Synaptic and morphological characteristics of temperature-sensitive and -insensitive rat hypothalamic neurones.

1. Intracellular recordings were made from neurones in rat hypothalamic tissue slices, primarily in the preoptic area and anterior hypothalamus, a thermoregulatory region that integrates central and peripheral thermal information. The present study compared morphologies and local synaptic inputs of warm-sensitive and temperature-insensitive neurones. 2. Warm-sensitive neurones oriented their dendrites perpendicular to the third ventricle, with medial dendrites directed toward the periventricular region and lateral dendrites directed toward the medial forebrain bundle. In contrast, temperature-insensitive neurones generally oriented their dendrites parallel to the third ventricle. 3. Both warm-sensitive and temperature-insensitive neurones displayed excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs). In most cases, EPSP and IPSP frequencies were not affected by temperature changes, suggesting that temperature-insensitive neurones are responsible for most local synapses within this hypothalamic network. 4. Two additional neuronal groups were identified: silent neurones having no spontaneous firing rates and EPSP-driven neurones having action potentials that are primarily dependent on excitatory synaptic input from nearby neurones. Silent neurones had the most extensive dendritic trees, and these branched in all directions. In contrast, EPSP-driven neurones had the fewest dendrites, and usually the dendrites were oriented in only one direction (either medially or laterally), suggesting that these neurones receive more selective synaptic input.