Influence of medial preoptic‐anterior hypothalamic area stimulation of the excitability of mediobasal hypothalamic neurones in the rat.

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Abstract
Extracellular action potentials recorded from 798 neurons in the mediobasal hypothalamus (MBH) of pentobarbitone anesthetized male rats were analyzed for a change in excitability following stimulation in the medial preoptic and anterior hypothalamic areas. An increase in excitability characteristic of orthodromic excitation was observed from 11.5% (n = 92) of MBH neurons. Latencies for excitation were shorter for cells tested with anterior hypothalamic area stimulation (n = 42; mean 5.4 .+-. 2.6 ms SD) than for cells tested with medial preoptic stimulation (n = 50; mean 15.2 .+-. 7.2 ms SD). With spontaneously active neurons, excitation was followed by a decrease in excitability lasting 150-250 ms. An initial decrease in excitability, suggestive of post-synaptic inhibition, over a wide latency range (4-30 ms) and with durations of 100-400 ms was observed from 3.6% of MBH neurons. Features of antidromic invasion were observed from 149 MBH neurons. From the medial preoptic area, the latency range was 0.5-38 ms (mean 7.8 .+-. 5.5); from the anterior hypothalamic area the latency range was 0.4-9.5 ms (mean 3.1 .+-. 2.3). Occasionally an abrupt decrease in latency followed an increase in stimulus intensity. Most cells followed paired stimuli at frequencies up to 500 Hz. Axon conduction velocities were estimated to be under 2.0 m/s. Antidromic invasion was usually followed by a decrease in excitability lasting approximately 100-150 ms. Twenty MBH neurons displayed antidromic invasion from both the medial preoptic or anterior hypothalamic areas and one other stimulation site: the median eminence (5 cells); the amygdala (6 cells); the region of thalamic nucleus medialis dorsalis (3 cells) and the midbrain periaqueductal gray (6 cells). Interaction studies indicated that the axons of these cells branched close to the origin of the axon itself. Antidromic invasion from the surface of the median eminence identified 39 tuberoinfundibular neurons. Stimulation in the medial preoptic and anterior hypothalamic area produced orthodromic excitatory (n = 5) and inhibitory (n = 4) actions on HVM [ventromedial hypothalamic nucleus] neurons, but was without an action on most other neurons (n = 30). Tuberoinfundibular neurons in the ventromedial nucleus responded to stimulation in the amygdala, but usually at latencies greater than that for medial preoptic area evoked responses. Apparently there is a close relationship between MBH neurons and cells located in both the amygdala and the medial preoptic-anterior hypothalamic area. The data for tuberoinfundibular neurons indicates that several extrahypothalamic areas may send fibers to these cells. These pathways may be important for the adaptive neuroendocrine responses reported in the literature.