Prolonged Fear Conditioning in Rats Elicits a New Type of Aversive 44-kHz Ultrasonic Vocalization

Core Concepts

Rats emit a previously unrecognized type of long, high-pitched, and unmodulated ultrasonic vocalization, termed "44-kHz calls", during prolonged fear conditioning, which are distinct from the established aversive 22-kHz and appetitive 50-kHz calls.

Abstract

The study investigates a new type of ultrasonic vocalization emitted by rats during prolonged fear conditioning. The authors first describe the characteristics of these "44-kHz calls" - they are long (>150 ms), high-pitched (32-51.5 kHz), and unmodulated in frequency. These calls were observed in Wistar and spontaneously hypertensive rats (SHR) subjected to fear conditioning protocols, and were distinct from the previously known aversive 22-kHz and appetitive 50-kHz vocalizations.
The 44-kHz calls were particularly prevalent during the latter half of the 10-trial fear conditioning sessions, gradually replacing the 22-kHz calls. The frequency, duration, and amplitude of the 44-kHz calls were analyzed and compared to the 22-kHz calls. The 44-kHz calls were associated with increased freezing behavior, suggesting they communicate an aversive emotional state.
Playback experiments further demonstrated that the behavioral and physiological responses to 44-kHz calls were similar to those elicited by the aversive 22-kHz calls, despite the 44-kHz calls occupying the frequency range of the appetitive 50-kHz calls. This indicates that the frequency band alone does not determine the emotional valence of the calls, and that rats have a wider vocal repertoire than previously believed.
The authors propose that the 44-kHz calls and the established 22-kHz calls constitute a "supertype" of long, unmodulated aversive vocalizations, with a consistent 1.5-fold frequency ratio between them. They suggest that the 44-kHz calls have been overlooked in the past due to methodological limitations and the focus on the canonical 22-kHz and 50-kHz call types. Further research is needed to fully understand the significance and underlying mechanisms of this newly described vocalization type.

Stats

Rats emitted up to 19.4% of 44-kHz calls during the latter half of the 10-trial fear conditioning sessions.
The frequency of 22-kHz calls gradually increased from 24.5 ± 0.1 to 27.9 ± 0.4 kHz during the training.
The duration of 22-kHz calls decreased from 969.6 ± 43.1 ms to 794.6 ± 39.8 ms during the first four trials.
The amplitude of 22-kHz calls decreased in the latter half of the training session, while the amplitude of 44-kHz calls remained stable.

Quotes

"Rats have a wider vocal repertoire than previously believed, and current definitions of major call types may require reevaluation."
"We propose that these long 22-kHz and 44-kHz vocalizations constitute a supertype group of long unmodulated aversive calls ("long 22/44-kHz vocalizations")."

Key Insights Distilled From

Rats emit aversive 44-kHz ultrasonic vocalizations during prolonged Pavlovian fear conditioning

by Olszynski,K.... at www.biorxiv.org 04-08-2023

https://www.biorxiv.org/content/10.1101/2023.04.06.535936v2

Deeper Inquiries

How do the neural mechanisms underlying the production of 44-kHz calls differ from those of 22-kHz and 50-kHz calls?

The neural mechanisms underlying the production of 44-kHz calls in rats are likely to differ from those of 22-kHz and 50-kHz calls due to the distinct emotional states and contexts in which these vocalizations are emitted. The production of 22-kHz calls is associated with aversive states and distress, typically signaling fear or anxiety. On the other hand, 50-kHz calls are emitted in appetitive situations, signaling positive or rewarding states. In contrast, 44-kHz calls, as described in the context provided, are emitted in intensified aversive states during prolonged fear conditioning.
Neurobiologically, the production of these different types of vocalizations is likely to involve different neural circuits and neurotransmitter systems. For example, the production of 22-kHz calls has been linked to the limbic system, particularly the amygdala, which plays a key role in emotional processing and fear responses. In contrast, the production of 50-kHz calls has been associated with the mesolimbic dopamine system, involved in reward processing and motivation.
Given that 44-kHz calls represent a novel type of aversive vocalization emitted in specific contexts, it is plausible that they involve a unique neural circuitry that integrates both aversive and possibly rewarding aspects of the emotional state. Further research is needed to elucidate the specific neural mechanisms underlying the production of 44-kHz calls and how they differ from 22-kHz and 50-kHz calls.

How do the potential evolutionary origins and adaptive functions of the 44-kHz calls in the rat vocal repertoire?

The potential evolutionary origins and adaptive functions of 44-kHz calls in the rat vocal repertoire can be understood in the context of communication and social behavior. In the wild, rats use ultrasonic vocalizations to communicate with conspecifics, convey emotional states, and coordinate social interactions. The emergence of 44-kHz calls may have evolutionary significance in enhancing the rats' ability to communicate specific emotional states in response to prolonged aversive stimuli.
One possible adaptive function of 44-kHz calls could be to signal heightened distress or fear in situations of prolonged threat or danger. By emitting these high-pitched vocalizations, rats may alert others in their social group to the presence of a significant aversive stimulus and coordinate defensive or avoidance behaviors. The distinct frequency and duration characteristics of 44-kHz calls may serve as a more salient and specific signal of intense fear compared to 22-kHz calls.
Furthermore, the presence of 44-kHz calls in the rat vocal repertoire may reflect the adaptive flexibility of their communication system. By expanding their vocal repertoire to include novel types of vocalizations, rats may enhance their ability to convey nuanced emotional states and adapt their responses to varying environmental challenges. The evolution of 44-kHz calls could thus represent a mechanism for improving social cohesion, group coordination, and survival in aversive conditions.

Could the 44-kHz calls be used as a biomarker for studying emotional dysregulation in rat models of human psychiatric disorders?

The 44-kHz calls emitted by rats in response to prolonged fear conditioning could potentially serve as a valuable biomarker for studying emotional dysregulation in rat models of human psychiatric disorders. These vocalizations represent a specific and quantifiable behavioral response to aversive stimuli, making them a potentially sensitive indicator of emotional states and stress responses in rats.
In the context of psychiatric disorders characterized by emotional dysregulation, such as anxiety disorders or post-traumatic stress disorder (PTSD), the analysis of 44-kHz calls could provide insights into the underlying neural mechanisms and behavioral manifestations of these conditions. By measuring the frequency, duration, and context of 44-kHz vocalizations in rat models, researchers can assess the impact of genetic, environmental, and pharmacological manipulations on emotional processing and stress reactivity.
Furthermore, the use of 44-kHz calls as a biomarker in preclinical studies may help in the development of novel therapeutic interventions for psychiatric disorders. By targeting the neural circuits and neurotransmitter systems involved in the production of 44-kHz calls, researchers can identify potential drug targets and behavioral interventions to modulate emotional responses and improve emotional regulation in rat models of human psychiatric disorders.