Midline thalamic inputs to the amygdala: Ultrastructure and synaptic targets

Area of Science:

• Neuroscience
• Cellular Biology
• Neuroanatomy

Background:

• The basolateral nucleus of the amygdala (BL) receives significant subcortical input from midline dorsal thalamic nuclei.
• The central medial (CMT) and paraventricular (PVT) nuclei are key sources of this input to the BL.
• The precise synaptic connections mediating these thalamic influences on BL activity remain largely uncharacterized.

Purpose of the Study:

• To elucidate the ultrastructural properties of CMT and PVT axon terminals in the rat BL.
• To identify the specific synaptic targets of CMT and PVT inputs within the BL.
• To determine the contribution of these thalamic inputs to BL neuronal activity.

Main Methods:

• Phaseolus vulgaris-leucoagglutinin (PHAL) anterograde tracing was employed to label CMT and PVT projections.
• Electron microscopy was used to examine the ultrastructure of axon terminals and their synaptic contacts.
• Calcium/calmodulin-dependent protein kinase II (CAMKIIα) immunoreactivity identified principal BL neurons.

Main Results:

• PHAL-labeled CMT and PVT axon terminals predominantly formed asymmetric synapses in the BL.
• These terminals contacted both dendritic spines and a substantial number of dendritic shafts.
• A majority of dendritic shafts postsynaptic to CMT and PVT terminals were CaMKIIα-immunopositive, indicating they belong to principal neurons.

Conclusions:

• CMT and PVT inputs primarily synapse onto principal neurons in the basolateral amygdala.
• These thalamic inputs exert excitatory influence on principal BL neurons with minimal feed-forward inhibition.
• CMT and PVT pathways are likely major determinants of basolateral amygdala activity and emotional processing.