Different control mechanisms of growth hormone (GH) secretion between gamma-amino- and gamma-hydroxy-butyric acid: neuroendocrine evidence in Parkinson's disease

Microbes, Immunity, and Behavior: Psychoneuroimmunology Meets the Microbiome

There is now a large volume of evidence to support the view that the immune system is a key communication pathway between the gut and brain, which plays an important role in stress-related psychopathologies and thus provides a potentially fruitful target for psychotropic intervention. The gut microbiota is a complex ecosystem with a diverse range of organisms and a sophisticated genomic structure. Bacteria within the gut are estimated to weigh in excess of 1 kg in the adult human and the microbes within not only produce antimicrobial peptides, short chain fatty acids, and vitamins, but also most of the common neurotransmitters found in the human brain. That the microbial content of the gut plays a key role in immune development is now beyond doubt. Early disruption of the host-microbe interplay can have lifelong consequences, not just in terms of intestinal function but in distal organs including the brain. It is clear that the immune system and nervous system are in continuous communication in order to maintain a state of homeostasis. Significant gaps in knowledge remain about the effect of the gut microbiota in coordinating the immune-nervous systems dialogue. However, studies using germ-free animals, infective models, prebiotics, probiotics, and antibiotics have increased our understanding of the interplay. Early life stress can have a lifelong impact on the microbial content of the intestine and permanently alter immune functioning. That early life stress can also impact adult psychopathology has long been appreciated in psychiatry. The challenge now is to fully decipher the molecular mechanisms that link the gut microbiota, immune, and central nervous systems in a network of communication that impacts behavior patterns and psychopathology, to eventually translate these findings to the human situation both in health and disease. Even at this juncture, there is evidence to pinpoint key sites of communication where gut microbial interventions either with drugs or diet or perhaps fecal microbiota transplantation may positively impact mental health.

Sodium oxybate: neurobiology and clinical efficacy

Sodium oxybate is the sodium salt of γ-hydroxybutyrate (GHB), an endogenous short-chain fatty acid that is speculated to function as a neurotransmitter in the mammalian CNS. Pharmacodynamic effects of exogenously-administered sodium oxybate may include modulating the release of neurotransmitters, including γ-aminobutyric acid, dopamine, endogenous opioids and serotonin, and stimulating release of growth hormone. It is rapidly absorbed, with approximately 25% bioavailability and a plasma half-life of 40–60 min, necessitating twice-nightly dosing. Sodium oxybate is indicated for the treatment of cataplexy and excessive daytime sleepiness in patients with narcolepsy, and has been shown to improve disrupted night-time sleep and increase Stage 3 and 4 (slow-wave restorative) sleep in this patient population. The most common adverse events reported in clinical trials in patients with narcolepsy include headache, nausea, dizziness, nasopharyngitis, somnolence, vomiting and urinary incontinence.

Compromised peripheral immunity of mice injected intrastriatally with six-hydroxydopamine

Intracisternal or intracerebroventricular administration of six-hydroxydopamine (6-OHDA), which results in decreased norepinephrine (NE) and dopamine (DA) levels throughout the brain, causes impaired peripheral immunity. However, in vivo immunocompetence following selective striatal depletion of DA by 6-OHDA has not been investigated. Thus, we sought to determine whether striatal DA depletion compromises host resistance to Listeria monocytogenes (LM) and impairs the immune response to keyhole limpet hemocyanin (KLH). Mice treated with 6-OHDA (90% decrease in striatal DA) had (i) increased LM colonization in liver and spleen, (ii) lower primary IgM and IgG(1) antibody titers, as well as secondary IgM titers, and (iii) compromised DTH response compared to controls. Co-administration of a DA uptake inhibitor partially (40%) spared striatal DA depletion and completely prevented the increase in LM burden, but was ineffective in preventing any of the 6-OHDA-induced suppressions of the immune responses to KLH. Thus, striatal DA is suggested to play a response-specific role in peripheral immunological functions.

Different control of GH secretion by gamma-amino- and gamma-hydroxy-butyric acid in 4-year abstinent alcoholics

The present study attempted to establish whether long term abstinence from alcohol restores the stimulatory effects on GH secretion of gamma-aminobutyric acid (GABA) and gamma-hydroxybutyric acid (GHB) that are absent during the first month of alcohol withdrawal. Six 4-year abstinent alcoholic subjects--already tested with GHB 4 years earlier, and seven age- and weight-matched normal controls, were tested p.o. with 800 mg sodium valproate (a drug that enhances endogenous GABA activity), 10 mg baclofen (a GABA(B) receptor agonist), 25 mg/kg body-weight GHB, or a placebo. The blood samples for growth hormone (GH) assay were taken every 30 min for the next 150 min. Placebo administration did not modify GH secretion in any subject. All drugs induced a significant increase in serum GH levels in normal controls. GH secretion in abstinent alcoholics did not change after baclofen or sodium valproate administration, whereas the GH response to GHB was similar to that observed in normal controls. The data confirm previous observations which suggest that different neuroendocrine mechanisms underlie GABA and GHB control of GH secretion. The data also indicates that the GHB--but not the GABA--stimulated pathway returns to normal in alcoholics after 4 years abstinence.

Gamma-hydroxybutyric acid and growth hormone secretion studies in rats and dogs

Gamma-hydroxybutyric acid, a gamma-aminobutyric acid metabolite, and baclofen, a gamma-aminobutyric acid B agonist, are endowed with a small growth hormone-releasing activity in human beings. In this study, we have investigated the reciprocal interactions of gamma-hydroxybutyric acid and the gamma-aminobutyric acid B system by evaluating the growth hormone-releasing activity of the two compounds and their respective antagonists in in vivo and in vitro experiments performed in rats and dogs. In in vivo experiments, neither gamma-hydroxybutyric acid (25, 100, 150, and 300 mg/kg, SC) nor baclofen (0.25, 1, 2, 4, and 8 mg/kg, SC) significantly modified growth hormone secretion in 9-day-old rat pups. Similarly, no growth hormone and prolactin release was observed in adult anesthetized rats after administration of gamma-hydroxybutyric acid (100 mg/kg, IP) or baclofen (10 mg/kg IP). Equally ineffective on the somatotropic response was the administration of gamma-hydroxybutyric acid (200 mg/kg, IP) alone or associated with its specific receptor antagonist NCS-382 (150 mg/kg, IP) given to adult anesthetized rats. In addition, a toxicological dose of gamma-hydroxybutyric acid (1500 mg/kg, IP) did not alter baseline growth hormone levels in adult conscious rats. gamma-Hydroxybutyric acid (50 mg/kg, IP) given for 10 days to adult conscious rats did not alter the growth hormone response to the same gamma-hydroxybutyric acid dose given acutely. In conscious dogs, gamma-hydroxybutyric acid (20 and 50 mg/kg, IV) and baclofen (0.15, 0.30 mg/kg, IV) also were ineffective in stimulating growth hormone secretion. In this species, growth hormone response to hexarelin (31.25 microg/kg, IV), a potent growth hormone-releasing peptide, was not modified by coadministration of gamma-hydroxybutyric acid (50 mg/kg, IV). In in vitro experiments, increasing doses of gamma-hydroxybutyric acid (10(-7), 10(-5), and 10(-3) M) did not alter growth hormone concentrations in media of rat pituitary cell cultures. In contrast, growth hormone-releasing hormone (10(-7) M) induced a significant growth hormone release into the media. In conclusion (1) gamma-hydroxybutyric acid is not an effective growth hormone secretagogue; (2) the reciprocal functional interactions between gamma-hydroxybutyric acid and the gamma-aminobutyric acid B system could not be investigated, due to the ineffectiveness of gamma-hydroxybutyric acid and baclofen to stimulate growth hormone release; and (3) short-term administration of gamma-hydroxybutyric acid does not induce adverse effects amenable to activation of the somatotropic function.

Muscarinic cholinergic mediation of the GH response to gamma-hydroxybutyric acid: neuroendocrine evidence in normal and parkinsonian subjects

We have recently reported that parkinsonian patients show a significant GH response to gamma-hydroxybutyric acid (GHB), but not to gamma-aminobutyric acid (GABA)-ergic drug administration. In order to establish whether muscarinic cholinergic receptors mediate the GH secretion induced by GHB, normal men and parkinsonian patients were tested with GHB both in the absence and in the presence of the anticholinergic agent, pirenzepine. Both normal controls and parkinsonian patients showed a significant serum GH rise in response to GHB (25 mg/kg body weight p.o.) even though a slightly, but significantly lower response was observed in parkinsonian patients. Pretreatment with pirenzepine (100 mg p.o. 2 h before GHB) completely suppressed the GHB-induced GH release in both normal controls and parkinsonian patients. These data indicate that a cholinergic mechanism mediates the GH response to GHB in normal men. In addition the data indicate that this pathway is preserved in the parkinsonian brain.

Long-term administration of GHB does not affect muscular mass in alcoholics

Gamma-hydroxybutyric acid (GHB) is a drug recently utilized for alcoholism management. It has been shown that GHB has anabolic effects since it can increase growth hormone (GH) release in healthy subjects. At present, there are no studies investigating body composition in alcoholics during long-term GHB treatment. In this study body composition and GH secretion in alcohol dependent subjects was evaluated during addiction and at different time of GHB administration and alcohol abstinence. A total of 45 male alcohol dependent patients (mean age 39.7+/-9.8 yrs, mean height: 171+/-6.8 cm, body mass index--BMI--22.1+/-1.6 kg/m(-2)) were consecutively enrolled. Body composition was assessed by anthropometry, bioimpedance analysis and tritiated water method. A 7-day food diary was collected. Plasma GH levels were determined by radioimmunoassay. A 6-month total abstinence was obtained in 22 patients, by means of psychological support counseling and self-help groups in 9 subjects and also by 50 mg/kg/day of GHB in 13 subjects. At 1, 2, 3 and 6 months of abstinence, the biochemical assessment and metabolic variables were re-examined. Fat-free mass (FFM) and basal GH secretion were similar at the different times of follow-up in both groups of patients. GHB treated patients and those receiving psychological support alone showed similar values in FFM and GH. Both groups of patients did not differ in FFM and plasma GH level from healthy controls at any of the times evaluated. Waist-to-hip ratio did not differ between patient groups, while higher values were shown in alcoholics with respect to control subjects. The present study shows that long-term administration of GHB did not affect muscular mass and did not determine an increase of GH release in chronic alcoholics. This findings could be due to an impairment of the hypothalamic-limbic system and of GABAergic neurotransmission in alcoholics' brain.