Activation of supraoptic magnocellular neurons by gamma 2-melanocyte stimulating hormone (gamma 2-MSH).

The effects of intracarotid infusions of the peptide gamma 2-melanocyte stimulating hormone (gamma 2-MSH) on electrophysiologically and immunohistochemically identified supraoptic nucleus (SON) units were investigated. Over a wide dose range this agent always excited SON units, while control infusions of vehicle had no effect. Because neural responses invariably preceded blood pressure elevation, it appears that gamma 2-MSH excitation of the magnocellular system was due to a direct effect on the central nervous system and was not a result of systemic cardiovascular responses. These results suggest a forebrain gamma 2-MSH sensitive site in the activation of SON magnocellular neurons.

The supraoptic nucleus: afferents from areas involved in control of body fluid homeostasis.

Physiological evidence indicates that the supraoptic nucleus may be an important integrating region for information relating to body fluid homeostasis. It is known that the supraoptic nucleus receives neural influences from brain receptive zones for plasma osmolality and angiotensin II, as well as from relay centers for blood pressure and blood volume. It is also known that these influences interact to modulate vasopressin release from the supraoptic nucleus. Therefore, a detailed investigation of the neurochemical afferents to the supraoptic nucleus from regions of the lamina terminalis and the brainstem was undertaken. Injection of a fluorescent retrograde tracer, doxorubicin, into the supraoptic nucleus was combined with histochemistry of angiotensin II and catecholamines. Following supraoptic nucleus injection, retrograde label was found in forebrain neurons of the subfornical organ, median preoptic nucleus, and organum vasculosum of the lamina terminals. Some labeled cells in the subfornical organ and organum vasculosum of the lamina terminalis were also found to contain angiotensin II immunoreactivity. In the brainstem, retrograde label was found in neurons of the A1, A2 and A6 cell groups. Many of these cells were also found to contain catecholamine fluorescence or tyrosine hydroxylase immunoreactivity. Corroboration of the A2 projection was obtained by lesions of this nucleus, which reduced catecholamine fluorescence in the supraoptic nucleus. These findings provide an anatomical basis for the functional observations that the supraoptic nucleus plays a key integrative role in the maintenance of body fluid homeostasis.

Forebrain and brainstem afferents to the arcuate nucleus in the rat: potential pathways for the modulation of hypophyseal secretions.

Doxorubicin, an anti-oncogenic agent, was used as a retrograde marker to identify arcuate nucleus afferent projections. Injections of this tracer into the arcuate nucleus indicated that the subfornical organ, the organum vasculosum of the lamina terminalis, the nucleus raphe dorsalis and median raphe send projections to the arcuate nucleus. Immunocytochemical procedures were used to demonstrate that the raphe projections to the arcuate nucleus are serotoninergic. This anatomical investigation provides evidence that neural pathways exist between forebrain body fluid and mineral nuclei, mesencephalic serotonin nuclei and the arcuate nuclei.

Response of supraoptic magnocellular neurons to stimulation of forebrain alpha-adrenoceptors.

Effects of alpha-adrenoceptor agents on electrophysiologically and immunohistochemically identified supraoptic nucleus (SON) vasopressin (VP) units were investigated by intracarotid infusion. Clonidine, an alpha 2-adrenoceptor agonist always excited SON units and alpha 2-adrenoceptor antagonists consistently inhibited them. alpha 1-Adrenoceptor agents produced inconsistent responses. The results implicate forebrain alpha 2-adrenergic receptors in the excitation of SON VP neurons.

Demonstration of regional blood-brain barrier permeability in human brain.

The brain of a 78-year-old woman with argyria was examined at autopsy. Silver nitrate deposition was observed in circumventricular organs (CVO) and in the paraventricular and supraoptic nuclei of the hypothalamus. These findings parallel animal experiments of other investigators and are the best demonstration so far of regional absence of the blood-brain barrier in humans. These observations demonstrate similarities between humans and other mammals of CVO anatomy, permeability to blood-borne agents, and perhaps neural connections between CVOs and magnocellular nuclei.

Systemic angiotensin II, blood pressure and supraoptic neuronal activity.

Recordings of SON single unit activity and systemic arterial blood pressure (B.P.) were taken from 10 rats while systemic infusions of angiotensin II (AII), 1-1000 ng/kg body weight/min in 7 steps, or phenylephrine, 1-100 ng in 3 steps were administered. The relationship between AII concentrations and neuronal activity was biphasic. Within the physiological range (1 ng to 100 ng) AII excited single units in a dose dependent manner, but it had little effect on B.P. At higher concentrations, B.P. rose and neuronal activity was decreased. Phenylephrine, however, did not excite neuronal activity. With increasing phenylephrine concentrations, B.P. rose and neuronal activity slowed. We conclude that increased B.P. may dampen the SON neuronal output by baroreceptor inhibition. Under physiological conditions, therefore, AII may serve to reinforce tonic vasopressin release while inhibiting vasopressin release at pressor doses. This further suggests a role for plasma AII as an important link of the renal-hypothalamic-hormonal feedback loop.

Recovery from decamethonium in rat muscle and denervated guinea-pig diaphragm.

1. In denervated guinea-pig diaphragm the depolarization produced by decamethonium (100 microM) was followed by an initial phase of recovery, and then by a slow restoration of membrane potential in the presence of the drug, with hyperpolarization. Membrane potentials were measured by repeated insertions. The slow phase of spontaneous recovery was not found in the absence of potassium or in the presence of ouabain (100 microM). 2. With 1 microM-decamethonium the net loss of potassium from denervated muscle was 17% by wet weight in 20 min as compared with controls, which represents a loss of over 30 mM in internal concentration. Similar results were obtained with 100 microM-decamethonium. Spontaneous recovery of potassium occurred in the succeeding 2 h in the presence of 1 microM and 100 microM-decamethonium. With 5 nM-decamethonium muscles exposed for 20 min had a potassium content which was not reduced as compared with controls. 3. In rat diaphragm decamethonium (100 microM) also produced depolarization and slow spontaneous recovery which was not seen in the absence of potassium or the presence of ouabain. With 3 mM-decamethonium spontaneous recovery of potential was complete in 5 min. 4. Change from 5 mM-potassium to potassium-free solution produced consistent hyperpolarization in denervated guinea-pig diaphragm. In rat diaphragm at 38 degrees C the results were variable, with some fibres showing hyperpolarization while others showed depolarization.

Two possible actions for circulating angiotensin II in the control of vasopressin release.

The supraoptic-hypophyseal tract is a primary system for the synthesis and release of vasopressin. Angiotensin II (AII) has been shown to release vasopressin when injected into the cerebral ventricles (IVT). However, intravenous (IV) AII injections have not produced consistent results. The present studies were conducted to examine the effects of AII delivered by either route on the unit activity of supraoptic nucleus (SON) magnocellular neurons. Rats were prepared with intracranial cannulas to insure delivery of drugs to the left lateral ventricle and with polyethylene catheters in the left jugular vein, femoral vein, and femoral artery for systemic injections and arterial pressure recordings. A ventral approach permitted recording from the SON without violating the ventricular-SON partition. Magnocellular neurons were electrophysiologically identified. In the majority of identified cells, IVT AII increased activity. In others pressor doses of AII IV inhibited firing while blood pressure was elevated. After sino-aortic denervation, AII IV excited SON neurons. Based on latency, and the fact that lesioning the anteroventral third ventricle blocked the action of AII IVT, the results indicate that AII IVT acts on a periventricular site to influence SON magnocellular neurons. Furthermore, systemic AII may have two effects on SON neurons: a central excitatory action, and an inhibition due to a baroreceptor reflex.

Spontaneous recovery from depolarizing drugs in rat diaphragm.

1. The end-plate region in surface fibres of rat diaphragm was located by the use of polarizing filters. 2. Carbachol (100 microM) produced depolarization at the end-plate to -55 mV, as shown by continuous recording, with some indication of spontaneous recovery in the presence of the drug. The miniature end-plate potentials disappeared and remained absent. 3. By repeated sampling it was found that the resting potential at the end-plate had largely recovered after 45 min in the presence of carbachol. Individual fibres showed much variation in the rate of recovery, and in some fibres the repolarization was rapid. 4. In the absence of K, carbachol produced depolarization at the end-plate without significant recovery, as shown by repeated sampling. 5. When muscles were exposed to ouabain (100 microM) in addition to carbachol the end-plate remained depolarized without recovery for 60 min. The effect of ouabain was reversible: withdrawal of ouabain (in the presence of carbachol) led to substantial recovery. 6. Suberyldicholine (100 microM) gave results which were similar to those produced by carbachol. 7. It was inferred that the spontaneous recovery of membrane potential in the presence of carbachol and of suberyldicholine is a process which is sensitive to K and to ouabain.

Pharmacological properties of kryptopyrrole and its oxidation products on isolated sciatic nerve of rat and on guinea-pig ileum.

1 Kryptopyrrole (2, 4-dimethyl, 3-ethylpyrrole) inhibited conduction in rat sciatic nerve by a local anaesthetic action. 2 Tone and both spontaneous and electrically-induced contractions of guinea-pig ileum were also inhibited by kryptopyrrole. The concentration of kryptopyrrole required for 50% inhibition of a maximum twitch tension (ID50) was 0.085 mM. 3 Oxidation products of kryptopyrrole with chromatographic properties similar to those of urinary constituents reported in schizophrenia and hepatic porphyrias had little or no effect at similar concentrations. 4 Dose-response curves to exogenous acetylcholine in guinea-pig ileum were shifted to the right by kryptopyrrole, with loss of parallelism and reduction in the maximum contraction. 5 Acetylcholine overflow from ileal segments at rest and during electrical stimulation was reduced by kryptopyrrole. 6 These results on ileal segments are consistent with kryptopyrrole having both a post-junctional site of action, presumably directly on the muscle, and a pre-junctional site reducing the output of acetylcholine from the myenteric plexus. 7 The significance of these findings is discussed in relation to a possible clinical pathological role for these compounds.

Sodium entry in rat diaphragm induced by depolarizing drugs.

1. An increased uptake of labelled sodium was found in the end-plate region of rat diaphragm following brief exposure to solution containing (24)Na plus carbachol (100 muM), with a wash in inactive saline. Tetrodotoxin (0.1 muM) was also present. Comparable results were obtained with decamethonium and suberyldicholine.2. With carbachol (100 muM) the influx of labelled sodium at the end-plate region was increased by a factor of at least three as compared with that at the end of the fibres.3. After entry the labelled sodium spread along the fibres with an apparent diffusion coefficient which was half that expected in the external solution.4. The dose-response curve for the effect of carbachol gave a half-maximal value of 72 muM.5. In muscles depolarized by potassium methyl sulphate the effect of carbachol on the entry of sodium was reduced although demonstrable.6. The entry of labelled sodium at the end-plate was maintained during prolonged exposure to carbachol (100 muM) or decamethonium (100 muM).7. The rate of entry of (24)Na obtained with carbachol, after corrections for the wash, was estimated as 1.5 x 10(3) ions channel(-1) sec(-1), measured over a period of 15 sec.8. Labelled decamethonium and labelled carbachol also accumulated at the end-plate region. After extrapolation to allow for the effects of the wash the entry of decamethonium when expressed as a clearance (pl. mg(-1) sec(-1)) was comparable to that of sodium, as expected if decamethonium and sodium enter through the same channels.