Comparative analysis of the sensitivity of single neurons of the central nervous system to thyrotropin-releasing hormone and its structural analog.

Abstract

There are now data indicating the presence of a number of biologically active oligopeptide substances in various regions of the brain and spinal chord, which were hitherto considered as strictly specific only for structures of the hypothalamus and participated in the neuroendocrine sphere as regulators of the release of tropic hormones of the pituitary. Such oligopeptides, detected in regions of the brain other than the hypothalamus, including primarily thyrotropin-releasing hormone (TRH) and gonadotropin-releasing hormone. According to the literature data [ I-4], of their total content in the central nervous system,the mediobasal hypothalamus accounts for only 30 and 20%, respectively. Until now, the question of the role of the extrahypothalamic-releasing hromones has not been entirely elucidated, and the available data are contradictory [5-8]. In the present work we undertook an attempt to approach a resolution of this question by evaluating the sensitivity of single neurons of various regions of the brain to TRH (Pyr-His-Pro-NH 2 ) and a structural analog of it (TRH-AN) - Pyr-Ser-Leu-NH 2, under conditions of microiontophoretic delivery of these preparations to nerve cells in various regions of the central nervous system. MATERIALS AND METHODS The investigations were conducted on male rats weighing 250-300 g under conditions of urethane anesthesia (120 mg per 100 g of body weight) and stereotaxic fixation of the animals. The activity" of the cortical [L-0.2 H-0(+3), AP5.2] hippocampal [L-0.2, H-0 (+1.7), AP-5.2], medial-thalamic [L-0.2, H-0 (0.7),AP--5.2], paraventricular [L-0.2, H-0 (-2.7), AP-5.2] and arcuate [L-0.2, H-0 (-3.5), AP-5.2] neurons was studied. These regions were selected for investigation according to data indicating a relatively high content of TRH in these structures in comparison with other regions of the brain. For extraceUular recording of the impulse activity and microiontophoresis we used multichannel glass electrodes with a 5-6/1 thickness of the tip. The central, recording stem of the microelectrode fiUed with a 2% solution of pontamine azure in a 2 M NaC1 solution had a resistance~ of 3-10 MI2. The side stems were filled with a 0.15 M NaC1 solution or with a solution of TRH (1 mg/ml), pH 6.5, or TRH-AN (1 mg/ml), pH 7.5. Preparations of TRH and TRH-AN, the synthesis of which was developed and carried out at the laboratory of the chemistry of protein hormones of the Institute of Experimental Endocrinology and Hormone Chemistry, Academy of Medical Sciences of the USSR, were used in the work. The use of TRH pH 6.5 and TRH-AN pH 7.5 was associated with the fact that ionization in solutions proceeds more intensively in weakly acid medium for the first preparation and in weakly alkaline medium for the second. These differences are due to structural peculiarities of the preparations. As a control, one of the stems of the microelectrode was filled with TRH solution (1 mg/ml), pH 7.5. The resistance of the side stems was 30-100 M ~2. The electrodes were filled