Synaptic inhibition: its role in suprachiasmatic nucleus neuronal thermosensitivity and temperature compensation in the rat.

1. Whole-cell patch clamp recordings of neurones in the suprachiasmatic nucleus (SCN) from rat brain slices were analysed for changes in spontaneous synaptic activity during changes in temperature. While recent studies have identified temperature-sensitive responses in some SCN neurones, it is not known whether or how thermal information can be communicated through SCN neural networks, particularly since biological clocks such as the SCN are assumed to be temperature compensated. 2. Synaptic activity was predominantly inhibitory and mediated through GABAA receptor activation. Spontaneous inhibitory postsynaptic potentials (IPSPs) and currents (IPSCs) were usually blocked with perifusion of 10-50 microM bicuculline methiodide (BMI). BMI was used to test hypotheses that inhibitory synapses are capable of either enhancing or suppressing the thermosensitivity of SCN neurones. 3. Temperature had opposite effects on the amplitude of IPSPs and IPSCs. Warming decreased IPSP amplitude but increased IPSC amplitude. This suggests that thermally induced changes in IPSP amplitude are primarily influenced by resistance changes in the postsynaptic membrane. The thermal effect on IPSP amplitude contributed to an enhancement of thermosensitivity in some neurones. 4. In many SCN neurones, temperature affected the frequency of IPSPs and IPSCs. An increase in IPSP frequency with warming and a decrease in frequency during cooling made several SCN neurones temperature insensitive, allowing these neurones to maintain a relatively constant firing rate during changes in temperature. This temperature-adjusted change in synaptic frequency provides a mechanism of temperature compensation in the rat SCN.