Increase in ACh release from the rat brain cortex slices by 3',5'-cyclic adenosine monophosphate

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.

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

Effects of catecholamines microinjected into the paraventricular nucleus in the hypothalamus on urine outflow in a rat which was loaded with water and anesthetized with ethanol were studied. L-Norepinephrine, L-epinephrine and L-isoproterenol induced potent antidiuresis with similar time courses to each other. The ED50 values for L-norepinephrine, L-epinephrine and L-isoproterenol were approximately 5, 10 and 5 nmol, respectively. The D-isomer of isoproterenol demonstrated no significant antidiuretic activity. The effect of L-norepinephrine was inhibited strongly by premicroinjection of alpha- and beta-adrenoceptor antagonists. The effect of DL-isoproterenol was inhibited strongly by beta-adrenoceptor antagonists, but not affected by alpha-adrenoceptor antagonist. Premicroinjection of a muscarinic antagonist, atropine, partially inhibited anti-diuretic effects induced by L-norepinephrine and DL-isoproterenol. Visceral functions other than urine outflow such as mean blood pressure, respiration rate, heart rate and rectal temperature were not significantly altered when the urine outflow decreased down to 20-30% of the control by microinjection of L-norepinephrine and DL-isoproterenol. The results demonstrated that stimulation of alpha- and beta-adrenoceptor in the paraventricular nucleus induced potent antidiuretic effects, partial inhibition of which by atropine suggested a possible presynaptic facilitation of the release of ACh by the stimulation of the adrenoceptors.

Effects of adenylate cyclase inhibitors, phosphodiesterase inhibitors, and dibutyryl cyclic AMP on spontaneous and various stimuli-induced acetylcholine release from guinea pig ileum myenteric plexus

In order to examine a possible contribution of cyclic AMP to acetylcholine (ACh) release from guinea pig ileum myenteric plexus, effects of adenylate cyclase inhibitors, phosphodiesterase (PDE) inhibitors and dibutyryl cyclic AMP on the spontaneous and the various stimuli-induced ACh release were investigated. A PDE inhibitor, theophylline (1 mM) increased the ACh release induced by nicotine (6.16 microM) significantly. Another PDE inhibitor, 3-isobutyl-1-methylxanthine (IBMX, 1 mM) and dibutyryl cyclic AMP (4 mM) had no effect. The adenylate cyclase inhibitors dithiobisnitrobenzoic acid (DTNB, 1 mM) and alloxan (4 mM) both decreased the nicotine-induced ACh release remarkably. PDE inhibitors increased and adenylate cyclase inhibitors decreased the high-K+-induced ACh release. Dibutyryl cyclic AMP brought about a slight but significant increase of the high-K+-induced ACh release. All the drugs failed to alter the ACh release induced by electrical field stimulation (EFS) at 10 Hz. Effects of all drugs except dibutyryl cyclic AMP on the spontaneous ACh release were the same as those on the nicotine-induced one. Dibutyryl cyclic AMP decreased it significantly. These results suggest that the cyclic AMP system is involved in the spontaneous, the nicotine-induced and the high-K+-induced ACh release and that the EFS-induced ACh release is independent of cyclic AMP.

Inhibitory effect of dibutyryl cyclic GMP on potassium-stimulated 45Ca uptake by synaptosomes from rat brain

The effects of dibutyrl cyclic GMP (db-cGMP) and dibutyryl cyclic AMP (db-cAMP) on potassium-stimulated 45Ca uptake by the P2 fraction of Gray and Whittaker were investigated with the following results. (1) db-cGMP inhibited the initial rate of potassium-stimulated 45Ca-uptake in a dose-dependent manner in 0.1 mM Ca0 medium, but had no effect on the uptake in low K+ medium. In 0.1 mM Ca0 medium, the concentration of db-cGMP causing 50% inhibition was about 3 mM. db-cAMP (5 mM) had no effect on the uptake. (2) db-cGMP-inhibited potassium-stimulated 45Ca uptake in 1 mM Ca0 medium, though less than in 0.1 mM Ca0 medium. (3) db-cGMP inhibited potassium-stimulated 45Ca uptake by synaptosomes (pinched-off nerve terminals) more than the uptakes by other subfractions of P2 fraction. It is suggested from the results that cGMP inhibits the Ca influx resulting from depolarization of the nerve endings in situ.

Effect of cyclic nucleotide derivatives on the release of ACh from cortical slices of the rat brain

Atropine (5 nM to 5 muM), but not D-tubocurarine or hexamethonium, enhanced the release of acetylcholine (ACh) from rat brain cortical slices exposed to high K+. The enhancement was dose-dependent, and it could be partially antagonized by oxotremorine (50-500 muM) or by dibutyryl or 8-bromo-cyclic GMP (1 mM), but not by tetrodotoxin (200 nM) or by dibutyryl cyclic AMP (1 mM). The effects of oxotremorine and the cyclic GMP derivatives were not due to diminished ACh synthesis, since these compounds did not influence the reduction of tissue ACh resulting from treatment with K+ and atropine. These results suggest that cyclic GMP may mediate the regulation of ACh release by presynaptic muscarinic receptors.