Three classes of GABA-like immunoreactive neurons in the mushroom body of the cockroach.

The mushroom body (MB) is a higher center of the insect brain and is critical to some forms of associative memory. Each MB consists of calyces connected to alpha and beta lobes via pedunculus. In the calyces, input neurons make synaptic connections with intrinsic neurons. In the pedunculus and lobes, intrinsic neurons make synaptic connections with output neurons. Here, the distribution of gamma-aminobutyric acid (GABA)-like immunoreactivity in the MB of the cockroach Periplaneta americana was investigated, using an antiserum against a GABA-protein conjugate, to elucidate inhibitory pathways of the MB. We report that three classes of extrinsic neurons of the MB exhibit GABA-like immunoreactivity. The first is four large neurons which arborize in a diffuse neuropil surrounding the alpha lobe and project into whole areas of the calyces. Their cell bodies are 30-50 micron in diameter, among the largest in the brain. The second group is 7-9 neurons ascending from the circumesophageal connective and projecting into the calyces, which probably represent inhibitory input neurons. The third group is ca. 40 neurons with dendritic arborizations in the junction between the pedunculus and the lobes, which probably represent inhibitory output neurons.