Objective: To identify the afferent link in the neural pathway mediating emotional responses conditioned to acoustic stimuli by evaluating whether autonomic and behavioral responses to conditioned acoustic stimuli depend on auditory cortex or medial geniculate nucleus (MG) input
Materials & Equipment Checklist
6 items1 from ConductScience
Gather these items before starting the experiment. Check off items as you prepare.
Equipment5
Not specified • Not specified • Not specified • Not specified
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Protocol Steps
View Abstract
The purpose of this study was to identify the afferent link in the neural pathway which mediates emotional responses coupled to auditory stimuli. We evaluated whether autonomic and behavioral responses elicited by acoustic conditioned emotional stimuli are based on afferent information derived from the auditory cortex or from the auditory thalamic relay station, the medial geniculate nucleus (MG), in rats. The rat auditory cortex was defined through anterograde neuroanatomical tracing studies involving the injection of HRP into MG. Lesions were then placed in the auditory cortex or in MG. After 10 to 20 days the rats were subjected to classical fear conditioning trials involving the pairing of a pure tone with electric footshock. Changes in mean arterial pressure and heart rate and the duration of immobilization (“freezing”) and drink suppression elicited by presentation during extinction trials (no footshock) of the acoustic conditioned emotional stimulus were measured. Auditory cortex lesions did not affect the magnitude of the mean arterial pressure or heart rate conditioned responses nor the duration of freezing or drink suppression. In contrast, lesions of MG suppressed the magnitude of both the autonomic and somatomotor (behavioral) conditioned emotional responses but did not affect either autonomic or somatic responses elicited by the footshock unconditioned stimulus. Lesions of the inferior colliculus, the primary source of afferent input to MG, replicated the effects of MG lesions. These findings demonstrate that lesions of MG and lower auditory centers, but not lesions of the auditory cortex, block autonomic and behavioral conditioned emotional responses coupled to acoustic stimuli and indicate that subcortical rather than cortical efferents of MG sustain these responses. Our concurrent observation that MG projects to several subcortical areas (central and lateral amygdala; caudate-putamen; ventromedial hypothalamus) involved in emotional behavior and autonomic function suggests hypotheses concerning subsequent links in this emotional processing pathway.
1
Neuroanatomical mapping of auditory cortex
Conduct anterograde neuroanatomical tracing studies by injecting HRP into the medial geniculate nucleus to define the rat auditory cortex
Not specifiedNot specified
Note: This mapping step precedes lesion placement
View evidence from paper
“The rat auditory cortex was defined through anterograde neuroanatomical tracing studies involving the injection of HRP into MG”
2
Lesion placement
Place lesions in the auditory cortex, medial geniculate nucleus (MG), or inferior colliculus in separate groups of rats
Not specifiedNot specified
Note: Three lesion groups were tested: auditory cortex, MG, and inferior colliculus
View evidence from paper
“Lesions were then placed in the auditory cortex or in MG”
3
Post-lesion recovery period
Allow rats to recover following lesion placement before behavioral testing
10 to 20 daysNot specified
Note: Recovery period allows stabilization of lesion effects
View evidence from paper
“After 10 to 20 days the rats were subjected to classical fear conditioning trials”
4
Classical fear conditioning
Subject rats to fear conditioning trials pairing a pure tone acoustic stimulus with electric footshock
Not specifiedNot specified
Fear Conditioning System
Matches: Fear conditioning apparatus with electric footshock delivery system
Note: Extinction trials test conditioned responses without reinforcement
View evidence from paper
“Changes in mean arterial pressure and heart rate and the duration of immobilization (freezing) and drink suppression elicited by presentation during extinction trials (no footshock) of the acoustic conditioned emotional stimulus were measured”
6
Unconditioned stimulus response testing
Present electric footshock unconditioned stimulus to measure direct autonomic and somatic responses independent of conditioning
Not specifiedNot specified
Fear Conditioning System
Matches: Fear conditioning apparatus with electric footshock delivery system
