Stefan M. Brudzynski

Ethotransmission: communication of emotional states through ultrasonic vocalization in rats

Adult rats emit two categories of ultrasonic vocalizations, 22 kHz calls and 50 kHz calls. These vocalizations communicate animals emotional state to other members of the social group. Production of social vocalizations is an evolutionary old activity in vertebrates, and is regulated by well-preserved brain circuitries. The 22 kHz calls express negative, aversive state and are initiated by activity of the mesolimbic cholinergic system originating from laterodorsal tegmental nucleus. The 50 kHz calls express positive, appetitive state and are initiated by activity of the mesolimbic dopaminergic system originating from the ventral tegmental area. The 22 kHz calls serve as warning and alarm calls, while the 50 kHz calls serve as affiliative and social-cooperating calls. These specie-specific vocalizations play role of ethological transmitters, termed ethotransmitters, that is, they are species-specific signals that are selectively recognized by receivers and have capability of changing emotional state of the receivers.

Ultrasonic calls of rats as indicator variables of negative or positive states: Acetylcholine–dopamine interaction and acoustic coding

Adult rats produce two distinct types of ultrasonic vocalizations referred to as 22- and 50-kHz calls, respectively. Emission of the respective calls represents signaling a negative or positive state of the rat organism. The signaling has an adaptive value for survival and/or well-being of rats and their social groups. Literature is reviewed from studies on cats and rats, which indicates that the positive or negative states constitute a complex and integrated set of somatic, autonomic, endocrine, affective, and cognitive correlates. The basic states and their correlates are initiated, integrated, and maintained by activity of the subsets of the ascending cholinergic and dopaminergic systems originating from the reticular brainstem core. The cholinergic and dopaminergic systems interact mutually to form a dynamic balance, which is involved in a decision-making process of initiating and maintaining one or the other of these states. Activation of the relevant portion of the ascending cholinergic system invariably induces the negative state and releases 22-kHz calls while activation of the ascending dopaminergic system induces the positive state with 50-kHz calls. The 22- and 50-kHz calls have distinct and mostly non-overlapping acoustic parameters, which ensure unambiguous recognition of the calls and thus, the state of the emitter. The animal may only signal one of the states at any given time and emission of 22- or 50-kHz calls is mutually exclusive. It is postulated, therefore, that these two main types of ultrasonic calls are reliable indicator variables of two opposing states of the adult rat organism: negative or positive.

Amphetamine-induced 50 kHz calls from rat nucleus accumbens: A quantitative mapping study and acoustic analysis

Emission of 50 kHz ultrasonic calls in rats is known to be associated with appetitive behavioural situations and positive social interactions. The purpose of the study was to pharmacologically characterize amphetamine-induced 50 kHz calls and to perform quantitative mapping of this response in the nucleus accumbens. Injections of amphetamine into the nucleus accumbens induced species-typical 50 kHz calls in adult rats. The acoustic parameters of the calls were not affected by different amphetamine doses or combination of agents. The increase in the number of calls occurred predominantly from the accumbens shell and to a lesser degree from the core region. This effect was dose-dependent within the range of 1-20 microg of amphetamine and was reversed by pretreatment with D1 or D2 dopamine antagonists (SKF-83566 or raclopride) administered to the same brain site. However, another D2 dopamine receptor antagonist, haloperidol, which is known to increase the accumbens dopamine level, was ineffective in reversing the increase in call number at the dose studied. On the contrary, intraacumbens haloperidol, when injected alone, caused an increase in 50 kHz calls. It is concluded that the release of dopamine, predominantly in the accumbens shell region, is responsible for production of 50 kHz calls and the calls may indicate an appetitive state compatible with anticipation of reward and positive affect. Both D1 and D2 subtypes of dopamine receptors may be necessary to induce 50 kHz calls and signal the appetitive state.

Behavioural responses of laboratory rats to playback of 22 kHz ultrasonic calls

It has been demonstrated that cholinergic stimulation of the anterior hypothalamic-preoptic region induces 22 kHz ultrasonic vocalization in rats. Acoustic features of the cholinergically induced vocalization did not differ from those of 22 kHz calls emitted in natural situations and, therefore, could have a behavioural significance for other conspecifics. The 22 kHz calls induced by intracerebral injection of carbachol were played back to rats and their responses were compared with responses to playback of 22 kHz calls induced by tactile stimuli and to those with background noise. Animal responses were measured by an accelerometric sensor as an average ergometric activity. The average activity count was not changed during presentation of acoustic stimuli, however, striking differences were found in animal responses immediately after discontinuation of the sound. Activity of the rats consistently and significantly decreased after presentation of 22 kHz calls induced by tactile stimuli or by injection of carbachol. Animal responses to calls induced by carbachol were indistinguishable from responses to calls induced by tactile stimuli. No significant changes in the general activity of the animals were observed after presentation of the background noise or during the sessions without stimuli. The results demonstrate that carbachol-induced ultrasonic calls have behavioural significance for other conspecifics and could serve as an alarm call in a similar way to naturally produced 22 kHz vocalization.

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.

Ultrasonic vocalization of laboratory rats in response to handling and touch

The goal of the study was to investigate the ultrasonic vocalization induced in freely behaving, naive rats by gentle touch with a human hand. Thirty-nine rats were tested in an unfamiliar experimental cage with repeatable hand touch. Vocalization appeared with an average latency of 4.6 +/- 5.0 s (SD). The nape of the neck was the most effective area, and after a couple of stimuli applied, 66.7% of rats emitted 21-32 kHz ultrasonic vocalization. It consisted of multiple series of long calls, about 70% of which exceeded 300 ms. The responses quickly habituated from session to session to extinction. Significantly more rats housed in single cages vocalized ultrasonically than animals housed in community cages. The long latencies of the vocalization, their appearance in multiple series to a single touch, and quick habituation to the stimuli indicate that 22 kHz ultrasonic vocalization of rats reflects a distress caused by a potential danger to the animal and it does not necessarily reflect physical discomfort or pain. This vocalization may, therefore, play an adaptive role in increasing chances of survival by conveying information about potential threats to other conspecifics.

Analysis of 22 kHz ultrasonic vocalization in laboratory rats : long and short calls

There is a remarkable variation in the length of single ultrasonic calls emitted by adult rats. The duration of calls is likely to convey information for conspecifics. The goal of the present study was to analyze 22 kHz calls emitted by naive laboratory rats in response to contact with the human hand and to measure their acoustic features, with a particular emphasis on call duration. Repeated hand touch applied to the nape of the neck of rats induced ultrasonic calls, 97.4% of which were within the range of 20-29 kHz and 2.6% of which were within 44-67 kHz. Distribution of duration of 6765 calls revealed two subpopulations of 22 kHz calls: 20-300 ms calls with its peak at 150 ms and calls above 310 ms with highest values at approximately 500-600 ms without a clear peak. These two call populations were referred to as short and long calls, respectively. The short and the long vocalizations contained 80% and 100% of calls within the range of the 22 kHz frequency, respectively. The findings indicated that, in the situation studied, the 22 kHz vocalization of adult rats consists of two distinguishable subpopulation of calls: short and long with the boundary between them at 300 ms.

The related roles of dopamine and glutamate in the initiation of 50-kHz ultrasonic calls in adult rats.

Effects of amphetamine on the production of 50-kHz ultrasonic calls were studied. Calls were emitted spontaneously or were induced by an intrahypothalamic-preoptic injection of glutamate. Sonographic analysis of recorded calls revealed that they were within the 35-70-kHz sound frequency range reported for the 50-kHz call type. Systemic amphetamine (AMPH, 2 mg/kg) significantly increased the number of spontaneously emitted 50-kHz calls and the effect of AMPH was dose-dependent. Low dose of intracerebral glutamate (17 microg) had no additive effect on the number of AMPH-induced calls. Higher dose of intracerebral glutamate alone (34 microg) significantly increased the number of 50-kHz calls, which was completely reversed by systemic application of haloperidol (2 mg/kg), a dopamine antagonist. The results suggest that glutamate-induced or spontaneously occurring 50-kHz calls in adult rats are dependent upon dopaminergic transmission. It is postulated that this type of calls may be indicative of dopamine mediated affective state in adult rats.

Release of Dopamine in the Nucleus Accumbens Caused By Stimulation of the Subiculum in Freely Moving Rats

Stimulation of the ventral subiculum of the hippocampus activates the hippocampal-accumbens pathway and increases locomotor activity. Dopaminergic terminals in the nucleus accumbens have also been implicated in initiation of locomotor activity, and the release of dopamine in the nucleus accumbens is critical for locomotor responses initiated from the subiculum to occur. We have demonstrated release of dopamine in the nucleus accumbens using in vivo microdialysis after stimulation of the ventral subiculum with NMDA. Extracellular dopamine level in the nucleus accumbens was significantly increased by 40% over baseline as a result of NMDA stimulation of the ventral subiculum. This stimulation also caused more than a 40-fold increase in horizontal activity and total distance covered by the rats. Injection of saline into the subiculum caused neither a change in the dopamine level nor an increase in animals activity. The dynamics of the measured changes in dopamine overflow correlated with the time course of locomotor changes. The results demonstrate that stimulation of the ventral subiculum causes release of dopamine in the nucleus accumbens which parallels the increase in locomotor activity.

Involvement of neuronal cell bodies of the mesencephalic locomotor region in the initiation of locomotor activity of freely behaving rats

The locomotor activity of freely-moving rats was increased substantially by injections of L-sodium glutamate or of picrotoxin, a GABA antagonist, into the region of the tegmental pedunculopontine nucleus. The onset of hyper-motility was more rapid with L-glutamate than with picrotoxin and the duration shorter. Locomotor activity from injecting amphetamine unilaterally into the nucleus accumbens was reduced by injections of GABA into the ipsilateral pedunculopontine nucleus. These observations provide additional evidence implicating neurons of the MLR and possibly GABA synaptic inputs to these neurons in locomotor activity and suggest that they may mediate indirect inputs from the nucleus accumbens.