Localization and functional roles of corticotropin-releasing factor receptor type 2 in the cerebellum

Neuropeptides and Their Roles in the Cerebellum

Although more than 30 different types of neuropeptides have been identified in various cell types and circuits of the cerebellum, their unique functions in the cerebellum remain poorly understood. Given the nature of their diffuse distribution, peptidergic systems are generally assumed to exert a modulatory effect on the cerebellum via adaptively tuning neuronal excitability, synaptic transmission, and synaptic plasticity within cerebellar circuits. Moreover, cerebellar neuropeptides have also been revealed to be involved in the neurogenetic and developmental regulation of the developing cerebellum, including survival, migration, differentiation, and maturation of the Purkinje cells and granule cells in the cerebellar cortex. On the other hand, cerebellar neuropeptides hold a critical position in the pathophysiology and pathogenesis of many cerebellar-related motor and psychiatric disorders, such as cerebellar ataxias and autism. Over the past two decades, a growing body of evidence has indicated neuropeptides as potential therapeutic targets to ameliorate these diseases effectively. Therefore, this review focuses on eight cerebellar neuropeptides that have attracted more attention in recent years and have significant potential for clinical application associated with neurodegenerative and/or neuropsychiatric disorders, including brain-derived neurotrophic factor, corticotropin-releasing factor, angiotensin II, neuropeptide Y, orexin, thyrotropin-releasing hormone, oxytocin, and secretin, which may provide novel insights and a framework for our understanding of cerebellar-related disorders and have implications for novel treatments targeting neuropeptide systems.

Cdk5-dependent rapid formation and stabilization of dendritic spines by corticotropin-releasing factor

The neuropeptide corticotropin-releasing factor (CRF) exerts a pivotal role in modulating neuronal activity in the mammalian brain. The effects of CRF exhibit notable variations, depending on factors such as duration of exposure, concentration, and anatomical location. In the CA1 region of the hippocampus, the impact of CRF is dichotomous: chronic exposure to CRF impairs synapse formation and dendritic integrity, whereas brief exposure enhances synapse formation and plasticity. In the current study, we demonstrate long-term effects of acute CRF on the density and stability of mature mushroom spines ex vivo. We establish that both CRF receptors are present in this hippocampal region, and we pinpoint their precise subcellular localization within synapses by electron microscopy. Furthermore, both in vivo and ex vivo data collectively demonstrate that a transient surge of CRF in the CA1 activates the cyclin-dependent kinase 5 (Cdk5)-pathway. This activation leads to a notable augmentation in CRF-dependent spine formation. Overall, these data suggest that upon acute release of CRF in the CA1-SR synapse, both CRF-Rs can be activated and promote synaptic plasticity via activating different downstream signaling pathways, such as the Cdk5-pathway.

Spatial and temporal expression profiles of urocortin 3 mRNA in the brain of the chicken (Gallus gallus)

Urocortin 3 (UCN3) is a neuropeptide believed to regulate stress-coping responses by binding to type 2 corticotropin-releasing hormone receptors. Here, we report the cloning and brain distribution of UCN3 mRNA in a sauropsid-the chicken, Gallus gallus. Mature chicken UCN3 is predicted to be a 40-amino acid peptide showing high sequence similarity to human (93%), mouse (93%), and Xenopus (88%) UCN3. During the last third of embryonic development, UCN3 mRNA levels changed differentially in the various brain parts. In all brain parts, UCN3 mRNA levels tended to increase toward hatching, except for caudal brainstem, where a gradual decrease was observed during the last week of embryonic development. In cerebellum, a rapid increase in gene expression occurred between embryonic days 17 and 19. Using in situ hybridization, UCN3 mRNA was found to be expressed predominantly in the hypothalamus, pons, and medulla of posthatch chick brains, but not in some areas that are among the main expression sites in rodents, such as the brain areas where in mammals the median preoptic nucleus and the medial amygdala are located. This suggests that the roles of UCN3 in chicken, and perhaps sauropsids in general, are not all identical to those in rodents.

Corticotropin-releasing factor receptors and urocortins, links between the brain and the heart

Corticotropin-releasing factor (CRF), a 41 amino acid peptide, was discovered as a key signal in mediating neuroendocrine, autonomic, and behavioral responses to stress. It was revealed later that there exist additional CRF-like peptides, termed urocortins. The CRF receptor subtype 1 (CRF1 receptor) is predominant in the brain whereas subtype 2 (CRF2 receptor) is highly expressed in the brain and the heart. Both centrally and peripherally administered CRF and urocortins produce significant hemodynamic effects via activation of CRF receptors in the brain and the heart. CRF and urocortins are important neural and cardioactive hormones, and are potentially useful therapy for heart failure.

Hormonal regulation of cerebellar development and plasticity

Cerebellar development and plasticity is involved in various epigenetic processes that activate specific genes at different time point. The epigenetic influences include humoral influences from endocrine cells of peripheral organs. A number of hormone receptors are expressed in cerebellum, and cerebellar function is greatly influenced by hormonal status. Furthermore, recent studies have shown that some of such substances are produced locally and affect through their specific hormone receptors. The aim of this special issue was to introduce several key features of hormones and their receptors to regulate cerebellar development and plasticity. The contribution covers thyroid/steroid hormone systems including orphan receptors and co-regulators, neurosteroids, and transporters. It also covers environmental signal that may affect cerebellar hormonal environment. Furthermore, several neuropeptides, which are initially found as neuroendocrine hormones but later identified as neurotransmitters that play an important role in cerebellar function, are also covered.

Functional roles of neuropeptides in cerebellar circuits

Whereas the cerebellum contains 22 different types of neuropeptides as presently known, their expression is generally weak and diffusely dispersed in cerebellar tissues, which often makes their functional significance doubtful. Nevertheless, our knowledge about certain neuropeptides has advanced to the extent that we can figure out their unique functional roles in cerebellar circuits. Throughout the cerebellum, CRF is contained in climbing fibers and its spontaneous release is required for the induction of cerebellar long-term depression (LTD), a cellular mechanism of motor learning. Corticotropin-releasing factor (CRF) is also expressed in the paraventricular nucleus-pituitary system and amygdala-lower brainstem system, both of which are involved in coping responses to stress. In view that motor learning requires stressful efforts for correcting errors in repeated trials, CRF in climbing fibers may imply that the olivocerebellar system is part of a large CRF-operated functional system that acts to cope with various stressors. Orexin, on the other hand, is contained in beaded fibers, which, originating from the hypothalamus, project to various brainstem nuclei and also to the cerebellum, exclusively the flocculus. Currently available evidence suggests that, in fight-or-flight situations, orexinergic neurons switch the state of cardiovascular control systems including the flocculus to secure blood supply to working muscles. Considerable knowledge has also been accumulated about angiotensin II, galanin, and cerebellin, but there is still a gap in defining their unique functional roles in cerebellar circuits.