Resistance of hypothalamic dopaminergic neurons to neonatal 6-hydroxydopamine toxicity

Developing brain as a source of circulating norepinephrine in rats during the critical period of morphogenesis

The development of individual organs and the whole organism is under the control by morphogenetic factors over the critical period of morphogenesis. This study was aimed to test our hypothesis that the developing brain operates as an endocrine organ during morphogenesis, in rats during the perinatal period (Ugrumov in Neuro Chem 35:837-850, 2010). Norepinephrine, which is a morphogenetic factor, was used as a marker of the endocrine activity of the developing brain, although it is also secreted by peripheral organs. In this study, it was first shown that the concentration of norepinephrine in the peripheral blood of neonatal rats is sufficient to ensure the morphogenetic effect on the peripheral organs and the brain itself. Using pharmacological suppression of norepinephrine production in the brain, but not in peripheral organs, it was shown that norepinephrine is delivered from the brain to the general circulation in neonatal rats, that is, during morphogenesis. In fact, even partial suppression of norepinephrine production in the brain of neonatal rats led to a significant decrease of norepinephrine concentration in plasma, suggesting that at this time the brain is an important source of circulating norepinephrine. Conversely, the suppression of the production of norepinephrine in the brain of prepubertal rats did not cause a change in its concentration in plasma, showing no secretion of brain-derived norepinephrine to the bloodstream after morphogenesis. The above data support our hypothesis that morphogenetic factors, including norepinephrine, are delivered from the developing brain to the bloodstream, which occurs only during the critical period of morphogenesis.

Hypothalamic overexpression of mutant huntingtin causes dysregulation of brown adipose tissue

Expression of mutant huntingtin (htt) protein has been shown to cause metabolic imbalance in animal models of Huntington disease (HD). The pathways involved are not fully understood but dysfunction of both the hypothalamus and brown adipose tissue (BAT) has been implicated. Here we show that targeted expression of mutant HTT in the hypothalamus leads to loss of the A13 dopaminergic cell group located in the zona incerta and reduced mRNA expression of neuropeptide Y1 receptor in the hypothalamus. Furthermore, this is accompanied by downregulation of uncoupling protein 1 expression and PPARγ coactivator-1 alpha in BAT and a rapid body weight gain. Taken together, our data might provide a mechanistic link between expression of mutant HTT, reduced activity of a hypothalamic dopaminergic pathway and dysfunction of BAT and in part explain the development of an obese phenotype in HD mouse models.

Protection from neuronal damage evoked by a motivational excitation is a driving force of intentional actions

Motivation may be understood as an organism's subjective attitude to its current physiological state, which somehow modulates generation of actions until the organism attains an optimal state. How does this subjective attitude arise and how does it modulate generation of actions? Diverse lines of evidence suggest that elemental motivational states (hunger, thirst, fear, drug-dependence, etc.) arise as the result of metabolic disturbances and are related to transient injury, while rewards (food, water, avoidance, drugs, etc.) are associated with the recovery of specific neurons. Just as motivation and the very life of an organism depend on homeostasis, i.e., maintenance of optimum performance, so a neuron's behavior depends on neuronal (i.e., ion) homeostasis. During motivational excitation, the conventional properties of a neuron, such as maintenance of membrane potential and spike generation, are disturbed. Instrumental actions may originate as a consequence of the compensational recovery of neuronal excitability after the excitotoxic damage induced by a motivation. When the extent of neuronal actions is proportional to a metabolic disturbance, the neuron theoretically may choose a beneficial behavior even, if at each instant, it acts by chance. Homeostasis supposedly may be directed to anticipating compensation of the factors that lead to a disturbance of the homeostasis and, as a result, participates in the plasticity of motivational behavior. Following this line of thought, I suggest that voluntary actions arise from the interaction between endogenous compensational mechanisms and excitotoxic damage of specific neurons, and thus anticipate the exogenous compensation evoked by a reward.

The hypothalamus and hypertension

Most forms of hypertension are associated with a wide variety of functional changes in the hypothalamus. Alterations in the following substances are discussed: catecholamines, acetylcholine, angiotensin II, natriuretic peptides, vasopressin, nitric oxide, serotonin, GABA, ouabain, neuropeptide Y, opioids, bradykinin, thyrotropin-releasing factor, vasoactive intestinal polypeptide, tachykinins, histamine, and corticotropin-releasing factor. Functional changes in these substances occur throughout the hypothalamus but are particularly prominent rostrally; most lead to an increase in sympathetic nervous activity which is responsible for the rise in arterial pressure. A few appear to be depressor compensatory changes. The majority of the hypothalamic changes begin as the pressure rises and are particularly prominent in the young rat; subsequently they tend to fluctuate and overall to diminish with age. It is proposed that, with the possible exception of the Dahl salt-sensitive rat, the hypothalamic changes associated with hypertension are caused by renal and intrathoracic cardiopulmonary afferent stimulation. Renal afferent stimulation occurs as a result of renal ischemia and trauma as in the reduced renal mass rat. It is suggested that afferents from the chest arise, at least in part, from the observed increase in left auricular pressure which, it is submitted, is due to the associated documented impaired ability to excrete sodium. It is proposed, therefore, that the hypothalamic changes in hypertension are a link in an integrated compensatory natriuretic response to the kidney's impaired ability to excrete sodium.

Mechanism of 6-hydroxydopamine neurotoxicity

The catecholaminergic neurotoxin 6-hydroxydopamine (6-OHDA) has recently been found to be formed endogenously in patients suffering from Parkinson's disease. In this article, we highlight the latest findings on the biochemical mechanism of 6-OHDA toxicity. 6-OHDA has two ways of action: it easily forms free radicals and it is a potent inhibitor of the mitochondrial respiratory chain complexes I and IV. The inhibition of respiratory enzymes by 6-OHDA is reversible and insensitive towards radical scavengers and iron chelators with the exception of desferrioxamine. We conclude that free radicals are not involved in the interaction between 6-OHDA and the respiratory chain and that the two mechanisms are biochemically independent, although they may act synergistically in vivo.

Differential expression of neurotensin receptor mRNA in the dopaminergic cell groups of the rat diencephalon and mesencephalon

Several lines of evidence support interactions between neurotensin (NT) and dopaminergic (DAergic) neurons in the brain. In order to obtain further knowledge about the anatomical substrate for such interactions, the distribution of cells expressing the cloned neurotensin receptor (NTR) mRNA was examined in relation to tyrosine hydroxylase (TH) mRNA-expressing cells within different subnuclei of the diencephalon and ventral mesencephalon of the male rat. In situ hybridization was performed on consecutive sections labeled with 33P-labeled oligonucleotide probes. In the hypothalamus, NTR mRNA signals were mostly found in the suprachiasmatic, dorsomedial, dorsal premammillary, and supramammillary nuclei. On the other hand, DAergic cells of the periventricular nucleus of the hypothalamus and dorsal aspect of the arcuate nucleus, revealed by TH in situ hybridization, did not exhibit NTR mRNA even though dense NT binding sites have been previously described in this nuclei. In the zona incerta, TH mRNA-containing cells were concentrated in the medial part, with little overlap with NTR mRNA-expressing cells located mainly in its mediolateral extent. In contrast, the distribution of both markers was superimposable within the different subdivisions of the ventral tegmental area and substantia nigra, as previously suggested, but also in the retrorubral field. These anatomical data further support the NT-dopamine interactions on both mesocorticolimbic and nigrostriatal DAergic systems. Moreover, the results suggest that diencephalic DAergic neurons do not synthesize the cloned NTR mRNA or express it at considerably lower levels than DAergic mesencephalic cells.

Baroreceptor-aortic nerve-mediated release of endogenous L-3,4-dihydroxyphenylalanine and its tonic depressor function in the nucleus tractus solitarii of rats

We have proposed that L-3,4-dihydroxyphenylalanine (L-DOPA) is a neurotransmitter and/or neuromodulator in the central nervous system [Misu Y. and Goshima Y. (1993) Trends pharmac. Sci. 14, 119-123]. This study aimed to explore whether or not endogenous L-DOPA, as a neurotransmitter candidate of the primary baroreceptor afferents, tonically functions to activate depressor neurons in the nucleus tractus solitarii of anesthetized rats. By parallel microdialysis in bilateral nucleus tractus solitarii areas, the basal L-DOPA release was in part inhibited by tetrodotoxin perfusion (1 microM) or Ca2+ deprivation, and was markedly reduced by alpha-methyl-p-tyrosine (200 mg/kg, i.p.), a tyrosine hydroxylase inhibitor. Forty to 100 mM K+ concentration-dependently released L-DOPA. Fifty millimoles K+ repetitively and constantly released L-DOPA. This release was Ca(2+)-dependent. Stimulation of the left aortic nerve (100 Hz, 8 V) repetitively and constantly released L-DOPA and this release was tetrodotoxin-sensitive. Phenylephrine i.v. infused produced L-DOPA release and reflex bradycardia, temporally associated with a rise and subsequent recovery of blood pressure. This release and bradycardia were abolished by denervation of the bilateral carotid sinus and aortic nerves. In addition, L-DOPA methyl ester, a competitive L-DOPA antagonist, when microinjected into depressor sites of the left nucleus tractus solitarii, antagonized depressor responses to mild stimulation (20 Hz, 3 V) of the ipsilateral aortic nerve. This antagonist alone, microinjected bilaterally, elicited a dose-dependent hypertension, which was abolished by alpha-methyl-p-tyrosine. Furthermore, by immunocytochemical analysis seven days after denervation of the left aortic nerve, tyrosine hydroxylase- and L-DOPA-, but not dopamine- and dopamine-beta-hydroxylase-immunoreactivities decreased in the ipsilateral nucleus tractus solitarii and dorsal motor vagus nucleus complex area. In the left ganglion nodosum, denervation decreased staining and number of L-DOPA-immunoreactive cells and staining of tyrosine hydroxylase-immunoreactive cells, but no modification of dopamine-immunoreactive cells was seen. Taken together with previous findings that L-DOPA itself is stereoselectively responsible for cardiovascular control in this nucleus, it is probable that L-DOPA is a neurotransmitter of the primary baroreceptor afferents terminating directly in depressor neurons and/or indirectly in some neurons within a microcircuit, including depressor neurons of the nucleus tractus solitarii. Endogenously released L-DOPA itself tonically functions to activate depressor neurons for regulation of blood pressure in the rat nucleus tractus solitarii.

Circadian rhythms of norepinephrine in the rat suprachiasmatic nucleus

Norepinephrine (NE) contents in the suprachiasmatic nucleus (SCN) in rats kept under light-dark (LD) or constant dark (DD) conditions displayed significant variations over one day with a peak during the subjective day and a trough during the subjective night. These results indicate that rhythmic variation of NE in the SCN is driven by an endogenous pacemaker and independent of external light. In agreement with these observations, immunocytochemical study localized tyrosine hydroxylase immunopositive fibers in the medial part of the SCN. Unlike serotonin, histamine and neuropeptide Y projections to the ventrolateral SCN, NE innervation may represent the general activity level of the brainstem, from where these fibers arise.

Organotypic slice culture of the rat suprachiasmatic nucleus: sustenance of cellular architecture and circadian rhythm

The suprachiasmatic nucleus of the mammalian brain is thought to be the anatomical locus of circadian rhythms. To examine the functional organization of the suprachiasmatic nucleus in vitro with intact intercellular connections for a prolonged period, we have established an organotypic slice culture system using a roller-tube technique. Brain slices (400 microns in thickness) containing the bilateral suprachiasmatic nuclei, were obtained from newborn rats at four to seven days old and were maintained in vitro for more than three weeks. During this three-week period, the slices flattened to one to three cell layers and two tightly packed neuronal cell-masses (neuronal zones), with diameters of about 1 mm were formed, which were surrounded by a peripheral glial cell-dispersed zone. In the neuronal cell zones, peptides and their messenger RNAs were found cytochemically with characteristic patterns similar to the suprachiasmatic nucleus in the brain. In situ hybridization and immunocytochemistry showed that vasoactive intestinal peptide messenger RNA expressing and vasoactive intestinal peptide-immunoreactive neurons were detected predominantly in the ventrolateral part of the neuronal zones in the suprachiasmatic nucleus slice culture. Vasopressin messenger RNA-expressing and vasopressin-immunoreactive cells were localized in the dorsomedial neuronal zones near the ependymal cell zone. The distribution of cell bodies and fibers containing these neuropeptides and their messenger RNAs in the neuronal zones of suprachiasmatic nucleus organotypic slice culture were similar to that of the suprachiasmatic nucleus in vivo. This suggests that the suprachiasmatic nucleus in these organotypic slice cultures retains the biological characteristics of these cells in vivo as the cells did develop, form compact neuronal masses and did establish connections. To examine the possibility that suprachiasmatic nucleus neurons in slice cultures show a persistent rhythmic activity, we also measured the amount of vasopressin released into the culture medium. Sampling at 4-h intervals combined with enzyme immunoassay revealed that vasopressin concentration in the medium embracing suprachiasmatic nucleus slice cultures fluctuated with a period of approximately 24 h. The present findings suggest that the intranuclear neuronal networks of the suprachiasmatic nucleus are maintained in vitro for a long duration and that organotypic cultures of the suprachiasmatic nucleus produce and release bioactive substances in an oscillating manner. The suprachiasmatic nucleus in slice cultures may be useful for future analysis of circadian rhythms in vitro.