Joan C. Speh

Rest and its corepressors mediate plasticity of neuronal gene chromatin throughout neurogenesis

Regulation of neuronal gene expression is critical to central nervous system development. Here, we show that REST regulates the transitions from pluripotent to neural stem/progenitor cell and from progenitor to mature neuron. In the transition to progenitor cell, REST is degraded to levels just sufficient to maintain neuronal gene chromatin in an inactive state that is nonetheless poised for expression. As progenitors differentiate into neurons, REST and its co-repressors dissociate from the RE1 site, triggering activation of neuronal genes. In some genes, the level of expression is adjusted further in neurons by CoREST/MeCP2 repressor complexes that remain bound to a site of methylated DNA distinct from the RE1 site. Expression profiling based on this mechanism indicates that REST defines a gene set subject to plasticity in mature neurons. Thus, a multistage repressor mechanism controls the orderly expression of genes during development while still permitting fine tuning in response to specific stimuli.

GABA is the principal neurotransmitter of the circadian system.

The circadian timing system imposes a temporal organization on physiological processes and behavior. The two major nuclei of the system are the intergeniculate leaflet (IGL) of the lateral geniculate complex and the suprachiasmatic nucleus (SCN) of the hypothalamus. In this study, we demonstrate that neurons of both nuclei colocalize GABA with peptides. In the IGL, GABA is colocalized with neuropeptide Y in neurons projecting to the SCN and with enkephalin in neurons projecting to the contralateral IGL. In the SCN, GABA is colocalized with vasopressin and vasoactive intestinal polypeptide. All, or nearly all, of the neurons in the IGL and SCN are GABA-producing. Thus, GABA should be considered the principal neurotransmitter of the circadian system.

Suprachiasmatic nucleus organization

Abstract. The suprachiasmatic nucleus (SCN) of the hypothalamus is a dominant circadian pacemaker in the mammalian brain controlling the rest-activity cycle and a series of physiological and endocrine functions to provide a foundation for the successful elaboration of adaptive sleep and waking behavior. The SCN is anatomically and functionally organized into two subdivisions: (1) a core that lies adjacent to the optic chiasm, comprises predominantly neurons producing vasoactive intestinal polypeptide (VIP) or gastrin-releasing peptide (GRP) colocalized with GABA and receives dense visual and midbrain raphe afferents, and (2) a shell that surrounds the core, contains a large population of arginine vasopressin (AVP)-producing neurons in its dorsomedial portion, and a smaller population of calretinin (CAR)-producing neurons dorsally and laterally, colocalized with GABA, and receives input from non-visual cortical and subcortical regions. In this paper, we present a detailed quantitative analysis of the organization of the SCN core and shell in the rat and place this in the context of the functional significance of the subdivisions in the circadian control of regulatory systems.

Comparative anatomy of the mammalian hypothalamic suprachiasmatic nucleus.

A detailed analysis of the cytoarchitecture, retinohypothalamic tract (RHT) pro jections, and immunohistochemical localization of major cell and fiber types within the hy pothalamic suprachiasmatic nuclei (SCN) was conducted in five mammalian species: two species of opossum, the domestic cat, the guinea pig, and the house mouse. Cytoarchitectural and immunohistochemical studies were conducted in three additional species of marsupial mammals and in the domestic pig. The SCN in this diverse transect of mammalian taxonomy bear striking similarities. First, the SCN are similar in location, lying close to the third ventricle (3V) dorsal to the optic chiasm (OC), with a cytoarchitecture characterized by small, tightly packed neurons. Second, in all groups studied, the SCN receive bilateral retinal input. Third, the SCN contain immu nohistochemically similar elements. These similarities suggest that the SCN developed char acteristic features early in mammalian phylogeny. Some details of SCN organization vary among the species studied. In marsupials, vaso pressin-like immunoreactive (VP-LI) and vasoactive intestinal polypeptide-like immunoreac tive (VIP-LI) cells codistribute primarily in the dorsomedial aspects of the SCN, while in eutherians, VP-LI and VIP-LI cells are separated into SCN subnuclei. Furthermore, the marsupial RHT projects to the periventricular dorsomedial region, whereas the eutherian RHT projects more ventrally in the SCN into the zone that typically contains VIP-LI perikarya.

Retinohypothalamic tract development in the hamster and rat

The development of the retinohypothalamic tract (RHT) in the albino rat and golden hamster was studied using anterograde transport of cholera toxin conjugated to horseradish peroxidase (CT-HRP). The RHT has three components in the adult: (1) a dense projection to the ventrolateral subdivision of the suprachiasmatic nucleus (SCN) with some fibers extending into the dorsomedial SCN; (2) a projection to adjacent areas, the anterior hypothalamic area (AHA) and retrochiasmatic area (RCA) and in the hamsters, into the preoptic area (POA); (3) a projection to the lateral hypothalamic area (LHA). In the rat, the projection to the SCN and adjacent areas first appears as scattered varicosities at the ventral border of the SCN at postnatal day 1 (P1) and gradually increases until the adult pattern is achieved at approximately P10. The projections to the AHA and RCA are seen first at P2-P3 and gradually increase to the adult appearance by P15. Both the projection to the SCN and adjacent areas and to the LHA, initially are more extensive than in the adult. Many of the axons extend well beyond the zone of the adult pattern but these anomalous fibers are eliminated by P6-P10. The LHA projection first appears at embryonic day 21-22 (E 21-22) and gradually increases in density from P1-P6. In the hamster the projections to the SCN, AHA and LHA appear first on P4 and gradually increase in density to reach the adult pattern by P15. The projections to the RCA and POA are present by P6 and reach the adult pattern by P15. None of the RHT projections in the hamster has the initial extended growth followed by pruning back that characterizes RHT development in the rat. Thus, the development of the RHT in both the rat and the hamster is complex with components of the projection appearing at different times with differing patterns of development that indicate specialized interactions of the developing axons with their target neurons. Synaptogenesis in the hamster hypothalamus was analyzed using an antiserum to synapsin I. Few synapses are present at E16, the last day of gestation, in the LHA, SCN and AHA. From P1-P3, synaptogenesis proceeds rapidly and the adult pattern is achieved in all three areas by P4.

Serotonin innervation of the primate suprachiasmatic nucleus

The suprachiasmatic nucleus (SCN) in rodents receives a dense innervation from serotonin neurons of the midbrain raphe. This projection overlaps the terminal field of the retinohypothalamic tract in the SCN core, the central part of the nucleus characterized by a population of vasoactive intestinal polypeptide (VIP)-containing neurons. To determine whether a similar pathway is present in primates, we carried out an immnunocytochemical investigation of the primate SCN using antisera against either serotonin (monkey) or the serotonin transporter (human). This demonstrated a dense serotonergic plexus over the SCN core in both species. As in rodents, the distribution of the serotonin innervation of the primate SCN overlaps that of the retinohypothalamic input and the VIP neuronal population. We also find a supraependymal plexus of serotonin axons in the third and lateral ventricles of the human and monkey brains that is similar in distribution, but less dense, than the one reported in rodents.

Morphological and functional development of the suprachiasmatic nucleus in transplanted fetal hypothalamus

The development of the suprachiasmatic nucleus (SCN) in fetal rat hypothalamus transplanted to the adult brain was studied using morphological and functional methods. Anterior hypothalamic tissue was transplanted into the third ventricle, lateral ventricle or subarachnoid space of intact, adult hosts from E17 fetuses. These transplants developed the cytoarchitectonic and immunohistochemical staining characteristics of SCN, clusters of parvocellular neurons expressing vasopressin- and vasoactive intestinal polypeptide-like immunoreactivity in adjacent cellular populations, irrespective of the exact location of the transplanted tissue in the host brain. The functional status of the transplants placed in the rostral third ventricle and the foramen of Monroe was analyzed and compared to host SCN using in vitro recording of neuronal firing rate and measurement of metabolism using the 2-deoxyglucose (2-DG) technique. During subjective day, neuronal firing rates and 2-DG uptake were high in discrete cell groups within the transplants which were subsequently demonstrated to exhibit the cytoarchitectonic and immunohistochemical characteristics of SCN. The firing rates and 2-DG uptake in these areas were lower during the subjective night. This pattern of activity closely resembles that of the intact SCN. In contrast, neither transplanted anterior hypothalamic area, lacking an identifiable SCN-like structure, nor posterior hypothalamic area showed day-night differences in firing rate or 2-DG uptake. These observations indicate that SCN transplanted into intact adult hosts exhibits morphological and functional differentiation nearly identical to the host and that the transplanted SCN maintains circadian function which is probably entrained to the host SCN.

A putative retinohypothalamic projection containing substance P in the human

The retinohypothalamic tract (RHT) is the principal pathway mediating the entraining effects of light on the circadian pacemaker, the suprachiasmatic nucleus (SCN). In the rat, the RHT has two components, one which projects to the SCN and the intergeniculate leaflet of the thalamus and has no known peptide content and one which projects to the SCN and, perhaps, to the olivary pretectal nucleus and contains substance P (SP). Both terminate predominantly in a zone of the SCN that contains vasoactive intestinal polypeptide (VIP)-producing neurons. In the human, there is a similar dense axonal plexus of SP-immunoreactive axons in the SCN located largely in the area occupied by VIP-immunoreactive neurons and distinct from other SP-immunoreactive axons in the area. We propose that this SP plexus represents a component of the RHT in the human brain.