The animals were continuously observed on closed circuit television. Control recordings were made from each sheep during the day and evening for a one or two week pre-exposure period, after which exposure to orally administered dieldrin was begun.

The EEG changes observed prior to clinical toxicity consisted of bursts or spindles of high amplitude (200-300 μv), slow waves (4-5 Hz) occurring in one or more leads (Figure 1). These bursts usually occurred simultaneously in all leads and were of one to three seconds duration. The occurrence of these slow waves was related to dosage. As exposure continued the slow wave activity occurred more frequently and was of greater duration. With sufficient exposure convulsive seizures occurred and the EEG consisted of high amplitude fast waves with some spike-wave complexes similar to those seen in petit mal epileptic seizures. The post convulsive EEG was irregular with high amplitude, slow frequency components present.

The presentation of high intensity, variable frequency strobe has precipitated convulsive seizures in exposed sheep prior to the occurrence of clinical toxicity. Further investigation with the use of repetitive auditory stimulation is under way.

Before discussing the behavioral work done in our laboratory a comment on the necessity of utilizing behaviorial techniques in the investigation of the effects of neurotoxicants is appropriate. It must be remembered that it is the ability to behave in an appropriate manner in a particular situation which determines in large measure the survival of the animal, and to an extent that of the species. Therefore, if neurotoxicants are capable of affecting behavior it is important that we be aware of this. Subjects must be studied with a variety of behavioral measures which test both their learning and performance

capabilities. It is only when this is done that the behavioral effects of neurotoxicants may be evaluated.

In an avoidance experiment 20 sheep were trained to a 90% criterion to make a conditioned avoidance response (CAR) following the presentation of a conditioned stimulus (CS) in order to avoid an unconditioned stimulus (UCS). The CAR consisted of jumping a 12-inch barrier, the CS was a light and the UCS was a mild electric shock. Ten of the sheep were then exposed to dieldrin, and extinction trials were conducted. Dieldrin had no effect on the number of trials to extinction, response latency, or the number of spontaneous barrier crossings.

The effect of dieldrin on detour behavior has also been investigated. In a detour problem the animal encounters a barrier along the most direct route to a goal object, in this case a food reward. Dieldrin had no effect on the time it took the sheep to circumvent the barrier and reach the food reward.

The effect of dieldrin on visual discrimination learning in sheep was investigated through the use of a Y maze (Figure 2). Six adult female sheep were used in this study. In this situation the animal was placed in the start box and when the guillotine door was opened had to approach one of two geometric symbols, presented simultaneously in the ends of the arms of the maze, in order to avoid a mild electric shock. One symbol was arbitrarily designated as correct. The animals were given 20 trials per day and training was continued until a criterion of 70% correct responses for three successive days was reached. The sheep were trained to criterion on two problems, matched for performance, and one member of each pair was exposed daily to an oral dose of 10 mg dieldrin/kg B.W. The sheep were then trained on a third problem. If criterion was reached on this prob

lem in less than 30 days a fourth problem was presented. Only one sheep met criterion on problem three and received training on problem four. The other five sheep failed to master problem three in the 30 days. All sheep were then tested for recall of problem one.

An analysis of variance (Table 1) performed on the number of trials required to learn, and then to relearn, problem one indicated that there was a highly significant (p<.01) interaction between dieldrin exposure and relearning (Figure 3). This means that the number of trials necessary to relearn problem one was dependent upon whether or not the animals were exposed to dieldrin. It should be noted that the number of sheep used in this experiment was small, three in each group, and for that reason it is being replicated. Thus, the present data must be viewed as preliminary, and care must be taken in generalizing from these findings.

Operant conditioning has proved to be a useful and flexible tool in psychopharmacological research. An operant conditioning apparatus suitable for use with sheep has been developed and is being used to study several types of behavior. It should be emphasized that the experiments reported here represent pilot work. This work was undertaken to study the

feasibility and effectiveness of operant conditioning in the investigation of the toxic effects of dieldrin. These studies provide an indication of the techniques used and the results found. It must be emphasized that the results, while intriguing, are not definitive. More work is necessary and is being conducted.

The sheep were tested individually in the test chamber as shown in Figure 4. The apparatus was under programmed control. Prior to being brought into this situation the sheep were reduced to 85% of their free feeding body weights. They were .then trained to eat from the food hopper, and were habituated to the presentation and retraction of the hopper. Behavior compatible with the pedal-pressing response was selectively reinforced by the use of shaping techniques. Pressure on the pedal closed a switch which caused the food hopper to be presented to the animal for a predetermined period of time. In a short time the animal began to press the pedal.

One animal was trained to respond on a fixed ratio schedule of 40:1. That is, the pedal had to be pressed 40 times before the food hopper was presented. After establishing a baseline of stable responding on this schedule the animal was exposed daily to an oral dose of 15 mg dieldrin/kg B.W. This study is in progress and no results are available.

It has been mentioned that hypersynchronous slow waves occur in the EEG of sheep exposed to dieldrin. Mirsky and Rosvold1 have reported that this type of EEG activity is associated with impaired attentiveness on a vigilance task. Because of the similarity of the EEG activity reported by Mirsky and Rosvold and that observed in our laboratory an attempt was made to determine whether this type of EEG activity observed in dieldrin toxicity was also associated with impaired attentiveness.

Through the use of successive approximation techniques two sheep were trained to bar press for food. The animals were then trained to bar press only in the presence of a 5 KHz tone of 6 db intensity. As training progressed the duration of the tone was gradually reduced to 0.1 second. If the animals bar pressed within five seconds after the presentation of the tone there was an automatic presentation of food (corn) for 3.5 seconds. The electronic control circuitry was programmed so that a period of 30 seconds had to elapse between bar presses before the tone was presented. If the sheep bar pressed during this interval, the control circuitry automatically reset the 30 second interval. When the sheep reached a stable response level to tone presentations, bilateral dural recording electrodes were surgically implanted over the frontal, parietal, and occipital cortex. After a two-week postoperative recovery period, three days of control EEG and vigilance performance data were collected. Then the animals were exposed to a daily oral dose of 20 mg dieldrin/kg body weight for three and four days, respectively. The sheep were given a minimum of 100 tone presentations per day and their EEGs were recorded continually throughout the exposure period. Upon termination of exposure the animals were given 50 tone presentations per day until they responded at their pre-exposure rates.

Se señala la importancia que tiene determinar funciones

tales como la memoria, y sentido de descriminación después de la administración de dieldrín y otros insecticidas. Se observan en animales de experimento alteraciones en el encefalograma siete horas después de la acción de los insecticidas.

During the pre-exposure period the sheep responded to an average of 82% of the tone presentations. By the second day of exposure, response decrements of 63% and 58% were noted.

The following points should be emphasized: (1) the animals showed no signs of clinical toxicity, (2) when food was presented in the operant chamber they ate eagerly indicating that the performance decrement was not due to loss of appetite and, (3) the animals manifested a definite orienting response (head and ear movements) when the tone was presented, indicating that although the bar press was not made the tone was heard. By the eighth day after termination of exposure the sheep were again responding at their pre-exposure

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