Autonomic system control of the pineal gland and the role of this complex in the integration of body function

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.

Vasopressin potentiation of the melatonin synthetic pathway via specific V1a receptors in the rat pineal gland

The pineal gland releases the "time-keeping' hormone melatonin following a rhythmic sympathetic input which translates light information. The aim of this work was to study the role and mechanism of action of the central vasopressinergic input on pineal cAMP-dependent melatonin synthesis in the rat. The pineal was found to display vasopressin receptors of the V1a subtype, as the V1a antagonist [125I]HO-LVA bound in a saturable manner to pineal membranes with a high affinity (kd = 10 pM) and a maximal binding capacity (B(max)) of 13 fmol/mg protein. Vasopressin was able to displace [125I]HO-LVA binding in a dose-dependent manner (k(i) = 1.9 nM). Vasopressin had no effect on the basal cAMP level and melatonin secretion in cultured rat pinealocytes. However, it clearly and dose-dependently (EC50 = 7 nM) potentiated by 2-3 times cAMP accumulation and by 1.5-2.5 times melatonin secretion induced by moderate noradrenergic stimulation. On strongly stimulated pinealocytes, however, vasopressin could potentiate cAMP accumulation, but not melatonin secretion. The potentiatory effect of vasopressin was inhibited in the presence of the V1a antagonist. These results indicate that vasopressin is a potent modulator of rat pineal synthetic activity.

Neurochemical effects of static magnetic field exposure

BACKGROUND

There has been considerable interest in both the lay and scientific media concerning the putative effects of exposure to electromagnetic fields. An assessment of the effects of static magnet exposure on neurochemistry was undertaken to determine potential risks to patients and staff involved with magnetic resonance imaging and spectroscopy.

METHODS

One set of rats were exposed to weak static field (800 gauss [G]) in an otherwise normal laboratory surrounding. Another set of rats were exposed to 7-Tesla fields, both with suitable controls.

RESULTS

Exposure of rats (n=8) to weak static fields for periods between 12 hours and 8 days produced no significant change in nighttime pineal or serum melatonin levels, as compared to controls, nor did it significantly influence levels of pontine medullary 5-hydroxytryptamine [5-HT] and hypothalamic 5-hydroxyindoleacetic acid [5-HIAA]. Placing rats in a 7-Tesla MRI magnet for 45 minutes produced similar results.

CONCLUSIONS

These experiments suggest that daily light/dark cycle has much greater influence on levels of melatonin, catecholamines, serotonin, or their metabolites than does exposure to a static magnetic field.

Ontogeny of the pineal response to norepinephrine

The Syrian hamster pineal displays age-dependent changes in melatonin output measured in vitro. Between the ages of 4 and 19 days, pineal melatonin generation in response to 10 microM norepinephrine (NE) increased 34-fold. Production of melatonin by cultured pineals from 1-week-old hamsters showed a clear dose responsiveness to NE: The most effective dose was 10 microM and the response declined at both higher and lower doses. When cultured pineals from 7-day-old animals were exposed to four cycles of NE in the medium (10 hr 10 microM NE: 14 hr 0 M NE), the melatonin output followed the driving rhythm with a rising lag time of 8 hr and a falling lag of 4 hr. This time course is consistent with the conclusion [Santana et al., 1990; Gonzalez-Brito et al., 1990] that transcription events lead to a long lag time between the stimulus and the onset of melatonin synthesis. In the absence of exogenous NE, melatonin output from most glands dropped to undetectable levels in just over 2 days; however, even after 3 days without NE, glands could be stimulated to produce melatonin when NE was added to the medium. When incubated with 10 microM NE for 6 hr in static culture, glands from 11- versus 4-day-old neonates produced 20 times more melatonin and had over three times higher NAT specific activity. After a 15 min incubation with 10 microM NE, cAMP content was three-fold higher in 11-compared to 4-day-old pineals. Our results demonstrate that although the pineal's response to NE increases with age, its response time is invariant throughout postnatal development.

Pineal microdialysis in freely moving rats

We describe a surgical technique to implant the guide cannula for in vivo microdialysis in the rat pineal gland. This technique has the following features and advantages: (a) does not require ligation of the superior or transverse sinus, (b) minimizes bleeding from the dural veins, (c) does not disturb the sympathetic innervation originating from superior cervical ganglia, which is essential for pineal function. This new technique makes it possible to carry out chronic pineal microdialysis of freely moving rats.

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.

Pineal involvement in the alimentary behavior and taste preferences in the rat

The purpose of this experiment was to explore the effects of pinealectomy or sham pinealectomy on circadian rhythms of taste preferences, food and fluid intake and body weight gain in rats. We compared the body weight gain, the amounts of food eaten and that of deionized water, total fluid, salt, sour, sweet and bitter near-threshold solutions consumed by rats before all surgical manipulations and after pinealectomy or sham pinealectomy. The results showed that the pineal gland does not exert a major influence on circadian organization of taste preferences, drinking, feeding and body weight modifications in the rat. The failure of pinealectomy to modify light/dark rhythms of taste preferences and other related alimentary behaviors may be explained by the fact that pinealectomy does not completely eliminate circulating melatonin.

Characterization of the light response in the pineal gland of intact and sympathectomized rats

Electrophysiological recordings were performed in the pineal gland of pigmented rats BD9 to investigate the possible contribution of a direct neural connection of the gland with the central nervous system in the transmission of photic information. Extracellular potentials were recorded during brief photic stimulation of the eyes before and after bilateral sympathectomy. Two types of responses could be distinguished in intact as well as in sympathectomized rats: spontaneously active units which were unresponsive to light flashes and units responding to photic stimulation with ON and ON/OFF discharges. Spectral sensitivity curves recorded from the pineal organ of dark adapted rats showed a maximum at 500 nm. Interruption of the sympathetic innervation by bilateral denervation or removal of the superior cervical ganglia did not alter the spectral properties of pineal units. Additionally, response-intensity curves and response threshold (about 0.0017 microW/cm2) were not changed after sympathectomy. These results suggest that the pineal gland of the rat receives projections from the visual system via a central pinealopetal innervation.

Day-night differences in the sensitivity of adrenoceptors in the Syrian hamster pineal gland: an in vivo iontophoretic study

Investigations on the regulation of pineal melatonin synthesis in the Syrian hamster revealed distinct differences compared to this well-understood mechanism in rat. E.g., a circadian profile of pineal norepinephrine (NE) is absent, there is no beta-adrenoceptor sensitivity during daytime and adrenergic receptor supersensitivity is not easily achieved. To elucidate the action of NE on pineal receptor sites, the effects of iontophoretic application of adrenergic compounds on spontaneous electrical discharge rates of pinealocytes were investigated during day- and nighttime. Following application of either NE, isoproterenol or clonidine, cells were activated, inhibited or not affected. Whereas about one-third of the units responded to iontophoretic application of sympathomimetics at daytime, the number of affected cells was doubled during the night. These results demonstrate the involvement of adrenoceptors in the regulation of circadian rhythms in electrophysiological properties of Syrian hamster pinealocytes. Since relatively few cells responded during daytime and inhibitory adrenergic mechanisms dominated at night, the classification of receptors by means of iontophoresis may provide a basis to explain the difficulty to influence pineal melatonin synthesis by sympathomimetics in the Syrian hamster.

Local glucose utilization of the brain and pineal gland during stimulation of the cervical sympathetic trunk

The quantitative autoradiographic 2-[14C]deoxyglucose method was employed to map the metabolic activity of the superior cervical ganglion and the entire brain during unilateral electrical stimulation of the cervical sympathetic trunk in the urethane-anesthetized rat. Stimulation of the cervical sympathetic trunk increased glucose utilization in the ipsilateral superior cervical ganglion (+95%) but did not produce side-to-side differences in glucose utilization in any of the brain structures examined in this study. Compared to the control nonstimulated animals, the rate of glucose metabolism in the pineal gland was increased 71% following stimulation of the cervical sympathetic trunk. The pineal gland was the only brain region out of 87 structures examined in which glucose utilization was increased by electrical stimulation of its sympathetic innervation.

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.

Innervation of the pineal gland in the rat: an HRP study

Injections of horseradish peroxidase were made stereotaxically and directly (surgically exposed) into the pineal gland of Sprague-Dawley and Long-Evans rats weighing 30 to 500 g (20 to 130 days of age). Retrogradely labeled cells were seen in the superior cervical ganglia. Anterogradely labeled fibers were observed within the pineal stalk, lamina intercalaris, and the medial habenular nuclei. Terminal fields were identified in lamina intercalaris and medial habenular nuclei. Labeled cells were not seen within the central nervous system. These results suggest that in the rat the pineal gland is not centrally innervated but in fact is innervated solely by the sympathetic postganglionic fibers.

Persistence of photic evoked responses in pineal gland after its pedunculotomy and superior cervical ganglionectomy

The present study analyzes the participation of pineal stalk and superior cervical ganglia (SCG) in the conduction of photic evoked potentials (PEP) to the pineal body (PB) in unanesthetized freely moving rats implanted with semimicroelectrodes. The PEPs were recorded in PB, lateral hypothalamus (LH), and habenular complex (HC). In some rats the pineal stalk was lesioned either by electrolysis or by surgical means. Recordings were begun 3 days after electrode implantation; three recording sessions were performed before and after extirpation of SCG or lesion of pineal stalk. The PEPs recorded in LH and HC were characterized by 3 components (positive-negative-positive: PNP). The average latency of the first wave was 35.3 +/- 1.7 and 38.8 +/- 1.6 ms for LH and HC, respectively. The peaks of PEPs in PB showed different polarity (NPN), and the first component had a latency of 36.1 +/- 1.2 ms. The PEPs persisted in all three structures after extirpation of SCG and after electrolytic lesion or transection of the pineal stalk. These results show that the PEPs recorded in PB are not generated in this structure, since they persist after interrupting its connections with the rest of the central nervous system; this suggests that PEPs could be the result of electrotonic transmission from neighboring structures.

Magnetic field effects on the rat pineal gland: role of retinal activation by light

In view of the reported involvement of the retinae in mediating magnetic field effects on pineal function in rats, the present study sought to test the hypothesis - based on theoretical calculations - that dim light activation of photoreceptors is necessary for magnetoreception by the retinae. Adult male rats were exposed to a single nocturnal inversion of the earth's magnetic field in the presence or absence of dim red light. Pineal gland N-acetyltransferase and hydroxyindole-O-methyltransferase activities were measured as indices of magnetosensitivity. In animals exposed to dim red light, pineal enzyme activities were inhibited significantly by the magnetic stimulus in comparison to controls (dim red light only). In contrast, the pineal response to a magnetic stimulus was absent in total darkness. These results support the notion that photoreceptor stimulation by dim light is necessary for the perception of weak magnetic fields.

Rat pineal exhibits two electrophysiological patterns of response to microiontophoretic norepinephrine application

The spontaneous activity of 117 pineal units was recorded in urethane-anesthetized rats. The pineal units exhibited a wide range of firing rates of which 50% were on average slower than 14 spikes per second. Superior cervical ganglion (SCG) stimulation was studied in 76 pineal units; this stimulation caused excitation in 55% of the units. Microiontophoretic application of norepinephrine (NE) induced changes of firing rates in 61% of the pineal units tested. Two patterns of activity following NE microiontophoresis was observed: increase in firing rate (64%) and decrease in firing rate (36%). NE-induced excitation was observed only in those units excited by SCG stimulation. When NE and SCG stimulation were applied together, partial summation of the excitation induced by each one alone was observed. None of the units in which NE depressed the firing rate responded to SCG stimulation. Local application of propranolol blocked the excitation initiated by SCG stimulation as well as the excitation and the depression induced by NE microiontophoresis.

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.

Electrophysiological properties of rat pinealocytes: evidence for circadian and ultradian rhythms

Extracellular single-unit recordings were made during day- and night-time in the pineal gland of urethane-anesthetized adult male Sprague-Dawley rats. All cells exhibiting spontaneous electrical activity had firing frequencies from less than 1 Hz to about 100 Hz, and their discharge patterns were characterized as regular, irregular or bursting. While most of the spontaneously active cells (n = 163) showed a uniform activity level throughout the recording period (30-120 min), a group of 9 cells exhibited oscillatory rhythms with periods of 4-8 min. In addition, long-term recordings across day- and night-time from five cells revealed increasing activity during night-time in three cells, while the remaining two units showed constant activity throughout the recording time (8-20 h). Comparison of day- and night-data in general indicated an overall higher level of activity at night.

Melanin: the organizing molecule

The hypothesis is advanced that (neuro)melanin (in conjunction with other pigment molecules such as the isopentenoids) functions as the major organizational molecule in living systems. Melanin is depicted as an organizational "trigger" capable of using established properties such as photon-(electron)-phonon conversions, free radical-redox mechanisms, ion exchange mechanisms, and semiconductive switching capabilities to direct energy to strategic molecular systems and sensitive hierarchies of protein enzyme cascades. Melanin is held capable of regulating a wide range of molecular interactions and metabolic processes primarily through its effective control of diverse covalent modifications. To support the hypothesis, established and proposed properties of melanin are reviewed (including the possibility that (neuro)melanin is capable of self-synthesis). Two "melanocentric systems"--key molecular systems in which melanin plays a central if not controlling role--are examined: 1) the melanin-purine-pteridine (covalent modification) system and 2) the APUD (or diffuse neuroendocrine) system. Melanin's role in embryological organization and tissue repair/regeneration via sustained or direct current is considered in addition to its possible control of the major homeostatic regulatory systems--autonomic, neuroendocrine, and immunological.

Electrophysiological study of evoked electrical activity in the pineal gland

Experiments were carried out in rats, unanesthetized, paralyzed and artificially respirated. The electrical activity from the pineal gland was recorded with bipolar electrodes. Field potentials were evoked in the pineal after peripheral (photic and sciatic nerve) and central (septal area, habenular complex and optic tract) stimulations. In general these potentials were biphasic with the exception of that evoked by the sciatic nerve and optic tract, which exhibited a complex response and a triphasic field potential, respectively. Bilateral sympathectomy did not modify the pineal evoked responses, but when the pineal stalk was sectioned all the responses were immediately suppressed after the lesion. On the basis of the above experimental data one could conclude that the bulk of the inputs to the pineal gland come through its stalk. At the present, the physiological significance of these findings is not clear.

Dihydroergocryptine binding sites in bovine and rat pineal glands

High affinity binding of [3H]dihydroergocryptine (DHE) to crude membrane preparations of bovine and rat pineal glands was examined by a rapid filtration procedure through Whatman GFB paper. Scatchard analysis of bovine pineal membranes revealed a single population of binding sites with Kd = (4.8 +/- 1.9) X 10(-8) M (mean +/- S.E.M., n = 3) and binding site concentration = 834 +/- 103 fmol/mg of protein. When various alpha- and beta-adrenergic agonists or antagonists were tested for their ability to inhibit [3H]DHE binding the following Ki values (microM) were obtained: DHE (0.039), phentolamine (0.102), phenoxybenzamine (0.262), methysergide (0.670), L-epinephrine (2.27), serotonin (3.09), L-norepinephrine (3.20), octopamine (30.3), L-propranolol (1710), L-isoproterenol (greater than 2000). In order to obtain information on the pre- or postsynaptic location of pineal DHE binding sites, pineal 900 g supernatants were prepared from rats subjected to bilateral superior cervical ganglionectomy or sham-operation 7 days earlier. Ganglionectomy caused a 53% increase in the number of binding sites without changing significantly their Kd (5.2 X 10(-8) M and 5.5 X 10(-8) M in ganglionectomized and control rats, respectively). These results are compatible with the view that postsynaptic alpha-adrenoceptors are present in the pineal gland.

Electrophysiological evidence for circadian rhythmicity in a mammalian pineal organ

Long-term electrophysiological recordings from the guinea-pig pineal organ show that three types of intrinsic cells can be distinguished: (i) Cells showing constant firing rates over periods of up to 24 hours. (ii) Cells which are highly active during the day and show a low firing rate during the night. (iii) Cells which exhibit low activity during the day and enhanced activity during the night; these cells can be strongly inhibited by 1 min of light given during the night. Both the light- and darkness-activated cells show activity patterns which closely follow season-dependent differences in day- and night-lengths. In addition, both cell types show an oscillatory pattern in maintained activity.

The pineal gland as an APUD organ: supporting evidence and implications

It is only in recent years that the pineal gland has emerged from being thought of as non-functional and unimportant. The rise from obscurity has been the result of the interest of investigators of multidisplinary origins; such approaches, whilst clearly advancing understanding, also tend to leave knowledge fragmentary. In the last decade, a new neuroendocrine axis known as the APUD system has been delineated and scientifically accepted. The essence of the concept has been to link a physically widely dispersed and apparently unconnected series of glandular structures by a common embryological derivation and their secretion of closely similar functional products. The present proposal collates evidence for the inclusion of the pineal gland within the APUD system. In addition, because of the proven connections and response of the pineal gland to environmental changes, it is hypothesised that pineal products may provide the link between the environment and other members of the widely dispersed APUD system and thereby temper body homeostasis.

Failure of urethane anesthetic to block induction of pineal serotonin N-acetyltransferase activity in the rat

Ability of urethane anesthetic to block induction of pineal serotonin N-acetyltransferase (SNAT; E.C.2.3.1.5) activity was measured in individual rat pineal glands in animals receiving urethane (25% w/v, IP, 1.2 g/kg) or saline, 6 hr prior to sacrifice. Using a radioenzymatic assay, SNAT determinations were made twice daily (at 1200 or 2400 hr) immediately after the sacrifice of each animal. The results show that urethane had no effect on the induction of SNAT activity: (1) implying that the neural activity of those structures involved in induction of SNAT activity (e.g., suprachiasmatic nucleus) is not substantially altered by this anesthetic and (2) suggesting that the central blockade of ovulation by urethane does not include alterations in suprachiasmatic nucleus activity.

Sleep and motor activity of the rat during ultra-short light-dark cycles

The vigilance states and motor activity of the rat were recorded during ultrashort light-dark (LD) cycles. The LD schedules were the following: 10 min light -- 10 min dark (LD 10:10), LD 7.5:7.5, LD 5:5 and LD 2.5:2.5. With all short LD schedules, paradoxical sleep (PS) and high motor activity occurred more frequently during dark periods than during light. PS and motor activity were rapidly enhanced by darkness and inhibited by light, whereas the percentages of total sleep and slow wave sleep were only gradually decreased during the dark periods and increased during light. Compared to the LD 12h:12h control days, sleep was enhanced by all short LD schedules, an effect that was most marked in the circadian phase of maximum waking (corresponding to the LD 12h:12h dark period). The short LD cycles interacted with an intrinsic sleep cycle, whose period is approximately 10 min. Interval histograms of PS episodes showed that an optimal synchronization of the ultradian sleep rhythm is achieved when the period of the LD cycle or its multiple correspond also to 10 min. It is suggested that the response of PS and motor activity to changes in illumination may be due to common mechanisms.

The role of suprachiasmatic nuclei of the hypothalamus in the production of circadian rhythm

(1) Action potentials were recorded from single neurons in suprachiasmatic nuclei of hypothalamus in urethane anesthetized male rat. The rate of firing ranged from less than 1 to over 10/sec but was generally 4-8/sec, and it varied from one cell to another in the same animal. (2) Repetitive stimulation of optic nerve or light acting on the eye augmented the activity of approximately half of the the suprachiasmatic neurons examined (67 out of 159 cells) while 37 neurons (23%) showed clear inhibition by the same stimuli. (3) Stimulation of the suprachiasmatic nuclei strongly inhibited the electrical activity of cervical sympathetic nerves. Light or optic nerve stimulation also inhibited activity of cervical sympathetic nerves. (4) The rate of discharge of suprachiasmatic nuclei showed, at times, some short duration oscillations occurring every 3-5 min, but at other times the same neuron showed a steady low frequency of firing. (5) Projection of optic nerves to the suprachiasmatic nuclei was demonstrated by anterograde migration of horseradish peroxidase placed in the vitreous body. (6) It was suggested that light excites certain groups of neurons in the suprachiasmatic nuclei which exert inhibitory action on cervical sympathetic nerve. This, in turn, caused a reduction in norepinephrine release by nerve fibers innervating the pineal and a reduction in pineal enzyme production. Thus, neurons in these nuclei contribute to the suppression of pineal enzyme production produced by light.