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Stability of intracranial self-stimulation following hypophysectomy
Physiology and Behavior, Vol. 10, pp. 351-353, Brain Research Publications Inc., 1973. Printed in U.S.A.
Stability of Intracranial Self-Stimulation Following Hypophysectomy ANTHONY G. PHILLIPS AND MAURICE SHAPIRO
Department o f Psychology, University of British Columbia, Vancouver, Canada (Received 24 July 1972) PHILLIPS, A. G. AND M. SHAPIRO. Stability of intracranial self-stimulation following hypophysectomy. PHYSIOL. BEHAV. 10(2) 351-353, 1973.- The possibility of a hypothalamo-pituitary involvement in the intracranial self-stimulation phenomenon was investigated. Threshold and optimal stimulation currents were identified by means of the method of limits, in rats with bipolar electrodes implanted in the lateral hypothalamus. Total or sham hypophysectomies were performed after the establishment of these current values, and found to have no effect on reinforcing brain stimulation. It was concluded that a hypothalamo-pituitary mechanism does not subserve reinforcing brain stimulation. Intracranial self-stimulation
Positive reinforcement
Lateral hypothalamus
Pituitary
Hormones
METHOD
ELECTRICAL stimulation of many subcortical structures will serve to reinforce a wide variety of responses such as lever pressing [ 11 ], runway activity [25], and immobility [13], in species ranging from goldfish [1] to man [7]. Although it has long been maintained that this phenomenon of self-stimulation provides an appropriate model for studying the neural basis of positive reinforcement [9, 10, 24], the recent findings that reinforcing brain stimulation has much in common with natural rewards [2, 4, 16, 17, 18, 22] has added credence to this assumption. As to the way in which activity in these subcortical substrates of reinforcement could serve to reinforce behaviour, Olds and Olds [ 12] have hypothesized the release of neurohumoral factors capable of influencing the neurophysiological characteristics of those neurons subserving the behavioural response leading to activity in the reward systems. Pfaff [15] has rearticulated this position by proposing a simple mechanism of reinforcement which causes the release of an activating substance into the bloodstream which in turn would influence recently activated neurons during initial learning and maintain high levels of performance of well established responses. In view of the intimate relationship between the pituitary gland and those diencephalic structures from which self-stimulation can be obtained [5, 6, 19, 21], together with the known effects of hormones on selfstimulation [2,8], a hypothalamo-pituitary system would appear to be a prime candidate for the proposed reinforcement mechanism. Consequently, the following experiment was designed to examine the effects of hypophysectomy on stable self-stimulation behaviour in the rat.
Animals Hooded rats of the Royal Victoria strain (obtained from the Quebec Breeding Farms) weighing 3 0 0 - 3 5 0 g at the time of surgery, were housed individually in stainless steel cages, with food and water available ad lib. Colony lighting was controlled on a 12 hr light/dark cycle.
Surgery and Histology Twisted bipolar nichrome electrodes (Plastic Products, Co. MS 3 0 3 - 0 . 0 1 ) were implanted into the lateral hypothalamus at the level of the fornix (Bregma - - 2 . 0 , lateral 1.7, ventral from dura - 8.6) in accordance with standard stereotaxic procedure [20]. A seven day post-operative recovery period intervened between surgery and experimental testing. Following the elicitation and stabilization of self-stimulation behaviour, the animals were prepared for complete or sham hypophysectomy. All animals were anesthetized with sodium pentobarbital (50 mg/kg IP) and a 19 gauge needle was positioned under the hypophysis in accordance with the procedure for transauricular hypophysectomy in rats [3]. Hypophysectomy was accomplished by vacuum suction in four animals. The vacuum was not connected in the four remaining control animals leaving the hypophysis intact. Upon completion of experimental testing, all animals were sacrificed with ether and their brains removed for microscopic examination of the sella turcica. Effects of hypophysectomy were also assessed by comparing the
1This research was supported by research grant No. APA-7808 from the National Research Council of Canada. The authors gratefully acknowledge the histological assistance of Mr. Bryan MacNeill. 351
352
Pllll_i,lP% :\NI) SIIAPtR()
weights of adrenals and testes in experimental and control animals. All brains were stored in buffered formalin, before embedding in paraffin. The brains were sectioned at 20 **, stained with luxol fast blue and counterstained with thionin.
Apparatus All animals were screened for self-stimulation in a shuttle box [25] (80 cm long x 25 cm wide x 35 cm high) constructed from sheet metal, with a Plexiglas front wall. A 60 cycle a.c. sine wave constant current stimulator was activated via p h o t o sensitive relays m o u n t e d in the walls of the shuttle box 15 cm from each end wall, and 4 cm above the grid floor. When the animal was attached to the stimulator via a flexible cable (Plastic Products Co.) it could control the stimulus duration by first breaking a photobeam at one end of the shuttle box to initiate stimulation then crossing through the second set of photocells to terminate it. The n u m b e r of stimulations and the duration of the stimulus was recorded for each test session.
stimulation rate, h y p o p h y s e c t o m y appeared to enhance the crossing rate at optimal intensities for animals 1.1t-51 and LH-53. Although the mean response rate f~r the h y p o p h y s e c t o m i z e d group increased from :J preoperative score of X=51 crosses/5 rain to a postoperative level of X=61 crosses in 5 min at optimal current levels, neither this nor any o t h e r comparison of pre- and postoperative response rates proved to be significant (1:(2,(,)=1.5330, p > 0 . 2 5 ) . Data from only three of the four hypophysectomized animals were obtained during the last two days ot testing, as LH-42 died on postoperative Day S It is (~f interest to note that this animal displayed high rates of self-stimulation (i.e., optimal=77 crosses/5 r a i n ) j u s t a few hours prior to succumbing to the effects of the ~peration.
~v~x
Procedure °
Animals were initially screened for self-stimulation on three consecutive days. Most animals began shuttling back and forth after several priming stimuli and those animals which failed to exceed their operant crossing rate by more than 100% in a 10 min trial, after 3 days of training, were rejected. Animals identified as self-stimulators were then given seven days training at an intensity which maintained consistent crossing in the shuttle box (Y~=32 uA rms). Each daily test consisted of a 5 min operant period with no stimulation, and a 10 rain stimulation period. The animals were then tested for stability, i.e., less than 20% variation in crossing rate on three consecutive days. Eight animals exhibited stable runway performance, and were retained for the main experiment. On the f o u r days prior to surgery, threshold and o p t i m u m stimulus intensity levels were established using an ascending and descending m e t h o d of limits presented in a counterbalanced order. Each ascending test session started with the stimulus intensity at 0 uA for a 5 min operant measure. The intensity was then raised in 2 uA steps, every 5 min until the crossing rate was 50% greater than the operant rate. The intensity that resulted in the initial 50% increase in crossing, was defined as threshold intensity. After defining threshold, the current was raised 5 uA every 5 min until the crossing rate reached a s y m p t o t e (i.e., less than 10% variation over three consecutive increments), declined by 20%, or the animal tried to j u m p from the apparatus. The intensity at which the rate reached a s y m p t o t e was defined as the optimal intensity. On the descending trials, the session was again started with a 5 min operant period and the intensity during the 5 min stimulation periods descended in 5 #A steps, from the optimal level established on the ascending trials. The animals were tested in this manner on the four days prior to h y p o p h y s e c t o m y , on the four days following surgery, and final ascending and descending trials were given on postoperative days seven and eight.
•
tURRErCr ~¢r~SfTY tea
FIG. 1. Mean crossing rates as a function of stimulus intensity on preoperative Days 1 4 (Pre-op), postoperative Days 1-4 (Post-op 1), and postoperative Days 7 - 8 (Post-op 2), for each hypophysectomized (upper panels) and control animal (lower panels). Scores depicted are for operant level, threshold, and 10 uA increments above threshold.
Aa~
RESULTS
The data depicted in Fig. 1 illustrate that b o t h h y p o p h y s e c t o m y and sham h y p o p h y s e c t o m y failed to disrupt intracranial self-stimulation. Rather than attenuate self-
FIG. 2. Electrode placements of four hypophysectomized (closed circle) and two control animals (open circle), on coronal sections taken from Pellegrino and Cushman [ 14 ].
INTRACRANIAL SELF-STIMULATION
353
P o s t m o r t e m e x a m i n a t i o n of the sella turcica revealed no trace of hypophyseal tissue in the experimental animals, and intact h y p o p h y s e s in all sham operated animals. Histological e x a m i n a t i o n showed the electrode tracts terminating b o t h dorso- and ventrolateral to the fornix in the region of the lateral h y p o t h a l a m u s b e t w e e n anterior planes 3.8 and 4.8 [14]. These placements are shown in Fig. 2. DISCUSSION F r o m the results of this e x p e r i m e n t and the recent observation of unaffected resting discharge patterns of h y p o t h a l a m i c single units up to 6 days after hypophysect o m y [23] it is d o u b t f u l that h y p o p h y s e c t o m y results in an i m m e d i a t e functional disturbance of the hypothalamus. F r o m this we may conclude that the positive r e i n f o r c e m e n t obtained from electrical stimulation of the lateral hypothalamus is not subserved by a h y p o t h a l a m o - p i t u i t a r y mechanism. This statement may also be generalized to electrical stimulation of the ventral tegmental area, as we have observed that h y p o p h y s e c t o m y has no effect on selfstimulation in this region of the brainstem (Phillips and Shapiro, unpublished observations). The mechanism involved in positive reinforcement, if n e u r o h u m o r a l at all, must involve structures that are endogenous to the brain. One reason for suspecting pituitary involvement in the
self-stimulation p h e n o m e n o n was the k n o w n effects of h o r m o n e s on this behaviour. Olds [8] and Campbell [2] have shown castration to be accompanied by a permanent decline in self-stimulation rates in rat and rabbit respectively, and the reinstatement of normal response rates after testosterone proprionate replacement therapy. A n d r o g e n injections also p r o d u c e d an i m m e d i a t e increase in response rate in intact male and female rabbits while estrogen injections antagonized self-stimulation behaviour [2]. In a t t e m p t i n g to define the principal active substances Campbell [2] first d e m o n s t r a t e d the facilitatory effects of luteinizing h o r m o n e (LH) injections on self-stimulation in ovariectomized rabbits, thus explicitly raising the possibility of anterior pituitary h o r m o n e participation. A further refinement involved duplicating the effects seen with LH with injections of luteinizing h o r m o n e releasing factor ( L R F ) , a substance of h y p o t h a l a m i c origin that causes the release of LH from the anterior pituitary gland. On the basis of these results it was uncertain w h e t h e r the increase in self-stimulation resulted from the secondary release of LH from the anterior pituitary or the direct influence of L R F on the neurons subserving self-stimulation. The stability of self-stimulation following h y p o p h y s e c t o m y supports the role of the h y p o t h a l a m i c releasing factor and raises the possibility of its involvement in positive reinforcement.
REFERENCES 1. Boyd, E., and L. Gardner. Positive and negative reinforcement from intracranial self-stimulation in teleosts. Science 136: 648-649, 1962. 2. Campbell, H. J. The effect of steroid hormones on self-stimulation, central and peripheral. Steroidologia 1 : 8 - 2 4 , 1970. 3. Falconi, G., and G. I. Rossi. Transauricular hypophysectomy in rats and mice. Endocrinology 74: 301-303, 1964. 4. Gibson, W. E., L. D. Reid, M. Sakai and F. B. Porter. |ntracranial reinforcement compared with sugar water reinforcement. Science 148: 1357-1359, 1965. 5. Green, J. D. Neural pathways to the hypophysis: Anatomical and Functional. In: The Hypothalamus, edited by W. Haymaker, E. Anderson and W. J. H. Nauta. Springfield: C. C. Thomas, 1969. 6. Haymaker, W. Hypothalamo-pituitary neural pathways and the circulatory system of the pituitary. In: The Hypothalamus, edited by W. Haymaker, E. Anderson and W. J. H. Nauta. Springfield: C. C. Thomas, 1969. 7. Heath, R. G. Pleasure responses of human subjects to direct stimulation of the brain: Physiologic and psychodynamic considerations. In: The Role of Pleasure in Behavior - A Symposium by Twenty-two Authors, edited by R. G. Heath, New York: Harper and Row, 1964. 8. Olds, J. Effects of hunger and male sex hormone on selfstimulation of the brain. J. comp. physiol. Psychol. 51: 320-324, 1958. 9. Olds, J. Hypothalamic substrates of reward. Physiol. Rev. 42: 554-604, 1962. 10. Olds, J. The central nervous system and the reinforcement of behavior. Am. Psychol. 24: 114-132, 1969. 11. Olds, J. and P. M. Milner. Positive reinforcement produced by electrical stimulation of septal area and other regions of rat brain. J. comp. physiol. Psychol. 47: 419-427, 1954. 12. Olds, J. and M. E. Olds. Drives, rewards and the brain. In: New Directions in Psychology. Vol. 2. New York: Holt, 1965. 13. Paxinos, G. and D. Bindra. Rewarding intracranial stimulation, movement and the hippocampal theta rhythm. Physiol. Behav. 5: 227-231, 1970.
14. PeUegrino, L. J. and A. J. Cushman. A Stereotaxic Atlas o f the Rat Brain. New York: Appleton-Centruy-Crofts, 1967. 15. Pfaff, D. W. Biological models of memory and reinforcement. Psychol. Rev. 71: 70-81, 1969. 16. Phillips, A. G. Enhancement and inhibition of olfactory bulb self-stimulation by odours. Physiol. Behav. 5: 1127-1131, 1970. 17. Phillips, A. G. and G. J. Mogenson. Effects of taste on self-stimulation and induced drinking. J. comp. physiol. Psychol. 66: 654-660, 1968. 18. Phillips, A. G., C. W. Morgan and G. J. Mogenson. Changes in self-stimulation preference as a function of incentive of alternative rewards. Can. J. Psychol. 24: 289-297, 1970. 19. Raisman, G. and P. M. Field. Anatomical considerations relevant to the interpretation of neuroendocrine experiments. In: Frontiers in Neuroendocrinology. edited by L. Martini and W. F. Ganong. London: Oxford University Press, 1971. 20. Skinner, J. E. Neuroscience: A Laboratory Manual. Philadelphia, Pa.: W. G. Saunders, Co., 1971. 21. Stutinsky, F. S. Hypothalamic .neurosecretion. In: The Hypothalamus, edited by L. Martini, M. Motta and F. Fraschini. New York: Academic Press, 1970. 22. Trowill, J. A., J. Panksepp and R. Gandelman. An incentive model of rewarding brain stimulation. Psychol. Rev. 76: 264-281, 1969. 23. van Delft, A. M. L. and J. I. Kitay. Effect of ACTH on single unit activity in the diencephalon of intact and hypophysectomized rats. Neuroendocrinology 9: 188-196, 1972. 24. Valenstein, E. S. The anatomical locus of reinforcement. In: Progress in Physiological Psychology. Vol. 1. edited by E. Stellar and J. J. Sprague, New York: Academic Press, 1966. 25. Valenstein, E. S. and W. J. Meyers. Rate independent test of reinforcing consequences of brain stimulation. J. comp. physiol. Psychol. 57: 52--60, 1964.
Stability of Intracranial Self-Stimulation Following Hypophysectomy ANTHONY G. PHILLIPS AND MAURICE SHAPIRO
Department o f Psychology, University of British Columbia, Vancouver, Canada (Received 24 July 1972) PHILLIPS, A. G. AND M. SHAPIRO. Stability of intracranial self-stimulation following hypophysectomy. PHYSIOL. BEHAV. 10(2) 351-353, 1973.- The possibility of a hypothalamo-pituitary involvement in the intracranial self-stimulation phenomenon was investigated. Threshold and optimal stimulation currents were identified by means of the method of limits, in rats with bipolar electrodes implanted in the lateral hypothalamus. Total or sham hypophysectomies were performed after the establishment of these current values, and found to have no effect on reinforcing brain stimulation. It was concluded that a hypothalamo-pituitary mechanism does not subserve reinforcing brain stimulation. Intracranial self-stimulation
Positive reinforcement
Lateral hypothalamus
Pituitary
Hormones
METHOD
ELECTRICAL stimulation of many subcortical structures will serve to reinforce a wide variety of responses such as lever pressing [ 11 ], runway activity [25], and immobility [13], in species ranging from goldfish [1] to man [7]. Although it has long been maintained that this phenomenon of self-stimulation provides an appropriate model for studying the neural basis of positive reinforcement [9, 10, 24], the recent findings that reinforcing brain stimulation has much in common with natural rewards [2, 4, 16, 17, 18, 22] has added credence to this assumption. As to the way in which activity in these subcortical substrates of reinforcement could serve to reinforce behaviour, Olds and Olds [ 12] have hypothesized the release of neurohumoral factors capable of influencing the neurophysiological characteristics of those neurons subserving the behavioural response leading to activity in the reward systems. Pfaff [15] has rearticulated this position by proposing a simple mechanism of reinforcement which causes the release of an activating substance into the bloodstream which in turn would influence recently activated neurons during initial learning and maintain high levels of performance of well established responses. In view of the intimate relationship between the pituitary gland and those diencephalic structures from which self-stimulation can be obtained [5, 6, 19, 21], together with the known effects of hormones on selfstimulation [2,8], a hypothalamo-pituitary system would appear to be a prime candidate for the proposed reinforcement mechanism. Consequently, the following experiment was designed to examine the effects of hypophysectomy on stable self-stimulation behaviour in the rat.
Animals Hooded rats of the Royal Victoria strain (obtained from the Quebec Breeding Farms) weighing 3 0 0 - 3 5 0 g at the time of surgery, were housed individually in stainless steel cages, with food and water available ad lib. Colony lighting was controlled on a 12 hr light/dark cycle.
Surgery and Histology Twisted bipolar nichrome electrodes (Plastic Products, Co. MS 3 0 3 - 0 . 0 1 ) were implanted into the lateral hypothalamus at the level of the fornix (Bregma - - 2 . 0 , lateral 1.7, ventral from dura - 8.6) in accordance with standard stereotaxic procedure [20]. A seven day post-operative recovery period intervened between surgery and experimental testing. Following the elicitation and stabilization of self-stimulation behaviour, the animals were prepared for complete or sham hypophysectomy. All animals were anesthetized with sodium pentobarbital (50 mg/kg IP) and a 19 gauge needle was positioned under the hypophysis in accordance with the procedure for transauricular hypophysectomy in rats [3]. Hypophysectomy was accomplished by vacuum suction in four animals. The vacuum was not connected in the four remaining control animals leaving the hypophysis intact. Upon completion of experimental testing, all animals were sacrificed with ether and their brains removed for microscopic examination of the sella turcica. Effects of hypophysectomy were also assessed by comparing the
1This research was supported by research grant No. APA-7808 from the National Research Council of Canada. The authors gratefully acknowledge the histological assistance of Mr. Bryan MacNeill. 351
352
Pllll_i,lP% :\NI) SIIAPtR()
weights of adrenals and testes in experimental and control animals. All brains were stored in buffered formalin, before embedding in paraffin. The brains were sectioned at 20 **, stained with luxol fast blue and counterstained with thionin.
Apparatus All animals were screened for self-stimulation in a shuttle box [25] (80 cm long x 25 cm wide x 35 cm high) constructed from sheet metal, with a Plexiglas front wall. A 60 cycle a.c. sine wave constant current stimulator was activated via p h o t o sensitive relays m o u n t e d in the walls of the shuttle box 15 cm from each end wall, and 4 cm above the grid floor. When the animal was attached to the stimulator via a flexible cable (Plastic Products Co.) it could control the stimulus duration by first breaking a photobeam at one end of the shuttle box to initiate stimulation then crossing through the second set of photocells to terminate it. The n u m b e r of stimulations and the duration of the stimulus was recorded for each test session.
stimulation rate, h y p o p h y s e c t o m y appeared to enhance the crossing rate at optimal intensities for animals 1.1t-51 and LH-53. Although the mean response rate f~r the h y p o p h y s e c t o m i z e d group increased from :J preoperative score of X=51 crosses/5 rain to a postoperative level of X=61 crosses in 5 min at optimal current levels, neither this nor any o t h e r comparison of pre- and postoperative response rates proved to be significant (1:(2,(,)=1.5330, p > 0 . 2 5 ) . Data from only three of the four hypophysectomized animals were obtained during the last two days ot testing, as LH-42 died on postoperative Day S It is (~f interest to note that this animal displayed high rates of self-stimulation (i.e., optimal=77 crosses/5 r a i n ) j u s t a few hours prior to succumbing to the effects of the ~peration.
~v~x
Procedure °
Animals were initially screened for self-stimulation on three consecutive days. Most animals began shuttling back and forth after several priming stimuli and those animals which failed to exceed their operant crossing rate by more than 100% in a 10 min trial, after 3 days of training, were rejected. Animals identified as self-stimulators were then given seven days training at an intensity which maintained consistent crossing in the shuttle box (Y~=32 uA rms). Each daily test consisted of a 5 min operant period with no stimulation, and a 10 rain stimulation period. The animals were then tested for stability, i.e., less than 20% variation in crossing rate on three consecutive days. Eight animals exhibited stable runway performance, and were retained for the main experiment. On the f o u r days prior to surgery, threshold and o p t i m u m stimulus intensity levels were established using an ascending and descending m e t h o d of limits presented in a counterbalanced order. Each ascending test session started with the stimulus intensity at 0 uA for a 5 min operant measure. The intensity was then raised in 2 uA steps, every 5 min until the crossing rate was 50% greater than the operant rate. The intensity that resulted in the initial 50% increase in crossing, was defined as threshold intensity. After defining threshold, the current was raised 5 uA every 5 min until the crossing rate reached a s y m p t o t e (i.e., less than 10% variation over three consecutive increments), declined by 20%, or the animal tried to j u m p from the apparatus. The intensity at which the rate reached a s y m p t o t e was defined as the optimal intensity. On the descending trials, the session was again started with a 5 min operant period and the intensity during the 5 min stimulation periods descended in 5 #A steps, from the optimal level established on the ascending trials. The animals were tested in this manner on the four days prior to h y p o p h y s e c t o m y , on the four days following surgery, and final ascending and descending trials were given on postoperative days seven and eight.
•
tURRErCr ~¢r~SfTY tea
FIG. 1. Mean crossing rates as a function of stimulus intensity on preoperative Days 1 4 (Pre-op), postoperative Days 1-4 (Post-op 1), and postoperative Days 7 - 8 (Post-op 2), for each hypophysectomized (upper panels) and control animal (lower panels). Scores depicted are for operant level, threshold, and 10 uA increments above threshold.
Aa~
RESULTS
The data depicted in Fig. 1 illustrate that b o t h h y p o p h y s e c t o m y and sham h y p o p h y s e c t o m y failed to disrupt intracranial self-stimulation. Rather than attenuate self-
FIG. 2. Electrode placements of four hypophysectomized (closed circle) and two control animals (open circle), on coronal sections taken from Pellegrino and Cushman [ 14 ].
INTRACRANIAL SELF-STIMULATION
353
P o s t m o r t e m e x a m i n a t i o n of the sella turcica revealed no trace of hypophyseal tissue in the experimental animals, and intact h y p o p h y s e s in all sham operated animals. Histological e x a m i n a t i o n showed the electrode tracts terminating b o t h dorso- and ventrolateral to the fornix in the region of the lateral h y p o t h a l a m u s b e t w e e n anterior planes 3.8 and 4.8 [14]. These placements are shown in Fig. 2. DISCUSSION F r o m the results of this e x p e r i m e n t and the recent observation of unaffected resting discharge patterns of h y p o t h a l a m i c single units up to 6 days after hypophysect o m y [23] it is d o u b t f u l that h y p o p h y s e c t o m y results in an i m m e d i a t e functional disturbance of the hypothalamus. F r o m this we may conclude that the positive r e i n f o r c e m e n t obtained from electrical stimulation of the lateral hypothalamus is not subserved by a h y p o t h a l a m o - p i t u i t a r y mechanism. This statement may also be generalized to electrical stimulation of the ventral tegmental area, as we have observed that h y p o p h y s e c t o m y has no effect on selfstimulation in this region of the brainstem (Phillips and Shapiro, unpublished observations). The mechanism involved in positive reinforcement, if n e u r o h u m o r a l at all, must involve structures that are endogenous to the brain. One reason for suspecting pituitary involvement in the
self-stimulation p h e n o m e n o n was the k n o w n effects of h o r m o n e s on this behaviour. Olds [8] and Campbell [2] have shown castration to be accompanied by a permanent decline in self-stimulation rates in rat and rabbit respectively, and the reinstatement of normal response rates after testosterone proprionate replacement therapy. A n d r o g e n injections also p r o d u c e d an i m m e d i a t e increase in response rate in intact male and female rabbits while estrogen injections antagonized self-stimulation behaviour [2]. In a t t e m p t i n g to define the principal active substances Campbell [2] first d e m o n s t r a t e d the facilitatory effects of luteinizing h o r m o n e (LH) injections on self-stimulation in ovariectomized rabbits, thus explicitly raising the possibility of anterior pituitary h o r m o n e participation. A further refinement involved duplicating the effects seen with LH with injections of luteinizing h o r m o n e releasing factor ( L R F ) , a substance of h y p o t h a l a m i c origin that causes the release of LH from the anterior pituitary gland. On the basis of these results it was uncertain w h e t h e r the increase in self-stimulation resulted from the secondary release of LH from the anterior pituitary or the direct influence of L R F on the neurons subserving self-stimulation. The stability of self-stimulation following h y p o p h y s e c t o m y supports the role of the h y p o t h a l a m i c releasing factor and raises the possibility of its involvement in positive reinforcement.
REFERENCES 1. Boyd, E., and L. Gardner. Positive and negative reinforcement from intracranial self-stimulation in teleosts. Science 136: 648-649, 1962. 2. Campbell, H. J. The effect of steroid hormones on self-stimulation, central and peripheral. Steroidologia 1 : 8 - 2 4 , 1970. 3. Falconi, G., and G. I. Rossi. Transauricular hypophysectomy in rats and mice. Endocrinology 74: 301-303, 1964. 4. Gibson, W. E., L. D. Reid, M. Sakai and F. B. Porter. |ntracranial reinforcement compared with sugar water reinforcement. Science 148: 1357-1359, 1965. 5. Green, J. D. Neural pathways to the hypophysis: Anatomical and Functional. In: The Hypothalamus, edited by W. Haymaker, E. Anderson and W. J. H. Nauta. Springfield: C. C. Thomas, 1969. 6. Haymaker, W. Hypothalamo-pituitary neural pathways and the circulatory system of the pituitary. In: The Hypothalamus, edited by W. Haymaker, E. Anderson and W. J. H. Nauta. Springfield: C. C. Thomas, 1969. 7. Heath, R. G. Pleasure responses of human subjects to direct stimulation of the brain: Physiologic and psychodynamic considerations. In: The Role of Pleasure in Behavior - A Symposium by Twenty-two Authors, edited by R. G. Heath, New York: Harper and Row, 1964. 8. Olds, J. Effects of hunger and male sex hormone on selfstimulation of the brain. J. comp. physiol. Psychol. 51: 320-324, 1958. 9. Olds, J. Hypothalamic substrates of reward. Physiol. Rev. 42: 554-604, 1962. 10. Olds, J. The central nervous system and the reinforcement of behavior. Am. Psychol. 24: 114-132, 1969. 11. Olds, J. and P. M. Milner. Positive reinforcement produced by electrical stimulation of septal area and other regions of rat brain. J. comp. physiol. Psychol. 47: 419-427, 1954. 12. Olds, J. and M. E. Olds. Drives, rewards and the brain. In: New Directions in Psychology. Vol. 2. New York: Holt, 1965. 13. Paxinos, G. and D. Bindra. Rewarding intracranial stimulation, movement and the hippocampal theta rhythm. Physiol. Behav. 5: 227-231, 1970.
14. PeUegrino, L. J. and A. J. Cushman. A Stereotaxic Atlas o f the Rat Brain. New York: Appleton-Centruy-Crofts, 1967. 15. Pfaff, D. W. Biological models of memory and reinforcement. Psychol. Rev. 71: 70-81, 1969. 16. Phillips, A. G. Enhancement and inhibition of olfactory bulb self-stimulation by odours. Physiol. Behav. 5: 1127-1131, 1970. 17. Phillips, A. G. and G. J. Mogenson. Effects of taste on self-stimulation and induced drinking. J. comp. physiol. Psychol. 66: 654-660, 1968. 18. Phillips, A. G., C. W. Morgan and G. J. Mogenson. Changes in self-stimulation preference as a function of incentive of alternative rewards. Can. J. Psychol. 24: 289-297, 1970. 19. Raisman, G. and P. M. Field. Anatomical considerations relevant to the interpretation of neuroendocrine experiments. In: Frontiers in Neuroendocrinology. edited by L. Martini and W. F. Ganong. London: Oxford University Press, 1971. 20. Skinner, J. E. Neuroscience: A Laboratory Manual. Philadelphia, Pa.: W. G. Saunders, Co., 1971. 21. Stutinsky, F. S. Hypothalamic .neurosecretion. In: The Hypothalamus, edited by L. Martini, M. Motta and F. Fraschini. New York: Academic Press, 1970. 22. Trowill, J. A., J. Panksepp and R. Gandelman. An incentive model of rewarding brain stimulation. Psychol. Rev. 76: 264-281, 1969. 23. van Delft, A. M. L. and J. I. Kitay. Effect of ACTH on single unit activity in the diencephalon of intact and hypophysectomized rats. Neuroendocrinology 9: 188-196, 1972. 24. Valenstein, E. S. The anatomical locus of reinforcement. In: Progress in Physiological Psychology. Vol. 1. edited by E. Stellar and J. J. Sprague, New York: Academic Press, 1966. 25. Valenstein, E. S. and W. J. Meyers. Rate independent test of reinforcing consequences of brain stimulation. J. comp. physiol. Psychol. 57: 52--60, 1964.