A comparison of the development of renal hypertension in male and female rats

1. The objective of this study was to determine the influence of gender on the development of renal hypertension in Sprague-Dawley rats using the Goldblatt two-kidney, one-clip (2K1C) model. In addition, this study examined the effect of ovariectomy upon the development of hypertension in female rats.2. At 10 weeks of age, male, intact female and ovariectomized female rats underwent clipping of the right renal artery or sham operation. Tail-cuff plethysmography was used to monitor the systolic blood pressure of all animals for 7 weeks post-clipping or sham operation. Rats were sub-grouped according to whether or not they developed hypertension (systolic blood pressure >=150 mmHg).3. Within 2 to 3 weeks of clipping, hypertension was induced in only 53% (n=120) of the intact female 2K1C rats, but in 83% (n=18) of the male and 78% (n=18) of the ovariectomized female rats.4. Seven weeks after right renal artery clipping, plasma renin activity was determined in a subset of each group and was found to be 5-6 fold higher in male (17.29+/-4.04 ng angiotensin I.h-1.ml-1) and ovariectomized female (9.71+/-1.25 ng angiotensin I.h-1.ml-1) hypertensive rats compared with their respective normotensive or sham-operated counterparts (3.39+/-0.58 ng angiotensin I.h-1.ml-1 and 1.60+/-0.41 ng angiotensin I.h-1.ml-1 respectively) (P<0.05, analysis of variance). In contrast, the plasma renin activity measured in intact female hypertensive rats was not significantly different from that measured in the corresponding 2K1C normotensive or sham-operated groups.5. These results indicate that the success rate of inducing renal hypertension in Sprague-Dawley rats is higher in males than in intact females. Furthermore, these results suggest that the induction of 2K1C hypertension may be influenced by ovarian hormones.

Nicotinic acetylcholine receptor mediated modulation of evoked excitatory amino acid release in the nucleus tractus solitarius of the rat: evidence from in vivo microdialysis

In vivo microdialysis was used to measure release of endogenous l-glutamate and l-aspartate in the nucleus tractus solitarius of the anaesthetised rat evoked by baroreceptor loading. Aortic constriction, the method of loading, elicited a reproducible increase in extracellular levels of l-glutamate to 322+/-139% of basal levels, which could be attenuated by concomitant local administration of the nicotinic acetylcholine receptor antagonist mecamylamine (100 microM).

Autoradiographic visualisation of axonal transport of adenosine A1 receptors along the rat vagus nerve and characterisation of adenosine A1 receptor binding in the dorsal vagal complex of hypertensive and normotensive rats

The present study had employed in vitro receptor autoradiography with [3H]DPCPX to visualise the presence of adenosine A1 receptors on the rat nodose ganglion, which contains the perikarya of vagal afferent neurons projecting the the nucleus tractus solitarius (NTS). In addition, unilateral vagal ligation resulted in an accumulation of [3H]DPCPX binding adjacent to the ligatures, indication that adenosine A1 receptors are subject to axoplasmic flow along the rat vagus nerve. Radioligand binding assays were utilised to characterise the properties of adenosine A1 receptors in the dorsal vagal complex (NTS, area postrema and dorsal motor nucleus of the vagus) of pup and adult normotensive (Wistar Kyoto, WKY) and hypertensive (spontaneously hypertensive, SHR) rats. Saturation binding indicated that the affinity (KD) of [3H]DPCPX, and the binding site density (Bmax) were not different between the adult WKY and SHR, although the pup SHR had a lower KD value than the pup WKY rat. Competition binding assays revealed complex differences between the two rat strains; however, with respect to hypertension, the affinity of the selective adenosine A1 agonist, cyclohexyladenosine (CHA), was markedly reduced in the membranes from SHR (Ki approximately 93 nM) compared to WKY (approximately 6 nM). Such an observation is consistent with the attenuated responses of SHRs to intra-NTS injections of adenosine.

The distribution of nitric oxide synthase-, adenosine deaminase- and neuropeptide Y-immunoreactivity through the entire rat nucleus tractus solitarius: Effect of unilateral nodose ganglionectomy

The present study has employed immunocytochemistry on free-floating sections of adult rat medulla oblongata to characterise the distribution of nitric oxide synthase- (NOS), adenosine deaminase- (ADA) and neuropeptide Y- (NPY) immunoreactivity (IR) throughout the entire rostro-caudal axis of the nucleus tractus solitarius (NTS). In addition, unilateral nodose ganglionectomy was performed in a group of rats to determine whether any observed immunoreactivity was associated with central vagal afferent terminals. NOS-IR was found throughout the entire NTS, in cells, and both varicose and non-varicose fibres. Furthermore, unilateral nodose ganglionectomy resulted in a clear reduction in NOS-IR (visualised with diaminobenzidine) in a highly restricted portion of the ipsilateral medial NTS. Similarly, ADA- and NPY-containing cells, fibres and terminals were also found throughout the adult rat NTS. However, following unilateral nodose ganglionectomy, there was no apparent reduction in either ADA-IR or NPY-IR on the denervated side of the NTS. These data indicate a role for nitric oxide, purines and neuropeptide Y as neuromodulators within the rat NTS, although only nitric oxide appears to be primarily associated with vagal afferent input. Adenosine deaminase and neuropeptide Y-containing neurons appear to be predominantly postsynaptic to vagal input, although their possible association with vagal afferents cannot be completely excluded.

Distribution of opioid peptide gene expression in the limbic system of Fawn-Hooded (alcohol-preferring) and Wistar-Kyoto (alcohol-non-preferring) rats

Preprodynorphin and preproenkephalin mRNA expression was examined in the CNS of two rat strains, the alcohol-preferring Fawn-Hooded (FH) and the alcohol-non-preferring Wistar-Kyoto (WKY), using in situ hybridisation histochemistry. Relative to the WKY, the FH showed significantly lower levels of preproenkephalin mRNA in the striatum and nucleus accumbens (-24% and -17% respectively), but a higher level of preprodynorphin mRNA in the hippocampus (+33%). The depressed level of preproenkephalin mRNA in the nucleus accumbens may be implicated in alcohol-seeking behaviour.

Bradykinin B2 receptors in nodose ganglia of rat and human

The present study has employed in vitro electrophysiology to characterise the ability of bradykinin to depolarise the rat isolated nodose ganglion preparation, containing the perikarya of vagal afferent neurons. Both bradykinin and kallidin elicited a concentration-dependent (1-100 nM) depolarisation when applied to the superfusate bathing the nodose ganglia, whereas the bradykinin B1 receptor agonist, des-Arg9-bradykinin, was only effective in the micromolar range. Furthermore, the electrophysiological response to bradykinin was antagonised by the bradykinin B2 receptor antagonist, D-arginyl-L-arginyl-L-prolyl-trans-4-hydroxy-L-prolylglycyl-3-(2-t hienyl)-L-alanyl-L-seryl-D-1,2,3,4-tetrahydro-3-isoquinolinecarbonyl+ ++-L-(2alpha,3beta,7abeta)-octahydro-1H-indole-2-carbonyl-L- arginine (Hoe 140), in a concentration-related manner. To determine the anatomical location of functional bradykinin B2 receptors, in vitro autoradiography with [125I]para-iodophenyl Hoe 140 was performed on sections of rat and human inferior vagal (nodose) ganglia and confirmed the presence of binding over vagal perikarya. Collectively, these data provide evidence for functionally relevant bradykinin B2 receptors on vagal afferent neurons, which are apparently also present on human vagal perikarya.

A reliable procedure for comparison of antioxidants in rat brain homogenates

Lipid peroxidation is a major consequence of oxidative stress and an important cause of neuronal damage in ischaemic injuries and neurodegenerative disorders such as Parkinson's disease. Recent research has focused on the development of antioxidant drugs which may delay or minimize neurodegeneration. Rapid and reliable assays are therefore necessary in order to evaluate novel antioxidant compounds. A widely adopted method for measurement of lipid peroxidation is the thiobarbituric acid reacting substances (TBARS) assay. Several variations of this method have appeared in the literature, some of which have been tested by us without success. We have therefore established a reliable procedure which takes into account the most important factors previously found to influence the TBARS method. Briefly, various concentrations of drug were added to rat brain homogenates (10% w/v in 20 mM Tris-HCl buffer, pH 7.4) and incubated at 37 degrees C for 10 min before addition of ammonium ferric sulphate (100 or 1000 microM) and a further incubation at 37 degrees C for 30 min. Proteins were then precipitated with 8.1% sodium dodecyl sulphate, the reaction stopped with 20% acetic acid, and the samples were then centrifuged for 15 min. Aliquots of supernatant were added to an equal volume of thiobarbituric acid (0.8%), samples were heated at 95 degrees C for 30 min, and then cooled on ice before reading at 532 nm. The present adaptation represents a simple and highly reproducible assay which does not require difficult extraction procedures with hazardous chemicals and results in a stable chromagen. The method has been evaluated using a number of structurally distinct antioxidants and iron chelators. IC50 values (microM) for percentage inhibition of TBARS formation were as follows: desferroxamine (1.1), U83836E (1.7), butylated hydroxytoluene (13), U74500A (20), LY231617 (22), idebenone (89), and Trolox (110). This order of potency was comparable to that found with a commercially available, but expensive kit designed to specifically measure malondialdehyde (Spearman's rank correlation coefficient, p < 0.01).

Nicotinic acetylcholine receptors in the rat and primate nucleus tractus solitarius and on rat and human inferior vagal (nodose) ganglia: evidence from in vivo microdialysis and [125I]alpha-bungarotoxin autoradiography

The nucleus tractus solitarius is a key brain centre involved in the regulation of numerous autonomic functions. The present study has employed in vitro autoradiography and in vivo microdialysis to investigate the presence and function of nicotinic acetylcholine receptors located in the medial nucleus tractus solitarius of the rat. Autoradiography using [125I]alpha-bungarotoxin (0.5 nM) enabled visualization of binding sites on sections of rat and monkey brainstem. Specific binding was highest in the medial nucleus tractus solitarius. The presence of binding sites was also apparent on sections of rat nodose ganglia/vagus nerve and human inferior vagal ganglia. Subsequent to unilateral ligation of the vagus nerve in the rat, an accumulation of binding sites was visualized adjacent to the ligature. Unilateral nodose ganglionectomy in the rat caused an approximate 97% reduction in [125I]alpha-bungarotoxin binding site density in the medial nucleus tractus solitarius from 814 +/- 183 to 27 +/- 2 d.p.m./mm2. Microdialysis results indicated that local administration of nicotine (1 mM) into the nucleus tractus solitarius of the rat resulted in increases of extracellular L-glutamate of 146 +/- 9% of basal levels. This effect was not reproducible following a second stimulation and was also blocked by prior and co-administration of the nicotinic acetylcholine receptor antagonist mecamylamine (100 microM). Extracellular levels of L-aspartate exhibited a similar pattern although results were not significant. Taken together, these results are supportive of the presence of a population of [125I]alpha-bungarotoxin binding sites located presynaptically with respect to vagal afferent terminals in the medial nucleus tractus solitarius of the rat. It is possible that these binding sites are the site of action of locally administered nicotine on extracellular levels of L-glutamate, the favoured neurotransmitter at primary baroreceptor afferent fibres. These data are discussed in relation to the functional pharmacology of acetylcholine and nicotinic acetylcholine receptors in this region of the brain.

Distribution of GABAA receptors in the limbic system of alcohol-preferring and non-preferring rats: in situ hybridisation histochemistry and receptor autoradiography

The present study has employed quantitative receptor autoradiography and in situ hybridisation histochemistry to compare the expression of the mRNA encoding the alpha 1 and alpha 2 subunits of the GABAA receptor and the binding density of mature GABAA receptors in the limbic system of alcohol-preferring Fawn-Hooded rats (FH) with Wistar-Kyoto rats (WKY). Quantifiable levels of mRNA encoding the alpha 1 subunit were found in cortical regions, ventral pallidum, substantia nigra, horizontal limb of the diagonal band and the hippocampus of both rat strains. Interestingly, expression of the alpha 1 subunit mRNA was decreased by approximately 30% in the hippocampus of FH compared to WKY rats. Following a 28-day period with free access to 10% ethanol, expression of the alpha 1 subunit transcript, was significantly increased in the piriform cortex and horizontal limb of the diagonal band, unaltered in the hippocampus but decreased in the substantia nigra of FH rats. Quantifiable levels of mRNA encoding the alpha 2 subunit were found in nucleus accumbens, amygdala, cortical regions, lateral septal nucleus, hippocampus, medial habenula and ventral pallidum of both strains. Expression of the alpha 2 subunit mRNA was decreased by approximately 35% in both the hippocampus and occipital cortex of FH compared to WKY rats. However, consumption of 10% ethanol in FH rats had no impact upon expression of the mRNA encoding the alpha 2 subunit in any region examined. Mature GABAA receptors were studied by autoradiography utilising the antagonist radioligand [3H]SR95531 and the agonist radioligand [3H]muscimol. Topographic binding throughout the limbic system of both strains was observed for both radioligands. Specifically, [3H]SR95531 binding was higher in the occipital cortex, hippocampus, lateral septal nucleus, superior colliculus and ventral pallidum of the FH rats compared to WKY rats; however, in the nucleus accumbens [3H]SR95531 binding was lower in FH compared to WKY. Ethanol consumption had no measurable effect on the binding of [3H]SR95531 in FH rats. In the case of [3H]muscimol, binding was higher in the cortex, lateral septum and ventral pallidum of FH compared to WKY. Furthermore, ethanol consumption resulted in a 25-30% increase in [3H]muscimol binding in the lateral septum and striatum of FH rats. These data provide evidence for differential expression of GABAA receptor subunits in FH and WKY rats, and additionally indicate anatomically defined variations in GABAA receptor binding between the two rat strains.

Effects of acute ethanol on GABA release and GABA(A) receptor density in the rat mesolimbic system

The present study has addressed whether acute ethanol administration mediates changes in GABA release and GABA(A) receptor density in the rat mesolimbic system. In vivo microdialysis was performed in the ventral pallidum and between the ventral tegmental area and substantia nigra of conscious ethanol-naive rats. Extracellular levels of endogenous GABA were stable in both brain regions and not significantly affected following administration of 5 ml of 20% ethanol by gavage, despite clear overt behavioral signs of intoxication. Two hours following ethanol administration, animals were decapitated and the brains processed for autoradiography. Adjacent tissue sections were incubated with [3H]SR95531 or [3H]muscimol and the resulting autoradiograms quantified. Binding of both radioligands was significantly reduced in the striatum of rats treated with ethanol compared to vehicle (15 +/- 2% for [3H]SR95531 and 33 +/- 6% for [3H]muscimol). In contrast, ethanol had no effect on [3H]SR95531 binding in hippocampus, while that of [3H]muscimol was increased by 100 +/- 30%. Ethanol had no effect on the binding of either radioligand in all other areas examined. Therefore, while acute ethanol had no effect on the release of GABA in either the ventral pallidum or ventral tegmental area, changes in the binding density of GABA(A) ligands were observed in selected brain regions.

[3H]zolpidem binding in alcohol-preferring and non-preferring rat brain

The present study has employed in vitro autoradiography to study the distribution and density of [3H]zolpidem binding sites, which are regarded as an index of ethanol-sensitive gamma-aminobutyric acid (GABA)A receptors, in the brains of alcohol-preferring Fawn-Hooded (FH) rats compared to non-alcohol preferring Wistar-Kyoto (WKY) rats. Binding of [3H]zolpidem showed a similar distribution profile in both rat strains examined and included cerebellum, globus pallidus, nucleus of the solitary tract and a number of midbrain/hindbrain nuclei. Densitometric quantitation of binding revealed that FH rats possessed a significantly higher density of [3H]zolpidem binding compared to WKY rats in cortical regions, substantia nigra pars reticulata and the ventral pallidum. These data indicate that FH rats may have an increased number of ethanol-sensitive GABA(A) receptors in regions intimately involved in reward processes, and may partially explain the alcohol-seeking nature of the FH rat.

Prepro-neuropeptide Y mRNA and NPY binding sites in human inferior vagal ganglia

The inferior vagal ganglia contain the cell bodies of centrally projecting vagal afferent neurones. Using in situ hybridization-histochemistry with a combination of two antisense neuropeptide Y (NPY) oligonucleotides, we have demonstrated that a population of human inferior vagal perikarya express mRNA encoding prepro-NPY, the precursor of NPY. In vitro receptor autoradiography, using both [125I]Bolton Hunter-NPY ([125I]BH-NPY, 15 pM) and [125I]peptide YY ([125I]PYY, 25 pM), enabled visualization of NPY binding sites. Competition binding with NPY (1 microM), PYY (1 microM) and [Leu31,Pro34]NPY (100 nM), suggest that both Y1 and Y2 receptor subtypes are present on human vagal afferent neurones. These observations suggest a potential role for NPY in neuromodulation of vagal transmission in humans.

Complex interactions between nitric oxide and adenosine receptors in the rat isolated nodose ganglion

The present study has employed in vitro electrophysiology, utilising the isolated rat nodose ganglion preparation, to determine whether nitric oxide (NO) and adenosine interact with each other in vagal afferent neurons. The nucleophile NO donor, diethylamine-NO, caused reproducible, concentration-related depolarisations of the isolated rat nodose ganglia. Pre-incubation of the isolated rat nodose ganglia with the adenosine A2A receptor agonists CGS 21680 (2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine hydrochloride) and DPMA (N6-[2-(3,5-dimethoxyphenyl)-2-(2-methylphenyl)-ethyl]adenosine) (both 1 microM) resulted in a functional antagonism of the ability of diethylamine-NO to depolarise the preparation. A similar effect was observed with adenosine (10 microM) only in the presence of the adenosine A1 receptor antagonist PACPX (1,3-dipropyl-8-(2-amino-4-chlorophenyl)-xanthine, 100 nM). Conversely, the adenosine A1 receptor agonists ENBA (N6-[2-endo-norbomyl]adenosine, 1 microM) and cyclohexyladenosine (100 nM) potentiated the effect of diethylamine-NO on isolated rat nodose ganglia. Inclusion of either adenosine A3 agonists or ATP had no effect on the diethylamine-NO concentration-response curve. These data suggest an ability of NO to interact, in opposing manner, with adenosine A2A and A1 receptors in rat vagal afferent neurons. On the other hand, neither A3 receptors nor ATP appear capable of interacting with NO.

The release of immunoreactive neuropeptide Y in the spinal cord of the anaesthetized rat and cat

The release of immunoreactive (ir-) neuropeptide Y (NYP) was studied in the anaesthetized rat and cat by means of microprobes bearing immobilized antibodies to the C terminus of NPY. An extensive basal release of ir-NYP was detected throughout the dorsal and upper ventral horn of the rat. This spontaneous release was not significantly altered by sectioning the spinal cord at the thoraco-lumbar junction nor by electrical stimulation of peripheral nerves. Since NPY is virtually absent in primary afferents it is probable that spontaneous release within the spinal cord comes from active NPY-containing intrinsic spinal neurones. In the spinal cat spontaneous release of ir-NPY was detected in the mid-dorsal horn and this was unaltered by peripheral noxious thermal or noxious mechanical stimuli. As in the rat, release from intrinsic spinal neurones is most probable. The extensive spontaneous release of ir-NPY in both species suggests a widespread role in spinal cord function.

Adenosine-dopamine receptor interactions in the isolated rat nodose ganglion but not in membranes of dorsal vagal complex

The present study has employed in vitro electrophysiology and radioligand binding assays to determine whether dopamine and adenosine receptors interact with each other on rat vagal afferent neurons. Preincubation of the isolated rat nodose ganglion with the adenosine A2a agonists CGS 21 680 or DPMA (Both 1 microM) resulted in a functional antagonism of the ability of dopamine to depolarise the preparation. Specifically, the concentration-response curve to dopamine was significantly shifted to the right in the presence of CGS 21 680 and DPMA. On the other hand, adenosine itself, A1 and A3 receptor agonists and ATP were all incapable of modulating the electrophysiological response to dopamine. In contrast to the nodose ganglion, CGS 21 680 did not significantly affect the ability of the dopamine D2 ligands quinpirole or raclopride to displace [125I]NCQ298 binding to dopamine D2 receptors in membranes prepared from rat dorsal brain stem. These data indicate the presence of an interaction between high affinity adenosine A2 receptors and dopamine D2 receptors on the soma of rat vagal afferent neurons, whereas the situation in the brain stem remains less clear.

Functional GABAA receptors on rat vagal afferent neurones

1. In the present study, in vitro electrophysiology and receptor autoradiography were used to determine whether rat vagal afferent neurones possess gamma-aminobutyric acid (GABA)A receptors. 2. GABA (1-100 microM) and isoguvacine (3-100 microM) caused a concentration-dependent depolarization of the rat isolated nodose ganglion preparation at room temperature. When applied to the tissue 20 min before the agonist, SR95531 (3 microM) and bicuculline (3 microM) caused a parallel shift to the right of the GABA and isoguvacine concentration-response curves, yielding shifts of 81 fold and 117 fold for SR95531 and 4 fold and 12 fold for bicuculline, respectively. 3. Baclofen (10 nM-100 microM) was unable to elicit a depolarization of the rat isolated nodose ganglion preparation at either room temperature or at 36 degrees C, whilst 5-aminovaleric acid (10 microM), a GABAB receptor antagonist, was unable to antagonize significantly the GABA-induced depolarization at either room temperature or at 36 degrees C. 4. [3H]-SR95531 (7.2 nM), a GABAA receptor-selective antagonist, bound topographically to sections of rat brainstem. Specific binding was highest in the medial nucleus tractus solitarius (NTS) and dorsal motor nucleus of the vagus nerve (DMVN). Binding was also observed in certain medullary reticular nuclei, in particular the parvocellular reticular nucleus. 5. Unilateral nodose ganglionectomy caused a reduction in GABAA binding site density in the medial NTS from 93 +/- 7 to 68 +/- 6 d.p.m./mm2. This procedure also caused a reduction in GABAA binding site density in the side of the NTS contralateral to the lesion, from 151 +/- 12 to 93 +/- 7 d.p.m./mm2. Sham surgery had no effect on the binding of [3H]-SR95531 in rat brainstem. 6. The present data provide evidence for the presence of GABAA receptors located on the soma and central terminals of rat vagal afferent neurones. Additionally, a population of GABAA receptors is evidenced postsynaptically in the rat NTS with respect to vagal afferent terminals. These data are discussed in relation to the functional pharmacology of GABA in this region of the NTS.

Markers of adenosine removal in normotensive and hypertensive rat nervous tissue

Adenosine mechanisms are altered in brain stem nuclei associated with cardiovascular control in spontaneously hypertensive rats (SHR). Therefore, in the present study we used a number of techniques to compare the binding of the adenosine transport inhibitor [3H]nitrobenzylthioinosine ([3H]NBMPR) as well as adenosine deaminase immunoreactivity (ADA-IR) in brain stems and nodose ganglia of SHR and age-matched normotensive Donryu rats (DRY). Saturation binding revealed a single class of [3H]NBMPR binding sites in the dorsal brain stem of both strains, with Kd and Bmax values of 65 +/- 9 pmol/L and 282 +/- 31 fmol/mg protein, respectively, in SHR and 129 +/- 2 pmol/L and 217 +/- 23 fmol/mg protein in DRY. The Kd for [3H]NBMPR was significantly lower in SHR than in DRY. In competition assays, NBMPR, dilazep, dipyridamole, and adenosine displaced [3H]NBMPR binding, with Kd values of 0.21 +/- 0.04, 57.16 +/- 16.20, 1340 +/- 100, and 87000 +/- 12500 nmol/L, respectively, in DRY and 0.17 +/- 0.04, 28.24 +/- 3.60, 621 +/- 100, and 32000 +/- 6820 in SHR. Kd values for all displacers were lower in SHR; however, only values for dipyridamole and adenosine reached statistical significance. Autoradiography of adenosine transport sites with [3H]NBMPR revealed that unilateral nodose ganglionectomy reduced [3H]NBMPR binding on the denervated side of the nucleus tractus solitarius by 20.6 +/- 1.1% in DRY and 18.7 +/- 2.3% in SHR. The density of [3H]NBMPR binding in nodose ganglia was significantly lower in SHR (0.99 +/- 0.06 Bq/mm2) than in DRY (1.25 +/- 0.08). Immunohistochemical studies demonstrated ADA-IR in the dorsal vagal complex, associated with both nerve cells and fibers. Measurement of ADA-IR in the dorsal vagal complex with an 125I-labeled secondary antibody revealed a significantly higher level of ADA-IR in SHR (122%) than in DRY. In the nodose ganglia, ADA-IR was associated with a population of vagal perikarya. The present study helps provide a molecular explanation for the previously reported impaired cardiovascular responses to intra-nucleus tractus solitarius microinjection of adenosine in hypertensive rats.

Actions of nitric oxide and expression of the mRNA encoding nitric oxide synthase in rat vagal afferent neurons

The present study has investigated whether nitric oxide (NO) is involved in neurotransmission of rat vagal afferent neurons. The diethylamine-NO complex (diethylamine-NO, 10-100 microM) and S-nitroso-N-acetylpenicillamine (3-100 microM) both elicited a concentration-dependent depolarisation of the isolated rat nodose ganglion preparation. Pre-treatment with 1 H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ, 300 nM), 6-(phenylamino)-5,8-quinolinedione (LY83,583, 30 microM) and Methylene blue (100 microM) all caused a significant shift to the right in the concentration-response curve to diethylamine-NO. Incubation of rat nodose ganglion sections with a 35S-labeled antisense oligonucleotide to neuronal NO synthase resulted in visualisation of the mRNA encoding NO synthase over vagal afferent perikarya. The anatomical findings, therefore, suggest that a number of rat vagal afferent perikarya possess the ability to produce the enzyme required for the biosynthesis of NO. Collectively, these data suggest that NO may be functionally important as a neuromodulator of rat vagal afferent neurons.

Loss of [125I]-pindolol binding to beta-adrenoceptors on rat nodose ganglion after chronic isoprenaline treatment

The nodose ganglion contains the cell bodies of afferent nerves which convey predominantly sensory information from the viscera to the central nervous system (CNS). Autoradiographic studies show binding sites for beta-adrenoceptor ligands are present on sections of the rat nodose ganglion and also on the corresponding inferior vagal ganglion in humans, indicating the presence of beta-adrenoceptors in these ganglia. Since prolonged stimulation of beta-adrenoceptors in rats with the nonselective beta-adrenoceptor agonist isoprenaline (400 micrograms kg-1 day-1 s.c.) for 14 days results in desensitisation and/or down-regulation of receptors in peripheral tissues, such as heart, kidney and blood vessels, the effects of this treatment on the beta-adrenoceptor population on the nodose ganglion have been examined. Using [125I]-pindolol as a radioligand, autoradiographic studies revealed that specific binding was reduced by 74% in ganglia from isoprenaline-pretreated rats compared to that in ganglia from vehicle-pretreated rats, demonstrating down-regulation of receptors by isoprenaline. [125I]-Pindolol binding was sensitive to inhibition by ICI 118.551 (selective beta 2-adrenoceptor antagonist) but not to atenolol (selective beta 1-adrenoceptor antagonist), indicating receptors are predominantly of the beta 2-adrenoceptor subtype. No change in binding was apparent over the vagus nerve. The nodose ganglion appears to be an additional site at which beta 2-adrenoceptors may be down-regulated in vivo, possibly interfering with normal baro-, chemo- and sensory reflexes.

Radioligand binding and autoradiographic visualization of adenosine transport sites in human inferior vagal ganglia and their axonal transport along rat vagal afferent neurons

The present study has employed membrane-binding studies and in vitro autoradiography to demonstrate the presence of adenosine transport sites in human inferior vagal ganglia using [3H]nitrobenzylthioinosine ([3H]NBMPR), a potent inhibitor of adenosine transport. In addition, [3H]NBMPR was used to determine whether adenosine transport sites are subject to axonal transport along the rat vagus nerve. Binding of [3H]NBMPR to human inferior vagal ganglia membranes was saturable and reversible. Saturation experiments revealed a single class of high affinity-binding sites with a Kd of 93.73 +/- 23.13 pM and Bmax of 413.50 +/- 50.40 fmol/mg protein. In displacement experiments, the adenosine transport inhibitor dipyridamole was the most potent displacer of [3H]NBMPR binding (Ki = 42.7 +/- 28.0 nM). Adenosine itself was able to fully displace [3H]NBMPR binding with a Ki of 115.0 +/- 34.0 microM. The A1/A2a adenosine receptor agonist 5'-(N-ethylcarboxamido)-adenosine (NECA) was able to fully displace [3H]NBMPR binding in only one experiment at a concentration of 100 microM, yielding an affinity 1000-fold higher than its affinity for adenosine receptors. All competition curves obtained from displacement experiments displayed monophasic profiles, indicating the presence of a single class of [3H]NBMPR binding sites. Incubation of human inferior vagal ganglia sections with [3H]NBMPR (0.7 nM) revealed dense binding which appeared to be consistent with the distribution of neuronal cell bodies in this tissue. Following unilateral ligation of the vagus nerve in the rat, accumulation of [3H]NBMPR binding sites occurred both proximal and distal to the vagal ligatures. These results suggest that [3H]NBMPR binds with high affinity to a single class of adenosine transport sites, and that these sites are present on vagal afferent neurons in the human and undergo bidirectional axonal transport along the rat vagus nerve.

Neurochemical modulation of cardiovascular control in the nucleus tractus solitarius

The central control of cardiovascular function has been keenly studied for a number of decades. Of particular interest are the homeostatic control mechanisms, such as the baroreceptor heart-rate reflex, the chemoreceptor reflex, the Bezold-Jarisch reflex and the Breuer-Hering reflex. These neurally-mediated reflexes share a common termination point for their respective centrally-projecting sensory afferents, namely the nucleus tractus solitarius (NTS). Thus, the NTS clearly plays a critical role in the integration of peripherally initiated sensory information regarding the status of blood pressure, heart rate and respiratory function. Many endogenous neurochemicals, from simple amino acids through biogenic amines to complex peptides have the ability to modulate blood pressure and heart rate at the level of the NTS. This review will attempt to collate the current knowledge regarding the roles of neuromodulators in the NTS, the receptor types involved in mediating observed responses and the degree of importance of such neurochemicals in the tonic regulation of the cardiovascular system. The neural pathway that controls the baroreceptor heart-rate reflex will be the main focus of attention, including discussion of the identity of the neurotransmitter(s) thought to act at baroafferent terminals within the NTS. In addition, this review will provide a timely update on the use of recently developed molecular biological techniques that have been employed in the study of the NTS, complementing more classical research.

Neuropeptide Y gene expression and receptor autoradiography in hypertensive and normotensive rat brain

Neurones containing neuropeptide Y (NPY) may participate in central cardiovascular control by tonically influencing barosensitive neurones within the nucleus tractus solitarius. The present study has employed both in situ hybridisation histochemistry and receptor autoradiography, to visualise the expression of prepro-NPY mRNA in the forebrain and to determine the NPY receptor subtype(s) in the brainstem, respectively. Prepro-NPY gene expression was visualised in the hypothalamus, cortex, dentate gyrus and lateral reticular thalamus from age-matched spontaneously hypertensive rats (SHR) and normotensive Don Ryu rats (DRY) and Wistar Kyoto rats (WKY). Quantitative densitometry revealed an increase in the NPY transcript in the arcuate nucleus of SHR rats compared to their normotensive counterparts. Autoradiography using [125I]Bolton-Hunter-NPY (BH-NPY, 15 pM) demonstrated NPY binding sites in the area postrema, the commissural nucleus tractus solitarius (cNTS) and the inferior olivary complex. NPY (1 microM) and peptide YY (1 microM), but not [Leu31,Pro34]NPY (10-100 nM), fully inhibited the binding of [125I]BH-NPY. These results indicate that NPY receptors of the Y2 subtype predominate in the dorsal vagal complex. Unilateral nodose ganglionectomy resulted in a partial loss of NPY binding sites in the commissural NTS, but not the area postrema, suggesting that a proportion of binding sites (Y2 subtype) are present on central vagal terminals. While all three rat strains appear to have the same relative proportions of NPY receptor subtypes in the brainstem, the relevance of the differential NPY gene expression in the arcuate nucleus regarding central cardiovascular control mechanisms and/or the pathogenesis of hypertension remains to be elucidated.

Visualization of beta-adrenoceptor binding sites on human inferior vagal ganglia and their axonal transport along the rat vagus nerve

DESIGN

Because of uncertainties regarding the complete antihypertensive mechanism of action of beta-adrenoceptor antagonists, the present study determined whether vagal afferent neurons of humans and rats possess beta-adrenoceptors. Such a location would provide an appropriate target for beta-blockers to modulate neurotransmission of barosensitive neurons, thereby affecting blood pressure. Therefore, in vitro receptor autoradiography of high-affinity beta-adrenoceptor binding sites was performed on slices of human and rat inferior vagal (nodose) ganglia with [125I]-pindolol.

METHODS

Slide-mounted sections of human and rat inferior vagal ganglia were incubated with [125I]-pindolol in the absence or presence of propranolol (10 mumol/l) to define non-specific binding, atenolol (10 mumol/l) to inhibit binding to beta 1-adrenoceptors, or ICI 118551 (3 nmol/l) to inhibit binding to beta 2-adrenoceptors. Unilateral vagal ligation was also performed in the rat to study whether beta-adrenoceptors are subject to axonal transport along the vagus nerve.

RESULTS

[125I]-pindolol bound with > 90% specific binding to sections both of human and of rat inferior vagal ganglia. Specific binding occurred over both neuronal perikarya and nerve fibres. In both species the beta 2-adrenoceptor subtype appeared to predominate, as defined by the differential ability of ICI 118551 (beta 2) and atenolol (beta 1) to inhibit the binding of [125I]-pindolol. Furthermore, unilateral vagal ligation in the rat caused an accumulation of specific binding adjacent to the ligature sites.

CONCLUSIONS

We conclude that human and rat vagal afferent (and efferent) neurons possess beta-adrenoceptors that potentially could explain the mechanism of action of beta-adrenoceptor antagonists in the therapy of hypertension.

5-Hydroxytryptamine3 receptor modulation of excitatory amino acid release in the rat nucleus tractus solitarius

In vivo microdialysis was employed to measure release of endogenous L-glutamate (GLU) and L-aspartate (ASP) in the medial nucleus tractus solitarius of urethane anaesthetised rats. Basal extracellular levels of these amino acids were stable following a 90 min equilibration period (6.3 +/- 0.24 and 3.4 +/- 0.6 pmol/20 microliters sample of GLU and ASP, respectively). Basal levels of endogenous extracellular GLU and ASP were increased over 2-fold and 3-fold, respectively, following local administration of the selective 5-hydroxytryptamine (5-HT3) receptor agonist phenylbiguanide (300 microM). Intracerebral administration of the selective 5-HT3 receptor antagonist ondansetron (30 microM) blocked the effect of phenylbiguanide on GLU release whilst the effect on ASP was variable and complex. These data suggest that 5-HT3 receptor activation in the rat nucleus tractus solitarius can affect excitatory amino acid neurotransmission in this region of the medulla oblongata.

Functional dopamine D2 receptors on rat vagal afferent neurones

1. In the present study in vitro electrophysiology and receptor autoradiography were used to determine whether rat vagal afferent neurones possess dopamine D2 receptors. 2. Dopamine (10-300 microM) elicited a temperature- and concentration-dependent depolarization of the rat isolated nodose ganglion preparation. When applied to the tissue 15 min prior to agonist, raclopride (10 microM), clozapine (10 microM) or a mixture of raclopride and clozapine (10 microM each) all produced a threefold parallel shift to the right of the dopamine concentration-response curve. In contrast, SCH 23390 (100 nM), phentolamine and propranolol (1 microM each) failed to antagonize the dopamine-mediated depolarization. 3. [125I]-NCQ 298 (0.5 nM), a D2 selective radioligand, bound topographically to sections of rat brainstem. Densitometric quantification of autoradiograms revealed 93.8 +/- 0.5% specific binding of this salicylamide radioligand, as determined by raclopride (10 microM, n = 10 animals). Binding was highest in the nucleus tractus solitarius (NTS), particularly the medial and gelatinous subnuclei. In addition, specific binding was also observed in the interpolar spinal trigeminal nucleus and the inferior olive. 4. Unilateral nodose ganglionectomy caused a 36.6 +/- 3.0% reduction in specific binding in the denervated NTS compared to the contralateral NTS. Furthermore, the loss of binding was confined to the dorsal aspect of the medial subnucleus of the NTS. Sham surgery had no effect on the binding of [125I]-NCQ 298 in rat brainstem. 5. The present data provide evidence for the presence of functionally relevant dopamine D2 receptors on both the soma and central terminals of rat vagal afferent neurones. In addition, the majority of D2 receptors in the rat NTS appear to be located postsynaptically with respect to vagal terminals, and are presumably located either on ascending glossopharyngeal terminals, descending terminals from higher brain regions or on neuronal cell bodies within the NTS.