Tectothalamic inhibitory projection neurons in the avian torus semicircularis

Area of Science:

• Neuroscience
• Auditory System Research
• Comparative Neuroanatomy

Background:

• Inhibitory feedforward projections are crucial in mammalian auditory systems, originating from the inferior colliculus (IC).
• Large GABAergic (LG) neurons in the IC receive excitatory input (VGLUT2+) and project to the auditory thalamus.
• The avian torus semicircularis (TS), the IC homolog, lacks established cell type homology and inhibitory feedforward mechanisms.

Purpose of the Study:

• To investigate the presence and characteristics of LG neurons in the avian TS.
• To determine if avian LG neurons receive similar excitatory input as their mammalian counterparts.
• To assess the projection targets of GABAergic neurons in the avian TS.

Main Methods:

• Neuroanatomical techniques were employed in pigeons and chickens.
• Immunohistochemistry was used to identify GABAergic and VGLUT2-positive neurons.
• Retrograde tracing was performed by injecting tracers into the auditory thalamus.
• Ultrastructural analysis examined synaptic connections on GABAergic neurons.

Main Results:

• The avian TS contains two types of GABAergic neurons, with larger ones receiving dense VGLUT2+ axosomatic terminals.
• Ultrastructural analysis confirmed that over 30% of large GABA+ neuron somata were contacted by presumptive excitatory terminals.
• Retrogradely labeled tectothalamic neurons were identified, with a subset being large, GABAergic, and receiving dense VGLUT2+ input.

Conclusions:

• This study demonstrates the existence of LG neurons in birds, analogous to those in mammals.
• The similar morphology and input patterns suggest a conserved function of tectothalamic inhibition across amniotes.
• These findings contribute to understanding the evolutionary basis of auditory processing circuits.