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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.

The hypothalamic supraoptic nucleus (SON), as well as the paraventricular nucleus (PVN), contains cell bodies of the magnocellular vasopressinergic neurons that synthe size and release arginine vasopressin (AVP). Stimulation of the AVP neurons by individual neurotransmitters and neuromodulators can induce the release of AVP, eventual ly causing antidiuretic effects through an activation of renal AVP (V2) receptors (1,2). Electrophysiological and immunohistochemical examinations (3 5) indicate that the AVP neurons receive inputs from cholinergic and adrenergic neurons.
Using the microinjection technique, the cholinergic (muscarinic) and adrenergic (a and p-subtypes) receptors in both the SON and PVN are shown to be involved in the urine outflow (6 8). It is known that adenosine triphosphate (ATP) is co-localized with acetylcholine (ACh) and noradrenaline (NA) in cholinergic and adrener gic nerve terminals, respectively, and this purine is also co-released with these neurotransmitters (9,10). It is, thus, expected that ATP may act as a neurotransmitter or neuromodulator in the peripheral and central nervous sys tem (11)(12)(13). We previously found that ATP injected into the PVN induced antidiuretic effects through an activa tion of P2-purinoceptors (14). Therefore, such may also be the case in the SON, and ATP may act on the AVP neu rons through an activation of a specific receptor. ATP will then modulate cholinergic and adrenergic agonists induced antidiureses.
In the present studies, we measured the effects of ATP injection into the SON on the urine outflows, urine osmot ic pressure and the visceral functions. The effects of an tagonist pretreatment of the AVP receptor, purinoceptor and muscarinic receptor on these ATP actions were also examined, the objective being to determine the possible in volvement of these individual receptors on the effect of ATP. The effects of some purine compounds (ADP, AMP and adenosine) on urine outflows were also com pared with those of ATP to characterize the purinoceptor subtype involved in the effects of ATP.

Measurement of outflow and osmotic pressure of urine
The urine outflow was measured by Dicker's method, with some modifications (6,15). Male Wistar rats, weigh ing 280-320 g, were starved overnight for approx. 17 hr but allowed free access to water. The animals were loaded orally through a catheter with a volume of water equiva lent to 5% of their body weight. Then they were anesthe tized by oral administration of a volume of 12% ethanol, equivalent to 5% of their body weight. Cannulae were in serted into the trachea, bladder, and external jugular vein. The rats were fixed in a stereotaxic instrument (Taka hashi Co., Tokyo). The ethanol-anesthetized rats did not move during the experiments. Drops of urine, flowing from the urinary cannula, were counted by a photoelec tric drop counter (DCT 102; Unique Medical, Inc., Tokyo) and recorded as single pulses. Ethanol (3 % in Locke solution) was infused at a constant rate of 0.10 ml/min through the cannula in the jugular vein, in order to maintain a constant level of anesthesia and a constant rate of urine outflow. The Fiske Osmometer (Model G 62; Fiske Associates, Inc., Uxbridge, MA, USA) was used to measure urine osmotic pressure by the freezing point depression method. Urine, which was collected by a catheter in the bladder and then diluted to 10-fold by dis tilled water, was used (minimum volume: 200,ul) for meas uring osmotic pressure. The initial control osmotic pres sure was 278:1-34 mOsm. Vehicle alone, when injected into the SON, changed neither the urine outflow nor the urine osmotic pressure.

Injection of drugs
A stainless steel cannula (outer diameter: 200 pm) was unilaterally inserted stereotaxically into the SON (A, 6.28; L, 1.3; H, 8.8 mm) according to the atlas of Konig and Klippel (16). The position of the tip of the cannula within the SON was histochemically confirmed by locali zation in a group of magnocellular cells, positively stained by the method of Gomori (17). Injection of a 1-pl volume of the effective doses of antidiuretic agonists, such as oxo tremorine (0.4 nmol) or NA (80 nmol), at a distance of 1 mm outside the SON did not induce any significant effects on the urine outflow. Therefore, the spread of the drug to sites in the brain outside the SON by injection of a 1-pl volume of the drug was estimated to be less than ap prox. 1 mm. The pH of the solutions containing adenine nucleotides. and adenosine were adjusted to approx. 7.0 by adding NaOH. After the urine outflow had reached a constant rate of 0.078-0.133 ml/min, 1 pl of the drug so lution was injected through a microsyringe connected to the cannula. Then 2 pl of an artificial cerebrospinal fluid (C.S.F.: 128 mM NaCI, 3.0 mM KCI, 1.2 mM CaC12i 0.8 MM MgC12, 0.65 mM NaH2PO4 and 4.8 mM NaHCO3, pH 7.4) was infused at a rate of approx. 0.3 p1/min for 10 min (total of 3 p1). The outflow of urine was measured every 10 min before and after the injection of drugs, and it was expressed as a percentage of the control outflow. In the experiments to test the effect of pretreatment with theophylline or atropine, ATP, theophylline or atropine, and ATP were injected into the SON in turn. All the injec tions were performed after the urine outflow had re covered to the initial level.

Pretreatment with an A VP antagonist
The effect of pretreatment with d(CH2)5-D-Tyr(Et) VAVP (an AVP receptor antagonist) was tested in order to check the release of AVP. The effect of microinjection of ATP into the SON was first observed, and then the AVP antagonist (50 pg/kg) was applied intravenously through a cannula in the external jugular vein 40 min be fore the second injection of ATP. The urine outflows were not detectably changed after injection of the AVP antagonist. The inhibitory effects of the antagonist were estimated as described above for the inhibitory effects of theophylline or atropine.

Statistical analyses
Student's t-test was used for the statistical analysis of paired tests. Differences were considered significant at P < 0.05. The ED50 values and 95% confidence limit of the ED50 values for ATP were calculated from dose-response curves (Fig. 2), drawn by the least squares method. Figure 1 shows the effect of single injections of ATP (30-150 nmol) into the SON on the urine outflow. ATP decreased the urine outflow in a dose-dependent manner. The injection of ATP induced a decrease in urine out flow; the decrease reached the maximal at 20 min after in jection of ATP at any concentration of ATP. The effects of ATP injection on the urine outflow were reversible, and the initial level of urine outflow was produced at ap prox. 1 hr after the injection of lower concentrations of ATP (< 100 nmol). The injection of 150 nmol ATP caused a slower recovey of urine outflow, and the recov ery remained incomplete even at 100 min after the injec tion. Figure 2 shows the dose-response relationship of the peak value of urine outflow inhibited by a single injection of ATP into the SON. The relationship was linear, and the median effective dose (ED50 value) estimated from the relationship was 56 nmol (values ranged from 38 to 84 nmol, n = 3 8) nmol.

Effects of injection of A TP on the urine outflow
Effects of injection of A TP on the urine osmotic pressure Figure 3 shows the effects of injection of ATP (100 nmol) into the SON on both the urine osmotic pressure and the urine outflow measured at 20 and 60 min after the injection (n = 6). At 20 min after the injection of ATP, the osmotic pressure was increased to 210%, and the urine outflow was decreased to 34±7% of the control. At 60 min, when the urine outflow had recovered to close to the initial level (63 ± 11 %), osmotic pressure was nearly recovered to the initial control level (135±24%).
Effects of pretreatment with theophylline or atropine on the antidiuretic effect of A TP Theophylline (50 nmol, a P1-purinoceptor antagonist), injected into the SON significantly inhibited the anti diuretic effect of ATP (100 nmol) (Fig. 5). Atropine (300 nmol), a muscarinic receptor antagonist when inject ed into the SON, also inhibited the effect of ATP (100 nmol) (Fig. 6). Neither 50 nmol theophylline nor 300 nmol atropine alone changed the urine outflow. The urine outflows, expressed as a percentage of the control, at 10, 20 and 30 min after the injection of theophylline into the SON were 103 ± 5, 96 ± 7 and 93 ± 4, respectively (n = 3). The urine outflows at 10, 20 and 30 min after the injection of atropine into the SON were 83±19, 94±29 and 114±28010 of the control, respectively (n=5).   Effects of injection of various adenine nucleotides and adenosine Figure 7 shows the effects of microinjection of ADP, AMP and adenosine (100 nmol) into the SON on the urine outflow measured at 20-30 min after the injections. None of the three caused a significant change in the urine outflow.

Cardiovascular and respiratory effects of the injection of A TP
The effects of injection of ATP into the SON were ob served on blood pressure, heart rate, respiration rate and rectal temperature. The injection of ATP (100 nmol) into the SON had no detectable effect on these parameters. The blood pressure, heart rate and respiration rate were 112 -1:17 (116 ± 9) mmHg, 420 ± 24 (425 ± 13) /min and 87± 13 (81 ±9) /min at 20 min after injection of ATP, respectively, when the urine outflow was 37±10% of the initial urine outflow (100%) (each control value during the 10 min before the injection is indicated in paren theses).

DISCUSSION
The present experiments showed that injection of ATP into the SON caused a marked decrease in urine outflow with a concomitant increase in the urine osmotic pressure, in a concentration-dependent manner. These effects of ATP on urine outflow appeared slowly, and the time courses were comparable to those induced by cholinergic (6) or adrenergic agonists injected into either the SON or PVN (7,8) and also those for ATP injected into the PVN (14). The ATP-induced increase in urine osmotic pressure indicated that the decrease in urine outflow by ATP im plied an antidiuretic effect probably due to a release of AVP. No significant change was found in the cardiovascu lar and respiratory parameters following the injection of ATP into the SON. Following the injection of ATP into the SON, the urine osmotic pressure was increased, and the antidiuretic effect of ATP was completely blocked by an intravenous injection of d(CH2)5-D-Tyr(Et)VAVP, an AVP-receptor antagonist (50 pg/kg). The small distance of the diffusion of drugs microinjected into the SON (less than 1 mm) suggests that the action site of ATP is inside the SON. These results suggest that microinjected ATP ac tivates the magnocellular cells and then elicits a release of AVP from the neurohypophysis into the circulation, caus ing an antidiuretic effect through an activation of the renal AVP (V2) receptors.
The results suggest that the antidiuretic effect of ATP injected into the SON may be mediated through an activa tion of P1-purinoceptors since the antidiuretic effect of ATP (100 nmol) was antagonized by pretreatment with theophylline (50 nmol), a P1-purinoceptor antagonist (12). On the other hand, the absence of any effect by ADP, AMP or adenosine injected into the SON on the urine outflow suggests that the effects of ATP are mediat ed mainly through an activation of P2-purinoceptors (11,18,19). These results also contrasts with those observed in the PVN, in which microinjection of ATP causes a more potent antidiuretic effect than that of adenosine and is not antagonized by theophylline (14). The purinoceptor involved in the antidiuretic effect of ATP in the SON may not be a typical P1 or P2-subtype. Further studies using a P2-purinoceptor selective antagonist are needed to charac terize the subtype of purinoceptor in the SON. The antidiuretic effect of ATP was inhibited by the pre injection of atropine into the SON, suggesting that the effect may be mediated through an activation of muscarin ic receptors. We previously reported that oxotremorine in jected into the SON causes an antidiuretic effect, possibly through elevated AVP release (6). The presence of choline acetyltransferase and acetylcholinesterase (4,5) in the SON and the findings that iontophoretic application of ACh excites the vasopressinergic neurons of the SON (20,21) suggested that the cholinergic neurons may play im portant roles in the AVP release. It has been recently reported that ATP potentiated a release of neurotransmit ters such as ACh (22,23), NA (24) and glutamate (25). Therefore, it is speculated that ATP microinjected into the SON may activate purinoceptors on the presynaptic terminals of cholinergic neurons, and enhance the release of ACh, and thus elevate the release of AVP through an activation of muscarinic receptors.
In conclusion, ATP injected into the SON induced a potent antidiuretic effect. The results suggest that ATP stimulates a theophylline-sensitive purinoceptor, pro motes the release of AVP and then induces the antidiuret ic effect through a mediation of renal AVP (V2) receptors. It is also suggested that cholinergic neurons in the SON may participate in this ATP-induced antidiuretic effect.