Hypothalamocerebellar and cerebellohypothalamic projections--circuits for regulating nonsomatic cerebellar activity?

Cerebellar involvement in visceral and affective responses is known from physiological and behavioral studies, but the pathways involved in these responses have remained enigmatic. Over the last ten years neuroanatomical studies have shown that the cerebellum and hypothalamus are interconnected by direct hypothalamocerebellar and cerebellohypothalamic projections and by a multitude of indirect pathways. The hypothalamocerebellar projection terminates in the cerebellar nuclei and in all layers of the cerebellar cortex as multilayered fibres. This projection is, at least in part, histaminergic. New immunocytochemical experiments indicate that small numbers of hypothalamocerebellar neurones may contain GABA- or glycine-like immunoreactivity. GABA may function as a transmitter in hypothalamocerebellar fibres, probably in conjunction with histamine, but it is not clear whether glycine may also function as a transmitter or only serve metabolic functions. The bidirectional pathways between the cerebellum and hypothalamus may be part of the circuits through which the cerebellum participates in the modulation of a variety of nonsomatic events. In addition, new observations on patients with well localized cerebellar lesions reveal simultaneous somatic and visceral dysfunction. Recent research on direct hypothalamocerebellar pathways and on other connections between hypothalamus and cerebellum is reviewed. It is hypothesized that the cerebellum may act as a general modulator and coordinator of a wide range of central nervous activities, somatic as well as nonsomatic.