The action of the A1 noradrenergic region on phasically firing neurons in the rat paraventricular nucleus

Norepinephrine dialysate levels in the hypothalamic paraventricular nucleus: influence on photoperiod-driven prolactin levels in the female siberian hamster

Basal prolactin (PRL) levels in Siberian hamsters are modulated by ambient photoperiod via the nocturnal melatonin signal. Recent evidence from our laboratory has demonstrated that norepinephrine (NE), a putative neurochemical regulator of PRL secretion shows photoperiod-dependent fluctuations in the hypothalamic paraventricular nucleus (PVN) that are independent of the pineal melatonin signal. NE content in the PVN is elevated under a short-day photoperiod with a time course that follows the short-day-induced decline in PRL. NE could thus modulate the release of a prolactin-releasing factor that subsequently drives PRL synthesis and release rates. In order to determine whether NE release in the PVN correlates with basal PRL levels, this study evaluated the relationship between extracellular NE levels in the PVN and basal PRL under long- and short-day photoperiod conditions. In addition, the profile of NE during the proestrous surge of PRL was investigated. Female Siberian hamsters were housed either in long-day or short-day photoperiods for 8 weeks. After 6 weeks of photoperiod exposure, the animals were implanted with a stainless steel guide cannula aimed at the PVN. Two weeks later, the animals were fitted with a jugular cannula for serial blood sampling and implanted with an indwelling microdialysis probe. Dialysis samples were collected every 20 min for 5 h (12.00-17.00 h) from short-day-exposed animals and from long-day-exposed animals classified as being either in diestrus or proestrus. Blood samples were collected every hour and analyzed for PRL levels by radioimmunoassay. NE and methoxy-4-hydrophenylglycol (MHPG) levels were significantly higher in short-day-exposed animals, correlating with the suppressed basal secretion of PRL. Both long-day groups had lower NE and MHPG levels, and higher PRL, than the short-day group. However, long-day animals showed a significant decline in NE in proestrus just prior to the onset of the afternoon surge of PRL. These data suggest that NE released within the PVN participates in the regulation of photoperiod effects upon PRL secretion, as well as that of the afternoon surge of PRL on proestrus.

CCK-8 excites oxytocin-secreting neurons in the paraventricular nucleus in rats--possible involvement of noradrenergic pathway

Systemic administration of CCK-8 increased plasma oxytocin (OXT) level in rats anesthetized with a mixture of urethane and alpha-chloralose. Extracellular recordings were made from magnocellular neurosecretory neurons in the paraventricular nucleus (PVN) of the hypothalamus in anesthetized rats to examine the effects of CCK-8 on the firing of PVN neurons. Thirteen out of 16 nonphasic neurons (putative OXT-secreting neurons) were excited by intravenous and/or intraperitoneal administration of CCK-8. By contrast, 8 out of 10 phasic cells, vasopressin(AVP)-secreting cells, were inhibited by systemic administration of CCK-8. Four out of five nonneurosecretory neurons in the PVN were excited by the administration of CCK-8. Moreover, microiontophoretically applied phentolamine blocked the excitatory responses induced by CCK-8 in nonphasic neurons. We measured extracellular noradrenaline (NA) level in the PVN, using in vivo microdialysis. Intravenous administration of CCK-8 induced NA release in the PVN. These results suggest that CCK-8 activates the excitatory afferent pathway to putative OXT-secreting neurons in the PVN which may, at least in part, be involved in the central noradrenergic projection.

Collateral input to the paraventricular and supraoptic nuclei in rat. II. Afferents from the ventral lateral medulla and nucleus tractus solitarius

In the rat, medullary afferents to the hypothalamic magnocellular nuclei mediate the baroreceptor reflexes of vasopressinergic neurons and the cholecystokinin- or gastric distention-induced excitation of oxytocinergic neurons. One strategy that reflexes such as these may use to coordinate the activity of magnocellular neuroendocrine neurons is collateral branching of input. Previous work has shown that the distributions of medullary neurons projecting to the paraventricular and the supraoptic nuclei overlap and that their axons branch. Thus, we hypothesized that single neurons in the ventral lateral medulla and/or the nucleus tractus solitarius would project to both the paraventricular and supraoptic nuclei via collateral branches of their axons. Medullary afferent neurons were retrogradely labeled after injection into the paraventricular and the supraoptic nucleus on one side of the brain with two different fluorescent tracers: Fluoro-Gold or rhodamine-labeled latex microspheres. The topographic distribution of labeled cells in the medulla containing either a single fluorescent tracer or both tracers were plotted. Of these labeled neurons, a small percentage (7%) contained both dyes, suggesting that they send collateral branches to both of the magnocellular neuroendocrine nuclei injected. Single labeled cells were both ipsi- and contralateral to the injected side (53% ipsilateral), but most double-labeled cells were ipsilateral (84%). In rats, areas that project to both the paraventricular and the supraoptic nuclei may act upon both nuclei together. Thus, afferent inputs, in conjunction with the known inter- and intracellular changes that take place within the magnocellular nuclei, may be involved with the coordinated responses throughout magnocellular neuroendocrine system during medullary reflexes, i.e., the baroreceptor-mediated reflexes or the gastric distention reflexes.

Emerging concepts of structure-function dynamics in adult brain: the hypothalamo-neurohypophysial system

As the first known of the mammalian brain's neuropeptide systems, the magnocellular hypothalamo-neurohypophysial system has become a model. A great deal is known about the stimulus conditions that activate or inactivate the elements of this system, as well as about many of the actions of its peptidergic outputs upon peripheral tissues. The well-characterized actions of two of its products, oxytocin and vasopressin, on mammary, uterine, kidney and vascular tissues have facilitated the integration of newly discovered, often initially puzzling, information into the existing body of knowledge of this important regulatory system. At the same time, new conceptions of the ways in which neuropeptidergic neurons, or groups of neurons, participate in information flow have emerged from studies of the hypothalamo-neurohypophysial system. Early views of the SON and PVN nuclei, the neurons of which make up approximately one-half of this system, did not even associate these interesting, darkly staining anterior hypothalamic cells with hormone secretion from the posterior pituitary. Secretion from this part of the pituitary, it was thought, was neurally evoked from the pituicytes that made the oxytocic and antidiuretic "principles" and then released them upon command. When these views were dispelled by the demonstration that the hormones released from the posterior pituitary were synthesized in the interesting cells of the hypothalamus, the era of mammalian central neural peptidergic systems was born. Progress in developing an ever more complete structural and functional picture of this system has been closely tied to advancements in technology, specifically in the areas of radioimmunoassay, immunocytochemistry, anatomical tracing methods at the light and electron microscopic levels, and sophisticated preparations for electrophysiological investigation. Through the judicious use of these techniques, much has been learned that has led to revision of the earlier held views of this system. In a larger context, much has been learned that is likely to be of general application in understanding the fundamental processes and principles by which the mammalian nervous system works.(ABSTRACT TRUNCATED AT 400 WORDS)

Re-evaluation of the effects of norepinephrine on the single-unit activity of paraventricular neurosecretory neurons

Electrical stimulation of the A1 noradrenergic region elicited predominantly an excitatory orthodromic response from paraventricular neurosecretory neurons recorded in urethane-anesthetized male rats. In a majority of cases, locally applied norepinephrine (NE) as well as A1 region stimulation excited paraventricular neurosecretory neurons. However, in some neurons including a subpopulation of putative vasopressin neurons, iontophoresed NE was inhibitory. Both excitatory and inhibitory effects of NE were selectively blocked by the alpha-antagonist, phentolamine, but not by the beta-antagonist, timolol. In general, the results support the notion that the A1 noradrenergic input to paraventricular neurosecretory neurons is excitatory. Nevertheless, the results do not exclude the possibility that NE exerts a specific, alpha-receptor-mediated, inhibitory effect on a subpopulation of paraventricular neurons.

Function of the ventrolateral medulla in the control of the circulation

The CNS control of the cardiovascular system involves the coordination of a series of complex neural mechanisms which integrate afferent information from a variety of peripheral receptors and produce control signals to effector organs for appropriate physiological responses. Although it is generally thought that these control signals are generated by a network of neural circuits that are widely distributed in the CNS, over the last two decades a considerable body of experimental evidence has accumulated suggesting that several of these circuits involve neurons found on or near the ventral surface of the medulla oblongata. Neurons in the VLM have been shown to be involved in the maintenance of vasomotor tone, in baroreceptor and chemoreceptor (central and peripheral) reflex mechanisms, in mediating the CIR and somatosympathetic reflexes and in the control of the secretion of vasopressin. These physiological functions of VLM neurons have been supported by neuroanatomical and electrophysiological studies demonstrating direct connections with a number of central structures previously implicated in the control of the circulation, including the IML, the site of origin of sympathetic preganglionic axons, and the SON and PVH, the site of origin of neurohypophyseal projecting axons containing AVP. Considerable suggestive evidence has also been obtained regarding the chemical messengers involved in transmitting information from VLM neurons to other central structures. There have been developments suggesting a role for monoamines and neuropeptides in mediating the neural and humoral control of SAP by neurons in the VLM. This review presents a synthesis of the literature suggesting a main role for VLM neurons in the control of the circulation.

Inhibiting the rabbit caudal ventrolateral medulla prevents baroreceptor-initiated secretion of vasopressin

The A 1 noradrenergic neurones are known to project from the caudal ventrolateral medulla to the vasopressin-secreting neuroendocrine cells in the hypothalamus. They therefore represent a possible central pathway from the medulla to the hypothalamus for baroreceptor-initiated secretion of vasopressin. We tested this hypothesis in the anaesthetized rabbit. Muscimol, a gamma-aminobutyric-acid-receptor agonist, was injected into the caudal ventrolateral medulla to inhibit the A 1 noradrenergic neurones. Secretion of vasopressin, measured by radioimmunoassay, was initiated either by arterial haemorrhage or by constriction of the inferior vena cava. After injection of vehicle into the caudal ventrolateral medulla, or after injection of muscimol into nearby control areas, both haemorrhage and constriction of the inferior vena cava produced the expected elevation in plasma vasopressin. After injection of muscimol into the caudal ventrolateral medulla, secretion of vasopressin in response to haemorrhage and to constriction of the inferior vena cava, was completely abolished. The A 1 noradrenergic neurones may be the sole pathway transmitting the reflex for baroreceptor-initiated secretion of vasopressin from the medulla to the hypothalamus.

Inputs from the A1 noradrenergic region to hypothalamic paraventricular neurons in the rat

Electrical stimulation of the rat A1 noradrenergic region produced excitation (77%) of the activity of putative vasopressin (VP)-secreting neurons in the paraventricular nucleus (PVN) and produced excitation (4%), inhibition (26%) and excitation-inhibition (11%) of the activity of PVN neurons that were not antidromically identified by neurohypophysial stimulation. The excitatory response of putative VP-secreting neurons was blocked by microiontophoretically applied phentolamine, an alpha-adrenoceptor antagonist, but not by timolol, a beta-adrenoceptor antagonist. The inhibitory response of unidentified PVN neurons, on the other hand, was blocked by timolol, but not by phentolamine.