Co-culture of hypothalamic neurons and melanotrope cells: a model to study synaptogenesis between central neurons and endocrine cells

Hybrid synapse formation between spinal motoneurons and superior cervical ganglion neurons in vitro: a study of the functional reconstruction of visceral organs

Spinal motoneurons (SMNs) were co-cultured with labeled superior cervical ganglion neurons (SCGNs) in complete medium on coverslips prepared by the microisland technique. "Hybrid" synapses formation between SMNs and SCGNs was examined by double immunostaining, reverse transcription-polymerase chain reaction (RT-PCR), and electrophysiological assays. The number of synaptic puncta was significantly higher in co-cultured cells compared to controls (P<0.01). Spontaneous synaptic currents (SSCs) were also significantly increased in SCGNs within the co-cultured population of cells (P<0.01). These SSCs could be blocked by a specific nicotinic receptor blocker, but not by a glutamate receptor antagonist. These observations clearly demonstrate the functional synapses formation increased in our co-culture system, suggesting that the "hybrid" synapse formation occurred between somatic and automatic neurons. Our data also indicate that acetylcholine and nicotinic receptors may be involved in mediating these processes.

Cerebrospinal fluid-contacting neurons in the rat spinal cord, a gamma-aminobutyric acidergic system expressing the P2X2 subunit of purinergic receptors, PSA-NCAM, and GAP-43 immunoreactivities: light and electron microscopic study

Cerebrospinal fluid-contacting neurons (CSFcNs) occur in various brain regions of lower vertebrates. In mammals, they are restricted to medullospinal areas, and little is known about their projection sites. In the present work, we investigated some morphofunctional characteristics of such neurons in the rat spinal cord by light and electron microscopic immunocytochemistry. CSFcNs expressing the P2X(2) subunit of purinergic receptors were present throughout the spinal cord, though more numerous at lower thoracolumbar and sacral levels. These neurons coexpressed GAD and the polysialylated neural cell adhesion molecule (PSA-NCAM), a marker of cellular plasticity. From low thoracic levels downward, tiny amyelinic axons (less than 200 nm in diameter) were tightly packed in bundles, which ran along the ependyma and extended ventrally, eventually concentrating against the walls of the ventral median fissure. In addition to P2X(2), GAD, gamma-aminobutyric acid (GABA), and PSA, these axons expressed GAP-43 immunoreactivity. Moreover, they were labelled along their entire lengths with antibodies against synaptotagmin and synaptophysin, but these failed to reveal intraspinal terminal fields. Taken together, our observations indicate the presence in the rat spinal cord of a highly plastic system of GABAergic CSFcNs that express the P2X(2) subunit of purinergic receptors. The function of this original system remains open to question. In these neurons, the P2X(2) receptors may confer a sensitivity to ATP either present in the CSF or released by nearby neurons of the central autonomic area.

The mammalian pituitary intermediate lobe: an update on innervation and regulation

The pituitary intermediate lobe (IL) in mammals is an area of uniform endocrine cells which synthesize and release specific peptide products of the proopiomelanocortin gene. The lobe receives direct synaptic connections onto the endocrine cells from hypothalamic dopaminergic neurons. This review updates information on the dopaminergic as well as the gamma-aminobutyric acid inhibitory neuroregulation for the IL. It also provides a discussion of stimulatory molecules which are likely to affect peptide release, particularly the neurotransmitter serotonin, which may be present via uptake into the dopaminergic nerve terminals. Other stimulatory molecules discussed which are likely to significantly affect peptide secretion are norepinephrine, corticotropin-releasing factor, and several opiate peptides. A new direction of study involves the potential interaction of neurotrophic factors, which are present in all areas of the pituitary, and may be suggested to have a supportive role for the neural elements of the IL. The endocrine cells of the IL and their direct hypothalamic innervation are considered to be an easily accessed peripheral model for study of both neural-endocrine and neurotrophic-target cell interactions.

Regulation of GABAA receptor by protein tyrosine kinases in frog pituitary melanotrophs

The effects of protein tyrosine kinase (PTK) and PTK inhibitors on the GABAA receptor function were studied in cultured frog pituitary melanotrophs by using the patch-clamp technique. Extracellular application of the PTK inhibitors genistein (10-9 to 10-5 M) or lavendustin A (10-12 to 10-7 M) provoked a bell-shaped potentiation of the whole-cell current induced by GABA (3x10-6 M). In contrast, at high concentrations, genistein (10-4 M) and lavendustin A (10-5 M) reversibly reduced the GABA-evoked current. Daidzein and lavendustin B, the inactive analogs of genistein and lavendustin A, respectively, did not modify the current induced by GABA. In the inside-out configuration, bath application of the recombinant PTK pp60c-src (75 U/ml) inhibited the GABA-activated chloride current, and the inhibitory effect of pp60c-src was prevented by genistein (10-7 M). Immunoblotting revealed that genistein, at doses of 10-7 M or 10-4 M, markedly inhibited tyrosine phosphorylation of the beta2/beta3 subunits of the GABAA receptor. Extracellular application of the PKA activator Bt2cAMP (10-3 M), the PKA/PKC inhibitor H7 (10-5 M) and the Cam KII inhibitor W7 (10-5 M) reversibly diminished the whole-cell GABA-induced current. Internal application of H7 and W7 (10-4 M) did not modify the dose-dependent effects of genistein. Internal application of sodium orthovanadate (10-4 M), a protein tyrosine phosphatase inhibitor, decreased the GABA-evoked current and markedly reduced the potentiating effect of genistein. The present study provides the first evidence that, in frog pituitary melanotrophs, the GABAA receptor is phosphorylated at least on its beta2/beta3 subunits by an endogenous PTK. Our data also demonstrate that tyrosine phosphorylation exerts an inhibitory effect on GABAA receptor function.

Differential ultrastructural distribution of synapsin and synaptophysin proximal to a ligation in bovine splenic nerve

Synaptophysin and synapsin, closely correlated on synaptic vesicles in terminals, may show a differential distribution at synapse formation and maturation. In order to disclose the fine structural details of these differences, synapsin and synaptophysin distribution was studied by immunocytochemistry on ligated bovine splenic axons in vitro and compared with terminals in the vas deferens. In the synaptic differentiations taking place proximally synapsin could only be detected on the accumulating elements of the axonal reticulum. Large dense granules and clusters of small synaptic vesicles were negative. Synaptophysin was restricted to these clusters. In the vas deferens, co-localization of synapsin and synaptophysin could be seen on small vesicles. From their formation small synaptic vesicles carry synaptophysin. Synapsin may be involved in the dynamic membrane changes taking place at the ligation. At a functional terminal, synapsin shifts to small synaptic vesicles.

Differentiation of rat hypothalamic dopaminergic neurons is stimulated in vitro by target cells: the melanotrophs

We have investigated in vitro the influence of pituitary intermediate lobe melanotrophs on the differentiation of their afferent hypothalamic dopaminergic neurons. The presence of melanotrophs in primary cultures of foetal hypothalamic neurons induces an increase of the number of dopaminergic neurons (while the total neuronal population remains unchanged) and induces a stimulation of their neuritic outgrowth. These effects are mediated by diffusible factors since they are reproduced by application of conditioned medium issued from co-cultures with intermediate lobe cells from newborn rats. Moreover, by immunoneutralization of alpha-melanocyte-stimulating hormone (alphaMSH) in the co-culture or conditioned medium, or by application of the peptide itself, we demonstrate that the neuritotrophic effect on dopaminergic neurons is mediated by alphaMSH, the main secretory product of melanotrophs, whereas the inductive effect on the number of dopaminergic neurons is attributable to another diffusible neurotrophic factor(s) present in foetal, but not adult, adenohypophysis. Similar effects are observed on cultures of newborn hypothalamic neurons. However, at this stage of neuronal development, alphaMSH also increases the number of dopaminergic neurons, which could be due to a change of neuronal receptivity. We show that the neuritotrophic influence of alphaMSH is restricted to the dopaminergic neurons connected to the melanotrophs, and that in addition, these neurons systematically co-express the tyrosine hydroxylase and glutamate decarboxylase as the neurons innervating the melanotrophs in situ. These findings indicate that the differentiation of dopaminergic hypothalamic neurons is influenced by the target cells, melanotrophs, and that this trophic influence implicates alphaMSH.

Target-dependent synaptogenesis: inductive effect of pituitary melanotrophs on their central innervation

The glandular activity of the vertebrate pituitary intermediate lobe (IL) is regulated by direct cellular innervation, in contrast with the purely humoral regulation of adjacent pituitary anterior lobe (AL). Thus in the rat IL, melanotrophs receive a dopaminergic and GABAergic innervation from the basal hypothalamus, which tonically inhibit their glandular activity. We studied this model of neuron-target interactions in cocultures in defined medium of fetal hypothalamic neurons with neonate pituitary glandular cells. In the cocultures with IL cells, neuroglandular contacts occurred after 4 days in vitro (DIV) but required another 8 DIV to exhibit ultrastructural and immunocytochemical features of fully differentiated functional synapses; by contrast, neuroneuronal synapses developed much faster and could already be detected after 4 DIV. In the cocultures with AL cells, neuroglandular contacts never mature in differentiated synapses. Confocal microscope observation revealed that dopaminergic neurons, which represented less than 1% of total neurons in the cocultures, established 50% of the synapses detected on the melanotrophs. These cells are thus able, contrary to the AL cells, to promote the establishment of functional synapses and, to some extent, to select their specific innervation.