Presynaptic localization of a truncated isoform of the type 2 corticotropin releasing factor receptor 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.

Opposing actions of CRF-R1 and CB1 receptor on facial stimulation-induced MLI-PC plasticity in mouse cerebellar cortex

BACKGROUND

Corticotropin-releasing factor (CRF) is the major neuromodulator orchestrating the stress response, and is secreted by neurons in various regions of the brain. Cerebellar CRF is released by afferents from inferior olivary neurons and other brainstem nuclei in response to stressful challenges, and contributes to modulation of synaptic plasticity and motor learning behavior via its receptors. We recently found that CRF modulates facial stimulation-evoked molecular layer interneuron-Purkinje cell (MLI-PC) synaptic transmission via CRF type 1 receptor (CRF-R1) in vivo in mice, suggesting that CRF modulates sensory stimulation-evoked MLI-PC synaptic plasticity. However, the mechanism of how CRF modulates MLI-PC synaptic plasticity is unclear. We investigated the effect of CRF on facial stimulation-evoked MLI-PC long-term depression (LTD) in urethane-anesthetized mice by cell-attached recording technique and pharmacological methods.

RESULTS

Facial stimulation at 1 Hz induced LTD of MLI-PC synaptic transmission under control conditions, but not in the presence of CRF (100 nM). The CRF-abolished MLI-PC LTD was restored by application of a selective CRF-R1 antagonist, BMS-763,534 (200 nM), but it was not restored by application of a selective CRF-R2 antagonist, antisauvagine-30 (200 nM). Blocking cannabinoid type 1 (CB1) receptor abolished the facial stimulation-induced MLI-PC LTD, and revealed a CRF-triggered MLI-PC long-term potentiation (LTP) via CRF-R1. Notably, either inhibition of protein kinase C (PKC) with chelerythrine (5 µM) or depletion of intracellular Ca²⁺ with cyclopiazonic acid (100 µM), completely prevented CRF-triggered MLI-PC LTP in mouse cerebellar cortex in vivo.

CONCLUSIONS

The present results indicated that CRF blocked sensory stimulation-induced opioid-dependent MLI-PC LTD by triggering MLI-PC LTP through CRF-R1/PKC and intracellular Ca²⁺ signaling pathway in mouse cerebellar cortex. These results suggest that activation of CRF-R1 opposes opioid-mediated cerebellar MLI-PC plasticity in vivo in mice.

Corticotrophin-Releasing Factor Modulates the Facial Stimulation-Evoked Molecular Layer Interneuron-Purkinje Cell Synaptic Transmission in vivo in Mice

Corticotropin-releasing factor (CRF) is an important neuromodulator in central nervous system that modulates neuronal activity via its receptors during stress responses. In cerebellar cortex, CRF modulates the simple spike (SS) firing activity of Purkinje cells (PCs) has been previously demonstrated, whereas the effect of CRF on the molecular layer interneuron (MLI)–PC synaptic transmission is still unknown. In this study, we examined the effect of CRF on the facial stimulation–evoked cerebellar cortical MLI-PC synaptic transmission in urethane-anesthetized mice by in vivo cell-attached recording, neurobiotin juxtacellular labeling, immunohistochemistry techniques, and pharmacological method. Cell-attached recordings from cerebellar PCs showed that air-puff stimulation of ipsilateral whisker pad evoked a sequence of tiny parallel fiber volley (N1) followed by MLI-PC synaptic transmission (P1). Microapplication of CRF in cerebellar cortical molecular layer induced increases in amplitude of P1 and pause of SS firing. The CRF decreases in amplitude of P1 waveform were in a dose-dependent manner with the EC₅₀ of 241 nM. The effects of CRF on amplitude of P1 and pause of SS firing were abolished by either a non-selective CRF receptor antagonist, α-helical CRF-(9-14), or a selective CRF-R1 antagonist, BMS-763534 (BMS, 200 nM), but were not prevented by a selective CRF-R2 antagonist, antisauvagine-30 (200 nM). Notably, application CRF not only induced a significant increase in spontaneous spike firing rate, but also produced a significant increase in the number of the facial stimulation–evoked action potential in MLIs. The effect of CRF on the activity of MLIs was blocked by the selective CRF-R1 antagonist, and the MLIs expressed the CRF-R1 imunoreactivity. These results indicate that CRF increases excitability of MLIs via CRF-R1, resulting in an enhancement of the facial stimulation–evoked MLI-PC synaptic transmission in vivo in mice.

Oxytocin facilitates adaptive fear and attenuates anxiety responses in animal models and human studies-potential interaction with the corticotropin-releasing factor (CRF) system in the bed nucleus of the stria terminalis (BNST)

Despite its relatively well-understood role as a reproductive and pro-social peptide, oxytocin (OT) tells a more convoluted story in terms of its modulation of fear and anxiety. This nuanced story has been obscured by a great deal of research into the therapeutic applications of exogenous OT, driving more than 400 ongoing clinical trials. Drawing from animal models and human studies, we review the complex evidence concerning OT's role in fear learning and anxiety, clarifying the existing confusion about modulation of fear versus anxiety. We discuss animal models and human studies demonstrating the prevailing role of OT in strengthening fear memory to a discrete signal or cue, which allows accurate and rapid threat detection that facilitates survival. We also review ostensibly contrasting behavioral studies that nonetheless provide compelling evidence of OT attenuating sustained contextual fear and anxiety-like behavior, arguing that these OT effects on the modulation of fear vs. anxiety are not mutually exclusive. To disambiguate how endogenous OT modulates fear and anxiety, an understudied area compared to exogenous OT, we survey behavioral studies utilizing OT receptor (OTR) antagonists. Based on emerging evidence about the role of OTR in rat dorsolateral bed nucleus of stria terminalis (BNST) and elsewhere, we postulate that OT plays a critical role in facilitating accurate discrimination between stimuli representing threat and safety. Supported by human studies, we demonstrate that OT uniquely facilitates adaptive fear but reduces maladaptive anxiety. Last, we explore the limited literature on endogenous OT and its interaction with corticotropin-releasing factor (CRF) with a special emphasis on the dorsolateral BNST, which may hold the key to the neurobiology of phasic fear and sustained anxiety.

Corticotrophin-Releasing Factor Modulates Cerebellar Purkinje Cells Simple Spike Activity in Vivo in Mice

Corticotropin-releasing factor (CRF) is a major neuromodulator that modulates cerebellar neuronal activity via CRF receptors during stress responses. In the cerebellar cortex, CRF dose-dependently increases the simple spike (SS) firing rate of Purkinje cells (PCs), while the synaptic mechanisms of this are still unclear. We here investigated the effect of CRF on the spontaneous SS activity of cerebellar PCs in urethane-anesthetized mice by in vivo electrophysiological recording and pharmacological methods. Cell-attached recordings from PCs showed that micro-application of CRF in cerebellar cortical molecular layer induced a dose-dependent increase in SS firing rate in the absence of GABA_A receptor activity. The CRF-induced increase in SS firing rate was completely blocked by a nonselective CRF receptor antagonist, α-helical CRF-(9–14). Nevertheless, application of either a selective CRF-R1 antagonist, BMS-763534 (BMS, 200 nM) or a selective CRF-R2 antagonist, antisauvagine-30 (200 nM) significantly attenuated, but failed to abolished the CRF-induced increase in PCs SS firing rate. In vivo whole-cell patch-clamp recordings from PCs showed that molecular layer application of CRF significantly increased the frequency, but not amplitude, of miniature postsynaptic currents (mEPSCs). The CRF-induced increase in the frequency of mEPSCs was abolished by a CRF-R2 antagonist, as well as protein kinase A (PKA) inhibitors. These results suggested that CRF acted on presynaptic CRF-R2 of cerebellar PCs resulting in an increase of glutamate release through PKA signaling pathway, which contributed to modulation of the cerebellar PCs outputs in Vivo in mice.

Topographical distribution of corticotropin-releasing factor type 2 receptor-like immunoreactivity in the rat dorsal raphe nucleus: co-localization with tryptophan hydroxylase

Corticotropin-releasing factor (CRF) and CRF-related neuropeptides are involved in the regulation of stress-related physiology and behavior. Members of the CRF family of neuropeptides bind to two known receptors, the CRF type 1 (CRF₁) receptor, and the CRF type 2 (CRF₂) receptor. Although the distribution of CRF₂ receptor mRNA expression has been extensively studied, the distribution of CRF₂ receptor protein has not been characterized. An area of the brain known to contain high levels of CRF₂ receptor mRNA expression and CRF₂ receptor binding is the dorsal raphe nucleus (DR). In the present study we investigated in detail the distribution of CRF₂ receptor immunoreactivity throughout the rostrocaudal extent of the DR. CRF₂ receptor-immunoreactive perikarya were observed throughout the DR, with the highest number and density in the mid-rostrocaudal DR. Dual immunofluorescence revealed that CRF₂ receptor immunoreactivity was frequently co-localized with tryptophan hydroxylase, a marker of serotonergic neurons. This study provides evidence that CRF₂ receptor protein is expressed in the DR, and that CRF₂ receptors are expressed in topographically organized subpopulations of cells in the DR, including serotonergic neurons. Furthermore, these data are consistent with the hypothesis that CRF₂ receptors play an important role in the regulation of stress-related physiology and behavior through actions on serotonergic and non-serotonergic neurons within the DR.

Alterations in colonic corticotropin-releasing factor receptors in the maternally separated rat model of irritable bowel syndrome: differential effects of acute psychological and physical stressors

Early-life stress is a key predisposing factor to the development of functional gastrointestinal (GI) disorders. Thus, changes in stress-related molecular substrates which influence colonic function may be important in understanding the pathophysiology of such disorders. Activation of peripheral corticotropin-releasing factor (CRF) receptors is thought to be important in the maintenance of GI function homeostasis. Therefore, immunofluorescent and Western blotting techniques were utilized to investigate colonic expression of CRF receptors in the maternal separation (MS) model as compared to non-separated (NS) rats. Receptor expression was also assessed following exposure to two different acute stressors, the open field (OF) and colorectal distension (CRD). Immunofluorescent dual-labeling demonstrated increased activation of both CRFR1 (MS: 79.6+/-4.4% vs. NS: 43.8+/-6.8%, p<0.001) and CRFR2 (MS: 65.9+/-3.2% vs. NS: 51.6+/-5.8%, p<0.05) positive cells in MS rats. Protein expression of CRFR1 and CRFR2 in the proximal colon was similar under baseline conditions and not affected by exposure to an OF stressor in either cohort. In contrast, distal CRFR1 and CRFR2 levels were higher in MS rats but were significantly reduced post OF stress. Moreover, decreases in expression of CRFR1 in the proximal and distal colon of NS rats following exposure to CRD were blunted in MS rats. CRD also caused an increase in the functional isoform of CRFR2 in the distal colon of MS rats with no effect in NS colons. This study demonstrates that acute stressors alter colonic CRF receptor expression in a manner that is determined by the underlying stress sensitivity of the subject.

Early life social isolation alters corticotropin-releasing factor responses in adult rats

Stress induced by early life social isolation leads to long-lasting alterations in stress responses and serotonergic activity. Corticotropin-releasing factor (CRF) is a neurotransmitter that mediates stress responses and alters serotonergic activity. We tested the hypothesis that the stress of early life isolation enhances responses to CRF in adulthood by determining the effect of CRF infusions into the dorsal raphe nucleus (dRN) on 5-HT release in the nucleus accumbens (NAc) of adult rats using in vivo microdialysis. Juvenile male rats were either isolated or housed in groups of three for a 3-week period beginning on postnatal day 21 after which, all rats were group-reared for an additional 2 weeks. Following the isolation/re-socialization procedure, infusion of 100 ng CRF into the dRN decreased 5-HT release in the NAc of group-reared rats. This treatment did not significantly affect 5-HT release in the NAc of isolation-reared animals. In contrast, infusion of 500 ng CRF into the dRN transiently increased 5-HT release in the NAc of both group-reared and isolated animals with isolated animals showing a more prolonged serotonergic response. Western blot and immunofluorescent staining for CRF receptors in the dRN showed that CRF(2) receptor levels were increased in the dRN of isolation-reared animals when compared with group-reared rats. Taken together, the results suggest that isolation during the early part of development causes alterations in both CRF receptor levels and CRF-mediated serotonergic activity. These effects may underlie the increased sensitivity to stress observed in isolates.

Cellular localization of the full-length isoform of the type 2 corticotropin releasing factor receptor in the postnatal mouse cerebellar cortex

Corticotropin releasing factor (CRF) and its cognate receptors, defined as Type 1 and Type 2 have been localized within the cerebellum. The Type 2 CRF receptor (CRF-R2) is known to have both a full length (CRF-R2alpha) and a truncated (CRF-R2alpha-tr) isoform. A recent study documented CRF-R2alpha primarily in Bergann glia and astrocytes, as well as in populations of Purkinje cells in the adult cerebellum. The goal of the present study is to determine if CRF-R2alpha is present in the postnatal cerebellum, and if so to describe its cellular distribution. RT-PCR data showed that CRF-R2alpha is expressed in the mouse cerebellum from birth through postnatal day 21. Between birth and P14, CRF-R2alpha-immunoreactivity was localized within the somata of Purkinje cells, and migrating GABAergic interneurons. GFAP-immunoreactive astrocytes, including Bergmann glia, also expressed CRF-R2alpha-immunoreactivity from P3-P14. There is a change, however, in CRF-R2alpha immunolabeling within neurons as the cerebellum matures. Compared to its expression in the adult cerebellum, Purkinje cells, and GABAergic interneurons showed more extensive CRF-R2alpha immunolabeling during early postnatal development. We postulate that CRF-R2alpha could be involved in developmental events related to the survival and differentiation of Purkinje cells and GABAergic neurons, whereas in the adult, this isoform of the CRF receptor family is likely involved in modulating Bergmann glia that have been shown to play a role in regulating the synaptic environment around Purkinje neurons.

Evidence for the presence of the type 2 corticotropin releasing factor receptor in the rodent cerebellum

Corticotropin releasing factor (CRF), localized in afferent inputs to the cerebellum, binds to two receptors defined as the Type 1 (CRF-R1) and the Type 2 (CRF-R2alpha). CRF-R1 has been localized to the cerebellum, as has a truncated isoform of CRF-R2alpha. Evidence for the presence of the full length isoform of CRF-R2alpha in the cerebellum is conflicting. We used RT-PCR, immunohistochemical, and physiologic techniques to resolve this conflict. RT-PCR data show low levels of CRF-R2alpha in the vermis and hemisphere of the cerebellum. These observations were confirmed by the Gene Expression Nervous System Atlas (GENSAT) database. A CRF-R2alpha antibody was used to determine the cellular distribution of the receptor in the cerebellum. The vast majority of the receptors are localized to Bergmann glial cells located throughout the cerebellum, as well as astrocytes in the granule cell layer. Neuronal labeling is present in sub-populations of Purkinje cells, Golgi cells, basket cells, and cerebellar nuclear neurons. Physiologic data show that urocortin II, which binds selectively to CRF-R2alpha, increases the firing rate of both Purkinje cells and nuclear neurons; this response can be blocked by the CRF-R2alpha-specific antagonist, antisauvagine-30. The present results confirm that CRF-R2alpha is present in the cerebellum and functions in circuits that modulate the firing rate of Purkinje cells and cerebellar nuclear neurons. A comparative analysis showed that the patterns of distribution of CRF-R1, CRF-R2alpha and CRF-R2alpha-tr are distinct. These data indicate that the CRF family of peptides modulates cerebellar output by binding to multiple CRF receptors.