Generally, AD length increased gradually from day 1 to day 6-9 of stimulation, followed by a more rapid increase for several days. All animals developed C5 seizures by the twelfth stimulation (Table I). Naloxone reliably reduced the mean number of days taken to achieve C5 seizures from 10.5 to 8.0 days (t -- 3.3, P < 0.01). Another consistent observation was that in all instances, animals treated with naloxone during the kindling phase displayed C5 seizures before that stimulation which evoked the longest duration amygdaloid AD (mean i S.D. == 2.5 ~_ 1.38 days before the stimulation which produced maximal AD), whereas the control animals exhibited C5 seizures on or after that stimulation which resulted in peak AD duration (0.75 ± 0.5). This difference proved reliable (t = 4.19, P < 0.005). There was, however, no reliable difference between groups in the mean number of stimulations taken to maximize the duration of the AD. These results indicate that blocking endogenous opioids with the narcotic antagonist naloxone facilitates the development of behavioral seizures. Naloxone was shown to have only a minor effect on the EEG indices of seizure activity measured in the stimulated amygdala. Naloxone thus appears to have increased the animals' sensitivity to the epileptogenic properties of the primary focus AD. in addition, this conclusion is supported by the observation that the duration of AD, during the first C5 seizure for animals who received naloxone during the kindling phase (43.7 ~ 16.1) tended to be briefer than for controls (66.3 26.7), although this difference was not shown to be statistically reliable.
295 TABLE II
Mean (4- S.D.) afterdischarge duration and mean (4- S.D.) latency to the first step o f class 5 seizure rats before and after drug treatment Drug day 1
Drug days 1-6
Morphine 5 mglkg (n = 5) Mean A D 56.5 (4-15.7) Mean 'first step' 46.2 (4-29.3)
64.0 (4-15.1) 203.0 (4-137.0)*
63.5 (4-21.4) 122.1 (::t-90.2)**
Naloxone 5 mg/kg (n = 4) Mean A D 55.7 (4-10.9) Mean 'first step' 37.6 (4-24.4)
58.5 (2:7.9) 22.5 (4-18.5)
Baseline days 1~6
55.9 (4-12.7) 31.1 (4-21.4)
* P < 0.005. ** All Ps < 0.05.
In the testing phase, neither morphine (5 mg/kg) nor naloxone (5 mg/kg) reliably altered AD duration or the complexity of the AD waveform (Table II). Although Frenk et al. reported the gradual build-up of AD in the primary focus in rats receiving naloxone for the first time after having been kindled to C5 seizures 8, no such abnormal spiking patterns were observed in the AD of animals that received naloxone during the kindling phase or in the testing phase. Three rats which received morphine in the testing phase developed low amplitude repetitive spiking (Fig. 1) bursts which occurred throughout the duration of the 5 min postictal recording session. No animal receiving naloxone in the test phase exhibited such spiking patterns. Morphine increased the duration of the behavioral depression with 3 of 5 rats failing to initiate a stepping response within the allotted 5 min recording period (t = 5.96, P < 0.0005; Table II). Naloxone failed to produce a reliable effect on postictal behavioral depression in the testing phase when compared to the baseline~period. CONTROL
S #i
.
M O R P H I N E
.
(
5
.
MG/KG
.
.
.
*
....
i
.....
_
._
l.J
--
~L
I
)
10
SEC.
Fig. 1. Representative development of spiking bursts recorded from the amygdala (170-230 sec poststimulation) of the same rat on its final baseline day (upper trace) and after its first treatment with morphine (i.p., 5 mg/kg, lower trace).
296 Neither naloxone nor morphine was shown to have an effect on AD duration or complexity of the A D waveform for the remainder of the testing phase. The increase in postictal depression exhibited by the morphine-treated rats persisted throughout the testing phase (lowest t ~ 2.8, P ~ 0.025; Table II). No novel AD or behavioral patterns were observed on the seventh testing day when only the kindling stimulation was administered. The present results indicate that blocking endogenous opioids with the narcotic antagonist, naloxone, facilitates the development of kindled behavioral seizures in rats without markedly altering the development of electroencephalographic brain seizure activity in the stimulated amygdala. This conclusion is supported by the observation that naloxone reliably reduced the number of stimulations necessary to evoke C5 amygdaloid kindled seizures. Furthermore, in each case, rats treated with naloxone during the actual kindling process exhibited C5 seizures before the stimulation which produced the AD of the longest duration, while control rats exhibited generalized seizures on or after the day of peak AD duration. This suggests that endogenous opioids might normally function to decrease the susceptibility of the brain to epileptogenic activity. Furthermore, based on our measures, naloxone-induced facilitation of the temporal development of the behavioral seizures is apparently not dependent on the increase in the duration of AD in the primary focus. In conclusion, the endogenous opioids, while capable of inducing brain seizure activity in large doses 5-7A3,a9,22 are apparently not necessary for this, and appear to subserve an inhibitory capacity in the development of amygdaloid-kindled seizures in the rat. This suggests that the opiates possess anticonvulsant properties, an idea consonant with recent findings16, TM. It is concluded that the epileptogenic potential of the endogenous opiates is a transient rather than tonic phenomenon, and accordingly, relatively unimportant in the sustaining processes which produce the permanent changes in brain function reflected in kindling. This research was supported by N S F - U R P Grant SP1-7726217 (C.H.), N I H Grant AM17157, and Research Scientist Development Award MH-00086 (J.P.).
1 Atweh, S. and Kuhar, M., Autoradiographic localization of opiate receptors in the rat brain. Ill. The telencephalon, Brain Research, 134 (1977) 393-405. 2 Burnham, W. M., Primary and 'transfer' seizure development in the kindled rat, Canad. J. neurol. Sci., 2 (1975) 417-428. 3 Corcoran, M. E. and Wada, J. A., Naloxone and the kindling of seizures, Life Sci., 24 (1979) 791-795. 4 Elde, R., H6kfelt, T., Johansson, O. and Terenius, L., Immunohistochemical studies using antibodies to leucine-enkephalin : initial observations on the nervous system of the rat, Neuroscience, 1 (1976) 349-351. 5 Frenk, H., Urca, G. and Liebeskind, J. C., Enkephalin-induced cortical seizures in the rat, Neurosci. Abstr., 3 (1977) 291. 6 Frenk, H., Urca, G. and Liebeskind, J. C., Epileptic properties of leucine- and methionineenkephaline in comparison with morphine and reversibility by naloxone, Brain Research, 147 (1978) 327-337. 7 Frenk, H., McCarty, B. C. and Liebeskind, J. C., Different brain areas mediate the analgesic and epileptic properties of enkephalin, Science, 200 (1978) 335-337.
297 8 Frenk, H., Engle, J., Ackerman, R. F., Shavit, Y. and Liebeskind, J. C., Endogenous opioids may mediate postictal behavioral depression in amygdaloid-kindled rats, Brain Research, 167 (1979) 435-440. 9 Goddard, G. V., Mclntyre, D. C. and Leech, C. L., A permanent change in brain function resulting from daily electrical stimulation, Exp. Neurol., 25 (1969) 295-330. 10 Henriksen, S. J., Bloom, F. E., Ling, N. and Gulliemin, R., fl-Endorphin induces nonconvulsive limbic seizures, Proc. nat. Acad. Sci. (Wash.), 75 (1978) 5221-5225. 11 H~Skfelt,T., Elde, R., Johansson, O., Terenius, L. and Stein, L., The distribution of enkephalinimmunoreactive cell bodies in the rat central nervous system, Neurosci. Lett., 5 (1977) 25-31. 12 K6nig, J. F. R., and Klippel, R. A., The Rat Brain: A Stereotaxie Atlas of the Forebrain and Lower Parts o f the Brain Stem, Williams and Wilkins, Baltimore, 1963. 13 Le Gal La Salle, G., Calvino, B. and Ben-Ari, Y., Morphine enhances amygdaloid seizures and increases interictal spike frequency in kindled rates, Neurosci. Lett., 6 (1977) 255-268. 14 Racine, R. J., Modification of seizure activity by electrical stimulation: II. Motor seizure, Electroenceph, clin. Neurophysiol., 32 (1972) 281-294. 15 Racine, R. J., Modification of seizure activity by electrical stimulation: cortical areas, Electroenceph, clin. Neurophysiol., 38 (1975) 1-12. 16 Schreiber, R. A., The effect of naloxone on audiogenic seizures, Psychopharmacology, 66 (1979) 205-206. 17 Sherman, J. E. and Liebeskind, J. C., An endorphinergic, centrifugal substrate of pain modulation: recent findings, current concepts and complexities, Proc. Assoc. Res. nerv. ment. Disorders, in press. 18 Simantov, R., Kuhar, M. J., Uhl, G. R. and Snyder, S. H., Opioid peptide enkephalin: immunohistochemical mapping in the rat central nervous system, Proc. nat. Acad. Sci. (Wash.), 74 (1974) 2167-2171. 19 Tortella, F. C., Cowan, A. and Adler, M. W., D-ala2-methionine enkephinamide and morphine are anticonvulsant after central administration in rats, Neurosci. Abstr., 5 (1979) 542. 20 Uhl, G. R., Kuhar, M. J. and Snyder, S. H., Enkephalin-containingpathway: amygdaloid efferents in the stria terminalis, Brain Research, 149 (1978) 223-228. 21 Urca, G., Frenk, H., Liebeskind, J. C. and Taylor, A. N., Morphine and enkephalin: analgesic and epileptic properties, Science, 197 (1977) 83-86. 22 Vosu, H. and Wise, R. A., Cholinergic seizure kindling in the rat: comparison of caudate, amygdala, and hippocampus, Behav. Biol., 13 (1975) 491495. 23 Wada, J. A., and Oswa, T., Generalized convulsive seizure state induced by daily electrical amygdaloid stimulation in Senegalese baboons, Papio papio, Neurology (Minneap.), 26 (1976) 273286.
293
Naloxone facilitates amygdaloid kindling in rats
C H E R Y L HARDY, J A A K PANKSEPP, J O H N ROSSI III and A. J. ZOLOVICK
Department of Psychology, Bowling Green State University, Bowling Green, Ohio 43403 (U.S.A.) (Accepted March 13th, 1980)
Key words: amygdala - - naloxone - - kindling --epilepsy
The aim of the present investigation was to assess the possible contribution of the brain opioid system to the development of epileptic seizures following amygdaloid stimulation in rats treated with the narcotic antagonist naloxone. Since both narcotic alkaloids and endorphins have been found to induce EEG seizure activity, brain opioids have been implicated in the etiology of epileptic brain activity7,1°,13,17,21. Kindled seizures, produced by repeated low intensity electrical stimulations of certain brain sites, have been used as one potential experimental model of epilepsy 9,22. In the kindling paradigm repeated daily administration of brain stimulation, which is initially unable to evoke a behavioral response, leads to a progressive development of brain seizure activity accompanied by bilateral clonic convulsions 15. In the present study the amygdala was chosen as the site for stimulation because it has been demonstrated to be the most easily kindled brain area 2,9,15,22 as well as a site of high opiate receptor concentrationl,a,al,la, 20. Bipolar stainless-steel electrodes, with a tip separation of 1 mm, were unilaterally implanted into the right amygdala of 10 female Wistar rats. Coordinates were anterior 6.5 mm, lateral 4.4 mm, and 7.9 mm below the dural surfacOL Following a one-week post-operative period for recovery and handling, 6 subjects received injections of naloxone (i.p., 1 mg/kg) and 4 subjects received an equivalent amount of water vehicle 15-20 min prior to each daily stimulation (1 sec train of 235 /zA 1 msec bipolar rectangular pulses at 62.5 Hz). Amygdaloid EEG was monitored on an oscilloscope and recorded using a Narco-Biosystems Physiograph (filter bandpass: 0.3-100 Hz). Behavioral manifestations were simultaneously noted. All rats were kindled at least until a Class 5 (C5) seizure, as defined by Racine 14, was elicited. After the 12-day kindling period all injections were terminated. Following 6 additional baseline days when the animals received kindling stimulations only, a 6-day testing phase was conducted. Half of each original group received injections of naloxone (i.p., 5 mg/kg) and the remaining animals received injections of morphine (i.p., 5 mg/kg) 15-20 min prior to the daily kindling stimulations. A seventh test day was conducted in which only the kindling stimulations were given. Data were analyzed for mean afterdischarge (AD) duration, number of stimula-
294 TABLE I Percentage o f animals with Class 5 seizures during the kindling period and mean ( :L S.D.) afterdischarge duration (see) Kindling day
Naloxone with Class 5 seizures AD duration Control with Class 5 seizures AD duration
6
7
8
9
10
II
12
0~ 28.5 (±28.5)
50~/oo 41.7 (±26.1)
67~o 55.3 (±33.1)
83~o 45.3 (±21.2)
lOOK 44.8 (~ 13.8)
10OK, 54.5 (~ 18.4)
100~ 59.0 (±19.1)
0~o 19.8 (±12.9)
0~ 26.8 (±13.3)
0~o 35.5 (±22.9)
0~ 56.3 (±24.0)
75~o 65.3 (±28.1)
75 'k,~ 58.5 (±10.9)
100~ 49.3 (±12.6)
tions to a C5 seizure, and postictal behavioral depression (defined as the number of seconds taken by the rat to initiate a stepping response following cessation of the AD). Standard histological techniques verified that all electrode placements were in the amygdala. All animals exhibited a systematic development of post-stimulation afterdischarges. Although the variability between AD durations among animals was large (Table I), each animal exhibited a systematic lengthening of AD durations across successive test days. Generally, AD length increased gradually from day 1 to day 6-9 of stimulation, followed by a more rapid increase for several days. All animals developed C5 seizures by the twelfth stimulation (Table I). Naloxone reliably reduced the mean number of days taken to achieve C5 seizures from 10.5 to 8.0 days (t -- 3.3, P < 0.01). Another consistent observation was that in all instances, animals treated with naloxone during the kindling phase displayed C5 seizures before that stimulation which evoked the longest duration amygdaloid AD (mean i S.D. == 2.5 ~_ 1.38 days before the stimulation which produced maximal AD), whereas the control animals exhibited C5 seizures on or after that stimulation which resulted in peak AD duration (0.75 ± 0.5). This difference proved reliable (t = 4.19, P < 0.005). There was, however, no reliable difference between groups in the mean number of stimulations taken to maximize the duration of the AD. These results indicate that blocking endogenous opioids with the narcotic antagonist naloxone facilitates the development of behavioral seizures. Naloxone was shown to have only a minor effect on the EEG indices of seizure activity measured in the stimulated amygdala. Naloxone thus appears to have increased the animals' sensitivity to the epileptogenic properties of the primary focus AD. in addition, this conclusion is supported by the observation that the duration of AD, during the first C5 seizure for animals who received naloxone during the kindling phase (43.7 ~ 16.1) tended to be briefer than for controls (66.3 26.7), although this difference was not shown to be statistically reliable.
295 TABLE II
Mean (4- S.D.) afterdischarge duration and mean (4- S.D.) latency to the first step o f class 5 seizure rats before and after drug treatment Drug day 1
Drug days 1-6
Morphine 5 mglkg (n = 5) Mean A D 56.5 (4-15.7) Mean 'first step' 46.2 (4-29.3)
64.0 (4-15.1) 203.0 (4-137.0)*
63.5 (4-21.4) 122.1 (::t-90.2)**
Naloxone 5 mg/kg (n = 4) Mean A D 55.7 (4-10.9) Mean 'first step' 37.6 (4-24.4)
58.5 (2:7.9) 22.5 (4-18.5)
Baseline days 1~6
55.9 (4-12.7) 31.1 (4-21.4)
* P < 0.005. ** All Ps < 0.05.
In the testing phase, neither morphine (5 mg/kg) nor naloxone (5 mg/kg) reliably altered AD duration or the complexity of the AD waveform (Table II). Although Frenk et al. reported the gradual build-up of AD in the primary focus in rats receiving naloxone for the first time after having been kindled to C5 seizures 8, no such abnormal spiking patterns were observed in the AD of animals that received naloxone during the kindling phase or in the testing phase. Three rats which received morphine in the testing phase developed low amplitude repetitive spiking (Fig. 1) bursts which occurred throughout the duration of the 5 min postictal recording session. No animal receiving naloxone in the test phase exhibited such spiking patterns. Morphine increased the duration of the behavioral depression with 3 of 5 rats failing to initiate a stepping response within the allotted 5 min recording period (t = 5.96, P < 0.0005; Table II). Naloxone failed to produce a reliable effect on postictal behavioral depression in the testing phase when compared to the baseline~period. CONTROL
S #i
.
M O R P H I N E
.
(
5
.
MG/KG
.
.
.
*
....
i
.....
_
._
l.J
--
~L
I
)
10
SEC.
Fig. 1. Representative development of spiking bursts recorded from the amygdala (170-230 sec poststimulation) of the same rat on its final baseline day (upper trace) and after its first treatment with morphine (i.p., 5 mg/kg, lower trace).
296 Neither naloxone nor morphine was shown to have an effect on AD duration or complexity of the A D waveform for the remainder of the testing phase. The increase in postictal depression exhibited by the morphine-treated rats persisted throughout the testing phase (lowest t ~ 2.8, P ~ 0.025; Table II). No novel AD or behavioral patterns were observed on the seventh testing day when only the kindling stimulation was administered. The present results indicate that blocking endogenous opioids with the narcotic antagonist, naloxone, facilitates the development of kindled behavioral seizures in rats without markedly altering the development of electroencephalographic brain seizure activity in the stimulated amygdala. This conclusion is supported by the observation that naloxone reliably reduced the number of stimulations necessary to evoke C5 amygdaloid kindled seizures. Furthermore, in each case, rats treated with naloxone during the actual kindling process exhibited C5 seizures before the stimulation which produced the AD of the longest duration, while control rats exhibited generalized seizures on or after the day of peak AD duration. This suggests that endogenous opioids might normally function to decrease the susceptibility of the brain to epileptogenic activity. Furthermore, based on our measures, naloxone-induced facilitation of the temporal development of the behavioral seizures is apparently not dependent on the increase in the duration of AD in the primary focus. In conclusion, the endogenous opioids, while capable of inducing brain seizure activity in large doses 5-7A3,a9,22 are apparently not necessary for this, and appear to subserve an inhibitory capacity in the development of amygdaloid-kindled seizures in the rat. This suggests that the opiates possess anticonvulsant properties, an idea consonant with recent findings16, TM. It is concluded that the epileptogenic potential of the endogenous opiates is a transient rather than tonic phenomenon, and accordingly, relatively unimportant in the sustaining processes which produce the permanent changes in brain function reflected in kindling. This research was supported by N S F - U R P Grant SP1-7726217 (C.H.), N I H Grant AM17157, and Research Scientist Development Award MH-00086 (J.P.).
1 Atweh, S. and Kuhar, M., Autoradiographic localization of opiate receptors in the rat brain. Ill. The telencephalon, Brain Research, 134 (1977) 393-405. 2 Burnham, W. M., Primary and 'transfer' seizure development in the kindled rat, Canad. J. neurol. Sci., 2 (1975) 417-428. 3 Corcoran, M. E. and Wada, J. A., Naloxone and the kindling of seizures, Life Sci., 24 (1979) 791-795. 4 Elde, R., H6kfelt, T., Johansson, O. and Terenius, L., Immunohistochemical studies using antibodies to leucine-enkephalin : initial observations on the nervous system of the rat, Neuroscience, 1 (1976) 349-351. 5 Frenk, H., Urca, G. and Liebeskind, J. C., Enkephalin-induced cortical seizures in the rat, Neurosci. Abstr., 3 (1977) 291. 6 Frenk, H., Urca, G. and Liebeskind, J. C., Epileptic properties of leucine- and methionineenkephaline in comparison with morphine and reversibility by naloxone, Brain Research, 147 (1978) 327-337. 7 Frenk, H., McCarty, B. C. and Liebeskind, J. C., Different brain areas mediate the analgesic and epileptic properties of enkephalin, Science, 200 (1978) 335-337.
297 8 Frenk, H., Engle, J., Ackerman, R. F., Shavit, Y. and Liebeskind, J. C., Endogenous opioids may mediate postictal behavioral depression in amygdaloid-kindled rats, Brain Research, 167 (1979) 435-440. 9 Goddard, G. V., Mclntyre, D. C. and Leech, C. L., A permanent change in brain function resulting from daily electrical stimulation, Exp. Neurol., 25 (1969) 295-330. 10 Henriksen, S. J., Bloom, F. E., Ling, N. and Gulliemin, R., fl-Endorphin induces nonconvulsive limbic seizures, Proc. nat. Acad. Sci. (Wash.), 75 (1978) 5221-5225. 11 H~Skfelt,T., Elde, R., Johansson, O., Terenius, L. and Stein, L., The distribution of enkephalinimmunoreactive cell bodies in the rat central nervous system, Neurosci. Lett., 5 (1977) 25-31. 12 K6nig, J. F. R., and Klippel, R. A., The Rat Brain: A Stereotaxie Atlas of the Forebrain and Lower Parts o f the Brain Stem, Williams and Wilkins, Baltimore, 1963. 13 Le Gal La Salle, G., Calvino, B. and Ben-Ari, Y., Morphine enhances amygdaloid seizures and increases interictal spike frequency in kindled rates, Neurosci. Lett., 6 (1977) 255-268. 14 Racine, R. J., Modification of seizure activity by electrical stimulation: II. Motor seizure, Electroenceph, clin. Neurophysiol., 32 (1972) 281-294. 15 Racine, R. J., Modification of seizure activity by electrical stimulation: cortical areas, Electroenceph, clin. Neurophysiol., 38 (1975) 1-12. 16 Schreiber, R. A., The effect of naloxone on audiogenic seizures, Psychopharmacology, 66 (1979) 205-206. 17 Sherman, J. E. and Liebeskind, J. C., An endorphinergic, centrifugal substrate of pain modulation: recent findings, current concepts and complexities, Proc. Assoc. Res. nerv. ment. Disorders, in press. 18 Simantov, R., Kuhar, M. J., Uhl, G. R. and Snyder, S. H., Opioid peptide enkephalin: immunohistochemical mapping in the rat central nervous system, Proc. nat. Acad. Sci. (Wash.), 74 (1974) 2167-2171. 19 Tortella, F. C., Cowan, A. and Adler, M. W., D-ala2-methionine enkephinamide and morphine are anticonvulsant after central administration in rats, Neurosci. Abstr., 5 (1979) 542. 20 Uhl, G. R., Kuhar, M. J. and Snyder, S. H., Enkephalin-containingpathway: amygdaloid efferents in the stria terminalis, Brain Research, 149 (1978) 223-228. 21 Urca, G., Frenk, H., Liebeskind, J. C. and Taylor, A. N., Morphine and enkephalin: analgesic and epileptic properties, Science, 197 (1977) 83-86. 22 Vosu, H. and Wise, R. A., Cholinergic seizure kindling in the rat: comparison of caudate, amygdala, and hippocampus, Behav. Biol., 13 (1975) 491495. 23 Wada, J. A., and Oswa, T., Generalized convulsive seizure state induced by daily electrical amygdaloid stimulation in Senegalese baboons, Papio papio, Neurology (Minneap.), 26 (1976) 273286.