Research reportInvolvement of the cerebellum in classical fear conditioning in goldfish
Introduction
It is well known that the limbic system, especially the amygdala, is critically involved in fear-related classical conditioning in mammals [5], [6], [10], [14], [16]. On the other hand, ablation of the teleost telencephalon, which is phylogenetically related to the limbic system in land vertebrates, does not disrupt classical autonomic conditioning using an aversive procedure similar to that used in mammals [7], [16], [18], [27].Recently, there is growing evidence that the mammalian cerebellum, in addition to being a motor coordination center, is involved in emotional behavior [2], [3], [7], [25], [29], [30]. The vermal part of the cerebellum has been implicated in emotional or fear-related behaviors [2], [30]. Bobée et al. [3] have reported that the cerebellar vermis is involved in attentional capabilities and emotional behavior as well as in motor control. Although the mammalian cerebellum shows marked elaboration compared with that in fish, the basic design of internal circuitry is shared with fish [1], [4], [17]. The cerebellum of fish consists of three main parts: corpus and valvulla cerebelli and lobus vestibulolateralis. The piscine cerebellar corpus is thought to be homologous with the vermal part of the cerebellum of higher vertebrates [9]. The cerebellum of fish has been shown to be involved in motor coordination, including the regulation of locomotor movement, DLR and VOR [20], [21], [22], [23], [24], [26]. However, the complete functions of the fish cerebellum have yet to be determined.
In the present study, to investigate the cognitive role of the cerebellum of fish, we conducted experiments examining the effects of cerebellar manipulations on fear-related classical heart rate conditioning. We performed two types of manipulations, total ablation of the corpus cerebelli (CC) and localized cooling of the CC for reversible inactivation of the cerebellar function. Effects of these manipulations on delay classical conditioning using aversive procedure were examined. In the present experiments, we used goldfish paralyzed by an injection of curare. Since the expression of conditioned responses of goldfish in fear-related classical conditioning is established quickly, we were able to obtain a series of data from a naive subject in a paralyzed condition.
Section snippets
Animals
Commercially obtained goldfish (Carassius auratus), 45.9–90.0 mm in body length, were kept in our laboratory on a 14 h/10 h light/dark cycle at controlled room temperature (18–24 °C) for 3–4 weeks before use.
Ablation experiment
Goldfish were anesthetized in MS-222 (0.015%), and the cerebellar corpus (CC) was removed by aspiration through a small hole made in the skull. The hole was then covered with dental cement. Sham control fish were surgically operated in the same manner to the CC-ablated fish except for the
Effects of CC ablation on classical heart rate conditioning
There were no significant differences in the heart rate prior to the start of the conditioning session between normal, sham-operated, and CC-ablated goldfish. Thus, surgical operation had no effect on the heart rate under resting conditions. Fig. 2 shows the cardiac response to the first presentation of a US in normal and CC-ablated goldfish. The goldfish responded to a mild electrical shock in the trunk with cardiac deceleration (bradycardia) followed by slight acceleration (tachycardia),
Discussion
Classical conditioning in which emotional associative learning and systemic conditioned responses are involved is established relatively quickly compared with that involving discrete somatic motor learning [13], [15]. In the present study, we found that the establishment of a conditioned bradycardic response was remarkably rapid in our preparations, supporting our belief that the present situation was not discrete somatic motor learning but so-called fear-related emotional learning. Although
Acknowledgments
This research was partly supported by Japan Society for the Promotion of Science.
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