Physiology & Behavior, Vol. 15, pp. 253-257. Pergamon Press and Brain Research Pubi., 1975. Printed in
the U.S.A.
Acceleration and Pacing of Copulatory Performance of Male Rats by Repeated, Aversive Brain Stimulation 1 ROBERT D. EIBERGEN AND ANTHONY R. CAGGIULA2
Psychobiology Program, Departments o f Pharmacology and Psychology University o f Pittsburgh, Pittsburgh, PA 15260
(Received 20 December 1974) EIBERGEN, R. D. AND A. R. CAGGIULA. Acceleration and pacing of copulatory performance of male rats by repeated, aversive brain stimulation. PHYSIOL. BEHAV. 15(3) 253-257, 1975. - Repetitive electric shocks applied to the dorsal midbrain (DM) paced the copulatory behavior of sexually experienced male rats. DM stimulation also accelerated the rate of copulation, the achievement of ejaculation and the resumption of copulation after ejaculation and increased the number of ejaculations in an hour test. Dorsal midbrain
Averslvestimulation
Copulation
COPULATORY performance of sexually experienced male rats can be dramatically altered by peripherally applied aversive stimulation. Brief electric shocks delivered to the back every 30 sec reduced the postejaculatory interval and paced copulation of experienced males, i.e., 70 percent of all copulatory responses occurred within 5 sec after a shock [ 3]. More recently, a similar shock pattern applied to either the back [ 14] or tail [ 10] of male rats accelerated the rate of copulation, the achievement of ejaculation, and the resumption of copulation after ejaculation. In an attempt to extend these findings, we questioned whether the tactile component of peripheral shock was a necessary factor in producing these changes, or whether other methods of delivery were also effective. A preliminary attempt to induce or facilitate copulation with another form of aversive, arousing stimulation, loud noise, failed [8]. However, this was not comparable to electric shock in terms of either the intensity of behavioral arousal produced or resistance to habituation. A better candidate would be aversive brain stimulation. In the present experiment, experienced male rats were permitted to copulate while receiving brief, repetitive pulses of stimulation within an area of the dorsal midbrain (DM) previously shown to produce high rates of escape responding [13]. We asked whether DM stimulation applied in this manner could (1) pace copulatory activity of male rats, (2) influence the rate of copulation or the achievement of ejaculation, and (3) affect the number of ejaculations achieved in an hour.
METHOD
Animals Six sexually experienced male Sherman rats, approximately 1 0 0 - 1 5 0 days old and weighing 2 5 0 - 3 0 0 g were used in the experiment. The animals were housed in individual cages with free access to food and water throughout the experiment, and were maintained on a reversed 14 hr l i g h t - 1 0 hr dark schedule. Females used as copulatory partners were brought into behavioral receptivity by long term, subcutaneous implants of fused estradiol (approximately 20 mg) and subcutaneous injection of 1 mg progesterone 6 - 1 2 hr before testing.
Surgery All surgery was performed under 40 mg/kg sodium pentobarbital anesthesia. Monopolar platinum electrodes, 0.009 in. in dia., insulated to within 0.5 mm of the tips were implanted unilaterally into the dorsal midbrain. With the skull level, a small hole was made 2 . 5 - 3 . 0 mm anterior to the intraaural line and 1.0 mm lateral to the saggital suture. The electrode was lowered through the hole to a point 4 . 5 - 5 . 0 mm below the surface of the skull. These co-ordinates were chosen because pilot studies showed that this location yielded consistent aversive responses such as jumping and escaping. A 0.25 mm dia. uninsulated wire was wrapped around seven jeweler's screws embedded in the
1Supported by PHS grant MH 16581. The authors would like to thank Drs. H. Szechtman and M. Hartz for assistance with the statistical analysis. Hormones were generously provided by Dr. Preston Perlman, Schering Corporation, Bloomfield, N. J. a Reprint requests should be sent to Anthony R. Caggiula, Department of Psychology, University of Pittsburgh, Pittsburgh, PA 15260. 253
254 skull, and served as the indifferent lead. The electrode and the indifferent were then fastened to a connecting plaque, which was fastened to the skull with dental acrylic.
EIBERGEN AND CAGGIULA photographic negatives and printed at an enlargement of 12 ×. RESULTS
Apparatus The testing chamber was an elevated, circular Plexiglas cage, 18 in. in dia., with a wire mesh floor and opaque panels on 3 sides. Stimulation consisted of trains of biphasic 100 hz pulses of 1 msec in duration, passed through a revolving commutator and through flexible leads attached to the electrode assembly on the skull of the animal. All tests were conducted during the dark phase of the reverse day-night cycle. A dim red light above the chamber provided light for observation. Mounts, mounts with intromission, and ejaculations were recorded on an Esterline-Angust event recorder.
Testing Procedure Seven to 10 days following surgery, each animal was adapted to the testing chamber for 15 min and then given several 0.5 sec pulses of brain stimulation. The intensity was successively increased until it was just capable of reliably eliciting jumping reactions. These intensities ranged from 200 to 350 uA for the 6 males. The animal was then returned to its home cage. This threshold intensity was determined for each animal and was used in all subsequent brain stimulation tests for that animal. Each animal was tested in 6 one hr sessions run at 10 day intervals. The tests alternated between shock and no-shock sessions. Before each test the animal was adapted to the chamber for 15 min. A receptive female was then introduced into the cage. In shock sessions, an automatic timer delivered a 0.5 sec pulse every 20 sec beginning 5 sec after the first intromission. (Pilot tests showed that a shock delivered before the first intromission inhibited copulation.) Otherwise no-shock and shock tests were identical. The following measures of sexual activity were taken: mount frequency (MF), the number of mounts without intromission preceeding ejaculation; intromission frequency (IF), intromissions preceeding ejaculation; ejaculatory latency (EL), time from the first intromission of a series to the ejaculation; inter-intromission interval (III), the average time between intromissions in a given ejaculatory series; post ejaculatory interval (PEI), the tirfie from one ejaculation to the first intromission of the next series. For the analysis of possible pacing effects of brain shock, each 20 sec postshock period was divided into four 5 sec periods, and the total number of copulatory responses (mounts with and without intromissions, and ejaculations), occurring in each of these periods was calculated. For control (i.e., no-shock) tests, successive 20 sec intervals, also divided into 5 sec periods were superimposed on the behavioral data, with the first interval starting 5 sec after the first intromission.
Histology At the completion of testing, the animals were deeply anesthesized with sodium pentobarbital, and perfused intracardially with 0.9 percent saline followed by 10 percent Formalin solution. The brains were removed, further fixed in Formalin, frozen and sliced in 50 micron sections. Alternate sections were stained using the cresyl violet and Weil methods. The Weil sections were used as
Brief electric shocks delivered to the dorsal midbrain at twenty second intervals paced the copulatory behavior of male rats (Fig. 1). Seventy-nine percent of the copulatory responses of male rats occurred within l0 sec after a shock, and 47 percent were clustered within the first 5 sec following shock. By contrast, the copulatory activity of these same animals was evenly distributed across the four 5 sec periods of the control tests. The differences between the distributions of copulatory responses over twenty second intervals for shock and no-shock sessions were statistically significant (X2 = 15.76, df = 3; p
the U.S.A.
Acceleration and Pacing of Copulatory Performance of Male Rats by Repeated, Aversive Brain Stimulation 1 ROBERT D. EIBERGEN AND ANTHONY R. CAGGIULA2
Psychobiology Program, Departments o f Pharmacology and Psychology University o f Pittsburgh, Pittsburgh, PA 15260
(Received 20 December 1974) EIBERGEN, R. D. AND A. R. CAGGIULA. Acceleration and pacing of copulatory performance of male rats by repeated, aversive brain stimulation. PHYSIOL. BEHAV. 15(3) 253-257, 1975. - Repetitive electric shocks applied to the dorsal midbrain (DM) paced the copulatory behavior of sexually experienced male rats. DM stimulation also accelerated the rate of copulation, the achievement of ejaculation and the resumption of copulation after ejaculation and increased the number of ejaculations in an hour test. Dorsal midbrain
Averslvestimulation
Copulation
COPULATORY performance of sexually experienced male rats can be dramatically altered by peripherally applied aversive stimulation. Brief electric shocks delivered to the back every 30 sec reduced the postejaculatory interval and paced copulation of experienced males, i.e., 70 percent of all copulatory responses occurred within 5 sec after a shock [ 3]. More recently, a similar shock pattern applied to either the back [ 14] or tail [ 10] of male rats accelerated the rate of copulation, the achievement of ejaculation, and the resumption of copulation after ejaculation. In an attempt to extend these findings, we questioned whether the tactile component of peripheral shock was a necessary factor in producing these changes, or whether other methods of delivery were also effective. A preliminary attempt to induce or facilitate copulation with another form of aversive, arousing stimulation, loud noise, failed [8]. However, this was not comparable to electric shock in terms of either the intensity of behavioral arousal produced or resistance to habituation. A better candidate would be aversive brain stimulation. In the present experiment, experienced male rats were permitted to copulate while receiving brief, repetitive pulses of stimulation within an area of the dorsal midbrain (DM) previously shown to produce high rates of escape responding [13]. We asked whether DM stimulation applied in this manner could (1) pace copulatory activity of male rats, (2) influence the rate of copulation or the achievement of ejaculation, and (3) affect the number of ejaculations achieved in an hour.
METHOD
Animals Six sexually experienced male Sherman rats, approximately 1 0 0 - 1 5 0 days old and weighing 2 5 0 - 3 0 0 g were used in the experiment. The animals were housed in individual cages with free access to food and water throughout the experiment, and were maintained on a reversed 14 hr l i g h t - 1 0 hr dark schedule. Females used as copulatory partners were brought into behavioral receptivity by long term, subcutaneous implants of fused estradiol (approximately 20 mg) and subcutaneous injection of 1 mg progesterone 6 - 1 2 hr before testing.
Surgery All surgery was performed under 40 mg/kg sodium pentobarbital anesthesia. Monopolar platinum electrodes, 0.009 in. in dia., insulated to within 0.5 mm of the tips were implanted unilaterally into the dorsal midbrain. With the skull level, a small hole was made 2 . 5 - 3 . 0 mm anterior to the intraaural line and 1.0 mm lateral to the saggital suture. The electrode was lowered through the hole to a point 4 . 5 - 5 . 0 mm below the surface of the skull. These co-ordinates were chosen because pilot studies showed that this location yielded consistent aversive responses such as jumping and escaping. A 0.25 mm dia. uninsulated wire was wrapped around seven jeweler's screws embedded in the
1Supported by PHS grant MH 16581. The authors would like to thank Drs. H. Szechtman and M. Hartz for assistance with the statistical analysis. Hormones were generously provided by Dr. Preston Perlman, Schering Corporation, Bloomfield, N. J. a Reprint requests should be sent to Anthony R. Caggiula, Department of Psychology, University of Pittsburgh, Pittsburgh, PA 15260. 253
254 skull, and served as the indifferent lead. The electrode and the indifferent were then fastened to a connecting plaque, which was fastened to the skull with dental acrylic.
EIBERGEN AND CAGGIULA photographic negatives and printed at an enlargement of 12 ×. RESULTS
Apparatus The testing chamber was an elevated, circular Plexiglas cage, 18 in. in dia., with a wire mesh floor and opaque panels on 3 sides. Stimulation consisted of trains of biphasic 100 hz pulses of 1 msec in duration, passed through a revolving commutator and through flexible leads attached to the electrode assembly on the skull of the animal. All tests were conducted during the dark phase of the reverse day-night cycle. A dim red light above the chamber provided light for observation. Mounts, mounts with intromission, and ejaculations were recorded on an Esterline-Angust event recorder.
Testing Procedure Seven to 10 days following surgery, each animal was adapted to the testing chamber for 15 min and then given several 0.5 sec pulses of brain stimulation. The intensity was successively increased until it was just capable of reliably eliciting jumping reactions. These intensities ranged from 200 to 350 uA for the 6 males. The animal was then returned to its home cage. This threshold intensity was determined for each animal and was used in all subsequent brain stimulation tests for that animal. Each animal was tested in 6 one hr sessions run at 10 day intervals. The tests alternated between shock and no-shock sessions. Before each test the animal was adapted to the chamber for 15 min. A receptive female was then introduced into the cage. In shock sessions, an automatic timer delivered a 0.5 sec pulse every 20 sec beginning 5 sec after the first intromission. (Pilot tests showed that a shock delivered before the first intromission inhibited copulation.) Otherwise no-shock and shock tests were identical. The following measures of sexual activity were taken: mount frequency (MF), the number of mounts without intromission preceeding ejaculation; intromission frequency (IF), intromissions preceeding ejaculation; ejaculatory latency (EL), time from the first intromission of a series to the ejaculation; inter-intromission interval (III), the average time between intromissions in a given ejaculatory series; post ejaculatory interval (PEI), the tirfie from one ejaculation to the first intromission of the next series. For the analysis of possible pacing effects of brain shock, each 20 sec postshock period was divided into four 5 sec periods, and the total number of copulatory responses (mounts with and without intromissions, and ejaculations), occurring in each of these periods was calculated. For control (i.e., no-shock) tests, successive 20 sec intervals, also divided into 5 sec periods were superimposed on the behavioral data, with the first interval starting 5 sec after the first intromission.
Histology At the completion of testing, the animals were deeply anesthesized with sodium pentobarbital, and perfused intracardially with 0.9 percent saline followed by 10 percent Formalin solution. The brains were removed, further fixed in Formalin, frozen and sliced in 50 micron sections. Alternate sections were stained using the cresyl violet and Weil methods. The Weil sections were used as
Brief electric shocks delivered to the dorsal midbrain at twenty second intervals paced the copulatory behavior of male rats (Fig. 1). Seventy-nine percent of the copulatory responses of male rats occurred within l0 sec after a shock, and 47 percent were clustered within the first 5 sec following shock. By contrast, the copulatory activity of these same animals was evenly distributed across the four 5 sec periods of the control tests. The differences between the distributions of copulatory responses over twenty second intervals for shock and no-shock sessions were statistically significant (X2 = 15.76, df = 3; p
