Scand. J . Psychol., 1975, 16, 7-10
Self-stimulation and mating behavior in the male rat SVEN G. CARLSSON KNUT LARSSON
Absrract.-A presumed relationship between selfstimulation of the brain and sexual behavior was studied in two experiments. In the first experiment the male rat was allowed self-stimulation during a period of 5 minutes and thereafter transferred to a mating cage with a female. NO effects of the self-stimulation were recorded in the mating behavior. In the second experiment the male was allowed one intromission and thereafter presented with a lever for self-stimulation, still having access t o the female. The sexual responses were completely inhibited, and selfstimulation at normal rate resumed. It was concluded that there is no necessary link between self-stimulation of the brain and sexual behavior.
A relationship between responses evoked by selfstimulation of the brain and sexual behavior has often been speculated upon. The frequent occurrence of seminal emissions during self-stimulation was early noted (Miller, 1957) and there is an apparent resemblance between the highly goal-directed activity exhibited by the rat during copulation and the vigorous pedal-pulling shown in a selfstimulation test. Experimental evidence for such a relationship was reported by Olds (1958), who observed an androgen dependency of self-stimulation in rats. Herberg (1963) confirmed this observation stimulating in a medial forebrain area. This study represents a n attempt t o explore, further, the relationship between self-stimulation and sexual behavior. Two experiments were conducted. In the first experiment the male was allowed to perform self-stimulation for a specified period of time and thereafter presented with a receptive female. In the second experiment, the male rat was allowed to start copulatory behavior, whereupon it was presented with a lever for self-stimulation. METHOD Subjecrs In experiment 1 participated nine and in experiment 2 six male rats, all from a locally bred strain of albinos aged
University of Goteborg, Sweden
approximately 6 months. They were selected on the basis of their performance in preliminary tests for showing a consistently high level of sexual behavior.
Mating tests The technique used in observing and recording the mating behavior was the same a s has been described in detail elsewhere (Larsson, 1963). During each test the following items were measured; ( a )mounting frequency-number of mounts without intromission; (b) intromission frequency-number of mounts with intromission; ( c )intromission latency-time in minutes from the entrance of the female into the mating cage until the first intromission; (d) ejaculation latency-time in minutes from the first intromission until ejaculation; ( e ) postejaculation intervaltime in minutes from an ejaculation to the next intromission. Females were brought into estrus by injections of estrogen and progesterone according to a schedule described previously (Larsson, 1963).
Self-stimulation technique Elecirodes and operation technique. Bipolar electrodes were used, They were prepared of dental steelsuture (0.20 mm). The threads were straightened, insulated and glued together. Suitable lengths were cut leaving only the crosssection of the tips uninsulated. The electrode was fixed in a stereotactic manipulator and lowered into the medial forebrain bundle-lateral hypothalamic area. A typical electrode placement is shown in Fig. 1. Barbiturate anesthesia was used. While fixed in the manipulator, the electrode was cemented to the skull and to three anchoring screws with dental acrylic, which was carefully applied with dental fillingtubes. Stimulation parameters and testing. A stimulator, built according to the description of Petrohilos, Simpson, Welfod and Valenstein, was used. It generates bipolar square pulses with a frequency of 100 pulses pairs per sec, each pulse of 0.2 m sec duration and with 0.2 m sec delay between negative and positive pulses. The current was fed intoacircuitwithtkeratin series withaZOOKohmresistor, keeping current level approximately constant. An oscirloscope, measuring voltage drop across the resistor, was used to monitor stimulating current. The output voltage from the generator was regulated until the rat performed slight orienting responses, which occurred at 50-200 &Amp. When self-stimulating, the rat was trained to press a Scand. J . Psychoi. I6
8
S . G . Carlsson
Fig. I . Typical electrode placement. The figure shows lower part of an electrode track, with electrode tip at arrow.
pedal in an arena, 30x50 cm. Each time the pedal was pressed the rat received brain stimulation for 0.3 sec. Lifting the pedal within 0.3 sec stopped the current. The rats were trained in daily sessions and all had reached their individual plateaus of rate of self-stimulation at the time the experiments began.
in the self-stimulation cage (with the pedal covered) and achieve one ejaculation. This was intended to allow the males sexual practice in the experimental situation. Before the start of the experimental period, the rats achieved a stable baseline in self-stimulation frequency.
Experimental procedure Experiment I . Mating after self-stimulation: In the experimental tests, S was allowed self-stimulation for a period of 5 min and was then immediately transferred to the mating cage and presented with a sexually receptive female. In the control tests the same procedure was used with the exception that S was not allowed to perform any self-stimulation. The mating test was ended, (a)when no intromission had taken place within IS min after presentation of the female, ( b )when no ejaculation occurred within 20 min, ( c )after the first intromission following ejaculation, or (d) after a maximum of 20 minutes after ejaculation. The rats were observed at least six times in each c o n d tion. Each test was separated from the next by an interval of at least 24 hours. Experimental and control tests alternated. Experiment 2. Selfstimulation of the brain after mating: Following an adaptation period of 5 minutes to the self-stimulation cage with the pedal covered, a receptive female was presented to the male. After the first intromission, the pedal was uncovered. Testing was ended 10 minutes after the pedal had been presented to the male. Between each test the subjects were allowed to copulate
RESULTS
Scand. J . Psychol. 16
Experiment 1
The median response rate in the five minutes selfstimulation session ranged from 48 to 575. Some rats showed seminal deposits in several of these self-stimulation periods. Table 1 shows individual median performances of various measures of the sexual behavior under experimental and control conditions for each of the nine rats participating in this experiment. Only sporadic significant differences (in either direction) were found. No significant correlations were observed between individual lever-pressing frequency and subsequent sexual behavior (Spearman’s correlation by ranks a=0.05). Further, comparing experimental and control conditions for all nine animals, no significant differences were found (Wilcoxon Matched-pairs signed ranks test, a=0.05). Thus, activation through self-
Selfistimulation and mating
9
Table 1. Individual medians of copulatory characteristics f o r the rats in Experiment 1 with and without preceding 5-minutes periods of self-stimulation. Comparisons performed by the Mann- Whitney U test f o r each individual rat Intromission latency in min
Ejaculation latency in min
Rat no
With Md
Without Md
P
With Md
1 2 3 4 7 10 13
0.8 0.7 15.0 0.2 0.4 0.3 1.0 0.3 0.6
1.2 0.5 1.6 0.1 0.2 0.2 0.5 0.3 0.4
NS NS NS NS NS NS NS NS NS
15
500
Without Md
3.1
5.9
5.6
5.6 15.5
25.0 4.1 6.3 1.0
6.0 4.9 2.1
Intromission preceding ejaculation
Postejaculation later.iy in min
With Md
Without Md
With Md
Without Md
NS NS NS
10
10 12 10
Self-stimulation and mating behavior in the male rat SVEN G. CARLSSON KNUT LARSSON
Absrract.-A presumed relationship between selfstimulation of the brain and sexual behavior was studied in two experiments. In the first experiment the male rat was allowed self-stimulation during a period of 5 minutes and thereafter transferred to a mating cage with a female. NO effects of the self-stimulation were recorded in the mating behavior. In the second experiment the male was allowed one intromission and thereafter presented with a lever for self-stimulation, still having access t o the female. The sexual responses were completely inhibited, and selfstimulation at normal rate resumed. It was concluded that there is no necessary link between self-stimulation of the brain and sexual behavior.
A relationship between responses evoked by selfstimulation of the brain and sexual behavior has often been speculated upon. The frequent occurrence of seminal emissions during self-stimulation was early noted (Miller, 1957) and there is an apparent resemblance between the highly goal-directed activity exhibited by the rat during copulation and the vigorous pedal-pulling shown in a selfstimulation test. Experimental evidence for such a relationship was reported by Olds (1958), who observed an androgen dependency of self-stimulation in rats. Herberg (1963) confirmed this observation stimulating in a medial forebrain area. This study represents a n attempt t o explore, further, the relationship between self-stimulation and sexual behavior. Two experiments were conducted. In the first experiment the male was allowed to perform self-stimulation for a specified period of time and thereafter presented with a receptive female. In the second experiment, the male rat was allowed to start copulatory behavior, whereupon it was presented with a lever for self-stimulation. METHOD Subjecrs In experiment 1 participated nine and in experiment 2 six male rats, all from a locally bred strain of albinos aged
University of Goteborg, Sweden
approximately 6 months. They were selected on the basis of their performance in preliminary tests for showing a consistently high level of sexual behavior.
Mating tests The technique used in observing and recording the mating behavior was the same a s has been described in detail elsewhere (Larsson, 1963). During each test the following items were measured; ( a )mounting frequency-number of mounts without intromission; (b) intromission frequency-number of mounts with intromission; ( c )intromission latency-time in minutes from the entrance of the female into the mating cage until the first intromission; (d) ejaculation latency-time in minutes from the first intromission until ejaculation; ( e ) postejaculation intervaltime in minutes from an ejaculation to the next intromission. Females were brought into estrus by injections of estrogen and progesterone according to a schedule described previously (Larsson, 1963).
Self-stimulation technique Elecirodes and operation technique. Bipolar electrodes were used, They were prepared of dental steelsuture (0.20 mm). The threads were straightened, insulated and glued together. Suitable lengths were cut leaving only the crosssection of the tips uninsulated. The electrode was fixed in a stereotactic manipulator and lowered into the medial forebrain bundle-lateral hypothalamic area. A typical electrode placement is shown in Fig. 1. Barbiturate anesthesia was used. While fixed in the manipulator, the electrode was cemented to the skull and to three anchoring screws with dental acrylic, which was carefully applied with dental fillingtubes. Stimulation parameters and testing. A stimulator, built according to the description of Petrohilos, Simpson, Welfod and Valenstein, was used. It generates bipolar square pulses with a frequency of 100 pulses pairs per sec, each pulse of 0.2 m sec duration and with 0.2 m sec delay between negative and positive pulses. The current was fed intoacircuitwithtkeratin series withaZOOKohmresistor, keeping current level approximately constant. An oscirloscope, measuring voltage drop across the resistor, was used to monitor stimulating current. The output voltage from the generator was regulated until the rat performed slight orienting responses, which occurred at 50-200 &Amp. When self-stimulating, the rat was trained to press a Scand. J . Psychoi. I6
8
S . G . Carlsson
Fig. I . Typical electrode placement. The figure shows lower part of an electrode track, with electrode tip at arrow.
pedal in an arena, 30x50 cm. Each time the pedal was pressed the rat received brain stimulation for 0.3 sec. Lifting the pedal within 0.3 sec stopped the current. The rats were trained in daily sessions and all had reached their individual plateaus of rate of self-stimulation at the time the experiments began.
in the self-stimulation cage (with the pedal covered) and achieve one ejaculation. This was intended to allow the males sexual practice in the experimental situation. Before the start of the experimental period, the rats achieved a stable baseline in self-stimulation frequency.
Experimental procedure Experiment I . Mating after self-stimulation: In the experimental tests, S was allowed self-stimulation for a period of 5 min and was then immediately transferred to the mating cage and presented with a sexually receptive female. In the control tests the same procedure was used with the exception that S was not allowed to perform any self-stimulation. The mating test was ended, (a)when no intromission had taken place within IS min after presentation of the female, ( b )when no ejaculation occurred within 20 min, ( c )after the first intromission following ejaculation, or (d) after a maximum of 20 minutes after ejaculation. The rats were observed at least six times in each c o n d tion. Each test was separated from the next by an interval of at least 24 hours. Experimental and control tests alternated. Experiment 2. Selfstimulation of the brain after mating: Following an adaptation period of 5 minutes to the self-stimulation cage with the pedal covered, a receptive female was presented to the male. After the first intromission, the pedal was uncovered. Testing was ended 10 minutes after the pedal had been presented to the male. Between each test the subjects were allowed to copulate
RESULTS
Scand. J . Psychol. 16
Experiment 1
The median response rate in the five minutes selfstimulation session ranged from 48 to 575. Some rats showed seminal deposits in several of these self-stimulation periods. Table 1 shows individual median performances of various measures of the sexual behavior under experimental and control conditions for each of the nine rats participating in this experiment. Only sporadic significant differences (in either direction) were found. No significant correlations were observed between individual lever-pressing frequency and subsequent sexual behavior (Spearman’s correlation by ranks a=0.05). Further, comparing experimental and control conditions for all nine animals, no significant differences were found (Wilcoxon Matched-pairs signed ranks test, a=0.05). Thus, activation through self-
Selfistimulation and mating
9
Table 1. Individual medians of copulatory characteristics f o r the rats in Experiment 1 with and without preceding 5-minutes periods of self-stimulation. Comparisons performed by the Mann- Whitney U test f o r each individual rat Intromission latency in min
Ejaculation latency in min
Rat no
With Md
Without Md
P
With Md
1 2 3 4 7 10 13
0.8 0.7 15.0 0.2 0.4 0.3 1.0 0.3 0.6
1.2 0.5 1.6 0.1 0.2 0.2 0.5 0.3 0.4
NS NS NS NS NS NS NS NS NS
15
500
Without Md
3.1
5.9
5.6
5.6 15.5
25.0 4.1 6.3 1.0
6.0 4.9 2.1
Intromission preceding ejaculation
Postejaculation later.iy in min
With Md
Without Md
With Md
Without Md
NS NS NS
10
10 12 10
