Localization of acetylcholinesterase and choline acetyltransferase in the rat pituitary gland

Pituitary-adrenal responses to oxotremorine and acute stress in male and female M1 muscarinic receptor knockout mice: comparisons to M2 muscarinic receptor knockout mice

Both within the brain and in the periphery, M(1) muscarinic receptors function primarily as postsynaptic receptors and M(2) muscarinic receptors function primarily as presynaptic autoreceptors. In addition to classical parasympathetic effectors, cholinergic stimulation of central muscarinic receptors influences the release of adrenocorticotrophic hormone (ACTH) and corticosterone. We previously reported that oxotremorine administration to male and female M(2) receptor knockout and wild-type mice increased ACTH to a significantly greater degree in knockout males compared to all other groups, and that M(2) knockout mice of both sexes were significantly more responsive to the mild stress of saline injection than were wild-type mice. These results accord with the primary function of M(2) receptors as presynaptic autoreceptors. In the present study, we explored the role of the M(1) receptor in pituitary-adrenal responses to oxotremorine and saline in male and female M(1) knockout and wild-type mice. Because these mice responded differently to the mild stress of saline injection than did the M(2) knockout and wild-type mice, we also determined hormone responses to restraint stress in both M(1) and M(2) knockout and wild-type mice. Male and female M(1) knockout and wild-type mice were equally unresponsive to the stress of saline injection. Oxotremorine increased both ACTH and corticosterone in M(1) wild-type mice to a significantly greater degree than in knockout mice. In both M(1) knockout and wild-type animals, ACTH responses were greater in males compared to females, and corticosterone responses were greater in females compared to males. Hormone responses to restraint stress were increased in M(2) knockout mice and decreased in M(1) knockout mice compared to their wild-type counterparts. These findings suggest that M(1) and M(2) muscarinic receptor subtypes differentially influence male and female pituitary-adrenal responses to cholinergic stimulation and stress. The decreased pituitary-adrenal sensitivity to oxotremorine and restraint stress noted in M(1) knockout mice is consistent with M(1) being primarily a postsynaptic receptor. Conversely, the increased pituitary-adrenal sensitivity to these challenges noted in M(2) knockout mice is consistent with M(2) being primarily a presynaptic autoreceptor.

Distribution of α7 and α4 nicotinic acetylcholine receptor subunits in several tissues of Triturus carnifex (Amphibia, Urodela)

The distribution of neuronal and non-neuronal mRNAs for alpha7 and alpha4 nicotinic acetylcholine receptor subunits was investigated in Triturus carnifex tissues using the in situ hybridization approach. The findings reveal a composite pattern of expression only partially overlapping for the two subunits; subunit alpha7 seems to be expressed widely throughout nervous, gastrointestinal and skin tissues, while alpha4 is present in a restricted number of cells of nervous and gastrointestinal tissue. We also found a specific pattern for each subunit; alpha7 and alpha4 associated exclusively to the epidermal glands and hypophysis, respectively; this is probably due to alternative roles that nicotinic acetylcholine receptors play in regulating physiological functions of non-neuronal amphibian tissues, rather than as mere neurotransmitters in the nervous system.

Choline acetyltransferase immunoreactivity in the developing brain of Xenopus laevis

The spatiotemporal sequence of the appearance of cholinergic structures in the brain of Xenopus laevis during development was studied by means of choline acetyltransferase (ChAT) immunohistochemistry. The first ChAT labeling in the central nervous system of Xenopus was obtained at late embryonic stages in the spinal motoneurons, the cranial nerve motor nuclei of the brainstem, and in amacrine cells of the retina. During premetamorphosis, these cholinergic structures maturated significantly and new ChAT-immunoreactive cells were observed in several other nuclei such as the solitary tract nucleus, isthmic nucleus, laterodorsal and pedunculopontine tegmental nuclei, epiphysis, dorsal habenular nucleus, medial amygdala, bed nucleus of the stria terminalis, and dorsal pallidum. Further maturation continued through prometamorphosis and the climax of the metamorphosis together with the appearance of new cell groups in the efferent octaval nucleus, ventral hypothalamic nucleus, anterior preoptic area, suprachiasmatic nucleus, and medial septum. Transient expression of ChAT was only seen in the large Mauthner cells that showed moderate ChAT labeling during pre- and prometamorphosis but became immunonegative at the end of the metamorphosis. The gradual appearance, in general from caudal to rostral brain levels, of ChAT immunoreactivity in Xenopus, was correlated with other developmental events to get insight into the possible roles of acetylcholine during ontogeny. Comparison with the developmental pattern of cholinergic systems in other vertebrates shows that Xenopus possesses abundant features in common with amniotes, suggesting a conservative developmental plan for tetrapods.

Distribution of choline acetyltransferase-immunoreactive structures in the lamprey brain

The distribution of cholinergic neurons and fibers was studied immunohistochemically in the brain of two species of lampreys (Petromyzon marinus and Lampetra fluviatilis), by using an antiserum against choline acetyltransferase (ChAT). The results obtained in both species were similar, but there appeared some interspecies differences. In the forebrain, cholinergic cells were present in the striatum, preoptic region, paraventricular nucleus, pineal and parapineal organs, habenula, and pretectum. The cranial nerve motoneurons (III, IV, V, VI, VII, IX, and X), the first and second spino-occipital nerves (so), and the ventral horn of the spinal cord showed a strong ChAT immunoreactivity. Additional cholinergic neurons were observed: the mesencephalic M5 nucleus of Schober, two different cell populations in the isthmic region, the efferent component of the eighth nerve, putative preganglionic parasympathetic cells, cells in the solitary tract nucleus, and the rhombencephalic reticular formation. Cholinergic fibers were widely distributed in the brain. Comparison with previous studies in other vertebrates suggests that major cholinergic pathways, like tectal innervation from the isthmic region, are also present in lampreys. Of particular interest was the prominent projection to the neurohypophysis from cholinergic neurons in the preoptic region and paraventricular nucleus. Present data were analyzed within the segmental paradigm, as was previously done in other vertebrates. Our results reveal that the organization of many cholinergic systems in the lamprey as, for example, in the striatal, preoptic, and isthmic regions, comprises features of the anamniote brain that remain common to all living amniotes studied so far, thus being conservative to a surprisingly high degree. Therefore, the distribution of ChAT-immunoreactive structures in the lamprey brain is, in general, comparable to that previously described in other vertebrate species.

Prolactin: structure, function, and regulation of secretion

Prolactin is a protein hormone of the anterior pituitary gland that was originally named for its ability to promote lactation in response to the suckling stimulus of hungry young mammals. We now know that prolactin is not as simple as originally described. Indeed, chemically, prolactin appears in a multiplicity of posttranslational forms ranging from size variants to chemical modifications such as phosphorylation or glycosylation. It is not only synthesized in the pituitary gland, as originally described, but also within the central nervous system, the immune system, the uterus and its associated tissues of conception, and even the mammary gland itself. Moreover, its biological actions are not limited solely to reproduction because it has been shown to control a variety of behaviors and even play a role in homeostasis. Prolactin-releasing stimuli not only include the nursing stimulus, but light, audition, olfaction, and stress can serve a stimulatory role. Finally, although it is well known that dopamine of hypothalamic origin provides inhibitory control over the secretion of prolactin, other factors within the brain, pituitary gland, and peripheral organs have been shown to inhibit or stimulate prolactin secretion as well. It is the purpose of this review to provide a comprehensive survey of our current understanding of prolactin's function and its regulation and to expose some of the controversies still existing.

Muscarinic M1 and M3 receptors are present and increase intracellular calcium in adult rat anterior pituitary gland

Physiological and biochemical evidence indicates the existence of functional muscarinic cholinergic receptors in the anterior pituitary. The selectivity of these receptors has been characterised by studying the binding of [3H]quinuclidinyl benzilate ([3H]QNB) and [3H]diphenyl-acetoxy-N-methyl-piperidine ([3H]4-DAMP) in membrane preparation of male rat anterior pituitary at 25 degrees C. Competition experiments with receptor selective muscarinic antagonists were used to characterise specific selective muscarinic receptor binding. Both [3H]QNB and [3H]4-DAMP bound to anterior pituitary membranes at low concentrations, binding was saturable and was potently displaced by 4-DAMP (M1, M3 subtypes selective antagonist) > atropine (general) > pirenzepine (M1). Methoctramine (M2) didn't antagonise the [3H]QNB binding efficiently. Acetylcholine and carbachol increased the intracellular Ca2+ level in 62% and 65% of cultured rat anterior pituitary cells in a dose-dependent manner, and this effect was prevented by pirenzepine. Based on these results we suggest that both M1 and M3 muscarinic receptors are present and active in the majority of cells in the rat anterior pituitary gland, but their physiological role in the adult rat remains to be examined.

Cholinergic regulation of the pituitary: autoexcitatory control by acetylcholine of melanotrope cell activity in Xenopus laevis

In the studies on Xenopus laevis presented here, the role of acetylcholine (ACh) in the regulation of pituitary activity was examined. The results are discussed in connection with other results on cholinergic regulation of pituitary functioning in vertebrates. It is demonstrated that dissociated melanotropes can synthesize ACh in vitro. In addition, immunocytochemical studies show the presence of the ACh-synthesizing enzyme choline acetyltransferase (ChAT) in the cytoplasm of melanotrope cells. ACh concentration-dependently raises the intracellular free calcium concentration as well as the release of proopiomelanocortin (POMC)-derived peptides. Selective muscarinic receptor antagonists showed that the actions of ACh are mediated through an M1-subtype muscarinic receptor. Immunofluorescence cytochemistry visualized muscarinic receptors on the surface of melanotrope cells. It is concluded that ACh stimulates the release of POMC-derived peptides from the Xenopus melanotrope cell in an autocrine, excitatory way. Finally, the possible mechanism through which ACh is released from melanotrope cells and the physiological significance of ACh in the pituitary gland are discussed.

Distribution of choline acetyltransferase immunoreactivity in the brain of anuran (Rana perezi, Xenopus laevis) and urodele (Pleurodeles waltl) amphibians

Because our knowledge of cholinergic systems in the brains of amphibians is limited, the present study aimed to provide detailed information on the distribution of cholinergic cell bodies and fibers as revealed by immunohistochemistry with antibodies directed against the enzyme choline acetyltransferase (ChAT). To determine general and derived features of the cholinergic systems within the class of Amphibia, both anuran (Rana perezi, Xenopus laevis) and urodele (Pleurodeles waltl) amphibians were studied. Distinct groups of ChAT-immunoreactive cell bodies were observed in the basal telencephalon, hypothalamus, habenula, isthmic nucleus, isthmic reticular formation, cranial nerve motor nuclei, and spinal cord. Prominent plexuses of cholinergic fibers were found in the olfactory bulb, pallium, basal telencephalon, ventral thalamus, tectum, and nucleus interpeduncularis. Comparison of these results with those obtained in other vertebrates, including a segmental approach to correlate cell populations, reveals that the cholinergic systems in amphibians share many features with amniotes. Thus, cholinergic pedunculopontine and laterodorsal tegmental nuclei could be identified in the amphibian brain. The finding of weakly immunoreactive cells in the striatum of Rana, which is in contrast with the condition found in Xenopus, Pleurodeles, and other anamniotes studied so far, has revived the notion that basal ganglia organization is more preserved during evolution than previously thought.

Acetylcholine stimulates growth hormone secretion in the neonatal rat pituitary

The direct effect of acetylcholine on pituitary growth hormone secretion during the postnatal period of the rat was studied using a superfusion system. Acetylcholine elicited a dose related stimulatory effect on growth hormone (GH) secretion in the pituitaries from 2-day old rats. M1 muscarinic agonist McN A343 mimicked the GH releasing effect of acetylcholine, nicotine was ineffective. The GH release elicited by acetylcholine diminished with postnatal development, it was small by the end of the third postnatal week and was not demonstrable in the adult pituitaries. The effect of acetylcholine was potently antagonized by pirenzepine (M1 antagonist) and 4-DAMP (M3 and M1 antagonist) but not by methoctramine (M2 antagonist). It is concluded that unlike in the adult, in the newborn rat the cholinergic regulation of pituitary GH secretion plays a prominent role directly at pituitary level most likely via M1 receptors.

Acetylcholine synthesis in a primary culture of porcine intermediate lobe cells

Abstract Previous pharmacological studies with the pituitary gland have suggested that acetylcholine (ACh) might be involved in the regulation of intermediate lobe (IL) function. Whether ACh is endogenous to the IL cells or provided from an extrinsic source is unclear. The present experiments tested the possibility that the endocrine cells of the IL may be a source of ACh by measuring certain cholinergic markers in a primary culture of dissociated porcine cells. The endogenous ACh content was readily measurable in both the freshly dissociated IL cells and in 2- or 4-day primary cultures. Choline acetyltransferase activity was also measurable in the freshly dissociated and cultured IL cells and was reduced by 53% in the presence of a specific inhibitor, napthylvinylpyridine (50 muM). IL cells incubated in the presence of [(14)C]choline (1 muM) were able to synthesize [(14)C]ACh and the accumulation of the new ACh was inhibited by hemicholinium-3 (30 muM), a competitive inhibitor of high affinity choline uptake at cholinergic nerve terminals. In conclusion, these results demonstrate that the endocrine cells of the IL are capable of synthesizing and storing ACh.

Acetylcholine in the rat pituitary: a possible humoral factor

Significant amounts of acetylcholine (ACh) were detected in each of the 3 lobes of the rat pituitary (3-6 pmol/anterior lobe, 3 pmol/intermediate lobe and 1.8 pmol/posterior lobe). In the anterior lobes of cyclic rats the levels of ACh varied with the estrous cycle, with daily peaks being observed on the days of proestrus and estrus. The occurrence of ACh, apparently as a humoral factor, appears to be unique to the anterior pituitary.

Acetylcholinesterase in the rat neurohypophysis is decreased after dehydration and released by stimulation of the pituitary stalk

Specific acetylcholinesterase and non-specific cholinesterases are present in all three lobes of the rat pituitary gland. This paper describes two new observations on hypophyseal acetylcholinesterase. Firstly, a prolonged increase of neurohormone secretion evoked by dehydration and sodium loading was accompanied by a decrease in the acetylcholinesterase activity localized to the neural lobe, where acetylcholinesterase has previously been demonstrated in fine nerve fibres. Secondly, electrical stimulation of the pituitary stalk in vitro elicited the release of acetylcholinesterase and non-specific cholinesterases from the combined neural and intermediate lobe indicating that the enzyme can be released from nerve endings in the hypophysis by action potentials. The observed loss of enzyme activity during dehydration may be the consequence of a prolonged activation of cholinergic nerves in the gland, leading to an increased release of acetylcholinesterase, which is not immediately replaced by fresh enzyme. The decrease in acetylcholinesterase in the neural lobe during dehydration may also be connected with its peptidase function and thus with the previously observed loss of substance P from the neural lobe during dehydration [Holzbauer et al. (1984) Neurosci. Lett. 47, 23-28].