3 CLASSES OF GABA-LIKE IMMUNOREACTIVE NEURONS IN THE MUSHROOM BODY OFTHE COCKROACH

The mushroom body (MB) is a higher center of the insect brain and is c ritical to some forms of associative memory. Each MB consists of calyc es connected to alpha and beta lobes via pedunculus. In the calyces, i nput 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 (GA BA)-like immunoreactivity in the MB of the cockroach Periplaneta ameri cana was investigated, using an antiserum against a GABA-protein conju gate, to elucidate inhibitory pathways of the MB. We report that three classes of extrinsic neurons of the MB exhibit GABA-like immunoreacti vity. The first is four large neurons which arborize in a diffuse neur opil surrounding the alpha lobe and project into whole areas of the ca lyces. Their cell bodies are 30-50 mu m in diameter, among the largest in the brain. The second group is 7-9 neurons ascending from the circ umesophageal connective and projecting into the calyces, which probabl y represent inhibitory input neurons. The third group is ca. 40 neuron s with dendritic arborizations in the junction between the pedunculus and the lobes, which probably represent inhibitory output neurons. (C) 1998 Elsevier Science B.V.