Structural evidence for functional domains in the rat hippocampus.

The hippocampus has two major outputs: multisynaptic pathways to the cerebral cortex and a massive descending projection directly to the lateral septal part of the basal ganglia. Here it is shown that the descending output is organized in such a way that different hippocampal regions map in an orderly way onto hypothalamic systems mediating the expression of different classes of goal-oriented behavior. This mapping is characterized by a unidirectional hippocampo-lateral septal projection and then by bidirectional lateral septo-hypothalamic projections, all topographically organized. The connectional evidence predicts that information processing in different regions of the hippocampus selectively influences the expression of different classes of behavior.

Hippocampo-hypothalamic connections: origin in subicular cortex, not ammon's horn.

An autoradiographic study of the subcortical projections of the rat hippocampal formation shows that the efferent fibers of the hippocampus proper (fields CA1-4 OF Ammon's horn) do not project to the hypothalamus but are confined to the precommissural fornix, ending primarily in the septum. The fibers that are distributed by way of the fornix system to the hypothalamus (principally the arcuate-ventromedial region and the mammillary nuclei) and the anterior thalamus arise from the subicular region of the cerebral cortex (that is, the subiculum, presubiculum, and parasubiculum).

Central noradrenergic pathways for the integration of hypothalamic neuroendocrine and autonomic responses.

Immunohistochemical and axonal transport methods were used to describe the organization of a series of central noradrenergic pathways that interrelate the nucleus of the solitary tract, which receives primary visceral sensory information, and the paraventricular and supraoptic nuclei of the hypothalamus, which participate in autonomic and neuroendocrine modes of homeostatic control. The results indicate that pathways arising from noradrenergic cells in the dorsal vagal complex, the ventrolateral medulla, and the locus coeruleus end in specific subdivisions of the paraventricular and supraoptic nuclei which are involved in the regulation of responses from the pituitary gland and from both divisions of the autonomic nervous system. This circuitry may play an important role in the integration of hypothalamic responses to visceral stimuli.

Functional expression of a new pharmacological subtype of brain nicotinic acetylcholine receptor.

A new type of agonist-binding subunit of rat neuronal nicotinic acetylcholine receptors (nAChRs) was identified. Rat genomic DNA and complementary DNA encoding this subunit (alpha 2) were cloned and analyzed. Complementary DNA expression studies in Xenopus oocytes revealed that the injection of messenger RNAs (mRNAs) for alpha 2 and beta 2 (a neuronal nAChR subunit) led to the generation of a functional nAChR. In contrast to the other known neuronal nAChRs, the receptor produced by the injection of alpha 2 and beta 2 mRNAs was resistant to the alpha-neurotoxin Bgt3.1. In situ hybridization histochemistry showed that alpha 2 mRNA was expressed in a small number of regions, in contrast to the wide distribution of the other known agonist-binding subunits (alpha 3 and alpha 4) mRNAs. These results demonstrate that the alpha 2 subunit differs from other known agonist-binding alpha-subunits of nAChRs in its distribution in the brain and in its pharmacology.

Expression in brain of a messenger RNA encoding a novel neuropeptide homologous to calcitonin gene-related peptide.

As a consequence of alternative RNA processing events, a single rat gene can generate messenger RNA's (mRNA's) encoding either calcitonin or a neuropeptide referred to as alpha-type calcitonin gene-related peptide (alpha-CGRP). An mRNA product of a related gene has been identified in rat brain and thyroid encoding the protein precursor of a peptide differing from alpha-CGRP by only a single amino acid. The RNA encoding this peptide, which is referred to as beta-CGRP, appears to be the only mature transcript of the beta-CGRP gene. Hybridization histochemistry reveals a similar distribution of alpha- and beta-CGRP mRNA's, but their relative levels of expression vary in different cranial nerve nuclei. Thus beta-CGRP is a new member of a family of related genes with potential functions in regulating the transduction of sensory and motor information.

The neuronal mineralocorticoid receptor as a mediator of glucocorticoid response.

The cloning of the mineralocorticoid receptor (MR) and the glucocorticoid receptor (GR) cDNAs provides a basis for understanding the actions of glucocorticoids in the central nervous system. Structural evidence is presented for the identity of the type I corticosteroid binding site as the MR expressed in the brain. This identification is supported by the anatomical distribution of MR mRNA, determined by in situ hybridization histochemistry, which parallels the steroid autoradiographic localization of the type I sites. An in vitro assay for MR and GR function demonstrates that these receptors respond to different levels of glucocorticoid, suggesting that together they confer a larger dynamic range of sensitivity to this hormone. These studies lead to a new hypothesis for glucocorticoid action in the central nervous system.

Primary structure and expression of beta 2: a novel subunit of neuronal nicotinic acetylcholine receptors.

A new subunit, beta 2, of the neuronal nicotinic receptor family has been identified. This subunit has the structural features of a non-agonist-binding subunit. We provide evidence that beta 2 can substitute for the muscle beta 1 subunit to form a functional nicotinic receptor in Xenopus oocytes. Expression studies performed in oocytes have demonstrated that three different neuronal nicotinic acetylcholine receptors can be formed by the pairwise injection of beta 2 mRNA and each of the neuronal alpha subunit mRNAs. The beta 2 gene is expressed in PC12 cells and in areas of the central nervous system where the alpha 2, alpha 3, and alpha 4 genes are expressed. These results lead us to propose that the nervous system expresses diverse forms of neuronal nicotinic acetylcholine receptors by combining beta 2 subunits with different agonist-binding alpha subunits.

Neuronal expression of chimeric genes in transgenic mice.

Gene expression may occur in unexpected ectopic sites when diverse genetic elements are juxtaposed as chimeric genes in transgenic mice. To determine the specific contribution of the promoter and reporter gene in ectopic expression, we have analyzed the expression of 14 different fusion genes in transgenic mice. Chimeric genes containing the mouse metallothionein-I promoter linked to either the rat or human growth hormone gene or the calcitonin/CGRP gene are expressed in a very similar pattern of neuronal regions. This ectopic expression is not a unique feature of the metallothionein promoter, since transferring the human growth hormone gene to four other heterologous promoters resulted in varying degrees of ectopic expression in overlapping subsets of cortical and hypothalamic neurons. The novel pattern of ectopic expression suggests that these otherwise unrelated neurons share a common developmental regulatory machinery for activation of gene transcription.

Tst-1, a member of the POU domain gene family, binds the promoter of the gene encoding the cell surface adhesion molecule P0.

Tst-1, a member of the POU domain gene family, is expressed in specific neurons and in myelinating glia in the mammalian nervous system. Bacterially expressed Tst-1 binds specifically to the promoter of the gene encoding myelin protein P0, a Schwann cell surface adhesion molecule. In cotransfection assays, Tst-1 can specifically repress the P0 promoter. The N-terminal part of Tst-1 protein is highly glycine- and alanine-rich, a structural feature shared by the helix-loop-helix protein TFEB.

What is the brain?

From a structural perspective, there are ten basic parts of the vertebrate CNS that are almost universally agreed upon. These parts have been grouped in at least five different ways corresponding to five different theories about its basic plan or architecture. Two classical models that remain popular today are derived from (1) comparative anatomy and the body's segmental organization, and (2) comparative embryology and the neural tube's transverse and longitudinal organization. A new approach is concerned with deciphering the genetic program that assembles the nervous system during embryogenesis; how it will correspond to the other models remains to be determined. The simplest current model to explain the organization of the mammalian nervous system involves a segmental trunk that mediates reflex sensory-motor functions, and suprasegmental cerebral hemispheres and cerebellum.

Mapping the human brain: past, present, and future.

The ability to map functional activity in the living human brain has rekindled interest in the organization of human neural circuitry specifically and in animal neural circuitry generally. Faced with incredible complexity, researchers are turning to the power of computer graphics for two- and three-dimensional interactive mapping, to the mathematical modeling of dynamics in proposed circuits, and to databases with powerful discovery engines.

What is the amygdala?

'Amygdala' and 'amygdalar complex' are terms that now refer to a highly differentiated region near the temporal pole of the mammalian cerebral hemisphere. Cell groups within it appear to be differentiated parts of the traditional cortex, the claustrum, or the striatum, and these parts belong to four obvious functional systems--accessory olfactory, main olfactory, autonomic and frontotemporal cortical. In rats, the central nucleus is a specialized autonomic-projecting motor region of the striatum, whereas the lateral and anterior basolateral nuclei together are a ventromedial extension of the claustrum for major regions of the temporal and frontal lobes. The rest of the amygdala forms association parts of the olfactory system (accessory and main), with cortical, claustral and striatal parts. Terms such as 'amygdala' and 'lenticular nucleus' combine cell groups arbitrarily rather than according to the structural and functional units to which they now seem to belong. The amygdala is neither a structural nor a functional unit.

Dramatic pituitary hyperplasia in transgenic mice expressing a human growth hormone-releasing factor gene.

A transgenic animal model system was used to analyze the mitogenic effects of GRF on its target cell, the pituitary somatotroph. We have previously established a strain of mice that express a mouse metallothionein-I/human GRF (hGRF) fusion gene, and that grow to be abnormally large due to GH hypersecretion. We show here that chronic GRF production in these mice leads to the development of enormous pituitary glands. The increase in pituitary size appears to be largely the result of a selective proliferation (hyperplasia) of somatotrophs, the GH-producing cells. This observation provides direct evidence that a neuropeptide may act as a specific trophic factor for its target cell. In addition to this effect on pituitary development, we find that the pituitary is a major site of expression of mouse metallothionein-I/hGRF mRNA, and of hGRF peptide. This tissue specificity was unexpected in that neither component of the fusion gene is highly expressed in the normal pituitary. It suggests that pituitary somatotrophs might produce and respond to GRF in an essentially autocrine fashion in these transgenic animals.

Diurnal variations in the content of preprocorticotropin-releasing hormone messenger ribonucleic acids in the hypothalamic paraventricular nucleus of rats of both sexes as measured by in situ hybridization.

The content of mRNA coding for the precursor of CRH in the hypothalamic paraventricular nucleus (PVH) was measured in intact male and estrogen-treated ovariectomized female rats at different times of the day using in situ hybridization histochemistry. Plasma corticosterone concentrations were measured in separate groups of animals at the same time points, with females showing higher levels than males at all times, and both sexes exhibiting the well characterized diurnal rhythm. There was a significant decrease (greater in females) in the content of prepro-CRH (ppCRH) mRNA between midday and midnight in rats of both sexes. Previous studies have demonstrated that the content of ppCRH mRNA in the PVH responds inversely to changes in the concentration of plasma corticosteroid in a concentration-dependent manner. The present study suggests that ppCRH mRNA in the PVH may respond in a similar way to fluctuating concentrations of plasma corticosterone throughout the day, and that although the exact mechanism remains to be elucidated, diurnal changes in the rate of CRH synthesis may be involved in the expression of diurnal rhythms within the hypothalamo-hypophysial-adrenal axis.

Cushing's syndrome due to ectopic production of corticotropin-releasing factor.

A patient with Cushing's syndrome is described who had a metastatic medullary carcinoma of the thyroid which contained corticotropin-releasing factor. ACTH was found by an immunohistochemical method in the patient's pituitary, but not in the thyroid tumor. This is the second report demonstrating corticotropin-releasing factor in tumor tissue in this syndrome. Wider use of immunohistological methods can help distinguish this variety from other tumors associated with the ectopic ACTH syndrome when bioassays are not available.

An autoradiographic study of the efferent connections of the preoptic region in the rat.

The normal morphology of the rat preoptic region has been briefly described on the basis of Nissl- and silver-stained preparations and its efferent connections have been studied autoradiographically in over 50 rat brains with single small injections of 3H-proline, or various mixtures of 3H-proline, 3H-leucine, and 3H-lysine. Injections in the anteroventral part of the lateral preoptic area labeled fibers projecting through, and perhaps to, the anterior and lateral hypothalamic areas and ending in the supramammillary region, and ventral fiber lamina of the mammillary complex; other labeled fibers ended in the periventricular hypothalamic gray and the internal lamina of the median eminence. The posteromedial lateral preoptic area projects to the same regions, as well as to the medial septal-diagonal band complex, and to the lateral habenula through the stria medullaris. Injections of the posterolateral lateral preoptic area labeled each of the above fiber systems as well as fibers to the main olfactory bulb, anterior olfactory nucleus and taenia tecta. Other fibers coursed over the genu of the corpus callosum, through the stria terminalis and ansa peduncularis to the medial, cortical and basal amygdaloid nuclei and the anterior amygdaloid area, and through the medial forebrain bundle to the substantia nigra. The transition region between the lateral preoptic and lateral hypothalamic areas at the level of the supraoptic nucleus has widespread connections as a whole (a) with the medial septal-diagonal band complex, lateral septum and bed nucleus of the stria terminalis, (b) through or to most of the hypothalamus, the substantia nigra, central tegmental field, central gray, superior central nucleus, and the locus coeruleus, (c) through the stria medullaris to the lateral habenula (bilaterally), parataenial, paraventricular, and mediodorsal nuclei of the thalamus, (d) through the stria terminalis and ansa peduncularis to the central, medial and cortical nuclei of the amygdala, and (e) to the main olfactory bulb, anterior olfactory nucleus, cingulate bundle, olfactory tubercle, medial septal-diagonal band complex and the lateral septum.

Projections of the medial preoptic nucleus: a Phaseolus vulgaris leucoagglutinin anterograde tract-tracing study in the rat.

The projections of the medial preoptic nucleus (MPN) were examined by making injections of the anterogradely transported lectin Phaseolus vulgaris leucoagglutinin (PHA-L) into the MPN and charting the distribution of labeled fibers. The evidence indicates that the MPN projects extensively to widely distributed regions in both the forebrain and brainstem, most of which also supply inputs to the nucleus. An important neuroendocrine role for the MPN is underscored by its extensive projections to almost all parts of the periventricular zone of the hypothalamus, including the anteroventral periventricular, anterior part of the periventricular, paraventricular (PVH), and arcuate nuclei, and a role in autonomic mechanisms is indicated by projections to such regions as the dorsal and lateral parvicellular parts of the PVH, the lateral parabrachial nucleus, and the nucleus of the solitary tract. Other projections of the MPN suggest participation in the initiation of specific motivated behaviors. For example, inputs to two nuclei of the medial zone of the hypothalamus, the ventromedial and dorsomedial nuclei, may be related to the control of reproductive and ingestive behaviors, respectively, although the possible functional significance of a strong projection to the ventral premammillary nucleus is presently unclear. The execution of these behaviors may involve activation of somatomotor regions via projections to the substantia innominata, zona incerta, ventral tegmental area, and pedunculopontine nucleus. Similarly, inputs to other regions that project directly to the spinal cord, such as the periaqueductal gray, the laterodorsal tegmental nucleus, certain medullary raphe nuclei, and the magnocellular reticular nucleus may also be involved in modulating somatic and/or autonomic reflexes. Finally, the MPN may influence a wide variety of physiological mechanisms and behaviors through its massive projections to areas like the ventral part of the lateral septal nucleus, the bed nucleus of the stria terminalis, the lateral hypothalamic area, the supramammillary nucleus, and the ventral tegmental area, all of which have extensive connections with regions along the medial forebrain bundle. Although the PHA-L method does not allow a clear demonstration of possible differential projections from each subdivision of the MPN, our results suggest that each of them does give rise to a unique pattern of outputs.(ABSTRACT TRUNCATED AT 400 WORDS)

The organization of neural inputs to the medial preoptic nucleus of the rat.

There is general agreement that the medial preoptic nucleus (MPN) receives projections from widespread regions of the brain, although there are significant discrepancies in the literature with regard to certain specific inputs. Therefore, we have reexamined the inputs to this nucleus with both retrograde and anterograde axonal transport techniques. First, injections of the retrograde tracers true blue, SITS, or wheat germ agglutinin were made into the region of the MPN and the distribution of retrogradely labeled cells was charted. Then, autoradiographic material was used to confirm the results of the retrograde studies, to identify the route taken by fibers projecting to the MPN, and to describe the distribution of projections with respect to the three cytoarchitectonic subdivisions of the nucleus. The results indicate that the MPN receives inputs from widely distributed areas in both the forebrain and brainstem, and that these inputs appear to be distributed topographically within the three cytoarchitectonic subdivisions of the nucleus. Direct inputs to the MPN arise from all major areas of the hypothalamus (except for the median and magnocellular preoptic nuclei, the supraoptic and suprachiasmatic nuclei, and the medial and lateral mammillary nuclei). Projections from nuclei within the periventricular zone of the hypothalamus end primarily in the medial part of the MPN, while inputs from the lateral zone are mainly confined to the lateral part of the nucleus, as are projections from the nuclei within the medial zone, except for those from the anterior and ventromedial nuclei, which appear to be more widespread. The MPN receives major inputs from limbic regions including the amygdala, ventral subiculum, and ventral lateral septal nucleus, all of which end preferentially in the lateral part of the MPN. In contrast, the projection from the encapsulated part of the bed nucleus of the stria terminalis appears to end preferentially in the central part of the MPN and in immediately adjacent regions of the medial subdivision. In addition, the MPN may receive relatively sparse inputs from infralimbic and insular cortical areas, the nucleus accumbens, and the substantia innominata. Finally, ascending serotoninergic projections from the raphe nuclei appear to terminate principally in the lateral part of the MPN, whereas inputs from regions containing noradrenergic cell groups are chiefly distributed to the central and medial parts of the nucleus. Other brainstem regions that appear to provide modest inputs include the ventral tegmental area, central tegmental field, periaqueductal gray, pedunculopontine nucleus, and the peripeduncular nucleus.(ABSTRACT TRUNCATED AT 400 WORDS)

Differential steroid hormone and neural influences on peptide mRNA levels in CRH cells of the paraventricular nucleus: a hybridization histochemical study in the rat.

The three major classes of neurons in the paraventricular nucleus (PVH) provide a rich model for studying hormonal and neural influences on multiple neuropeptides expressed in individual cells. A great deal of previous work has examined this problem at the immunohistochemical level, where hormonal and neural influences on peptide levels have been established. In situ hybridization methods were used here to determine whether these effects are accompanied by measurable changes in neuropeptide mRNA levels. In the first series of experiments, the time-course of corticosterone replacement effects on corticotropin-releasing hormone (CRH) mRNA levels in parvicellular neuroendocrine cells of adrenalectomized animals were determined, and a dose-response curve was established. CRH mRNA hybridization remains maximal with plasma levels of steroid up to about 50 ng/ml, then declines sharply between about 60-130 ng/ml, and is just detectable at higher levels. We confirmed that corticosterone decreases vasopressin mRNA levels in this cell group and showed that levels of preproenkephalin mRNA are also decreased, whereas no significant changes in cholecystokinin, beta-preprotachykinin, and angiotensinogen mRNA levels could be detected. Thus, corticosterone decreases some neuropeptide mRNA levels and has no influence on others in this cell group. Tyrosine hydroxylase mRNA hybridization is also unaffected in this part of the nucleus. In a second group of experiments, the cell-type specificity of corticosterone influences was examined. It was found that while the hormone depresses CRH mRNA levels in parvicellular neurons, it increases such levels in PVH neurons with descending projections, in certain magnocellular neurosecretory neurons, and in a part of the central nucleus of the amygdala, whereas no influence was detected in the rostral lateral hypothalamic area. Furthermore, the stimulatory effects of corticosterone have different threshold levels in different cell groups. Thus, in different types of neurons, corticosterone may increase, decrease, or have no influence on CRH mRNA levels. In contrast, while corticosterone depresses vasopressin mRNA levels in parvicellular CRH neurons, it has no obvious effects on vasopressin mRNA levels in magnocellular or descending neurons; as with CRH, the effects of corticosterone on vasopressin mRNA levels are cell-type specific. In a third series of experiments it was shown that glucocorticoid receptor and mineralocorticoid receptor mRNAs are found in all three cell types in the PVH and that corticosterone tends to produce modest increases in mRNA levels for both receptors. Finally, it was shown that unilateral catecholamine-depleting knife cuts do not change mRNA levels for any of the neuropeptides (or steroid hormone receptors) examined here, although dramatic changes in neuropeptide levels themselves have been shown.4+

Projections of the ventral subiculum to the amygdala, septum, and hypothalamus: a PHAL anterograde tract-tracing study in the rat.

The projections of the ventral subiculum are organized differentially along the dorsoventral (or septotemporal) axis of this cortical field, with more ventral regions playing a particularly important role in hippocampal communication with the amygdala, bed nuclei of the stria terminalis (BST), and rostral hypothalamus. In the present study we re-examined the projection of the ventral subiculum to these regions with the Phaseolus vulgaris leucoagglutinin (PHAL) method in the rat. The results confirm and extend earlier conclusions based primarily on the autoradiographic method. Projections from the ventral subiculum course either obliquely through the angular bundle to innervate the amygdala and adjacent parts of the temporal lobe, or follow the alveus and fimbria to the precommissural fornix and medial corticohypothalamic tract. The major amygdalar terminal field is centered in the posterior basomedial nucleus, while other structures that appear to be innervated include the piriformamygdaloid area, the posterior basolateral, posterior cortical, posterior, central, medial, and intercalated nuclei, and the nucleus of the lateral olfactory tract. Projections from the ventral subiculum reach the BST mainly by way of the precommissural fornix, and provide rather dense inputs to the anterodorsal area as well as the transverse and interfascicular nuclei. The medial corticohypothalamic tract is the main route taken by fibers from the ventral subiculum to the hypothalamus, where they innervate the medial preoptic area, "shell" of the ventromedial nucleus, dorsomedial nucleus, ventral premammillary nucleus, and cell-poor zone around the medial mammillary nucleus. We also observed a rather dense terminal field just dorsal to the suprachiasmatic nucleus that extends dorsally and caudally to fill the subparaventricular zone along the medial border of the anterior hypothalamic nucleus and ventrolateral border of the paraventricular nucleus. The general pattern of outputs to the hypothalamus and septum is strikingly similar for the ventral subiculum and suprachiasmatic nucleus, the endogenous circadian rhythm generator.