Acetylcholine and related enzymes in the neural lobe and anterior hypothalamus of the rabbit

Comparative studies on the release of vasopressin and 7B2 from isolated hypothalami

A new protein, 7B2, has been found in the anterior, intermediate and posterior pituitary and the hypothalamus. It has been previously suggested that 7B2 is synthesized in the hypothalamus and is transported to the posterior lobe in a similar way to vasopressin (AVP). We examined the in vitro release of AVP and 7B2, using rat hypothalami in a perifused column system. Membrane depolarization with KCl (56 mM) caused a marked stimulation of AVP release (from 116% to 263% above basal values). Release of 7B2 was also stimulated by potassium from 106% to 212% above basal values. Potassium pulses in calcium-free medium failed to release AVP or 7B2. Osmotic and cholinergic stimulation increased AVP secretion by 205% and 282% above basal values, respectively, but had no significant effect on 7B2 secretion. Chromatographic profiles of perifusion media revealed one immunoreactive 7B2 peak eluting at a coefficient of 0.34 corresponding to that of rat hypothalamic 7B2. Similarly AVP coeluted with AVP standard. Thus AVP and 7B2 are differentially released by cholinergic and osmotic stimuli in vitro.

The effect on vasopressin release of microinjection of cholinergic agonists into the paraventricular nucleus of conscious rats

Abstract We have studied in conscious unrestrained rats under basal conditions the effect of activation of muscarinic and nicotinic receptors in the paraventricular nucleus on vasopressin secretion and mean arterial blood pressure. The microinjection of oxotremorine (0.2, 2 or 20 ng), a specific muscarinic agonist, produced a substantial, dose-dependent, transient increase in the plasma vasopressin concentration. There was also a rise in mean arterial blood pressure and a bradycardia that followed the same time-course as the change in plasma vasopressin levels. The microinjection of nicotine (0.1, 1 or 10 HQ) into the paraventricular nucleus had only questionable effects on vasopressin release and mean arterial blood pressure; heart rate was unaffected. These findings suggest that muscarinic receptors may be of primary importance in the paraventricular nucleus in the Cholinergic stimulation of vasopressin release.

Low-affinity uptake of [3H]choline by rat neurointermediate lobe in vitro

Uptake of [3H]choline by rat neurointermediate lobes in vitro was investigated. The rate of uptake showed saturation with concentration of [3H]choline above 120 microM. Lowering of the incubation temperature from 32 to 4 degrees C, or increasing the concentration of K+ in the incubation medium from 5 to 100 mM, diminished the rate of uptake by 83.8% (SEM 6.9%, n = 3) or 43.1% (SEM 13.5%, n = 3), respectively. Following preloading with [3H]choline, a slow efflux (1% of the content every 20 min) of [3H]radioactivity was observed from the perifused glands. This efflux was enhanced 10-fold by increasing the concentration of K+ in the perifusion medium to 100 mM. Neither the uptake of [3H]choline, nor the subsequent basal or potassium-enhanced efflux of [3H]radioactivity were affected by reducing the concentration of Na+ from 125 to 19 mM, or by including 10 microM hemicholinium-3 in the medium during preincubation and perifusion. Replacing Ca2+ by 0.5 mM EGTA during perifusion resulted in a minor decrease (28%, SEM 7.3%, n = 7) in the potassium-enhanced [3H]radioactivity efflux. This decrease occurred only in one of the two high-potassium periods during the perifusion. In conclusion, uptake of [3H]choline by rat neurointermediate lobes was due to a low-affinity, saturable mechanism, with the efflux of [3H]radioactivity most likely representing the depolarization-facilitated outflow of [3H]choline. Autoradiography of the tissue sections showed this uptake to be localized to both neuronal and glial elements of the neural lobe, in contrast to sparse labelling of pars intermedia.

Localization of acetylcholinesterase and choline acetyltransferase in the rat pituitary gland

Experiments were conducted to determine the presence of two cholinergic biomarkers, acetylcholinesterase (AChE) and choline acetyltransferase (ChAT) in the rat pituitary. A histochemical procedure for AChE was used to provide visualization of structures containing this enzyme. Radiochemical methods provided a sensitive assay for measuring ChAT activity. Nerve fibres staining for AChE activity were observed in the neurointermediate lobe, with the greatest concentrations appearing at the junction region with the pituitary stalk. Cells staining for AChE were found in the pars distalis and pars intermedia. ChAT activity correlated well with AChE distribution in pars nervosa and pars intermedia but not in pars distalis. The greatest levels of ChAT activity were in pars intermedia and the region where the stalk joins the pituitary. Significant values were also found for the pars nervosa. The presence of AChE and ChAT in pars intermedia and pars nervosa is evidence for a cholinergic innervation to these regions. In pars distalis, where other investigators have found muscarinic receptors, intense staining for AChE and absence of ChAT activity may indicate non-innervated, acetylcholine-sensitive sites.

Binding of [3H]quinuclidinyl benzilate to regions of rat pituitary and hypothalamus

Muscarinic ligand binding sites in fragments of rat hypothalamus and pituitary were studied using [3H]quinuclidinyl benzilate (QNB). In the hypothalamus, the highest amount of specific QNB binding was to n. paraventricularis and n. dorsomedialis. Specific QNB binding in other hypothalamic regions varied within a relatively narrow range. Fragments of whole pituitary also bound QNB but to a much smaller degree than brain. Pituitary binding of QNB was blocked by atropine but not by hexamethonium or d-tubocurarine. Within the pituitary, specific QNB binding to posterior pituitary was three times greater than to anterior pituitary. These findings are consistent with the operation of cholinergic mechanisms in hypothalamic and pituitary function.

An electron microscopic study on the structural development of the neural lobe in the human fetus

Structural development of the neural lobe of the hypophysis was studied by light and electron microscopy in five human fetuses ranging from 7.5 to 19 weeks of ovulation age. Clusters of small clear vesicles were found occasionally in axon profiles in a fetus of 7.5 weeks. Granular vesicles in axon profiles first accumulated in a fetus of 8.5 weeks. Concurrently, fenestration of capillary walls and expansion of perivascular spaces were observed. After 8.5 weeks, three types of vesicle-containing axon profiles were found, which contained predominantly granular vesicles (GP), small clear vesicles (SP), and a mixed population of both granular and small clear vesicles (MX), respectively. Their numbers increased significantly during two periods, 7.5-8.5 weeks and 15.5-19 weeks. Ratios of GP, SP, and MX to the total number of the three types of axon were almost the same in all the fetuses studied, i.e., about 40%, 40%, and 20%, respectively. Developmental changes in the diameter of granular vesicles were examined in GP, SP, and MX of each fetus. Apart from a few exceptions, the mean diameter of granular vesicles was larger in GP than in MX, which was in turn larger than in SP. The diameters increased with development and those in GP reached almost the same value as in the adult by 19 weeks.

Antibodies to synaptic vesicles purified from Narcine electric organ bind a subclass of mammalian nerve terminals

Antibodies were raised in rabbits to synaptic vesicles purified to homogeneity from the electric organ of Narcine brasiliensis, a marine electric ray. These antibodies were shown by indirect immunofluorescence techniques to bind a wide variety of nerve terminals in the mammalian nervous system, both peripheral and central. The shared antigenic determinants are found in cholinergic terminals, including the neuromuscular junction, sympathetic ganglionic and parasympathetic postganglionic terminals, and in those synaptic areas of the hippocampus and cerebellum that stain with acetylcholinesterase. They are also found in some noncholinergic regions, including adrenergic sympathetic postganglionic terminals, the peptidergic terminals in the posterior pituitary, and adrenal chromaffin cells. They are, however, not found in many noncholinergic synapse-rich regions. Such regions include the molecular layer of the cerebellum and those laminae of the dentate gyrus that receive hippocampal associational and commissural input. We conclude that one or more of the relatively small number of antigenic determinants in pure electric fish synaptic vesicles have been conserved during evolution, and are found in some but not all nerve terminals of the mammalian nervous system. The pattern of antibody binding in the central nervous system suggests unexpected biochemical similarities between nerve terminals heretofore regarded as unrelated.

Central cholinergic control of cerebral blood flow in the baboon. Effect of cholinesterase inhibition with neostigmine on autoregulation and CO2 responsiveness

Cerebral autoregulation and vastomotor responsiveness to carbon dioxide (CO2) were measured quantitatively by the use of the autoregulation index and chemical index, respectively, in normal baboons before and after intravertebral and intracarotid infusion of the anticholinesterase agent, neostigmine methylsufate (Prostigmin). Continuous measurements were made of cerebral blood flow (measured as bilateral internal jugular venous outflow), arterial and cerebral venous pO2 and pCO2, cerebral arteriovenous oxygen differences, and endotracheal CO2. The effect of intravertebral infusion of neostigmine (12.5 mug/kg body weight) was compared to intravertebral infusion of neostigmine (25 mug/kg body weight) for assessment of any specific action of the drug on a hypothetical cholinergic vasomotor center, presumed to be located in the territory of the vertebrobasilar supply. No significant or persistent changes in cerebral blood flow (CBF) and cerebral metabolic rate for oxygen (CMRO2) followed either intravertebral or intracarotid infusion of neostigmine. Cerebral vascular resistance (CVR) and cerebral perfusion pressure (CPP), however, decreased significantly after intravertebral infusion. Cerebral autoregulatory vasoconstriction during increases of CCP was significantly reduced following both intravertebral and intracarotid infusion. Cerebral autoregulatory vasodilatation was not altered as CPP was lowered. Cerebral vasodilatory reactivity to CO2 inhalation was significantly enhanced following intravertebral neostigime but not following intracarotid neostigmine. Cerebral vasoconstrictive response to hyperventilation was not influenced by neostigmine. These results support the view that central cholinergic cerebrovascular influences exist, and are vasodilatory in nature.

Morphology of the perivascular regions of the rat neural lobe in relation to hormone release

Neural lobes of rats subjected to dehydration by drinking 2% saline for four days were examined electron microscopically and compared to untreated controls. The ultrastructure of the blood vessels and the tissues surrounding them were examined and it was found that, although few exocytotic figures could be seen in either group of animals, a significantly larger (P smaller than 0.01) number os small vesicles were found in nerve endings adjacent to the perivascular space in the saline treated group when compared to nerve endings not closely associated with blood vessels. No differences were found in the control group of animals, which supports the suggestion that the vesicles could arise from a membrane recapture process.

The role of brain acetylcholine in the pressor response to centrally infected neostigmine

In unanaesthetized rats, neostigmine (2.5 mug) injected into the cerebral ventricles produces a rise in arterial blood pressure. The effect on the rise was examined after lowering the acetylcholine (ACh) level of the brain by hemicholinium (HC-3) similarly injected.The pressor response was not significantly altered 2 h after injection of 50 mug HC-3 when the ACh level was reduced by 54%. It was greatly attenuated 2 h after injection of 75 mug when the ACh level was reduced by 76%. However, a similar attenuation was observed as early as 30 min after this injection when the ACh level was reduced by 18% only.It is concluded that the pressor response is not mediated by undestroyed ACh but is a direct central effect of neostigmine.

Preferential release of newly synthesized 3 H-acetylcholine from rat cerebral cortex slices in vitro

1. Slices of rat cerebral cortex after treatment with the irreversible cholinesterase inhibitor soman, were incubated for 5 min in a Krebs-Henseleit solution containing 25 mM KCl and (3)H-choline. Subsequently incubation was continued in a medium containing non-radioactive choline and this medium was replaced at 5 min intervals. The amounts of labelled and total acetylcholine (ACh) released into the medium and extracted from the slices were determined at intervals.2. After the initial 5 min contact with (3)H-choline, 44% of the newly synthesized ACh contained a choline moiety originating from the choline in the medium. During the initial 5 min and the subsequent incubation part of the labelled ACh was released. While the rate of total ACh release remained constant, that of the release of labelled ACh was highest in the 5 min period following the initial incubation with (3)H-choline and then declined exponentially.3. The ratio of labelled ACh/total ACh in the ACh released during the initial 5 min incubation with (3)H-choline and during the subsequent 5 min was about three times as high as that in the ACh extracted from the slices at the end of these incubation periods.4. The ratio of labelled ACh/total ACh in superficial layers of the slices was not higher than that in the total slices.5. The rates of release of labelled and unlabelled ACh decreased when calcium was omitted from the incubation medium and were restored when the calcium was added. This suggests that both labelled and unlabelled ACh were released from nerve endings. The efflux of (3)H-choline was not calcium dependent.6. It is concluded that labelled ACh newly synthesized from externally applied (3)H-choline does not exchange immediately with all other ACh in the tissue and has a greater chance of being released than unlabelled ACh.

Noradrenaline and acetylcholine responses of supraoptic neurosecretory cells

1. Neurones in the supraoptic nucleus were examined for their responsiveness to microiontophoretically applied monoamines and cholinomimetic agents. One hundred and sixty-one of the 749 neurones recorded were antidromically identified as neurosecretory cells.2. The monoamines, dopamine, noradrenaline and serotonin, reduced the activity of all cells which responded.3. Reduction in activity following noradrenaline administration was antagonized by the beta-adrenergic blocking agent MJ-1999 and potentiated by desmethylimipramine.4. Acetylcholine produced either a decrease or an increase in activity of responsive cells with depression the predominant result. Both the depressant and the excitatory response to acetylcholine were seen in individual neurosecretory cells which were depressed by noradrenaline.5. Application of acetyl-beta-methylcholine and carbaminoylcholine consistently resulted in depression of all responsive cells, while nicotine excited the majority of responsive cells. Both types of response were observed on the same neurosecretory cell.6. The depressant response was antagonized by the muscarinic blocking agent atropine while the excitatory response was antagonized by the nicotinic blocking agent dihydro-beta-erythroidine. Responses to acetylcholine were potentiated by the acetylcholinesterase inhibitor physostigmine.7. The data indicate that noradrenaline-containing terminals on these neurosecretory cells are likely to inhibit their discharge rate, while the presumed cholinergic terminals might function as either excitatory or inhibitory, depending on the receptor activated.8. These results support the hypothesis that ADH release is related to the neuronal activity of the supraoptic neurosecretory cell.

Supraoptic neurosecretory cells: adrenergic and cholinergic sensitivity

Adrenergic and cholinergic agonists and antagonists were applied microelectrophoretically to over 700 neurons in the cat supraoptic nucleus, 20 percent of which were antidromically identified as neurosecretory cells. Norepinephrine uniformly depressed all sensitive cells. Acetylcholine caused both muscarinic depression and nicotinic excitation which were antagonized by atropine and dihydro-beta-erythroidine, respectively. These results support the hypothesis that norepinephrine and acetylcholine are directly involved in controlling the release of antidiuretic hormone.