The effect of brainstem stimulation on the evoked potentials in the intergeniculate leaflet

On the intrinsic regulation of neuropeptide Y release in the mammalian suprachiasmatic nucleus circadian clock

Timing of the circadian clock of the suprachiasmatic nucleus (SCN) is regulated by photic and non-photic inputs. Of these, neuropeptide Y (NPY) signaling from the intergeniculate leaflet (IGL) to the SCN plays a prominent role. Although NPY is critical to clock regulation, neither the mechanisms modulating IGL NPY neuronal activity nor the nature of regulatory NPY signaling in the SCN clock are understood, as NPY release in the SCN has never been measured. Here, microdialysis procedures for in vivo measurement of NPY were used in complementary experiments to address these questions. First, neuronal release of NPY in the hamster SCN was rhythmic under a 14L : 10D photocycle, with the acrophase soon after lights-on and the nadir at midday. No rhythmic fluctuation in NPY occurred under constant darkness. Second, a behavioral phase-resetting stimulus (wheel-running at midday that induces IGL serotonin release) acutely stimulated SCN NPY release. Third, bilateral IGL microinjection of the serotonin agonist, (+/-)-2-dipropyl-amino-8-hydroxyl-1,2,3,4-tetrahydronapthalene (8-OH-DPAT) (another non-photic phase-resetting stimulant), at midday enhanced SCN NPY release. Conversely, similar application of the serotonin antagonist, metergoline, abolished wheel-running-induced SCN NPY release. IGL microinjection of the GABA agonist, muscimol, suppressed SCN NPY release. These results support an intra-IGL mechanism whereby behavior-induced serotonergic activity suppresses inhibitory GABAergic transmission, promoting NPY activity and subsequent phase resetting. Collectively, these results confirm IGL-mediated NPY release in the SCN and verify that its daily rhythm of release is dependent upon the 14L : 10D photocycle, and that it is modulated by appropriately-timed phase-resetting behavior, probably mediated by serotonergic activation of NPY units in the IGL.

Medial vestibular connections with the hypocretin (orexin) system

The mammalian medial vestibular nucleus (MVe) receives input from all vestibular endorgans and provides extensive projections to the central nervous system. Recent studies have demonstrated projections from the MVe to the circadian rhythm system. In addition, there are known projections from the MVe to regions considered to be involved in sleep and arousal. In this study, afferent and efferent subcortical connectivity of the medial vestibular nucleus of the golden hamster (Mesocricetus auratus) was evaluated using cholera toxin subunit-B (retrograde), Phaseolus vulgaris leucoagglutinin (anterograde), and pseudorabies virus (transneuronal retrograde) tract-tracing techniques. The results demonstrate MVe connections with regions mediating visuomotor and postural control, as previously observed in other mammals. The data also identify extensive projections from the MVe to regions mediating arousal and sleep-related functions, most of which receive immunohistochemically identified projections from the lateral hypothalamic hypocretin (orexin) neurons. These include the locus coeruleus, dorsal and pedunculopontine tegmental nuclei, dorsal raphe, and lateral preoptic area. The MVe itself receives a projection from hypocretin cells. CTB tracing demonstrated reciprocal connections between the MVe and most brain areas receiving MVe efferents. Virus tracing confirmed and extended the MVe afferent connections identified with CTB and additionally demonstrated transneuronal connectivity with the suprachiasmatic nucleus and the medial habenular nucleus. These anatomical data indicate that the vestibular system has access to a broad array of neural functions not typically associated with visuomotor, balance, or equilibrium, and that the MVe is likely to receive information from many of the same regions to which it projects.

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.

Ultradian rhythmic neuronal oscillation in the intergeniculate leaflet

Our paper is the first to describe ultradian rhythmic neuronal oscillation in the intergeniculate leaflet (IGL) of the rat. We recorded a multiple-unit neuronal activity (MUA) from dorsal to ventral parts of the lateral geniculate nucleus (LGN) in anaesthetized rats. In all the subdivisions of the lateral geniculate complex we observed spontaneous irregular firing rates of cells. However only at the anatomical localisation of the IGL, after the light was on, those responses exhibited burst firing with a constant interburst interval, which lasted several hours until the light was off. The duration of that rhythmic oscillation obtained by means of Fourier's analysis was approximately 124 s. To date we have not had sufficient data to discuss possible mechanisms of this neuronal rhythmicity. We can only conclude that light is the most important stimulus not only for suprachiasmatic nuclei (SCN), but also for the IGL. On the other hand, we can neither exclude nor confirm that in order to evoke ultradian rhythmical oscillation in the IGL, in addition to light also non-photic information is necessary.