Role of central serotonergic systems in the development of hypertension in spontaneously hypertensive rats.

The spontaneously hypertensive rat (SHR) maintained a higher blood pressure level at and after 8 weeks old than the genetical control Wistar-Kyoto strain (WKY). At 10 weeks old, the turnover rate of 5-hydroxytryptamine (5-HT) was lower in the hypothalamus of SHR than of WKY. Following portacaval anastomosis (PCA) in SHR, the blood pressure was significantly decreased in comparison with that of sham-operated control SHR. In WKY, no significant change in the blood pressure response was observed. PCA treatment increased the 5-HT turnover including that in SHR. If the SHR with PCA was bred with food pellets containing higher concentrations of leucine and isoleucine, the blood pressure increased and the 5-HT turnover decreased. These findings suggest that the central serotonergic system is involved in the development of hypertension.     

Lesions of the central nucleus of the amygdala decrease taste threshold for sodium chloride in rats

Previous studies reported that NaCl intake was down-regulated in rats with bilateral lesions of the central nucleus of the amygdala (CeA). In line with the evidence from anatomical and physiological studies, such an inhibition could be the result of altered taste threshold for NaCl, one of the important factors in assessing taste functions. To assess the effect of CeA on the taste threshold for NaCl, a conditioned taste aversion (CTA) to a suprathreshold concentration of NaCl (0.1M) in rats with bilateral lesions of CeA or sham lesions was first established. And then, two-bottle choice tests between water and a series of concentrations of NaCl were conducted. The taste threshold for NaCl is defined as the lowest concentration at which there is a reliable difference scores between conditioned and control subjects. Rats with CeA lesions acquired a taste aversion for 0.1M NaCl when it was paired with LiCl and still retained the aversion after the two-bottle choice test. The results of the two-bottle choice test showed that the taste threshold for NaCl was 0.0006M in rats with CeA lesions, whereas in rats with sham lesions the threshold was 0.005M, which was identical to that of normal rats. The conditioned results confirm the claim that CeA is not essential in the profile of conditioned taste aversion. Our findings demonstrate that lesions of the CeA increased the sensitivity to NaCl taste in rats, indicating that the CeA may be involved in encoding the intensity of salty gustation elicited by NaCl.     

Alteration of catecholamine uptake in cerebral cortex from rats fed a saturated fat diet

The transport of [³H]norepinephrine into chopped cerebral cortex of neonates was changed by feeding pregnant rats with semisynthetic diets enriched in saturated fat (coconut oil) as compared to polyunsaturated fat (sunflower oil). There was a significant decrease in the uptake of norepinephrine in neonates from dams fed coconut oil compared to neonatal pups from animals fed sunflower oil. Differences were observed on days 7, 11, 24, and 37 but not in adults which had been maintained on the diet since birth. Kinetic analysis of norepinephrine uptake on day 7 neonates demonstrated a 5-fold increase in K_m and a 2-fold increas in V_max for the coconut oil fed group as compared to control or the sunflower oil fed rats. [³H]Dopamine uptake kinetics revealed no significant difference in day 7 neonates but a doubling of the K_m and V_max in day 37 rats and in adults fed coconut oil. Lipid analysis of cerebral cortex synaptosome membrane fractions revealed significant differences in the fatty acyl composition of the phospholipid. The results indicate that the norepinephrine and dopamine transport systems can be differentially affected by dietary lipid composition in the developing rat. It is suggested that the changes may be due to alterations in membranes fluidity in the local environment of the transport system.     

Contribution of central amiloride-sensitive transport systems to the development of hypertension in spontaneously hypertensive rats

This study was conducted to examine if central amiloride-sensitive transport systems are involved in the development and/or maintenance of hypertension in spontaneously hypertensive rats (SHR). Either amiloride (75 microg/60 microl/day) or artificial cerebrospinal fluid (aCSF, 60 microl/day) was infused centrally (i.c.v.) for 4 weeks to development (4-5-weeks-old) and maintenance (10-12-weeks-old) phases of hypertension in SHR. In development phase, amiloride i.c.v. (n=14) blunted the elevation of blood pressure (BP) compared to aCSF i.c.v. (n=9) (amiloride vs. aCSF; after 3 weeks of i.c.v., 146+/-3 vs. 166+/-5 mmHg, P<0.001). The difference of BP at 3 weeks of i.c.v. was canceled after ganglionic block with hexamethonium (115+/-4 vs. 117+/-5 mmHg). Further, pressor responsiveness to norepinephrine was augmented in amiloride i.c.v. rats (amiloride, n=11 vs. aCSF, n=6; %Delta BP at 800 ng/kg/min.: 16.9+/-1.3 vs. 10.8+/-1.4 mmHg, P<0.05) and this augmentation disappeared after ganglionic block. Pressor responsiveness to angiotensin II and cumulative sodium balance did not differ in the two groups. Intravenous administration of amiloride at the same dose did not attenuate the development of hypertension. On the other hand, in maintenance phase, amiloride i.c.v. by the same protocol as in development phase had no effect on BP in SHR. Also, amiloride i.c.v. did not affect BP in normotensive Wistar-Kyoto rats. These results suggest that central amiloride-sensitive transport systems are involved in the development, but not in the maintenance, of hypertension in SHR through the modulation of autonomic neural mechanisms.     

Adverse effect of combination of chronic psychosocial stress and high fat diet on hippocampus-dependent memory in rats

The combined effects of high fat diet (HFD) and chronic stress on the hippocampus-dependent spatial learning and memory were studied in rats using the radial arm water maze (RAWM). Chronic psychosocial stress and/or HFD were simultaneously administered for 3 months to young adult male Wister rats. In the RAWM, rats were subjected to 12 learning trials as well as short-term and long-term memory tests. This procedure was applied on a daily basis until the animal reaches days to criterion (DTC) in the 12th learning trial and in memory tests. DTC is the number of days that the animal takes to make zero error in two consecutive days. Groups were compared based on the number of errors per trial or test as well as on the DTC. Chronic stress, HFD and chronic stress/HFD animal groups showed impaired learning as indicated by committing significantly (P < 0.05) more errors than untreated control group in trials 6 through 9 of day 4. In memory tests, chronic stress, HFD and chronic stress/HFD groups showed significantly impaired performance compared to control group. Additionally, the stress/HFD was the only group that showed significantly impaired performance in memory tests on the 5th training day, suggesting more severe memory impairment in that group. Furthermore, DTC value for above groups indicated that chronic stress or HFD, alone, resulted in a mild impairment of spatial memory, but the combination of chronic stress and HFD resulted in a more severe and long-lasting memory impairment. The data indicated that the combination of stress and HFD produced more deleterious effects on hippocampal cognitive function than either chronic stress or HFD alone.     

Effects of central administration of resistin on renal sympathetic nerve activity in rats fed a high‐fat diet: a comparison with leptin

Similar to leptin, resistin acts centrally to increase renal sympathetic nerve activity (RSNA). In high‐fat fed animals, the sympatho‐excitatory effects of leptin are retained, in contrast to the reduced actions of leptin on dietary intake. In the present study, we investigated whether the sympatho‐excitatory actions of resistin were influenced by a high‐fat diet. Further, because resistin and leptin combined can induce a greater sympatho‐excitatory response than each alone in rats fed a normal chow diet, we investigated whether a high‐fat diet (22%) could influence this centrally‐mediated interaction. Mean arterial pressure (MAP), heart rate (HR) and RSNA were recorded before and for 3 hours after i.c.v. saline (control; n=5), leptin (7 μg; n=4), resistin (7 μg; n=5) and leptin and resistin combined (n=6). Leptin alone and resistin alone significantly increased RSNA (71±16%, 62±4%, respectively). When leptin and resistin were combined, there was a significantly greater increase in RSNA (195±41%) compared to either hormone alone. MAP and HR responses were not significantly different between hormones. When the responses in high‐fat fed rats were compared to normal chow fed rats, there were no significant differences in the maximum RSNA responses. The findings indicate that sympatho‐excitatory effects of resistin on RSNA are not altered by high‐fat feeding, including the greater increase in RSNA observed when resistin and leptin are combined. Our results suggest that diets rich in fat do not induce resistance to the increase in RSNA induced by resistin alone or in combination with leptin.     

Regional quantitative permeability of blood-brain barrier lesions in rats with chronic renal hypertension.

Blood-brain transfer constants for a small, neutral amino acid tracer, [14C]alpha-aminoisobutyric acid (AIB), were measured by quantitative autoradiography and image analysis in 15 individual brain structures of 2-kidney, 1-clip renal hypertensive rats (RHR) and age-matched normotensive controls (NR). Mean arterial pressures (MAP) for 4 month-old RHR and NR were 182 +/- 19 and 121 +/- 3 mmHg, respectively. Most brain structures in RHR had very low [14C]AIB transfer constants similar to those in NR (1-3 microliters.g-1.min-1), indicative of normal blood-brain barrier (BBB) function. Focal lesions, however, having transfer constants 2-7x normal and measuring less than 1.7 mm2 in area, appeared in RHR primarily in the cerebellar vermian and cerebral cortices. Chronic unilateral cervical sympathectomy did not influence the incidence or magnitude of BBB lesions in the denervated hemisphere of RHR. Acute arterial hypertension produced by systemic infusion of phenylephrine (elevation of MAP in RHR by 43%) increased the incidence and magnitude of lesions by 48% and 2-12x, respectively, although many brain regions in acutely hypertensive RHR retained normal permeability to [14C]AIB. The results demonstrate normal BBB permeability for much of the brain in chronic renal hypertension, with focal lesions having 7x or less the normal rate of blood-brain transfer for a small physiological probe.     

Contribution of the central interaction between calcium and sodium to hemodynamic regulation in spontaneously hypertensive rats.

The contribution of the central interaction between calcium and sodium to hemodynamic regulation was assessed in spontaneously hypertensive rats (SHRs) and Wistar-Kyoto (WKY) rats. The effect of a high calcium solution (Ca2+, 130 mg/dl, 10 microliters) infused into the cerebral ventricle (i.c.v.) on hemodynamic responses induced by a high sodium solution (Na+, 1,000 mEq/1, 10 microliters) i.c.v. and the mechanism by which high Ca2+ affects the hemodynamic responses induced by high Na+ i.c.v. were studied. High Na+ i.c.v. induced a pressor response with tachycardia in the SHRs, but induced a pressor response with reflex bradycardia in the WKYs. Prior treatment with high Ca2+ i.c.v. attenuated the pressor response induced by high Na+ i.c.v. (+55.6 +/- 4.4 to +33.1 +/- 3.2 mmHg, P < 0.01) and restored reflex bradycardia (+86.4 +/- 7.7 to -26.7 +/- 7.6 bpm, P < 0.01) in SHRs. Whereas prior treatment with high Ca2+ i.c.v. attenuated the pressor response (+35.7 +/- 2.0 to +22.2 +/- 4.0 mmHg, P < 0.05), it did not alter the degree of reflex bradycardia (-81.7 +/- 7.1 to -69.2 +/- 120 bpm, n.s.) in WKYs. Ganglionic blockade attenuated the pressor response (+56.9 +/- 3.5 to +42.9 +/- 2.3 mmHg, P < 0.05) and restored reflex bradycardia (+82.1 +/- 10.3 to -65.9 +/- 11.0 bpm, P < 0.01) in SHRs, whereas, inhibition of arginine vasopressin attenuated the pressor response without modification of the tachycardic response.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of dihydroergotoxine on central cholinergic neuronal systems and discrimination learning test in aged rats.

We evaluated changes in the cholinergic neuronal system and learning ability with aging. Choline acetyltransferase (ChAT) activity, a presynaptic index of the cholinergic system, was decreased in the cerebral cortex, hippocampus, striatum, and hypothalamus in the brain of aged rats compared with young adults. Muscarinic cholinergic binding sites (receptors, MCR), a postsynaptic index of the cholinergic system, were markedly decreased in all areas of the brain. However, intraperitoneal injection of 1 mg/kg of dihydroergotoxine (DHET) for 14 days normalized both ChAT and MCR in the cerebral cortex and hippocampus. In the striatum, ChAT was normalized, but MCR did not recover. Aged rats showed marked learning impairment in a 30-day operant type brightness discrimination learning test. Daily DHET administration restored the discrimination ability in the aged rats to nearly the young adult level. DHET had no effects on central cholinergic indices or learning test results in young adult rats. These findings suggest that learning is impaired in aged rats due to impairment in the central cholinergic neuronal system, and that DHET normalizes the decreased function in this system, restoring the learning ability.     

Reduced brain DHA content after a single reproductive cycle in female rats fed a diet deficient in N-3 polyunsaturated fatty acids.

BACKGROUND: Low levels of n-3 polyunsaturated fatty acids (PUFAs), particularly docosahexaenoic acid (DHA, 22:6n3), are implicated in postpartum depression. METHODS: The effects of pregnancy and lactation on brain phospholipid fatty acid content were determined in female rats fed diets containing sufficient (control) or negligible (deficient) alpha-linolenic acid (18:3n-3), the dietary precursor of DHA, beginning at conception. Female virgins, fed the diets for 6 weeks, served as control animals. Whole brain total phospholipid composition was determined at weaning by GC. RESULTS: Brain DHA content of postpartum dams fed the deficient diet was decreased by 21% compared with age-matched virgin control animals, with a reciprocal increase in docosapentaenoic acid (22:5n6) to 243%. CONCLUSIONS: Under dietary conditions supplying inadequate n-3 PUFAs, maternal brain DHA content can be reduced after a single reproductive cycle. This depletion may affect neuronal function and thus the sensitivity of the postpartum organism to stress.     

Partial protection from salt-induced stroke and mortality by high oral calcium in hypertensive rats.

BACKGROUND AND PURPOSE: Repeated demonstration of an antihypertensive effect of high oral calcium in stroke-prone spontaneously hypertensive rats led us to determine whether it also protects such rats from premature mortality and stroke-related lesions. METHODS: Female stroke-prone rats (11-13 per diet) were fed high- and low-calcium (2.0% and 0.4%, respectively) diets with both high and low salt (7.0% and 0.3%, respectively) content from age 4 weeks until spontaneous death. In addition to life span, other variables measured included blood pressures, plasma chemistries, and histological characterization of stroke-related lesions. RESULTS: Life span was increased from 51 +/- 4 to 68 +/- 1 weeks (p less than 0.05) by high versus low oral calcium in rats fed high-salt diets; it was further increased to greater than or equal to 82 weeks (p less than 0.05) in rats fed low-salt (+/- added calcium) diets. As seen previously, high oral calcium attenuated salt-induced hypertension but did not affect blood pressure in rats fed low-salt diets. High versus low oral calcium exerted contrasting effects (p less than 0.05) on brain lesions (hemorrhages and infarctions) in rats fed high-salt diets, decreasing lesion size (242 +/- 21 versus 712 +/- 276 microns per rat [diameters seen in histological sections]) but increasing lesion number (8.9 +/- 2.4 versus 3.4 +/- 2.2 per rat); it exerted little influence on the few brain lesions that appeared in rats fed low-salt diets. CONCLUSIONS: High oral calcium may protect stroke-prone hypertensive rats from early salt-induced mortality at least partially by decreasing severity (size) of stroke-related lesions, an effect which may relate to decreased blood pressure. However, this protection may be limited by increased number (incidence) of such lesions, an effect which suggests that high oral calcium may increase the number of brain vessels susceptible to stroke-related injury independent of change in blood pressure.     

Effect of chronic exposure to high magnesium on neuron survival in long-term neocortical explants of neonatal rats in vitro.

In order to assess the effect of elevated magnesium, neuronal morphology and physiology was studied in chronically cultured organotypic neonatal rat occipital neocortex. Explants grown in 10 mM magnesium were found to experience an approximate 30% cell loss (as shown by cell count and DNA-protein analysis), while 12.5 and 15 mM magnesium showed ca. 47 and 60% cell losses, respectively. Intracellular recording from 10 mM magnesium explants revealed that measurable postsynaptic potentials and action potentials could occur, apparently depending on the type of cell examined. All postsynaptic activities ceased in 12.5 mM magnesium cultures, though action potentials could be elicited by current stimulation. The effects of known depolarizing agents, viz. potassium and N-methyl-D-aspartate, on 12.5 mM magnesium-grown explants were also examined. Explants grown in the presence of 12.5 mM magnesium plus 10 mM potassium showed a dramatic increase in the loss of neurons. The simultaneous addition of 6,7-dinitro-quinoxaline-2,3-dione showed this to be due to an increase in non-N-methyl-D-aspartate mediated cell death in response to glutamate release brought about by the depolarizing effects of the potassium. The addition of 10 μM N-methyl-D-aspartate to 12.5 mM magnesium-grown cultures, on the other hand, improved cell survival to control levels. The mechanism of this reciprocal neuroprotective effect of N-methyl-D-aspartate against magnesium has yet to be elucidated. We conclude that these findings are consistent with regard to the opposing actions of N-methyl-D-aspartate and magnesium on calcium influx and various metabolic processes within the explants.     

Long-lasting cardiovascular depressor response following sciatic stimulation in spontaneously hypertensive rats. Evidence for the involvement of central endorphin and serotonin systems

A naloxone-reversible long-lasting depressor response induced by a prolonged low frequency stimulation of the sciatic nerve in conscious spontaneously hypertensive rats (SHRs) was reported in a previous paper. In the present study pharmacological tools were used to further investigate the neurotransmitters involved in this phenomenon. Naloxone infusion (20–25 mg/kg/h following a bolus dose of 10 mg/kg i.v.) attenuated significantly the depressor response, while dexamethasone pretreatment had no such effect, suggesting an important role of the brain endorphin system, but not of the pituitary β-endorphin, in this depressor response. Since the concomitant increase in pain threshold produced by the sciatic stimulation exhibited a different time course of development and naloxone reversibility, it is suggested that the depressor response and the hypalgesic effect produced by the same stimulation are mediated via different types of opiate receptors in the brain. On the other hand, PCPA abolished the post-stimulatory depressor response whereas 5-HTP and zimelidine had additive effects on the sciatic stimulation-induced depressor response, suggesting the involvement of central serotonin systems in the mechanism of the response. The interaction between the central endorphin and the serotonin systems in the mediation of the post-stimulatory depressor response is discussed.     

Ascending projections from the area around the spinal cord central canal: A Phaseolus vulgaris leucoagglutinin study in rats.

A single small iontophoretic injection of Phaseolus vulgaris leucoagglutinin labels projections from the area surrounding the spinal cord central canal at midthoracic (T6-T9) or lumbosacral (L6-S1) segments of the spinal cord. The projections from the midthoracic or lumbosacral level of the medial spinal cord are found: 1) ascending ipsilaterally in the dorsal column near the dorsal intermediate septum or the midline of the gracile fasciculus, respectively; 2) terminating primarily in the dorsal, lateral rim of the gracile nucleus and the medial rim of the cuneate nucleus or the dorsomedial rim of the gracile nucleus, respectively; and 3) ascending bilaterally with slight contralateral predominance in the ventrolateral quadrant of the spinal cord and terminating in the ventral and medial medullary reticular formation. Other less dense projections are to the pons, midbrain, thalamus, hypothalamus, and other forebrain structures. Projections arising from the lumbosacral level are also found in Barrington's nucleus. The results of the present study support previous retrograde tract tracing and physiological studies from our group demonstrating that the neurons in the area adjacent to the central canal of the midthoracic or lumbosacral level of the spinal cord send long ascending projections to the dorsal column nucleus that are important in the transmission of second-order afferent information for visceral nociception. Thus, the axonal projections through both the dorsal and the ventrolateral white matter from the CC region terminate in many regions of the brain providing spinal input for sensory integration, autonomic regulation, motor and emotional responses, and limbic activation.     

Quantitation of preproenkephalin mRNA levels in brain regions from male Fischer rats following chronic cocaine treatment using a recently developed solution hybridization assay.

Quantitative solution hybridization assays were used to determine the picogram amounts of preproenkephalin mRNA (PPenk mRNA) and the microgram quanities of total rat RNA in extracts of eight brain regions from rats which had received three daily intraperitoneal injections of cocaine (10 or 30 mg/kg/day) or saline for 14 days. The young adult male Fischer rats were sacrificed 30 min after the final injection. The highest density of PPenk mRNA (pg PPenk mRNA/micrograms total cellular RNA) was found in extracts of striatum (34.08 +/- 1.79 pg/micrograms for 11 saline-treated rats), followed by extracts of nucleus accumbens (10.08 +/- 0.81 pg/micrograms), and extracts of hypothalamus (2.99 +/- 0.31 pg/micrograms). Extracts of frontal cortex (1.78 +/- 0.24 pg/micrograms), pituitary (1.39 +/- 0.08 pg/micrograms), central grey (1.31 +/- 0.16 pg/micrograms), and cerebellum (1.24 +/- 0.09 pg/micrograms) had intermediate values. Extracts of hippocampus (0.53 +/- 0.03 pg/micrograms) had the lowest density. No significant differences were found among the treatment groups in any brain area investigated. Therefore, chronic cocaine treatment as administered in this protocol did not alter expression of the gene encoding proenkephalin.     

Hyperthermia-enhanced serotonin (5-HT) depletion resulting from D-fenfluramine (D-Fen) exposure does not evoke a glial-cell response in the central nervous system of rats.

D-Fen-induced hyperthermia has been shown to coincide with an enhanced depletion of 5-HT and 5-hydroxyindole acetic acid (5-HIAA). Because these observations have relied on D-Fen exposure at multiple environmental temperatures, some have questioned the validity of the findings. Therefore, this experiment was designed to determine if the correlation between elevated body temperature and 5-HT depletion could be observed when D-Fen exposure occurred in one warm environment (28 degrees C) and to determine if a hyperthermia-enhanced glial-cell response could be evoked by D-Fen exposure. Hyperthermia-enhanced 5-HT and 5-HIAA depletion resulting from D-Fen exposure was dependent on body temperature during drug exposure. In the frontal cortex, 5-HT concentrations ranged from 3 to 45% of control values. Likewise, in the striatum and hippocampus, 5-HT concentrations ranged from 13 to 53% and 6 to 40%, respectively. The 5-HIAA concentrations had a wider range than the 5-HT concentrations for each brain region. In the frontal cortex, striatum and hippocampus, 5-HIAA ranged from 0 to 93%, 15 to 72% and 0 to 83% of control, respectively. In spite of the substantial reductions in 5-HT, there was no detectable glial-cell response. D-Fen-induced hyperthermia does not appear to cause generalized damage to neurons in the frontal cortex, striatum and hippocampus.