We used patch-clamp recordings in slice preparations from Sprague-Dawley rats to evaluate responses of 20 spinal-projecting neurons in the dorsal paraventricular nucleus (PVN) to electrical stimulation in suprachiasmatic nucleus (SCN). Neurons containing a retrograde label transported from the thoracic (T(1)-T(4)) intermediolateral column displayed three intrinsic properties that collectively allowed distinction from neighboring parvocellular or magnocellular cells: a low-input resistance, a hyperpolarization-activated time-dependent inward rectification, and a low-threshold calcium conductance. Twelve of fifteen cells tested responded to electrical stimulation in SCN. All of 10 cells tested in media containing 2,3,-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[f]quinoxaline-7-sulfonamide disodium (5 microM) and D(-)-2-amino-5-phosphonopentanoic acid (20 microM) responded with constant latency (11.4 +/- 0.7 ms) inhibitory postsynaptic potentials, able to follow 20- to 50-Hz stimulation and blockable with bicuculline (20 microM). By contrast, all eight cells tested in the presence of bicuculline demonstrated constant latency (9.8 +/- 0.6 ms) excitatory postsynaptic potentials that followed at 20-50 Hz and featured both non-N-methyl-D-aspartate (NMDA) and NMDA receptor-mediated components. We conclude that both GABAergic and glutamatergic neurons in SCN project directly to spinal-projecting neurons in the dorsal PVN.

In the rat, projections from the suprachiasmatic nucleus (SCN) to the supraoptic nucleus (SON) of the hypothalamus were characterized in vivo using extracellular recordings and in slice preparations using both extracellular and whole‐cell patch clamp recording. Of 117 magnocellular neurones recorded in the SON in vivo, fifteen (13%) displayed a short latency excitation, sixty‐eight (58%) a short latency inhibition, six (5%) were unresponsive and twenty‐eight (24%) gave long latency responses following SON stimulation. The responses of putative vasopressin cells in the SON to SCN stimulation in vivo (4 out of 61 cells, 7% excited; 49 out of 61 cells, 80% inhibited) were significantly different from those of putative oxytocin cells (10 out of 50 cells, 20% excited and 16 out of 50 cells, 32% inhibited; P < 0.02, test for differences between proportions). Recordings in vitro using patch technology in whole‐cell mode showed both inward and outward currents in SON cells at holding potentials near resting membrane potential following stimulation of the SCN region. The outward currents could be blocked by bicuculline (10 μm; n= 7) and the inward currents were blocked by the non‐NMDA antagonist 6‐nitro‐7‐sulphamoylbenzo(f)quinoxaline‐2,3‐dione (5 μm; n= 4). We conclude that there is a strong projection from the SCN to the SON with both inhibitory (GABAergic) and excitatory (glutamatergic) components which may regulate the daily changes in neurohypophysial hormone secretion.

1. Single cell extracellular recordings were made from the suprachiasmatic nucleus (SCN) in urethane-anaesthetized Syrian hamsters at different times of the light-dark cycle. Peristimulus time histograms (PSTHs) were created following stimulation of the optic nerve. 2. Both short-latency (<50 ms) and long-latency (>50 ms) excitatory responses were seen.Almost all inhibitory responses had a short latency. 3. A total of 288 SCN neurones were recorded. Taking all types of response together, 55 (36f9%) of the 149 neurones tested in the dark period responded to optic nerve stimulation while only 23 (16f6%) of the 139 neurones tested in the light period responded. The difference between the proportion of all responsive and non-responsive neurones in the dark and light periods was highly significant (P < 0 01, Fisher's exact probability test). The difference in the proportion of excitatory responses was also significant (P < 0 01).4. During the dark period, the mean spontaneous firing rate (5 00 + 0-88 spikes s-'; mean + S.E.M., n = 55) of the responsive cells was significantly higher than that of the non-responsive cells (2'65 + 0 33 spikes s-; mean +S.E.M., n = 74; P < 0 01; Student's unpaired t test).5. Injection of APV (20 mm, 2,1u, I.c.v.; n = 6), an antagonist for the NMDA receptor, or CNQX (10 mm, 2 ul, i.c.v.; n = 5), an antagonist of the non-NMDA receptor, significantly reduced the responses of all the neurones tested. 6. We conclude that there is daily variation in the firing of SCN neurones in vivo and the variation is restricted to those cells receiving optic nerve inputs. The change in the responsiveness of the SCN to optic nerve stimulation at different times of day suggests that there is a rapidly changing cycle of synaptic function in the SCN. The action of the antagonists suggests that the excitatory retinal projections to the SCN which show this variation are mediated by glutamate and that both NMDA and non-NMDA receptors are involved.

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