The suprachiasmatic nucleus (SCN) of the hypothalamus generates endogenous rhythms that control the daily cycles of many mammalian functions (Hastings & Maywood, 2000). The SCN needs to be entrained by external cues in order to adjust its activity to the environment. The cycles are entrained by both photic and non-photic cues. A number of inputs to the SCN have been described anatomically (Moga & Moore, 1997; Krout et al. 2002), amongst them inputs from the retina and from the intergeniculate leaflet (IGL) in the caudal thalamic region, deep to the lateral geniculate nucleus.

We attempted to characterise some of the connections using conventional extracellular recording techniques to monitor the response of single cells in the SCN following stimulation of the IGL area or the optic nerve (OptN). Recordings were made from single units in the SCN of urethane-anaesthetised (1.3 g kg^-1 I.V.) male rats while stimulating the contralateral OptN exposed within the orbit. The effectiveness of stimulation of the optic nerve was confirmed by observing twitches in the rectus muscles of the eye. Another stimulating electrode was placed within the ipsilateral IGL area. The position of the electrodes was confirmed by histological reconstruction after the experiments. The responses of the SCN cells were assessed by creating peristimulus time histograms. All experiments were carried out in accordance with the Animals (Scientific Procedures) Act, 1986, and the rats were killed humanely at the end of the experiments.

Of a total of 85 SCN cells recorded, 20 (23.5 %) showed an excitatory response, 21 (24.7 %) an inhibitory response, and 11 (12.9 %) a complex response (with both excitatory and inhibitory components) after IGL stimulation. In contrast, only 14 (18.2 %) cells out of 77 showed a response to OptN stimulation. Of these, 10 (13 %) showed an excitatory response and 4 (5.2 %) were inhibited. In 8 of the 85 cells recorded no OptN response was tested. Whereas the responses seen after stimulating IGL were either long (> 20 ms; 40 out of 52, 77 %) or short (▓le│ 20 ms; 12/52, 23 %) in latency, only long latency responses were seen after stimulating the OptN.

We conclude that there is a strong functional projection from the IGL to the SCN comprising both inhibitory and excitatory pathways. The different latencies of the responses suggest the existence of both monosynaptic and polysynaptic pathways. Since all the OptN responses were delayed, it seems probable that the projections to the SCN from OptN are polysynaptic and probably do not involve the IGL.

J.A.G. was supported by a Benefactors’ Scholarship from St John’s College, Cambridge.