Corticotropin releasing factor receptor type 1 signaling in epilepsy and traumatic brain injury

Stress increases the frequency by which epileptic seizures occur. Corticotropin-releasing factor (CRF) coordinates neuroendocrine, autonomic and behavioral response to stress. This thesis sought to study the cellular and molecular mechanisms by which CRF regulates the activity of neural circuits in the piriform cortex (PC) in normal and epileptic states. The PC is richly innervated by CRF and 5-HT containing axons arising from the central amygdala and raphe nucleus. CRFR1 and 5-HT2A/CRs have been shown to interact in a manner where CRFR activation subsequently potentiates the activity of 5-HT2A/CRs. The first purpose of this thesis was to determine how the activation of CRFR1 and/or 5-HT2Rs modulates PC activity at both the circuit and cellular level. Voltage-sensitive dye imaging showed that CRF acting through CRFR1 dampened activation of layer II in the PC and interneurons of the endopiriform nucleus. Application of the selective 5-HT2A/CRagonist 2,5-dimethoxy-4-iodoamphetamine (DOI) following CRFR1 activation potentiated this effect. Blocking the interaction between CRFR1 and 5-HT2R with a Tat-CRFR1-CT peptide abolished this potentiation. Application of forskolin did not mimic CRFR1 activity but instead blocked it, while a protein kinase A antagonist had no effect. However, activation and antagonism of protein kinase C (PKC) either mimicked or blocked CRF modulation respectively. DOI had no effect when applied alone indicating that the prior activation of CRFR1 receptors was critical for the DOI activity. This data shows that CRF and 5-HT, acting through 5-HT2A/CRs, reduce the activation of the PC. This modulation may be an important blunting mechanism of stressor behaviors mediated through the olfactory cortex. Anxiety and stress conditions induce neurons arising from the central amygdala and local interneurons to release CRF in PC, where it normally dampens excitability. The second aim of this thesis was to determine the role of CRF in stress associated epilepsy. We showed that CRF increased the excitability of PC in rats subjected to kindling, a model of temporal lobe epilepsy. In non-kindled rats, CRF activates its receptor, a G protein-coupled receptor (GPCR) and signals through a Gaq/11 mediated pathway as identified in the first aim of this thesis. After seizure induction, CRF signaling occurred through a pathway involving Gas. This change in signaling was associated with reduced abundance of regulator of G-protein signaling protein-2 (RGS2), which promotes the switch in CRFR1 signaling cascade to a Gas dependent mechanism. RGS2 knockout mice responded to CRF in a similar manner as epileptic rats. These observations indicate that seizures produce changes in neuronal signaling that can increase seizure occurrence by converting a beneficial stress response into an epileptic trigger. People with traumatic brain injury often develop epileptic seizures. The mechanisms underlying this are poorly understood. Considerable evidence suggests that association of stressful life experiences in brain injured patients lead them to develop post-traumatic stress disorder. CRF release in brain regions that are implicated in epileptogenesis make these situations worse. The third aim of this thesis was to understand the role of CRF in inducing excitability in PC after brain injury. We found that CRF has variable effects on the interneurons of ipsilateral and contralateral PC. Altogether, its actions lead to increased excitability of PC compared to healthy rat PC. The extent of excitability produced by CRF and the signaling mechanism of CRFR1 after brain injury were similar to CRF actions and CRFR1 signaling mechanism in kindling induced epilepsy. Overall, this thesis study provides the basic mechanisms by which certain forms of epilepsy, both stress and injury induced develops. It also points out the important discovery of this project that is, the capability of GPCRs to switch signaling cascades depending on the pathological condition of the brain

Suppression of piriform cortex activity in rat by corticotropin-releasing factor 1 and serotonin 2A/C receptors

The piriform cortex (PC) is richly innervated by Corticotropin-releasing factor (CRF) and Serotonin (5-HT) containing axons arising from central amygdala and Raphe nucleus. CRFR1 and 5-HT2A/2CRs have been shown to interact in manner where CRFR activation subsequently potentiates the activity of 5-HT2A/2CRs. The purpose of this study was to determine how the activation of CRFR1 and/or 5-HT2Rs modulates PC activity at both the circuit and cellular level. Voltage sensitive dye imaging showed that CRF acting through CRFR1 dampened activation of the layer II of PC and interneurons of endopiriform nucleus. Application of the selective 5-HT2A/CR agonist 2,5-dimethoxy-4-iodoamphetamine (DOI) following CRFR1 activation potentiated this effect. Blocking the interaction between CRFR1 and 5-HT2R with a Tat-CRFR1-CT peptide abolished this potentiation. Application of forskolin did not mimic CRFR1 activity but instead blocked it, while a protein kinase A antagonist had no effect. However, activation and antagonism of protein kinase C (PKC) either mimicked or blocked CRF modulation respectively. DOI had no effect when applied alone indicating that the prior activation of CRFR1 receptors was critical for DOI to show significant effects similar to CRF. Patch clamp recordings showed that both CRF and DOI reduced the synaptic responsiveness of layer II pyramidal neurons. CRF had highly variable effects on interneurons within layer III, both increasing and decreasing their excitability, but DOI had no effect on the excitability of this group of neurons. These data show that CRF and serotonin, acting through both CRFR1 and 5-HT2A/CRs, reduce the activation of the PC. This modulation may be an important blunting mechanism of stressor behaviours mediated through the olfactory cortex

BK Channels Mediate Synaptic Plasticity Underlying Habituation in Rats

Habituation is a basic form of implicit learning and represents a sensory filter that is disrupted in autism, schizophrenia, and several other mental disorders. Despite extensive research in the past decades on habituation of startle and other escape responses, the underlying neural mechanisms are still not fully understood. There is evidence from previous studies indicating that BK channels might play a critical role in habituation. We here used a wide array of approaches to test this hypothesis. We show that BK channel activation and subsequent phosphorylation of these channels are essential for synaptic depression presumably underlying startle habituation in rats, using patch-clamp recordings and voltage-sensitive dye imaging in slices. Furthermore, positive modulation of BK channels in vivo can enhance short-term habituation. Although results using different approaches do not always perfectly align, together they provide convincing evidence for a crucial role of BK channel phosphorylation in synaptic depression underlying short-term habituation of startle. We also show that this mechanism can be targeted to enhance short-term habituation and therefore to potentially ameliorate sensory filtering deficits associated with psychiatric disorders

CRF Mediates Stress-Induced Pathophysiological High-Frequency Oscillations in Traumatic Brain Injury

Copyright © 2019 Narla et al. It is not known why there is increased risk to have seizures with increased anxiety and stress after traumatic brain injury (TBI). Stressors cause the release of corticotropin-releasing factor (CRF) both from the hypothalamic pituitary adrenal (HPA) axis and from CNS neurons located in the central amygdala and GABAergic interneurons. We have previously shown that CRF signaling is plastic, becoming excitatory instead of inhibitory after the kindling model of epilepsy. Here, using Sprague Dawley rats we have found that CRF signaling increased excitability after TBI. Following TBI, CRF type 1 receptor (CRFR1)-mediated activity caused abnormally large electrical responses in the amygdala, including fast ripples, which are considered to be epileptogenic. After TBI, we also found the ripple (120-250 Hz) and fast ripple activity (\u3e250 Hz) was cross-frequency coupled with θ (3-8 Hz) oscillations. CRFR1 antagonists reduced the incidence of phase coupling between ripples and fast ripples. Our observations indicate that pathophysiological signaling of the CRFR1 increases the incidence of epileptiform activity after TBI. The use for CRFR1 antagonist may be useful to reduce the severity and frequency of TBI associated epileptic seizures