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Lesion of the temporo-ammonic perforant path facilitates self-stimulation of the lateral entorhinal cortex in mice
Brain Research, 344 (1985) 377-381 Elsevier
377
BRE 21075
Lesion of the temporo-ammonic perforant path facilitates self-stimulation of the lateral entorhinal cortex in mice CLAUDE DESTRADE L,MONIQUE GAUTHIER l, PIERRE CAZALA l and MAURO CAUDARELLA2 ILaboratoire de Psychophysiologie, U.A. CNRS No 339, Institut de Biologie Anirnale, Universit~ de Bordeaux 1, Avenue des Facult(s, 33405 Talence, Cedex (France) and 2Department of Psychology, Queen's University, Kingston, Ont., K7L 3N6 (Canada) (Accepted May 14th, 1985) Key words: perforant path lesion - - self-stimulation - - entorhinal cortex - - phenobarbital
The effect of a lesion of the perforant path (PP) on self-stimulation (SS) of the lateral entorhinal cortex (LEC) was tested in mice between 8 and 21 days after surgery. The current intensities tested ranged between 0 and 80¢tA (peak to peak 100 Hz sine-wave). The PP lesion led to a two-fold increase in SS rates at intensities above 30/tA without affecting the baseline SS rates (0 #A) and SS threshold (30 #A). The lesion also led to a significant increase in LEC after-discharge (AD) threshold and eliminated behavioral convulsions during SS testing. The suppression of AD by i.p. Na phenobarbital injection (10 mg/kg) led to a similar increase in SS rates in sham-lesioned mice; there was no difference in PP-lesioned animals. These results might be interpreted as evidence in favor of an independence of the neuronal processes mediating entorhinal and hippocampal reward-related behaviors.
Self-stimulation (SS) has been observed in rats and mice with electrodes implanted in the entorhinal cortex 2,t4. Self-stimulation was obtained with low current intensities, whereas high intensities disturbed entorhinal SS probably as a consequence of seizure after-discharges induced in the hippocampus by entorhinal stimulation 1~. This interfering effect of seizures has been already observed with SS electrodes located in septum or hypothalamus and Reid et a!.16 have shown that an injection of anticonvulsive drugs before the training session facilitates self-stimulation behavior for high current intensities. Besides a single interfering side effect of seizures on SS performance 16, recent results indicate that some intrinsic hippocampal circuits may exert an inhibitory influence on entorhinal self-stimulation1. For example we have shown that in spite of the intimate and reciprocal connections between these brain regions, self-stimulation of the entorhinal cortex and the hippocampus are, at least partly, i n d e p e n d e n t p h e n o m e n a 1. In view of these results, we hypothesized that a lesion of the perforant path, i.e. the m a j o r input from
the entorhinal cortex to the hippocampus It.19, might facilitate entorhinal self-stimulation performance. As an additional control, SS was tested when the hippocampal seizures were suppressed by an injection of an anticonvulsive dose of phenobarbital. Sixteen male BALB/c Orl mice were used at about 10 weeks of age. They were housed in plastic cages and had ad libitum access to food and water u n d e r standard conditions of constant t e m p e r a t u r e and a 12 h/12 h light/dark cycle. U n d e r sodium thiopental anaesthesia (90 mg/kg), 16 animals u n d e r w e n t two simultaneous stereotaxic procedures: implantation of a chronic bipolar electrode in the lateral entorhinal cortex (LEC) and acute lowering of a probe for sham (n = 8) or electrolytic lesion of the perforant path (n = 8). Bipolar electrodes were made of twisted strands of 0.09 mm insulated platinum wire. PP lesions were produced by passing anodal high frequency current (500 kHz, 0.05 mA) for 12 s through the tip of an epoxylite-coated stainless-steel electrode (0.35 m m in diameter, 0.1 mm baretip). The stereotaxic coordinates for PP were: - 3 . 6 mm
Correspondence: C. Destrade, Laboratoire de Psychophysiologie, Universit6 de Bordeaux I, Avenue des Facultds, 33405 Talence Cedex, France. 0006-8993/85/$03.30 © 1985 Elsevier Science Publishers B.V. (Biomedical Division)
378 posterior to b r e g m a ( A P ) , _+3.0 lateral to the midline (L), and 2.4 below the surface of the skull (V); for LEC: A P - 3 . 2 mm, L +3.0 mm, V - 4 . 9 tnm. In both cases, the incisor bar was level with the interaural line. For L E C , the electrode was i m p l a n t e d at an angle of 12 ° to the midsagittal plane. A n example of histological control is shown in Fig. t. Five days after surgery, the 16 mice were tested for L E C stimulation in o r d e r to d e t e r m i n e their individual after-discharge ( A D ) threshold value according to a previously described procedure'). Each electrode could be connected either to the stimulator or to a polygraph. The current intensities ranged between 30 and 120 # A (peak to peak) and the stimulation (100 Hz, sinewave) was delivered in trains of 0.2 s O N - O F F for a total time of 4 s. In sham-lesioned mice, the threshold values ranged b e t w e e n 45 and 70 ,uA (mean value: 55 # A ) ; the values were significantly increased (U = 57.5, P < 0.05) in PP-lesioned animals, the intensity of 120 MA failing to induce A D
in 4 of the 8 subjects (see Fig. 2, for an cxampte ol en .... torhinal and hippocampal EEG recording, before and after PP lesion). Between 8 and 21 days after surgery, tile animals were tested for L E C self-stimulation. All mice were placed in a Plexiglas Skinner box (14 x 7 x 13 cm high) where they had constant access to a lever (3 x 6 cm) located on the short side of the box. which delivered a 0.2 s train of 100 tfz sine-wave stimulation. The mice were assigned with the aid of a random n u m b e r table to one of 3 identical Skinner boxes where they were tested in 3 daily 20-min SS sessions. Lever-presses were automatically recorded and the behavioral convulsions induced by electrical stimulation noted by the experimenter. The current intensities varied between 0 and 8()j~A (peak to peak). They were presented in an ascending series (10 u A steps). Each intensity was p r e s e n t e d 3 times. The SS threshold was individually d e t e r m i n e d by
~i!~i
Fig. 1. Horizontal sections showing histologically verified electrode site in the lateral entorhinal cortex fleft I and lesion of the perlorant path (right).
379
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STIMULATION (80,uA)
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1 oo~,v I____ lsec
Fig. 2. Suppression of lateral entorhinal cortex (LEe) and hippocampal (HPC) after-discharges after lesion of the perforant path. After-discharges were induced by electrical stimulation of the LEe. In this example, recording electrodes were the L E e and the CA1 field of the dorsal hippocampus. L E e electrical stimulation (10 trains of 0.2 s ON-OFF, 80/,A) was applied in the same animal 1 day before (top) and 8 days after lesion of the perforant path {bottom).
comparing the scores recorded at 0/~A (operant level) and at each intensity by means of correlated nondirectional (two-tailed) t-tests; significant difference between PP-lesioned and control animals were evaluated by means of t-tests for independent samples. During the last 5 days of the experiment (between 17 and 21 days), each animal received an injection, in balanced order, of either Na phenobarbital (Gardehal, 10 mg/kg) or saline. All injections (1 ml/100 g of b. wt.) were performed intraperitoneally (i.p.) 15 rain before testing. The animals were tested for L E e self-stimulation at 80 # A current intensity. Each daily treatment was administered twice on consecutive days. The last day, using the experimental procedure described above, an E E G recording was taken in order to control the effect of the 10 mg/kg Gar-
denal i.p. injection in after-discharges elicited by L E e stimulation. The means of 20-min SS rates are presented in Fig. 3 for all current intensities. Compared to the operant level (0 uA), the first significant difference was observed in both groups at the intensity of 30 ~A (P < 0.02). No behavioral convulsions were observed at this intensity. Beginning at 40 ~A intensity, significant differences between PP-lesioned and sham-subjects were observed (P < 0.05 for 40~A; P < 0.01 for 50, 60 and 80~A). There was no convulsion at 40/~A; from 50 to 80 ~A, behavioral convulsions were systematically observed in all sham-lesioned animals, while convulsions were noted in only one PP-lesioned animal at 80uA intensity.
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INTENSITY (juA)
Fig. 3. Mean 20-min rates (+ S.E.M.) for lateral entorhinal cortex SS in control and perforant path-lesioned animals. The current intensities ranged from 0 to 80/~A (peak to peak). The means of 20-min SS rates after Na phenobarbital injection are presented in Table I. There were no significant differences in self-stimulation rates in PPlesioned animals, Sham-lesioned mice, on the other hand, showed a significant increase in SS rates (t(7) = 4.70; P < 0.01) accompanied by the disappearance of behavioral convulsions. In addition, the after-discharges induced by L E C stimulation were abolished in sham-lesioned animals which received i.p. injection of 10 mg/kg phenobarbital. The present results show that entorhinal cortex self-stimulation can be significantly facilitated by a prior lesion of the perforant-path. The PP lesion led to about a two-fold increase in SS rates at all current TABLE I Mean 20-rain rates (+_ S. E. M.) for lateral entorhinal cortex SS in control and perforant path-lesioned animals, Na phenobarbital or saline injection and high current intensity (80 t~A)
The statistical analyses are pairwise comparisons between the drug dose (10 mg/kg) and the corresponding saline mean SS rate within the same group by means of correlated non,directional (two-tailed) t-test. Saline
Phenobarbital
PP lesion (n = 8)
1098 + 38
1089 + 65
Control (n = 8)
550 +_47
* P < 0.01.
1017 +_ 132"
intensities above 30 u A , without affecting baseline SS rates ((I/~A) and SS threshold (30.uAt. ()n the other hand, the suppression of after-discharges by i.p. phenobarbital injection led to a similar increase of SS rates in sham-lesioned mice. As a general rule, these results confirm our assumption that self-stimulation of the entorhinal cortex and self-stimulation of the hippocampus are at least partly independent phenomena. Moreover, since PP lesion also led to increased SS rates at a rehttively low current intensity (40 # A ) , which does not trigger after-discharges in the hippocampus, we may assume that the lesion, similarly to the Na phenobarbital injection, may have suppressed an inhibition of the entorhinal cortex originating in hippocampal circuits. This conclusion is congruent with recent data showing a dissociation of hippocampus and entorhinal cortex by pharmacological effects of diazepam on electrical self-stimulation. Low doses of this benzodiazepine tranquilizer (0.5 and 1 mg/kg injected i.p.) produced significant increases in L E C self-stimulalion, whereas no effect was observed in hippocampal self-stimulated animals 1. However, this conclusion necessitates some qualification. Firstly, the perforant path lesion suppresses the entorhinal efferents to the hippocampus, but not necessarily the hippocampal efferents to the entorhinal region ~°. Also one needs to know whether the PP lesion entirely blocks transmission to the hippocampus: if, for example, a percentage of fibers is spared, this may be sufficient to maintain some degree of transmission. Moreover, it should be pointed out that there are numerous projections of the entorhinal cortex to cortical or subcortical areas other than the hippocampus ~2,1:~-17-~s. In a recent study from our laboratory, an attempt was made in order to describe the neural pathways activated by LEC stimulation. We examined the uptake of 2-deoxyglucose (2-DG) following stimulation of L E C after prior unilateral lesion of the perforant path 6. We found that the 2-DG radioactivity disappeared ipsilaterally in subiculum, hippocampus and in some thalamic nuclei, but all extra-hippocampal labelling was unchanged. For instance, the metabolic activity was intact in amygdala, diagonal band and lateral septal nucleus, suggesting the possible involvement of a nonfornix amygdalofugal hippocampal pathway 7,15. Collier and Routtenberg3 recently found that rats
381 self-administer electrical stimulation to the granule
tion of hippocampus and entorhinal cortex in mem-
cells of the dentate gyms. M o r e o v e r , they showed
ory formation presents certain difficulties since the
that this post-trial granule cells stimulation affected
lesion of the t e m p o r o - a m m o n i c perforant path does
certain types of m e m o r y 4. In the same m a n n e r we
not abolish the delayed effect of L,E C post-training
have shown that the rewarding stimulation of the
stimulation on long-term memoryS, and additionally
L E C acted on long-term m e m o r y processes 9. Thus,
as presently reported, facilitates self-stimulation of
the localization of reward and m e m o r y effects in
LEC. Thus, in conclusion, though our results provide
two neuronal populations with intimate connections
some evidence for the hypothesis of an independence
might suggest that these functions are distributed
of the neuronal processes mediating entorhinal and-
a mong the same n eu ro a n a t o m i c a l systems 3. Con-
hippocampal reward-related behaviors, we cannot
cerning this latter point, we have shown a sequential
presently rule out the possibility that the increase in
and t i m e - d e p e n d e n t intervention of h ip p o c a m p u s
ICSS rates is simply due to a partial reduction of sei-
and entorhinal cortex in m e m o r y formation. Post-
zure activity.
training stimulation of the d e n ta te gyrus modified the retention of a food-reinforced discriminative learn-
This work was p e r f o r m e d during the stay of M.
ing task when applied i m m e d i a t e l y after the acquisi-
Caudarella in the University of Bordeaux I (support-
tion session. Lateral entorhinal cortex stimulation
ed by a Fellowship from the Fondation Fyssen, Pa-
was uneffective at this short acquisition-treatment in-
ris). We thank A. M. Perret, J. Ducout, A. Zielinski
terval, but surprisingly the L E C stimulation im-
and T. Durkin for assistance with histology and prep-
proved retention when delayed b e t w e e n 15 and 30
aration of the manuscript. The work was supported
min after training 5,9.
by the C . N . R . S . ( U . A , 339) and by a grant from the
H o w e v e r , the hypothesis of a synergistic interven1 Caudarella, M., Destrade. C., Cazala, P. and Gauthier, M., Dissociation of limbic structures by pharmacological effects of diazepam on electrical self-stimulation in the mouse, Brain Research, 302 (1984) 196-200. 2 Collier, T. J., Kurtzman, S. and Routtenberg, A., Intracranial self-stimulation derived from entorhinal cortex, Brain Research, 137 (1977) 188-196. 3 Collier, T. J. and Routtenberg, A., Electrical self-stimulation of dentate gyrus granule cells, Behav. Neural Biol.. 42 (1984) 85-90. 4 Collier, T. J. and Rounenberg, A., Selective impairment of declarative memory following stimulation of dentate gyms granule cells: a naloxone-sensitive effect, Brain Research, 310 (1984) 384-387. 5 Destrade, C., Gauthier, M. and Sif, J., Sequential intervention of different limbic structures in memory processes. In B. E. Will, P. Schmitt and J. C. Dalrymple-Alford (Eds.), Brain Plasticity, Learning and Memory, Plenum Publishing Co., New York, in press. 6 Destrade, C., Sif, J., Gauthier, M. and Calas, A., A functional anatomical study of neural pathways involved in memory mechanisms using 14C-2-deoxy-D-glucose: effects of entorhinal cortex electrical stimulation, Soc. Neurosci. Abstr., 10 (Abstr. 39.4) (1984) 134. 7 Gage, F. H., Bj6rklund, A. and Stenevi, U., Cells of origin of the ventral cholinergic septo-hippocampal pathway undergoing compensatory collateral sprouting following timbria-fornix transection, Neurosci. Lett., 44 (1984) 211-216. 8 Gauthier, M. and Destrade, C., Late post-learning effect of entorhinal cortex electrical stimulation persists despite destruction of the perforant path, Brain Research, 310 (1984) 174-179. 9 Gauthier, M., Destrade, C. and Soumireu-Mourat, B., Late post-learning participation of entorhinal cortex in memory processes, Brain Research, 233 (1982) 255-264.
I N S E R M (no 81.6.014). 10 Hjorth-Simonsen, A., Hippocampal affcrcnts to the ipsilateral entorhinal area: an experimental study in the rat, J. comp. Neurol., 142 (1971) 417-438. 11 Hjorth-Simonsen, A. and Jeunc, B., Origin and termination of the hippocampal perforant path in the rat studied bv silver impregnation, J, comp. Neurol., 144 (1972) 215-232. 12 Kosel, K. C., Van Hoesen, G. W. and Rosene, D. L., Nonhippocampal cortical projections from the entorhinal cortex in the rat and rhesus monkey, Brain Research, 244 (1982) 201-214. 13 Meibach, R. C. and Siegel, A.. Subicular projections to the posterior cingulate cortex in rats, Exp. Neurol., 57 (1977) 264-274. 14 Ott, T., Destrade, C. and R0,thrich, H., Induction of selfstimulation behavior derived from a brain region lacking in dopaminergic innervation, Behav. Neural Biol., 28 (1980) 512-516. 15 Poletti, C. E., Kliot, M, and Boytim, M., Metabolic influence of the hippocampus on hypothalamus, preoptic and basal forcbrain is exerted through amygdalofugal pathways, Neurosci. Lett., 45 (1984) 211-216. 16 Reid, L. D., Gibson, W. E., Gledhill. S. M, and Poricr, P. B., Anticonvulsivant drugs and self-stimulating behavior, J. Comp. Physiol. Psychol.. 57 (1964) 353-356. 17 Sorensen, K. E., lpsilateral projection from the subiculum to the retrosplenial cortex in the Guinea pig, ,1. cornp. Neurol., 113 (1980) 893-911. 18 Sorensen. K. E. and Witter, M. P., Entorhinal efferents reach the caudate-putamen, Neurosci. Lett., 35 (1983) 259-264. 19 Steward. O., Topographic organization of the projections from the entorhinal area to the hippocampal formation of the rat, J. comp. Neurol.. 167 (1976) 285-314.
377
BRE 21075
Lesion of the temporo-ammonic perforant path facilitates self-stimulation of the lateral entorhinal cortex in mice CLAUDE DESTRADE L,MONIQUE GAUTHIER l, PIERRE CAZALA l and MAURO CAUDARELLA2 ILaboratoire de Psychophysiologie, U.A. CNRS No 339, Institut de Biologie Anirnale, Universit~ de Bordeaux 1, Avenue des Facult(s, 33405 Talence, Cedex (France) and 2Department of Psychology, Queen's University, Kingston, Ont., K7L 3N6 (Canada) (Accepted May 14th, 1985) Key words: perforant path lesion - - self-stimulation - - entorhinal cortex - - phenobarbital
The effect of a lesion of the perforant path (PP) on self-stimulation (SS) of the lateral entorhinal cortex (LEC) was tested in mice between 8 and 21 days after surgery. The current intensities tested ranged between 0 and 80¢tA (peak to peak 100 Hz sine-wave). The PP lesion led to a two-fold increase in SS rates at intensities above 30/tA without affecting the baseline SS rates (0 #A) and SS threshold (30 #A). The lesion also led to a significant increase in LEC after-discharge (AD) threshold and eliminated behavioral convulsions during SS testing. The suppression of AD by i.p. Na phenobarbital injection (10 mg/kg) led to a similar increase in SS rates in sham-lesioned mice; there was no difference in PP-lesioned animals. These results might be interpreted as evidence in favor of an independence of the neuronal processes mediating entorhinal and hippocampal reward-related behaviors.
Self-stimulation (SS) has been observed in rats and mice with electrodes implanted in the entorhinal cortex 2,t4. Self-stimulation was obtained with low current intensities, whereas high intensities disturbed entorhinal SS probably as a consequence of seizure after-discharges induced in the hippocampus by entorhinal stimulation 1~. This interfering effect of seizures has been already observed with SS electrodes located in septum or hypothalamus and Reid et a!.16 have shown that an injection of anticonvulsive drugs before the training session facilitates self-stimulation behavior for high current intensities. Besides a single interfering side effect of seizures on SS performance 16, recent results indicate that some intrinsic hippocampal circuits may exert an inhibitory influence on entorhinal self-stimulation1. For example we have shown that in spite of the intimate and reciprocal connections between these brain regions, self-stimulation of the entorhinal cortex and the hippocampus are, at least partly, i n d e p e n d e n t p h e n o m e n a 1. In view of these results, we hypothesized that a lesion of the perforant path, i.e. the m a j o r input from
the entorhinal cortex to the hippocampus It.19, might facilitate entorhinal self-stimulation performance. As an additional control, SS was tested when the hippocampal seizures were suppressed by an injection of an anticonvulsive dose of phenobarbital. Sixteen male BALB/c Orl mice were used at about 10 weeks of age. They were housed in plastic cages and had ad libitum access to food and water u n d e r standard conditions of constant t e m p e r a t u r e and a 12 h/12 h light/dark cycle. U n d e r sodium thiopental anaesthesia (90 mg/kg), 16 animals u n d e r w e n t two simultaneous stereotaxic procedures: implantation of a chronic bipolar electrode in the lateral entorhinal cortex (LEC) and acute lowering of a probe for sham (n = 8) or electrolytic lesion of the perforant path (n = 8). Bipolar electrodes were made of twisted strands of 0.09 mm insulated platinum wire. PP lesions were produced by passing anodal high frequency current (500 kHz, 0.05 mA) for 12 s through the tip of an epoxylite-coated stainless-steel electrode (0.35 m m in diameter, 0.1 mm baretip). The stereotaxic coordinates for PP were: - 3 . 6 mm
Correspondence: C. Destrade, Laboratoire de Psychophysiologie, Universit6 de Bordeaux I, Avenue des Facultds, 33405 Talence Cedex, France. 0006-8993/85/$03.30 © 1985 Elsevier Science Publishers B.V. (Biomedical Division)
378 posterior to b r e g m a ( A P ) , _+3.0 lateral to the midline (L), and 2.4 below the surface of the skull (V); for LEC: A P - 3 . 2 mm, L +3.0 mm, V - 4 . 9 tnm. In both cases, the incisor bar was level with the interaural line. For L E C , the electrode was i m p l a n t e d at an angle of 12 ° to the midsagittal plane. A n example of histological control is shown in Fig. t. Five days after surgery, the 16 mice were tested for L E C stimulation in o r d e r to d e t e r m i n e their individual after-discharge ( A D ) threshold value according to a previously described procedure'). Each electrode could be connected either to the stimulator or to a polygraph. The current intensities ranged between 30 and 120 # A (peak to peak) and the stimulation (100 Hz, sinewave) was delivered in trains of 0.2 s O N - O F F for a total time of 4 s. In sham-lesioned mice, the threshold values ranged b e t w e e n 45 and 70 ,uA (mean value: 55 # A ) ; the values were significantly increased (U = 57.5, P < 0.05) in PP-lesioned animals, the intensity of 120 MA failing to induce A D
in 4 of the 8 subjects (see Fig. 2, for an cxampte ol en .... torhinal and hippocampal EEG recording, before and after PP lesion). Between 8 and 21 days after surgery, tile animals were tested for L E C self-stimulation. All mice were placed in a Plexiglas Skinner box (14 x 7 x 13 cm high) where they had constant access to a lever (3 x 6 cm) located on the short side of the box. which delivered a 0.2 s train of 100 tfz sine-wave stimulation. The mice were assigned with the aid of a random n u m b e r table to one of 3 identical Skinner boxes where they were tested in 3 daily 20-min SS sessions. Lever-presses were automatically recorded and the behavioral convulsions induced by electrical stimulation noted by the experimenter. The current intensities varied between 0 and 8()j~A (peak to peak). They were presented in an ascending series (10 u A steps). Each intensity was p r e s e n t e d 3 times. The SS threshold was individually d e t e r m i n e d by
~i!~i
Fig. 1. Horizontal sections showing histologically verified electrode site in the lateral entorhinal cortex fleft I and lesion of the perlorant path (right).
379
LEe
STIMULATION
,HPC
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~
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1
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STIMULATION (80,uA)
HPC
1 oo~,v I____ lsec
Fig. 2. Suppression of lateral entorhinal cortex (LEe) and hippocampal (HPC) after-discharges after lesion of the perforant path. After-discharges were induced by electrical stimulation of the LEe. In this example, recording electrodes were the L E e and the CA1 field of the dorsal hippocampus. L E e electrical stimulation (10 trains of 0.2 s ON-OFF, 80/,A) was applied in the same animal 1 day before (top) and 8 days after lesion of the perforant path {bottom).
comparing the scores recorded at 0/~A (operant level) and at each intensity by means of correlated nondirectional (two-tailed) t-tests; significant difference between PP-lesioned and control animals were evaluated by means of t-tests for independent samples. During the last 5 days of the experiment (between 17 and 21 days), each animal received an injection, in balanced order, of either Na phenobarbital (Gardehal, 10 mg/kg) or saline. All injections (1 ml/100 g of b. wt.) were performed intraperitoneally (i.p.) 15 rain before testing. The animals were tested for L E e self-stimulation at 80 # A current intensity. Each daily treatment was administered twice on consecutive days. The last day, using the experimental procedure described above, an E E G recording was taken in order to control the effect of the 10 mg/kg Gar-
denal i.p. injection in after-discharges elicited by L E e stimulation. The means of 20-min SS rates are presented in Fig. 3 for all current intensities. Compared to the operant level (0 uA), the first significant difference was observed in both groups at the intensity of 30 ~A (P < 0.02). No behavioral convulsions were observed at this intensity. Beginning at 40 ~A intensity, significant differences between PP-lesioned and sham-subjects were observed (P < 0.05 for 40~A; P < 0.01 for 50, 60 and 80~A). There was no convulsion at 40/~A; from 50 to 80 ~A, behavioral convulsions were systematically observed in all sham-lesioned animals, while convulsions were noted in only one PP-lesioned animal at 80uA intensity.
'
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•
Control
10OO
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70
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INTENSITY (juA)
Fig. 3. Mean 20-min rates (+ S.E.M.) for lateral entorhinal cortex SS in control and perforant path-lesioned animals. The current intensities ranged from 0 to 80/~A (peak to peak). The means of 20-min SS rates after Na phenobarbital injection are presented in Table I. There were no significant differences in self-stimulation rates in PPlesioned animals, Sham-lesioned mice, on the other hand, showed a significant increase in SS rates (t(7) = 4.70; P < 0.01) accompanied by the disappearance of behavioral convulsions. In addition, the after-discharges induced by L E C stimulation were abolished in sham-lesioned animals which received i.p. injection of 10 mg/kg phenobarbital. The present results show that entorhinal cortex self-stimulation can be significantly facilitated by a prior lesion of the perforant-path. The PP lesion led to about a two-fold increase in SS rates at all current TABLE I Mean 20-rain rates (+_ S. E. M.) for lateral entorhinal cortex SS in control and perforant path-lesioned animals, Na phenobarbital or saline injection and high current intensity (80 t~A)
The statistical analyses are pairwise comparisons between the drug dose (10 mg/kg) and the corresponding saline mean SS rate within the same group by means of correlated non,directional (two-tailed) t-test. Saline
Phenobarbital
PP lesion (n = 8)
1098 + 38
1089 + 65
Control (n = 8)
550 +_47
* P < 0.01.
1017 +_ 132"
intensities above 30 u A , without affecting baseline SS rates ((I/~A) and SS threshold (30.uAt. ()n the other hand, the suppression of after-discharges by i.p. phenobarbital injection led to a similar increase of SS rates in sham-lesioned mice. As a general rule, these results confirm our assumption that self-stimulation of the entorhinal cortex and self-stimulation of the hippocampus are at least partly independent phenomena. Moreover, since PP lesion also led to increased SS rates at a rehttively low current intensity (40 # A ) , which does not trigger after-discharges in the hippocampus, we may assume that the lesion, similarly to the Na phenobarbital injection, may have suppressed an inhibition of the entorhinal cortex originating in hippocampal circuits. This conclusion is congruent with recent data showing a dissociation of hippocampus and entorhinal cortex by pharmacological effects of diazepam on electrical self-stimulation. Low doses of this benzodiazepine tranquilizer (0.5 and 1 mg/kg injected i.p.) produced significant increases in L E C self-stimulalion, whereas no effect was observed in hippocampal self-stimulated animals 1. However, this conclusion necessitates some qualification. Firstly, the perforant path lesion suppresses the entorhinal efferents to the hippocampus, but not necessarily the hippocampal efferents to the entorhinal region ~°. Also one needs to know whether the PP lesion entirely blocks transmission to the hippocampus: if, for example, a percentage of fibers is spared, this may be sufficient to maintain some degree of transmission. Moreover, it should be pointed out that there are numerous projections of the entorhinal cortex to cortical or subcortical areas other than the hippocampus ~2,1:~-17-~s. In a recent study from our laboratory, an attempt was made in order to describe the neural pathways activated by LEC stimulation. We examined the uptake of 2-deoxyglucose (2-DG) following stimulation of L E C after prior unilateral lesion of the perforant path 6. We found that the 2-DG radioactivity disappeared ipsilaterally in subiculum, hippocampus and in some thalamic nuclei, but all extra-hippocampal labelling was unchanged. For instance, the metabolic activity was intact in amygdala, diagonal band and lateral septal nucleus, suggesting the possible involvement of a nonfornix amygdalofugal hippocampal pathway 7,15. Collier and Routtenberg3 recently found that rats
381 self-administer electrical stimulation to the granule
tion of hippocampus and entorhinal cortex in mem-
cells of the dentate gyms. M o r e o v e r , they showed
ory formation presents certain difficulties since the
that this post-trial granule cells stimulation affected
lesion of the t e m p o r o - a m m o n i c perforant path does
certain types of m e m o r y 4. In the same m a n n e r we
not abolish the delayed effect of L,E C post-training
have shown that the rewarding stimulation of the
stimulation on long-term memoryS, and additionally
L E C acted on long-term m e m o r y processes 9. Thus,
as presently reported, facilitates self-stimulation of
the localization of reward and m e m o r y effects in
LEC. Thus, in conclusion, though our results provide
two neuronal populations with intimate connections
some evidence for the hypothesis of an independence
might suggest that these functions are distributed
of the neuronal processes mediating entorhinal and-
a mong the same n eu ro a n a t o m i c a l systems 3. Con-
hippocampal reward-related behaviors, we cannot
cerning this latter point, we have shown a sequential
presently rule out the possibility that the increase in
and t i m e - d e p e n d e n t intervention of h ip p o c a m p u s
ICSS rates is simply due to a partial reduction of sei-
and entorhinal cortex in m e m o r y formation. Post-
zure activity.
training stimulation of the d e n ta te gyrus modified the retention of a food-reinforced discriminative learn-
This work was p e r f o r m e d during the stay of M.
ing task when applied i m m e d i a t e l y after the acquisi-
Caudarella in the University of Bordeaux I (support-
tion session. Lateral entorhinal cortex stimulation
ed by a Fellowship from the Fondation Fyssen, Pa-
was uneffective at this short acquisition-treatment in-
ris). We thank A. M. Perret, J. Ducout, A. Zielinski
terval, but surprisingly the L E C stimulation im-
and T. Durkin for assistance with histology and prep-
proved retention when delayed b e t w e e n 15 and 30
aration of the manuscript. The work was supported
min after training 5,9.
by the C . N . R . S . ( U . A , 339) and by a grant from the
H o w e v e r , the hypothesis of a synergistic interven1 Caudarella, M., Destrade. C., Cazala, P. and Gauthier, M., Dissociation of limbic structures by pharmacological effects of diazepam on electrical self-stimulation in the mouse, Brain Research, 302 (1984) 196-200. 2 Collier, T. J., Kurtzman, S. and Routtenberg, A., Intracranial self-stimulation derived from entorhinal cortex, Brain Research, 137 (1977) 188-196. 3 Collier, T. J. and Routtenberg, A., Electrical self-stimulation of dentate gyrus granule cells, Behav. Neural Biol.. 42 (1984) 85-90. 4 Collier, T. J. and Rounenberg, A., Selective impairment of declarative memory following stimulation of dentate gyms granule cells: a naloxone-sensitive effect, Brain Research, 310 (1984) 384-387. 5 Destrade, C., Gauthier, M. and Sif, J., Sequential intervention of different limbic structures in memory processes. In B. E. Will, P. Schmitt and J. C. Dalrymple-Alford (Eds.), Brain Plasticity, Learning and Memory, Plenum Publishing Co., New York, in press. 6 Destrade, C., Sif, J., Gauthier, M. and Calas, A., A functional anatomical study of neural pathways involved in memory mechanisms using 14C-2-deoxy-D-glucose: effects of entorhinal cortex electrical stimulation, Soc. Neurosci. Abstr., 10 (Abstr. 39.4) (1984) 134. 7 Gage, F. H., Bj6rklund, A. and Stenevi, U., Cells of origin of the ventral cholinergic septo-hippocampal pathway undergoing compensatory collateral sprouting following timbria-fornix transection, Neurosci. Lett., 44 (1984) 211-216. 8 Gauthier, M. and Destrade, C., Late post-learning effect of entorhinal cortex electrical stimulation persists despite destruction of the perforant path, Brain Research, 310 (1984) 174-179. 9 Gauthier, M., Destrade, C. and Soumireu-Mourat, B., Late post-learning participation of entorhinal cortex in memory processes, Brain Research, 233 (1982) 255-264.
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