Enkephalin-like immunoreactivity in the chick brainstem: possible relation to the cochlear efferent system

Area of Science:

• Neuroscience
• Auditory System Research
• Neuroanatomy

Background:

• Mammalian lateral olivocochlear (LOC) neurons expressing choline acetyltransferase (ChAT) also show enkephalin (Enk) immunoreactivity.
• The presence of Enk-like immunoreactivity (Enk-LI) in avian cochlear efferent neurons remains uncharacterized.

Purpose of the Study:

• To investigate whether avian cochlear efferent neurons contain Enk-LI.
• To map the distribution of Enk-LI and ChAT-immunoreactive (ChAT-I) neurons and terminals in the domestic chicken's auditory brainstem.

Main Methods:

• Immunohistochemistry was performed on the domestic chicken's auditory brainstem using antisera against ChAT, leucine-enkephalin (L-Enk), and methionine-enkephalin (M-Enk).
• Colchicine injections into the lateral ventricle were used to identify perikarya.
• Distribution of Enk-LI and ChAT-I perikarya and terminals was analyzed.

Main Results:

• Enk-LI terminals were observed around, but not within, the superior olivary nucleus (SO) and nucleus of the lateral lemniscus, pars intermedia (LLi).
• A moderate concentration of Enk-LI terminals was found near the ventrolateral group of ChAT-I cochlear efferent neurons.
• Intensely stained Enk-LI perikarya were located in the nucleus of the lateral lemniscus, pars ventralis (LLv), and scattered in the LLi and nucleus subceruleus ventralis (SCv), with no overlap with ChAT-I somata.

Conclusions:

• In chickens, enkephalin-like immunoreactivity is not present in the somata of cochlear efferent neurons.
• Enkephalinergic terminals are densely concentrated in areas associated with cochlear efferent neurons.
• The nucleus of the lateral lemniscus, pars ventralis (LLv) is a potential source of these enkephalinergic terminals.