mise à jour du
31 décembre 2002
1971; 232; 274-275
 Yawning and Penile Erection induced in Rats by Cortical Spreading Depression
Joseph Houston
Institute of Physiology, Czeehosloyak Academy of Sciences, Prague


Behaviour can be stimulated as well as disrupted by cortical spreading depression (CSD) the massive wave of neural depolarization and subsequent EEG depression which can be induced to spread slowly over a cortical hemisphere by injection of KCl. Eating, drinking and locomotor behaviour have been elicited in rats by administering single waves of CSD (ref. 1). Consummatory behaviour was most likely to be elicited if an animal had extensive feeding experience in the testing chamber.I wish to describe two further effects of single waves of CSD, the induction of penile erections and of stretching and yawning responses.
Waves of spreading depression were generated either by injecting K+ electrophoretically into the occipital cortices of male hooded rats, or by pressure injection of about 1 µl. of 6% KCl solution. For electrophoresis, refillable glass cannulae containing 25% KCl and a coiled silver wire were implanted chronically. An anodal current of 1 mA, passed between the wire and an indifferent electrode over the cerebellum for about 10s, released sufficient K + to trigger one wave of CSD (ref. 2). For pressure injection, steel cannulae were placed over the occipital cortices, and KCI was injected from smaller needles attached to flexible polyethylene tubing. The corresponding steady potential changes (SPCs) were recorded bilaterally across Ag-AgCl electrodes placed 2 mm lateral to bregma. The reference electrode was 10 mm anterior to bregma at the midline. The animals were able to eat and drink freely inside a box with one transparent wall. Waves of CSD were administered at least 5 mn after any manifestation of the behaviour we were monitoring. Waves were separated by at least 15 mn.
Although much less consummatory behaviour was elicited by CSD than when rats were pretrained to drink, the rats yawned frequently after a wave of CSD. Yawning was never observed in the experimental chamber except after CSD. Typically, the response consists of stretching, with extension and raising of a forepaw, accompanied by wide opening of the mouth. Frequently chewing movements preceded a yawning response by 1-30 s. Yawning was observed in seventeen out of twenty rats tested.
Fig. 2 (top) shows the distribution of onset times of seventythree cases of elicited yawning in six rats. Of these 25% were elicited by CSD applied to both cortical hemispheres. This histoggram shows the proportion of times yawning commenced in any of the 1 mn intervals following injection of K+. Onsets were measured from an arbitrary synchronization point of maximal depolarization at the anterior (frontal cortex) recording electrode. Thus,whenever yawning accompanied a wave of CSD, it was most likely to occur first about 5 min after initiation of CSD. The mean number of yawns elicited by each of the seventythree CSDs was 4.3. Fig. 2 (bottom) shows the distribution of all the 314 elicited yawns. At least three waves of CSD usually necessary before a rat yawned. On four occasions yawning followed the first wave of CSD, but in each case the animals had experience of CSD on the previous day. Apparently facilitation of yawning after CSD persists for some time. Drowsiness generally coincided with yawning.
When yawning followed waves of CSD it was often possible to elicit the chewing-stretching-yawning sequence by startling the animal with a sudden noise or by tapping on the chamber.
Thus CSD has two effects, namely, repeated waves induce a disposition to yawn, and subsequently single waves can the yawning response. The metabolic, hormonal, and physiological changes which accompany CSD (ref. 3) may provide insight into the central mechanism of yawning, of which little is known.
The distribution of the onset of yawning corresponds closely to the distribution of the other behaviours elicited by CSP in slightly different conditions. This fact, in conjunction with the data on the induction of yawning in startled animals, suggests that, given a readiness to yawn, the yawning response occurs an after effect of arousing stimulation, possibly when arousal value of the stimulation has dissipated (the lateencies of yawning elicited by startling varied from 1to 20 s). ln such scheme, a yawn could have some regulatory function in reponse to a sudden mobilization of neural resources.

During and between bouts of CSD which induced yawning six rats frequently exhibited penile erections. Typically they arched their backs, trembled, reared up on their hind legs, and exhibited a remarkable erection, often with waving of the penis. This was followed by licking and grooming of the genital area. The relationship in these experiments between yawning and sexual excitement is not clear, but it is interesting in the light of recent reports of yawning and sexual arousal induced concurently by injection of adrenocorticotropin into the brain of rats, rabbits and cats and by the release of erections and yawning in monkeys in response to their mirror images.