Synaptic connections between GABA-immunoreactive neurons and uniglomerular projection neurons within the antennal lobe of the cockroach, Periplaneta americana.

Synapses within deutocerebral glomeruli between GABA-immunoreactive, putatively inhibitory local interneurons and uniglomerular projection (output) neurons were demonstrated by means of a combination of GABA-immunogold labeling and intracellular HRP injection. The following connections were identified. 1) GABA-immunoreactive (GABAir) neurons form output synapses in a dyadic fashion onto a uniglomerular projection neuron and, in addition, a second GABAir neuron. A uniglomerular projection neuron in turn forms dyadic output synapses onto two GABAir neurons. Several examples of reciprocal connections have been identified between, first, GABAir neurons and uniglomerular projection neurons, and, second, GABAir neurons themselves. 2) GABAir neurons are serially connected with uniglomerular projection neurons via interposed GABAir processes. In some cases, also the first GABAir process of such a polysynaptic connection formed an output synapse onto the projection neuron. Such serial connections may form the structural basis for both, the feedforward inhibition as well as the feedforward disinhibition of uniglomerular projection neurons by GABAergic neurons. The reciprocal contacts may serve as control devices that modulate the output activity of the projection neurons.