The effects of external stimuli on the emotional-aversive response evoked by intrahypothalamic carbachol injections

The aim of this investigation was to examine the effects of threatening (the sight and barking of the dog) or ‘neutral’ (flash and tone) stimuli on the emotional-aversive response evoked by direct intracerebral application of carbachol in cats. Unilateral microinjections of carbachol in 10 μg doses and 2 μl volume into anterior hypothalamic/preoptic area caused a full-blown emotional response with marked vocalization. All stimuli which might be threatening for cats caused a significant increase of vocalization. ‘Neutral’ stimuli of comparable intensity were without effect. To investigate the effects of stimulation intensity two clicks: low and loud (difference of 27 dB), were used. The loud click significantly increased vocalization level while the low click of the same intensity as a dog's barking had no significant influence on the carbachol-induced response. Cholinergic stimulation of the anterior hypothalamic/preoptic area seems to be specifically responsible for the obtained change of perception towards increased sensitivity to any potentially threatening alteration of the environment.