Ependymal tanycytes projecting to the ventromedial hypothalamic nucleus as demonstrated by retrograde and anterograde transport of HRP

Hypothalamic effective connectivity at rest is associated with body weight and energy homeostasis

Hunger and satiety drive eating behaviours via changes in brain function. The hypothalamus is a central component of the brain networks that regulate food intake. Animal research parsed the roles of the lateral hypothalamus (LH) and medial hypothalamus (MH) in hunger and satiety, respectively. Here, we examined how hunger and satiety change information flow between human LH and MH brain networks, and how these interactions are influenced by body mass index (BMI). Forty participants (16 overweight/obese) underwent two resting-state functional MRI scans while being fasted and sated. The excitatory/inhibitory influence of information flow between the MH and LH was modelled using spectral dynamic causal modelling. Our results revealed two core networks interacting across homeostatic state and weight: subcortical bidirectional connections between the LH, MH and the substantia nigra pars compacta (prSN), and cortical top-down inhibition from fronto-parietal and temporal areas. During fasting, we found higher inhibition between the LH and prSN, whereas the prSN received greater top-down inhibition from across the cortex. Individuals with higher BMI showed that these network dynamics occur irrespective of homeostatic state. Our findings reveal fasting affects brain dynamics over a distributed hypothalamic-midbrain-cortical network. This network is less sensitive to state-related fluctuations among people with obesity.

Circadian and light regulation of oxytocin and parvalbumin protein levels in the ciliated ependymal layer of the third ventricle in the C57 mouse

The walls of the third ventricle have been proposed to serve as a bidirectional conduit for exchanges between the neural parenchyma and the cerebrospinal fluid. In immunohistochemical studies of mice, we observed that light exposure and circadian phase affected peptide staining surrounding the third ventricle at the level of the suprachiasmatic nuclei. Under high magnification, we observed robust staining for the neurohormone oxytocin and the calcium-binding protein parvalbumin associated with cilia extending into the third ventricle from the surrounding ventricular wall; no similar staining was observed for vasopressin or calbindin. Retinal illumination had opposite effects on levels of parvalbumin and oxytocin in the cilia: light exposure during late subjective night increased oxytocin staining, but decreased parvalbumin staining in the cilia. Preventing cellular transport with colchicine eliminated immunohistochemical staining for oxytocin in the cilia. There was also a significant daily rhythm of oxytocin immunostaining in the third ventricle wall, and in magnocellular neurons in the anterior hypothalamus. The results suggest that environmental lighting and circadian rhythms regulate levels of oxytocin in the cerebrospinal fluid, possibly by regulating movement of oxytocin through the third ventricle wall.

Studies of ventromedial hypothalamic afferents in the rat using three methods of HRP application

The afferent neural connections of the ventromedial nucleus of the rat hypothalamus (VMH) have been studied in detail using three horseradish peroxidase (HRP) application methods: HRP crystal implants, HRP-gel implants, and iontophoretic deposition of the enzyme. Examination of the cases in which the retrograde tracer was best confined to various subdivisions of the nucleus revealed that the septal area projects only to the ventrolateral VMH, and that the medial preoptic area, rostral lateral hypothalamus, and the ventral subiculum project mainly to the ventrolateral VMH. Thus, the subdivision of the VMH that contains the highest density of estradiol-concentrating neurons (Morrell et al. 1986) receives a larger set of inputs than the rostral and central parts of the nucleus. The central subdivision receives a more restricted set of projections than either the medial or the lateral regions. These studies suggest that there may be partial anatomical segregation of neural inputs to the various subdivisions of the VMH.

The ventricular ependyma of mice infected with Trypanosoma brucei

The fine structure of ependymal cells lining the cerebral ventricles of normal mice and of mice infected with Trypanosoma brucei was examined by transmission electron microscopy. Most of the ependymal cells had been stripped from the ventricular surface of the brain in infected animals but some of the remaining ependymal cells contained intracellular trypomastigotes. The same process of stripping had occurred in a single human brain that was included in the series, but intracellular forms were not found. The significance of the intracellular forms and the implication of the stripping of ventricular ependymal cells are discussed in relation to the pathogenesis of sleeping sickness.

Interference of the nutritional condition of the rat with peripheral glucose regulation determined by CNS mechanisms

Glucose and insulin levels of the cerebrospinal fluid (CSF) were changed by an infusion of either glucose, insulin or a mixture of glucose and insulin in the third ventricle of freely moving undisturbed rats. Before, during and after infusions venous blood samples were withdrawn to determine insulin- and glucose concentrations. In rats on carbohydrate rich food neither plasma insulin nor blood glucose changed during infusion of either glucose, insulin or a mixture of both. However, in rats on carbohydrate free food an immediate decline in blood glucose of about 7 mg/dl occurred 15 min after the start of the infusion. During infusion of a mixture of glucose and insulin, blood glucose rose slightly and reached a value which was in between those attained when glucose and insulin were infused separately. Plasma insulin did not change following central infusions. It is argued that the central nervous system (CNS) is sensitive to changes in CSF levels of glucose and insulin and affects peripheral glucose homeostasis mainly by changing glucose output from the liver. This action of the CNS, however, is dependent on the nutritional condition of the animal.

Tanycytes in the medial habenular nucleus of the rat

Tanycytes with foot processes contacting capillary basal membranes were identified in the rat medial habenular nucleus. They constitute a relatively small but constant population of cells among the conventional ependymal cells. In contrast to tanycytes in most circumventricular organs, habenular tanycytes possess cilia. Superimposed upon the cells are nerve fibers belonging to the serotonergic supraependymal axon plexus. Their ultrastructure differs in many respects from that of hypothalamic tanycytes.

Hypothalamic pathways involved in metabolic regulatory functions, as identified by track-tracing methods

The present review of the fiber connections of the hypothalamus has been concerned basically with recent data obtained by the aid of the autoradiographic and HRP tracer techniques. Evidence presented has shown that, besides confirming many of the older data, recent studies have resulted in the introduction of several conceptual modifications into the classic picture of hypothalamic hodological relationships. Among these conceptual modifications, the following can be mentioned: (1) the medially placed nuclei of the hypothalamus have a great number of long efferent and afferent connections with many extrahypothalamic structures; (2) many hypothalamic nuclei send direct projections to cell territories in the brainstem and spinal cord that contain preganglionic autonomic motor neurons; (3) several neural districts that lie caudal to the mesencephalon send direct projections to the hypothalamus; (4) in addition to the olfactory channel, other sensory pathways (including interoceptive and gustatory conduction lines) have a relatively direct access to hypothalamic mechanisms; (5) the hypothalamus sends fibers to several brainstem territories that give rise to widespread monoaminergic projections; and (6) there are anatomical pathways that establish reciprocal connections between the hypothalamus and the basal ganglia. Some of the possible physiological correlates of these anatomical findings in the context of metabolic regulatory functions have been briefly indicated.

Neuronal control of the ventromedial nucleus from the periventricular nucleus in the hypothalamic slice of the rat

A neuronal pathway between periventricular nucleus and ventromedial hypothalamus (VMH) was investigated by both anatomical and electrophysiological methods. Horseradish peroxidase (HRP)-labelled clusters were observed in the periventricular nucleus after application of HRP into the VMH. Periventricular nucleus stimulation produced mainly excitation of VMH neurones with a latency of 3 msec. These results suggest that periventricular nucleus sends information in the cerebro-spinal fluid to the VMH.