The lateral hypothalamic area. An ultrastructural analysis

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.

Discharge properties of neurons recorded in the parvalbumin-positive (PV1) nucleus of the rat lateral hypothalamus

This study reports for the first time the extracellular activity recorded, in anesthetized rats, from cells located in an identified cluster of parvalbumin (PV)-positive neurons of the lateral hypothalamus forming the PV1-nucleus. Random-like firing characterized the majority (21/30) of the cells, termed regular cells, with a median firing rate of 1.7 spikes/s, Fano factor equal to 1, and evenly distributed along the rostro-caudal axis. Four cells exhibiting an oscillatory activity in the range 1.6-2.1Hz were observed only in the posterior part of the PV1-nucleus. The asynchronous activity of PV1 neurons is likely to produce a "network-driven" effect on their main target within the periaqueductal gray matter. The hypothesis is raised that background random-like firing of PV1-nucleus is associated with functional network activity likely to contribute dynamic information related to condition transitions of awareness and non-conscious perception.

Efferent connections from the lateral hypothalamic region and the lateral preoptic area to the hypothalamic paraventricular nucleus of the rat

The lateral preoptic and lateral hypothalamic regions contain the majority of the cell groups embedded in the fibre trajectories of the medial forebrain bundle on its course through the hypothalamus. Recent studies have extended considerably the parcellation of the lateral hypothalamic region, and therefore, the need to emphasize new insights into the anatomical organisation of projections from the neurons of the lateral hypothalamic region. In the present study we describe the anatomical organisation of efferent projections from the lateral preoptic and lateral hypothalamic regions to the hypothalamic paraventricular nucleus (PVN) on the basis of retrograde- and anterograde-tracing techniques. Iontophoretic injections of the retrograde tracer, cholera toxin subunit B, into the PVN revealed that most hypothalamic nuclei project to the PVN. Within the lateral hypothalamic region, retrogradely labelled cells were concentrated in the intermediate hypothalamic area, the lateral hypothalamic area, and the perifornical nucleus, whereas fewer retrogradely labelled cells were found in the lateral preoptic area. To determine the distribution of terminating fibres in subnuclei of the heterogeneous PVN, iontophoretic injections of the anterograde tracer Phaseolus vulgaris-leucoagglutinin were delivered into distinct areas of the lateral hypothalamic region. Neurons of the intermediate hypothalamic area projected mainly to the PVN subnuclei, which contained parvicellular neuroendocrine cells. In contrast, neurons of the rostral and tuberal parts of the lateral hypothalamic area and the perifornical nucleus projected to the PVN subnuclei, which contained parvicellular neurons that send descending projections to preganglionic cell groups in the medulla and spinal cord. The perifornical nucleus was the only area within the lateral hypothalamic region that consistently innervated magnocellular perikarya of the PVN. Finally, all areas of the lateral hypothalamic region contributed substantially to fibres terminating in the perinuclear shell of the PVN. These results demonstrate that anatomically distinct areas of the lateral hypothalamic region have distinct projections to subnuclei of the PVN and further substantiate the view that the lateral hypothalamic region as well as the PVN constitute anatomically and functionally heterogeneous structures.

Medial forebrain bundle of the rat: IV. Cytoarchitecture of the caudal (lateral hypothalamic) part of the medial forebrain bundle bed nucleus

In the preceding study (Geeraedts et al.: J. Comp. Neurol. 294:507-536, '90), the rostral or telencephalic portion of the rat's bed nucleus of the medial forebrain bundle (MFB) has been parcellated into several cytoarchitectonically distinct cellular groups and subgroups. The purpose of the present investigation is to subject the caudal or lateral hypothalamic (LH) portion of the MFB bed nucleus to a detailed cytoarchitectonic analysis. This analysis is based on the same materials, methods, and cytoarchitectonic criteria that were also employed in the preceding study. In contrast to descriptions in the literature, it was found that the LH-region constitutes a very heterogeneous population of neurons with an evident arrangement into groups, several of which have not been identified previously. Many of these cellular groups are partly or entirely located within the boundary of the LH-trajectory of the MFB as previously established by Nieuwenhuys et al. (J. Comp. Neurol. 206:49-81, '82). These groups are designated here as the MFB-related cellular groups. They appear to be arranged into two longitudinal zones. Both zones are caudally replaced by the ventral tegmental area (VTA) and a part of the mesencephalic tegmentum (TEGM1). The lateral zone lies in close proximity to the internal capsule/cerebral peduncle and comprises the following cellular groups: the ventrolateral subarea of the lateral hypothalamic area (LHVL), the anterolateral subarea of the lateral hypothalamic area (LHAL), the lateral tuberal nucleus (TUL), the pre-subthalamic nucleus (PSUT), the retro-subthalamic nucleus (RSUT), the anterodorsal subarea of the lateral hypothalamic area (LHAD), and the lateral hypothalamic nucleus (LHN). The medial zone consists of the following cellular groups: the intermediate hypothalamic area (IHA), the medial tuberal nucleus (TUM), the perifornical nucleus (PFX), the lateral supramammillary nucleus (SUL), the submammillothalamic nucleus (SMT), and the nucleus geminus posterior (GEP). The cellular groups of the medial zone together with the tuberomammillary nucleus groups of the medial zone together with the tuberomammillary nucleus (TUMM) are positioned at the interface between the lateral and the medial hypothalamus, and form an array of cellular groups indicated in our study as the intermediate division of the hypothalamus. The MFB-related cellular groups are dorsally, medially, ventrally, and laterally surrounded by rather well-known brain structures. Both the MFB-related cellular groups and the surrounding structures have been identified and delimited. This resulted in a new, elaborate cytoarchitectonic atlas of the rat's lateral hypothalamic region.(ABSTRACT TRUNCATED AT 400 WORDS)

Two substrates for medial forebrain bundle self-stimulation: myelinated axons and dopamine axons

The directly activated substrates for medial forebrain bundle (MFB) self-stimulation are primarily low threshold, myelinated axons with absolute refractory periods of 0.4 to 1.2 msec, conduction velocities of 1 to 8 m/sec and current-distance constants of 1000 to 3000 microA/mm2. When small electrode tips or high currents are used, however, a second population of long refractory period (1.2 to 5 msec) axons is added. The excitability properties of this second population are almost identical with those of dopamine (DA) axons. Furthermore, the long-refractory period effects of MFB self-stimulation are reduced, but not completely blocked, by peripheral injections of alpha-flupenthixol, suggesting that dopamine axons make small contributions to MFB self-stimulation when small tips are used. Collision data, strength-duration data and refractory period data in various self-stimulation experiments are compared. Asymmetric collision effects, recently observed in cortical and striatal sites mediating electrically evoked turning, may help determine where synapses are located in circuits mediating electrically evoked behaviors. A neural model of symmetric, asymmetric and mixed collision is proposed.

Morphometry of cytoarchitecture of the lateral hypothalamic area in the rat

The cytoarchitecture of the lateral hypothalamic area (LHA) in rats was studied with cresyl violet-stained coronal celloidin sections and with sections of the brain impregnated by a Golgi method. Unimodality was established in the frequency distribution histogram of both the somatic cross-sectional area of the LHA neurons and somatic shape (elongation and circularity). The predominant somatic orientation was in the dorsomedial-ventrolateral direction: bimodality of the frequency distribution of somatic orientation was denied. These findings suggest that the LHA neurons examined in the present study are not subdivisible on the basis of the somatic area, shape or orientation. Although the neurons were classified into eight types based upon the dendritic pattern, those in the LHA largely consisted of only three of them; Type III (dendrites extending in two directions along the long axis of soma), Type IV (three directions) and Type VIII (four directions) jointly accounted for 97.1 percent of the total neurons examined. This finding suggests that the parameter of dendritic pattern serves an important purpose in the typing of the rat LHA neurons. The orientation of intrinsic dendrites and intrinsic axons in the LHA has also been described.

The medial forebrain bundle of the rat. I. General introduction

This paper is the first of a projected series of studies on the structure and composition of the medial forebrain bundle (MFB) of the rat and the relations of this fiber system to its bed nucleus, the lateral hypothalamic area. The first part of the paper comprises an extensive review of literature on the MFB from its discovery by Ganser in 1882 to the present. This review serves as the basis for an evaluation of our present-day knowledge of the organization of the MFB, which is presented in the second part of this paper. Despite the wealth of information available on the origins and sites of termination of the axons that constitute the MFB, surprisingly little attention has been given to the bundle itself, to its topographic boundaries, its fiber composition, or to the spatial arrangement of its constituent components. These features of the MFB as it extends through the lateral preoptic and lateral hypothalamic areas have been analyzed in normal Klüver-Barrera- and Bodian-stained material. From this analysis, a detailed atlas of the MFB and some of the surrounding structures has been prepared. This atlas, which forms the third section of this paper, illustrates the appearance and organization of the MFB at ten equidistant levels through the lateral preoptic and lateral hypothalamic continuum.

Serotonergic innervation of the lateral hypothalamus: evidence from synaptosomal uptake studies

Uptake of tritiated serotonin by synaptosomes prepared from rat lateral hypothalamus was examined. Uptake of serotonin into lateral hypothalamic synaptosomes occurred by both saturable and non-saturable processes. The saturable process was a high affinity transport with kinetic parameters that agree closely with those previously reported for serotonin uptake into synaptosomes prepared from other brain regions. Fluoxetine, a selective inhibitor of uptake into serotonergic neurons, was a potent inhibitor of serotonin uptake into lateral hypothalamic synaptosomes. Desipramine and benztropine, noradrenergic and dopaminergic uptake inhibitors respectively, were much less effective. Damage to the ascending serotonergic system, by either electrolytic lesion of the dorsal or median raphe nucleus, or by 5,7 dihydroxytryptamine injections into the midbrain serotonergic pathways, significantly reduced the uptake of serotonin by lateral hypothalamic synaptosomes. Taken together, these data provide further evidence for a serotonergic terminal field within the lateral hypothalamus.

Cell types within the medial forebrain bundle: a Golgi study of preoptic and hypothalamic neurons in the rat

The morphology of lateral preoptic (POL) and lateral hypothalamic (HLA) neurons was studied in 14- to 200-day-old rats with the chlorate-formaldehyde modification of the Golgi method. Drawings of 91 POL and HLA neurons revealed three distinct neuronal types within the MFB based on somatic size and shape and dendritic morphology. Class I neurons, which accounted for 75-80% of the neurons in the MFB, has fusiform or multipolar somata averaging 21 X 14 micron and 2-5 sparsely branched dendrites with a moderate number of sticklike spines. The extensive dendritic domains of Class I neurons ranged from 700 to 1,500 micron and were usually oriented perpendicular to the longitudinal fibers of the MFB. Both nonoriented and oriented Class I neurons were encountered. Nonoriented Class I neurons had expansive dendritic arbors which reached nearly all regions of the MFB in the coronal plane. Oriented Class I neurons had dendritic domains which were confined to specific regions (e.g., ventral-lateral) of the MFB. Class II neurons, which made up approximately 10% of the MFB neurons, had large multipolar somata averaging 30 X 17 micron and 2-5 stout dendrites which were densely covered with hairlike spines. Class II neurons also exhibited spines on their somata and proximal dendritic trunks and had dendritic domains of 700-1,000 micron. Class III neurons had small somata averaging 15 X 12 micron and restricted dendritic arbors of 500-700 micron in diameter. Class III neurons exhibited both spiny and spine-free dendrites and made up 10% of MFB neurons. Because of the parcellation of chemically coded fiber systems within the MFB, individual POL and HLA neurons may not be homogeneous in the type of afferents they receive from other brain areas.

Altered neuroanatomical organization in the central nervous system of the genetically obese (ob/ob) mouse

Genetically obese (C57BL/6J ob/ob) mice have significantly reduced brain weights (-14.6%) and cortical brain volumes (-7.9%) compared to lean control mice (C57BL/6J +/+). Morphometric analyses of soma cross-sectional areas of individual neurons in select brain region also reveals significant alterations in the ob/ob mouse. Neurons from 8 out of 9 brain regions, including the ventromedial hypothalamic nucleus, show significantly decreased soma cross-sectional areas in ob/ob mice compared to controls. Only lateral hypothalamic area neurons have equivalent soma cross-sectional areas for these two mouse strains. The decreased brain weight and volume coupled with the observed morphometric changes in individual neuronal soma size suggest that the ob/ob mouse brain differs considerably from that of controls. These differences may underlie some of the endocrine abnormalities seen in this genetic obesity syndrome.