Behavioral and Neurochemical Consequences of Cortical Spreading Depression in Freely Moving Rats

Abstract

Cortical Spreading Depression (CSD) is characterized by a wave of neuronal and glial depolarization followed by depression of bioelectrical activity that slowly propagates through the cortex of many species, including humans. CSD is associated with brain disorders such as stroke, head trauma and migraine. Many earlier studies have provided compelling evidence that CSD is the underlying mechanism of aura in migraine; however, whether CSD can elicit headache associated with migraine is not fully understood. Cutaneous allodynia is highly prevalent in the peri-orbital area and extracephalic sites of migraine patients, suggesting that sensitization of primary afferents and central trigeminovascular neurons in these patients could be initiated by the underlying mechanism of aura.Unlike previous reports on the interaction between CSD and the trigeminal system, in which nociceptive behavior could not be measured since they employed anesthetized animals, we designed a model in which freely moving rats could be monitored for both CSD events and behavior responses due to pinprick plus KCl injection to the occipital cortex. We show that significant tactile hypersensitivity of the periorbital region of the face and hindpaws develop in a time-dependent manner following CSD. Enhanced expression of Fos protein and increased mRNA levels of the inflammatory cytokines IL-1beta and IL-6 are found within the trigeminal nucleus caudalis (TNC) two hours following cortical injection. We further show that systemic administration of anti-migraine drugs such as sumatriptan, naproxen and CGRP(8-37) (a CGRP antagonist) attenuate the generalized allodynia that ensue following cortical stimulation by KCl. Microinjection of bupivacaine in the ipsilateral trigeminal ganglion or in the rostral ventromedial medulla (RVM) prior to cortical pinprick plus KCl injection reversibly diminishes tactile hypersensitivity, suggesting that RVM pain-facilitating cells become activated by a trigeminal-RVM pathway following CSD. In addition we demonstrate that cortical pinprick plus KCl injection induced CSD events in 24/28 (85%) rats, among which 66% and 87% developed allodynia in the face and hindpaw, respectively.These studies suggest a potential association between CSD and development of hypersensitivity in rats, indicating that this model can be used to investigate the role of CSD-evoked migraine-related pain and to explore novel therapeutic strategies.