Responses of Adenohypophyseal Hormones to Substance P Administration in Geldings: Comparison to Responses After Brief Exercise and Sulpiride Administration

Abstract

Nine adult geldings were used in three experiments to study the possible role of substance P in the prolactin responses to non-dopaminergic stimuli. Experiment 1 was performed as an incomplete Latin square design to determine the secretory responses of prolactin, growth hormone (GH), adrenocorticotropic hormone (ACTH), thyroid stimulating hormone (TSH), luteinizing hormone (LH) and follicle stimulating hormone (FSH) to intravenous administration of substance P. Doses tested and compared to no peptide (0 dose, control) were 62, 125, 250, and 500 ug of substance P. The three highest doses of peptide caused an immediate rise in heart rate, sweating, salivation, rhinorrhea, stretching of hind legs, and defecation. The lowest dose (62 ug) caused minor sweating, some rhinorrhea, and a rise in heart rate. Recovery from these physical responses was complete in approximately 30 min. All doses of substance P caused an immediate rise (P < 0.01) in plasma prolactin concentrations, with the three highest doses producing similar responses, and the 62 ug dose producing a minimal response (P < 0.05). Concentrations of ACTH (P < 0.01) and GH (P = 0.05) also increased after substance P administration; concentrations of LH, FSH, and TSH were unaffected. Experiment 2 compared the effects of brief exercise on hormonal characteristics. Two minutes of trotting increased (P < 0.01) plasma concentrations of GH, ACTH, and prolactin, as well as LH (P = 0.055). Experiment 3 determined the relative responses of prolactin to a fixed dose of sulpiride (0.1 mg/kg of body weight). In general, the prolactin responses to substance P were similar to those after exercise, which were both generally less than after sulpiride. These data are consistent with a possible role of substance P in the prolactin response to stressful stimuli.