Differential effects of electrical stimulation, blockade of neuronal amine uptake and activation of α2-adrenoceptors on the release of endogenous noradrenaline and 5-hydroxytryptamine from the isolated rat pineal gland

Abstract

Isolated rat pineal glands were incubated in vitro and the release of endogenous noradrenaline or 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) was determined by HPLC with electrochemical detection. In the absence of test drugs, the spontaneous outflow of noradrenaline was about 10 fmol/10 min and electrical stimulation (5 Hz, 1500 pulses) evoked the release of about 70 fmol noradrenaline. Nomifensine enhanced the spontaneous outflow of noradrenaline about threefold and the electrically evoked release of noradrenaline about sixfold. In the presence of nomifensine, the alpha 2-adrenoceptor antagonist yohimbine markedly increased the electrically evoked release of noradrenaline, whereas the alpha 1-adrenoceptor antagonist prazosin had no effect. Clonidine inhibited the electrically evoked release of noradrenaline by about 65%, and this was antagonized by yohimbine in a competitive manner. In the absence of drugs, the initial spontaneous outflow of 5-HT was (compared with noradrenaline) very high 64 pmol/10 min). It declined by 80% within 1 h of incubation in vitro. The outflow of 5-HIAA amounted initially to 38 pmol 10 min and declined by 40% within 1 h of incubation. Addition of L-tryptophan (10 mumol/l) after 1 h of incubation in vitro largely enhanced the outflow of 5-HT and 5-HIAA within 30 min of incubation (about ten- and fourfold, respectively). When L-tryptophan was present from the onset of incubation the initial outflow of 5-HT and 5-HIAA was only slightly elevated, but the decline was largely attenuated. Neither omission of calcium nor addition of nomifensine, clonidine or yohimbine significantly affected the spontaneous outflow of 5-HT or 5-HIAA.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for a Regulatory Role of Melatonin on Serotonin Release and Uptake in the Pineal Gland

“…After this time, 5-HT and 5-HIAA output showed a progressive decrease of approximately 5-8% per h. Similar biphasic secretory patterns of indoleamine secretion from perifused pineal glands were previously reported ( 10,34). Indoleamine secretory profiles obtained by incubating pineal glands removed from SCGx rats run in parallel with those taken from control pineal glands.…”

“…Recently it has been reported that the release of 5-HT from the pineal gland is calciumindependent and increases in presence of L-tryptophan, suggesting a preferential efflux of newly synthesized 5-HT (34). In our study, release of 5-HT is likely to occur in absence of de novo 5-HT synthesis, since the lack of tryptophan in the medium.…”

Sympathetic denervation and chronic serotonin uptake blockade by fluoxetine do not affect pineal gland 5-hydroxyindole acetic acid: evidence that oxidative deamination of pineal serotonin is a property of the pinealocyte

“…The interesting possibility that other mechanisms of 5HT deamination exist was recently suggested by Murphy et al (1990) who reported normal levels of urinary 5HIAA in two individuals with Norrie disease which includes a chromosomal deletion of the gene encoding both MAO enzymes. 5HT catabolism by the pinealocyte is consistent with two observations: 1) electrical stimulation with and without the amine uptake inhibitor nomifensine, which enhanced NE release from nerve terminals, did not alter 5HT and 5HIAA release in vitro (Racke et al, 1991); and 2) the high positive correlation of 5HIAA vs 5HT was further enhanced in individual pineal glands during the rapid increase in 5HT following acute light exposure at night (McNulty et al, 1986). If 5HT was shuttled from the pinealocyte and deaminated in a second cellular compartment during this short period (1-30 min), the correlation of 5HIAA vs 5HT would be expected to decrease rather than increase because of the time lag in making the 5HT accessible to the MAO enzyme.…”

“…First, significant levels of the oxidative metabolite 5-hydroxyindole acetic acid (5HIAA) were reported in the pineal gland following removal of the NE terminals by superior cervical sympathectomy (Vassilieff et al, 1982;McIntyre et al, 1985). Second, administration of the neuronal amine uptake blocker nomifensine enhanced release of NE, but did not significantly affect outflow of 5HIAA from the cultured pineal gland (Racke et al, 1991). Finally, the ratio of 5HIAA/5HT exhibited a very high correlation in individual glands during rapid metabolic changes (1-10 min) that occur following acute light exposure during darkness (McNulty et al, 1986) suggesting a single compartment for both substrate and enzyme.…”

The role of the intracellular and extracellular serotonin in the regulation of melatonin production in rat pinealocytes

“…Pineal 5-HT release has been found to increase in the presence of NE, which probably acts through a-adrenoceptor mechanisms [Benson et al, 1986;Aloyo and Walker, 1987, 19881. However, the role of the P-adrenergic receptors remains controversial [Aloyo and Walker, 1988;Racke et al, 1991;Olcese and Munker, 19941. Recent studies, indeed, indicate that pineal extracellular 5-HT may interact to modulate the P-adrenergically-induced NAT activity [Sugden, 1990;Olcese and Munker, 19941, sug-gesting that 5-HT may play an autocrine role in the pineal gland.…”