Retinohypothalamic projection: electrophysiological evidence for the existence in female rats

Electrophysiology of the suprachiasmatic circadian clock

In mammals, an internal timekeeping mechanism located in the suprachiasmatic nuclei (SCN) orchestrates a diverse array of neuroendocrine and physiological parameters to anticipate the cyclical environmental fluctuations that occur every solar day. Electrophysiological recording techniques have proved invaluable in shaping our understanding of how this endogenous clock becomes synchronized to salient environmental cues and appropriately coordinates the timing of a multitude of physiological rhythms in other areas of the brain and body. In this review we discuss the pioneering studies that have shaped our understanding of how this biological pacemaker functions, from input to output. Further, we highlight insights from new studies indicating that, more than just reflecting its oscillatory output, electrical activity within individual clock cells is a vital part of SCN clockwork itself.

Pinealectomy reduces optic nerve but not intergeniculate leaflet input to the suprachiasmatic nucleus at night

The suprachiasmatic nucleus (SCN) of the hypothalamus regulates circadian rhythms in mammals. It receives, among others, direct inputs from the retina and from the thalamic intergeniculate leaflet (IGL). The former sends photic signals to the SCN, whereas the latter probably integrates photic and nonphotic information. To characterise these inputs in vivo, extracellular single-unit recordings were made from the SCN of rats under urethane anaesthesia during electrical stimulation of the optic nerve (OptN) or the IGL region. Cell responses were evaluated by creating peri-stimulus time histograms. Because humoral signals such as melatonin might modulate the activity of the SCN in addition to neural inputs, recordings were also made using pinealectomised (Px) rats to test for a possible role of this hormone in regulating inputs to the SCN. A significantly greater number of cells responded to IGL (60 of 90, 67%) than to OptN (35 of 75, 47%) stimulation in intact animals (chi(2) = 5.905, P = 0.015). The same was true when Px animals were tested (IGL, 82 of 131, 63%; OptN, 31 of 111, 28%; chi(2) = 27.637, P < 0.001). In intact animals, the proportion of cells responsive to IGL stimulation during the day and during the night was not significantly different from the proportion responsive in Px animals. The same was true for OptN stimulation during the day. However, during the night, the proportion of cells responsive to OptN stimulation in intact animals was significantly greater than the proportion responsive in Px animals (chi(2) = 7.127, P = 0.008). Our findings suggest that a lack of melatonin modulates OptN but not IGL inputs to the SCN.

The eye as metronome of the body

Vision is much more than just resolving small objects. In fact, the eye sends visual information to the brain that is not consciously perceived. One such pathway entails visual information to the hypothalamus. The retinohypothalamic tract (RHT) mediates light entrainment of circadian rhythms. Retinofugal fibers project to several nuclei of the hypothalamus. These and further projections to the pineal via the sympathetic system provide the anatomical substrate for the neuro-endocrine control of diurnal and longer rhythms. Without the influence of light and dark, many rhythms desynchronize and exhibit free-running periods of approximately 24.2-24.9 hours in humans. This review will demonstrate the mechanism by which the RHT synchronizes circadian rhythms and the importance of preserving light perception in those persons with impending visual loss.

Neuropeptide Y and optic chiasm stimulation affect suprachiasmatic nucleus circadian function in vitro

The retinohypothalamic tract (RHT) is a direct pathway from the retina to the suprachiasmatic nucleus (SCN). Electrical stimulation of the optic nerve or optic chiasm activates the RHT and produces shifts in phase of a circadian rhythm in SCN neuron activity in rat hypothalamic slices in vitro. The phase response curve (PRC) for this effect is very similar to that obtained from administration of light pulses to intact animals maintained in constant darkness. The effect of optic chiasm stimulation is blocked by tetrodotoxin. In addition to the RHT, there is a second entraining pathway, the geniculohypothalamic tract, which arises from neuropeptide Y (NPY)-containing neurons of the intergeniculate leaflet of the lateral geniculate complex. In contrast to optic chiasm stimulation. NPY produces phase shifts in the rhythms of SCN neuron firing rate in vitro with a PRC that similar to that for NPY infusion into the SCN in intact animals as well as that produced by a series of treatments that induce locomotor activity. These results indicate that phase shifts of the circadian rhythm of SCN neuron activity may be produced by activation of two different entraining pathways and that the physiological actions of these pathways on pacemaker function are markedly different.

The relation between light-induced discharge in the suprachiasmatic nucleus and phase shifts of hamster circadian rhythms

The role of neurophysiological activation of suprachiasmatic nucleus (SCN) cells in phase shifting the circadian pacemaker of the hamster was investigated in a combined behavioural and electrophysiological study. An electrophysiological study examined the relation between the pattern of light presentation and the induced discharge rate in the SCN. Behavioural experiments examined the relation between the pattern of light presentation and the magnitude of phase shift induced. The combination of these results provides an indirect assay of the relations between induced neural discharge in the SCN and phase shifts of the circadian activity rhythm. The data indicate that the magnitude of phase shifts is monotonically, but not linearly, related to photically induced changes in discharge rate.

Effect of substance P on circadian rhythms of firing activity and the 2-deoxyglucose uptake in the rat suprachiasmatic nucleus in vitro

The suprachiasmatic nuclei (SCN) have been identified as a pacemaker for many circadian rhythms in mammals. Although substance P (SP) fibers from retina are found to terminate the SCN, the physiological role of this peptide is uncertain. The 2-deoxyglucose (2-DG) uptake and firing activity in the SCN show a robust circadian change. SP causes an increase in 2-DG uptake by SCN during the subjective night but not during subjective day. SP-induced increase in 2-DG uptake is blocked by co-treatment with the SP receptor antagonist, spantide. Treatment with SP produces phase shifts of circadian rhythm in spontaneous neural activity in SCN neurons with a phase-response curve that is similar to the effect of light pulses to animals under constant darkness. SP-induced phase change is also blocked by pretreatment with spantide. SP-induced increase in 2-DG uptake and phase changes in firing activity occur only during subjective night, at circadian times when photic phase shifting of activity occurs. The present results suggest that SP may be an important transmitter for conveying environmental light-dark information from retina to the SCN.

Factors affecting slow regular firing in the suprachiasmatic nucleus in vitro

In isolated slices of hypothalamus, suprachiasmatic nucleus (SCN) neurons were recorded intracellularly. Blockade of Ca++ channels increased spike duration, eliminating an early component of the afterhyperpolarization (AHP) that followed evoked spikes. The duration and reversal potential of AHPs were, however, unaffected, suggesting that only an early, fast component of the AHP was Ca(++)-dependent. Unlike other central neurons that exhibit pacemaker activity, therefore, SCN neurons do not display a pronounced, long-lasting Ca(++)-dependent AHP. Extracellular Ba++ and intracellular Cs+ both revealed slow depolarizing potentials evoked either by depolarizing current injection, or by repolarization following large hyperpolarizations. They had different effects on the shape of spikes and the AHPs that followed them, however. Cs+, which blocks almost all K+ channels, dramatically reduced resting potential, greatly increased spike duration (to tens of milliseconds), and blocked AHPs completely. In contrast, Ba++ had little effect on resting potential and produced only a small increase in spike duration, depressing an early Ca(++)-dependent component and a later Ca(++)-independent component of the AHP. The relatively weak pacemaker activity of SCN neurons appears to involve voltage-dependent activation of at least one slowly inactivating inward current, which brings the cells to firing threshold and maintains tonic firing; both Ca(++)-dependent and Ca(++)-independent K+ channels, which repolarize cells after spikes and maintain interspike intervals; and Ca++ channels, which contribute to activation of Ca(++)-activated K+ currents and may also contribute to slow depolarizing potentials. In the absence of powerful synaptic inputs, SCN neurons express a pacemaker activity that is sufficient to maintain an impressively regular firing pattern. Slow, repetitive activation of optic input, however, increases local circuit activity to such an extent that the normal pacemaker potentials are overridden and firing patterns are altered. Since SCN neurons are very small and have large input resistances, they are particularly susceptible to synaptic input.

Photic responses of geniculo-hypothalamic tract neurons in the Syrian hamster

The putative neural pacemaker controlling circadian rhythms in mammals is contained in the suprachiasmatic nuclei of the hypothalamus. These nuclei receive a projection, the geniculo-hypothalamic tract (GHT), from neurons in the intergeniculate leaflet (IGL) and portions of the ventral lateral geniculate nucleus (vLGN) of the thalamus. We examined the responses of putative GHT neurons to diffuse illumination using extracellular electrophysiological recordings. The great majority of IGL neurons showed sustained ON responses to diffuse retinal illumination; vLGN neurons showed more variation in their responses. Discharge rates of sustained ON neurons increased monotonically as light intensity was increased and saturated over 2-3 log units of intensity changes. Many IGL neurons had binocular input, and input from the ipsilateral eye was often inhibitory. These results indicate that GHT neurons may provide information about ambient light intensity to the suprachiasmatic nuclei.

The suprachiasmatic nuclei: circadian phase-shifts induced at the time of hypothalamic slice preparation are preserved in vitro

Neurons of the suprachiasmatic nuclei (SCN) of the hypothalamus compose a primary oscillator which organizes circadian rhythms in mammals. In cultured hypothalamic slices from rat brain, the SCN diurnal oscillation in neuronal firing rate continued unperturbed when slices were prepared during the light phase of the donor's light/dark cycle. However, when slices were prepared during the donor's dark period, the rhythm was phase-shifted. The sign and shape of the phase-response relationship for resetting in the isolated oscillator is very similar to that for intact animals, except that in isolation the SCN oscillator undergoes large shifts during the first cycle. The finding that a phase-shifting stimulus at the time of brain slice preparation causes normal phase readjustment in vitro demonstrates that the underlying mechanism is endogenous to the SCN and can be probed in the brain slice.

Retarded growth of the suprachiasmatic nucleus and pineal body in dw and lit dwarf mice

The suprachiasmatic nucleus (SCN) and the pineal body in 3 types of inherited hormone-deficient mice, the dw, lit and hyt mice were examined by morphological, morphometric and biochemical techniques. In the dw and lit mice the SCN was underdeveloped. In the ventral part of the SCN, where most of the retinal fibers appeared to terminate, both cell number and cell size were decreased, although the size of the SCN was unaltered. In addition, the pineal bodies of both mice were morphologically underdeveloped and showed low levels of N-acetyltransferase activity. In contrast, the hyt SCN was comparable to the normal controls in every respect. The hyt pineal was well developed and showed levels of enzyme activity comparable to the controls. However, in all the deficient mice, the optic nerve appeared to be normal in morphological and biochemical studies. These results suggest that the underdevelopment of the pineal body, the reduced levels of spontaneous locomotion and the indistinct diurnal periodicity of the dw and lit mice might be related to the retarded neuronal growth of the SCN, and that growth hormone likely is indispensable for the development of the SCN.

A second hypothalamic nucleus receiving retinal input in man: the paraventricular nucleus

A retinofugal projection to the suprachiasmatic nucleus of the hypothalamus has been described in man by means of a newly developed staining technique (PPD) for tracing degenerated fibers in the human brain. We applied the PPD method to the chiasmal/hypothalamic area of human autopsy brains from patients who had incurred prior optic nerve damage. We followed degenerated fibers from the optic nerve through the optic chiasm and the optic tract. At the optic chiasm/tract junction, some fibers were seen to diverge and to form an optic fascicle which traversed the lateral preoptic-anterior hypothalamic area towards the third ventricle. These degenerated fibers terminated in the paraventricular nucleus of the hypothalamus. We suggest that there are at least two retinohypothalamic pathways in man. Some of the neuroendocrine imbalances in blind persons may be attributed to the disruption of the retinal input to the paraventricular and suprachiasmatic nuclei of the hypothalamus. These retinohypothalamic pathways may be the anatomical substrates for light/dark entrainment of human neuroendocrine and autonomic regulatory processes.

Regular firing patterns of suprachiasmatic neurons maintained in vitro

Cells of the suprachiasmatic nucleus (SCN) recorded in vitro display a characteristic firing pattern. Unlike many other central neurons, they have the ability to fire at a constant low rate with fixed interspike interval. This regularity is most pronounced at firing rates above 3-5 spikes/s. Spontaneous firing below 3 spikes/s was less regular but became increasingly regular as the firing rate was increased. Similarly, regular discharges became irregular when the firing rate was reduced below 3-5 spikes/s. The mean spontaneous firing rate was 5.6 +/- 1.6, range less than 1 to 12 spikes/s and cells were resistant to attempts to increase their rate of firing beyond 15-20 spikes/s. Statistical analysis showed that the firing patterns of all the cells studied formed a single continuous population in terms of their interspike interval distributions, and that these distributions were a function of the firing rate. Addition of either of two commonly used anesthetics, urethane or sodium pentabarbitone, disrupted previously stable, regular activity.

The electrical properties of neurones of the rat suprachiasmatic nucleus recorded intracellularly in vitro

Stable intracellular recordings were obtained from 22 suprachiasmatic neurones in isolated brain slices. These cells were characterized by resting potentials of about -60 mV, high input resistances, relatively short time constants and action potentials of short duration. The action potentials were preceded by a slow depolarization and followed by a relatively brief afterhyperpolarization and long-lasting increase in membrane conductance. Current-voltage relations were usually linear between 0 and 80 mV negative to the resting potential. Postsynaptic potentials were evoked in these cells by electrical stimulation of the optic chiasm or contralateral suprachiasmatic nucleus. Both excitatory postsynaptic potentials, which evoked action potentials, and inhibitory postsynaptic potentials were recorded. Synaptic potentials were associated with an increase in membrane conductance. Action potentials evoked by synaptic activation were sometimes followed by up to three small, fast potentials. Small fast potentials were not seen to occur spontaneously, or to follow spontaneous, or current-evoked spikes, nor were they evoked by synaptic potentials that failed to evoke action potentials. The suprachiasmatic nucleus is essential for the generation of normal biological rhythms in mammals. The input it receives from the optic nerve is thought to be important in this role. It is hoped that these preliminary intracellular studies will form a basis for further work on the inherent properties of suprachiasmatic neurones and their responses to visual input.

Influence of environmental light-dark cycle and enucleation on activity of suprachiasmatic neurons in slice preparations

The influence of environmental light-dark cycle (LD) and bilateral enucleation on single neuronal activity in the suprachiasmatic nucleus (SCN) was examined using a hypothalamic slice preparation. Firstly, we reconfirmed previous results that the discharge rate in slices taken from animals kept on normal LD was higher during the light than during the dark period. Secondly, the day time discharge rate in the ventrolateral part of the SCN was decreased by bilateral enucleation and DD housing, while in the dorsomedial part it was unaffected. Thirdly, LL housing suppressed the discharge rates in both parts during the day and night periods. The present results suggest that the dorsomedial part of the SCN is more important in regulation of the circadian rhythm of SCN neuronal activity than the ventrolateral.

Light responsiveness of the suprachiasmatic nucleus within the island with the retino-hypothalamic tract spared

To investigate daily variation in responsiveness of the suprachiasmatic nucleus (SCN) to exposure to light, the evoked change in multiple unit activity of the SCN within the retino-hypothalamic island with the retino-hypothalamic tract (RHT) spared, was measured in the rat at 6 h intervals across a 24 h day. It is found that SCN multiple unit discharge rates increase during a 1 h exposure to light which interrupts constant darkness irrespective of the time of day. A non-parametric statistical analysis did not indicate a significant daily variation in light-responsiveness of the SCN. The present results suggest that the phase-dependent shift of the SCN rhythm after a light exposure is induced not by a rhythm in light-responsiveness of the SCN mediated by the RHT, but by a more complicated machinery within the SCN.

Electrophysiological studies of the development of suprachiasmatic neuronal activity in hypothalamic slice preparations

The rate and pattern of neuronal discharge in the suprachiasmatic nucleus (SCN) during developmental stages were studied and compared with those of the ventromedial (VMH) and the lateral hypothalamus (LHA) using rat hypothalamic slices. The firing rate of the SCN neurons was low on the 7th and 11th days; however, it dramatically increased by the 14th day to reach the adult rate, while firing rates of VMH and LHA neurons increased gradually with age. The discharge rate of neurons in the ventrolateral part of the SCN (VL-SCN) was higher than that of the dorsomedial SCN (DM-SCN) neurons in 14-, 21- and 70-100-day-old rats. Activity of the DM-SCN neurons on day 21 were unaffected by bilateral enucleation on the third day, while activity in the VL-SCN decreased; that of both parts was significantly decreased by a constant light schedule.

Reversal of multiunit activity within and outside the suprachiasmatic nucleus in the rat

Multiunit activity (MUA) in the suprachiasmatic nucleus (SCN) and adjacent hypothalamic areas were compared in albino rats anesthetized with thiopental, immobilized with succinylcholine chloride and artificially respired. During adaptation to light lasting up to 2 h, MUA in the SCN increased gradually, whereas MUA in the other locations outside the SCN decreased. During adaptation to the dark, MUA in the SCN decreased whereas MUA outside the SCN increased. This reversal was apparent not only when recording in the SCN and adjacent hypothalamus simultaneously, but also while advancing an electrode from the optic chiasm through the SCN to the anterior hypothalamus.

Ontogeny of the vasopressinergic neurons of the suprachiasmatic nucleus and their extrahypothalamic projections in the rat brain--presence of a sex difference in the lateral septum

Immunocytochemical studies have revealed the presence of extensive vasopressinergic projections from the suprachiasmatic nucleus to the limbic system and other brain areas. Vibratome sections and the unlabeled antibody enzyme method were used to investigate the ontogeny of the vasopressinergic neurons of the suprachiasmatic nucleus and their exohypothalamic fibers in the rat brain. The first immunopositive neurons of this nucleus were revealed on the 2nd postnatal day. An adult appearance of the suprachiasmatic nucleus was detected on day 14. Although fibers appeared on the periventricular nucleus already on the 7th postnatal day, such fibers were visible in the lateral septum and lateral habenular nucleus only on day 10. From the 12th postnatal day onwards a marked sex difference developed with respect to the density of the vasopressin fibers in the lateral septum and, to a lesser extent, in the lateral habenular nucleus. In male rats the fiber density was higher in both areas. This sex difference persisted in adulthood.

In vivo metabolic activity of a putative circadian oscillator, the rat suprachiasmatic nucleus

The suprachiasmatic nucleus (SCN) has been proposed as a site for an endogenous circadian oscillator in mammals, since lesions of the nucleus abolish a wide spectrum of overt-circadian rhythms. To demonstrate that a directly measurable property of the SCN itself in intact (unlesioned) animals is affected by environmental light and exhibits circadian rhythmicity, we used the autoradiographic 2-deoxy-D-[14C]glucose method to determine glucose utilization of rat SCN under a variety of lighting conditions. Our experiments indicate an important role for the SCN in circadian rhythm organization, and we believe the deoxyglucose method will prove useful as a tool for better understanding the functions and mechanisms of circadian clocks. Key words: suprachiasmatic nucleus, circadian rhythm, 2-deoxy-D-[14C]glucose.

Plasticity in synaptic appositions of optic nerve afferents under different lighting conditions

The influence of constant light and darkness (14 days) on the ultrastructure of synaptic appositions of optic nerve afferents has been studied in the suprachiasmatic nucleus (SCN) of hooded rats. Light exposure causes a general loss of postsynaptic density (PD) material, whereas after constant darkness the postsynaptic densities are thicker and the number of subjunctional bodies is higher. Accordingly, there are changes in the relative number of 'asymmetrical' (excitatory?) and 'symmetrical' (inhibitory?) synaptic appositions in optic and non-optic synapses. This phenomenon seems to reflect an adaptive response of the postsynaptic neurons to a long-term change of input activity.

Suprachiasmatic nucleus neurones: excitation and inhibition mediated by the direct retino-hypothalamic projection in female rats

The suprachiasmatic nucleus (SCN) of female rats was surveyed with microelectrodes under urethane anaesthesia. In rats with bilateral transection of the optic tracts, repetitive three pulses of 100 Hz applied to the contralateral optic nerve excited 8 and inhibited 11 other of the 86 SCN units examined. Transection of the optic tract did not significantly influence frequency of occurrence of the SCN units that were excited or inhibited by stimulation of the optic nerve. Certain SCN units responded to both of contralateral and ipsilateral stimulations of the optic nerve, indicating that bilateral visual inputs converge on the same single SCN neurones. Oscillatory responses with a period of 100--200 msec were occasionally produced by stimulation of the optic nerve. Flash stimuli with relatively weak intensity, even insufficient for producing wavelets in electroretinograms, produced an excitation and inhibition in SCN units. The mean firing rates were significantly altered by either electrical or flash stimuli repeated 500 times at 0.97 Hz in those units which showed no transitory response. Some of the SCN neurones receiving visual inputs were identified to be the tuberoinfundibular neurone and some other SCN neurones were found to receive converging inputs both from the optic nerve and from the axon collaterals of tuberoinfundibular neurones.

Retinohypothalamic tract development: alteration by suprachiasmatic lesions in the neonatal rat

The suprachiasmatic nucleus (SCN), the terminal nucleus of the retinohypothalamic (RH) tract, was ablated electrolytically in 2-day-old rats. Since previous studies have demonstrated that the earliest retinal fibers arrive on postnatal day 3 to 4 (refs. 4, 17), the lesions were inflicted prior to the formation of retinal connections. At day 25, [3H]proline was injected into the eye and autoradiography performed to determine if developing RH fibers would exhibit plasticity and innervate any hypothalamic nucleus other than the SCN. No evidence was found for the formation of anomalous retinal connections after complete, bilateral SCN lesions. Incomplete lesions, however, result in some alteration in the distribution of retinal fibers to the SCN. If a part of the caudal three-quarters of the SCN remains, RH projections form in a pattern dependent upon the size and location of the intact SCN fragment. These results indicate a high degree of specificity in developing RH fibers for their normal target tissue and a minimal capacity for plasticity in contrast with the various forms of neuronal reorganization observed after early destruction of other components of the visual system.

Suprachiasmatic nucleus responsiveness to photic and basal hypothalamic stimulation

In view of the demonstrated role of light and of the suprachiasmatic nucleus (SCN) in the maintenance of circadian rhythms related to endocrine functions and as the mediobasal hypothalamus (MBH) controls neuroendocrine activity, the effects of light and MBH stimulation on the electrical activity of SCN neurons were studied in rats. Out of 253 cells studied in the SCN, 32 cells were antidromically activated, while 65 cells responded orthodromically to MBH stimulation. In another series of experiments out of 95 suprachiasmatic neurons, 11 were antidromically activated by MBH stimulation and 18 different SCN cells responded to continuous light. The present data, which demonstrate photic responses in suprachiasmatic neurons and direct projections from the SCN to MBH, are discussed in view of the current knowledge on the role of the retinohypothalamic tract and the SCN in the control of circadian rhythms.

Participation of limbic-hypothalamic structures in circadian rhythm of slow wave sleep and paradoxical sleep in the rat

The effect of brain lesion or surgical isolation of the neural circuit on SWS and PS circadian rhythm have been studied in female rats under a 14/10 light-dark schedule. Cortical EEG'S AND DORSAL NECK EMG were used to monitor SWS, PS and alertness in female rats. Intact and operated controls showed regular 4-5-day vaginal cycles and nocturnal sleep rhythm, but the night PS value on proestrus was lower than in other cycles. Following septal lesion, MPO roof cut, vaginal cycles and SWS rhythm were regularly maintained; however, the PS appearance at night, except during proestrus, increased (night PS peak). These results were similar to those for pinealectomized or ovariectomized female rats. A frontal cut of the MBH produced persistent estrus and disturbed both SWS and PS circadian rhythm. The suprachiasmatic-lesioned rats showed persistent estrus and disrupted SWS rhythm, but regularly maintained the circadian PS rhythm. The vaginal cycles and SWS rhythm in the fornical-transected rats were regularly maintained, but the PS rhythm was disturbed during diestrus and showed ultradian rhythm. From these results, it is suggested that the pineal hormone and the gonadal feedback mechanisms may be involved in the night PS peak and this mechanism may involve the septal- and amygdaloid-hypothalamic systems. A different neural mechanism exist for SWS and PS circadian rhythm; SWS rhythm involves the suprachiasmatic-basal hypothalamic system and PS circadian rhythm is related, in part, to the hippocampal-hypothalamic system.

Suprachiasmatic nucleus: use of 14C-labeled deoxyglucose uptake as a functional marker

Glucose consumption of the rat suprachiasmatic nuclei (SCN) was studied under various experimental conditions by means of the [14C]deoxyglucose (DG) technique. The results show that glucose consumption of the SCN, in contrast to other brain structures, is a function of both the time of day and environmental lighting conditions. These data are consistent with the hypothesis that the SCN have an essential role in circadian rhythm regulation and indicate that the DG technique may provide a novel approach for the study of the central neural mechanisms underlying circadian rhythm regulation.