Electrophysiological actions of norepinephrine in rat lateral hypothalamus. I. Norepinephrine-induced modulation of LH neuronal responsiveness to afferent synaptic inputs and putative neurotransmitters.

The present studies were conducted as part of an ongoing investigation of the effects of norepinephrine (NE) in neuronal circuits of the mammalian brain. In this report, we describe noradrenergic actions in the lateral hypothalamus (LH), an area which has been implicated in the central integration of cardiovascular regulatory mechanisms, fluid balance and ingestive behaviors. Microiontophoretically applied NE was interacted with extracellularly recorded responses of LH neurons to iontophoretically applied putative neurotransmitters gamma-aminobutyric acid (GABA), acetylcholine (ACh) and glutamate (Glu); and activation of known input pathways from the reticular thalamus (RT) and the lateral preoptic area (LPO). Peri-event histograms of cell responses were computed before, during and after NE microiontophoresis (5-50 nA) and used to quantitatively evaluate monoamine-induced effects on spontaneous and stimulus evoked activity of LH neurons. In 16 of 23 LH neurons, RT-stimulus-induced inhibition was markedly prolonged from a mean of 28.3 +/- 4.8 ms to 44.7 +/- 5.2 ms, during iontophoretic application of NE. In 22 of 38 LH cells, LPO-stimulus-induced excitatory responses were enhanced above control levels during NE administration. In further tests, inhibitory responses of LH cells to iontophoretic pulses of GABA were potentiated during NE administration in 69% (24 of 35) of the cases tested. ACh-induced excitation was potentiated in 9 of 21 cells. In 4 of these cases, otherwise subthreshold doses of ACh caused marked increases in cell firing during the period of NE administration. By contrast, Glu-evoked excitation was antagonized by NE iontophoresis in 65.5% (17 of 26) of LH cells tested. These findings indicate that, as in other noradrenergic target regions of the CNS, NE can facilitate synaptically mediated responses of LH neurons. Taken together these observations suggest that NE may play an important regulatory role in the synaptic transfer of information within LH circuits, and consequently exert considerable influence over the homeostatic functions mediated by this structure.