Effect of Intraventricular Injection of Dopamine, Norepinephrine, Acetylcholine, and 5-Hydroxytryptamine on Immunoreactive Somatostatin Release into Rat Hypophyseal Portal Blood*

The effect of several putative neurotransmitters in the regulation of somatostatin release from the median eminence into hypophyseal portal blood was investigated in male rats under urethane anesthesia. Using a specially designed microfraction collector, hypophyseal portal blood was collected at 15-min intervals into five separate tubes containing 2 N acetic acid. Immunoreactive somatostatin (IRS) in the acid-blood mixtures was extracted with acetone, washed with organic solvents, and determined by RIA. Dopamine, norepinephrine, 5-hydroxytryptamine, or acetylcholine in a dose of 10-8 mol/5 μl freshly prepared 5% glucose solution or 5 nl glucose solution alone was injected into the third ventricle immediately after collection of the first blood sample. In 10 control animals, the mean (±SE) secretion rate and concentration of IRS were 3.90=0.66 pg/min and 628 ± 126 pg/ml before injection of 5% glucose, 2.12 ± 0.28 pg/min and 286 ± 22 pg/ml 15 min after the injection, 1.93 ± 0.17 pg/min and 261 ± 18 pg/ml at 30 min, 1.53 ± 0.21 pg/min and 240 ± 14 pg/ml at 45 min, and 2.02 ± 0.31 pg/min and 221 ± 33 pg/ml at 60 min. Portal IRS values increased significantly to 8.87 ± 1.94 pg/min and 1411 ± 166 pg/ml 30 min after dopamine injection, 5.93 ± 0.90 pg/min and 1055 ± 152 pg/ml 15 min after norepinephrine injection, and 5.44 ± 0.90 pg/min and 579 ± 88 pg/ml 45 min after acetylcholine injection. The injection of 5-hydroxytryptamine did not change portal IRS significantly. The stimulatory effect of dopamine on IRS release was also observed in Althesin-anesthetized rats. These results suggest that the secretion of IRS into hypophyseal portal blood is stimulated through the activation of catecholaminergic and cholinergic mechanisms in the rat.