Animals and Procedure
Twenty male rats of the highly inbred DA agouti strain were housed individually. The rats were 6 to 18 months of age and between 250 and 350 grams.
The sequence of procedures is shown in Table 1. Nine animals were given preoperative experience with shock-induced fighting, while 11 others were not. Sham lesions or bilateral or unilateral lesions were then placed in the thalamus. Postoperative testing procedures included an initial test of shock-induced fighting. Succeeding tests of shock-induced fighting were then administered under conditions of sensory deprivation. A final abbreviated test of fighting was administered under normal conditions, i.e., without sensory deprivation, to make sure that all rats could still show their preoperative behavior .
Testing Apparatus Pairs of rats were tested for shock-induced defensive upright posture and boxing in a clear plastic chamber 20 cm high and with a 20 by 15 cm floor area. The steel grid floor of the chamber was connected to a 600 V AC power source by way of a scrambler switch. A mimimum resistance of 400,000 Ω in series with the animals was used to minimize the effects of alterations in resistance caused by different stances of the animals.
Shock-induced fighting tests were conducted on pairs of rats, one of which was always an experienced boxer with no neural or sensory impairment. Each test session consisted of 20 trials, each trial consisting of 20 shocks. The shocks were 0.5 sec in duration and delivered at a rate of l/sec; thus each trial was 20 sec in duration. Trials were spaced at 1 min intervals; thus each test session was 20 min in duration. A titration method was used to determine the shock intensity on each trial, i.e., if boxing occurred during a trial at a specific intensity, then the intensity of the shock was lowered one step on the succeeding trial; conversely, if boxing did not occur, it was raised one step. Shock intensities were 0.05, 0.10, 0.30, 0.50, 1.00 and 1.50 mA. The advantage of this method is that it allows a measure of the animal's boxing which is independent of the threshold shock intensity. Half of the trials are above threshold and half are below threshold for the response. If no boxing occurred, then ascending and descending series of shock intensities were alternated.
The fundamental dependent variable in all tests was the percent of shocks to which the experimental animal responded with boxing, defined as facing the opponent in an upright posture and sparring with the forelimbs. Shock threshold, defined as shock intensity eliciting boxing on 50% of trials, was also determined for initial postop tests.
All animals showed vigorous and stable levels of upright posture and boxing before they were tested under facial anesthesia. Those animals which were tested prior to surgery (experienced animals) all showed boxing on more than 20% of the shocks in each test session. Note that since half of the trials in each session were below threshold, this means that the animals boxed on at least 40% of the individual shocks administered above threshold. Experienced animals were tested again after surgery but before facial anesthesia (initial postop test), and they continued to show boxing on 18% or more of the shock presentations. Those animals which were not tested prior to surgery (naive animals) all showed boxing on more than 10% of the shocks in the initial postop test session (range: 11% to 41%).
In all tests, the experimental animal was tested against an experienced, intact partner; this procedure is a conservative one, since it has been found that a lowered rate of fighting by an opponent results in lowered fighting rates by the experimental animal . In those cases in which the experimental animal did not show boxing against the normal opponent, it was then suspended in a harness in front of the opponent in an upright posture, and both animals were given footshock. This procedure ensured that no animals failed to show boxing because they could not stand upright.
(Continued on next page)