Chronic exposure to a high-fat diet affects stress axis function differentially in diet-induced obese and diet-resistant rats.

OBJECTIVE: Consumption of a high-fat (HF) diet is a contributing factor for the development of obesity. HF diet per se acts as a stressor, stimulating hypothalamo-pituitary-adrenal (HPA) axis activity resulting in elevated glucocorticoid levels; however, the mechanism behind this activation is unclear. We hypothesized that consumption of an HF diet activates HPA axis by increasing norepinephrine (NE) in the paraventricular nucleus (PVN) of the hypothalamus, leading to elevation in corticotrophin-releasing hormone (CRH) concentration in the median eminence (ME) resulting in elevated serum corticosterone (CORT). SUBJECTS: To test this hypothesis, diet-induced obese (DIO) and diet-resistant (DR) rats were exposed to either chow or HF diet for 6 weeks. MEASUREMENTS: At the end of 6 weeks, NE in the PVN was measured using HPLC, CRH in the ME, and CORT and leptin levels in the serum were measured using RIA and ELISA, respectively. The gene expression of tyrosine hydroxylase (TH), the rate-limiting enzyme in NE synthesis, and leptin receptor in brainstem noradrenergic nuclei were also measured. RESULTS: HF diet increased PVN NE in both DIO and DR rats (P<0.05). However, this was accompanied by increases in CRH and CORT secretion only in DR animals, but not in DIO rats. Leptin receptor mRNA levels in the brainstem noradrenergic areas were not affected in both DIO and DR rats. However, HF diet increased TH mRNA levels only in DIO rats. CONCLUSION: Significant differences occur in all the arms of HPA axis function between DIO and DR rats. Further studies are needed to determine whether this could be a causative factor or a consequence to obesity.

Reduced GABA inhibition of sympathetic function in renal-wrapped hypertensive rats.

Animals with bilateral cannulas in the paraventricular nucleus were made hypertensive by a one-kidney, figure eight renal wrap procedure or sham operated. Femoral artery and vein catheters were inserted for arterial pressure measurement and plasma catecholamine determination. After recovery and 4 days after hypertension surgery, bicuculline methiodide or muscimol was microinjected into the paraventricular nucleus. In some rats, nitroprusside was infused intravenously to reflexly stimulate the sympathetic nervous system. In control rats, bicuculline increased blood pressure, heart rate, and plasma norepinephrine and epinephrine concentrations. In contrast, in hypertensive rats blood pressure did not change while the heart rate response was maintained. Plasma norepinephrine and epinephrine responses were reduced 75 and 68%, respectively. Muscimol injections decreased arterial pressure in the hypertensive rats. Heart rate responses to nitroprusside were similar in the two groups of rats, while the plasma catecholamine responses were attenuated in the hypertensive animals. These data suggest that GABA function in the paraventricular nucleus is reduced in renal wrap hypertension.

Measurement of the distribution of [3H]bicuculline microinjected into the rat hypothalamus.

The purpose of this study was to measure the distribution of a radiolabeled drug [3H]bicuculline methylchloride ([3H]BMC) following microinjection into the supraoptic nuclei (SON) and the dorsal hypothalamus of conscious rats. The anteroposterior (AP) distribution was measured using liquid-scintillation counting while computer-assisted densitometry was used to measure the mediolateral (ML) and dorsoventral (DV) distribution of silver grains on autoradiograms. Following a 50-nl microinjection into the SON, 90% of the detected tracer was found within 0.6 +/- 0.1 mm (n = 5) of the injection site. Using the same volume, the pattern of distribution of 90% of the detected tracer in the SON was not significantly altered when rats received a microinfusion over 10 min (n = 5) or a bolus microinjection with a 10 min waiting period (n = 6) prior to death. Following a 100-nl microinfusion over 20 min into the dorsal hypothalamus, 90% of the detected radiolabel was found within 0.6 +/- 0.1 mm (n = 7) of the injection site, in a spherical pattern of distribution. Although caution must be used in extrapolating these results to other drugs, these data suggest that intraparenchymal microinjections of 50- and 100-nl volumes are suitable for drug localization in studies using microinjection techniques for conscious rats.

Evidence of an endogenous forebrain GABAergic system capable of inhibiting baroreceptor-mediated vasopressin release.

In conscious rats, intracerebroventricular (i.c.v.) injections of gamma-aminobutyric acid (GABA), a GABA-uptake inhibitor (nipecotic acid), and artificial CSF (aCSF) were restricted to forebrain regions and their effect on baroreceptor-mediated arginine-vasopressin (AVP) release was studied. AVP release was stimulated by the hypotension resulting from combined treatment with a converting enzyme inhibitor (CEI) and chlorisondamine (CHLOR), a ganglionic blocking agent. CEI + CHLOR reduced mean arterial pressure (MAP) from 118 +/- 2 to 63 +/- 2 mm Hg, but pressure then rose to a compensated level of 78 +/- 1 mm Hg. The compensation in MAP was shown to be AVP-dependent at the end of the experiment since the vascular AVP antagonist, d(CH2)5Tyr(Me)AVP, reduced MAP from 78 +/- 1 to 63 +/- 1 mm Hg. While AVP was contributing to MAP maintenance, GABA (15, 50 and 150 micrograms) caused dose-related reductions in MAP (5 +/- 1.7 +/- 1 and 11 +/- 2 mm Hg, respectively). Nipecotic acid (3-350 micrograms) also caused dose-related reductions in MAP (from 3 +/- 1 to 15 +/- 2 mm Hg), while aCSF had no effect on MAP. Pretreatment with d(CH2)5Tyr(Me)AVP, antagonized completely the depressor effects of GABA and nipecotic acid. In other rats, blood samples were taken to measure the changes in plasma AVP concentrations (pAVP) induced by CEI + CHLOR and subsequent treatment with aCSF or nipecotic acid (175 micrograms). Hypotension induced by CEI + CHLOR caused a significant increase in pAVP. Forebrain-restricted nipecotic acid significantly suppressed pAVP (61 +/- 8% reduction; P less than 0.05 vs aCSF). These data provide evidence of an endogenous forebrain GABAergic system which, when activated, can inhibit baroreceptor-mediated AVP release.

GABA inhibition of central angiotensin II and hypertonic CSF pressor responses.

Angiotensin II (AII), hypertonic cerebrospinal fluid (CSF) and serotonin produced an increase in arterial pressure when administered intraventricularly (IVT) in conscious rats. Injection of 25 and 100 micrograms (IVT) of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA), while producing slight hypotension, reduced the pressor effect of centrally administered AII and hypertonic CSF, but not serotonin. IVT-administered muscimol, a potent GABA agonist, also attenuated the pressor effect of IVT AII and hypertonic CSF. Thus, in addition to the profound depressor effect of large doses of centrally administered GABA, very low doses (25-100 micrograms, IVT) of this amino acid can alter the pressor responses caused by IVT injection of AII and hypertonic CSF.

Preoptic-hypothalamic periventricular lesions reduce natriuresis to volume expansion.

The present experiment was designed to determine if electrolytic ablation of the periventricular tissue surrounding the anteroventral third ventricle (AV3V) altered the natriuresis typically seen during isotonic volume expansion. Control and AV3V-lesioned rats received intravenous infusions of 0.9% NaCl at 0.5 ml/min until 10% body weight was given. Arterial blood pressure was monitored, and urine was collected throughout the experiment. Following expansion, blood was processed for analysis of natriuretic hormonelike activity by chromatographic separation of plasma extracts followed by measuring antinatriferic activity across the isolated toad bladder. Urinary sodium excretion and urine volume during expansion were significantly less in rats with lesions surrounding the AV3V region than in control rats. Toad bladder bioassay showed a high level of natriuretic hormonelike activity in control animals following volume expansion, but no natriuretic hormonelike activity in plasma from volume-expanded rats with AV3V lesions. These data demonstrate that AV3V periventricular ablation attenuates the natriuresis induced by isotonic-volume expansion. In addition, preliminary results indicate the AV3V region may be a central site critical for natriuretic hormonelike activity and control of extracellular fluid volume.

Effect of lesions of the anteroventral third ventricle (AV3V) on the development of hypertension in spontaneously hypertensive rats.

Lesions of the anteroventral third ventricle (AV3V), an angiotensin and osmosensitive region of the anterior hypothalamus, prevent or abort hypertension in a number of rat models. To determine if AV3V lesions alter hypertension in spontaneously hypertensive rats (SHR), lesions and control sham lesions were made in young SHR at 28 days of age. AV3V lesions had no effect on the development of hypertension in SHR. However, lesioned rats demonstrated significantly reduced pressor responses to intracerebroventricular injections of angiotensin II (AII) and hypertonic NaCl, and drinking produced by centrally administered AII. The depressor effect of central AII receptor blockade was also significantly attenuated in lesioned SHR. These effects appeared to be of central origin since the lesion did not affect the pressor action of intravenous AII or norepinephrine (NE). It is concluded that unlike other models of experimental hypertension (steroid-salt, one-and two-kidney renal, neurogenic) the development of hypertension in SHR does not depend upon the integrity of the AV3V region.