Histogenesis of the subthalamic nucleus

Dorsoventral Arrangement of Lateral Hypothalamus Populations in the Mouse Hypothalamus: a Prosomeric Genoarchitectonic Analysis

The lateral hypothalamus (LH) has a heterogeneous cytoarchitectonic organization that has not been elucidated in detail. In this work, we analyzed within the framework of the prosomeric model the differential expression pattern of 59 molecular markers along the ventrodorsal dimension of the medial forebrain bundle in the mouse, considering basal and alar plate subregions of the LH. We found five basal (LH1-LH5) and four alar (LH6-LH9) molecularly distinct sectors of the LH with neuronal cell groups that correlate in topography with previously postulated alar and basal hypothalamic progenitor domains. Most peptidergic populations were restricted to one of these LH sectors though some may have dispersed into a neighboring sector. For instance, histaminergic Hdc-positive neurons were mostly contained within the basal LH3, Nts (neurotensin)- and Tac2 (tachykinin 2)-expressing cells lie strictly within LH4, Hcrt (hypocretin/orexin)-positive and Pmch (pro-melanin-concentrating hormone)-positive neurons appeared within separate LH5 subdivisions, Pnoc (prepronociceptin)-expressing cells were mainly restricted to LH6, and Sst (somatostatin)-positive cells were identified within the LH7 sector. The alar LH9 sector, a component of the Foxg1-positive telencephalo-opto-hypothalamic border region, selectively contained Satb2-expressing cells. Published studies of rodent LH subdivisions have not described the observed pattern. Our genoarchitectonic map should aid in systematic approaches to elucidate LH connectivity and function.

Tangential Intrahypothalamic Migration of the Mouse Ventral Premamillary Nucleus and Fgf8 Signaling

The tuberal hypothalamic ventral premamillary nucleus (VPM) described in mammals links olfactory and metabolic cues with mating behavior and is involved in the onset of puberty. We offer here descriptive and experimental evidence on a migratory phase in the development of this structure in mice at E12.5–E13.5. Its cells originate at the retromamillary area (RM) and then migrate tangentially rostralward, eschewing the mamillary body, and crossing the molecularly distinct perimamillary band, until they reach a definitive relatively superficial ventral tuberal location. Corroborating recent transcriptomic studies reporting a variety of adult glutamatergic cell types in the VPM, and different projections in the adult, we found that part of this population heterogeneity emerges already early in development, during tangential migration, in the form of differential gene expression properties of at least 2–3 mixed populations possibly derived from subtly different parts of the RM. These partly distribute differentially in the core and shell parts of the final VPM. Since there is a neighboring acroterminal source of Fgf8, and Fgfr2 is expressed at the early RM, we evaluated a possible influence of Fgf8 signal on VPM development using hypomorphic Fgf8^neo/null embryos. These results suggested a trophic role of Fgf8 on RM and all cells migrating tangentially out of this area (VPM and the subthalamic nucleus), leading in hypomorphs to reduced cellularity after E15.5 without alteration of the migrations proper.

The T-type calcium channel as a new therapeutic target for Parkinson's disease

Parkinson's disease (PD) is one of the most prevalent movement disorder caused by degeneration of the dopaminergic neurons in substantia nigra pars compacta. Deep brain stimulation (DBS) at the subthalamic nucleus (STN) has been a new and effective treatment of PD. It is interesting how a neurological disorder caused by the deficiency of a specific chemical substance (i.e., dopamine) from one site could be so successfully treated by a pure physical maneuver (i.e., DBS) at another site. STN neurons could discharge in the single-spike or the burst modes. A significant increase in STN burst discharges has been unequivocally observed in dopamine-deprived conditions such as PD, and was recently shown to have a direct causal relation with parkinsonian symptoms. The occurrence of burst discharges in STN requires enough available T-type Ca(2+) currents, which could bring the relatively negative membrane potential to the threshold of firing Na(+) spikes. DBS, by injection of negative currents into the extracellular space, most likely would depolarize the STN neuron and then inactivate the T-type Ca(2+) channel. Burst discharges are thus decreased and parkinsonian locomotor deficits ameliorated. Conversely, injection of positive currents into STN itself could induce parkinsonian locomotor deficits in animals without dopaminergic lesions. Local application of T-type Ca(2+) channel blockers into STN would also dramatically decrease the burst discharges and improve parkinsonian locomotor symptoms. Notably, zonisamide, which could inhibit T-type Ca(2+) currents in STN, has been shown to benefit PD patients in a clinical trial. From the pathophysiological perspectives, PD can be viewed as a prototypical disorder of "brain arrhythmias". Modulation of relevant ion channels by physical or chemical maneuvers may be important therapeutic considerations for PD and other diseases related to deranged neural rhythms.

Subdivisions of the turtle Pseudemys scripta hypothalamus based on the expression of regulatory genes and neuronal markers

The patterns of distribution of a set of conserved brain developmental regulatory transcription factors and neuronal markers were analyzed in the hypothalamus of the juvenile turtle, Pseudemys scripta. Combined immunohistochemical techniques were used for the identification of the main boundaries and subdivisions in the optic, paraventricular, tuberal, and mammillary hypothalamic regions. The combination of Tbr1 and Pax6 with Nkx2.1 allowed identification of the boundary between the telencephalic preoptic area, rich in Nkx2.1 expression, and the prethalamic eminence, rich in Tbr1 expression. In addition, at this level Nkx2.2 expression defined the boundary between the telencephalon and the hypothalamus. The dorsalmost hypothalamic domain was the supraoptoparaventricular region that was defined by the expression of Otp/Pax6 and the lack of Nkx2.1/Isl1. It is subdivided into rostral, rich in Otp and Nkx2.2, and caudal, only Otp-positive, portions. Ventrally, the suprachiasmatic area was identified by its catecholaminergic groups and the lack of Otp, and could be further divided into a rostral portion, rich in Nkx2.1 and Nkx2.2, and a caudal portion, rich in Isl1 and devoid of Nkx2.1 expression. The expressions of Nkx2.1 and Isl1 defined the tuberal hypothalamus, whereas only the rostral portion expressed Otp. Its caudal boundary was evident by the lack of Isl1 in the adjacent mammillary area, which expressed Nkx2.1 and Otp. All these results provide an important set of data on the interpretation of the hypothalamic organization in a reptile, and hence make a useful contribution to the understanding of hypothalamic evolution.

Cre fate mapping reveals lineage specific defects in neuronal migration with loss of Pitx2 function in the developing mouse hypothalamus and subthalamic nucleus

Establishment of neuronal diversity is a central topic in developmental neurobiology. Prior studies implicated Pitx2, a paired-like homeodomain transcription factor, in mouse subthalamic nucleus neuronal development, but precise stages of neuronal differentiation affected (migration, axon outgrowth, fate specification) and underlying mechanisms were unknown. Here we report lineage tracing experiments using Pitx2(cre/+), Pitx2(cre/null), and conditional nuclear lacZ reporter mice to track embryonic Pitx2 expressing neurons. Migration of subthalamic nucleus and hypothalamic neurons was severely arrested in Pitx2(cre/null) embryos, and subclasses of subthalamic nucleus neurons identified by Lmx1b, Foxp1, and Foxp2-gene expression revealed differing sensitivities to Pitx2 dosage. Interestingly, embryonic subthalamic nucleus development was unaffected in Lmx1b null mice, suggesting that Pitx2 and Lmx1b act via independent genetic pathways. These data provide the first direct evidence for Pitx2-dependent neuronal migration in the developing hypothalamus, and demonstrate that complex transcriptional networks regulate regional specialization of distinct hypothalamic and subthalamic nucleus neurons.

Genetics of subthalamic nucleus in development and disease

The subthalamic nucleus (STN) is a crucial node in the basal ganglia. Clinical success in targeting the STN for deep brain stimulation in Parkinson's disease patients has prompted increased interest in understanding STN biology. In this report, we discuss recent evidence for transcription factor mediated regulation of STN development. We also review STN developmental neurobiology and known patterns of gene expression in the developing and mature STN.

PITX2 is required for normal development of neurons in the mouse subthalamic nucleus and midbrain

Pitx2, a homeodomain transcription factor, is essential for normal development of the pituitary gland, craniofacial region, eyes, heart, abdominal viscera, and limbs. Complete loss of Pitx2 in mice (Pitx2(-/-)) results in embryonic lethality by approximately e15 due to cardiac defects, whereas embryos with partial loss of function (Pitx2(neo/-) or Pitx2(neo/neo)) survive until later in development (e17-e19). Pitx2 is expressed in discrete populations of postmitotic neurons in the mouse brain, but its role in mammalian central nervous system (CNS) development is not known. We undertook an analysis of Pitx2-deficient embryos to determine whether loss of Pitx2 affects CNS development. The CNS is normal in hypomorphic e16.5 Pitx2(neo/-) and e18.5 Pitx2(neo/neo) embryos, with no evidence of midline or other defects. Midgestation (e10.5) Pitx2(-/-) embryos have normally formed neural tube structures and cerebral vesicles, whereas older (e14.5) Pitx2(-/-) embryos exhibit loss of gene expression and axonal projections in the subthalamic nucleus (a group of cells in the ventrolateral thalamus) and in the developing superior colliculus of dorsal midbrain. Our results suggest a role for Pitx2 in regulating regionally specific terminal neuronal differentiation in the developing ventrolateral thalamus and midbrain.

The human brain at stages 21-23, with particular reference to the cerebral cortical plate and to the development of the cerebellum

The development of the human brain during the eighth embryonic week was studied in serial sections of 22 embryos, and graphic reconstructions were prepared. The cortical plate appears in stage 21 in the area of the future insula and is an excellent feature for staging. The internal capsule contains neocortical fibres. Its three main outlets begin to be present in stage 22 and lead to epithalamus, to dorsal thalamus, and to mesencephalon. At this time a well developed lateral olfactory tract can be seen. The anterior commissure appears in stage 23. A clear developmental relationship between claustrum and olfactory area is described for the first time in human embryos. The optic tract reaches the ventral area of the lateral geniculate body. Scattered fibres of the lateral lemniscus reach at least as far as the caudal mesencephalon, in which superior and inferior colliculi can be distinguished at stage 23; two caudal Blindsäcke containing ventricular recesses form in stage 23. The cerebellum is still present as a plate, but its internal bulge is considerably enlarged. It possesses radially- and tangentially-arranged cells; the latter form the external germinal layer. The dentate nucleus, as well as the inferior and superior cerebellar peduncles and some of the cerebellar commissures, are present. Compared with the highly developed and probably already functional remainder of the hindbrain, the cerebellar plate shows far less differentiation. Two caudal migratory streams (marginal and submarginal) are present and represent the corpus pontobulbare. The decussation of the pyramids appears in stage 23. This article concludes the study of the developing human brain during the embryonic period, from stage 8 to stage 23. The series was based on 340 serially-sectioned embryos and graphic reconstructions from 89 brains. No comparable investigation of the fetal brain is available.