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Development of brain stimulation reward in the medial prefrontal cortex: Facilitation by prior electrical stimulation of the sulcal prefrontal cortex
Physiology & Behavior, Vol. 28, pp. 869--872.Pergamon Press and Brain Research Publ., 1982. Printed in the U.S.A,
Development of Brain Stimulation Reward in the Medial Prefrontal Cortex: Facilitation by Prior Electrical Stimulation of the Sulcal Prefrontal Cortex I A. ROBERTSON, A. L A F E R R I I ~ R E A N D P. M. M I L N E R McGill University, Psychology D e p a r t m e n t 1205 Dr. Penfield Avenue, Montreal, Quebec H 3 A 1B1 Canada R e c e i v e d 2 D e c e m b e r 1981 ROBERTSON, A., A. LAFERRII~RE AND P. M. MILNER. Development of brain stimulation reward in the medial prefrontal cortex: Facilitation by prior electrical stimulation of the sulcal prefrontal cortex. PHYSIOL. BEHAV. 28(5) 869-872, 1982.--Electrical stimulation of the medial prefrontal cortex (MC) in rats delivered daily for seven days causes a marked improvement in the rate of acquisition of a self-stimulation response. In the present experiment, we looked at whether we could get the same facilitatory effect on self-stimulation of the MC by delivering pre-training stimulation to other points in the brain anatomically related to the MC. Electrical stimulation of the lateral hypothalamus was without effect. However, electrical stimulation of the sulcal prefrontal cortex (SC) either contralateral or ipsilateral to the MC electrode did facilitate acquisition of self-stimulation of the MC. Thus the SC and MC would appear to be part of the same substrate controlling the development of positive reinforcement in the MC. Self-stimulation
Medial prefrontal cortex
Reward
SELF-stimulation of the medial prefrontal cortex (MC) in rats is typically acquired slowly, over a period of several days, in contrast to the quick acquisition of self-stimulation in many other regions of the brain, particularly along the hypothalamic-tegmental axis [6]. Daily exposure to the MC stimulation seems to be critical: a seven day period of programmed (non-contingent) stimulation delivered through the MC electrode beforehand dramatically shortens the number of days it takes animals to start self-stimulating [3]. This suggests that the period of daily stimulation produces a relatively permanent change in MC function--a change that caused the development of a positively reinforcing stimulus from a neural one. One method of identifying the systems involved in the development of self-stimulation of the MC is to determine if programmed stimulation delivered to other brain sites can also have a facilitatory effect on acquisition of MC selfstimulation. One likely candidate is the other major division of the prefrontal c o r t e x - - t h e sulcal or suprarhinal zone (SC). In addition to having multiple parallel afferent and efferent connections to other brain structures, the SC and MC are reciprocally connected [1, 4, 5, 7, 9, 10]. The SC also supports self-stimulation [15] and, like self-stimulation of the MC, self-stimulation of the SC develops slowly over several
Sulcal prefrontal cortex
Lateral hypothalamus
days (personal observation). Moreover, there appears to .exist a functional relationship between the two areas with regard to self-stimulation. Bilateral destruction of the fibre connections between the MC and SC causes a severe and lasting disruption of MC self-stimulation response rates (Corbett, Laferri6re and Milner, submitted). Based on these data, it seems possible that the mechanism governing the development of self-stimulation in the MC might involve the SC. However, other efferent projections of the MC have also been implicated in self-stimulationof this site. F o r example, fibres coursing through the lateral hypothalamus (LH) and medial forebrain bundle have been hypothesized to form part of a descending self-stimulation system originating in the prefrontal cortex [8, 13, 14]. Therefore it might be expected that programmed stimulation of the L H area would have facilitatory effects on self-stimulation of the MC, similar to those obtained with programmed stimulation of the MC itself. In the present experiment, we looked at the effects of programmed pretraining stimulation of the SC or of the L H on the rate of acquisition of self-stimulation of the MC. Since the connection between the MC and the SC appears to be bilateral, we looked at the effects of both ipsilateral and contralateral stimulation of the SC in separate groups. These
1This research was supported by grants from the National Sciences and Engineering Research Council of Canada to Ann Robertson and Peter Milner. Ann Robertson is an N.S.E.R.C. Research Fellow.
C o p y r i g h t © 1982 B r a i n R e s e a r c h P u b l i c a t i o n s Inc.--0031-9384/82/080869-04503.00/0
STo
R O B E R T S ( ) N IAFt{RRIti',Rt 4 N I ) MII.Nt,]R
- A
~ B
•
C
4.2
.4
FIG. I. Coronal sections of rat brain adapted from a stereotaxic atlas (Ill showing locations of electrode tips. A. Self-stimulation electrodes in the MC. B. Programmed stimulation electrodes in the SC. C. Programmed stimulation electrodes in the LH. For all columns, closed stars represent Group I control rats (no programmed stimulation). Closed circles are Group II rats which received programmed stimulation in the ipsilateral SC. Open stars are Group IIl rats which received programmed stimulation in the contralateral SC. Closed triangles are Group IV rats which received programmed stimulation in the ipsilateral LH. The stars within circles in column B represent six rats from Groups II and III whose programmed stimulation electrodes missed the SC. Numbers beside each section represent distance in mm anterior (Columns A and B) or posterior (Column C) to bregma. Each symbol represents one or more subjects.
groups were compared to a control group of rats who received no programmed stimulation before being tested for self-stimulation of the MC. METHOD
Animals and Surgery Male albino rats, weighing 240--320 g at the time of surgery, were each implanted with a 127 p, diameter bipolar stainless steel electrode (Plastic Products, Roanoke, VA) under Nembutal anaesthesia (60 mg/kg, IP). There were four groups of rats: (I) fifteen rats received single electrodes implanted into the MC (4.5 mm anterior to bregma, 0.7 mm lateral to midline, and 3.5 mm ventral to skull surface); (II) twelve rats were implanted with one electrode into the MC and one electrode into the ipsilateral SC (3.8-3.9 mm anterior, 1.%2.2 mm lateral and 5.8 mm ventral, with an angle of 16 degrees from the vertical plane); (III) eleven rats were implanted with one electrode into the MC and one electrode into the contralateral SC; and (IV) ten rats received one electrode in the MC and one electrode in the ipsilateral LH (0.7 mm posterior, 1.5 mm lateral and 8.8 mm ventral). The incisor bar was set at +5.0 mm throughout. The rats were allowed to recover for seven days before testing began. Throughout the experiment, they were housed in single plastic cages with food and water available ad lib.
Phase I: Programmed Stimulation Group 1, the control group each imp(anted with a single MC electrode, did not receive programmed stimulation. Groups II, III and IV all received programmed stimulation every day for seven days, 20 min per day. Group II rats received stimulation through their ipsilateral SC electrode, Group III through their contralateral SC electrode, and Group IV through their LH electrode. The programmed stimulation consisted of 0.5 sec trains of 35/~A (RMS) 60 Hz sine wave delivered at a rate of one train every four sec (for a total of about 300 trains of stimulation every day). The test boxes were made of Plexiglas, 26x23x22 cm, and had grid floors.
Phase H: Acquisition of Self-stimulation The day after the final day of programmed stimulation, self-stimulation tests were begun. In this phase, all groups were treated identically. Testing took place in different boxes than those used in Phase I. These boxes were Plexiglas and wood (25x25x25 cm) and had Plexiglas levers (7.5 x 4 cm) situated on one wall, 5 cm above the wire mesh floor. Stimulation current again consisted of 0.5 sec trains of 60 Hz sine wave, and was pre-set at 40/~A (RMS). Animals were placed in the boxes for 20 rain/day. Each animal was actually placed on the lever when being put into the box, so
S E L F S T I M U L A T I O N A N D P R E F R O N T A L CORTEX
871
H&tology Follov~ing completion of testing, anaesthetized rats (Nembutal, 60 mg/kg, IP) were perfused trancardially with 0.9% saline followed by 10% Formalin. Frozen brain sections of 30 ~ thickness were cut and stained with thionin or neutral red and luxol fast blue and examined under a microscope for electrode tips. There were no differences amongst the locations of self-stimulation electrodes in the MC for the different groups (Fig. 1, column A). Most SC electrodes terminated in the lateral and dorsal insular portions of the sulcal cortex (column B). Six SC electrodes terminated outside the sulcal cortex in the far lateral part of the head of the caudate nucleus or the genu of the corpus callosum (circled stars in column B). Most LH electrodes were located in the perifornical area of the L H as well as zone incerta and tip of the internal capsule (column C).
-I
,,¢ Z
,,¢ I,I.
O III IJJ In
RESULTS
Z
IV
1
3
5 DAYS
7 TO
9
11
13
15
CRITERION
FIG. 2. Rate of acquisition of criterion number of responses (100/20 min) for each group. I=no programmed stimulation; II=programmed stimulation in the ipsilateral SC; III=programmed stimulation in the contralateral SC; and IV=programmed stimulation in the LH.
that each rat received at least one train of stimulation/session; however, other than that, no shaping procedures were employed-each rat had to find and press the bar itself to receive stimulation. Number of responses were recorded at the end of the 20 rain session. Animals were run for at least 14 consecutive days. Any animal that had not demonstrated serf-stimulation by that time was eliminated from the experiment. The predetermined criterion used to establish the presence of selfstimulation was at least 100 responses in 20 min. This figure was based on previous experiments which indicated that, having once attained a score of 100, no rat would fall below that score on successive tests. All animals received a minimum of five days of testing after reaching this criterion, in order to determine how response rates changed after an animal learned to self-stimulate. Following completion o f this phase, we tested to see if serf-stimulation could be obtained from the SC electrodes in Groups II and III and from the L H electrodes in Group IV. Testing took place in the same chambers used in phase II and consisted of between three and five 20 min sessions at the same programmed stimulation parameters used in the stimulation phase.
Of the 48 rats who started the experiment, seven were dropped because they failed to reach criterion in the allotted time. Additionally, six rats from Groups II and III (ipsilaterai and contralaterai SC programmed stimulation) were dropped because, as explained above, their electrodes were not located in the SC. These six animals were treated as a separate group. This left twelve rats in the control Group I, nine in the ipsilateral SC Group II, seven in the contralateral SC Group III and seven in the LH Group IV. The data were analyzed in terms of the first day on which any rat self-stimulated the MC at least 100 times in 20 min (Fig. 2). Distribution-free statistics were used on these data. There was a significant difference amongst the groups. Control rats who received no programmed stimulation took an average of 6.0 (_+S.E.M. 0.9) days to attain criterion. Prestimulation delivered to the ipsilateral SC electrode for seven days (Group II) caused a significant decrease (U=24.5, p =0.027) in the number of days to criterion: rats took, on the average, 3.2_+ 1.1 days. Group III, which received contralateral SC stimulation, similarly took an average of 2.6_+0.6 days to reach criterion, again significantly lower than the control group ( U = 10, p =0.008). In contrast, group IV, who received L H programmed stimulation, took almost the same number of days to reach criterion as control rats (6.1 _+1.4). The six rats from Groups II and IlI, whose SC electrodes landed outside the SC, did not differ from control rats, taking 6.3 (_+ 1.6) days to reach criterion. In addition to analyzing the data in terms of days to criterion, we also analyzed response rates in each group for the first five days after criterion was attained (analysis of variance followed by Newman-Keuls tests). All groups showed a significant increase in responding from the first to the second day (p<0.01), increasing from an overall average of 197 responses/20 rain to 271 responses/20 min. From the second to fifth days, responding was stable. There was also a significant (p<0.05) tendency for Group IV (LH) rats to respond at lower levels than any of the other three groups (at an average of 190 responses over the five days of testing, compared to the Group I (control) rats who attained the highest average rate of 314 responses over the five days). However, there were no differences in the rate of increases in responding between this and any of the other groups. Additionally, these rats, tested after a two week period, were not significantly different from other g r o u p s - - t h e y had attained the same asymptotic rate of responding.
872
ROBERTSON, I,AFERRI~Rti AND MII~NI'~b~
Animals in Groups 1I, II1 and IV were also tested to see if self-stimulation could be obtained from the SC and LH electrodes used in the programmed stimulation phase. All rats with LH electrodes and 94% of the rats with SC electrodes did self-stimulate at mean response rates of 969 (-+84) responses and 189 (-+ 17) responses/20 min respectively at the same current values used in the programmed stimulation phase. In contrast, of the 6 rats in Groups II and llI whose electrodes landed outside the SC, only one rat selfstimulated, at an average rate of 105 responses/20 rain. DISCUSSION Animals implanted with MC electrodes normally take several days to learn to self-stimulate, in contrast to the relatively quick acquisition of self-stimulation seen with many other self-stimulation sites [6]. A period of programmed stimulation delivered through the MC electrode over seven days produces a marked facilitation of acquisition [3]. The present report demonstrates that the same facilitation can be achieved (using the same parameters of programmed stimulation) by stimulating the SC either ipsilateral or contralateral to the self-stimulation electrode. Thus, not only is the SC critical in the maintenance of serf-stimulation of the MC (Corbett, Laferfi~re and Milner, submitted) but the SC may be involved in the acquisition of the rewarding effects of stimulation of the MC. These data suggest that there is a functional link between the SC and MC. The observation that both ipsilateral and contralateral stimulation were equally effective may not be surprising: bilateral destruction of the fibre tract connecting the SC and MC is necessary to eliminate serf-stimulation of the MC, suggesting that either of the two sides of the SC can maintain self-stimulation of this area. These observations are in line with anatomical studies demonstrating a bilateral projection from the MC to the SC [1]. One might hypothesize that electrical stimulation of the SC simply spreads to the MC and thus affects acquisition of self-stimulation of the MC. This seems unlikely because electrode placements from six animals in Groups II and lII which missed the SC and terminated between the MC and SC were not associated with a faster acquisition of selfstimulation. These six rats took much the same length of time to reach criterion as control rats in Group I.
Programmed stimulation in the area of ~he LIt had n~, effect on acquisition of self stimulation of the MC. Thus lhc LH may not be involved in the development of positive rein forcement in the MC. It is interesting to note that, allhough a link between the two sites has been proposed [ 13,141 ii would not appear to be essential for the maintenance of selfstimulation, as large lesions of the LH medial t\~rebrain bundle area do not affect self-stimulation of the ipsilateral MC [2]. The fact that programmed stimulation of the LH failed to affect acquisition of self-stimulation of the MC lends some specificity to the effectiveness of the SC stimulation in doing so. It would appear that neither is the facilitatory effect due to simply a general activation or arousal from brain stimulation nor is it caused by stimulation of any MC afferents or efferents. This is not to suggest, however, that other PFC projections might not be involved in the acquisition of MC self-stimulation. Additionally, the observation that stimulation of the SC is sufficient to facilitate MC self'stimulation does not imply its necessity. The present data simply indicate that the positively reinforcing effects of MC stimulation can be brought about by pre-training stimulation of a site for which both anatomical connections and functional relations to the MC have been demonstrated. In light of these observations, it is not surprising that programmed stimulation of the SC can facilitate not only acquisition of self-stimulation of the MC but also selfstimulation of the SC itself. We have observed that rats receiving programmed stimulation of the SC at the same parameters that were used in the present experiment took an average of 1.3(-+0.8) days to attain criterion during selfstimulation tests. In contrast, animals which did not receive such stimulation took 7.9 (-+ 1.8) days to self-stimulate the SC. There is little evidence at the present time concerning the nature of the anatomy of the proposed MC-SC link in the development of self-stimulation of the MC. One possibility presently under investigation is that repeated stimulation gradually causes a permanent potentiation of neural activity. similar to that proposed to underlie the kindling effect (e.g., [12]). If so, it may be the medial-sulcal connection which is involved in effecting the critical change underlying the development of positive reinforcement in the Mr'.
REFERENCES
I. Beckstead, R. M. Convergent prefrontal and nigral projections to the striatum of the rat. Neurosci. Lett. 12: 5%64, 1979. 2. Corbett, D., A. Laferrirre and P. M. Milner. Self-stimulation of the medial prefrontal cortex does not depend on the medial forebrain bundle. Soc. Neurosci. Abstr. 6: 725, 1980. 3. Corbett, D., A. Laferri~re and P. M. Milner. Behavioral sensitization effects following repeated electrical stimulation of the medial prefrontal cortex. Soc. Neurosci. Abstr. 7: 875, 1981. 4. Divac, 1., A. Kosmal, A. Bjorklund and O. Lindvall. Subcortical projections to the prefrontal cortex in the rat as revealed by the horseradish peroxidase technique. Neuroscience 3: 785-796, 1978. 5. Domesick, V. B. Thalamic relationships of the medial cortex in the rat. Brain Behav. Evolut. 6: 457-483, 1972. 6. Douglin, D. C. and R. B. Glassman. Gradual increase in selfstimulation response rates~effects of electrode loci. Physiol. Psychol. 7: 135-138, 1979. 7. Gerfen, C. R. and R. M. Clavier. Neural inputs to the prefrontal agranular insular cortex in the rat: horseradish peroxidase study. Brain Res. Bull. 4: 347-353, 1979.
8. lto, M. and J. Olds. Unit activity during self-stimulation behavior. J. Neurophysiol. 34: 263-273, 1971, 9. Krettek, J. E. and J. L. Price. The cortical projections of the mediodorsal nucleus and adjacent thalamic nuclei in the rat../. comp. Neurol. 171: 157-191, [977. 10. Leonard, C. M. The prefrontal cortex of the rat. 1. Cortical projections of the mediodorsal nucleus. 11. Efferent connections. Brain Res. 12: 321-343, 1969. I I. Pellegrino, L. J. and A. J. Cushman. A Stereotaxic Atlas t~/thc Rat Brain. New York: Appleton-Century-Crofts, 1967. 12. Racine, R. J. Modification of seizure activity by electrical stimulation: 1. After-discharge threshold. Electroenceph. din. Neurophysiol. 32: 269-279, 1972. 13. Rolls, E. T. and S. J. Cooper. Activation of neurones in the prefrontal cortex by brain-stimulation reward in the rat. Brain Res. 60: 351-368, 1973. 14. Routtenberg, A. Forebrain pathways of reward in Rattus norvegicus..I, comp. physiol. Psychol. 75: 26%276, 1971. 15. Routtenberg, A. and M. Sloan. Self-stimulation in the frontal cortex of Rattus norvegicus. Behav. Biol. 7: 567-572, 1972.
Development of Brain Stimulation Reward in the Medial Prefrontal Cortex: Facilitation by Prior Electrical Stimulation of the Sulcal Prefrontal Cortex I A. ROBERTSON, A. L A F E R R I I ~ R E A N D P. M. M I L N E R McGill University, Psychology D e p a r t m e n t 1205 Dr. Penfield Avenue, Montreal, Quebec H 3 A 1B1 Canada R e c e i v e d 2 D e c e m b e r 1981 ROBERTSON, A., A. LAFERRII~RE AND P. M. MILNER. Development of brain stimulation reward in the medial prefrontal cortex: Facilitation by prior electrical stimulation of the sulcal prefrontal cortex. PHYSIOL. BEHAV. 28(5) 869-872, 1982.--Electrical stimulation of the medial prefrontal cortex (MC) in rats delivered daily for seven days causes a marked improvement in the rate of acquisition of a self-stimulation response. In the present experiment, we looked at whether we could get the same facilitatory effect on self-stimulation of the MC by delivering pre-training stimulation to other points in the brain anatomically related to the MC. Electrical stimulation of the lateral hypothalamus was without effect. However, electrical stimulation of the sulcal prefrontal cortex (SC) either contralateral or ipsilateral to the MC electrode did facilitate acquisition of self-stimulation of the MC. Thus the SC and MC would appear to be part of the same substrate controlling the development of positive reinforcement in the MC. Self-stimulation
Medial prefrontal cortex
Reward
SELF-stimulation of the medial prefrontal cortex (MC) in rats is typically acquired slowly, over a period of several days, in contrast to the quick acquisition of self-stimulation in many other regions of the brain, particularly along the hypothalamic-tegmental axis [6]. Daily exposure to the MC stimulation seems to be critical: a seven day period of programmed (non-contingent) stimulation delivered through the MC electrode beforehand dramatically shortens the number of days it takes animals to start self-stimulating [3]. This suggests that the period of daily stimulation produces a relatively permanent change in MC function--a change that caused the development of a positively reinforcing stimulus from a neural one. One method of identifying the systems involved in the development of self-stimulation of the MC is to determine if programmed stimulation delivered to other brain sites can also have a facilitatory effect on acquisition of MC selfstimulation. One likely candidate is the other major division of the prefrontal c o r t e x - - t h e sulcal or suprarhinal zone (SC). In addition to having multiple parallel afferent and efferent connections to other brain structures, the SC and MC are reciprocally connected [1, 4, 5, 7, 9, 10]. The SC also supports self-stimulation [15] and, like self-stimulation of the MC, self-stimulation of the SC develops slowly over several
Sulcal prefrontal cortex
Lateral hypothalamus
days (personal observation). Moreover, there appears to .exist a functional relationship between the two areas with regard to self-stimulation. Bilateral destruction of the fibre connections between the MC and SC causes a severe and lasting disruption of MC self-stimulation response rates (Corbett, Laferri6re and Milner, submitted). Based on these data, it seems possible that the mechanism governing the development of self-stimulation in the MC might involve the SC. However, other efferent projections of the MC have also been implicated in self-stimulationof this site. F o r example, fibres coursing through the lateral hypothalamus (LH) and medial forebrain bundle have been hypothesized to form part of a descending self-stimulation system originating in the prefrontal cortex [8, 13, 14]. Therefore it might be expected that programmed stimulation of the L H area would have facilitatory effects on self-stimulation of the MC, similar to those obtained with programmed stimulation of the MC itself. In the present experiment, we looked at the effects of programmed pretraining stimulation of the SC or of the L H on the rate of acquisition of self-stimulation of the MC. Since the connection between the MC and the SC appears to be bilateral, we looked at the effects of both ipsilateral and contralateral stimulation of the SC in separate groups. These
1This research was supported by grants from the National Sciences and Engineering Research Council of Canada to Ann Robertson and Peter Milner. Ann Robertson is an N.S.E.R.C. Research Fellow.
C o p y r i g h t © 1982 B r a i n R e s e a r c h P u b l i c a t i o n s Inc.--0031-9384/82/080869-04503.00/0
STo
R O B E R T S ( ) N IAFt{RRIti',Rt 4 N I ) MII.Nt,]R
- A
~ B
•
C
4.2
.4
FIG. I. Coronal sections of rat brain adapted from a stereotaxic atlas (Ill showing locations of electrode tips. A. Self-stimulation electrodes in the MC. B. Programmed stimulation electrodes in the SC. C. Programmed stimulation electrodes in the LH. For all columns, closed stars represent Group I control rats (no programmed stimulation). Closed circles are Group II rats which received programmed stimulation in the ipsilateral SC. Open stars are Group IIl rats which received programmed stimulation in the contralateral SC. Closed triangles are Group IV rats which received programmed stimulation in the ipsilateral LH. The stars within circles in column B represent six rats from Groups II and III whose programmed stimulation electrodes missed the SC. Numbers beside each section represent distance in mm anterior (Columns A and B) or posterior (Column C) to bregma. Each symbol represents one or more subjects.
groups were compared to a control group of rats who received no programmed stimulation before being tested for self-stimulation of the MC. METHOD
Animals and Surgery Male albino rats, weighing 240--320 g at the time of surgery, were each implanted with a 127 p, diameter bipolar stainless steel electrode (Plastic Products, Roanoke, VA) under Nembutal anaesthesia (60 mg/kg, IP). There were four groups of rats: (I) fifteen rats received single electrodes implanted into the MC (4.5 mm anterior to bregma, 0.7 mm lateral to midline, and 3.5 mm ventral to skull surface); (II) twelve rats were implanted with one electrode into the MC and one electrode into the ipsilateral SC (3.8-3.9 mm anterior, 1.%2.2 mm lateral and 5.8 mm ventral, with an angle of 16 degrees from the vertical plane); (III) eleven rats were implanted with one electrode into the MC and one electrode into the contralateral SC; and (IV) ten rats received one electrode in the MC and one electrode in the ipsilateral LH (0.7 mm posterior, 1.5 mm lateral and 8.8 mm ventral). The incisor bar was set at +5.0 mm throughout. The rats were allowed to recover for seven days before testing began. Throughout the experiment, they were housed in single plastic cages with food and water available ad lib.
Phase I: Programmed Stimulation Group 1, the control group each imp(anted with a single MC electrode, did not receive programmed stimulation. Groups II, III and IV all received programmed stimulation every day for seven days, 20 min per day. Group II rats received stimulation through their ipsilateral SC electrode, Group III through their contralateral SC electrode, and Group IV through their LH electrode. The programmed stimulation consisted of 0.5 sec trains of 35/~A (RMS) 60 Hz sine wave delivered at a rate of one train every four sec (for a total of about 300 trains of stimulation every day). The test boxes were made of Plexiglas, 26x23x22 cm, and had grid floors.
Phase H: Acquisition of Self-stimulation The day after the final day of programmed stimulation, self-stimulation tests were begun. In this phase, all groups were treated identically. Testing took place in different boxes than those used in Phase I. These boxes were Plexiglas and wood (25x25x25 cm) and had Plexiglas levers (7.5 x 4 cm) situated on one wall, 5 cm above the wire mesh floor. Stimulation current again consisted of 0.5 sec trains of 60 Hz sine wave, and was pre-set at 40/~A (RMS). Animals were placed in the boxes for 20 rain/day. Each animal was actually placed on the lever when being put into the box, so
S E L F S T I M U L A T I O N A N D P R E F R O N T A L CORTEX
871
H&tology Follov~ing completion of testing, anaesthetized rats (Nembutal, 60 mg/kg, IP) were perfused trancardially with 0.9% saline followed by 10% Formalin. Frozen brain sections of 30 ~ thickness were cut and stained with thionin or neutral red and luxol fast blue and examined under a microscope for electrode tips. There were no differences amongst the locations of self-stimulation electrodes in the MC for the different groups (Fig. 1, column A). Most SC electrodes terminated in the lateral and dorsal insular portions of the sulcal cortex (column B). Six SC electrodes terminated outside the sulcal cortex in the far lateral part of the head of the caudate nucleus or the genu of the corpus callosum (circled stars in column B). Most LH electrodes were located in the perifornical area of the L H as well as zone incerta and tip of the internal capsule (column C).
-I
,,¢ Z
,,¢ I,I.
O III IJJ In
RESULTS
Z
IV
1
3
5 DAYS
7 TO
9
11
13
15
CRITERION
FIG. 2. Rate of acquisition of criterion number of responses (100/20 min) for each group. I=no programmed stimulation; II=programmed stimulation in the ipsilateral SC; III=programmed stimulation in the contralateral SC; and IV=programmed stimulation in the LH.
that each rat received at least one train of stimulation/session; however, other than that, no shaping procedures were employed-each rat had to find and press the bar itself to receive stimulation. Number of responses were recorded at the end of the 20 rain session. Animals were run for at least 14 consecutive days. Any animal that had not demonstrated serf-stimulation by that time was eliminated from the experiment. The predetermined criterion used to establish the presence of selfstimulation was at least 100 responses in 20 min. This figure was based on previous experiments which indicated that, having once attained a score of 100, no rat would fall below that score on successive tests. All animals received a minimum of five days of testing after reaching this criterion, in order to determine how response rates changed after an animal learned to self-stimulate. Following completion o f this phase, we tested to see if serf-stimulation could be obtained from the SC electrodes in Groups II and III and from the L H electrodes in Group IV. Testing took place in the same chambers used in phase II and consisted of between three and five 20 min sessions at the same programmed stimulation parameters used in the stimulation phase.
Of the 48 rats who started the experiment, seven were dropped because they failed to reach criterion in the allotted time. Additionally, six rats from Groups II and III (ipsilaterai and contralaterai SC programmed stimulation) were dropped because, as explained above, their electrodes were not located in the SC. These six animals were treated as a separate group. This left twelve rats in the control Group I, nine in the ipsilateral SC Group II, seven in the contralateral SC Group III and seven in the LH Group IV. The data were analyzed in terms of the first day on which any rat self-stimulated the MC at least 100 times in 20 min (Fig. 2). Distribution-free statistics were used on these data. There was a significant difference amongst the groups. Control rats who received no programmed stimulation took an average of 6.0 (_+S.E.M. 0.9) days to attain criterion. Prestimulation delivered to the ipsilateral SC electrode for seven days (Group II) caused a significant decrease (U=24.5, p =0.027) in the number of days to criterion: rats took, on the average, 3.2_+ 1.1 days. Group III, which received contralateral SC stimulation, similarly took an average of 2.6_+0.6 days to reach criterion, again significantly lower than the control group ( U = 10, p =0.008). In contrast, group IV, who received L H programmed stimulation, took almost the same number of days to reach criterion as control rats (6.1 _+1.4). The six rats from Groups II and IlI, whose SC electrodes landed outside the SC, did not differ from control rats, taking 6.3 (_+ 1.6) days to reach criterion. In addition to analyzing the data in terms of days to criterion, we also analyzed response rates in each group for the first five days after criterion was attained (analysis of variance followed by Newman-Keuls tests). All groups showed a significant increase in responding from the first to the second day (p<0.01), increasing from an overall average of 197 responses/20 rain to 271 responses/20 min. From the second to fifth days, responding was stable. There was also a significant (p<0.05) tendency for Group IV (LH) rats to respond at lower levels than any of the other three groups (at an average of 190 responses over the five days of testing, compared to the Group I (control) rats who attained the highest average rate of 314 responses over the five days). However, there were no differences in the rate of increases in responding between this and any of the other groups. Additionally, these rats, tested after a two week period, were not significantly different from other g r o u p s - - t h e y had attained the same asymptotic rate of responding.
872
ROBERTSON, I,AFERRI~Rti AND MII~NI'~b~
Animals in Groups 1I, II1 and IV were also tested to see if self-stimulation could be obtained from the SC and LH electrodes used in the programmed stimulation phase. All rats with LH electrodes and 94% of the rats with SC electrodes did self-stimulate at mean response rates of 969 (-+84) responses and 189 (-+ 17) responses/20 min respectively at the same current values used in the programmed stimulation phase. In contrast, of the 6 rats in Groups II and llI whose electrodes landed outside the SC, only one rat selfstimulated, at an average rate of 105 responses/20 rain. DISCUSSION Animals implanted with MC electrodes normally take several days to learn to self-stimulate, in contrast to the relatively quick acquisition of self-stimulation seen with many other self-stimulation sites [6]. A period of programmed stimulation delivered through the MC electrode over seven days produces a marked facilitation of acquisition [3]. The present report demonstrates that the same facilitation can be achieved (using the same parameters of programmed stimulation) by stimulating the SC either ipsilateral or contralateral to the self-stimulation electrode. Thus, not only is the SC critical in the maintenance of serf-stimulation of the MC (Corbett, Laferfi~re and Milner, submitted) but the SC may be involved in the acquisition of the rewarding effects of stimulation of the MC. These data suggest that there is a functional link between the SC and MC. The observation that both ipsilateral and contralateral stimulation were equally effective may not be surprising: bilateral destruction of the fibre tract connecting the SC and MC is necessary to eliminate serf-stimulation of the MC, suggesting that either of the two sides of the SC can maintain self-stimulation of this area. These observations are in line with anatomical studies demonstrating a bilateral projection from the MC to the SC [1]. One might hypothesize that electrical stimulation of the SC simply spreads to the MC and thus affects acquisition of self-stimulation of the MC. This seems unlikely because electrode placements from six animals in Groups II and lII which missed the SC and terminated between the MC and SC were not associated with a faster acquisition of selfstimulation. These six rats took much the same length of time to reach criterion as control rats in Group I.
Programmed stimulation in the area of ~he LIt had n~, effect on acquisition of self stimulation of the MC. Thus lhc LH may not be involved in the development of positive rein forcement in the MC. It is interesting to note that, allhough a link between the two sites has been proposed [ 13,141 ii would not appear to be essential for the maintenance of selfstimulation, as large lesions of the LH medial t\~rebrain bundle area do not affect self-stimulation of the ipsilateral MC [2]. The fact that programmed stimulation of the LH failed to affect acquisition of self-stimulation of the MC lends some specificity to the effectiveness of the SC stimulation in doing so. It would appear that neither is the facilitatory effect due to simply a general activation or arousal from brain stimulation nor is it caused by stimulation of any MC afferents or efferents. This is not to suggest, however, that other PFC projections might not be involved in the acquisition of MC self-stimulation. Additionally, the observation that stimulation of the SC is sufficient to facilitate MC self'stimulation does not imply its necessity. The present data simply indicate that the positively reinforcing effects of MC stimulation can be brought about by pre-training stimulation of a site for which both anatomical connections and functional relations to the MC have been demonstrated. In light of these observations, it is not surprising that programmed stimulation of the SC can facilitate not only acquisition of self-stimulation of the MC but also selfstimulation of the SC itself. We have observed that rats receiving programmed stimulation of the SC at the same parameters that were used in the present experiment took an average of 1.3(-+0.8) days to attain criterion during selfstimulation tests. In contrast, animals which did not receive such stimulation took 7.9 (-+ 1.8) days to self-stimulate the SC. There is little evidence at the present time concerning the nature of the anatomy of the proposed MC-SC link in the development of self-stimulation of the MC. One possibility presently under investigation is that repeated stimulation gradually causes a permanent potentiation of neural activity. similar to that proposed to underlie the kindling effect (e.g., [12]). If so, it may be the medial-sulcal connection which is involved in effecting the critical change underlying the development of positive reinforcement in the Mr'.
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