Cocaine effects on behavioral responding to a novel object placed in a familiar environment

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Abstract

It is well established that cocaine stimulant effects are potentiated in a novel environment. The relationship between cocaine and novel stimuli, however, remains poorly understood. In this study, we examined the effects of different dose levels of cocaine (5.0, 10.0 and 20.0 mg/kg) administered to separate groups of rats (N = 10) on attentional behavior to a small novel object stimulus placed within a central zone (CZ) of a familiar open-field environment. This method has been used to assess attentional function in young animals, brain damaged animals and drug treated animals. In previous studies, we have shown that attention to a novel object stimulus can be quantified by an animal's contact time with the object. Following a series of pre-exposures to the test environment without the novel object, we found that cocaine in a brief 10 min test session with the novel object present produced a dose related decrease in mean contact time with the novel object. In contrast caffeine (5.0, 10.0 and 20.0 mg/kg), which induced a locomotor stimulant effect equivalent to cocaine, did not impair novel object contact time. Correlational analyses indicated absence of significant negative correlation coefficients of locomotor activity and contact time with the novel object. These considerations indicate that the observed cocaine impairment of attention to the novel stimulus is not attributable to hyperactivity per-se. Furthermore, cocaine, but not caffeine, induced a dose related decrease in the duration of spontaneous grooming. Thus, cocaine appears to diminish an animal's overall capability to maintain a behavioral process (i.e., investigate a novel object stimulus and/or engage in spontaneous bodily directed activity such as grooming). Altogether, the findings obtained in the present study indicate that cocaine impairs an animal's ability to sustain attention to stimuli and suggest a behavioral state analogous to an attention deficit disorder.

Section snippets

Animals

Forty naive male Sprague–Dawley rats from Taconic Farms (Germantown, NY), 4 months old and weighing approximately 400 g at the start of the experiments were used. Upon arrival, the animals were housed in individual 48 × 27 × 20 cm clear polycarbonate cages in a climate-controlled room at 22–24 °C with a 12-h dark and 12 h light cycle. During the first week after arrival, all animals were handled and weighed daily for 7 days. During the second week, the animals received three injections I.P. of .9%

Results

The image analysis results of the three drug treatment test sessions with the object present in the central zone are shown in the first three figures. To provide a contrast, the results of the non-drug 10 min no-object session, which preceded each drug treatment test, are also shown in these figures. Fig. 1 presents (A) the locomotion distance scores and (B) average duration per central zone entry (mean CZ duration/entry) for each group in the no-object saline test and in the initial

Discussion

The cocaine and caffeine locomotor activation effects observed in the present study were in agreement with the expectations. Both drugs induced hyperlocomotion. The important finding, however, was that cocaine, but not caffeine, impaired responsiveness to a novel object in a familiar environment. The behavioral response to the novel object obtained in the present study for the non-drug treated animals were in agreement with our previous findings (Dai and Carey, 1994a, Dai and Carey, 1994b). The

Acknowledgement

This research was supported by a VA Merit Review Grant.

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