Pineal hyperactivity in spontaneously hypertensive rats: muscarinic regulation of indole metabolism

Ultrastructural and hormonal changes in the pineal-testicular axis following arecoline administration in rats

Arecoline is an alkaloid of betel nut of Areca catechu. Betel nut is chewed by millions of people in the world and it causes oral and hepatic cancers in human. It has therapeutic value for the treatment of Alzheimer and schizophrenia. Arecoline has immunosuppressive, mutagenic and genotoxic effects in laboratory animals. It also affects endocrine functions. The objective of this study was to investigate the effects of arecoline on pineal-testicular axis in rats. Since pineal activity is different between day and night, the current study is undertaken in both the photophase and scotophase. The findings were evaluated by ultrastructural and hormonal studies of pineal and testicular Leydig cells, with quantitations of fructose and sialic acid of sex accessories. Arecoline treatment (10 mg/kg body weight daily for 10 days) caused suppression of pineal activity at ultrastructural level by showing dilatation of the cisternae of the rough endoplasmic reticulum (RER), large autophagosome-like bodies with swollen mitochondrial cristae, numerous lysosomes, degenerated synaptic ribbons and reduced number of synaptic-like microvesicles. Moreover, pineal and serum N-acetylserotonin and melatonin levels were decreased with increased serotonin levels in both the gland and serum. In contrast, testicular Leydig cell activity was stimulated with abundance of smooth endoplasmic reticulum (SER), electron-dense core vesicles and vacuolated secretory vesicles, and increased testosterone level in the arecoline recipients. Consequently, the testosterone target, like prostate, was ultrastructurally stimulated with abundance of RER and accumulation of secretory vesicles. Fructose and sialic acid concentrations were also significantly increased respectively in the coagulating gland and seminal vesicle. These results were more significant in the scotophase than the photophase. The findings suggest that arecoline inhibits pineal activity, but stimulates testicular function (testosterone level) and its target organs presumably via muscarinic cholinergic receptor in rats.

Cholinergic innervation and function in the mammalian pineal gland

Besides the noradrenergic sympathetic system originating from the superior cervical ganglion, a cholinergic innervation of the mammalian pineal gland has been studied over the past three decades. In 1961, it was shown that lesion of the parasympathetic greater superficial petrosal nerve of the monkey resulted in degeneration of nerve fibers in the pineal gland. This was supported by ultrastructural studies of nerve terminals within the pineal gland, demonstrating the presence of cholinergic terminals containing small clear transmitter vesicles. Biochemical studies further showed the presence of the enzyme acetylcholinesterase in several mammalian species. During the last decade, several advanced and more elaborate technologies have been developed, allowing pinealogists to establish the presence of cholinergic fibers and their receptors. Thus, choline acetyltransferase was shown in bovine pineal by immunohistochemistry. Muscarinic and nicotinic receptors were identified, characterized, and localized. Gene expression of receptors was visualized, and the receptor-mediated effector systems and functions were elucidated. Taken together, the present data suggest the presence of a cholinergic innervation of the mammalian pineal gland originating in peripheral parasympathetic ganglia. However, some of the neuronal projections to the pineal gland with origin in the brain (the central innervation) might also be cholinergic. The cholinergic nerve fibers enter the gland, where they are located both in the perivascular spaces and between the pinealocytes. Some of the terminals make synapses on pinealocytes or intrapineal neurons. The released acetylcholine from the terminals interacts with the receptors, then alters the cascade of receptor-mediated events, which results in decreased N-acetyltransferase enzyme activity, thus leading to decreased melatonin synthesis. This counterbalance mechanism between the sympathetic noradrenergic and the cholinergic systems maintains the homeostasis of pineal functions.

Calcium responses of isolated, immunocytochemically identified rat pinealocytes to noradrenergic, cholinergic and vasopressinergic stimulations

Calcium responses of isolated rat pineal cells to noradrenergic, cholinergic and vasopressinergic stimulations were recorded by use of the fura-2 technique and an image analysis system. Subsequently the recorded cells were identified as pinealocytes by immunocytochemical demonstration of S-antigen, a pinealocyte-specific marker. S-antigen immunoreactive pinealocytes were shown to respond to norepinephrine stimulation with an elevation of the intracellular free calcium concentration ([Ca2+]i). This response was dose-dependent and consisted of a rapid increase in [Ca2+]i (primary phase) followed by a decrease to an elevated plateau well above the basal level (secondary phase). The plateau persisted for at least 1 h when cells were constantly exposed to norepinephrine and dropped to basal level upon removal of the stimulus. Analysis of the calcium responses of cells treated with caffeine or thapsigargin suggested that the primary phase reflects mobilization of calcium from inositol 1,4,5-trisphosphate-sensitive intracellular calcium stores. Depletion of these calcium stores was a decisive and sufficient prerequisite to evoke the secondary phase which was apparently elicited by calcium influx. These data suggest that a capacitative calcium entry is involved in pineal calcium signalling. Acetylcholine induced an increase in [Ca2+]i in rat pinealocytes. Experiments with different cholinergic agonists and antagonists provided evidence that the acetylcholine-induced calcium response was mediated via nicotinic acetylcholine receptors. Stimulation of isolated rat pineal cells with arginine-vasopressin caused a rise in [Ca2+]i in approx. 5% of the cells. However, these cells remained unidentified because they contained neither immunoreactive S-antigen nor immunoreactive glial fibrillary acidic protein, a marker for interstitial (glial) cells of the rat pineal organ. Taken together, the results underline the pivotal role of norepinephrine for the regulation of pineal signal transduction, but they also support the notion that other neurotransmitters and neuropeptides are involved in the modulation of pineal calcium signalling.

Cholinergic signaling in the rat pineal gland

1. Innervation of the mammalian pineal gland is mainly sympathetic. Pineal synthesis of melatonin and its levels in the circulation are thought to be under strict adrenergic control of serotonin N-acetyltransferase (NAT). In addition, several putative pineal neurotransmitters modulate melatonin synthesis and secretion. 2. In this review, we summarize what is currently known on the pineal cholinergic system. Cholinergic signaling in the rat pineal gland is suggested based on the localization of choline acetyltransferase (ChAT) and acetylcholinesterase (AChE), as well as muscarinic and nicotinic ACh binding sites in the gland. 3. A functional role of ACh may be regulation of pineal synaptic ribbon numbers and modulation of melatonin secretion, events possibly mediated by phosphoinositide (PI) hydrolysis and activation of protein kinase C via muscarinic ACh receptors (mAChRs). 4. We also present previously unpublished data obtained using primary cultures of rat pinealocytes in an attempt to get more direct information on the effects of cholinergic stimulus on pinealocyte melatonin secretion. These studies revealed that the cholinergic effects on melatonin release are restricted mainly to intact pineal glands since they were not readily detected in primary pinealocyte cultures.