Persistence of nonphotic phase shifts in hamsters after serotonin depletion in the suprachiasmatic nucleus

Serotonin-containing fibres (5-HT) project from the raphe complex to the suprachiasmatic nucleus (SCN). Previous studies have suggested that this pathway may be involved in nonphotic resetting of the circadian clock. For example, 5-HT agonists are capable of phase shifting the biological clock both in vivo and in vitro, producing phase response curves (PRCs) similar in shape to those of other nonphotic stimuli. Therefore we studied the role of the serotonergic projection to the SCN in nonphotic phase shifts by bilateral injection of the selective 5-HT neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT) onto the SCN of hamsters. About 50 days after the administration of the neurotoxin, the 5-HT and 5-HIAA (5-hydroxyindole acetic acid) levels were severely depleted in the SCN, as revealed by high performance liquid chromatography (HPLC), and immunocytochemistry (ICC). The average level of 5-HT depletion was 88% in Experiment 1 and 95% in Experiment 2. This treatment had no effect on the magnitude of phase shifts produced by 3 h of novelty-induced wheel-running starting at circadian time (CT) 4, the peak of the advance region of the PRC to this stimulus. The effect of 5-HT depletion on shifts produced by running at CT 22 were inconclusive because of changes in the behavior of control animals. No changes in the phase angle of entrainment of animals in a 14:10 light:dark (LD) cycle were detected in depleted animals. The results suggest that the 5-HT projection from the raphe to the SCN is not essential for activity-induced phase shifts in hamsters.     

Neonatal suprachiasmatic nucleus lesions: Effects on the development of circadian rhythms in the rat

Previous studies of the effects of suprachiasmatic nucleus (SCN) destruction and visual pathway transections in adult rodents have revealed the primary significance of the SCN and the retinohypothalamic (RH) projection in the generation and entrainment of circadian rhythms. In the present study we found that complete ablation of the SCN in 2-day-old rats, prior to its innervation by the RH projection, permanently eliminates circadian rhythms in spontaneous locomotor activity and drinking; activity and drinking appear randomly distributed over the light-dark cycle. In addition, females exhibit long periods of constant vaginal cornification and an absence of normal estrous cycles. These effects are independent of the animal's visual status; that is, they occur in blinded as well as sighted animals. Incomplete SCN lesions results in partial disruption of rhythmic functions such as damping of circadian rhythms in activity and/or drinking, irregular estrous cycling, and/or complete disruption of only one or two of these measures of rhythmicity. The absence of spared functions after early SCN destruction is consistent with the high degree of specificity for the SCN exhibited by developing RH fibers and further emphasizes the significance of the SCN in circadian rhythm generation. Neither morphological nor functional plasticity has been found following neonatal ablation of the SCN in the rat.     

Effect of 192 IgG-saporin on circadian activity rhythms,expression of P75 neurotrophin receptors,calbindin-D28K,and light-induced Fos in the suprachiasmatic nucleus in rats

Photic entrainment of circadian rhythms in mammals is mediated through a direct retinal projection to the core region of the suprachiasmatic nucleus (SCN), the circadian clock. A proportion of this projection contains the low-affinity p75 neurotrophic receptor (p75NTR). Neonatal monosodium glutamate (MSG) treatment, which dramatically reduces p75NTR immunoreactivity in the SCN has no impact on photic entrainment. In order to clarify the contribution of p75NTR fibers in photic entrainment, targeted lesions of the p75NTR-immunoreactive SCN plexus were performed using intracerebroventricular (ICV) or intrahypothalamic injections of the immunotoxin 192 IgG-saporin (SAP) in rats. SAP treatment effectively abolished p75NTR immunoreactivity within the SCN core. ICV SAP treatment produced three different behavioral activity patterns: Animals became arrhythmic, displayed a shorter free-running period, or remained rhythmic following the lesion. Arrhythmic animals had large hypothalamic lesion which encompassed the entire SCN. In rhythmic rats, ICV-SAP significantly reduced immunostaining for calbindin-D28k (CaBP) in the SCN, and rats with shortened free-running periods had the lowest number of CaBP immunoreactive cells. ICV SAP also attenuated light-induced Fos expression in the SCN core. Despite lack of p75NTR and reduced CaBP and Fos expression in the SCN, SAP-treated rhythmic rats displayed normal photic entrainment. Intrahypothalamic SAP treatment reduced CaBP expression in the SCN but had no effect on light-induced Fos expression, free-running rhythms, or photic entrainment. The data show that p75NTR-immunoreactive elements in the SCN are not required for photic entrainment.     

Loss of entrainment and anatomical plasticity after lesions of the hamster retinohypothalamic tract

The suprachiasmatic nuclei receive photic input information directly through a retinohypothalamic tract (RHT) and indirectly through a projection from the intergeniculate leaflet of the lateral geniculate complex, the geniculohypothalamic tract (GHT). Prior work has established that the RHT is sufficient for entrainment, but has not shown whether it is necessary because it has not been possible to transect that pathway. The present study addresses this problem by employing knife cuts to sever the RHT in male hamsters. Three knife cut procedures were used and one of these succeeded in separating the SCN from the optic chiasm in 8 animals with limited damage to the chiasm and the SCN. The effectiveness of the RHT lesion was confirmed by cholera toxin-HRP histochemistry which demonstrated that the knife cuts eliminate the normal retinal innervation of the SCN while sparing projections to thalamic and tectal visual centers. In a light-dark cycle, the lesioned animals exhibit free-running rhythms indicating that the RHT is necessary for entrainment. A surprising observation is the presence of extensive axonal sprouting of retinal fibers in brains of animals with RHT lesions. The newly-formed axons grow extensively into the SCN, anterior hypothalamus and basal forebrain, but form anomalous axonal plexuses which have no evident function.     

A Thalamic Contribution to Arousal-induced, Non-photic Entrainment of the Circadian Clock of the Syrian Hamster

It is well established that the circadian clock of the suprachiasmatic nuclei (SCN) is entrained by light. More recently, the potent effects of arousing, non-photic cues on the clock have been recognized. The neural mediators of non-photic entrainment are yet to be identified. To examine the contribution of the thalamic intergeniculate leaflet (IGL) and its NPY-immunopositive projection, the geniculo-hypothalamic tract to non-photic entrainment by arousal, male Syrian hamsters received lesions of the IGL (IGLX) which ablated NPY-immunoreactivity in the SCN. Their circadian responses to both photic and non-photic cues were then tested. Lesions resulted in a delay in the timing of activity onset following lights out, but had no effect on the behavioural or cellular circadian responses to phase-advancing light pulses presented at circadian time (CT) CT19 (where CT12 represents the time of activity onset). Injection with a benzodiazepine (chlordiazepoxide, 100 mg/kg) at CT6 suppressed wheel-running, increased general locomotion of intact controls and induced large phase advances of the circadian rhythm of wheel-running. Chlordiazepoxide also inhibited wheel-running in lesioned animals, but there was no significant increase in general locomotion and the lesioned animals did not phase advance. Serial arousal by injection of saline at intervals of 23.5 h for 6 days entrained the circadian rhythm of wheel-running of intact hamsters and was associated with an increase in general locomotor activity. Entrainment by serial arousal was abolished by IGLX. However, the lesioned animals did show a clear behavioural response to every presentation of the non-photic cue. These results show that the IGL is a necessary component of the neural pathways mediating both arousal- and benzodiazepine-induced non-photic entrainment.     

Serotonergic afferents of the pigeon accessory optic nucleus

Injections of a retrograde tracer into the accessory optic nucleus of the basal optic root (nBOR) of the pigeon, combined with 5-HT immunohistochemistry, revealed that serotonergic projections to the nBOR appeared to originate mainly from the median (MR) and paramedian (PMR) raphe nuclei. These projections were confirmed by the significant decrease in 5-HT immunoreactivity observed in nBOR after lesions in MR and PMR. These data characterize distinct sources of 5-HT innervation to the pigeon nBOR and suggest that those afferents could represent part of a modulatory system that contributes to the role of the nBOR in optokinetic mechanisms.     

A daily palatable meal without food deprivation entrains the suprachiasmatic nucleus of rats

Food is considered a potent Zeitgeber for peripheral oscillators but not for the suprachiasmatic nucleus (SCN), which is entrained principally by the light-dark cycle. However, when food attains relevant properties in quantity and quality, it can be a potent Zeitgeber even for the SCN. Here we evaluated the entrainment influence of a daily palatable meal, without regular food deprivation, on the circadian rhythm of locomotor activity and the c-Fos and PER-1 protein expression in the SCN. Rats fed ad libitum, in constant darkness, received a palatable meal for 6 weeks starting in the middle of the subjective day. Locomotor activity showed entrainment when the offset of activity coincided with the palatable meal-time. In the SCN, the peak expression of c-Fos was observed at palatable meal-time and PER-1 showed a peak during the onset of subjective night, as predicted according to the behavioural entrained pattern. In addition, c-Fos and PER-1 expression in the paraventricular thalamic nucleus (PVT) showed increased expression at palatable meal-time, while the intergeniculate leaflet did not, suggesting that the PVT may be involved as an input pathway of palatable food-entrainment to the SCN. These results demonstrate that daily access to a palatable meal can entrain the SCN; several stimuli can be implicated in this process, including motivation and arousal.     

Turning behaviour induced by stimulation of the 5-HT receptors in the subthalamic nucleus

The basal ganglia, which receive a rich serotonergic innervation, have been implicated in hyperkinetic and hypokinetic disorders. Moreover, a decrease in subthalamic nucleus (STN) activity has been associated with motor hyperactivity. To address the role of subthalamic serotonergic innervation in its motor function, turning behaviour was studied in rats with stimulation of the subthalamic serotonin (5-HT) receptors by intracerebral microinjections. The intrasubthalamic administration of 5-HT induced dose-dependent contralateral turning behaviour, with a maximal effect at a dose of 2.5 microg in 0.2 microL. Similar results were observed with microinjections of other 5-HT receptor agonists: quipazine (a 5-HT2B/C/3 agonist), MK-212 (a 5-HT2B/C agonist) and m-chlorophenylbiguanidine (a 5-HT3 agonist), while microinjections of 5-HT into the zona incerta or in the previously lesioned STN were ineffective. The effect of 5-HT was blocked by coadministration of the antagonist mianserin. Stimulation of subthalamic 5-HT receptors in animals bearing a lesion of the nigrostriatal pathway did not modify the motor response, which indicates that the dopamine innervation of the nucleus is not involved in this effect. Kainic acid lesion of the substantia nigra pars reticulata (SNr) suppressed the contralateral rotations elicited by stimulation of 5-HT2B/C/3 subthalamic receptors. This suggests a role of the subthalamic-nigral pathway in the turning activity. Furthermore, the partial blockade of glutamatergic receptors in the SNr by the antagonist DNQX increased the contralateral circling elicited by stimulation of 5-HT receptors in the STN. We concluded that the activation of the 5-HT2B/C and 5-HT3 subthalamic receptors elicited contralateral turning behaviour, probably via the subthalamic-nigral pathway.     

Serotonin antagonists do not attenuate activity-induced phase shifts of circadian rhythms in the Syrian hamster

A variety of observations from several rodent species suggest that a serotonin (5-HT) input to the suprachiasmatic nucleus (SCN) circadian pacemaker may play a role in resetting or entrainment of circadian rhythms by non-photic stimuli such as scheduled wheel running. If 5-HT activity within the SCN is necessary for activity-induced phase shifting, then it should be possible to block or attenuate these phase shifts by reducing 5-HT release or by blocking post-synaptic 5-HT receptors. Animals received one of four serotonergic drugs and were then locked in a novel wheel for 3 h during the mid-rest phase, when novelty-induced activity produces maximal phase advance shifts. Drugs tested at several doses were metergoline (5-HT_1/2 antagonist; i.p.), (+)-WAY100135 (5-HT_1A postsynaptic antagonist, which may also reduce 5-HT release by an agonist effect at 5-HT_1A raphe autoreceptors; i.p.), NAN-190 (5-HT_1A postsynaptic antagonist, which also reduces 5-HT release via an agonist effect at 5-HT_1A raphe autoreceptors; i.p.) and ritanserin (5-HT_2/7 antagonist; i.p. and i.c.v.). Mean and maximal phase shifts to running in novel wheels were not significantly affected by any drug at any dose. These results do not support a hypothesis that 5-HT release or activity at 5HT1, 2 and 7 receptors in the SCN is necessary for the production of activity-induced phase shifts in hamsters.     

Phenotype and function of raphe projections to the suprachiasmatic nucleus

The circadian clock, located in the suprachiasmatic nucleus (SCN), receives a major afferent from the median raphe nucleus (MRN). In the Syrian hamster, only about 50% of the cells giving rise to this afferent contain serotonin. There is mixed evidence as to whether the serotonergic portion of this projection is involved in non‐photic phase shifting of circadian locomotor rhythms. In order to better characterize the non‐serotonergic projections, we conducted retrograde tract tracing using the beta subunit of cholera toxin combined with multi‐label immunohistochemistry. Similar to previous findings, almost half of the retrogradely labeled cells contained serotonin. Additionally, approximately 30% of the retrogradely labeled cells contained vesicular glutamate transporter 3 (VGLUT3), but not serotonin. Surprisingly, some dorsal raphe cholera toxin labeling was also noted, particularly in animals with central‐SCN injections. To determine if the non‐serotonergic projections were important for non‐photic phase shifts elicited by MRN stimulation, the MRN was electrically stimulated in animals pretreated with SCN injection of either the serotonin neurotoxin 5,7‐dihydroxytryptamine or vehicle control. Intact animals phase advanced to midday electrical stimulation of the raphe while lesioned animals did not. Together, these results show that although some of the non‐serotonergic raphe projections to the SCN contain VGLUT3, it is the serotonergic raphe innervation of the SCN that is critical for non‐photic phase shifting elicited by MRN stimulation.     

Retinal input to the suprachiasmatic nucleus before and after puberty in Djungarian hamsters

Retinal projections to the suprachiasmatic nucleus (SCN) mediate the effect of photoperiod to entrain circadian rhythms and to control reproductive maturation in the Djungarian hamster. To determine whether the retinal innervation of the SCN had fully developed by the onset of puberty in this hamster species, prepubertal and postpubertal hamsters received an intraocular unilateral injection of horseradish peroxidase (HRP), and after 24 h, the anterograde transport of HRP to the SCN was studied. In prepubertal hamsters, the retinohypothalamic tract (RHT) was found to project to the medial and caudal SCN, principally the ventrolateral regions and, to an extent, the dorsomedial portion of the nucleus. RHT innervation was asymmetric; the SCN contralateral to the monocular injection received the dominant projection. A similar pattern of retinal projections was found postpubertally; however, the ipsilateral SCN was less extensively labelled with HRP and smaller as determined by Nissl counterstain compared to that in prepubertal hamsters. These findings indicate that modifications in the retinal innervation of the SCN occur as late as puberty, and may be part of a developmental change in the mechanism which processes photoperiodic information during sexual maturation.     

Involvement of 5-HT1A receptor mechanisms in the inhibitory effects of methamphetamine on photic responses in the rodent suprachiasmatic nucleus

We examined the role of serotonin 1A (5-HT_1A) receptors in the inhibitory effects of methamphetamine (MA) on photic entrainment to the circadian pacemaker in the suprachiasmatic nucleus (SCN) of rodents. MA inhibited optic nerve stimulation-evoked field potential in the SCN, light-induced Fos expression in the SCN and light-induced phase shift of hamster wheel-running rhythm. NAN-190, a 5-HT_1A receptor antagonist, eliminated the inhibitory effects of MA. NAN-190 has also been reported to antagonize ₁ adrenergic receptors. However, prazosin, which selectively antagonizes ₁ adrenergic receptors, did not affect the inhibitory action of MA on light-induced Fos expression. In addition, parachloroamphetamine, which is known to be a 5-HT releaser, dose-dependently inhibited light-induced phase shift of wheel-running rhythm. These findings suggest that elevation of endogenous 5-HT levels by MA inhibits the photic entraining responses of the circadian pacemaker in the SCN via 5-HT_1A receptor stimulation of the 5-HT released by MA.     

Effects of 6-hydroxydopamine lesions of the nigrostriatal pathway on striatal serotonin innervation in adult rats

The effects of 6-hydroxydopamine lesions of the nigrostriatal pathway on striatal serotonin (5-HT) innervation have been examined using immunohistochemistry in adult rats. One day after lesioning, a large number of swollen and densely stained 5-HT-immunoreactive fibers appeared around the lesion which was almost completely void of 5-HT-immunoreactivity. Four weeks after lesioning, a significant reduction in 5-HT innervation density was verified in the ventral portion of the rostral neostriatum and in the caudal neostriatum of the lesioned side. Eight weeks after lesioning, a similar decrease in 5-HT innervation density was observed in the neostriatum on the lesioned side. Some aberrant 5-HT-immunoreactive fibers were found around the lesion of the nigrostriatal pathway. These results indicate that 6-hydroxydopamine lesions of the nigrostriatal bundle of adult rats induce a reduction in striatal serotonin innervation density as well as aberrant morphology of 5-HT-immunoreactive fibers around the lesion.     

Role of the thalamic intergeniculate leaflet and its 5-HT afferences in the chronobiological properties of 8-OH-DPAT and triazolam in syrian hamster

The 5-HT(1A/7) receptor agonist 8-hydroxy-2-[di-n-propylamino]-tetralin (8-OH-DPAT) has chronobiological effects on the circadian system and, in the Syrian hamster, it is known that serotonergic (5-HT) projections connecting the median raphe nucleus to the suprachiasmatic nuclei (SCN) of the hypothalamus are a prerequisite for the expression of 8-OH-DPAT-induced phase advance of locomotor activity rhythm. We examined the possible involvement of the thalamic intergeniculate leaflet (IGL) in the phase-shifting properties of 8-OH-DPAT injections at CT7. Bilateral electrolytic lesions of the IGL blocked phase-shift responses to 8-OH-DPAT of the activity rhythm. Phase changes induced by injections of 8-OH-DPAT at CT7 and triazolam (Tz), a short-acting benzodiazepine, at CT6 were also studied after bilateral chemical lesion of the 5-HT fibres connecting the dorsal raphe nucleus (DR) to IGL. Destruction of 5-HT fibres within the IGL blocked the phase-shift response to Tz, but not the phase-shift response to 8-OH-DPAT. In conclusion, (a) IGL is essential for the phase-shifting effect of peripheral 8-OH-DPAT injections; (b) 5-HT fibres connecting DR to IGL are necessary for the expression of the phase-shifting effect of Tz but not of 8-OH-DPAT.     

Development of serotonin immunoreactivity in the rat spinal cord and its plasticity after neonatal spinal cord lesions

The postnatal maturation of spinal pathways may account for the gradual time course of postnatal development of behavior and also account for the greater anatomical reorganization which often follows damage to the developing CNS compared to the mature CNS. The purpose of the current study was to examine (1) the prenatal and postnatal development of the descending serotonergic (5-HT) projection to the spinal cord and (2) the effects of a neonatal spinal cord lesion on this development. In addition, we wished to determine (3) whether transplants of fetal spinal cord tissue placed into the neonatal lesion site alter the plasticity of the 5-HT projection to the cord. Peroxidase-antiperoxidase immunocytochemical techniques were used. At embryonic day 14 (E14), no 5-HT immunoreactive fibers could be identified at any spinal cord level. By E18 the first axons were identified in the white matter only at all spinal cord levels. At birth, 5-HT immunoreactive fibers were present both in the white matter and in the gray matter at all cord levels. The projection within the gray matter was diffuse and considerably less dense than in the adult. The postnatal maturation of the 5-HT projection within the gray matter of the spinal cord followed rostral to caudal and ventral to dorsal gradients. During the first weeks postnatal, the 5-HT immunoreactivity within the cord increased to attain an adult pattern and density by 14 days in the cervical cord and 21 days in the thoracic and lumbar cord. The effect of a spinal cord hemisection at birth on the anatomical reorganization of the descending serotonergic innervation of the cord was compared with the effect of the same lesion in the adult. In the adult animal, mid-thoracic hemisection decreased the 5-HT content of the ventral horn of the lumbar spinal cord caudal and ipsilateral to the lesion to 8% of that on the intact side. When this same lesion was made in the newborn animal, the innervation was 43% of that on the intact side. When a transplant of fetal spinal cord tissue was inserted into the lesion site in the newborn animals, there was even greater 5-HT innervation caudal to the lesion, 83% of that on the intact side. These results indicate that there is considerable postnatal development and plasticity of the descending serotonergic projection to the spinal cord, and this plasticity is enhanced by the presence of a spinal cord transplant at the site of the lesion.     

Entrainment of rat circadian rhythms by melatonin does not depend on the serotonergic afferents to the suprachiasmatic nuclei

Daily administration of melatonin (MEL) can entrain rat circadian rhythms free-running in constant darkness. The high MEL doses needed to obtain entrainment suggest the implication of other neural mechanisms than simply an effect on the hormone's specific receptors detected in the SCN. Administration of serotonin receptor agonists can phase-shift the rodent circadian clock, and MEL is known to modulate release and reuptake of serotonin in nerve endings. This raises the question of a critical involvement of 5-HT-fibres in the entraining properties of MEL. The aim of the present study was to test this hypothesis. Bilateral neurotoxic (5,7-dihydroxytryptamine) lesions of the serotonergic fibres in the SCN were performed in animals kept in LD 12:12. Following the post-operative period, the animals were transferred to constant darkness to free-run. MEL was then administered by a 1 h daily infusion. Both well lesioned and intact animals entrained to MEL. No differences were observed between lesioned and control animals on parameters such as the phase-angles between MEL onset and activity onset, and core body temperature acrophase, respectively. Entrainment of rat circadian rhythms to exogenous MEL is thus not directly dependent on the 5-HT fibres in the SCN.     

Indirect evidence for an association of 5-HT(1B) binding sites with retinal and geniculate axon terminals in the rat suprachiasmatic nucleus.

The purpose of the present study was to investigate the possible cellular location of 5-HT(1B) receptors on retinal and geniculate afferents in the rat suprachiasmatic nucleus (SCN). Biocular enucleation significantly decreased 5-HT(1B) binding site labeling (35%), specifically in the ventral part of the SCN, while monocular enucleation produced a decrease of smaller magnitude (12%), limited to the ventral part of the contralateral SCN, these results being consistent with the known distribution of retinal afferents in the nucleus. By contrast, bilateral geniculate lesion did not induce any significant variation of 5-HT(1B) binding site labeling in the SCN. Previously, we reported that serotonin (5-HT) synthesis inhibition by parachlorophenylalanine increases 5-HT(1B) binding site labeling in the SCN. Using saturation studies, we have now demonstrated that this upregulation reflected an increase in the total number of 5-HT(1B) binding sites (+41% in the dorsal and +67% in the ventral part of the SCN). Furthermore, we evaluated the effects of bilateral geniculate lesion after 5-HT stores depletion in order to overcome problems of technical resolution limits. The magnitude of upregulation was significantly decreased (27%) after bilateral geniculate lesion, suggesting that part of the 5-HT(1B) receptor population was located on geniculate axon terminals within the SCN. The possible involvement of 5-HT(1B) receptors, according to their cellular locations evidenced in the present study, in photic and nonphotic entrainment of the circadian clock is discussed.     

Suprachiasmatic nucleus and retinohypothalamic projections in moles.

The suprachiasmatic nucleus of the hypothalamus (SCN) and the retinohypothalamic projections were identified in one species of old-world moles, all of whom are blind as a result of natural loss of vision. A cyto-architectonic study revealed that the SCN is well developed, even though other visual nuclei in the dorsal thalamus and the midbrain are not. An immunohistochemical study showed that vasoactive intestinal polypeptide (VIP)-like immunoreactive cell bodies and fibers were distributed in the SCN, as has been reported in other mammals. Following intraocular injections of wheatgerm agglutinin conjugated to horseradish peroxidase (WGA-HRP), the central retinal projections were examined. The results indicated that the SCN receives a direct projection from the retina, as seen in many other mammals. In addition to the projection to the SCN, retinal fibers were seen to terminate in the anterior hypothalamic region and the retrochiasmatic area, as observed in some other mammals. In moles, retinohypothalamic projections are bilateral, with an ipsilateral predominance. Considering that the retinogeniculate and retinotectal projections are vestigial, it is highly probable that the optic pathway in moles primarily consists of retinohypothalamic projections, which are devoted to the entrainment of circadian and circannual rhythms.     

Dorsal raphe nucleus modulates neuronal activity in rat intergeniculate leaflet

Serotonergic input from midbrain raphe nuclei is believed to have a significant effect on mammalian circadian timing system. The suprachiasmatic nucleus (SCN) receives its serotonergic input from the median raphe nucleus, while the intergeniculate leaflet (IGL) receives serotonergic innervation from the dorsal raphe nucleus (DRN). The present paper was aimed at determining whether projection from the DRN affected rhythmic neuronal oscillations in the IGL of rats. We investigated the impact of electrolytic lesions and electric stimulation of the DRN on spontaneous isoperiodic (i.e. burst firing with a constant interburst interval) neuronal activity recorded in the IGL. In all our experiments a complete lesion of the DRN always caused a significant increase (ca. 100%) of spontaneous activity of IGL neurons, their oscillatory character having been maintained, though. On the other hand, electric stimulation of the DRN produced a transient decrease in firing rate oscillations of the IGL neurons. The obtained results indicate that the neuronal projection from the DRN has a substantial modulating effect on IGL activity-an important element of the mechanism of the circadian time-keeping system that mediates the transfer of non-photic information to the SCN by modulating its activity. The observed increase of isoperiodic activity in the IGL after DRN lesion and a transient decrease in this activity after electric stimulation indicate an inhibitory character of this effect. The present findings corroborate the hypothesis that the DRN is a one of the major and extremely important source of the modulatory inputs to the mammalian circadian time-keeping system.     

Does the ventromedial hypothalamic nucleus contain a self-sustained circadian oscillator associated with periodic feedings?

Multiple unit activities (MUAs) of the ventromedial hypothalamic nucleus (VMH) were recorded from intact free-moving male rats. Rhythmic activities observed in the VMH were not distinguishable from those recorded in other places of the brain. When exposed to restricted daily feeding schedules, animals with suprachiasmatic nucleus (SCN) lesions developed a rhythm in motor activity and a rhythm of the VMH with a peak of activity immediately after and a trough right before the feeding. While neural activities of the SCN revealed circadian rhythms even after a lesion of the VMH, circadian rhythms in MUA of the VMH were completely abolished by a lesion of the SCN. Although the VMH is involved in the synchronization of rhythms to periodic feeding, the present results indicate that, unlike the SCN which contains an oscillator associated with light, the VMH does not contain another self-sustained oscillator associated with food.