Effects of Temperature on the Preopticohypophyseal Neurosecretory System of Carp, Cyprinus carpio L.

Abstract

Effects of low (+4 to +10°C) and high (+34 to +40°C) temperatures (T) on the preoptico-hypophyseal neurosecretory system (PHNS) of carp were studied. All the animals, including controls, were kept in a tank at 20–27°C before the experiments. A tendency to a decrease in size of the perikarya and nucleoli of the neurosecretory cells (NSC) of the nucleus praeopticus (NP) is noted in fish exposed to T 4–10°C for 1–2 h. The size of the nuclei is not changed, but they become denser. The amount of the neurosecretory material (NSM) in the cells of NP increases up to 4–4.5 units (3–4 units in controls). The NSM is intensely stained and looks homogeneous. The amount of NSM in the proximal and distal parts of the neurohypophysis (Nh) increases up to 4–5 units (3–4 units in controls). Capillaries in the NP (in comparison to the controls) are not very pronounced; the hyperemia of the Nh increases up to 2.5 units (2 units in controls). A decrease in size of the perikarya, nuclei, and nucleoli is seen in the NP of fish exposed to a gradual fall of T to 4–10°C for 6 h, and after keeping them at this T for 2–2.5 h. The amount of NSM decreases to 1–2 units. In the proximal parts of Nh 2.5 units of NSM are contained; no change is found in the NSM content in the distal parts of Nh. NSM is loosely arranged, finely granulated, and palely stained. The capillaries are not pronounced in the PHNS. An increase in size of the perikarya, nuclei, and nucleoli is found in NP under the influence of high T, which has a lethal effect. Basophilia of the cytoplasm is lowered. NSM is poorly stained. Its amount is increased up to 4 units. NSM is displaced from the proximal into the distal parts of Nh, where its amount exceeds 5 units. NP and Nh are strongly hyperemic. Thus, under the influence of T changes, inhibition of discharge of the neurohormones takes place. At low T, inhibition of synthesis of NSM, and at high T an activation of synthesis of NSM occurs in the NP. Under the gradual influence of low T, an activation of release of neurohormones into the blood circulation is observed against the background of inhibition of the synthesis of NSM in NP.