Effects of sleep-promoting substance on rat hypothalamic neuron activity

The effects of electrophoretically applied sleep-promoting substance (SPS) on neuronal activity of the preoptic area (POA) and SPS interactions with some neurotransmitters were examined in the rat. In the POA, 14% of the tested neurons were excited and 13% were inhibited by SPS. Neurons that were excited by SPS were also excited by glutamate (Glu). The excitatory effect of Glu was blocked or attenuated by concurrent application of SPS in 35% of the neurons, and this modulatory effect was long-lasting. A significant number of SPS responsive neurons were inhibited by noradrenaline (NA). This inhibitory effect was attenuated, blocked or reversed by concurrent application of SPS in 28% of the tested neurons. There was no relation between the effects of acetylcholine (ACh) alone and those of SPS alone. The effects of ACh were modified by concurrent SPS application in 21% of the neurons tested; the effect was either enhancement or blockade of the ACh excitation. Possible contribution of SPS in the hypothalamus to the regulation of sleep and wakefulness is discussed.