Abstract
Serotonin is considered to play a significant role in anxiety-related behaviors in animals through actions of the median raphe nucleus (MRN) neurons. The habenular nucleus is also concerned with anxiety behaviors, but the connections between MRN neurons and lateral habenular nucleus (LHb) neurons are not yet understood completely. We investigated this point in rats anesthetized with urethane (i.p. 1.5g/kg) anatomically and electrophysiologically: anterograde as well as retrograde tracing methods and extratracellular unit recordings in MRN neurons on stimulation of LHb. Anatomical experiment with injection of fast blue (FB) and fluoro ruby (FR) into LHb and MRN, showed that LHb neurons project to MRN. Stimulation (0.3mA in strength, 0.3ms of duration) of LHb suppressed MRN neurons firing for 25ms to 50ms in post-stimulus time histogram, and stronger stimulation (1mA, 0.3ms) prolonged the duration of this inhibition. Picrotoxin (i.v. 1mg/kg) blocked a later part of the inhibition, but not an early one. Our results suppose that the early inhibitory effect of LHb stimulation is through a direct pathway and the later inhibition via GABAergic interneurons. [J Physiol Sci. 2008;58 Suppl:S149]
