The secretion of many hormones, including oxytocin, vasopressin and growth hormone, is not constant but shows a day-night rhythm. The suprachiasmatic nucleus (SCN) is thought to generate most mammalian biological rhythms and previous studies have reported suprachiasmatic efferents to the paraventricular nucleus (PVN) and the supraoptic nucleus (SON). We used in vivo extracellular electrophysiological techniques to show that the SCN also sends direct and indirect neural projections to the arcuate nucleus (ARC). This projection consisted of both excitatory and inhibitory components and may contribute to the entrainment of the rhythm in growth hormone secretion to the day-night cycle. Some SCN neurones appear to project to both the SON and the ARC. The SCN in turn receives excitatory and inhibitory inputs from the ARC and the peri-nuclear zone of the SON (peri-SON), which may provide feedback information, as well as allowing nonphotic entrainment of the SCN, for example, in response to feeding. Our data thus suggest extensive two-way connections between the SCN and its target nuclei which may contribute to the generation of day-night neuroendocrine rhythms. They also suggest the existence of indirect retinal projections to the ARC and PVN. We further investigated the retinal projection to the SCN. We were unable to demonstrate a significant difference in retinal input to those suprachiasmatic cells which had efferent projections to particular hypothalamic targets (SON and/or ARC), and those which did not.

Novel measures of coding based on interspike intervals were used to characterise the rhythms of single unit activity in the supraoptic nucleus during the day/night cycle in urethane-anaesthetised rats in vivo. Both continuously firing and phasic cells showed significant (P < 0.001) diurnal rhythms of spike frequency and in the irregularity of firing, as quantified by the log interval entropy (ENT). Comparison of rhythms in log interval ENT showed that the amplitude of the rhythms was greater for the continuously firing cells than for the phasic cells (P = 0.002). Rhythms persisted after hypertonic stimulation or pinealectomy and both treatments reduced the amplitude significantly only for the continuously firing cell group. By contrast, the mesor (i.e. mid-point of the rhythm) was reduced only for the phasic cell group. A similar analysis applied to the activity of cells of the suprachiasmatic nucleus showed that, after pinealectomy, there was a significant rhythm in ENT (P < 0.001) but not firing rate; however, the amplitude of the rhythm in ENT was attenuated (P = 0.047). Diurnal changes in the electrical activity of supraoptic cells are consistent with previously reported circadian changes in magnocellular neuropeptide release. Differences between continuous and phasic cell groups in the effects of osmotic stimulation on rhythmic activity indicate that the two cell types differ in their coding of osmolality and zeitgeber time information. The different effects of pinealectomy on the supraoptic and suprachiasmatic nuclei suggest that removal of endogenous melatonin unmasks a difference in circadian coding between the two nuclei.