Growth Hormone (GH) but not IGF-I-Secretion is Stimulated in Aged Rats by Chronic Hexarelin and GHRP-6 administration.

The present studies were designed to investigate in aged rats (22-24 months old) the effectiveness and specificity of acute hexarelin (HEXA) administration on the release of growth hormone (GH), the minimal dose of the peptide that produces the maximal GH secretion and the effects of long term treatment with HEXA or growth hormone releasing peptide (GHRP-6) on plasma levels of GH and IGF-1. To select the appropriate anesthetic to obtain serial blood samples, the GH releasing activity of HEXA (30 µg/kg i.v.) was tested in young rats anesthetized with tribromoethanol and ketamine-xylazine, and the results were compared with those obtained from conscious freely moving rats. When compared to the changes found in conscious rats, the GH-response to HEXA was increased by ketamine-xylazine anesthesia, while it was decreased by tribromoethanol. Therefore, all subsequent experiments were performed in conscious freely moving rats. To determine the minimal effective dose of HEXA and GHRP-6 in aged animals, different doses (5, 10, 20, 40, and 80 µg/kg) of either HEXA or GHRP-6 were administered subcutaneously to young and aged rats. It was found that 20 µg/kg bw of HEXA or GHRP-6 was the minimal dose producing the maximal GH response. Plasma PRL and LH determinations in the animals injected with the minimal dose showed that HEXA and GHRP-6 stimulate the GH release specifically. Based on the results obtained in the latter experiments, young and aged rats were chronically treated with s.c. injections of 20 µg/kg twice a day for 15 or 30 days. At the end of the experiments, animals were acutely tested with s.c. injection of the same dose (20 µg/kg). It was found that both young and aged animals released GH in response to the repeated administration of GH releasing peptides after 15 days of treatment. However, after 30 days of treatment plasma GH did not change when young and aged HEXA treated animals were acutely tested with such peptide, but increased significantly in the GHRP-6 treated animals when they were acutely tested with GHRP-6. In response to the long term treatment with the GH-releasing peptides, plasma IGF-I levels increased in young animals when they were acutely tested with the corresponding peptide. However, plasma IGF-I levels in aged rats were not modified by the acute administration of either HEXA or GHRP-6 after 15 or 30 days of treatment. This study showed that: 1. HEXA and GHRP-6 stimulated GH release in aged rats and the magnitude of GH responses was similar to that in young rats; 2. the minimal dose of HEXA and GHRP-6 resulting in maximal GH response was 20 g/kg and such dose of HEXA specifically stimulated GH secretion in aged rats; 3. fifteen but not 30 days treatment of aged rats with such small dose of HEXA resulted in a similar GH response to subsequent stimulation with the same peptide; 4. plasma IGF-I responses to an acute administration of HEXA or GHRP-6 in rats chronically treated with the same peptide vanished with aging.

Effect of long-term administration of Hexarelin on the somatotrophic axis in aged rats.

The growth hormone-releasing peptide Hexarelin (Hexa; 80 micrograms/kg-1, s.c.) was administered for 30 and 60 days to old rats. The GH-releasing effect of Hexa was maintained during chronic treatment. At the end of the treatment, old rats were administered once with Hexa which elicited a greater GH response in rats chronically treated with the peptide than in those receiving a placebo. Pituitary GHmRNA concentrations were significantly lower in the older rats than in the younger animals, irrespective of Hexa treatment, while the GH protein content was similar in all the groups studied. The same was true for hypothalamic GHRH, whose synthesis was reduced in all the older animals but not in the young, in the presence of maintained concentrations of the peptide. Somatostatin mRNA concentrations were significantly higher in the hypothalami of older rats and administration of Hexa for 30 or 60 days brought the concentrations of somatostatin mRNA of aged rats to 'young' levels. Treatments with Hexa failed to alter the circulating levels of IGF-1. The data reported in this article indicate that long-term treatment with Hexa normalized some biological indices of somatotrophic function in aged rats.

Effects of single and short-term administration of clonidine on hypothalamic-pituitary somatotropic function of the adult male rat: an in situ hybridization study.

The effects of the alpha-2 adrenoceptor agonist clonidine (CLO) on the growth hormone (GH) regulatory neuronal systems, growth hormone-releasing hormone (GHRH) and somatostatin (SS), were studied in adult male rats given a single or a short-term administration (1, 3 and 6 days) of the drug. Acute administration of CLO significantly decreased hypothalamic GHRH content [leaving unaltered GHRH messenger RNA (mRNA) levels] and increased plasma GH levels; hypothalamic SS content/mRNA levels and pituitary GH content/mRNA levels remained unchanged. In 1- and 3-day CLO-treated rats, by contrast, decreased hypothalamic GHRH content was coupled with a significant reduction in GHRH mRNA levels. In these rats, pituitary GH content and mRNA levels were also significantly increased, whereas hypothalamic SS content and mRNA levels remained unaltered. In 6-day CLO-treated rats, hypothalamic GHRH content and mRNA levels were still significantly reduced, plasma GH levels were increased, but to a lesser extent than in 1- and 3-day CLO-treated rats, and pituitary GH content and mRNA reverted to control levels. Hypothalamic SS content and mRNA levels remained unaltered. These results indicate that 1) functional activation of alpha-2 adrenergic receptors by CLO increases GHRH release from the hypothalamus, 2) CLO, via GHRH, increases GH secretion and biosynthesis, which in turn feeds back in the hypothalamus to reduce GHRH biosynthesis, and 3) reduction of hypothalamic GH-stimulatory activity tones down the initial pituitary somatotropic hyperfunction. Unaltered hypothalamic SS content and mRNA levels in all CLO-treated rats suggests that the somatostatinergic system is less sensitive than the GHRH system to changes in circulating GH levels.

NOS is present in the brain of Triatoma infestans and is colocalized with CCK.

Immunohistochemistry was used to establish the presence of nitric oxide synthase (NOS) in the central nervous system of Triatoma infestans which is the main vector for Chagas' disease in Argentina and neighbouring countries. In addition, we have investigated the presence of cholecystokinin (CCK) and studied the possible coexistence of these molecules. The results show NOS-like immunoreactivity (LI) in neurones of the soma rind of the protocerebrum, the optic lobe and in the lateral part of the sensory deutocerebrum with a few cells in the suboesophageal and the prothoracic ganglia. The distribution of CCK-LI was similar to that of NOS-LI and in several areas both molecules coexisted in neurones and fibres. The results suggest that nitric oxide may act as a neurotransmitter in the brain of insects.

Combined administration of growth-hormone-releasing hormone and clonidine restores defective growth hormone secretion in old dogs.

We have studied in old dogs the effects of short-term administration of growth hormone (GH)-releasing hormone (GHRH) alone or co-administered with clonidine (CLO), an alpha 2-adrenergic agonist, on the GH secretory pattern (cluster analysis), and GH responsiveness to an acute GHRH or GHRH + CLO challenge and plasma somatomedin C (SMC) levels. Dogs were given either GHRH alone twice daily for 10 days (treatment 1) or combined GHRH + CLO both given twice daily (treatment 2) or GHRH + CLO given once daily (treatment 3). Animals were sampled from 09.00 to 15.00 h, at 10-min intervals, both before and 14 h after treatments. At the end of the 6-hour sampling period, dogs were challenged with simultaneous administration of GHRH and CLO, while they were tested with GHRH alone on the morning of the following day. In dogs undergoing treatment 1, acute administration of GHRH or GHRH + CLO elicited mean GH peak responses higher than before treatment, but none of the GH secretory indices were modified during the 6-hour sampling period, except for the increase in mean GH peak amplitude. In dogs undergoing treatment 2, acute administration of GHRH elicited a mean GH peak response higher than that before treatment, whereas administration of GHRH + CLO induced a mean GH peak response not different from that elicited by GHRH + CLO before treatment or by GHRH alone after treatment. However, this treatment significantly augmented the frequency of spontaneous bursts of GH secretion, the mean GH peak amplitude and the total peak area.(ABSTRACT TRUNCATED AT 250 WORDS)

Central mechanisms subserving the impaired growth hormone secretion induced by persistent blockade of NMDA receptors in immature male rats.

Recently, we have reported in immature female rats that short-term blockade of glutamate receptors of the N-methyl-D-aspartic acid (NMDA) subtype by the noncompetitive antagonist MK-801 induced a reduction of growth rate, basal and stimulated growth hormone (GH) release and plasma somatomedin C levels. In the present study, we investigated in immature male rats the mechanism(s) through which agonists and antagonists of glutamate receptors affect GH secretion. In 21-day-old male rats, administration of MK-801 (0.2 mg/kg i.p.b.i.d.) for 10 days induced a significant impairment of growth rate, which was unrelated to a significant reduction of food intake. GH secretion from anterior pituitary fragments of MK-801-treated rats was not significantly reduced under basal conditions but was significantly less under stimulation by 40 mM K+. Incubation of dispersed pituitary cells of 31-day-old rats with N-methyl-aspartic acid (1 and 100 microM), alone or associated with MK-801 (1 microM) did not change GH secretion. Semi quantitative densitometric analysis of hypothalami of MK-801-treated rats evidenced a clearcut decrease in the intensity of GHRH-like immuno-reactivity (LI) staining in the median eminence (ME), whereas no difference was observed in the ME-somatostatin (SS)-LI. Finally, GHRH mRNA but not SS-mRNA, evaluated by slot-blot hybridization, was reduced in the hypothalamus of MK-801-treated rats. These and our previous data would demonstrate that NMDA glutamate receptors play an important role in the neuroendocrine control of GH secretion in the rat, and suggest an action mediated by GHRH-secreting neurons.

Release of prolactin and growth-hormone by pituitary grafts. Role of the neurointermediate lobe.

The release of prolactin (PRL) and growth hormone (GH) of pituitary grafts and in situ glands was investigated using perifusion techniques. Whole pituitary or anterior lobe grafts were used. The grafts or the in situ glands were incubated alone in a chamber. The hypophysis of dioestrous-1 glands were used as controls. Both types of grafts released PRL an GH to the medium, being the PRL release of the anterior lobe graft higher than that of the whole pituitary graft. The in situ pituitary glands of grafted animals released less PRL than the control dioestrous glands. Also, the in situ glands of whole pituitary grafted animals released less PRL than the hypophysis of animals which bore grafts of only the anterior lobe. No difference in GH secretion, either by the graft or by the in situ gland was observed when whole pituitary or anterior grafts were used. These results provide a further support to the hypothesis that a pituitary graft under the kidney capsule exerts profound modifications in the function of the in situ hypophysis. The presence of the neurointermediate lobe (NIL) in the graft is modulatory to the release of PRL.

Release of prolactin and growth-hormone by pituitary grafts role of the neurointermidiate lobe

Usando técnicas de perfurasión, se estudió la liberación de PRL y de GH de los transplantes hipofisarios y de las correspondientes hipófisis in situ. Se utilizaron transplantes de hipófisis total o de lóbulo anterior hipofisario solamente. Las hipófisis de hembras en el estadio Diestro-1 del ciclo estral se utilizaron como controles. Tanto los transplantes como las hipófisis in situ se incubaron solas en una cámara de perfusión. Ambos tipos de transplante liberaron PRL y GH al medio, siendo la liberación de PRL del transplante de lóbulo anteior mayor que la liberación por el transplante de hipófisis total. Las glándulas in situ de los animales transplantados liberaron menos PRL que las glándulas animales con transplante de hipófisis total menor que la de las hipófisis de animales con transplante de lóbulo anterior solamente. No se encontraron diferencias en la liberación de GH entre los dos tipos de transplante ni entre los dos tipos de hipófisis in situ. Estos resultados sugieren que la presencia de un transplante hipofisario bajo la cápsula renal produce profundas modificaciones en la función de las hipófisis in situ. :a presencia del lóbulo neurointermedio en el transplante modula la liberación de PRL (AU)

Release of prolactin and growth-hormone by pituitary grafts role of the neurointermidiate lobe

Usando técnicas de perfurasión, se estudió la liberación de PRL y de GH de los transplantes hipofisarios y de las correspondientes hipófisis in situ. Se utilizaron transplantes de hipófisis total o de lóbulo anterior hipofisario solamente. Las hipófisis de hembras en el estadio Diestro-1 del ciclo estral se utilizaron como controles. Tanto los transplantes como las hipófisis in situ se incubaron solas en una cámara de perfusión. Ambos tipos de transplante liberaron PRL y GH al medio, siendo la liberación de PRL del transplante de lóbulo anteior mayor que la liberación por el transplante de hipófisis total. Las glándulas in situ de los animales transplantados liberaron menos PRL que las glándulas animales con transplante de hipófisis total menor que la de las hipófisis de animales con transplante de lóbulo anterior solamente. No se encontraron diferencias en la liberación de GH entre los dos tipos de transplante ni entre los dos tipos de hipófisis in situ. Estos resultados sugieren que la presencia de un transplante hipofisario bajo la cápsula renal produce profundas modificaciones en la función de las hipófisis in situ. :a presencia del lóbulo neurointermedio en el transplante modula la liberación de PRL

Release of prolactin and growth-hormone by pituitary grafts. Role of the neurointermediate lobe.

The release of prolactin (PRL) and growth hormone (GH) of pituitary grafts and in situ glands was investigated using perifusion techniques. Whole pituitary or anterior lobe grafts were used. The grafts or the in situ glands were incubated alone in a chamber. The hypophysis of dioestrous-1 glands were used as controls. Both types of grafts released PRL an GH to the medium, being the PRL release of the anterior lobe graft higher than that of the whole pituitary graft. The in situ pituitary glands of grafted animals released less PRL than the control dioestrous glands. Also, the in situ glands of whole pituitary grafted animals released less PRL than the hypophysis of animals which bore grafts of only the anterior lobe. No difference in GH secretion, either by the graft or by the in situ gland was observed when whole pituitary or anterior grafts were used. These results provide a further support to the hypothesis that a pituitary graft under the kidney capsule exerts profound modifications in the function of the in situ hypophysis. The presence of the neurointermediate lobe (NIL) in the graft is modulatory to the release of PRL.

Release of prolactin and growth-hormone by pituitary grafts. Role of the neurointermediate lobe.

The release of prolactin (PRL) and growth hormone (GH) of pituitary grafts and in situ glands was investigated using perifusion techniques. Whole pituitary or anterior lobe grafts were used. The grafts or the in situ glands were incubated alone in a chamber. The hypophysis of dioestrous-1 glands were used as controls. Both types of grafts released PRL an GH to the medium, being the PRL release of the anterior lobe graft higher than that of the whole pituitary graft. The in situ pituitary glands of grafted animals released less PRL than the control dioestrous glands. Also, the in situ glands of whole pituitary grafted animals released less PRL than the hypophysis of animals which bore grafts of only the anterior lobe. No difference in GH secretion, either by the graft or by the in situ gland was observed when whole pituitary or anterior grafts were used. These results provide a further support to the hypothesis that a pituitary graft under the kidney capsule exerts profound modifications in the function of the in situ hypophysis. The presence of the neurointermediate lobe (NIL) in the graft is modulatory to the release of PRL.

Papilla palatina, nasopalatine duct and taste buds of young and adult rats.

The morphology of the papilla palatina, the nasopalatine ducts and the taste buds situated within these ducts was studied in pups and adult rats using light and electron microscopy. During development, the papilla palatina grew in width and depth, becoming a protuberance in weanlings and adults. The nasopalatine ducts enlarged and two folds of the lateral walls of the ducts differentiated, reducing the width of the tubes in the region of the oral openings. Taste buds appeared postnatally. Light, dark, and perigemmal cells were found in all stages studied, but light cells were scarce up to 8 days of age. The taste pore appeared between 11 and 13 days of age; it lacked electron-dense material of cellular origin. Synaptic-like images could be found only in relation to dark cells. The papilla palatina, the nasopalatine ducts and the taste buds were fully developed by the 3rd week of life.