Forskolin modulates acetylcholine release in the hippocampus independently of adenylate cyclase activation

[3H]Acetylcholine release from slices of rabbit hippocampus was elicited by electrical field stimulation (360 pulses/3 Hz). Both forskolin, commonly used as a specific activator of adenylate cyclase, as well as 1,9-dideoxy-forskolin, which fails to activate adenylate cyclase, increased the evoked transmitter release in an almost identical manner. In addition, the phosphodiesterase inhibitor, rolipram, and the membrane-permeable analogue of cAMP, 8-Br-cAMP, did not influence acetylcholine release. These data show that forskolin is not specific to adenylate cyclase and that the increase in acetylcholine release in the rabbit hippocampus occurs through a mechanism other than activation of adenylate cyclase.

Mechanism of renal effects of intracerebroventricular histamine in rabbits

Histamine, when given intracerebroventricularly (i.c.v.), has been reported to produce antidiuresis in the rabbit. In this study it was attempted to elucidate the mechanism involved in the effect. Histamine (H), 100 micrograms/kg i.c.v., produced antidiuresis with decreases in renal plasma flow and glomerular filtration rate in urethane-anesthetized rabbits. With larger doses, a tendency towards increased electrolyte excretion was noted in spite of decreased filtration. In the denervated kidney, marked diuresis and natriuresis were observed following i.c.v. H, whereas the contralateral innervated kidney responded with typical antidiuresis. Reserpinized rabbits also responded with marked natriuresis to i.c.v. H. Diphenhydramine (D), 250 micrograms/kg i.c.v., increased urine flow rate, sodium and potassium excretion, along with increase in renal perfusion. With 750 micrograms/kg i.c.v., marked natriuresis was observed in spite of decreased filtration. When H was given after D (250 micrograms/kg) the antidiuresis was completely abolished, and diuresis became more prominent. Cimetidine, 250 micrograms/kg i.c.v., elicited antidiuresis with decreases in renal hemodynamics, the pretreatment with cimetidine did not influence the antidiuresis by H and no natriuresis was noted. The present study suggests that histamine, given i.c.v., influences renal function in dual ways, i.e., antidiuresis by increasing the sympathetic tone to the kidney and diuresis due to some humoral natriuretic factor, the latter becoming apparent only when the former influence has been removed, and further suggests that H1-receptors might be involved in the nerve-mediated antidiuresis, whereas H2-receptors might mediate the humorally induced natriuresis and diuresis.

Influence of intracerebroventricular phenoxybenzamine on the renal action of intracerebroventricular morphine

Renal function is under regulatory influence of the central nervous system, either through mediation of humoral agents or by way of renal nerves, with the sympathetic nerve playing the most important role. As it was recently shown that morphine and enkephalin, when administered intracerebroventricularly (icv), affect renal function in the rabbit, indicating that the opiate receptors are also involved in the central regulation of renal function, it was attempted to clarify whether the central adrenergic system participates in the center-mediated renal action of morphine, employing phenoxybenzamine (PBZ). Morphine, 10 micrograms/kg icv, produced profound and long-lasting antidiuresis and antinatriuresis. PBZ, 250 micrograms/kg icv, elicited marked diuresis with natriuresis and kaluresis, along with significant increases of osmolar clearance and free-water reabsorption, and these effects were not related to a transient increase in glomerular filtration. When PBZ was given 10 min after morphine, the morphine antidiuresis immediately disappeared and the renal action of PBZ unfolded itself undiminished. Also, when morphine was given after PBZ, the PBZ-diuresis developed fully, unaffected by morphine. These observations indicate that the central adrenergic system is involved in the regulation of renal function through central opiate receptors.