Pharmacological and behavioral characteristics of 22 kHz alarm calls in rats

The present review is focused on the neural mechanisms and acoustic features of 22kHz alarm calls emitted by adult rats as a defensive measure in numerous behavioral situations. The alarm calls are initiated by activity of the cholinergic neurons of the laterodorsal tegmental nucleus (LDT) and a subsequent release of acetylcholine at the target areas, collectively termed as the medial cholinoceptive vocalization strip. Injection of carbachol, a predominantly muscarinic agent, into any portion of the cholinoceptive strip, or direct stimulation of the LDT, induced species-typical 22kHz calls comparable to those emitted in natural situations. The pharmacologically induced 22kHz calls contained their alarming properties for naïve rats. The 22kHz calls induced either by carbachol or by stimulation of the LDT could be antagonized by atropine, or scopolamine applied into the cholinoceptive strip. Our recent behavioral studies have shown that the combination of long call duration and constant sound frequency (20-30kHz) convey the alarming message. Anatomical and neurochemical organization of the vocalization strip and acoustic properties of the calls lead to the conclusion that 22kHz calls indicate a fundamental, negative affective state common for many behavioral situations.