UC San Diego

Inhibitory connections among neurons help to maintain dynamic processes such as production of oscillatory rhythmic behaviors, modulation of sensory input to enhance contrast, and spatio-temporal coordination of activity in a neuronal network. Inhibitory synaptic transmission based on gamma-amino butyric acid (GABA) is widely distributed in the animal kingdom. For example, in the leech, Hirudo verbana, GABA has been shown to mediate inhibitory neuronal connections shape many behaviors. In contrast to vertebrates, leeches--like many other invertebrates--also have GABAergic motor neurons that produce direct peripheral inhibition, in addition to producing central inhibition. Using immunohistochemistry, we probed ganglia along the leech ventral nerve cord to determine which neurons were recognized by antibodies against GABA. We worked with three different commercial antibodies to GABA and confirmed their specificity using dot blots. All three antibodies labeled the same set of neurons in leech ganglia, including some that were previously unidentified. (A surprising--and as yet unexplained--result was that some inhibitory motor neurons were not labeled by any of the three antibodies, which remains unexplained.). We explored the new neurons revealed by the anti-GABA antibodies and found that stimulating one of these neurons (cell-116) inhibited a very well-characterized excitatory motor neuron (DE-3), as well as two well-characterized inhibitory (DI-1 and VI-2) motor neurons. Simultaneous inhibition of both excitatory and inhibitory motor neurons based on excitation in just one motor neuron is a novel pattern in the leech nervous system, and future work will more fully explore this new and interesting set of connections