PubMed Health⌕ Search

Biomedical subjects

L Lima

Publications and source records attributed to L Lima.

At least 73 records · Page 4Linked to original sources
PubMed1994

8-[3H]hydroxy-2-(di-n-propylamino) tetralin binding sites in goldfish retina.

The binding sites of 8-[3H]hydroxy-2-(di-n-propylamino)tetralin ([3H]DPAT) were characterized in the retina of goldfish in order to evaluate the selectivity of the ligand for serotonin1A (5HT1A) receptors. Specificity of the binding was performed in the presence of serotonergic and dopaminergic agonists and antagonists. Buspirone, spiroxatrine and 5-methoxy-N,N-dimethyltryptamine were potent inhibitors, followed by propranolol, citalopram, imipramine and desipramine. Serotonin was not a potent inhibitor, and its interaction with the binding sites of [3H]DPAT was complex. Nomifensine displayed an important inhibition, however, other dopamine uptake blockers, such as bupropion and GBR-12909, were less potent. Haloperidol was also a good inhibitor, but the D1 receptor agonist, SKF-38393, the D2 receptor antagonist, sulpiride, and dopamine did not inhibit the binding. GppNHp inhibited the binding in the micromolar range. The analysis of saturation experiments by isotopic dilution, using buspirone to determine nonspecific binding, revealed two sites. The number of binding sites defined by buspirone were higher than the ones defined by nomifensine. The specific binding, using buspirone for definition, was reduced by the intraocular injection of 6-hydroxydopamine. This investigation demonstrates that [3H]DPAT labels 5HT1A receptors in goldfish retina, but also interacts with a non-5HT receptor site. These receptors seem to be localized in dopaminergic neurons.

Animals↗
PubMed1994

Characterization of serotonin transporter in goldfish retina by the binding of [3H]paroxetine and the uptake of [3H]serotonin: modulation by light.

The serotonin (5-HT) uptake system of goldfish retina was evaluated by the binding of [3H]paroxetine to membrane preparations and the uptake of [3H]5-HT into isolated cells from goldfish retina. The order of potency of inhibitors of [3H]paroxetine binding was imipramine > 5-methoxy-N,N- dimethyltryptamine > desipramine > fluoxetine > citalopram > 5-HT. The saturation experiments indicated a high-affinity binding site, and positive cooperativity with Hill coefficient higher than unity. The association reached equilibrium at about one hour of incubation and was efficiently displaced by imipramine. The equilibrium dissociation constants calculated by the antilog of the log concentration of ligand giving 50% of occupation, and by the ratio of dissociation/association constants, were similar: 5.84 and 2.34 nM, respectively. The binding was not significantly reduced by decreasing the temperature of incubation and was sodium dependent. The lesion with 5,7-dihydroxytryptamine reduced the binding to 60%. The uptake of [3H]5-HT into isolated cells also showed positive cooperativity. The order of potency of inhibitors was similar to the one obtained for the binding of [3H]paroxetine. Darkness increased the uptake of 5-HT. The allosteric regulation of the 5-HT transporter and the modulation by light could be related to the physiological role of the monoamine, as a neurotransmitter and as a precursor of melatonin synthesis in the retina.

Animals↗
PubMed1993

Studies on alpha 2-adrenergic modulation of hypothalamic somatostatin secretion in rats.

This study was undertaken to investigate whether or not alpha 2-adrenergic pathways would negatively modulate the hypothalamic somatostatin release in rats. To induce pharmacological changes in SS release, three groups of male Sprague-Dawley rats (n = 30/group) were separately anesthetized by ip administration of urethan (which increases SS tone; 1.2 g/kg), pentothal (which impairs SS release; 30 mg/kg), or ketamine (which does not affect spontaneous SS secretion; 40 mg/kg). Ten rats from each group were challenged with GRF (2 micrograms/kg iv), clonidine (40 micrograms/kg iv), or GRF plus clonidine. Administration of clonidine markedly increased the GH responsiveness to GRF in rats anesthetized with urethane or ketamine. In contrast, the GH response to GRF was not modified by clonidine in rats anesthetized with pentothal. These results show that alpha 2-adrenergic stimulation only modifies the GRF-induced GH rise when SS release is high. Therefore, in rats, central alpha 2-adrenergic pathways appear to play a main inhibitory effect on hypothalamic SS secretion.

Animals↗
PubMed1993

Modulation of 5HT1A receptors in the hippocampus and the raphe area of rats treated with clonazepam.

1. Clonazepam is one of the most potent benzodiazepines known to decrease the activity of the central serotonergic systems. The acute and subchronic administration of clonazepam reduced serotonin (5HT) turnover rate in the hippocampus of the rat, as determined by the ratio of the monoamine and its metabolite, 5-hydroxyindoleacetic acid. 2. The modulation of 5HT binding sites and 5HT1A receptors by the administration of clonazepam for various periods of time were studied in the hippocampus and the raphe area by experiments with radioligands. 3. The density of [3H]5HT recognition sites increased in the hippocampus of clonazepam-treated rats in a dose- and time-dependent manner. This increase was impaired by the simultaneous administration of the 5HT agonist 5-methoxy-N,N-dimethyltryptamine. The affinity of this binding did not significantly change. This observation might indicate an increase in some of the 5HT receptors or an increase of the uptake site. 4. The binding parameters for [3H]DPAT, Bmax and Kd, decreased in the hippocampus but not in the raphe area of clonazepam-treated rats. It seems that the presynaptic reduction in 5HT function, resulting in the decrease of its availability at the synaptic space, modifies the corresponding 5HT recognition sites. 5. These changes could be related to the anxyolitic activity or the withdrawal symptoms of benzodiazepines.

8-Hydroxy-2-(di-n-propylamino)tetralin↗
PubMed1993

Taurine-induced regeneration of goldfish retina in culture may involve a calcium-mediated mechanism.

A possible mechanism of action of taurine as a trophic substance was studied in goldfish retina by investigating the effect of extracellular and intracellular calcium chelators on in vitro outgrowth, and the effect of taurine on calcium influx into postcrush retinal cells in culture. The amino acid stimulated the outgrowth from goldfish retinal explants, an effect that was blocked by EGTA and 1,2-bis (o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid methyl ester (BAPTA). The influx of calcium into cultured cells from postcrush retina was increased by taurine by day 5 in culture, but not by day 10, supporting previous results indicating a critical period in which taurine stimulates outgrowth. The present results suggest that taurine partially exerts its regenerative effect on postcrush retinal explants by increasing calcium influx.

Animals↗
PubMed1993

Clonidine potentiates the growth hormone (GH) response to GH-releasing hormone in norepinephrine synthesis-inhibited rats: evidence for an alpha-2-adrenergic control of hypothalamic release of somatostatin.

The aim of this study was to investigate whether central alpha 2-adrenergic pathways act, in rats, by inhibiting somatostatin (SS) release, as it has been postulated to occur in other species. The growth hormone (GH) responses to GH-releasing hormone (GRF, 3 micrograms/kg i.v.) or clonidine (CLO, 100 micrograms/kg, i.v.), either given alone or in combination, were tested in male rats in which norepinephrine synthesis had been previously blocked with the dopamine-beta-hydroxylase inhibitor diethyldithio-carbamate (DDTC, 400 mg/kg i.p.). These experiments were also carried out in a control group of animals that had been given placebo (P) instead of DDTC. The GH responses to GRF in the presence of anti-SS serum given to other P and DDTC rats, allowed us to assess whether DDTC treatment had induced increased SS secretion. GH tests were carried out during spontaneous (P) or pharmacologically induced (DDTC) trough periods. Therefore, the mean (+/- SEM) GRF-induced GH peak was similarly low in both groups of rats (P: 66.5 +/- 16.6 micrograms/l; DDTC: 58 +/- 14 micrograms/l). The administration of anti-SS serum significantly (p < 0.01) increased these responses (P: 413 +/- 22; DDTC: 695 +/- 36; p < 0.01 vs. P). CLO administration elicited a maximal GH peak significantly higher (p < 0.05) in P (20.5 +/- 3) than in DDTC rats (4.1 +/- 2); however, the former was significantly lower than GH responses to GRF.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1993

Glucocorticoids may inhibit growth hormone release by enhancing beta-adrenergic responsiveness in hypothalamic somatostatin neurons.

The aim of this study was to investigate the mechanisms by which glucocorticoids inhibit GH secretion in man. In 10 normal volunteers subjects we compared the pattern of GHRH-induced GH release to that elicited by similar challenge given 60 min after a pretreatment with drugs affecting adrenergic and muscarinic cholinergic neurotransmission, both in basal situations and after having induced hypercortisolism. In a first study (P), synthetic GHRH [GRF-(1-29); 1 microgram/kg, i.v.] was administered 60 min after giving placebo. In other experiments, the administration of propranolol (PRO; 40 mg, orally), or clonidine (CLO; 0.300 mg, orally), or pyridostigmine (PD; 120 mg, orally) was followed by GHRH administration 60 min later. These experiments were repeated after giving a nocturnal dose of dexamethasone (DEX; 8 mg, orally at 2300 h). The administration of DEX significantly (P < 0.05) blunted the GH response to GHRH (peaks: 10.7 +/- 3.9 vs. 20.3 +/- 5.5 micrograms/L; DEX vs. P study, respectively). Conversely, either beta-adrenergic blockade (PRO), or alpha 2-adrenergic agonism (CLO), or the enhancement of muscarinic cholinergic tone (PD) significantly increased the GH response to GHRH (peaks: 43 +/- 4.6, 55.6 +/- 5.6 and 51.2 +/- 7, micrograms/L; PRO, CLO, and PD, respectively; P < 0.01 vs. P study). After nocturnal DEX administration, both PRO and CLO, but not PD, were able to reverse the inhibitory effect of DEX on GHRH-elicited GH release (peaks: 39 +/- 5.5, 25.9 +/- 3.9 and 12.9 +/- 3.1, micrograms/L; DEX + PRO, DEX + CLO, and DEX + PD, respectively). However, whereas the potentiating effect of PRO on GHRH-elicited GH release was still observed under hypercortisolism, it was lacking for both CLO and PD when these drugs were given in this situation. These data suggest that the inhibitory effect of glucocorticoid excess on GH release is due to increased hypothalamic somatostatin secretion which appears to be dependent on DEX-induced enhanced beta-adrenergic responsiveness. Moreover, the data further support a major role of hypothalamic alpha 2-adrenergic and beta-adrenergic activities in GH neuroregulation in man.

Adult↗
PubMed1992

[3H]serotonin binding sites in goldfish retinal membranes.

Serotonin (5HT) binding sites were studied in goldfish retinal membranes by radioligand experiments. The binding site of [3H]5HT was sensitive to pre-treatment of the membranes at 40 degrees or 60 degrees C. 5HT and 5-methoxy-N,N-dimethyltryptamine were the best inhibitors of [3H]5HT binding to retinal membranes. The 5HT2 agonist, 1-(-naphthyl)piperazine, was also a potent inhibitor, however, (+)-1-2,5-dimethoxy-4-iodophenyl-2-aminopropane was less efficient. The catecholaminergic agents haloperidol and clonidine did not display an important inhibition. Propranolol, also reported as 5HT1B antagonist, was a relatively potent blocker. Monoamine uptake blockers did not show potent inhibition. The GTP analog, GppNHp, inhibited the binding. The iterative analysis of saturation curves revealed two classes of binding sites, a high affinity component (B(max) 2.45 pmol/mg of protein, kd 6.86 nM), and a low affinity component (B(max) 53.46 pmol/mg of protein, Kd 232.07 nM). Analysis of the association and dissociation kinetics suggested a binding site (Kd 2 nM). The semilogarithmic plot of the dissociation kinetics gave curves concave to the upper side. The selectivity of the binding and the inhibition by GppNHp suggest the existence of 5HT1 receptors in goldfish retina. The low affinity interaction probably represents the transporter of 5HT or a subtype of receptor expressed in glial cells.

Animals↗
PubMed1992

Specific [3H]raclopride binding to neostriatal dopamine D2 receptors: role of disulfide and sulfhydryl groups.

Receptor binding studies were performed in rabbit neostriatum (caudate-putamen) using the dopamine D2 antagonist [3H]raclopride. Treatment of the membrane preparations with the reducing agent L-dithiothreitol (L-DTT) as well as with the alkylating compound N-ethylmaleimide (NEM), produced dose-dependent decreases of specific [3H]raclopride binding; the IC50 values were of 3.1 and 1.2 mM, respectively. Saturation experiments showed that the reduction of disulfide (-S-S-) bonds by L-DTT (1 mM) decreased the number of binding sites, with only a slight increase in the affinity. On the other hand, alkylation of sulfhydryl (-SH) groups by NEM (1 mM) decreased both receptor number and affinity. The properties of the remaining binding sites were examined in competition curves with the physiological substrate dopamine and the dopaminergic antagonist (+)butaclamol. The IC50 values for (+)butaclamol in control and in L-DTT and NEM treated membranes were between 3.4 and 4.8 nM, with Hill coefficients (nH) of 1, indicating that the remaining binding sites conserved a high affinity for antagonist binding. In the case of dopamine, the curves were shallow (nH 0.45-0.64) and both compounds increased the IC50 from 0.7 microM (control) to 8 microM and 11 microM, for L-DTT and NEM respectively. Iterative analysis revealed that L-DTT produced a very important (greater than 60%) decrease in the number of high-affinity (RH) binding. After NEM, there was a decrease in both the number of (RH) and the affinity (KH) of the high-affinity binding sites, and in the affinity (KL) of the low-affinity sites. These results demonstrate the participation of -S-S- and -SH groups in the agonist conformation of the primary ligand recognition site of the dopamine D2 receptor. Alternatively, -S-S- and -SH groups could be related to the coupling of the primary ligand recognition protein with adenylate cyclase by means of an inhibitory type of G protein.

Alkylation↗
PubMed1992

[3H]paroxetine binding and serotonin content of rat cortical areas, hippocampus, neostriatum, ventral mesencephalic tegmentum, and midbrain raphe nuclei region following p-chlorophenylalanine and p-chloroamphetamine treatment.

The agents p-chlorophenylalanine (PCPA) and p-chloroamphetamine (PCA) deplete brain serotonin (5-HT) levels by two different mechanisms; PCPA inhibits the enzyme tryptophan hydroxylase, whereas PCA has a neurotoxic action on certain 5-HT neurons. The parameters of [3H]paroxetine binding to homogenates prepared from the cerebral cortex of rats treated with PCPA, PCA, or saline; vehicle were investigated. The tissue concentrations of 5-HT and 5-hydroxyindole-3-acetic acid (5-HIAA) were also determined by HPLC in the same brain samples. After PCPA treatment, neither the maximum binding capacity (Bmax) nor the dissociation constant (KD) of [3H]paroxetine for the 5-HT uptake recognition site differed from controls despite a substantial reduction in the concentration of 5-HT and 5-HIAA. In contrast, significant changes in both the Bmax and KD values were observed in the cerebral cortex of rats treated with PCA. Furthermore, [3H]paroxetine binding and tissue concentrations of 5-HT and 5-HIAA were measured in the following different regions of the rat brain: cingulate, parietal, and visual cortical areas; dorsal and ventral hippocampus; rostral and caudal halves of neostriatum; ventral mesencephalic tegmentum; and midbrain raphe nuclei region after administration of PCPA, PCA, or saline vehicle. There was an excellent correlation between regional 5-HT levels and specific [3H]paroxetine binding in control and PCA-treated rats although this correlation was lost after PCPA treatment. Under these conditions, the 5-HT innervation remains unchanged whereas the concentration of 5-HT and 5-HIAA is greatly reduced. Thus, [3H]paroxetine binding appears to provide a reliable marker of 5-HT innervation density within the mammalian CNS.

Animals↗
PubMed1992

[Treatment of patients with panic disorder and an elevated urinary excretion of MHPG using imipramine or clonazepam].

The response to treatment with imipramine or clonazepam was evaluated in patients with panic disorder. These patients had a high urinary 3-methoxy-4-hydroxy-phenylglycol 24 h-excretion and normal plasma lactate levels. Both drugs were efficient as anti-panic agents, but clonazepam resulted more efficacious than imipramine by the scores of Hamilton Anxiety or Depression Scales. Moreover, clonazepam also reduced the anticipatory anxiety.

Adult↗
PubMed1991

Differential taurine uptake in central and peripheral regions of goldfish retina.

The transport system of taurine was investigated in fragments of goldfish retina prepared from the total tissue and from concentric regions: center and periphery. A high-affinity, saturable, sodium-dependent system was demonstrated in the three types of fragments. The Km for one-site analysis was similar in the two regions and the total retina. The analysis for two sites revealed a significant higher Km for the high-affinity site in fragments from the central region. The maximal uptake rate was higher in the central zone than in the total retina or the periphery. The Hill slopes obtained from saturation experiments of fragments of total retina, center, and periphery were similar to one other and near to 1. The slope of the time course uptake was intermediate for total retina and higher in the center than in the periphery. Hypotaurine and beta-alanine were found to inhibit taurine uptake, but GABA was a weak inhibitor. The values of Ki for hypotaurine by one- and two-site analysis were lower in the central region. The disruption of photoreceptors by shaking did not modify significantly the uptake of the amino acid. Remotion of endogenous taurine by dialysis of central and peripheral fragments increased the uptake in the periphery, but not in the center. The differences observed among the three samples revealed less affinity and high capacity for taurine uptake in the center, plus a higher sensitivity of inhibition. In addition, the peripheral zone had a greater affinity for taurine, and the maximal velocity of the entrance seems to be inhibited by the higher concentration of the amino acid in this zone. These observations may reflect differences between proliferating and non-proliferating regions of the retina (i.e., periphery and center).

Animals↗
PubMed1991

Region-selective reduction of brain serotonin turnover rate and serotonin agonist-induced behavior in mice treated with clonazepam.

Evidence supports a complex interaction between benzodiazepines and the central serotonergic system. This study attempts to correlate biochemical changes in the serotonin (5HT) system induced by clonazepam (CLON) with the behavioural response to a 5HT agonist. The acute administration of CLON to mice produced a time-dependent decrease in 5HT turnover rate in the raphe area (dorsal and medial raphe nuclei) and modified the serotonergic syndrome induced by 5-methoxy-N,N,-dimethyltryptamine (DMT). One hour after CLON administration, a dose-dependent increase in 5HT concentration was found in the raphe area, while 5-hydroxyindoleacetic acid (5HIAA) levels remained stable, leading to an increase in 5HT/5HIAA ratio, indicative of reduced 5HT turnover rate. No significant changes were detected in the frontal cortex of CLON-treated mice. After 4 days of CLON treatment, the 5HT turnover rate was still decreased in the raphe area and unchanged in the frontal cortex. Acute CLON administration produced dose-dependent alterations in locomotor activity, not observed after subchronic administration. Lateral head weaving, a motor manifestation of the serotonergic syndrome produced by DMT, was less intense in CLON-treated animals. The modifications in the 5HT system induced by CLON are region selective, suggesting differences in the receptors implicated in the interaction. Altered synaptic availability of 5HT as a result of CLON administration may be responsible for the differential response to DMT in control and CLON-treated mice.

Animals↗
PubMed1991

The role of sexual steroids in the modulation of growth hormone (GH) secretion in humans.

Sex steroids contribute to modulate GH secretion in man. However, both the exact locus and mechanism by which their actions are exerted still remain not clearly understood. We undertook a number of studies designed to ascertain: (1) whether or not sudden or chronic changes in circulating gonadal steroids may affect GH secretion in normal adults; and (2) the reason(s) for gender-related dimorphic pattern of GH release. The pituitary reserve of GH, as evaluated by means of a GHRH challenge, was similar in women with anorexia nervosa and in normally menstruating women. Estrogenic receptor blockade with tamoxifen (TMX) did not significantly change GHRH-induced GH response in these normal women. Therefore, acute or chronic hypoestrogenism apparently had no important effects at level of somatotrophs. In another group of normal women we tested the possibility that changes in circulating estrogens might induce changes in the hypothalamic-somatotroph rhythm (HSR). GHRH challenges were performed throughout a menstrual cycle, and again after having achieved functional ovarian blockade with a GnRH agonist treatment. Short-term ovarian blockade did not significantly affect the parameters of GH response to GHRH, although it was accompanied by an increase in the number of women in a refractory HSR phase at testing. This suggested a low potentiating effect on the basic pattern of somatostatin (SS) release occurring as a consequence of the decrease in circulating estrogens. In normal men, neither the GH response to GHRH nor the HSR were affected by functional testicular blockade (after GnRH agonist treatment). However, the administration of testosterone enanthate (250 mg) to another group of men increased both the GHRH-induced GH release and the number of subjects in a spontaneous secretory HSR phase at testing; these were reversed by estrogenic receptor blockade with TMS. In another group of normal men, the fraction of GH secreted in pulses (FGHP) during a nocturnal sampling period was significantly decreased by testicular blockade. Other parameters of GH secretion,such as the number of GH pulses and their mean amplitude (A), and the mean plasma GH concentration (MCGH), showed a slight, although not significant, decrease following the lack of androgens. The administration of testosterone enanthate (500 mg) reversed these parameters to values similar to those in the basal study. Interestingly, when tamoxifen was given after testosterone enanthate, A, MCGH and FGHP increased to values significantly higher than in any other experimental condition in that study.(ABSTRACT TRUNCATED AT 400 WORDS)

Androgens↗
PubMed1991

Clonidine pretreatment modifies the growth hormone secretory pattern induced by short-term continuous GRF infusion in normal man.

OBJECTIVE: The aim of this study was to investigate the effect of a single dose of clonidine on the pattern of GH release in response to a 10-hour continuous GRF infusion in normal man. DESIGN: Plasma GH was analysed in samples withdrawn at 20-minute intervals, from 0900 to 1900 h, according to the following protocols: in a control study, a placebo was given at 1000 h; in other experiments, clonidine (300 micrograms, orally) was given at 1000 h, alone or together with a continuous intravenous infusion of GRF 1-29 (0.3 micrograms/kg/h) starting at this time. In another experiment, the continuous infusion of GRF 1-29 was preceded by placebo administration at 1000 h. PATIENTS: Eight normal volunteers (four women and four men), aged 19-24 years were studied. MEASUREMENTS: Plasma GH levels were measured by RIA. Analysis of the pattern of GH secretion was performed using cluster analysis. RESULTS: Clonidine induced a slight but significant increase in plasma GH values, peaking 60 to 120 minutes later; however, no significant changes were observed in indices of total and pulsatile GH release for the whole sampling period in this study. Continuous GRF administration led to increased episodic GH secretion, by augmenting GH peak amplitude, although peak frequency was not modified. An increase in interpulse GH values was also observed during GRF infusion. Pretreatment with clonidine clearly changed the pattern of GH release during GRF infusion: the amount of GH secreted was significantly higher, the number of GH peaks significantly increased, and almost all the GH was secreted within them. CONCLUSIONS: These data concord with our previous demonstration that clonidine disrupts the hypothalamic-somatotroph rhythm by inhibiting the hypothalamic release of somatostatin. Given that clonidine pretreatment induced a more physiological episodic pattern of GRF-induced GH release, the possibility of combining clonidine and GRF therapy for short stature in children is envisaged.

Adult↗
PubMed1991

Role of central dopaminergic pathways in the neural control of growth hormone secretion in normal men: studies with metoclopramide.

The aim of this study was to gain further insight into the role that central dopaminergic pathways play in GH neuroregulation in man. Our experimental hypothesis was based on the possibility that most of the controversies on DA role could be due to the fact that the hypothalamic somatotroph rhythm (HSR) was not taken into account when interpreting the GH responses after pharmacological manipulations on dopaminergic pathways. In 10 normal subjects we monitored the effect of central dopaminergic blockade, achieved with metoclopramide (MCP; 10 mg, i.v. bolus), on the pattern of spontaneous GH secretion and the GH responses to a GHRH challenge (GRF1-29, 1 microgram/kg, i.v. bolus) administered together with MCP or 60 min after this drug was given. The study of HSR was made according to our previous postulate. Our results indicate that MCP administration, either prior to or together with the GHRH bolus, significantly increased GHRH-induced GH release during a refractory HSR phase; but not when the GHRH challenge took place during a spontaneous secretory phase. The strong relationship between pre-GHRH plasma GH values and GHRH-elicited GH peaks was lost when MCP was given. These data indicate that MCP was able to disrupt the intrinsic HSR by inhibiting the hypothalamic release of somatostatin (SS). While a main conclusion would be that central DA is a secretagogue for SS secretion, our results also suggest that this role could be dependent on its effects on the adrenergic inputs to SS neurons.

Adult↗
PubMed1991

Evidence that alpha 2-adrenergic pathways play a major role in growth hormone (GH) neuroregulation: alpha 2-adrenergic agonism counteracts the inhibitory effect of muscarinic cholinergic receptor blockade on the GH response to GH-releasing hormone, while alpha 2-adrenergic blockade diminishes the potentiating effect of increased cholinergic tone on such stimulation in normal men.

The aim of this study was to investigate the interrelationships between alpha 2-adrenergic and cholinergic pathways in the control of hypothalamic somatostatin (SRIF) secretion in humans. In eight normal volunteers subjects we compared the pattern of GHRH-induced GH release to that elicited by similar challenge given 60 min after a pretreatment with drugs affecting alpha 2-adrenergic and muscarinic cholinergic neurotransmission. In a control study, synthetic GHRH [GRF-(1-29); 1 microgram/kg, iv] was administered 60 min after giving placebo. In other experiments, the administration of atropine (1 mg, im), or clonidine (0.300 mg, orally), or atropine plus clonidine, or pyridostigmine (120 mg, orally), or yohimbine (30 mg, orally), or pyridostigmine plus yohimbine, at 0 min was followed by GHRH administration 60 min later. The administration of both clonidine and pyridostigmine significantly (P less than 0.01) enhanced the GH responses to GHRH compared to those elicited by this challenge when given after placebo. Conversely, atropine pretreatment significantly (P less than 0.01) blocked the GH response to GHRH challenge, whereas yohimbine did not significantly affect it. When atropine and clonidine were given together, the inhibitory effect of the former was overcame and mean GHRH-elicited GH peak response was significantly (P less than 0.05) higher than that in the control study. In contrast, pretreatment with yohimbine significantly (P less than 0.05) blunted the pyridostigmine-induced enhancement of GHRH-elicited GH release. These data confirm our previous postulate suggesting that the stimulatory effect of clonidine on GH release is mainly exerted by inhibiting the hypothalamic SRIF release. Moreover, the effect of cholinergic neurons on SRIF release seems to be, at least in part, dependent on alpha 2-adrenergic pathways. Based on these data, it can be proposed that the alpha 2-adrenergic system plays a major role in the control of hypothalamic SRIF release, and hence in GH neuroregulation, whereas the muscarinic cholinergic system would participate in such regulation by modulating the functional activity of the former.

Adult↗