Pineal N-acetyltransferase activity: effect of sympathetic stimulation

Neuromodulation of the Pineal Gland via Electrical Stimulation of Its Sympathetic Innervation Pathway

Stimulation of the pineal gland via its sympathetic innervation pathway results in the production of N-acetylserotonin and melatonin. Melatonin has many therapeutic roles and is heavily implicated in the regulation of the sleep-wake cycle. In addition, N-acetylserotonin has recently been reported to promote neurogenesis in the brain. Upregulation of these indoleamines is possible via neuromodulation of the pineal gland. This is achieved by electrical stimulation of structures or fibres in the pineal gland sympathetic innervation pathway. Many studies have performed such pineal neuromodulation using both invasive and non-invasive methods. However, the effects of various experimental variables and stimulation paradigms has not yet been reviewed and evaluated. This review summarises these studies and presents the optimal experimental protocols and stimulation parameters necessary for maximal upregulation of melatonin metabolic output.

Limited recovery of pineal function after regeneration of preganglionic sympathetic axons: evidence for loss of ganglionic synaptic specificity

The cervical sympathetic trunks (CSTs) contain axons of preganglionic neurons that innervate the superior cervical ganglia (SCGs). Because regeneration of CST fibers can be extensive and can reestablish certain specific patterns of SCG connections, restoration of end organ function would be expected. This expectation was examined with respect to the pineal gland, an organ innervated by the two SCGs. The activity of pineal serotonin N-acetyltransferase (NAT) exhibits a large circadian rhythm that is dependent on the sympathetic input of the gland, with high activity at night. Thirty-six hours after the CSTs were crushed bilaterally, nocturnal NAT was decreased by 99%. Three months later, enzyme activity had recovered only to 15% of control values, a recovery dependent on regeneration of CST fibers. Nevertheless, a small day/night rhythm was present in lesioned animals. Neither the density of the adrenergic innervation of the gland nor the ability of an adrenergic agonist to stimulate NAT activity was reduced in rats with regenerated CSTs. In addition, stimulation of the regenerated CST at a variety of frequencies was at least as effective in increasing NAT activity as seen with control nerves. These data suggest that the failure of pineal function to recover is not attributable to a quantitative deficit in the extent of reinnervation or synaptic efficacy. Rather, we suggest that there is some loss of specificity in the synaptic connections made in the SCG during reinnervation, resulting in a loss of the central neuronal information necessary for directing a normal NAT rhythm and thus normal pineal function.

Effects of central and peripheral inputs on single pineal cell activity in the rat

The influences exerted by central and peripheral afferents to the pineal gland have been studied in rats anesthetized with urethane (1.2 g/kg, i.p.). Spontaneous action potentials arising from the pinealocytes were recorded by means of glass micropipettes filled with 3 M NaCl containing a dye. The electrical stimulation of suprachiasmatic nucleus, superior cervical ganglia, sciatic nerve and retina evoked discharge changes in a significant number of pineal cells. However, a relatively higher proportion of pinealocytes failed to respond to these afferents. Three types of responses could be observed. Inhibitions were the predominant response patterns to suprachiasmatic nucleus, superior cervical ganglia and sciatic nerve, while excitations were mainly elicited following photic stimulation, whereas the remaining evoked activity was biphasic responses, which were observed in a small number of cells after stimulation of suprachiasmatic nucleus, superior cervical ganglia and retina. These data confirm some previous neural inputs to the pineal and demonstrate the existence of a modulatory effect of the suprachiasmatic nucleus on pinealocyte discharges as well as somatosensory afferents to the gland by way of the sciatic nerve.

Circadian variations of "synaptic" bodies in the pineal glands of Brattleboro rats

The function of the mammalian pineal gland is regulated primarily by the sympathetic system. Arginine-vasopressin (AVP) may also be involved in the regulation of pineal melatonin synthesis under experimental conditions. The present study was conducted in the AVP-deficient rat strain, the Brattleboro rat, to investigate whether the numbers and rhythms of pineal "synaptic" bodies in this strain are different from those found in intact rats. AVP or its non-vasoconstrictive analog, deamino-D-AVP, was also injected intra-arterially in Brattleboro or Sprague-Dawley rats to test whether this procedure influences "synaptic" body numbers. Brattleboro rats were killed at different time-points throughout the 24 h-cycle in March, June and September. "Synaptic" ribbons, spherules and intermediate structures were quantified and examined with regard to their intracellular location, with or without nocturnal AVP or D-AVP treatment. Numbers of ribbons were low during the day and high during the night (as in genetically intact rats), whereas spherules and intermediate structures numbers exhibited inconstant daily patterns. Night levels of "synaptic" ribbons were highest in June, lowest in March, whereas day levels did not differ significantly. No significant alterations in pineal "synaptic" body numbers were found following administration of AVP or D-AVP. Our results therefore indicate that AVP does not play a crucial role in the regulation of pineal "synaptic" body numbers in rats.

Elevation of pineal melatonin secretion by electrical stimulation of the cervical sympathetic trunk in rabbits

The influence of electrical stimulation of the unilateral sympathetic trunk on the release patterns of plasma melatonin in the confluens sinuum of anesthetized rabbits was studied. Plasma melatonin exhibited an episodic release pattern, with pulses superimposed on a basal level. The mean concentration, mean maximum and minimum levels of plasma melatonin in the confluens sinuum were found to be significantly elevated when the sympathetic trunk was electrically stimulated. The frequency of pulse peaks of plasma melatonin sampled at 4-min intervals was not affected by electrical stimulation. Our results suggest that the diurnal rhythmic secretory pattern of pineal melatonin into the confluens sinuum of rabbits is regulated by the cervical sympathetic system.

Effects of electrical stimulation of the superior cervical ganglia on the number of "synaptic" ribbons and the activity of melatonin-forming enzymes in the rat pineal gland

Melatonin metabolism in the mammalian pineal gland is under the clear influence of sympathetic fibers originating in the superior cervical ganglia (SCG). Previous studies suggested that pineal "synaptic" ribbons (SR) as well are regulated by the gland's sympathetic innervation. To gain more insight into the mechanisms involved, we examined the effects of sympathetic stimulation on SR number and on the activity of melatonin forming enzymes, serotonin N-acetyltransferase (NAT) and hydroxyindole-O-methyltransferase (HIOMT). The SCG in adult male rats were stimulated electrically during daytime for either 15 or 120 min. Immediately following stimulation, the glands were removed and processed for electron microscopy and for the determination of NAT and HIOMT activities. No differences in pineal SR number, size or location were found in rats stimulated with either parameters when compared with sham-stimulated or control animals. While the activity of HIOMT remained unchanged, the activity of NAT was also unaltered following 15 min of stimulation, but was augmented approximately three-fold in animals stimulated for 120 min. It is concluded that if SR in the rat pineal gland are under sympathetic control, the regulation is different from that involved in melatonin formation.

Electrophysiological characterization of the pineal gland of golden hamsters

In one of the most-widely used species in pineal gland research, the strongly photoperiodic golden hamster, Mesocricetus auratus, no electrophysiological data on pinealocytes are currently available. To fill this gap, in the present study 185 spontaneously active pinealocytes from male golden hamsters were recorded extracellularly, both during day- and night-time (light: dark cycle 12: 12, lights on at 07.00 h). As in other species, pinealocytes exhibited action potentials of 1-2 ms duration. An irregular firing pattern was observed in 95% of the pinealocytes, the remainder fired more regularly or showed a phasic discharge pattern. The firing frequencies ranged from 0.2 to 25 Hz and showed clear time-dependent differences. From 07.00 h to 22.00 h the mean firing frequencies were identical, i.e. in the range of 2 Hz; between 22.00 h and 01.00 h mean discharge rate increased to 5 Hz and exhibited a peak of 7 Hz between 01.00 h and 04.00 h, followed by a decrease to 4 Hz between 04.00 h and 07.00 h. Electrical stimulation of the superior cervical ganglion during day-time resulted in an augmentation of firing frequency in some pinealocytes and a decrease in others; during night-time, inhibitory responses only were observed. Photic stimulation, or electrical stimulation of either the optic chiasm or the habenular nuclei mostly decreased the firing rate of pinealocytes. Compared to other mammalian species, the electrophysiological properties of golden hamster pinealocytes appear to be basically similar.

A histochemical study of the adrenergic innervation of the rat pineal gland: evidence for overlap of the innervation from the two superior cervical ganglia and for sprouting following unilateral denervation

The rat pineal gland receives sympathetic innervation, via the right and left internal carotid nerves, from neurons whose cell bodies are located in the two superior cervical ganglia. Using fluorescence microscopy, we have examined the density and distribution of fluorescent profiles in the pineal gland after lesioning of the internal carotid nerves. Thirty-two hours after sectioning both internal carotid nerves, the density of the fluorescent profiles was 3% of that seen in sham-operated controls, indicating that the lesioned fibers had degenerated. Thirty-two hours after sectioning one internal carotid nerve, the density of the processes was decreased to approximately 50% of the control value. The magnitude of the decrease following a unilateral lesion was similar on both the right and left halves of the gland. Thus, the results suggest that each internal carotid nerve innervates both sides of the pineal gland. The implications of this overlap in the innervation from the two internal carotid nerves for recovery of pineal function after a unilateral lesion are discussed. Sections of pineal glands were also analyzed at later time points after a unilateral lesion. Two weeks after cutting one internal carotid nerve, the density of the fluorescent profiles had increased to greater than 80% of the control value. When the contralateral internal carotid nerve was cut 2 weeks after a unilateral lesion and the pineal gland was examined 32 h later, the density of the fluorescent profiles had decreased to 2% of the sham value. This suggests that all of the compensatory increase in adrenergic processes that takes place following the unilateral lesion is due to sprouting of the contralateral internal carotid nerve rather than to regeneration of the lesioned internal carotid nerve or to sprouting and ingrowth of other adrenergic neurons. It remains to be determined what, if any, functional significance this sprouting has, since the neurally dependent circadian rhythm in serotonin N-acetyltransferase activity in the pineal gland is restored to normal within 32 h after a unilateral lesion; that is, before significant sprouting has occurred.

Effects of chemical and surgical ganglionectomy on electrical activity of the pineal gland of male rats

In order to elucidate further the role of sympathetic innervation for pineal function, the influence of sympathectomy on the spontaneous electrical activity of single cells in the pineal gland of adult male rats was investigated. Extracellular single-unit recordings were made during nighttime in the pineal gland of urethane-anesthetized, blinded adult male rats that had been treated neonatally with 6-hydroxydopamine, or that were ganglionectomized either during, or 12-16 h or 36-40 h, prior to the recording experiment. These experiments revealed that the excitatory influence of the sympathetic system on pineal nocturnal electrical activity can be abolished by either chemical sympathectomy of neonatal rats or surgical superior cervical ganglionectomy in adult animals.

Changes in the electrical activity of the rat pineal gland following stimulation of the cervical sympathetic ganglia

In order to elucidate the role of sympathetic innervation for pineal function, the influence of both unilateral and bilateral electrical stimulation of the superior cervical ganglia on the electrical activity of single cells in the rat pineal gland was investigated. These experiments revealed a clear influence on spontaneous electrical activity of single pinealocytes. About half of the units tested by unilateral stimulation exhibited either a graded continuous augmentation or inhibitions of different magnitude. In addition, 'silent' cells without spontaneous activity could be activated by sympathetic stimulation. Sequential and simultaneous bilateral stimulations showed that only a few cells could be influenced by both ganglia and in these cases the influence seemed to be additive. Some pineal cells do not appear to be under the control of the sympathetic nervous system.

The influence of the frequency and pattern of sympathetic nerve activity on serotonin N-acetyltransferase in the rat pineal gland

The activity of the pineal enzyme arylamine: N-acetyltransferase (NAT) was determined following direct stimulation of the preganglionic or post-ganglionic nerves of the superior cervical ganglia. 1. Stimulation of the preganglionic trunks at 10 c/sec during the day or night was sufficient to increase NAT activity approximately 50-fold, to levels comparable to those observed at night in the intact animal. The time course of this effect of nerve stimulation differed between day and night. 2. The responses of pineal NAT to certain frequencies of stimulation were similar for preganglionic and post-ganglionic stimulation. In both cases the responses to stimulation at 5 c/sec appeared to be maximal, 10 c/sec causing no further increase. However, at 10 c/sec, stimulation was more effective post-ganglionically than preganglionically. 3. Various patterns of preganglionic stimulation, having the same average frequency, differed in their ability to increase the activity of NAT. Some, though not all, of these differences between patterns were observed during post-ganglionic stimulation. 4. Unilateral stimulation of the preganglionic nerves produced an increase in NAT activity that was less than half the increase produced by bilateral stimulation, suggesting that the innervation from the two ganglia interact within the pineal gland. 5. These data indicate that changes in the firing rates of sympathetic nerves innervating the pineal gland, within the range of frequencies typically observed for sympathetic neurones, would be sufficient to account for the circadian rhythm in NAT activity observed in the intact rat. Changes in the over-all pattern of sympathetic activity, in addition to changes in the total number of stimuli, could play a significant role in the pineal response.

Electrical stimulation of sympathetic nerves increases the concentration of cyclic AMP in rat pineal gland

Electrical stimulation of the superior cervical ganglia causes a rapid increase in the concentration of cyclic AMP in the pineal gland of rats. This effect is dependent upon the frequency, voltage, and duration of the stimulus and is markedly potentiated by pretreating the animals with desmethylimipramine. The increase in cyclic AMP is blocked by prior treatment of the rats with reserpine, bretylium, or propanolol but not with phentolamine. These results provide direct evidence that electrical stimulation of sympathetic nerves increases cyclic AMP in a target organ through the release of norepinephrine from presynaptic terminals acting on postsynaptic beta-adrenergic receptors.

Testing the TRH hypothesis of pineal function

In the Syrian hamster gonadal involution may be induced by exposure of the animal to darkness, short photoperiod, or blindness for several weeks. These conditions also influence body metabolism by inducing a substantial decrease in thyroxin concentrations. The role of the pineal gland in these phenomena has been demonstrated by experiments showing that if pinealectomized hamsters are subjected to darkness, short photoperiod, or blindness, their gonads do not involute and plasma thyroxin levels remain normal. Since thyrotrophin releasing hormone (TRH) causes the release of both prolactin and thyrotrophin from the pituitary we can predict on theoretical grounds that TRH mediates both pineal-gonad and pineal-thyroid phenomena. Melatonin may be the pineal hormone regulating hypothalamic TRH levels.

Pineal gland: 24-hour rhythm in norepinephrine turnover

There is a 24-hour rhythm in the turnover of norepinephrine in sympathetic nerves innervating the pineal gland. This rhythm persists in blinded animals but is suppressed in normnal rats by light. The rhythm in norepinephrine turnover generates the rhythums in pineal indoleamines and N-acetyltransferase.

Control of circadian change of serotonin N-acetyltransferase activity in the pineal organ by the beta--adrenergic receptor

Serotonin N-acetyltransferase (EC 2.3.1.5) activity in the rat pineal organ is enhanced 50-fold at night. Rats exposed to light at night or kept in darkness during the daytime do not show any elevation of enzyme activity. Treatment with reserpine, a compound that depletes norepinephrine from nerves, 1-propranolol, a beta-adrenergic blocking agent, or cycloheximide, an inhibitor of protein synthesis, abolishes the nocturnal increase in serotonin N-acetyltransferase activity, indicating that the enzyme activity is modulated by neural release of norepinephrine from sympathetic nerves via beta-adrenergic receptors, and that the increase in enzyme activity is due to synthesis of new enzyme molecules. When rats are exposed to light at night or injected with 1-propranolol, there is a precipitous fall in serotonin N-acetyltransferase activity (half-life 5 min). Cycloheximide administered at night results in a slow fall in enzyme activity (half-life 60 min). When rats are kept in darkness and then exposed to light for 10 min, L-isoproterenol rapidly initiates the elevation of serotonin N-acetyltransferase activity to the initial level in 60 min. On the other hand, when the rats are kept in continuous light, L-isoproterenol initiates an increase in serotonin N-acetyltransferase activity after a lag phase of 60 min. The results indicate that there are two types of changes in serotonin N-acetyltransferase activity; a rapid increase and decrease mediated by the beta-adrenergic receptor, and a slow increase and decrease in enzyme activity that appears to represent the turnover of the enzyme.

Induction and superinduction of serotonin N-acetyltransferase by adrenergic drugs and denervation in rat pineal organ

Activity of serotonin N-acetyltransferase (EC 2.3.1.5) in rat pineal organ is rapidly and markedly elevated in vivo after administration of beta-(3,4-dihydroxyphenyl)-L-alanine (L-DOPA), norepinephrine, epinephrine, isoproterenol, monoamine oxidase inhibitors, or theophylline. Serotonin or 5-hydroxytryptophan has no effect on the increase in activity of this enzyme. Inhibitors of protein synthesis or propranolol, a beta-adrenergic blocking agent completely inhibit(s) the increase in activity of serotonin N-acetyltransferase induced by drugs, indicating that new enzyme molecules are formed via stimulation of beta-receptors of pineal cells and adenosine 3':5'-cyclic monophosphate. When rat pineal organ is denervated by ganglionectomy, beta-(3,4-dihydroxyphenyl)-L-alanine induces much more serotonin N-acetyltransferase than in the innervated gland. This superinduction by denervation appears to be due to changes of the postsynaptic site, probably the beta-adrenergic receptor on the pineal cell.

Melatonin metabolism: neural regulation of pineal serotonin: acetyl coenzyme A N-acetyltransferase activity

There is a diurnal rhythm in the activity of serotonin N-acetyltransferase in the rat pineal gland. In the normal rat, the nocturnal enzyme activities are 15-to 30-fold greater than are daytime activities. This rhythm is abolished by decentralization or removal of the superior cervical ganglia, procedures that interrupt the only source of central neural input to the pineal gland. This effect of superior cervical sympathectomy on the N-acetyltransferase rhythm cannot be attributed to changes occurring in the denervated pineal parenchymal cells. When chronically denervated glands are placed in organ culture with norepinephrine, the neurotransmitter normally located in sympathetic terminals in the gland, N-acetyltransferase activity increases 10- to 20-fold. These data indicate that superior cervical sympathectomy abolishes the N-acetyltransferase rhythm by elimination of the input of central signals to the gland. These signals appear to regulate the N-acetyltransferase rhythm in the normal rat by regulation of the release of norepinephrine from the sympathetic terminals within the pineal gland.