P.G.M. Luiten

The course of paraventricular hypothalamic efferents to autonomic structures in medulla and spinal cord

By application of the anterograde transport technique of Phaseolus vulgaris leuco-agglutinin the descending autonomic projection of the paraventricular hypothalamic nucleus was investigated. The Phaseolus lectin technique allowed the detection of the cells of origin in the paraventricular PVN, the precise position of two distinct descending axon pathways and the detailed morphology of terminal structures in midbrain, medulla oblongata and spinal cord.

The Hypothalamus, Intrinsic Connections and Outflow Pathways to the Endocrine System in Relation to the Control of Feeding and Metabolism

Abbreviations

Cortical projection patterns of magnocellular basal nucleus subdivisions as revealed by anterogradely transported Phaseolus vulgaris leucoagglutinin

The present paper deals with a detailed analysis of cortical projections from the magnocellular basal nucleus (MBN) and horizontal limb of the diagonal band of Broca (HDB) in the rat. The MBN and HDB were injected iontophoretically with the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L). After immunocytochemical visualization of labeled efferents, the distribution of projections over the cortical mantle, olfactory regions and amygdala were studied by light microscopy. Based on differences in cortical projection patterns, the MBN was subdivided in anterior, intermediate and posterior portions (MBNa, MBNi and MBNp). All subdivisions maintain neocortical projections and are subject to an anterior to posterior topographic arrangement. In the overall pattern, however, the frontal cortex is the chief target. Furthermore, all MBN parts project to various regions of meso- and allocortex, which are progressively more dense when the tracer injection is more anteriorly placed. The most conspicuous finding, however, was a ventrolateral to dorsomedial cortical projection pattern as the PHA-L injection site moved from posterior to anterior. Thus, the posterior MBN projects predominantly to lateral neo- and mesocortex while the anterior MBN sends more fibers to the medial cortical regions. Furthermore, the MBNa is a source of considerable afferent input to the olfactory nuclei and as such should be regarded as a transition to the HDB. The HDB, apart from projecting densely to olfactory bulb and related nuclei, maintains a substantial output to the medial prefrontal cortical regions and entorhinal cortex, as well. Comparison of young vs aged cases indicate that aging does not appear to have a profound influence on cortical innervation patterns, at least as studied with the PHA-L method.

Ascending projections from the solitary tract nucleus to the hypothalamus. A Phaseolus vulgaris lectin tracing study in the rat

The course of the ascending pathways originating from the anterior gustatory and posterior visceral sensory part of the solitary tract nucleus and the topographic organization of the projections to the hypothalamus in the rat were studied with anterogradely transported Phaseolus vulgaris lectin. In general, the posterior visceral sensory part of the solitary tract nucleus has ascending projections as far as the septum-diagonal band complex and gives rise to heavy input to the bed nucleus of the stria terminals, and to the dorsomedial and paraventricular hypothalamic nuclei. A more moderate projection is aimed at a variety of other hypothalamic nuclei, to the medial and central amygdaloid nuclei and to the paraventricular nucleus of the thalamus. The ventromedial hypothalamic nucleus is strikingly missing an afferent input from the nucleus of the solitary tract. Furthermore, it was shown that whereas the caudal solitary tract nucleus has predominant long ascending connections, the projections from the anterior taste related region of the nucleus of the solitary tract have only limited forebrain projections which do not reach beyond the level of the anterior dorsal hypothalamic nucleus.

The projections of the dorsomedial hypothalamic nucleus in the rat.

The dorsomedial hypothalamic nucleus (DMH) output pathways are revealed by using autoradiographic tracing of tritium labeled Leucine and by the recently introduced Phaseolus vulgaris leuco-agglutinin immunocytochemical method. Terminal labeling appears in the dorsal motor nucleus of the vagus, nucleus ambiguus and in the parvocellular reticular formation at the lower medullary level. Mesencephalic labeling is found in the periaqueductal gray at the level of the oculomotor nucleus. In the hypothalamus labeled terminal boutons are identified in the lateral and ventromedial hypothalamic nuclei but also in the parvocellular paraventricular nucleus. Furthermore, the circumventricular organs are found to receive a dense DMH input, particularly the organum vasculosum of the lamina terminalis and the subfornical organ. These findings are discussed in relation to the dorsomedial nucleus involvement in the control of feeding and pancreatic hormone release. It appears that the DMH participates in this control via descending pathways to the preganglionic pancreas innervating neurons but also via a neuroendocrine route. The latter connection is indicated by terminal labeling in the parvocellular paraventricular nucleus in the area that contains the corticotropin-releasing factor positive cells.

Muscarinic acetylcholine receptors in the hippocampus, neocortex and amygdala : A review of immunocytochemical localization in relation to learning and memory

Immunocytochemical mapping studies employing the extensively used monoclonal anti-muscarinic acetylcholine receptor (mAChR) antibody M35 are reviewed. We focus on three neuronal muscarinic cholinoceptive substrates, which are target regions of the cholinergic basal forebrain system intimately involved in cognitive functions: the hippocampus; neocortex; and amygdala. The distribution and neurochemistry of mAChR-immunoreactive cells as well as behaviorally induced alterations in mAChR-immunoreactivity (ir) are described in detail. M35+ neurons are viewed as cells actively engaged in neuronal functions in which the cholinergic system is typically involved. Phosphorylation and subsequent internalization of muscarinic receptors determine the immunocytochemical outcome, and hence M35 as a tool to visualize muscarinic receptors is less suitable for detection of the entire pool of mAChRs in the central nervous system (CNS). Instead, M35 is sensitive to and capable of detecting alterations in the physiological condition of muscarinic receptors. Therefore, M35 is an excellent tool to localize alterations in cellular cholinoceptivity in the CNS. M35-ir is not only determined by acetylcholine (ACh), but by any substance that changes the phosphorylation/internalization state of the mAChR. An important consequence of this proposition is that other neurotransmitters than ACh (especially glutamate) can regulate M35-ir and the cholinoceptive state of a neuron, and hence the functional properties of a neuron. One of the primary objectives of this review is to provide a synthesis of our data and literature data on mAChR-ir. We propose a hypothesis for the role of muscarinic receptors in learning and memory in terms of modulation between learning and recall states of brain areas at the postsynaptic level as studied by way of immunocytochemistry employing the monoclonal antibody M35.

Hypoxia and brain development

Hypoxia threatens brain function during the entire life-span starting from early fetal age up to senescence. This review compares the short-term, long-term and life-spanning effects of fetal chronic hypoxia and neonatal anoxia on several behavioural paradigms including novelty-induced spontaneous and learning behaviours. Furthermore, it reveals that perinatal hypoxia is an additional threat to neurodegeneration and decline of cognitive and other behaviours during the aging process. Prenatal hypoxia evokes a temporary delay of ingrowth of cholinergic and serotonergic fibres into the hippocampus and neocortex, and causes an enhanced neurodegeneration of 5-HT-ir axons during aging. Neonatal anoxia suppresses hippocampal ChAT activity and up-regulates muscarinic receptor sites for 3H-QNB and 3H-pirenzepine binding in the hippocampus in the early postnatal age. The altered development of axonal arborization and pre- and postsynaptic cholinergic functions may be an important underlying mechanism to explain the behavioural deficits. As far as the cellular mechanisms of perinatal hypoxia is concerned, our primary aim was to study the putative importance of Ca2+ homeostasis of developing neurons by means of pharmacological interventions and by measuring the development of immunoexpression of Ca(2+)-binding proteins. We assessed that nimodipine, an L-type calcium channel blocker, prevented or attenuated the adverse behavioural and neurochemical effects of perinatal hypoxias, while it enhanced the early postnatal development of ir-Ca(2+)-binding proteins. The results are discussed in the context of different related research areas on brain development and hypoxia and ischaemia.

β-Amyloid neurotoxicity is mediated by a glutamate-triggered excitotoxic cascade in rat nucleus basalis

Whereas a cardinal role for β‐amyloid protein (Aβ) has been postulated as a major trigger of neuronal injury in Alzheimers disease, the pathogenic mechanism by which Aβ deranges nerve cells remains largely elusive. Here we report correlative in vitro and in vivo evidence that an excitotoxic cascade mediates Aβ neurotoxicity in the rat magnocellular nucleus basalis (MBN). In vitro application of Aβ to astrocytes elicits rapid depolarization of astroglial membranes with a concomitant inhibition of glutamate uptake. In vivo Aβ infusion by way of microdialysis in the MBN revealed peak extracellular concentrations of excitatory amino acid neurotransmitters within 20–30 min. Aβ‐triggered extracellular elevation of excitatory amino acids coincided with a significantly enhanced intracellular accumulation of Ca2+ in the Aβ injection area, as was demonstrated by 45Ca2+ autoradiography. In consequence of these acute processes delayed cell death in the MBN and persistent loss of cholinergic fibre projections to the neocortex appear as early as 3 days following the Aβ‐induced toxic insult. Such a sequence of Aβ toxicity was effectively antagonized by the N‐methyl‐d‐aspartate (NMDA) receptor ligand dizocilpine maleate (MK‐801). Moreover, Aβ toxicity in the MBN decreases with advancing age that may be associated with the age‐related loss of NMDA receptor expression in rats. In summary, the present results indicate that Aβ compromises neurons of the rat MBN via an excitotoxic pathway including astroglial depolarization, extracellular glutamate accumulation, NMDA receptor activation and an intracellular Ca2+ overload leading to cell death.

Detailed projection patterns of septal and diagonal band efferents to the hippocampus in the rat with emphasis on innervation of CA1 and dentate gyrus

The detailed patterns of afferentation to the ammons horn and dentate gyrus of the hippocampus in the rat were investigated employing the anterograde tracer Phaseolus vulgaris leuco-agglutinin (PHA-L) after punctate iontophoretic injections in the medial septum (MS) and vertical limb of the diagonal band of Broca (VDB). The topographically ordered innervation pattern was different in the regio superior (or CA1) vs. the regio inferior (or CA3) and in the dorsal vs. ventral aspects of ammons horn and dentate gyrus. The CA1 pyramidal and dentate granule cell layers in the dorsal hippocampus received afferent input almost exclusively from the VDB, whereas those cell layers in ventral hippocampus were supplied from both VDB and MS. The PHA-L labeled projecting fibers could be differentiated into two distinct fiber systems. One class of thick and coarse axons (tentatively called type I fibers) carried fewer but larger terminal boutons and were found to infiltrate the entire stratum oriens, dentate hilus, all layers of the regio inferior and the CA1 str. moleculare. A second, delicate thin (type II) fiber system provided with numerous and passant varicosities showed a much more restricted laminar innervation pattern and appeared to originate from areas in MS-VDB which are rich in AChE-positive neurons. The densest type II fiber networks could be observed in the CA1 subpyramidal and dentate supragranular zones, in the CA1 stratum lacunosum-moleculare and in the dentate middle third molecular layer. This laminar type II innervation pattern showed a remarkable coincidence with the reported distribution of cholinergic marker enzymes. The topographic and spatial organization of the projections described above will be discussed in relation to their possible functional significance.

Descending pathways from hypothalamus to dorsal motor vagus and ambiguus nuclei in the rat

The anatomical pathways between the hypothalamus and cell groups of the lower medulla that are involved in the neural control of endocrine pancreas activity were investigated. As part of this control system the descending pathways originating from lateral, dorsomedial and ventromedial hypothalamic nuclei towards the dorsal motor vagus and ambiguus nuclei, were studied by retrograde transport of horseradish peroxidase. Very small injections of the tracer, by means of the iontophoretic delivery method, were placed in the dorsal motor vagus, ambiguus and solitary tract nucleus as well as in the various nuclei of the medullary reticular formation. Subsequent retrograde labeling was studied in the hypothalamus and the brainstem. The appearance of considerable retrograde labeling in mesencephalic periventricular grey and rostral mesencephalic reticular formation indicated a possible role for these structures as intermediates in an indirect hypothalamo-medullary control circuitry. This led us to extend the peroxidase injections to these mesencephalic areas after which the hypothalamus was investigated for retrograde labeling. All data combined indicated the existence of three descending pathways, direct and indirect, between hypothalamus and the parasympathetic motor nuclei of the lower medulla.