The objective of our experiment was to use electrical stimulation of the dlPAG and of the VMH in rats as a model of panic attack to study the involvement of the DCbN. We have found that panic-like behaviour was accompanied by a decrease of c-Fos-ir cells in the DCbN, indicating deactivation. c-Fos expression was significantly lower in the DN of both treatment groups and in the FN of the dlPAG DBS group when compared to controls. In the VMH DBS group, c-Fos expression in the FN was lower compared to controls, with a trend towards significance. Using a semiquantitative analysis, the IN showed a similar trend towards decreased c-Fos expression in the stimulated rats compared to controls. This should be considered a preliminary finding since the method of counting was not performed using stereological principles.
There is anatomical and functional evidence supporting the role of the cerebellum in panic. Sakai et al. found a significantly higher glucose uptake in the cerebellum of patients with panic disorder compared to control patients [
20]. After clinical improvement of these patients due to cognitive-behavioural therapy, the glucose uptake in cerebellum had decreased [
21]. Several groups found increased activation of the cerebellum after CCK-4-induced panic attacks in healthy subjects [
24,
25]. This increase was not seen in these subjects during anticipatory anxiety. These studies were not initially designed to analyse changes in the cerebellum, and the changes found are usually unexpected. The findings are often simply mentioned as interesting or surprising, although several groups relate to the role of the cerebellum in fear conditioning as proposed by Sacchetti et al. [
38]. Sacchetti et al. reviewed the evidence showing that the cerebellar vermis plays a role in the fear response and in fear conditioning, especially in fear consolidation. The most important changes are thought to take place at the level of the Purkinje cell (PC). It has been shown that several forms of fear conditioning lead to increased PC excitability and an increased firing rate of the PCs. In contrast, heterozygous Lurcher mice, which show early and complete apoptosis of cerebellar PCs, show reduced inhibition to anxiety-provoking aversive areas [
39]. In summary, it seems that increased PC activity leads to more fear, and decreased or absent PC activity to less fear. Functional imaging is designed to analyse activation changes in the cerebral cortex; therefore, it is likely that the activation seen in the cerebellum is also located in the cortex. Increased activation found in functional imaging studies may therefore reflect the increased activity of the PCs. Since PCs are known to have an inhibitory action on the DCbN, these findings are in line with deactivation that we found in the DCbN in the stimulated rats which show panic-like behaviour.
In the cerebellum, the vermis, projecting through the fastigial nucleus, seems to be the most important structure in fear and panic. In the cases described by Schmahmann et al., the vermis was always involved in patients with changes of affect [
5]. In early experiments, vermal lesions were shown to attenuate a variety of fear behaviour, whereas vermal stimulation leads to increased fear-related responses [
40,
41] Other animal research shows a selective role for the fastigial nucleus in heart-rate conditioning [
40,
42,
43]. The vermis also contributes to consolidation of fear memory [
44]. There is also evidence of a role of the interposite nucleus and the dentate nucleus in fear. In studies investigating several aspects of fear conditioning, animals with lesions of the dentate and interposite nucleus do not acquire an aversive conditioned response but acquire an appetitive conditioned response [
45] and show unaltered heart-rate conditioning [
42] and vocalisation indicative of unspecific fear [
46]. Furthermore, there are clear bi-directional connections to the hypothalamus from (greatest to least concentration) the dentate nucleus, the interposite nucleus and the fastigial nucleus [
47‐
50], supporting a role of all DCbN in autonomic processes, for example, those related to fear. In addition, the DCbN have been shown to project to several parts of the fear network: Teune et al. injected tracers in the DCbN in rats and documented projections to the PAG from the fastigial nucleus and the dentate nucleus and, in lesser degree, from the interposite nucleus [
51]. Several groups have suggested projections from the fastigial nucleus to the amygdala, hippocampus, septal nuclei and the nucleus accumbens, based on behavioural changes following cerebellar stimulation or lesioning [
38,
41,
52]. Berntson and Torello, for example, showed that hyperemotionality caused by septal lesions was largely attenuated by lesions of the fastigial nucleus [
53]. An exact anatomical pathway has never been shown [
54]. However, these behavioural changes indicate that the involvement of the cerebellum in fear does not merely consist of regulation of an autonomic visceromotor response but that there is a place for the cerebellum in the network regulating fear processing [
38]. In a recent review, Stoodley and Schmahmann present a functional somatotopy of the non-motor functions of the cerebellum based on functional imaging [
55]. They conclude that the vermis of the posterior lobe seems to be specifically related to emotional processing, whereas activation in the posterior cerebellar hemispheres may be related to the decision-making aspects of the tasks used in the experimental setting [
55]. On the other hand, Timman et al. reviewed anatomical evidence for a role of the cerebellum in emotional and cognitive learning, and they conclude that with respect to fear, the vermis, projecting through the fastigial nucleus, contributes to the autonomic and somatic aspects, whereas the posterolateral cerebellar hemispheres, projecting through the dentate and interposite nucleus, play a role in the emotional content of fear processing [
40].
In summary, there is ample evidence that all cerebellar nuclei are involved in fear, in which the fastigial nucleus possibly mediates a different aspect of fear than the dentate nucleus and the interposite nucleus. This correlates with our findings in the present study showing a similar deactivation in the dentate nucleus and in the fastigial nucleus, and possibly also in the interposite nucleus. Increased fear is associated with increased PC activity in functional imaging studies, which is hypothesised to lead to inhibition of the DCbN and therefore of cerebellar output. We speculate that the cerebellum plays a role in regulating appropriate behaviour in response to any stimulus, and that a decreased cerebellar output may play a role in emergence of an inappropriate response, such as a panic attack. This decreased output may be in response to incorrect input, as suggested by Parvizi et al. [
31]; however, we suggest that the cerebellum also plays a direct role in the selection of relevant information on which an adequate behavioural response is based, and that deactivation of the DCbN then leads to inappropriate behaviour by inhibiting this selection process.