Lateralized hunger: ipsilateral attenuation of cortical spreading depression-induced feeding after unilateral 6-OHDA injection into the substantia nigra

Does single cortical spreading depression elicit pain behaviour in freely moving rats?

INTRODUCTION

Behavioural animal studies are critical, particularly to translate results to human beings. Cortical spreading depression (CSD) has been implicated in migraine pathogenesis. We aimed to investigate the effects of CSD on the behaviour of freely moving rats, since available CSD models do not include awake animals.

MATERIALS AND METHODS

We developed a new model to induce single CSD by applying topical N-methyl-D-aspartate (NMDA) and employed a combination of an automated behavioural analysis system, video camera and ultrasonic vocalisation (USV) calls for the first time. Electrocorticograms were also studied during CSD in freely moving rats. Behaviour associated with cephalic pain was assessed in a group of rats that received sumatriptan. Cortical c-fos immunoreactivity was performed in order to confirm CSD.

RESULTS

NMDA induced single CSD in ipsilateral cortex, evoked freezing behaviour (P < 0.01) and increased the number of wet dog shakes (WDS; P < 0.01). Grooming, locomotion, eating, drinking, and circling were not significantly altered among groups. Ultrasonic vocalisations compatible with pain calls (22-27 kHz) were only detected in 3 out of 25 rats. Sumatriptan did not significantly reduce the freezing behaviour. CSD induced significant c-fos expression in ipsilateral cerebral cortex and amygdala (P < 0.01).

CONCLUSIONS

CSD induces freezing behaviour by invoking anxiety/fear via amygdala activation in freely-moving rats. Single CSD is unlikely to lead to severe pain in freely-moving rats, though the development of mild or vague pain cannot be excluded. The relevance of rat behavioural responses triggered by CSD to migraine symptoms in humans needs further evaluation.

Electroencephalographic spreading depression and concomitant behavioral changes induced by intrahippocampal injections of ACTH1-24 and D-Ala2-Met-enkephalinamide in the rat

ACTH1-24 (0.5 or 10 micrograms = 0.17 or 3.45 nmol) and D-Ala2-Met-enkephalinamide (DAME; 10 micrograms = 17.05 nmol) were injected unilaterally into the hippocampus of freely moving rats to examine their effects on EEG activity, DC potentials and behavior. In 85% of the rats DAME elicited spreading depression (SD) with epileptiform discharges preceding and following the wave of SD. The following behavioral changes were recorded. DAME- and KCl-induced SD were accompanied by an increase in locomotor activity and wet-dog shaking behavior, which occurred only during the period of SD. After a wave of SD induced by DAME a biphasic pattern of activity, consisting of an initial depression in locomotion followed by hyperactivity, appeared in 59% of the rats. ACTH1-24 elicited SD in 13% of the rats tested. Neither the dosage of ACTH1-24 nor the strain of rats influenced the occurrence of SD and the incidence of ACTH-induced grooming behavior. SD induced by KCl also resulted in excessive grooming comparable to that induced by ACTH1-24. In the case of KCl-induced SD, grooming began directly after the injection of KCl and was frequently interrupted by short periods of locomotion. ACTH-induced grooming had a later onset and episodes of stretching and yawning were observed. It can be concluded that the behavioral effects of the injection of DAME are unspecific responses to SD and seizure activity. However, ACTH-induced grooming is not solely a byproduct of SD, since it occurred also in the absence of SD.