Antidiuretic effects of alpha- and beta-adrenoceptor agonists microinjected into the hypothalamic paraventricular nucleus in a water-loaded and ethanol-anesthetized rat

Clonidine antagonizes pressor effect of N-methyl-D-aspartate in the rostral ventrolateral medulla of rats

The purpose of the present study was to investigate the modulatory actions of adrenoreceptor agonists on N-methyl-D-aspartate (NMDA)-induced pressor effect in rostral ventrolateral medulla (RVLM). These drugs were locally applied into RVLM of urethane-anesthetized Sprague-Dawley rats through multibarrel pipettes. Microinjection of NMDA increased the arterial pressure, an effect which was abolished by pretreatment with clonidine, whereas neither the beta-adrenergic agonist isoproterenol nor the alpha 1-adrenergic agonist phenylephrine did alter this pressor response. Previous experiments demonstrated that clonidine binds to noradrenergic alpha 2 and imidazoline receptors in the RVLM. Norepinephrine, which has high affinity for the alpha 2 receptor and low affinity to the imidazoline receptor, partially antagonized NMDA-induced hypertension. On the other hand, administration of selective imidazoline receptor antagonist idazoxan partially reversed clonidine-mediated antagonism of NMDA. Taken together, these results suggest that clonidine may modulate the excitatory amino acid-induced pressor response through noradrenergic alpha 2 and imidazoline receptors in the RVLM.

Peptidase inhibitor-induced antidiuresis mediated through angiotensin and opioid receptors in the rat hypothalamus

We examined the effects on urine outflow rate after microinjections of thiorphan (a carboxypeptidase inhibitor) and bestatin (an aminopeptidase inhibitor) into the hypothalamic paraventricular (PVN) and supraoptic (SON) nuclei of anesthetized hydrated rats to determine the possible role of neuropeptides in the regulation of urine production. After individual microinjection of the peptidase inhibitors into the nuclei, only thiorphan at 100 nmol administered into the PVN significantly decreased the urine outflow rate. Two consecutive microinjections of the peptidase inhibitors at 100 nmol each into the nuclei induced potent antidiuresis. These effects after microinjections of the peptidase inhibitors into the PVN and SON were diminished by pretreatment with [Sar1,Ile8]angiotensin (ANG) II (an ANG II receptor antagonist) and naloxone (an opioid receptor antagonist) in the PVN and with [Sar1,Ile8]ANG II in the SON, respectively. A vasopressin (AVP) receptor antagonist, d(CH2)5-D-Tyr(Et)VAVP (i.v.), completely blocked the antidiuresis by microinjections of the peptidase inhibitors into both the nuclei. Urinary osmotic pressure was significantly increased by consecutive microinjections of the peptidase inhibitors into the PVN and SON. These results suggest that endogenously-released ANG II and opioid peptides in the PVN and ANG II in the SON regulate urine production mediated through increased AVP release.

Adrenergic neural connections between the bilateral supraoptic nuclei of the rat hypothalamus

Our previous study has demonstrated that unilateral microinjection of norepinephrine (NE) into the right supraoptic nucleus (SON) of anesthetized hydrated rats elicited dose-dependent decreases in the urine outflow rate. This was antagonized by pretreatment with phenoxybenzamine (an alpha-antagonist) and timolol (a beta-antagonist) in the same SON. In the present study, we examined the effects of NE, microinjected into the right, left and bilateral SON, on the urine outflow rate in order to investigate neural connections between the bilateral SON. NE administered by those three routes dose-dependently decreased the urine outflow rate. The order for the antidiuretic potency was as follows: the effect elicited by the intrabilateral-SON microinjection > the intra-left-SON microinjection = the intra-right-SON microinjection. The antidiuresis of NE microinjected into the right SON was inhibited by an electrolytic left-SON lesion and by pretreatment with phenoxybenzamine (20 nmol) and timolol (100 nmol), but not by atropine (300 nmol) in the left SON. These findings suggest adrenergic neural connections from the right to left SON, contributing to the regulation of urine production. Furthermore, there is a possibility that stimulation of endogenously-released NE in the bilateral SON is amplified through these neurons and elicits more potent effects than those produced in either the right or left nucleus.

Antidiuretic effects of ATP induced by microinjection into the hypothalamic supraoptic nucleus in water-loaded and ethanol-anesthetized rats

The effects of microinjection of purinoceptor agonists into the hypothalamic supraoptic nucleus (SON) on urination were examined in water-loaded and ethanol-anesthetized rats. Adenosine triphosphate (ATP), but neither adenosine diphosphate (ADP), adenosine monophosphate (AMP) nor adenosine, concentration-dependently decreased the urine outflow with concomitant increase in the urine osmotic pressure. The ED50 value for ATP was approx. 60 nmol. The antidiuretic effect of ATP was blocked either by prior injection of theophylline (an antagonist of the P1-type purinoceptor) or by intravenous administration d(CH2)5-D-Tyr(Et)-valine-arginine-vasopressin (VAVP). These results suggest that ATP injected into the SON has antidiuretic effects due to release of AVP through an activation of theophylline-sensitive purinoceptors.

Microinjections of angiotensin II into the supraoptic and paraventricular nuclei produce potent antidiureses by vasopressin release mediated through adrenergic and angiotensin receptors

We investigated the effects of angiotensin II (Ang II), microinjected into the supraoptic (SON) and paraventricular (PVN) nuclei of rats, on the urine outflow rate and underlying mechanisms. Ang II produced antidiuretic effects in a dose-dependent manner with ED50 values of 0.1 and 0.05 nmol in the SON and PVN, respectively. [Sar1, Ile8]Ang II at 0.1 nmol diminished the Ang II (0.5 nmol)-induced antidiureses in the SON more markedly than in the PVN. A high dose of [Sar1,Ile8]Ang II, 1 nmol, completely inhibited the effects in both the nuclei. In addition, the Ang II (1 nmol)-induced antidiuretic effects were partially inhibited by phenoxybenzamine (80 nmol) in the SON and by phenoxybenzamine, timolol (100 nmol) and propranolol (100 nmol) in the PVN. The microinjection of Ang II (1 nmol) into both the nuclei, after pretreatment with a vasopressin V1V2-antagonist, d(CH2)5-D-Tyr(Et)VAVP (i.v.) significantly increased the urine outflow rate. These findings suggest that 1) Two mechanisms account for the Ang II receptor-mediated antidiureses resulting from an increase in vasopressin release: direct stimulation on vasopressin-containing neurons and indirect stimulation on them through alpha-adrenoceptors in the SON and alpha- and beta-adrenoceptors in the PVN; 2) The Ang II-induced antidiuretic effect in the SON is slightly less potent than that in the PVN; and 3) Ang II receptors in the nuclei may possibly produce the diureses through mechanisms that are not presently understood.

Effects of beta 1- and beta 2-adrenoceptor agonists applied into the hypothalamic paraventricular nuclei of spontaneously hypertensive rats on urine production

We investigated effects of beta-adrenoceptor agonists (beta 1-selective: T-1583 and dobutamine, beta 2-selective: fenoterol, non-selective: isoproterenol) on urine outflow rate, blood pressure, heart rate, respiratory rate and rectal temperature. The drugs were applied into the paraventricular nuclei (PVN) of spontaneously hypertensive (SHR), Wistar-Kyoto (WKY) and Wistar rats. Fenoterol and isoproterenol markedly decreased the urine outflow rate, compared with T-1583 and dobutamine in the rats. There was no marked difference among the three strains in responsiveness to fenoterol and isoproterenol. The antidiuretic effects of fenoterol were inhibited by a beta 2-selective antagonist, butoxamine, more markedly than a beta 1-selective antagonist, atenolol, in SHR; and the inhibitory effects of these drugs were partial in WKY. In Wistar rats, the effect of fenoterol was inhibited by a non-selective beta-antagonist, timolol, but not by atenolol or butoxamine. A vasopressin antagonist (i.v.) did not diminish the antidiuretic effect of fenoterol. Fenoterol reduced the blood pressure in SHR and WKY, but not in Wistar rats. It was suggested that there were predominantly beta 2-adrenoceptors mediating antidiuresis in SHR. In WKY and Wistar rats, however, the beta-adrenoceptor subtypes mediating antidiuresis have yet to be determined. The ability of beta-adrenoceptor agonists to decrease urine outflow rates in SHR was not altered as compared to that in the control rats. beta-Adrenoceptor-mediated antidiuresis was not due to vasopressin release.

Microinjection of dynorphin into the supraoptic and paraventricular nuclei produces antidiuretic effects through vasopressin release

The mechanisms for the antidiuretic effects of dynorphin (DYN), an endogenous kappa-agonist, microinjected into the hypothalamic supraoptic (SON) and paraventricular (PVN) nuclei were investigated. DYN decreased the urine outflow rate dose-dependently from 5 to 20 nmol in the SON and PVN, and it increased vasopressin release. Microinjection of des-Tyr-DYN (a non-opioid peptide) into the SON produced antidiuretic effects with similar potency to that of the DYN-induced effects. However, in the PVN, the effects of des-Tyr-DYN were very markedly weaker than those of DYN. The DYN-induced antidiureses in the SON were partially inhibited by phenoxybenzamine, timolol and atropine, but not by naloxone. Those in the PVN were partially inhibited by naloxone, timolol and atropine, but not by phenoxybenzamine. Synthetic specific kappa-agonists, U50, 488H and Tyr-Gly-Gly-Phe-Leu-Arg-Arg-Ile-Arg-Pro- Arg-Leu-Arg-Gly 5-aminopentylamide (DAKLI), microinjected into the PVN also produced antidiuretic effects in a dose-dependent manner. The order of antidiuretic potency was DAKLI > DYN > U50,488H, which was the same as that of kappa-receptor binding affinity. The DAKLI-induced antidiureses in the PVN were not inhibited by naloxone. These results suggested that DYN caused antidiureses by vasopressin release, through adrenergic and cholinergic mechanisms in the SON and PVN. Only the DYN-induced effects in the PVN were mediated, at least partially, through opioid receptors, perhaps the kappa-subtype.

Effects of D-Ala2-D-Leu5-enkephalin, microinjected into the supraoptic and paraventricular nuclei, on urine outflow rate

The effects of D-Ala2-D-Leu5-enkephalin (DADL, a delta-opioid agonist), microinjected directly into the hypothalamic supraoptic (SON) and paraventricular (PVN) nuclei, on urine outflow rate, urinary osmotic pressure, blood pressure, heart rate, respiratory rate and rectal temperature were investigated in water-loaded and ethanol-anesthetized rats. The microinjection of DADL into both the nuclei decreased urine outflow rate in a dose-dependent manner with an increase in urinary osmotic pressure, but did not change the other recorded parameters. The DADL-induced antidiuretic effect in the SON was inhibited by naloxone, but not by atropine, phenoxybenzamine, timolol nor a vasopressin antagonist, d(CH2)5-D-Tyr(Et)VAVP. The effect in the PVN was inhibited by naloxone, atropine, timolol and d(CH2)5-D-Tyr(Et)VAVP, but not by phenoxybenzamine. These results suggest that DADL causes antidiuretic effects mediated through opioid receptors in both the SON and PVN, and the underlying mechanisms are different between them. Involvement of delta-opioid receptors in the DADL-induced antidiureses was discussed.

Alpha-1 adrenoceptor subtypes in canine aorta

The present study was designed to demonstrate the existence in canine aorta of alpha 1-adrenoceptor subtypes, alpha 1High and alpha 1Low, that have different binding affinities for 3H-prazosin and to assess the binding affinity of several drugs for each subtype by a displacement experiment. A radioligand binding assay with 3H-prazosin revealed the presence of two alpha 1-adrenoceptor subtypes in the canine aorta. One of them which has a high affinity for prazosin was designated as alpha 1High (Kd: 12.40 pM, Bmax: 21.88 fmol/mg protein), and the other type was designated as alpha 1Low (Kd: 506.03 pM, Bmax: 88.22 fmol/mg protein). The pKi values of several drugs for each subtype were determined, and all drugs used in the present study, except for benoxathian and chlorethylclonidine, showed significant differences between the pKi values for alpha 1High and those for alpha 1Low. Although it is difficult to characterize each alpha 1High and alpha 1Low into alpha 1A or alpha 1B by only the displacement potency, one structural characteristic to distinguish between alpha 1High and alpha 1Low could be evaluated.

Antidiuretic effects of purinoceptor agonists injected into the hypothalamic paraventricular nucleus of water-loaded, ethanol-anesthetized rats

The effects of injection of various purinoceptor agonists into the hypothalamic paraventricular nucleus in water-loaded and ethanol-anesthetized rats were investigated. Adenosine triphosphate (ATP), beta,gamma-methyleneadenosine 5'-triphosphate (AMP-PCP) and beta,gamma-imidoadenosine 5'-triphosphate (AMP-PNP) potently decreased the outflow of urine in a time- and dose-dependent manner. The ED50 values were approx 70 and 37 nmol for ATP and AMP-PCP, respectively. Adenosine diphosphate (ADP), AMP and adenosine reduced the outflow of urine much less than ATP. Adenosine triphosphate induced concomitant increases in the osmotic pressure of the urine and in the level of arginine-vasopressin (AVP) in plasma. The antidiuretic effect of ATP was blocked by prior injection of quinidine (a P2-purinoceptor antagonist) into the paraventricular nucleus, but not by the prior injection of theophylline (a P1-purinoceptor antagonist). The effect of ATP was also blocked by intravenous injection of an AVP(V1V2)-receptor antagonist, d(CH2)5-D-Tyr(Et)VAVP. The results suggest that ATP injected into the paraventricular nucleus may stimulate a purinoceptor, releasing AVP and inducing the antidiuretic effect through renal AVP(V2) receptors.

Mapping study of noradrenergic stimulation of vasopressin release

The precise role of hypothalamic norepinephrine (NE) in the control of vasopressin (AVP) release has remained unclear, due to reports of both inhibitory and excitatory effects of NE and only a few studies with direct hypothalamic manipulations. The present study utilized a chronically implanted swivel brain cannula to investigate, in undisturbed and freely behaving rats, the impact of acute hypothalamic infusions of monoamines on circulating AVP levels. The first study examined and compared the responsiveness of six hypothalamic sites to NE infusion through the swivel cannula. Results indicated that the excitatory effect of central noradrenergic stimulation on serum AVP is highly site specific, localized to the paraventricular (PVN) and supraoptic (SON) nuclei. These two nuclei appeared to be equally responsive to NE infusion, yielding a threefold rise in serum AVP over baseline levels. In contrast, NE in the dorsomedial nucleus produced a significantly smaller increase in AVP, and no response was observed in the ventromedial nucleus, posterior hypothalamus, or perifornical lateral hypothalamus. Further tests conducted in the PVN showed this nucleus to respond in a dose-dependent manner to NE infusion. In contrast, under similar test conditions, dopamine caused only a small increase in AVP at a relatively high dose, while a PVN injection of serotonin produced no response. These results support the existence of an excitatory noradrenergic system controlling AVP release and specifically demonstrate that this function of NE is localized to the PVN and SON, in contrast to other hypothalamic areas, and is mimicked to some extent by dopamine but not by serotonin.

Antidiuretic effects of methionine-enkephalin and 2-D-alanine-5-methionine-enkephalinamide microinjected into the hypothalamic supraoptic and paraventricular nuclei in a water-loaded and ethanol-anesthetized rat

Effects of methionine-enkephalin (ME) and 2-D-alanine-5-methionine-enkephalinamide (DAMEA) microinjected into the hypothalamic supraoptic (SON) and paraventricular (PVN) nuclei, which contain neurons synthesizing and releasing antidiuretic hormone, upon the outflow and the osmotic pressure of urine and the other visceral functions were studied in a rat which was loaded with water and anesthetized with ethanol. These opioid peptides when microinjected into the SON or PVN induced potent antidiuretic effects in dose-dependent and time-dependent manners with no significant effects on the other visceral functions. The approx. ED50 values for DAMEA were 1.3 (in the SON) and 0.7 (in the PVN) nmol, and the values for ME were 110 (in the SON) and 60 (in the PVN) nmol. The antidiuretic effects showed slow onset and long duration, with a minimal urine outflow at approx. 0.5 hr after microinjection and an approx. 2 hr-duration. The effects induced by the opioid peptides were inhibited by pretreatment with naloxone or atropine, without effects of pretreatment with alpha- or beta-adrenoceptor antagonists, suggesting that the antidiuretic effects were mediated through an opioid receptor having low sensitivity to naloxone and also possibly mediated through a muscarinic receptor which was stimulated probably by the ACh released by the opioid peptides.

Antidiuretic effects of dibutyryl-cyclic AMP microinjected into the hypothalamic supraoptic nucleus in water-loaded and ethanol-anesthetized rats

Effects of dibutyryl-cyclic AMP (db-cAMP) and cyclic AMP (cAMP) microinjected into the hypothalamic supraoptic nucleus (SON) which contains the neurons synthesizing and releasing antidiuretic hormone upon the outflow and the osmotic pressure of urine and the other visceral functions were investigated in water-loaded rats anesthetized with ethanol. When microinjected into the SON the dibutyryl analog of cAMP induced dose-dependent antidiuretic effects without significant effects on any other visceral functions. Dibutyryl-cAMP was much more effective than cAMP; The ED50 value for db-cAMP was approx. 200 nmol versus more than 500 nmol for cAMP. The time course of the antidiuretic effects was relatively slow with minimal urine outflow appearing only after more than 1/2 hour post-injection. The effects induced by db-cAMP demonstrated tachyphylaxis and were partially inhibited by pretreatment with atropine or theophylline, which suggests that the antidiuretic effects were mediated through muscarinic and adenosine receptors present in the nucleus.

Antidiuretic effects of dibutyryl-cyclic AMP microinjected into the hypothalamic paraventricular nucleus in a water-loaded and ethanol-anesthetized rat

Effects of dibutyryl-cyclic AMP (db-cAMP) and cyclic AMP (cAMP) when microinjected into the hypothalamic paraventricular nucleus (PVN) in a water-loaded and ethanol-anesthetized rat on the rate of urine outflow, urine osmotic pressure and other visceral functions were investigated. The microinjection of db-cAMP decreased the rate of urine outflow with concomitant increase in the urine osmotic pressure, but did not change mean blood pressure, heart rate, respiration rate and rectal temperature. The antidiuretic effect of db-cAMP was more potent than the effect of cAMP, the median effective doses (ED50) being approx. 40 nmol for db-cAMP and more than 300 nmol for cAMP, respectively. The time-courses for the antidiuretic effects and for the increase in the urine osmotic pressure showed a similar pattern, with the maximal effect at approx. 30 to 40 min and the duration of approx. one hour or longer. The effect of db-cAMP was potentiated by pretreatment with methylxanthines and inhibited by pretreatment with atropine. A second microinjection of db-cAMP induced a less potent antidiuretic effect than the first microinjection (tachyphylaxis). The results indicated the antidiuretic effects of microinjection of db-cAMP and cAMP into the PVN, and a possible mechanism for this was discussed.