Immunocytochemical and pharmacological evidence for GABAergic spiking local interneurons in the locust.

In the locust thoracic nervous system, spiking local interneurons within a ventral midline population are stained by a polyclonal antibody raised against GABA. Their cell bodies, their primary neurites in ventral commissure II, and their prominent neurites in the perpendicular tract linking ventral and dorsal fields of fine branches are all stained. Individual interneurons in this population were labeled with Lucifer yellow after their receptive fields had been determined physiologically. Alternate sections were stained with the antibody. Some, but not all, of the spiking local interneurons labeled with Lucifer yellow are also stained with the antibody. Apart from having somata that lie more posteriorly within the midline population, the antibody-stained interneurons cannot be distinguished on morphological or physiological grounds from those that are unstained. The way in which this cytochemical heterogeneity within an otherwise homogenous population might arise during development is discussed. A second group of spiking local interneurons with similar sensory input to that of the ventral midline population, but with cell bodies in the anterior lateral region of the ganglion and primary neurites in ventral commissure I, are not stained. Interneurons in the midline group receive direct inputs from sensory neurons and some directly inhibit particular leg motor neurons that mediate interjoint and tactile reflexes. Picrotoxin reversibly blocks the inhibition of the motor neurons and therefore abolishes these reflexes. Immunocytochemical and pharmacological experiments thus suggest that inhibition by some spiking local interneurons may be mediated by GABA.