Consummatory behavior and metabolic indicators after central ghrelin injections in rats.

Ghrelin, an endogenous ligand for the growth-hormone-secretagogue receptor, is a 28-amino acid peptide with a post-translational acyl modification necessary for its activity. It has central nervous system actions that affect appetite, body mass and energy balance. An intracerebroventricular (ICV) injection protocol of sub-nanomolar doses of ghrelin, known to alter the morphology of ACTH and GH producing pituicytes and plasma levels of these hormones, was used to provide an overview of metabolic changes linked to energy metabolism. Variables measured were: food intake (FI), water intake (WI), fecal mass, urine volume, body weight (BW), retroperitoneal (RP) and epididymal (EPI) white adipose tissue (WAT), and changes in serum leptin, insulin, triglycerides, cholesterol, and glucose. Five injections of rat ghrelin or PBS (n=8 per group) were given ICV every 24 h (1 microg/5 muL PBS) to adult male rats. Ghrelin had a positive and cumulative effect on FI, WI and BW (p<0.05), but not feces mass or urine volume (p>0.05). Centrally applied ghrelin clearly increased RP WAT (by 235%, p<0.001), EPI WAT (by 85%, p<0.05) and serum insulin levels (by 43%, p<0.05), and decreased serum leptin levels (by 77%, p<0.05) without (p>0.05) evoking changes in blood triglyceride cholesterol, or glucose levels. These data and the available literature clearly document that exposure of the brain of normal rats, over time, to sub-nanomolar doses of ghrelin results in metabolic dysregulation culminating in increased body mass, consummatory behavior, and lipid stores as well as changes in blood leptin/insulin levels. Thus, modulation of central ghrelin receptors may represent a pharmacological approach for controlling multiple factors involved in energy balance and obesity.

The effect of centrally administered ghrelin on pituitary ACTH cells and circulating ACTH and corticosterone in rats.

Ghrelin is a brain-gut peptide known for its growth hormone (GH)-releasing and appetite-inducing activities. This natural GH secretagogue (GHS) was originally purified from rat stomach, but it is expressed widely in different tissues where it may have endocrine and paracrine effects. The central effects of ghrelin on adrenocorticotropic hormone (ACTH) cells, ACTH release and subsequent corticosterone release from adrenal glands remains to be clarified. The aim of this study was to specifically determine the morphological features of ACTH-producing pituicytes and blood concentration of ACTH and corticosterone after central administration of ghrelin. Five doses of rat ghrelin or PBS (n=10 per group) were injected every 24 h (1 microg of ghrelin in 5 muL PBS), into the lateral cerebral ventricle of male rats. Results showed that ghrelin increased (p<0.05) absolute and relative pituitary weights compared to controls (58% and 41% respectively). Morphometric parameters, i.e. the volume of the ACTH cells, nuclear volume, and volume density were all increased (p<0.05), by 17%, 6% and 13%, respectively, 2 h after the last ghrelin treatment. Ghrelin increased circulating concentrations of ACTH and corticosterone (p<0.05) by 62% and 66%, respectively. The data provide clear documentation that intracerebroventricular ghrelin stimulates ACTH cell hypertrophy and proliferation, and promotes ACTH and corticosterone release. Determining the role of ghrelin in physiological stress responses and whether control of the peptide's activity would be useful for prevention and/or treatment of stress-induced diseases remain important research goals.

Central effects of ghrelin on serum growth hormone and morphology of pituitary somatotropes in rats.

Ghrelin, an endogenous ligand for the growth hormone (GH) secretagogue receptor, was originally purified from rat stomach; subsequently, ghrelin neurons were found in the arcuate nuclei of rats. Central effects of the peptide on GH release, however, remain to be clarified. The aim of the present study was to determine the morphologic features of GH-producing pituicytes and serum GH concentration after central administration of ghrelin. Five injections of rat ghrelin or phosphate-buffered saline (PBS; n = 10 rats/group) were given every 24 hrs (1 microg of ghrelin in 5 microl of PBS) into the lateral cerebral ventricle of male rats. Significant (P < 0.05) increases in absolute and relative pituitary weights occurred in ghrelin-treated rats versus controls (58% and 41%, respectively). Morphometric parameters (i.e., the volume of GH cells, volume of their nuclei, and volume density) all significantly (P < 0.05) increased by 17%, 18%, and 19%, respectively, in the ghrelin-treated group versus controls. Terminal serum concentration of GH was significantly (P < 0.05) increased by 15% with ghrelin treatment. The results clearly document that daily nanomolar doses of ghrelin into the lateral cerebral ventricle stimulate GH cell proliferation and promote GH release. Thus, achieving pharmacologic control of central ghrelin receptors is a promising modality to modulate the actions of GH.

Somatostatin-14 alters the thymus size and relation among the thymocyte subpopulations in peripubertal rats.

It is well known that somatostatin exerts a wide range of effects in the body, and acts as an autocrine or paracrine factor in the thymus. However, it has not been investigated yet whether somatostatin alters the thymus size and relation among the thymocyte subpopulations in the peripubertal rats. For this purpose, the peripubertal AO male rats were cannulated intracerebroventriculary and treated with repeated, low doses of somatostatin-14 (experimental group) or saline (control group). Twenty-four hours after the last treatment, we removed and prepared the thymuses for determination of thymocyte subpopulations by flow cytometry. After five days, animals were sacrificed and their thymuses taken for morphometrical analysis by stereological methods. We noticed that somatostatin-14 decreased volumes of thymus cortex and medulla, total number of thymocytes, number of thymocytes in the cortex and medulla and numerical density of thymocytes in deeper cortex. As a consequence of these changes, thymus size was also diminished. The phenotypic analysis of thymocyte subpopulations showed that somatostatin-14 decreased the percentage of CD4(+)CD8(+) cells with low level of TCR alphabeta expression, positively selected CD4(+)CD8(+)TCRalphabeta (high) cells and the most mature CD4(-)CD8(+)TCRalphabeta (high) cells, while the percentage of CD4(+)CD8(-)TCRalphabeta (high) thymocytes was slightly increased. Somatostatin-14 increased the relative proportion of the least mature CD4(-)CD8(-)TCRalphabeta (-/low), CD4(+)CD8(+)TCRalphabeta (-) cells and both of TCRalphabeta (-/low) single positive subpopulations. These results show that centrally applied somatostatin-14, induces hypotrophy of the thymus in peripubertal rats by changing the volumes and cellularities of the thymic compartments. Additionally, increased number of the least mature thymocytes and a deficiency of double positive cells indicate the involvement of somatostatin in the modulation of T cells maturation.

Effects of centrally applied somatostatin on pituitary adrenocorticotropes in female rats.

Effects of intracerebroventricular (i.c.v.) application of somatostatin (SRIH-14 or SRIH-28) on growth and function of pituitary adrenocorticotropes (ACTH cells) were examined in adult female Wistar rats. Animals were subjected to i.c.v. administration of three 1-microg doses of SRIH-14 or SRIH-28 dissolved in 5 microl saline every second day. Controls were treated in the same way with the same volume of saline only. ACTH-producing cells were studied using the peroxidase-antiperoxidase (PAP) immunohistochemical procedure; blood samples were collected for hormone analyses 5 days after the last injection. SRIH-28 treatment decreased (p < 0.05) all morphometric parameters compared to control rats. Volume of ACTH cells decreased by 10%, nuclei by 36% and volume density by 13%. No significant changes (p > 0.05) in these parameters occurred after SRIH-14 treatment. Plasma concentration of ACTH in SRIH-28-treated rats was significantly lower (p < 0.05) than in control rats by 35%. In SRIH-14-treated rats, plasma concentration of ACTH was slightly, but not significantly (p > 0.05) increased by 13% compared to saline treatment. These observations suggest that centrally administered somatostatin-28, but not somatostatin-14, is specifically involved in the control of growth and secretory activity of ACTH cells in female rats. Thus, selective pharmacological manipulation of SRIH-28 receptors reached from CSF may affect ACTH activity without altering actions usually attributed to receptors sensitive to SRIF-14.

Effects of ethacrynic acid on intraocular pressure of anesthetized rats.

Ethacrynic acid (ECA) lowers intraocular pressure (i.o.p.) by an effect usually ascribed to increased drainage of aqueous humor by the trabecular meshwork. Here, we describe the effects of a continuous 2-hr intracameral infusion of balanced salt solution (BSS), with or without 2 mM ECA (sodium salt), on IOP of pentobarbital anesthetized rats. The infusion was divided into a constant (0.05 microliter/min) and a periodic (0.25 microliter/min) component that cycled 4 min on then 4 min off. This permitted the calculation of dynamic changes in resistive (trabecular and uveoslceral drainage) and nonresistive (aqueous synthesis, episcleral venous pressure) components of IOP by fitting a second-order transfer function to the responses. ECA markedly blunted the BSS-induced rise in IOP (P < 0.01). The rise in resistive mechanisms (ocular impedance) was transiently blunted by ECA (P < 0.05) during the third and fourth 8-min cycles, and nonresistive mechanisms were reduced by ECA from cycles 3-10 (P < 0.05). Then, at the end of the infusion, the control and ECA dynamic values were similar (P < 0.05), although IOP of ECA-treated rats was still slightly reduced (P < 0.05). The most likely explanation is a summation of small changes in both resistive and nonresistive components of IOP dynamics. Systemic blood pressure was unchanged within either group. The well-known effects of ECA on the trabecular meshwork, alone, are insufficient to explain the dynamic changes in IOP observed in this model.

Aqueous humor dynamics in anesthetized rats infused with intracameral apraclonidine.

We studied the acute effects of the ocular hypotensive drug, apraclonidine (AP), on intraocular pressure (IOP) and aqueous humor dynamics of anesthetized rats during infusion-induced ocular hypertension. Two infusions were made into the anterior chamber of the eye: one was constant at a rate of 0.05 microl/min, the other was cyclic, at a rate of 0.25 microl/min, with the pump on for 4 min, then off for 4 min. Data were processed by complex demodulation and analysis of a second-order transfer function. This permitted separate calculations of resistive components (Ao), i.e., trabecular meshwork and uveoscleral outflows, and residual pressure (RP) estimating nonresistive components, i.e., aqueous synthesis and episcleral venous pressure. A balanced salt solution (BSS) and AP (0. 0005%) were tested. AP markedly delayed the within-group rise in IOP: 20 min for BSS vs. 60 min for AP (p < 0.001). IOP of AP rats was less than control for 100 min (p < 0.05). The infusions raised Ao in both groups (p < 0.05). AP initially had a transient inhibitory effect (p < 0.05). Infusions raised RP in both groups. AP had a strong inhibitory effect for the first 8 cycles (p < 0.05). These data document that the acute effects of AP in this in vivo rat model of ocular hypertension were to delay increases in IOP, mainly by reducing nonresistive components of aqueous humor dynamics. Transient inhibition of resistive mechanisms also occurred.

Timolol effects on aqueous humor dynamics in eyes of anesthetized rats.

Anterior chambers of the eyes of male rats were cannulated under pentobarbital anesthesia for intracameral infusions of balanced salt solution (BSS) and intraocular pressure (IOP) recording. Blood pressure was recorded from a femoral artery. IOP was recorded during a 2-h intracameral infusion composed of a constant component (0.05 microl/min) and a periodic component (0.25 microl/min), cycling at 4 min on and then 4 min off. After a 20-min baseline period, 1 drop of timolol (0.5%) or BSS was applied to the cornea and repeated 1 h later. Intracameral infusions of BSS and 0.05% timolol were also compared. Topical timolol slightly delayed the BSS-induced IOP rise (p <.05). Complex demodulation and the estimated gain parameter of a second-order transfer function fit to the periodic responses revealed that topical timolol also reduced (p <.05) passive outflow resistance. Intracameral timolol markedly delayed the BSS-induced rise in IOP. Initially, timolol decreased both outflow impedance and nonresistive components (p <.05) of IOP, but these effects dissipated by 2 h when IOPs were similar. In all experiments, within-group blood pressure was unchanged. Topical and intracameral timolol have different effects on IOP. The data support the opinion that, in vivo, timolol acts at beta-receptors that control both outflow impedance and nonresistive mechanisms, probably vascular, to lower IOP.

Morphometric and functional changes of rat pituitary somatotropes and lactotropes after central administration of somatostatin.

This study examined effects of intracerebroventricularly (i.c.v.) administered somatostatin (SRIH-14 and SRIH-28) on growth and function of pituitary somatotropes (GH cells) and lactotropes (PRL cells). Male rats received three i.c.v. injections (1 microgram/5 microliters) of SRIH-14 or SRIH-28 every second day. Blood samples were collected for hormone assays and pituitaries were removed for histological and morphometric evaluation 5 days after the last i.c.v. treatment. Compared to control animals, SRIH treatment decreased (p < 0.05) pituitary weight and all morphometric measurements obtained in GH and PRL cells. Concentrations of serum growth hormone (GH) and prolactin (PRL) in both SRIH-treated groups were also lower (p < 0.05) than in control rats. These findings suggest that centrally administered somatostatin is specifically involved in the control of growth and secretory activity of GH and PRL cells. Thus, pharmacological manipulation of SRIH receptors reached from cerebrospinal fluid may alter systemic effects of GH and PRL.

Mechanisms for vasopressin effects on intraocular pressure in anesthetized rats.

Continuous intracameral infusions of a balanced salt solution (0.175 microliter min-1) have been reported to raise intraocular pressure (IOP) in anesthetized rats. Palm et al. (1995) previously reported that this effect was attenuated significantly by inclusion of arginine-vasopressin (AVP, 10 ng 0.175 microliter-1) in the infusate. This study used experimental and computer simulation methods to investigate factors underlying these changes in IOP. First, constant intracameral infusions of artificial cerebrospinal fluid (aCSF) at different fixed rates (0.049-0.35 microliter min-1) were used to estimate the outflow resistance. Secondly, IOP responses were measured during an 2 hr intracameral infusion of either aCSF or AVP that was the sum of a small constant component (0.05 microliter min-1) and a larger periodic component (0.25 microliter min-1, cycling for 4 min on, then 4 min off); the mean infusion rate was 0.175 microliter min-1. As shown previously for 0.175 microliter min-1 constant infusions, the periodic aCSF infusion induced a significant rise in IOP that was attenuated by AVP administration. Complex demodulation analysis and the estimated gain parameter of a second order transfer function fit to the periodic responses indicated that outflow resistance increased significantly during the infusions in both aCSF and AVP groups, but that the indices of resistance did not differ significantly between aCSF and AVP infused eyes. This finding implies that changes in outflow resistance do not explain the difference in IOP responses to intracameral aCSF and AVP. The two responses differed significantly, though, in damping factors, such that the aCSF responses were considerably more underdamped than the AVP responses. It is hypothesized that aCSF-induced increase in IOP reflects both (1) a small component reflecting increased outflow resistance and (2) a larger non-resistive component. Since the non-resistive component is insensitive to pretreatment with acetazolamide, it is suggested that the aCSF-induced elevation in IOP reflects primarily vascular perfusion changes that are reduced by local vasoconstrictor actions of AVP. The latter mechanism likely maintains vascular perfusion of the globe when intraocular hypertension develops.

Inhibitory effects of centrally administered somatostatin on the adrenal zona glomerulosa in male rats.

This study examined the effects of intracerebroventricularly administered somatostatin (SRIF-28 and SRIF-14) on growth and steroidogenic capacity of the rat adrenal zona glomerulosa. Male adult Wistar rats were subjected to intracerebroventricular administration of three 1 microgram doses of SRIF-28 or SRIF-14 every other day. Five days after the last dose, the rats were sacrificed by decapitation. Blood samples were collected for hormonal analyses, and left adrenal glands were taken for histological and morphometric evaluation. In comparison with control animals, the SRIF-treated rats had decreased (p < 0.05) adrenal gland weight and volume. Stereological and morphometric analyses showed decreased (p < 0.05) absolute and relative volumes of the cells and nuclei only in the zona glomerulosa. The plasma levels of aldosterone, growth hormone, and prolactin in both SRIF-treated groups were lower (p < 0.05) than in the control group. The levels of adrenocorticotropic hormone after SRIF treatment did not differ significantly from those in control rats. These findings suggest that centrally administered somatostatin is specifically involved in the control of zona glomerulosa growth and secretion; this effect is probably mediated by inhibiting the secretion of the corresponding pituitary hormones.

Induced abortion as an independent risk factor for breast cancer: a comprehensive review and meta-analysis.

STUDY OBJECTIVE: To ascertain, from the published reports to date, whether or not a significantly increased risk of breast cancer is specifically attributable to a history of induced abortion, independent of spontaneous abortion and age at first full term pregnancy (or first live birth); to establish the relative magnitude of such risk increase as may be found, and to ascertain and quantify such risk increases as may pertain to particular subpopulations of women exposed to induced abortion; in particular, nulliparous women and parous women exposed before compared with after the first full term pregnancy. INCLUDED STUDIES: The meta-analysis includes all 28 published reports which include specific data on induced abortion and breast cancer incidence. Since some study data are presented in more than one report, the 28 reports were determined to constitute 23 independent studies. Overall induced abortion odds ratios and odds ratios for the different subpopulations were calculated using an average weighted according to the inverse of the variance. An overall unweighted average was also computed for comparison. No quality criteria were imposed, but a narrative review of all included studies is presented for the reader's use in assessing the quality of individual studies. EXCLUDED STUDIES: All 33 published reports including data on abortion and breast cancer incidence but either pertaining only to spontaneous abortion or to abortion without specification as to whether it was induced or spontaneous. These studies are listed for the reader's information. RESULTS: The overall odds ratio (for any induced abortion exposure; n = 21 studies) was 1.3 (95% confidence interval of 1.2, 1.4). For comparison, the unweighted overall odds ratio was 1.4 (1.3,1.6). The odds ratio for nulliparous women was 1.3 (1.0,1.6), that for abortion before the first term pregnancy in parous women was 1.5 (1.2,1.8), and that for abortion after the first term pregnancy was 1.3 (1.1,1.5). CONCLUSIONS: The results support the inclusion of induced abortion among significant independent risk factors for breast cancer, regardless of parity or timing of abortion relative to the first term pregnancy. Although the increase in risk was relatively low, the high incidence of both breast cancer and induced abortion suggest a substantial impact of thousands of excess cases per year currently, and a potentially much greater impact in the next century, as the first cohort of women exposed to legal induced abortion continues to age.

Comparison of anterior chamber infusates on the intraocular pressure of intact rat eyes.

1. Infusions of balanced salt solutions (BSS) into the eye often cause a delayed, gradual increase in intraocular pressure or outflow facility, known as the "washout" effect. The reason(s) is occult, especially at low input rates when drainage mechanisms are not overloaded by excessive volume input. However, direct, quantitative comparisons of BSSs used in ocular research have been reported infrequently. 2. We compared the effects of three BSSs on intraocular pressure and the estimated resistance to drainage after a 1-hr, low-volume infusion into the anterior chambers of the eyes of anesthetized rats. 3. The BSSs tested raised intraocular pressure (P < 0.05) after a 20-40-min delay, and the highest IOPs occurred at 1 hr. Recovery of intraocular pressure to baseline only occurred with one BSS (Dulbecco). 4. Fitting the ascending and descending portions of the mean pressure curves to an exponential revealed differences among the infusates. The Dulbecco solution resulted in minimal changes in time constant, gain, and offset during the ascending and descending periods. 5. The data obtained show that different BSSs yield pressure curves that appear grossly similar, even though there were large differences in composition and osmolality. However, the underlying changes in ocular dynamics were not identical. Thus, it may be prudent to test more than one solvent to study "washout," or to deliver drugs directly into the anterior chamber.

Interaction between head-down tilt and anterior chamber infusions on intraocular pressure of anesthetized rats.

Head-down tilt or infusions of a balanced salt solution into the anterior chamber of the eye raise intraocular pressure. We measured intraocular pressure directly in adult male Sprague-Dawley rats, anesthetized with pentobarbital, and subjected to 45 degrees head-down tilt alone, tilt with an anterior chamber infusion (0.087 microliter min-1), or tilt with an infusion containing arginine vasopressin. The intraocular pressure of the three groups differed during the 1 hr tilt and recovery periods. In the case of tilt alone, intraocular pressure quickly reached a peak after tilting, partially decreased during the tilt period, recovered to baseline immediately after tilt, then a secondary rise occurred. Combined infusion and tilt caused a slower rise to peak intraocular pressure, and only a partial recovery occurred during the 1 hr recovery period. Combined vasopressin infusion and tilt caused a gradual rise in intraocular pressure of a lesser magnitude than the other groups, followed by a rapid recovery to baseline pressure and no secondary rise. Systemic arterial pressure was stable within and between the groups. The underlying mechanism for these differing response patterns is unknown. However, some evidence indicates that infusions are independent of aqueous synthesis rate, and that vasopressin, acting on a V1 receptor subtype reached from the anterior chamber, exerts a vascular effect.

Induction of apoptosis by organotin compounds in vitro: neuronal protection with antisense oligonucleotides directed against stannin.

Immortalized cell lines and primary neuronal cultures were used to characterize the selective toxicity of trimethyltin (TMT),triethyltin (TET) and tributyltin (TBT). TBT and TET were cytotoxic at similar concentrations in the immortalized cell lines tested; the 50% toxic concentration (TC50) was 1 to 11 microM. In contrast, immortalized cell lines varied considerably in their sensitivity to TMT, with sensitive cell lines (neuroblastomas, T-, B-cell lines) showing TC50 values of 2 to 8 microM, whereas insensitive cells (NIH-3T3 fibroblast, HTB-14 glioma, TC-7 kidney cells) had TC 50 values > 100 microM. Primary neuronal cell cultures were very sensitive to organotins (TC50 values, 1-10nM), and showed patterns of selective toxicity with respect to neuronal and glial cells. Because organotin toxicity evolves over 24 to 48 hr. we determined whether these compounds induced apoptosis in primary cultures. TMT increased (P < .05) the fraction of apoptotic cells 6 and 12 hr after treatment with TMT at TC50 concentrations. Prior studies suggested that a protein, stannin, was localized in cells sensitive to organotins. Stannin was expressed in several TMT-sensitive cell lines (PC12, T, B cells) and in primary neurons in culture. Stannin was absent in the resistant HTB-14 glioma cell line. The role of stannin in mediating TMT toxicity in primary cultures was investigated by blocking stannin expression with specific antisense oligonucleotides. Treatment of primary cultures with antisense oligonucleotides for 48 hr before and during TMT treatment significantly protected neurons from the neurotoxic and apoptotic effects of TMT. This effect was not observed with scrambled oligonucleotide controls. Thus, TMT may induce apoptosis in sensitive cells, which is partly mediated by stannin. Based on the available data we conclude that stannin expression is necessary, but not sufficient for TMT toxicity.

Pentobarbital alters the response of cerebrospinal fluid pressure evoked by two provocative tests in rats.

The intracerbroventricular (ICV) route of administration is a commonly used pharmacologic procedure in rats. However, little attention is given to changes in cerebrospinal fluid pressure (CSFp) when the ventricular system is perturbed, especially in conscious animals. This communication examined CSFp in pentobarbital-anesthetized rats subjected to two pertubations where CSFp had been previously measured in conscious rats. Pentobarbital treatment resulted in marked, qualitative differences after both protocols. Whereas conscious rats responded to a 10-min ICV infusion (8 microliters/min) by a delayed, secondary increase in CSFp, this effect was completely eliminated (p < 0.001) by pentobarbital. Also, whereas conscious rats could maintain CSFp at normal values during an hour of 45 degrees head-down tilt, pentobarbital-treated rats could not. Anesthetized rats displayed an immediate fall in CSFp to near zero values (p < 0.001), followed by a rapid recovery (p < 0.001) after tilting. Such differences in CSF dynamics raise important issues. For example, what mechanism(s) underlie the changes in CSF dynamics? Also, are these differences in conscious and anesthetized rats potential variables for drug distribution when the ICV route is used?

Effects of angiotensin, vasopressin and atrial natriuretic peptide on intraocular pressure in anesthetized rats.

The effects of atrial natriuretic peptide (ANP), vasopressin (AVP) and angiotensin (ANG) on blood and intraocular pressures of pentobarbital anesthetized rats were evaluated following intravenous, intracerebroventricular or anterior chamber routes of administration. Central injections did not affect intraocular pressure. Equipressor intravenous infusions of ANG raised, whereas AVP decreased, intraocular pressure. Direct infusions of a balanced salt solution (0.175 microliter/min) raised intraocular pressure between 30 and 60 min. Adding ANG or ANP slightly reduced this solvent effect but AVP was markedly inhibitory. An AVP-V1 receptor antagonist reversed the blunting of the solvent-induced rise by the peptide, indicating receptor specificity. Acetazolamide pretreatment lowered intraocular pressure, but the solvent-induced rise in intraocular pressure and inhibition by AVP still occurred without altering the temporal pattern. Thus, these effects appear unrelated to aqueous humor synthesis rate. The data support the possibility of intraocular pressure regulation by peptides acting from the blood and aqueous humor.

Cardiac noradrenergic mechanisms mediate GABA-enhanced ouabain cardiotoxicity.

Peripherally administered gamma-aminobutyric acid (GABA) alters cardiovascular function and has been reported to enhance ouabain-induced cardiotoxicity in vivo. Control and reserpinized rat hearts were perfused in vitro to determine if GABA directly evokes bradycardia by GABAA receptors, interacts with ouabain, and if noradrenergic mechanisms are required. Also, double-staining immunohistochemistry was employed to determine whether GABA-ergic and noradrenergic synthetic enzymes were juxtaposed within atrial tissue. The main results were as follows. GABA produced a dose-dependent bradycardia (p < 0.05) by stimulating GABAA receptors in Langendorff-perfused hearts. Reserpinized hearts were unresponsive (p < 0.05) to GABA, except at the highest dose (20 mg/ml). Ouabain-induced cardiotoxicity was enhanced (p < 0.05) by GABA in isolated control, but not reserpinized hearts. Lastly, glutamic acid decarboxylase and tyrosine hydroxylase immunoreactivities were in close proximity in atrial slices. Collectively, the results document that GABA causes bradycardia and enhances ouabain cardiotoxicity by modulating noradrenergic mechanisms in the isolated rat heart. Since the synthetic enzymes for GABA and norepinephrine were in close proximity in atrial tissue, these transmitters may interact under physiological conditions.

Evidence for a noradrenergic projection to the subcommissural organ.

Several physiological studies have shown that the subcommissural organ (SCO) is influenced by catecholamines. This study provides immunohistochemical evidence for a noradrenergic input to the SCO of rats. A light plexus of tyrosine hydroxylase (TH)-and dopamine-beta-hydroxylase (D beta H)-positive axons present in the SCO of both Long-Evans and Sprague-Dawley rats. The innervation density was greatest in the hypendymal wing of the rostral aspect of the SCO and it declined both caudally in the hypendymal wing and medially in the hypendymal layer. Some TH- and D beta D beta H-immunoreactive fibers entered the lateral margin of the ependymal layer along the basal surface of ependymal cells; others coursed medially in the transverse plane to ramify along the base of the ependymal cells. These fibers are presumed to be noradrenergic because phenylethanolamine N-methyltransferase immunoreactivity was absent in adjacent sections through the SCO. Considering the potential role of the SCO region in sodium homeostasis, these data suggest that central noradrenergic input to the SCO may parallel peripheral catecholaminergic mechanisms that regulate sodium balance.