Manipulation of the Amygdalo-Auditory Cortex Circuit Influences the Perception of Tinnitus in Rats

The current study examined whether deactivation of the amygdalo-auditory cortex circuit impacts tinnitus behavior in rats. The perception of tinnitus is caused by cortical neuronal firing in the absence of an auditory stimulus. Amygdalar projections to cortex influence cortical firing based on the significance of incoming auditory stimuli. Because the tinnitus precept is not significant, we hypothesize that amygdalar circuits may be responsible for dampening these signals.

Methods

To identify whether amygdalo-auditory cortex circuits influence tinnitus perception, we injected an optogenetic inhibitory vector construct (AAV-CaMKIIa-eNpHR3.0-EYFP) into the amygdala and implanted a fiber-optic ferrule into auditory cortex. After surgery, animals were allowed to recover for 2-3 weeks to allow the optogenetic vector construct to be transported throughout the neurons and initiate the creation of light-sensitive chloride channels. These channels functionally deactivate affected neurons by keeping them in a hyperpolarize state. Such deactivation only occurs when the channels are activated by light.

To induce tinnitus, animals were presented with a 120 dB broadband noise located 10 cm from the cranium for one hour while under surgical anesthesia. Animals were tested for tinnitus with acoustic startle gap protocols before surgery (control), 1 day after surgery (so show tinnitus induction), and then immediately prior, during, and after light deactivation of the amygdalo-cortical circuit.

Results

Examination of the tissue indicated that the vector construct was transported both anterogradely and retrogradely throughout the brain. Within auditory cortex, a plethora of labeled axons and boutons were observed in both primary and secondary regions of cortex. Only a few labeled neurons were observed. These results indicate that the injections successfully labeled amygdalar neurons that project to auditory cortex.

Tinnitus induction was successful in all animals. It was observed throughout the duration of the experiment in control conditions. When the fiber optic light was turned on (deactivating the amygdalo-cortical circuit), acoustic startle responses reverted to the non-tinnitus state. This reversion was sustained only for the duration of the time in which the light was turned on. Immediately after laser cessation, acoustic startle results returned to levels indicating tinnitus. These results were repeated numerous times with each animal. 

Conclusion

Deactivation of the direct amygdalo-auditory cortex circuit can eliminate tinnitus behavior in rats. However, because tinnitus perception returned after the pathway was released from deactivation, it is likely that other circuits are responsible for the initiation of prolonged tinnitus activity.