Responses of the suprachiasmatic nucleus to retinohypothalamic tract volleys in a slice preparation of the mouse hypothalamus

The electrophysiological responses of the mouse suprachiasmatic nucleus (SCN) to stimulated synaptic input from the retinohypothalamic tract (RHT) were investigated using a hypothalamic slice preparation that includes the entire SCN, optic chiasm and optic nerves. Extracellular recordings of single-unit activity reveal a population of neurons in the ventrolateral SCN that are activated at a median latency of 10 ms after stimulation of the contralateral optic nerve. These neurons apparently receive direct excitatory input from RHT synapses. Other SCN neurons are activated at longer latencies, possibly through input from interneurons. The population field potentials evoked in the SCN by optic nerve volleys consist of a calcium-insensitive transient generated by optic tract axons in the chiasm, followed by calcium-sensitive waves generated by postsynaptic activity. The postsynaptic waves have the form of a field EPSP, negative in the dorsolateral SCN and positive in the ventrolateral SCN, upon which is superimposed a population spike of opposite polarity. The population spike occurs at the same latency as the monosynaptic single unit responses, which were all found near or ventral to the point of reversal of field potential. These findings suggest that neurons in the ventrolateral SCN are excited by synapses on dorsally extended dendrites. The conduction velocity of the RHT in the optic nerve was found to be 0.59 +/- 0.03 mm/ms, while that of the optic tract volley was 2.4 +/- 0.75 mm/ms. The low conduction velocity of the RHT indicates that, within the optic nerve, these axons are thin and/or unmyelinated in the optic nerve.