Monosynaptic inhibitory postsynaptic potentials from lateral habenula recorded in dorsal raphe neurons

The inhibitory response evoked in presumably serotonergic dorsal raphe neurons by stimulation of the lateral habenular nucleus was examined in the rat using intracellular recording techniques. Electrical stimulation of the lateral habenula produces a long-lasting hyperpolarization in dorsal raphe neurons having the slow spontaneous firing pattern (0.5-1.5 spikes/sec) and broad action potential (greater than 1 msec) indicative of serotonergic neurons. The hyperpolarizing response is reversed by hyperpolarizing current injection or by increasing intracellular Cl-concentration and is thus an inhibitory postsynaptic potential (IPSP) due to conductance increase to Cl-. The mean latency of the IPSP is 7.0 msec, which implies a mean conduction velocity for habenulo-raphe axons of 1.2 m/sec. The latency of the response could be demonstrated to be unaffected by changes in stimulus strength, indicating that the IPSP is monosynaptic, which is in agreement with recent anatomical data. Intracellular horseradish peroxidase labeling of responding neurons shows them to have a morphology typical of serotonergic dorsal raphe neurons.