Visual interneurons in the leech brain

Summary

• 1. The anterior visual (AV) cell is a bilaterally paired visual interneuron in the supraesophageal ganglion of the medicinal leech (Fig. 1).

• 2. The input resistance of the AV cell increases with depolarization from rest and decreases with hyperpolarization (Fig. 2); overall, the current-voltage relationship is sigmoid (Fig. 4A). These features persist when chemical synaptic transmission is blocked by high Mg saline (Figs. 4B, 5).

• 3. Because of this current-voltage relationship, for certain applied currents the AV cell has two steady-state membrane potentials. Under those conditions current pulses switch the AV cell from one steady state to the other (Fig. 5C).

• 4. The spontaneous spike-like depolarizations in the AV soma (Fig. 2) appear to be synaptic potentials rather than failed impulses, since injected current neither blocks nor stimulates them, nor does TEA affect them (Fig. 3). The current-voltage relationship of the AV cell explains the complicated dependence of these ‘large EPSPs’ on membrane potential (Fig. 6).

• 5. When any ipsilateral eye is illuminated an AV cell shows a graded depolarization that persists in high Mg saline (Figs. 8, 9D, 10). Moreover, the AV cell appears to be lucifer yellow dye-coupled to the photoreceptors of the ipsilateral eyes (Fig. 1).

• 6. When any contralateral eye is illuminated an AV cell receives a volley of large EPSPs (Fig. 10). This input does not persist in high Mg saline and is therefore probably polysynaptic.

• 7. Although there is no synaptic connection between the AV cells, the large EPSPs in an AV cell do match synaptic potentials in several other identified neurons in the supraesophageal ganglion (Fig. 12), including the other AV cell (Fig. 11C).

• 8. In addition, large EPSPs in an AV cell match EPSPs in the contralateral LVa cell (Fig. 13B) and IPSPs in the ipsilateral LVa cell (Fig. 13D). Depolarization of the LVa cell elicits large EPSPs in the contralateral AV cell (Fig. 13C).

• 9. These results suggest that the AV cells are second order visual neurons that are specialized to respond positively to slight increases in light intensity, and that the AV cells are tightly integrated with the other known visual interneurons.