Characterization of the Direct Amygdalo-Cortical Connection

Background

Stimulus significance can alter the attention of an animal to auditory cues in its environment. The amygdala receives inputs from sensory, memory, and association cortices. It is thought that the combination of these inputs contribute to an animal’s interpretation of sensory relevance. Projections from the amygdala back to sensory systems may alter the animal’s attention to sensory cues. Amygdalar projections to auditory cortex occur through both direct and indirect pathways. The indirect pathway through the nucleus basalis of Meynert has been reported to have long-term (minutes, hours) plastic effects on cortical firing patterns. These changes are thought to be associated with auditory conditioning. Little is knownabout the direct amygdalo-auditory cortex pathway. The current experiments describe the anatomy and physiology of this direct pathway.

Methods

Anterograde and retrograde tracing techniques were utilized to label the direct amygdalo-auditory cortex projection. To identify a potential functional significance for the pathway, frontal cortex was removed and single-unit electrophysiological recordings were obtained in primary auditory cortex before, during, and after amygdalar stimulation.

Results

Neurons were labeled throughout the basal and lateral nuclei of the amygdala. A high percentage of these neurons were pyramidal, indicating glutamatergic projections. However, many non-pyramidal neurons were also labeled. The neurons terminate throughout both primary and secondary

regions of auditory cortex with en passant and terminal boutons. Boutons were predominantly observed in layers V and VI, however less dense projections were observed throughout all cortical layers. Electrical stimulation of the amygdala (with the nucleus basalis pathway removed) had transient effects on the firing rate of neurons in auditory cortex.

Conclusion

The focus of amygdalo-auditory cortex terminals within layers V and VI indicate that the projection may target descending auditory pathways. Therefore it may also function to alter activity of lower-order auditory nuclei toward relevant sensory cues. Because the changes in cortical firing are transient, we hypothesize that the direct amygdalar projection may prime the cortical circuit and thereby influence the long-term plastic changes induced by the nucleus basalis of Meynert circuit.