Characterization of neurochemical phenotypes in cultured hypothalamic neurons with immunohistochemistry and in situ hybridization

Calcium-dependent regulation of cholinergic cell phenotype in the hypothalamus in vitro

Glutamate is a major fast excitatory neurotransmitter in the CNS including the hypothalamus. Our previous experiments in hypothalamic neuronal cultures showed that a long-term decrease in glutamate excitation upregulates ACh excitatory transmission. Data suggested that in the absence of glutamate activity in the hypothalamus in vitro, ACh becomes the major excitatory neurotransmitter and supports the excitation/inhibition balance. Here, using neuronal cultures, fura-2 Ca(2+) digital imaging, and immunocytochemistry, we studied the mechanisms of regulation of cholinergic properties in hypothalamic neurons. No ACh-dependent activity and a low number (0.5%) of cholinergic neurons were detected in control hypothalamic cultures. A chronic (2 wk) inactivation of N-methyl-D-aspartate (NMDA) ionotropic glutamate receptors, L-type voltage-gated Ca(2+) channels, calmodulin, Ca(2+)/calmodulin-dependent protein kinases II/IV (CaMK II/IV), or protein kinase C (PKC) increased the number of cholinergic neurons (to 15-24%) and induced ACh activity (in 40-60% of cells). Additionally, ACh activity and an increased number of cholinergic neurons were detected in hypothalamic cultures 2 wk after a short-term (30 min) pretreatment with bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid tetrakis(acetoxy-methyl) ester (BAPTA AM; 2.5 microM), a membrane permeable Ca(2+)-chelating agent that blocks cytoplasmic Ca(2+) fluctuations. An increase in the number of cholinergic neurons following a chronic NMDA receptor blockade was likely due to the induction of cholinergic phenotypic properties in postmitotic noncholinergic neurons, as determined using 5-bromo-2'-deoxyuridine (BrdU) labeling. In contrast, a chronic inactivation of non-NMDA glutamate receptors or cGMP-dependent protein kinase had little effect on the expression of ACh properties. The data suggest that Ca(2+), at normal intracellular concentrations, tonically suppresses the development of cholinergic properties in hypothalamic neurons. However, a decrease in Ca(2+) influx into cells (through NMDA receptors or L-type Ca(2+) channels), inactivation of intracellular Ca(2+) fluctuations, or downregulation of Ca(2+)-dependent signal transduction pathways (CaMK II/IV and PKC) remove the tonic inhibition and trigger the development of cholinergic phenotype in some hypothalamic neurons. An increase in excitatory ACh transmission may represent a novel form of neuronal plasticity that regulates the activity and excitability of neurons during a decrease in glutamate excitation. This type of plasticity has apparent region-specific character and is not expressed in the cortex in vitro; neither increase in ACh activity nor change in the number of cholinergic neurons were detected in cortical cultures under all experimental conditions.

Posttranslational processing of progrowth hormone-releasing hormone

The prepro-GH-releasing hormone (prepro-GHRH; 12.3 kDa) precursor, like other neuropeptide precursors, undergoes proteolytic cleavage to give rise to mature GHRH, which is the primary stimulatory regulator of pituitary GH secretion. In this study we present the first model of in vitro pro-GHRH processing. Using pulse-chase analysis, we demonstrate that at least five peptide forms in addition to GHRH are produced. The pro-GHRH (after removal of its signal peptide, 10.5 kDa) is first processed to an 8.8-kDa intermediate form that is cleaved to yield two products: the 5.2-kDa GHRH and GHRH-related peptide (GHRH-RP; 3.6 kDa). GHRH-RP is a recently described peptide derived from proteolytic processing of pro-GHRH that activates stem cell factor, a factor known to be essential for hemopoiesis, spermatogenesis, and melanocyte function. Further cleavage results in a 3.5-kDa GHRH and a 2.2-kDa product of GHRH-RP. Like GHRH, there is GHRH-RP immunostaining in hypothalamic neurons in the median eminence as detected by immunohistochemistry and immunoelectron microscopy. Based on deduced amino acid sequences of the pro-GHRH processing products, several peptides were synthesized and tested for their ability to stimulate the cAMP second messenger system. GHRH, GHRH-RP, and one of these peptides [prepro-GHRH-(75-92)-NH2] all significantly stimulated the PKA pathway. This work delineates a new model of pro-GHRH processing and demonstrates that novel peptides derived from this processing may have biological action.

Astrocytes rather than neurones mediate interleukin-1beta dependent nitric oxide and superoxide radical release in primary hypothalamic rat cell cultures

The cellular sources of nitric oxide in the hypothalamus are thought to be 'NOergic' neurones. Using free radical electrochemical sensors we investigated nitric oxide and superoxide radical release in primary hypothalamic cell cultures. We present evidence that under interleukin-1beta (IL-1beta) stimulation hypothalamic astrocytes rather than neurones release nitric oxide. Under L-arginine deprivation and IL-1beta stimulation a concentration-dependent release of superoxide was also observed, which was inhibited in the presence of nitric oxide synthase inhibitor nitro-L-argininemethyl-ester. These findings support the hypothesis that the balance between nitric oxide and superoxide may be of vital importance in hypothalamic pathophysiology.

Effects of intracerebroventricular leptin administration on food intake, body weight gain and diencephalic nitric oxide synthase activity in the mouse

1. Intracranial administration of leptin reduces both food intake and body weight gain in the mouse. Inhibitors of nitric oxide (NO) synthase produce similar effects. 2. To investigate the role of the brain L-arginine/NO pathway in mediating this effect of leptin, we have evaluated food intake and body weight gain after daily (5 days) intracerebroventricular (i.c.v.) administration of leptin (0.5-2 microg) alone or in association with L-arginine (10 microg). Moreover, we measured diencephalic nitric oxide synthase (NOS) activity after a single i.c.v. leptin (0.25-2 microg) injection and after consecutive doses of leptin (0.25-2 microg) over 5 days. The time course of the effect of leptin on NOS activity was also evaluated. 3. I.c.v. injected leptin (1 and 2 microg) significantly and dose-dependently reduced food intake and body weight gain with respect to vehicle (food intake: 5.97+/-0.16 g 24 h(-1) and 4.27+/-0.18 g 24 h(-1), respectively, vs 8.05+/-0.34 g 24 h(-1), P<0.001, n=6 for each group; body weight gain: -10.7+/-0.46% and -15.7+/-0.65%, respectively, vs 5.14+/-0.38%, P<0.001, n=6 for each group). This effect was antagonized by L-arginine (food intake: 7.90+/-0.37 g 24 h; body weight gain: 5.11+/-0.31%, n=6). Diencephalic NOS activity was significantly reduced by the highest doses of leptin with respect to vehicle (vehicle: 0.90+/-0.04 nmol citrulline min(-1) g(-1) tissue; leptin 1 microg: 0.62+/-0.03 nmol citrulline min(-1) g(-1) tissue, P<0.001; leptin 2 microg: 0.44+/-0.03 nmol citrulline min(-1) g(-1) tissue, P<0.001, n=6 for each group). Similar results were obtained in animals treated with daily consecutive doses of leptin. The inhibitory effect appeared rapidly (within 30 min) and was long lasting (up to 12 h). 4. Our results suggest that the brain L-arginine/NO pathway may be involved in the central effect of leptin on feeding behaviour and body weight gain in mice.

Synaptic modulation of oscillatory activity of hypothalamic neuronal networks in vitro

1. Rhythmic bursts of action potentials in neurosecretory cells are a key factor in hypothalamic neurosecretion. Rhythmicity and synchronization may be accomplished by pacemaker cells synaptically driving follower cells or by a network oscillator. 2. In this review we describe a hypothalamic cell culture which may serve as a model for a hypothalamic network oscillator. An overview is given of neurochemical phenotypes, synaptic mechanisms and their development, properties of receptors for fast synaptic transmission, and membrane properties of cells in dissociated rat embryonic hypothalamic culture. 3. Rhythmic activity spreads in the cultured network through synapses that release glutamate, activating a heteromultimeric AMPA-type receptor containing a GluR2 subunit which is associated with a high-conductance channel for Na+ and K+. Rhythmic activity is controlled by synapses that release GABA to activate GABAA receptors. The presumed function of the two receptor types is facilitated by their respective location, GABAA receptors predominating near the soma and AMPA receptors being abundant in dendrites. 4. Network oscillators may be more reliable for the presumed function than single-cell oscillators. They are controlled through synaptic modulation, which may prove to represent a process important for the release of hormones.

Synaptic connectivity in cultured hypothalamic neuronal networks

We have developed a novel approach to analyze the synaptic connectivity of spontaneously active networks of hypothalamic neurons in culture. Synaptic connections were identified by recording simultaneously from pairs of neurons using the whole cell configuration of the patch-clamp technique and testing for evoked postsynaptic current responses to electrical stimulation of one of the neurons. Excitatory and inhibitory responses were distinguished on the basis of their voltage and time dependence. The distribution of latencies between presynaptic stimulation and postsynaptic response showed multiple peaks at regular intervals, suggesting that responses via both monosynaptic and polysynaptic paths were recorded. The probability that an excitatory event is transmitted to another excitatory neuron and results in an above-threshold stimulation was found to be only one in three to four. This low value indicates that in addition to evoked synaptic responses other sources of excitatory drive must contribute to the spontaneous activity observed in these networks. The various types of synaptic connections (excitatory and inhibitory, monosynaptic, and polysynaptic) were counted, and the observations analyzed using a probabilistic model of the network structure. This analysis provides estimates for the ratio of inhibitory to excitatory neurons in the network (1:1.5) and for the ratio of postsynaptic cells receiving input from a single GABAergic or glutamatergic neuron (3:1). The total number of inhibitory synaptic connections was twice that of excitatory connections. Cell pairs mutually connected by an excitatory and an inhibitory synapse occurred significantly more often than predicted by a random process. These results suggests that the formation of neuronal networks in vitro is controlled by cellular mechanisms that favor inhibitory connections in general and specifically enhance the formation of reciprocal connections between pairs of excitatory and inhibitory neurons. These mechanisms may contribute to network formation and function in vivo.

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

As a first step towards elucidating mechanisms involved in neuroendocrine synaptogenesis, we developed a model of co-culture based on hypothalamic-intermediate pituitary interactions. Dissociated hypothalamic neurons from fetal rats at embryonic day 15 were cultured in a defined medium together with melanotrope cells of the pituitary intermediate lobe from neonatal rats. In these co-cultures, establishment of synaptic contacts between GABAergic or dopaminergic neurons and an endocrine target cell the melanotrope cell, was studied by morphofunctional approaches. Using double immunostaining with antibodies directed against glutamate decarboxylase or tyrosine hydroxylase and alpha-melanocyte-stimulating hormone, we demonstrated morphological contacts between GABAergic or dopaminergic neurons and melanotrope cells as early as three days in vitro. Furthermore, using an antibody directed against synapsin I, we showed a modification of synapsin I immunoreactivity from diffuse to punctate distribution correlated with the establishment of contacts and the observation of characteristic neuroendocrine synapses by electron microscopy. These results were further confirmed by electrophysiological studies. Patch-clamp recordings demonstrated that, at six days in vitro, some melanotrope cells displayed GABAergic synaptic currents, which occurred either spontaneously and/or could be evoked chemically by 50 mM KCl or 100 microM kainate. The proportion of the melanotrope cells receiving functional synaptic inputs increased until 10 days in culture, a stage at which virtually all melanotrope cells in contact with neurons possessed functional synapses. The results presented here describe the establishment of neuroendocrine synapses in vitro, studied by combining morphofunctional and electrophysiological approaches.

[3H]L-arginine transport and nitric oxide synthase activity in foetal hypothalamic cultures

[3H]L-arginine uptake and the conversion of [3H]L-arginine to [3H]citrulline were characterized in foetal hypothalamic cultures. [3H]L-arginine uptake was reduced by L-ornithine (10 microM-1 mM), high extracellular K+ (56 mM), L-glutamate (100 microM) and removal of extracellular Ca2+, but was increased by the nitric oxide synthase inhibitor, Nw-nitro-L-arginine benzyl ester (L-NABE; 1 mM). [3H]citrulline formation was inhibited by L-NABE (1 mM), increased by high extracellular K+ (56 mM) and unaffected by L-glutamate (100 microM). Removal of extracellular Ca2+ reduced [3H]citrulline formation by mixed (neurones and glia) and neurone-enriched cultures but not by glial-enriched cells. The results suggest that [3H]L-arginine uptake into hypothalamic cultures is mediated by the system y+ transporter and is dependent on extracellular Ca2+. [3H]citrulline production in hypothalamic neuronal, but not glial, cells is also dependent on extracellular Ca2+.

Neuropeptide Y depresses GABA-mediated calcium transients in developing suprachiasmatic nucleus neurons: a novel form of calcium long-term depression

In contrast to its inhibitory role in mature neurons, GABA can exert excitatory actions in developing neurons, including mediation of increases in cytosolic Ca2+. Modulation of this excitatory activity has not been studied previously. We used Ca2+ digital imaging with Fura-2 to test the hypothesis that neuropeptide Y (NPY) would depress GABA-mediated Ca2+ rises in neurons cultured from the developing suprachiasmatic nucleus (SCN). SCN neurons were chosen as a model system for this study because SCN neurons are primarily GABAergic, they express high levels of NPY and GABA receptors, and functionally, NPY causes profound phase-shifts in SCN-generated circadian rhythms. Vigorous GABA-mediated Ca2+ activity was found in young SCN neurons that were maintained in vitro for 4-14 d. NPY showed a dose-dependent rapid depression of the amplitude of Ca2+ rises generated by GABA released from presynaptic SCN axons. NPY exerted a long-term depression of cytosolic CA2+ in the majority of neurons tested, which lasted more than 1 hr after NPY washout. The magnitude of the NPY depression was dose-dependent. NPY did not affect Ca2+ levels when GABAA receptor activity was blocked by bicuculline; however, when bicuculline and NPY were withdrawn from the perfusion solution, the subsequent CA2+ rise was either significantly reduced or completely absent, suggesting that the NPY receptor was activated in the absence of elevated intracellular Ca2+ and GABAA receptor activity, and that the latent effect of NPY was revealed only after depolarizing GABA stimulation was renewed. Pretreating neurons with pertussis toxin greatly reduced the ability of NPY to depress GABAergic Ca2+ rises, suggesting that the NPY modulation of the GABA activity was based largely on a mechanism involving pertussis toxin-sensitive Gi/Go proteins. NPY receptor stimulation depressed (< 30%) postsynaptic Ca2+ rises evoked by GABA (20 microM) application in the presence of tetrodotoxin (TTX). The effects of NPY were mimicked by the NPY Y1 receptor agonist [Pro34,Leu31] NPY and the Y2 receptor agonist NPY 13-36 and by peptide YY (PYY). Together, our data suggest that the Y1 and Y2 type NPY receptors act both presynaptically and postsynaptically to depress GABA-mediated Ca2+ rises. If related mechanisms exist in peptide modulation of inhibitory GABA activity in mature neurons, this could underlie long-term changes in the behavior of neurons of the SCN necessary for phase-shifting the circadian clock by NPY, NPY also modulated GABA responses in neuroendocrine neurons from the hypothalamic arcuate nucleus. NPY thus can play an important role in evoking long-term depression of GABA-mediated Ca2+ activity in these developing neurons, allowing NPY-secreting cells to modulate the effects of GABA on neurite outgrowth, gene expression, and physiological stimulation. This is the first example of such a cellular memory: that is, long-term Ca2+ depression based on modulation of depolarizing GABA activity.

Phorbol ester or cAMP enhance thyrotropin-releasing hormone mRNA in primary cultures of hypothalamic cells

Thyrotropin releasing hormone (TRH) biosynthesis is subject to a multifactorial control. TRH mRNA levels are negatively regulated by thyroid hormones in the paraventricular hypothalamic nucleus, and positively in cold exposure or suckling. Effect of second messenger pathways stimulation, a known response to membrane receptors, was studied in vitro; cultures of rat embryonic hypothalami (18 day gestation) were treated with 12-O-tetradecanoylphorbol-13-acetate (TPA, 100 nM) or dibutiryl cAMP (dBcAMP, 1 mM) for various times. Levels of TRH mRNA were raised after the first hour of dBcAMP or 2 h of TPA treatment and were still increased at 24 h. These results suggest a neural regulation of TRH biosynthesis.