  We present a method for achieving temporally and spatially precise
photoactivation of neurons without the need for genetic expression of
photosensitive proteins. Our method depends upon conduction of thermal energy
via absorption by a dye or carbon particles and does not require the presence
of voltage-gated channels to create transmembrane currents. We demonstrate
photothermal initiation of action potentials in Hirudo verbana neurons and of
transmembrane currents in Xenopus oocytes. Thermal energy is delivered by
focused 50 ms, 650 nm laser pulses with total pulse energies between 250 and
3500 \muJ. We document an optical delivery system for targeting specific
neurons that can be expanded for multiple target sites. Our method achieves
photoactivation reliably (70 - 90% of attempts) and can issue multiple pulses
(6-9) with minimal changes to cellular properties as measured by intracellular
recording. Direct photoactivation presents a significant step towards
all-optical analysis of neural circuits in animals such as Hirudo verbana where
genetic expression of photosensitive compounds is not feasible.
