Search

Evidence suggests that somatostatin (SRIF) inhibits nonstress GH and TSH secretion and suppresses GH secretion in response to stress. The aims of this study were to determine whether placement of lesions in the hypothalamic periventricular (PV) nucleus, the location of SRIF neurons which seem to be responsible for most median eminence SRIF, causes elevation of nonstress plasma GH and TSH levels and blocks stress-induced suppression of these hormones. Electrolytic lesions were placed in female rats, and blood was obtained for assessing nonstress and stress plasma levels of GH and TSH at several intervals after surgery until autopsy, 56 weeks after surgery, when median eminences were collected. PV lesions produced only transient elevation of nonstress plasma GH levels and failed to block the suppression of GH secretion by stress. In contrast, PV lesions caused long term elevation of nonstress plasma TSH levels and blockade of stress-induced suppression of TSH secretion. The content and concentration of SRIF in the median eminence were reduced 90% in the PV-lesioned group. These data demonstrate that PV lesions, which result in marked depletion of median eminence SRIF, can cause long term elevations of plasma TSH levels and disruption of the TSH response to stress without producing alterations in GH secretion. Thus, the hypothalamic PV nucleus, its SRIF neurons and most median eminence SRIF are not essential for maintaining normal GH secretion, but seem to be involved in the regulation of TSH release. It appears that different brain structures are involved in inhibiting GH and TSH secretion.

To study the importance of neural connections of the medial basal hypothalamus (MBH) in mediating the prolactin (Prl) response to stress, adult male rats were subjected to complete, anterior or posterior surgical disconnection of the MBH. Complete MBH disconnection blocked, while anterior MBH disconnection significantly attenuated the effects of leg restraint and ether exposure on Prl responses. Similar effects were noted with regard to Prl responses to morphine and chlorpromazine, two potent stimulators of Prl secretion, indicating that the deficits were not specific for stressful stimuli. None of the surgical procedures significantly compromised the response of growth hormone or corticosterone to morphine, ruling out the possibility of surgically-induced panhypopituitarism. Pituitary stores of Prl were reduced in the groups with complete and anterior MBH disconnection at autopsy at 18 weeks. These results indicate that anterolateral neural connections of the MBH are important for maintaining adequate levels of pituitary Prl or mediating the stimuli for Prl secretory responses. These effects may involve alterations in the regulation of Prl-inhibiting factor or Prl-stimulating factor.

These experiments were designed to determine whether it is possible using in vitro perifusion to identify a sex difference in anterior pituitary (AP) release of growth hormone (GH) and, if so, to determine whether this difference is correlated with a sex difference in hypothalamic release or content of somatostatin (SRIF). Age-matched rats of both sexes were decapitated at approximately 09.00 h, and blood was collected for determination of non-stress plasma concentrations of GH. Each pituitary was rapidly removed and prepared for perifusion of the AP, and each preoptic-medial basal hypothalamus (PO-MBH) was removed and placed in a separate perifusion chamber. The effluent fractions from perifused APs were assayed for GH and prolactin (Prl), and those from PO-MBH blocks were assayed for SRIF. Non-stress plasma GH concentrations were similar in males and females. During perifusion, baseline GH release was higher (P less than 0.001) from male than from female APs. Release of GH from the APs of both sexes was similarly inhibited (P less than 0.001) by a 1-h administration of SRIF (10(-7) M), and high K+ (50 mM) caused larger (P less than 0.05) GH responses from male than from female APs. In contrast, baseline Prl release was higher (P less than 0.01) from female than from male glands, and Prl release was not affected by SRIF. Male and female PO-MBH tissues showed similar baseline release of SRIF and similar responses to high K+. After perifusion, GH content and concentration were higher in APs from males than from females, but SRIF content in the perifused male and female PO-MBH tissues was similar.(ABSTRACT TRUNCATED AT 250 WORDS)

These studies examined the effects of surgical interruption of anterolateral neural connections of the medial basal hypothalamus (MBH) on the subsequent ability of the perifused anterior pituitary (AP) to release prolactin (Prl). Anterolateral MBH cuts were made in adult male rats; sham-operated rats served as controls. An in vivo experiment performed at 5 weeks after the two types of surgery showed that the MBH cuts did not alter non-stress plasma Prl concentrations, but this surgery reduced (P less than 0.05) the Prl response to stress. At 4 months after surgery, the perifused APs from rats with MBH cuts released less (P less than 0.001) Prl under basal conditions and when perifused in series with blocks of preoptic-hypothalamic tissue than did APs from the sham-operated controls. The APs from the MBH-cut and control groups released similar amounts of Prl in response to an initial administration of 56 mM potassium (high K+). After 6 h of perifusion, the APs of the group with MBH cuts contained less (P less than 0.001) Prl than did those of the control group. These findings suggest that transecting anterolateral connections of the MBH causes a reduction in Prl synthesis and steady-state release without affecting the pool of Prl that is released in response to high K+.

Hypothalamic periventricular (PV) nucleus lesions reduce median eminence (ME) SRIF content by approximately equal to 80% without affecting non-stress plasma growth hormone (GH) levels or the GH response to stress. Our aim was to study the effects of PV lesions on SRIF released during perifusion of preoptic-anterior hypothalamic (PO-AH) tissue. Female rats received anterior or posterior PV lesions; sham-lesioned and intact rats served as controls. Non-stress and stress plasma GH levels were similar in all groups at 2, 4 and 16 weeks after surgery. At 18 weeks after surgery, the perifused PO-AHs of the PV- and sham-lesioned rats released similar amounts of SRIF, and these were higher (P less than 0.001) than amounts released from PO-AHs of intact rats. The PO-AHs from all groups showed similar increases in SRIF release after 56 mM K+. Two rats were chosen randomly from each group to assess ME SRIF content; PV lesions caused almost 80% depletion of SRIF, sham lesions did not. These results confirm that most SRIF neurons in the PV nucleus and 80% of ME SRIF content are not essential for the control of GH levels under non-stress conditions or for the GH response to stress and indicate that PV or sham lesions in the rostral forebrain enhance in vitro SRIF release, perhaps from neurons outside the PV nucleus.

Two experiments were performed to study the role of somatostatin (SRIF) neurons of the preoptic-anterior hypothalamic area (PO-AHA) in regulating growth hormone (GH) and thyrotropin (TSH) secretion in rats. Small lesions were placed in the periventricular (PV) zone and blood was collected at 24 h and 15 days after surgery. Blood samples were obtained at 3 min and at 15 min after ether exposure for assessing non-stress levels, respectively, of plasma GH and TSH. Non-stress blood samples were also collected at decapitation at 4 weeks. The brains from the first experiment were dissected and processed for measuring SRIF content in several regions. At 24 h and 15 days, non-stress GH and TSH levels were significantly elevated in rats with PV lesions. Stress-induced decrements in GH levels persisted in all groups. Although non-stress plasma GH and TSH levels returned to normal in lesioned rats at 4 weeks, SRIF content was decreased 83% in the median eminence and 33% in the hypothalamus. These results show that discrete lesions in the PV zone of the PO-AHA cause transient elevations in non-stress secretion of GH and TSH and that normal levels of such secretion can be reinstated despite reductions of SRIF in the median eminence and hypothalamus.

The purpose of this study was to determine the effects of destroying somatostatin (SRIF) neurons of the periventricular (PV) nucleus of the hypothalamus on the pulsatile pattern of growth hormone (GH) secretion in female rats. At 6-10 days after placement of PV lesions, blood samples collected every 15 min for 3-4 h showed an elevation in baseline GH levels and an increase in the amplitude of GH secretory peaks; the frequency of pulses was not affected. These changes were associated with an increase in mean integrated plasma GH levels. The alterations appeared transient because nonstress plasma GH levels were normal in two blood samples collected between 6 and 17 weeks after lesion placement and at autopsy at 17 weeks. Stress-induced suppression of GH secretion was also unaffected by the PV lesions. These lesions severely compromised the SRIF system that projects to the median eminence because the median eminence content of SRIF was approximately 85% depleted in the lesioned group. These results confirm that the hypothalamic PV nucleus is essential for maintaining most of the SRIF in the median eminence and demonstrate that damage to the PV nucleus causes transient alterations in the pulsatile pattern of GH secretion. However, the PV nucleus does not appear to play a major role in driving pulsatile GH secretion.

Enhanced nocturnal release of GH is decreased with aging in man, a change that may implicate GH in a general decline in anabolic metabolism associated with aging. The aim of this study was to determine whether nonhuman primates experience an age-related reduction in plasma GH levels by comparing the 24-h patterns of GH secretion in unrestrained young and aged male rhesus monkeys. Six young (8 yr old) and six aged (22+ yr old) intact rhesus males were fitted with indwelling jugular catheters, cranial platforms, and stainless steel cable tethers. Catheters passed from a swivel device at the top of each cage through a wall to an adjoining room. On four occasions, 1.0-ml blood samples were obtained from each male every 20 min for 24 h for plasma GH RIA. Plasma GH data were analyzed by the PULSAR program to detect hormone peaks. Mean 24-h plasma GH was less (P less than 0.0005) in aged males [0.84 +/- 0.04 ng/ml (+/-SEM)] than in young males (1.37 +/- 0.09 ng/ml). Likewise, the amplitudes of GH pulses were less (P less than 0.001) in aged males than in young males. Although no circadian pattern of GH concentrations was apparent in either age group, young males displayed more (P less than 0.05) nocturnal GH pulses (5.73 +/- 0.41 pulses/12 h) than those occurring during lights-on (3.09 +/- 0.32 pulses/12 h). The numbers of GH pulses in aged males (4.00 +/- 0.63 pulses/12 h) were similar to those in young males during lights-on, but aged males showed fewer (P less than 0.05) nocturnal GH pulses (4.27 +/- 0.47 pulses/12 h) than did young males. The duration of GH pulses in aged males (53.6 +/- 5.0 min) was similar to that in young males (50.6 +/- 5.0 min) during lights-off. Young males showed an extended (P less than 0.001) GH pulse duration (88.8 +/- 8.8 min) during lights-on that was not evident in aged males (66.2 +/- 5.4 min). These data demonstrate that unrestrained young rhesus males experience an enhanced nocturnal release of GH in terms of pulse frequency, and as in humans, this enhanced nocturnal release of GH is diminished with age. In addition, rhesus males experience, as do humans, a reduction in circulating GH levels as a function of age.

The chronic effects of lesions placed in the medial or lateral preoptic-anterior hypothalamic area (PO-AHA) on plasma growth hormone (GH) and corticosterone (cpd B) responses to stress, non-stress levels of these hormones and somatic growth were studied in adult female rats. Responses to 3-min immobilization--blood withdrawal stress uere tested at 2 and 11 weeks and to 3-min ether--blood withdrawal at 5 weeks after surgery. Non-stress plasma GH levels were further assessed in serial blood samples collected at 4-h intervals during a 24-h period at 15 weeks, and cpd B levels were studied similarly for a 44-h period at 22 weeks. Rats with lateral PO-AHA lesions showed normal GH suppression and cpd B elevation in response to both stress procedures. In contrast, neither stressor suppressed plasma GH levels in rats with medial PO-AHA lesions, but both produced normal increments in cpd B. The 24-h mean non-stress level of plasma GH in the group with medial lesions was higher and that of the group with lateral lesions was lower than those of controls. Cpd B levels were reduced during the 44-h study, and circadian rhythmicity was abolished in rats with medial lesions. The group with lateral lesions showed normal rhythmicity in plasma cpd B levels, Linear growth was not affected in either lesioned group. The GH findings suggest that the medial but not lateral PO-AHA is essential for stress-induced inhibition of GH secretion and that this region may exert a tonic inhibitory influence on non-stress GH secretion. The data also suggest that the medial PO-AHA is importantly involved in producing the circadian rhythm in pituitary-adrenal function.

The effects of surgical isolation of the medial basal hypothalamus (MBH) on linear growth and plasma growth hormone (GH) levels were studied in adult male and female rats. Whereas MBH isolation resulted in reduced linear growth in males, increased growth resulted in females. These effects were apparent in males 4 weeks after surgery and in females at 10 weeks. The altered growth rates persisted throughout the 28 weeks of the study. MBH isolation in both sexes led to increased food intake, obesity, and atrophy of reproductive tract organs. Non-stress plasma GH levels were unchanged at 4 weeks in both sexes but were elevated at 10 and 26 weeks in females and at 19 and 26 weeks in males. Serial samples of tail vein blood were obtained at 4-h intervals for 44 h from MBH-isolated and shamoperated females at 17 weeks postoperatively, and from similar groups of males at 19 weeks. GH levels were elevated in MBH-isolated rats of both sexes during the afternoons and evenings. Only sham-operated females showed evidence of a 24-h rhythmicity in circulating GH levels. The amplitudes of trough to peak excursions were similar in MBH-isolated and control females, but these excursions were reduced in MBH-isolated males. Although opposite effects on linear growth were noted in the two sexes, the resulting growth rates of operated males and females were similar, suggesting that MBH isolation disrupts mechanisms that normally regulate sex-specific growth patterns. Despite the sex difference in growth responses, MBH isolation caused an eventual increase in plasma GH levels in both sexes. Therefore, it is unlikely that the sex difference in growth responses is attributable exclusively to corresponding changes in circulating GH levels. A reduction in levels of circulating gonadal steroids may be an important factor. The elevations in plasma GH levels observed in both sexes suggest that MBH isolation interferes with mechanisms which inhibit GH secretion.

These experiments were designed to determine whether brain lesions which elevate nonstress plasma levels of GH and disrupt stress-induced suppression of GH secretion in female rats affect the median eminence content of somatostatin. Some rats received lesions in the medial or lateral preoptic-anterior hypothalamic area (PO-AHA), while others received anterior hypothalamic cuts. Sham-operated and intact rats served as controls. GH and somatostatin were measured by RIA. Medial but not lateral PO-AHA lesions caused elevated nonstress plasma GH levels at 2, 14, 17, and 23 weeks after surgery, but normal levels were obtained at autopsy at 27 weeks. These lesions compromised GH responses to stress at 14 and 23 weeks. Rats with anterior hypothalamic cuts showed elevated nonstress GH levels at 17, 23, and 27 weeks after surgery and loss of the GH response to stress at 14 and 23 weeks. Median eminence content of somatostatin was reduced approximately 80% in rats with medial PO-AHA lesions or anterior cuts. Whereas medial PO-AHA lesions were associated with normal body length and weight and evidence of estrogen secretion, anterior hypothalamic cuts produced increased linear growth and body weight and signs of functional castration. These results suggest that the effects of lesions which cause prolonged elevation of nonstress GH levels and disruption of the GH stress response are due to interference with somatostatin neurons located in the medial PO-AHA. Somatostatin content of the median eminence seems to depend largely on connections originating in or traversing the medial PO-AHA.

To determine whether residual inhibitory control of growth hormone (GH) secretion in rats with lesions of the periventricular nucleus (PVN) and depleted median eminence content of somatostatin (SRIF) is due to SRIF, PVN-lesioned rats were treated with SRIF antiserum. Such treatment, in contrast to normal sheep serum, caused increased (P less than 0.0001) plasma GH in lesioned and control groups. These results indicate that biologically important amounts of SRIF are available in the PVN-lesioned rat.

The aim of these experiments was to determine whether the telencephalon is essential for stress-induced suppression of plasma GH levels and, if so, which telencephalic structures are involved. In the first study, the telencephalon was completely removed from adult female rats by suction; intact and sham-operated rats served as controls. At 24 h after surgery, blood samples were collected at the initiation of and 15 min after a 3-min immobilization-blood withdrawal stress procedure and used for determining non-stress and stress plasma levels of GH, respectively. GH was measured by radioimmunoassay. In contrast to the normal stress-induced decrements in GH levels observed in controls, this response was blocked and stress caused a paradoxical increase (P less than 0.01) in GH levels in telencephalon-ablated rats. These results suggested that the telencephalon is essential for stress-induced inhibition of GH secretion. Therefore, the several components of the telencephalon were ablated individually in additional experiments to determine the structure(s) involved. Placement of large, bilateral lesions in the amygdala or complete removal by suction of hippocampus, septum or striatum did not interfere with the normal GH response to stress at 24 h after surgery. However, removal of the preoptic region duplicated the effects of complete telencephalon ablation i.e., the normal stress-induced decrement was not only blocked but also reversed. These findings suggest that the preoptic region is essential for mediating inhibition of GH secretion in response to immobilization-blood withdrawal stress and that other telencephalic structures are not essential for this neuroendocrine response.

The possibility that the gonadotrope population of the anterior pituitary experiences an age-related alteration in function was investigated by in vivo and in vitro methods in young (4- to 6-month) and old (18- to 20-month) male Sprague-Dawley rats. Plasma concentrations of testosterone were 50% lower in the old rats, but resting concentrations of luteinizing hormone (LH) were similar in the two age groups. After leg-restraint and blood-withdrawal stress, plasma LH levels were significantly elevated in both young and old males; however, LH levels achieved by aged males were 39% less than those achieved by young males. During perifusion of anterior pituitary, release of LH (ng/ml per 10 min) was stable for 7 h; old anterior pituitary released only 52% as much LH as young anterior pituitaries. The anterior pituitary LH content after perifusion was not altered with age. Castration 2 weeks prior to perifusion caused elevations in plasma LH and in LH released from anterior pituitary during perifusion that were similar in the two age groups. Implantation of testosterone-filled Silastic capsules 2 weeks prior to perifusion elevated plasma testosterone and reduced plasma LH levels in both age groups. The in vitro release of LH from anterior pituitaries was similarly reduced in both age groups. Administration of varying doses of LH-releasing hormone (LHRH) during perifusion caused similar releases of LH above baseline levels in young and old rats. These in vitro results show that aged male rat anterior pituitaries release less LH than anterior pituitaries from young males. However, the magnitude of the LH response of anterior pituitaries to LHRH is not reduced with aging. These findings suggest that the decline in androgen status in old rats is not attributable to a deficit in pituitary responsiveness to LHRH. The effects of manipulating testosterone levels failed to implicate a change in anterior pituitary sensitivity to feedback as a cause for the hormonal status of aged male rats.