Substance P (SP) affects gonadotropin release from the anterior pituitary gland. In the present study we tested whether SP exerts this effect through GnRH release into pituitary portal blood in intact male rats (INT), orchidectomized rats with s.c. chronically implanted empty Silastic capsule (ORCX), testosterone capsule (ORCX + T), and 17β-estradiol capsule (ORCX + E₂). The pituitary glands were exposed by the transpharyngeal approach under urethane-chloralose anesthesia. Then, the stalk portal vessels were cut and three 30-min portal blood samples were collected. Each first sample of blood was treated as a control before 0.2 ml injection of normal saline, 5 µg, or 25 µg of SP in 0.2 ml of normal saline into the internal carotid artery. GnRH concentration in the purified portal plasma were measured by RIA. Injection of SP into the internal carotid artery caused a significant increase in GnRH concentration in pituitary portal plasma only in INT rats. The higher dose of SP markedly increased GnRH concentration in the 1st blood sample (p < 0.001) and in the 2nd blood sample GnRH concentration was lower but still significant higher than prior SP injection (p < 0.05). The lower dose of SP increased GnRH concentration later, only in the 2nd portal blood sample after intracarotid SP injection (p < 0.001). Injection of normal saline had no effect on GnRH concentration in pituitary portal blood in INT rats. In ORCX, ORCX testosterone- and estrogen-implanted rats portal plasma GnRH concentrations were not changed significantly after injection of both doses of SP. These results indicate that SP stimulates GnRH release into pituitary portal blood and the influence of SP on GnRH neurons depends on the levels of circulating gonadal steroid hormones.

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Substance P (SP) and luteinizing hormone releasing hormone (LH-RH) content were measured by radioimmunoassay in median eminence of female rats, SP only in rats pituitary after decapitation on the day of oestrus (E), on the first and second day of dioestrus (D1 and D2), on the day of proestrus (P) and in "constant oestrus" (AD) caused by anterior hypothalamic deafferentation and in lactation (L). During four days of oestrus cycle the highest SP and LH-RH content of median eminence was found in dioestrus. Both during cyclic oestrus and "constant oestrus", SP and LH-RH were significantly lower than on the second day of dioestrus. These results may suggest the reciprocal functional relation of both hypothalamic peptides in the mechanism of the control of gonadotropin release. The highest LH-RH concentration of the median eminence was found in the animals on the 11th to 15th day of lactation, while the SP content in the median eminence of these animals is not significantly different from those of the remaining groups except the value for dioestrus (D2) rats. The SP content in pituitary of lactating rats was significantly higher than those of rats during cyclic or "constant oestrus".