Effects of microelectrophoretically applied acetylcholine- and angiotensin-antagonists on the paraventricular neurosecretory cells excited by osmotic stimuli

Direct catecholaminergic-cholinergic interactions in the basal forebrain. III. Adrenergic innervation of choline acetyltransferase-containing neurons in the rat

The central adrenergic neurons have been suggested to play a role in the regulation of arousal and in the neuronal control of the cardiovascular system. To provide morphological evidence that these functions could be mediated via the basal forebrain, we performed correlated light and electron microscopic double-immunolabeling experiments using antibodies against phenylethanolamine N-methyltransferase (PNMT) and choline acetyltransferase, the synthesizing enzymes for adrenaline and acetylcholine, respectively. Most adrenergic/cholinergic appositions were located in the horizontal limb of diagonal band of Broca, within the substantia innominata, and in a narrow band bordering the substantia innominata and the globus pallidus. Quantitative analysis indicated that cholinergic neurons of the substantia innominata receive significantly higher numbers of adrenergic appositions than cholinergic cells in the rest of the basal forebrain. In the majority of cases, the ultrastructural analysis revealed axodendritic asymmetric synapses. By comparing the number and distribution of dopamine beta-hydroxylase (DBH)/cholinergic appositions, described earlier, with those of PNMT/cholinergic interactions in the basal forebrain, it can be concluded that a significant proportion of putative DBH/cholinergic contacts may represent adrenergic input. Our results support the hypothesis that the adrenergic/cholinergic link in the basal forebrain may represent a critical component of a central network coordinating autonomic regulation with cortical activation.

Nicotinic cholinergic activation of magnocellular neurons of the hypothalamic paraventricular nucleus

The aim of the present work was to determine whether paraventricular neurons possess functional acetylcholine nicotinic receptors. Using infrared videomicroscopy and differential interference contrast optics, we performed whole-cell recordings in hypothalamic slices containing the paraventricular nucleus. Acetylcholine, locally applied by pressure microejection in the presence of the muscarinic antagonist atropine, evoked a rapidly rising inward current in paraventricular magnocellular endocrine neurons. This current persisted in the presence of blockers of synaptic transmission. It could be reversibly suppressed by nanomolar concentrations of methyllycaconitine, a selective antagonist of alpha 7-containing nicotinic receptors, but was insensitive to micromolar concentrations of dihydro-beta-erythroidine, an antagonist acting preferentially on non-alpha 7 nicotinic receptors. In addition, the effect of acetylcholine could be mimicked by exo-2-(2-pyridyl)-7-azabicyclo[2.2.1]heptane, a recently synthesized nicotinic agonist specific for alpha 7 receptors. Acetylcholine also desensitized paraventricular nicotinic receptors. Desensitization was pronounced and recovery from desensitization was rapid, consistent with the notion that paraventricular nicotinic receptors contain the alpha 7 subunit. Nicotinic currents could not be evoked in paraventricular parvocellular neurons, suggesting that these neurons are devoid of functional nicotinic receptors. The electrophysiological data were corroborated by light microscopic autoradiography, showing that [(125)I]alpha-bungarotoxin binding sites are present in all the magnocellular divisions of the paraventricular nucleus but are undetectable in other areas of this nucleus. Immunohistochemistry, performed using antibodies directed against vasopressin and oxytocin, indicated that responsiveness to nicotinic agonists was a property of vasopressin as well as of oxytocin magnocellular endocrine neurons, in both the paraventricular and the supraoptic nucleus. We conclude that nicotinic agonists can influence the magnocellular neurosecretory system by directly increasing the excitability of magnocellular neurons. By contrast, they are probably without direct effects on paraventricular parvocellular neurons.

Central angiotensin AT1 and muscarinic receptors in ITF expression on intracerebroventricular NaCl

In the present study, we investigated the expression pattern of the inducible transcription factors (ITF) c-Fos, c-Jun, JunB, JunD, and Krox-24 following intracerebroventricular injections of hyperosmolar saline (0.2, 0.3, and 0.6 M NaCl) and its mediation via angiotensin and/or muscarinic receptors. c-Fos, c-Jun, and Krox-24 were differentially expressed in organum vasculosum laminae terminalis, median preoptic area, subfornical organ (SFO), and paraventricular and supraoptic nuclei. Expression of c-Fos and c-Jun was inhibited by pretreatment with the angiotensin AT1 receptor antagonist losartan (10 and 20 nmol icv) following 0.20 and 0.30 M saline. Pretreatment with atropine (15 nmol icv) inhibited the 0.30 and 0.60 M NaCl-induced expression of c-Fos, c-Jun, and Krox-24 in all areas except the SFO. Coexpression of the ITF with vasopressin and oxytocin, the major effector peptides in osmoregulation, was demonstrated, implying the corresponding genes as putative target genes of the ITF. The results show a highly differentiated ITF expression pattern in the brain mediated by angiotensinergic and muscarinergic pathways, suggesting a finely tuned regulation of target genes.

Release of angiotensin in the paraventricular nucleus in response to hyperosmotic stimulation in conscious rats: a microdialysis study

Angiotensin peptides are thought to act as neurotransmitters or neuromodulators in central osmoregulation. We tested the hypothesis that angiotensin peptides are released in the paraventricular nucleus (PVN) of the hypothalamus upon local osmotic stimulation. Brain microdialysis and radioimmunoassay (RIA) techniques were used to measure the release of immunoreactive angiotensin II (irANG II) in the PVN following direct stimulation of this area with hyperosmotic solutions. In conscious rats, perfusion of the PVN with 0.3 M and 0.6 M NaCl in artificial cerebrospinal fluid (aCSF) elicited concentration-dependent increases in irANG II release to 5.52 +/- 0.53, (P < 0.01, n = 8) and 9.01 +/- 1.03 pg/100 microliters, (P < 0.001, n = 7), respectively, from basal values of 3.04 +/- 0.46 pg/100 microliters. Local perfusion of the PVN with 1.2 M glucose in aCSF also resulted in an increased release of irANG II from 3.07 +/- 0.87 to 6.24 +/- 0.45 pg/100 microliters (P < 0.05, n = 5). Fractionization of angiotensin peptides by HPLC followed by RIA revealed that ANG II (1-8) and ANG III (2-8) were released in similar amounts in the perfusate collected during 0.6 M NaCl stimulation (4.79 +/- 0.69 and 3.45 +/- 0.76 pg/100 microliters, respectively). Our results show that both, ANG II and ANG III are released in the PVN in response to local hyperosmotic stimulation. They support the concept that angiotensin peptides in the PVN are involved as neurotransmitters in central osmotic control.

The effect of neosurugatoxin on the release of neurohypophysial hormones by nicotine, hypotension and an osmotic stimulus in the rat

1. Experiments were carried out to test whether neosurugatoxin (NSTX) which blocks autonomic ganglia also acts centrally, like hexamethonium, on nicotinic cholinoceptors involved in the neural control of release of vasopressin and oxytocin from the neurohypophysis. 2. In the water-loaded rat under ethanol anaesthesia, nicotine 100 micrograms i.v. produced a pressor and an antidiuretic response accompanied by an increase in the urinary excretion of vasopressin and of oxytocin-like radioimmunoreactivity (OLRI). This indicates release of both vasopressin and oxytocin. 3. Under conditions in which tachyphylaxis was avoided, NSTX, 80 ng i.c.v., caused a prolonged inhibition of the release of both hormones by nicotine. 4. NSTX i.c.v. caused some reduction in the pressor response to nicotine. It is suggested that this response involves both central and peripheral stimulation of the sympathetic nervous system and that the central component is blocked by neosurugatoxin. 5. Muscarine, 40 ng i.c.v., produced a pressor and an antidiuretic response with increased urinary excretion of vasopressin and OLRI. All these effects were blocked by atropine but were not inhibited by NSTX. 6. Sodium nitroprusside (SN), 200 micrograms i.v., and hypertonic saline (HS; 1.54 M NaCl solution) 4 microliters i.c.v., both produced antidiuretic responses accompanied by increased urinary excretion of vasopressin and OLRI. The ratio of the excretion of vasopressin to that of OLRI was 5.1 +/- 1.3 (mean +/- s.e.: n = 8) for SN and 1.2 +/- 0.24 (mean +/- s.e.: n = 6) for HS.NSTX 80 ng i.c.v., caused a significant reduction in the antidiuretic response to the hypotension induced with SN: the increased urinary excretion of vasopressin was also significantly reduced but not that of OLRI. NSTX had no effect on the response to HS.7. We conclude that NSTX acts centrally on nicotinic cholinoceptors to block the release of vasopressin and oxytocin by nicotine and the release of vasopressin, but not that of oxytocin, by hypotension. It does not inhibit the release of either hormone by a central osmotic stimulus.

Hypothalamic osmoregulation for vasopressin release in streptozotocin-diabetic rats in vivo and in vitro

The central osmoregulation mechanism for vasopressin (VP) release was studied in the streptozotocin diabetic (STZ-DM) rat. Electrical activities of the VP-producing cell in the supraoptic nucleus (SON) were recorded extracellularly and compared with those in the control rat both in vivo and in vitro. Neuronal activities in the periventricular area (PVA) were also recorded in the in vitro experiment. Hyperosmolar stimulation which was done with an intraperitoneal injection of 1.0 M NaCl (4 ml/kg) resulted in an increase in plasma osmolality both of STZ-DM (increased by 14 +/- 2 mOsml/kg H2O) and control rats (15 +/- 3 mOsmol/kg H2O). Increased plasma osmolality caused significant increase in the mean discharge rate of VP-producing cells in the control animals, but only an insignificant change in STZ-DM rats. In the hypothalamic slice preparations incubated in the artificial cerebrospinal fluid (301 +/- 2 mOsml/kg H2O), VP-producing cells in control rats increased their discharge rate linearly as the osmolality (310 +/- 2 and 320 +/- 1 mOsmol/kg H2O) or concentration (10(-8) and 10(-6) M) of angiotensin II (AGII) of the perfusate was increased stepwise, but there was no change in response to either stimulus in STZ-DM rats. On the other hand, there was no difference in sensitivity to osmolality and AGII of PVA neurons in both animal groups. These data indicate that lower sensibility to osmotic change of VP-producing cells in STZ-DM rats may depend, at least partially, upon the disturbance of osmo- and AGII-sensitivity in VP-producing cells themselves, and that these changes seem to be restricted to SON VP-producing cells.

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)

Role of anterior peri-third ventricular structures in the regulation of supraoptic neuronal activity and neurohypophysical hormone secretion in the rat

Abstract Neurohypophysical hormone release, and the electrical activity of single neurons of the supraoptic nucleus, were monitored in urethane-anaesthetized rats. Immediately after electrolytic lesions of the region anterior and ventral to the third ventricle (AV3V region), supraoptic neurons showed little spontaneous activity and their responses to ip injection of hypertonic saline were severely impaired; corresponding deficits were found in the secretion of both oxytocin and vasopressin. Similar deficits in oxytocin secretion were also found in rats following electrolytic lesions which destroyed all or part of the subfornical organ; however the effects of the lesions were not additive: rats with lesions of both the AV3V region and the subfornical organ region showed a similar degree of impairment of osmotically stimulated oxytocin secretion to rats with lesions of either site alone. Such deficits might occur either as a result of destruction of osmoresponsive projections to the magnocellular nuclei, or as a result of destruction of an afferent input which is essential for the full expression of the innate osmosensitivity of supraoptic neurons. To test the latter possibility, supraoptic neurons in AV3V-lesioned rats were activated by continuous application of glutamate, and then tested with ip injection of hypertonic saline. Five of seven cells tested responded significantly to the hyperosmotic stimulus, though the responses were significantly weaker than observed in sham-lesioned rats. We suggest that the innate osmosensitivity of supraoptic neurons does contribute to their responses to systemic osmotic stimulation, but that expression of this innate osmosensitivity requires inputs from the AV3V region and/or the subfornical organ, some of which may also be osmoresponsive. Electrical stimulus pulses applied to the AV3V region influenced the electrical activity of most supraoptic neurons strongly: the predominant response was a short-latency, short-duration inhibition followed by long-latency, long-duration excitation. Whereas intracerebroventricular administration of the angiotensin II antagonist saralasin reduced spontaneous or osmotically induced activity of supraoptic neurons, the neuronal responses to AV3V stimulation were impaired only with relatively high doses of saralasin. We conclude that angiotensin ll-sensitive neurons are an important component of the afferent pathways that sustain the excitability of supraoptic neurons, but that angiotensin is probably not the major transmitter of the projection from the AV3V region to the supraoptic nucleus.

The role of the AV3V region in the control of magnocellular oxytocin neurons

The AV3V region is important in the control of body fluid and Na+ regulation and projects to the supraoptic and paraventricular nuclei. Oxytocin from the neurohypophysis mediates milk ejection and is involved in parturition, but has also been recently implicated as a candidate natriuretic hormone. We have studied the role of the AV3V region in the control of magnocellular oxytocin neurons in rats. Electrical stimulation of the AV3V region increased the firing rate of supraoptic oxytocin neurons and evoked a concomitant release of oxytocin. Acute electrolytic AV3V lesions silenced supraoptic neurons and abolished their excitation by hyperosmotic stimulation. The lesions also abolished osmotically-induced release of oxytocin. Re-activation of supraoptic neurons by local glutamate restored their osmoresponsiveness to about 50% normal. Thus, while supraoptic neurons are directly osmosensitive, the AV3V region is essential for their normal osmoresponsiveness. Electrolytic AV3V lesions did not affect suckling-induced oxytocin secretion or, in conscious rats, the release of oxytocin secretion during parturition. Thus the AV3V region is not involved in the activation of oxytocin neurons during suckling or parturition.

Neural inputs from the uterus to the paraventricular magnocellular neurons in the rat

Extracellular action potentials were recorded from antidromically identified, tonically firing cells in the hypothalamic paraventricular nucleus (PVN) of ovariectomized, estrogen-treated female rats under urethane anesthesia. Genital or somatic sensory stimuli, or electrical stimulation of the nerves innervating the pelvis were applied. Uterine horn or vaginal distension each excited 33% of the neurons tested. Probing of the cervix had no effect. Hindpaw pinch produced excitation in 39% and inhibition in 11% of the neurons tested. Non-noxious somatic stimuli had no effect. Stimulation of the uterine afferent nerves, the hypogastric and pelvic nerves, excited 55% and 30% of the neurons tested, respectively. Stimulation of a somatic nerve of the hindleg, the sciatic nerve, activated 80% of the neurons tested. These results indicate that specific sensory afferents arrive at the PVN from the uterus; in addition, somatic afferents converge in this hypothalamic nucleus.