ACTH and enkephalin axonal input to paraventricular neurons containing c-fos-like immunoreactivity

Volume transmission of beta-endorphin via the cerebrospinal fluid; a review

There is increasing evidence that non-synaptic communication by volume transmission in the flowing CSF plays an important role in neural mechanisms, especially for extending the duration of behavioral effects. In the present review, we explore the mechanisms involved in the behavioral and physiological effects of β-endorphin (β-END), especially those involving the cerebrospinal fluid (CSF), as a message transport system to reach distant brain areas. The major source of β-END are the pro-opio-melano-cortin (POMC) neurons, located in the arcuate hypothalamic nucleus (ARH), bordering the 3^rd ventricle. In addition, numerous varicose β-END-immunoreactive fibers are situated close to the ventricular surfaces. In the present paper we surveyed the evidence that volume transmission via the CSF can be considered as an option for messages to reach remote brain areas. Some of the points discussed in the present review are: release mechanisms of β-END, independence of peripheral versus central levels, central β-END migration over considerable distances, behavioral effects of β-END depend on location of ventricular administration, and abundance of mu and delta opioid receptors in the periventricular regions of the brain.

Genetic and experimental evidence supports the continuum of the central extended amygdala and a mutiple embryonic origin of its principal neurons

The central extended amygdala is the major output center for telencephalic control of ingestion, fear responses, stress, and anxiety. In spite of the abundant data supporting the similarity in neurochemistry, connections, and function along the extended amygdala, embryological support for this continuum is lacking. By using a combination of in vitro migration assays, in situ hybridization, and immunostaining, here we show that its major components, including central amygdala and lateral bed nucleus of the stria terminalis (BST), are mosaics formed by different proportions of dorsal lateral ganglionic eminence (LGE)-, ventral LGE-, and medial ganglionic eminence (MGE)-derived principal neurons. The dorsal LGE produces Pax6-expressing neurons that primarily populate lateral parts of the central extended amygdala, including the capsular and part of lateral central amygdala, but also produces a few cells for the lateral BST. Based on correlation with preproenkephalin, many of these cells are likely enkephalinergic. The ventral LGE produces Islet1-expressing neurons that populate primarily the central and medial parts of the central amygdala but also produces numerous neurons for the lateral BST. Correlation with corticotropin-releasing factor suggests that these neurons express this neuropeptide. The MGE produces the majority of neurons of the lateral BST, but its ventrocaudal subdivision also produces an important subpopulation of projection neurons containing somatostatin for medial aspects of the central amygdala. Thus, distinct principal neurons originate in different embryonic domains, but the same domains contribute neurons to most subdivisions of the central extended amygdala, which may explain the similarity in neurochemistry and connections along the corridor.

A semi-quantitative analysis of Fos expression by mustard oil

In the present study, a semi-quantitative analysis of Fos expression by mustard oil was performed. For this purpose, mustard oil was applied to the skin of the right hind foot of Wistar rats at various concentrations: 5%, 10%, 30%, 50%, 80% and 100% in liquid paraffin. The distribution and number of activated Fos-positive cells in the stimulated side (ipsilateral) and contralateral side of the spinal cord were investigated following the application. The ED50 of the response was also determined. The number of Fos-labelled cells gradually increased in a dose dependent manner in both sides of superficial layers (laminae I-II) of the spinal cord with increasing concentration of mustard oil. The increase between the doses was found significant in the ipsilateral superficial layers. The increase was significant in the contralateral superficial layers at concentrations above 50%. Very few Fos-labelled cells were observed around the central canal region in all concentrations. Higher doses of the mustard oil did not increase the number of activated cells in the deeper layers. However, the expression in the deeper layers (laminae III-X) does not show a consistent trend. Also, none of the concentrations used produced labelling in neurons of the deep ventral horn neurons or in motor neurons. Forty percent (40%) of mustard oil gave an approximately 1/2 maximum response i.e. an approximate ED 50. This may be important for studies using intrathecal application of antagonist following the mustard oil activation of skin nerve fibres.

Multiple neurotransmitter receptors contribute to the spinal Fos expression

The aim of this study is to identify the receptors which could potentially mediate the activation of c-Fos. Therefore, the effects of neurotransmitter receptor agonists in the activation of c-Fos in spinal neurons were studied by intrathecal injection of excitatory amino acid (EAA) receptor agonists: N-Methyl-D-Aspartate (NMDA), (S)-alpha-Amino-3-Hydroxy-5-Methyl-4-Isoxazolepropionic acid (AMPA), 2-Carboxyl-3-carboxymethyl-4-isopropenylpyMidine (Kainic acid, KA), (1S-3R)-1-Aminocyclopentane-1, 3-dicarboxylacid (ACPD), and substance-P receptor (neurokinin-1) agonist, [Sar9, Met (O2)11] SP (SarMet-SP). All drugs tested activated the production of c-Fos in spinal dorsal horn neurons. AMPA was found as the most potent agonist tested producing market production of c-Fos particularly in neurons of lamina II at doses of 10 pM per 10-microl injection. At this dose, other agonists were relatively ineffective. At higher doses, AMPA significantly increased the activated cells. NMDA significantly increased c-Fos production to a marked extent only at doses above 10 nM per 10-microl injection. KA and ACPD were least potent of the excitatory amino acid agonists. Injection of SarMet-SP at doses of 1 nM activated Fos selectively in neurons of lamina I. A dose-dependent increase in number of c-Fos-positive cells was observed for AMPA, KA, ACPD, and SarMet-SP, whereas NMDA gave a very strong expression after a high dose with no dose dependency. These finding suggest that multiple neurotransmitter receptors lead to c-Fos production in spinal neurons.

The inhibitory effect of hormones associated with stress on Na appetite of sheep

Stress is a large stimulus of Na appetite in rabbits, rats, and mice. This study investigated the influence of some peptides implicated in stress, i.e., adrenocorticotropin (ACTH), corticotropin-releasing factor (CRF), and the recently discovered member of the CRF family, urocortin, on the ingestive behavior of sheep. Intracerebroventricular infusion of these peptides over 4 days decreased the need-free Na intake of Na-repleted sheep. Intracerebroventricular infusion of urocortin, however, did not alter Na intake of Na-depleted sheep. Systemic infusion of ACTH increased, whereas systemic infusion of either urocortin or CRF decreased, Na intake of Na-repleted sheep. The increase in Na intake caused by the peripheral infusion of ACTH was blocked by concurrent i.v. infusion of urocortin, substantiating the inhibitory role of this peptide on Na appetite. Central administration of all peptides and i.v. administration of urocortin or urocortin and ACTH combined decreased food intake. Water intake was not directly influenced by the peptides. Rather, decreased water intake, when observed, was secondary to decreased food intake, as determined by pair-feeding experiments. Whereas systemic infusion of ACTH mimics the increase in Na intake observed in several different stressful situations, CRF and urocortin actually inhibit Na intake, indicating a direct central action overriding any effect of these peptides on ACTH release. Indeed, the inhibition of Na intake by urocortin occurred despite its stimulation of ACTH release and the subsequent increase in peripheral level of cortisol. Thus it would appear that hormones associated with stress have both excitatory and inhibitory influences on Na intake. Presumably, other physiological processes entrained by stress also will be important in determining the quantitative outcome on Na appetite.

Reciprocal circuits involved in nitroglycerin-induced neuronal activation of autonomic regions and pain pathways: a double immunolabeling and tract-tracing study

This study uses tract-tracing protocols to determine the circuitry of specific nuclei involved in nitroglycerin-induced activation. Combined retrograde and anterograde tracers were injected into nuclei which consistently demonstrate robust Fos expression following our systemic nitroglycerin injection paradigm. The nuclei, which conform to these criteria, that we have evaluated in this study are the locus coeruleus, parabrachial nucleus and paraventricular nucleus of the hypothalamus. Dual Fos/tracer immunocytochemistry in treated animals documented the existence of a subset of autonomic nuclei which are activated by nitroglycerin injection and have reciprocal connections. From the nature of this rich interconnection we suggest that nitroglycerin activates autonomic responses involved in cardiovascular pressor mechanisms. Nuclei which show strong Fos labeling following nitroglycerin administration, but not traced in this study, include the nucleus trigeminalis caudalis and the ventrolateral column of the periaqueductal gray, both of which mediate nociceptive modalities. These data confirm and expand on our previous findings and demonstrate that nitroglycerin activates a complex set of structures that are functionally and structurally interconnected to articulate an integrated response.

Neurochemical mechanisms of nitroglycerin-induced neuronal activation in rat brain: a pharmacological investigation

Nitroglycerin is a nitric oxide donor which induces sustained expression of Fos protein, a marker of neuronal activation, in specific neuronal groups in the central nervous system. The mechanisms which underlie nitroglycerin-induced neuronal activation are elusive at this time, although a precise role has been suggested for the pool of neurons containing nitric oxide synthase as well as for catecholaminergic and peptidergic pathways. The aim of this study was to provide further details on the central effect of nitroglycerin by means of a pharmacological manipulation of nitroglycerin-induced neuronal activation with inhibitors of the nitric oxide synthase, modulators of the sympathetic drive and mediators of pain perception. Adult male Sprague-Dawley rats received L-NGnitro-arginine methyl ester, 7-nitro-indazole, ephedrine sulfate, indomethacin, capsaicin or vehicle before the subcutaneous injection of nitroglycerin (10 mg/kg b.w.). They were sacrificed 4 hr after nitroglycerin administration and brain sections were processed for immunocytochemical visualization of Fos. All the pharmacological treatments administered before injecting nitroglycerin selectively influenced Fos expression in the different brain nuclei. The data obtained suggest that nitroglycerin-induced neuronal activation is mediated by nociceptive and barosensitive mechanisms. Nitric oxide seems to represent the most important mediator of this phenomenon. The sympathetic system and prostaglandin synthesis are also likely to be involved.

Effects of generalized convulsive seizures on corticotropin-releasing factor neuronal systems

Most stressors generate a set of endocrine and neural adaptations that form a stress response. The corticotropin-releasing factor neurons of the paraventricular nucleus of hypothalamus integrate endocrine and neural inputs, and cause a cascade of events with resultant increased levels of pituitary adrenocorticotropic hormone and adrenal hormones. Although activation of the hypothalamic-pituitary-adrenal axis is associated with a large variety of stressors, the effects of seizures on hypothalamic corticotropin-releasing factor neurons are essentially unknown. The goal of the present study was to elucidate the effects of generalized convulsive seizures on distinct and separate corticotropin-releasing factor cell populations in brain. Seizure-activated neurons were identified immunocytochemically through their expression of the Fos protein. Seizures were induced by intraperitoneal injection of kainic acid. In the paraventricular nucleus, the vast majority of corticotropin-releasing factor-like parvocellular neurons also expressed Fos-like protein following seizure elicitation. This response was specific to corticotropin-releasing factor neurons of the paraventricular nucleus, as corticotropin-releasing factor neurons in central nucleus of the amygdala or bed nucleus of the stria terminalis did not simultaneously localize Fos following seizures.

Enkephalin innervation of the paraventricular nucleus of the hypothalamus: distribution of fibers and origins of input

The opioid peptide enkephalin emerges as a major neuromodulator in the regulation and integration of the physiologic response in stressful conditions. The paraventricular nucleus of the hypothalamus is a coordinating center of neuroendocrine and autonomic functions. However, the detailed distribution of the enkephalin fibers and terminals in the paraventricular nucleus and the sources of enkephalinergic innervation are not well defined. In the present study, we used immunocytochemistry for the proenkephalin-derived octapeptide met-arg6-gly7-leu8 enkephalin to determine the distribution of enkephalin-immunoreactive fibers and somata within paraventricular nucleus. Without colchicine pretreatment, enkephalinergic fibers were prominent mainly in the ventromedial part of the parvicellular subdivision of the paraventricular nucleus, appearing in coronal sections as a dense collection of short segments of enkephalin-immunoreactive fibers. In the periventricular portion of the paraventricular nucleus, enkephalin-immunoreactive fibers produced a moderate plexus of short enkephalin-immunoreactive fibers dorsoventrally oriented. With colchicine treatment, a dense cluster of enkephalin-immunoreactive cell bodies was located in the dorsomedial and the dorsal parts of the parvicellular subdivisions. These enkephalin-immunoreactive neurons were small (< 10 microns) to medium sized (10-15 microns), with round and elongated shapes. Retrograde transport of wheat germ-conjugated gold particles, WGA-apoHRP-Au, from the paraventricular nucleus, combined with immunocytochemistry for enkephalin revealed that the major sources of extrahypothalamic enkephalin afferents to the paraventricular nucleus are provided by enkephalin neurons in the lateral reticular nucleus and the paragigantocellularis reticular nucleus of the medulla (approximately 20% of retrogradely labeled neurons within this nucleus were double labeled) and in the nucleus solitary tract (approximately 10% of retrogradely labeled neurons within this nucleus were double labeled). Retrogradely labeled enkephalin neurons were also observed in the medial preoptic area, median preoptic nucleus, dorsomedial hypothalamic nucleus, lateral septum and hypothalamic arcuate nucleus. These enkephalinergic pathways from the medulla and the forebrain could represent an anatomical substrate underlying the opioid effects on paraventricular neurons during physiological processes, such as cardiovascular regulation, feeding or stress responses.

NADPH-diaphorase activity and Fos expression in brain nuclei following nitroglycerin administration

Organic nitrates are considered nitric oxide donors in that they have been shown to form nitric oxide in vitro and in vivo. Nitroglycerin is an organic nitrate which possesses peculiar activities mediated, to some extent, by the central nervous system via the noradrenergic system. Previous reports have shown that systemic nitroglycerin is able to induce Fos expression in brain nuclei which are known to contain nitric oxide synthesizing enzyme. Neuronal NADPH-diaphorase has been shown to be a nitric oxide synthase. Thus, in this study we used NADPH-diaphorase histochemistry to evaluate the distribution of Fos-immunoreactive cells within neurons which contain nitric oxide synthase. The data showed co-localization of Fos with NADPH-diaphorase activity in numerous neurons of the paraventricular and supraoptic nuclei of the hypothalamus. In the brainstem, a few neurons were doubly labeled for Fos and NADPH-diaphorase activity, but NADPH-diaphorase positive fibers and Fos-immunoreactive neurons were consistently co-distributed in the locus coeruleus, parabrachial nucleus, nucleus tractus solitarius and spinal trigeminal nucleus caudalis. These findings demonstrate that nitroglycerin administration activates a selective group of neurons which are a source of nitric oxide or which are in close proximity with neuronal processes containing nitric oxide synthase, and suggest that the nitric oxide synthase synthesizing pathway may be involved at various levels in the central effect of nitroglycerin.

Systemic nitroglycerin induces Fos immunoreactivity in brainstem and forebrain structures of the rat

Nitroglycerin is a vasodilator which induces vascular relaxation by releasing nitric oxide in the wall of blood vessels. It has been suggested that the cardiovascular inhibitory responses which are induced by this drug are mediated by central structures. In this study, we evaluated the distribution and intensity of Fos immunoreactivity in rat brain nuclei following the systemic administration of nitroglycerin. In the medulla, a significant number of Fos-immunoreactive neurons were observed in the nucleus tractus solitarius, ventrolateral medulla, area postrema and spinal trigeminal nucleus caudalis. A robust staining was seen in the parabrachial nucleus, locus coeruleus and ventrolateral periaqueductal grey. In the hypothalamus, Fos-positive cells were densely packed in the paraventricular and supraoptic nuclei. Other areas where significant staining was observed include the central nucleus of the amygdala and the subfornical organ. These findings demonstrate that the systemic administration of nitroglycerin is capable of activating a spectrum of functionally diverse brain regions. This spectrum includes areas involved in reflex adjustments to nitroglycerin-induced hypotension, areas involved in sensory nociceptive perception and areas associated with integrative regulation of autonomic, behavioral and neuroendocrine functions.

c-fos expression in hypothalamic neurosecretory and brainstem catecholamine cells following noxious somatic stimuli

Noxious somatic stimuli elicit vasopressin secretion, an effect thought to result from activation of a facilitatory input from A1 catecholamine cells of the medulla oblongata. To better characterize the A1 cell response and effects on other neuroendocrine A1 projection targets, particularly within the paraventricular nucleus, we have now mapped c-fos expression in neurochemically identified catecholamine and neurosecretory cells following a noxious somatic stimulus. Unilateral hind paw pinch significantly increased c-fos expression in contralateral A1 cells whereas other brainstem catecholamine cell groups were unaffected. Expression of c-fos was also increased in the supraoptic nucleus, this effect being more pronounced for vasopressin than oxytocin neurosecretory cells and, as with A1 cells, primarily on the side contralateral to the stimulated paw. In contrast, the increase in the paraventricular nucleus was greater in oxytocin rather than in vasopressin cells. Additionally there was a significant rise in c-fos expression in medial parvocellular paraventricular nucleus cells of noxiously stimulated animals. Notably, the majority of tuberoinfundibular corticotropin-releasing factor cells are located in this medial parvocellular zone. These results are consistent with and expand on those previously reported from electrophysiological and anatomical studies. The finding of differing neurosecretory cell responses between supraoptic and paraventricular nuclei has interesting implications with regard to the afferent control of neurosecretory cell activity. For example, the substantially greater activation of supraoptic versus paraventricular nucleus vasopressin cells, despite being innervated by the same medullary noradrenergic cell group, raises the possibility of a differential input or differences in responsiveness. Furthermore, the activation of paraventricular nucleus parvocellular cells is consistent with suggestions that the A1 cell group provides an excitatory input to this population.

Neural and biochemical mediators of endotoxin and stress-induced c-fos expression in the rat brain

We and others have reported that c-fos protein is induced in the hypothalamus and brain stem of the rat following central and peripheral injections of endotoxin (lipopolysaccharide; LPS). We have now examined possible mechanisms through which LPS induces c-fos protein. The cyclooxygenase inhibitor indomethacin and the glutamate NMDA antagonist MK801 inhibited c-fos protein in the paraventricular nucleus (PVN), supraoptic nucleus (SON), and the A1/A2 regions of the brain stem induced by IP or IV injections of LPS (40 micrograms). The H1 histamine antagonist diphenhydramine, but not the H2 histamine antagonist cimetidine, reduced the amount of c-fos labeling. MK801 also attenuated the effects of stress (foot shock) on c-fos protein; however, indomethacin had no effect on c-fos protein induced by stress. We next examined the importance of visceral afferent innervation on the response to LPS or stress. Subdiaphragmatic vagotomy completely blocked the induction of c-fos protein following IP injections of LPS; however, vagotomy had a minimal effect on c-fos protein induced in the PVN and SON following IV injections of LPS, but potentiated c-fos induction following foot shock. Thus, prostaglandin synthesis, glutamate release, histamine receptors, and visceral afferents represent functional biochemical and neural pathways through which endotoxin activates c-fos protein in specific autonomic and neuroendocrine regulatory nuclei. Activation of NMDA glutamate receptors may represent a final common pathway for the induction of c-fos protein in the brain induced by both endotoxin and stress.