|[Frontiers in Bioscience 2, d88-125, March 1, 1997]|
CROSS-TALK SIGNALS IN THE CNS: ROLE OF NEUROTROPHIC AND HORMONAL FACTORS, ADHESION MOLECULES AND INTERCELLULAR SIGNALING AGENTS IN LUTEINIZING HORMONE-RELEASING HORMONE (LHRH)-ASTROGLIAL INTERACTIVE NETWORK|
Department of Pharmacology, Medical School, University of Catania, 95125 Catania, Laboratory of Biotech. Neuropharmacology, OASI Institute for Research and Care (IRCCS) on Mental Retardation and Brain Aging (IRCCS) Troina, (EN), Italy.
Received 8/2/96; Accepted 2/20/97; On-line 3/1/97
As a further step to verify the possible bi-directional communication between astroglial cells and GT1-1 neurons, a co-culture system was established (41). In these conditions, where the two cell-compartments were allowed to communicate with each other, but in the absence of cell contacts, a significant stimulation of basal LHRH release was observed, although GT1-1 proliferative potential was almost doubled, thus resulting in a net decrease of neuronal secretory capacity (Fig. 14).
Figure 14. Effect of astroglial (12 DIV)-GT1-1 neuron co-culture in the maturation of LHRH secretory potential. The technical procedure is described in details (41). Cortical glia was used in this experimental paradigm. LHRH secretion was examined every 2 days for 8 days. For measurement of LHRH release by RIA, the medium was replaced every two days, collected, centrifuged to remove cellular debris, and frozen at -80 °C. Results are the mean ± SEM of 2-3 different experimental manipulations. ** p < 0.01 vs. control.
This experimental paradigm revealed for the first time the presence of a bi-directional flow of informational molecules between the two cell populations, as observed by a doubling of the proliferative potential of each cell population, suggesting that LHRH and GFs released by glia, participate in GT1-1 neuron-astroglia crosstalk (Fig. 15).
Figure 15. Proliferative capacity of GT1-1 neurons and astroglial cells in co-culture conditions. At 2, 4, 6 and 8 days and for each respective cell type, GT1-1 or cortical astroglial cell proliferation were tested in triplicate by incubation of [Methyl-3H]Thymidine (1 mCi/ml of culture medium) for 2 h at 37 °C. Labeled DNA was collected and radioactivity was determined by liquid scintillation spectrophotometry, as described. Results are the mean ± SEM of 2-3 different experimental manipulations. * p < 0.01 vs. control.
In mixed cultures, both spontaneous LHRH release and GT1-1-astroglial cell proliferation were significantly increased. The inability to further stimulate LHRH release in the face of the presence of such a mitogenic effect on the GT1-1 neurons may have different explanations, depending on both 1. the autoregulatory actions of LHRH on its own secretion; 2. the nature of the GFs released by astroglial cells and their coupling to specific intracellular transducing pathways; and 3. the presence of cell-cell contacts interfering with LHRH inter/intracellular dynamics. It is possible that LHRH released in the medium could influence the further production of astroglial-derived factors either directly or indirectly, via receptor-mediated events and/or through second messenger-activated systems delivering signal molecules utilized by the neighboring cells, thus realizing a "cross-talk" between the separated cell compartments (see Fig. 6).
The ability of astrocytes to synthesize and release a number of prostaglandins (PGE2, PGF2alpha) and also tromboxane A2 (TX) in response to arachidonic acid (AA) or calcium ionophore (for review see previous sections) constitute a major link in LHRH-astroglial interactions, since PGE2 is an obligatory component in the phasic discharge of LHRH from the MBH. Another important connection, between the LHRH neuron and astroglia, as previously recalled, is their ability to use and to produce the novel "intercellular" diffusible modulator, NO and to express NO synthase (see previous sections). In the light of the host of receptors present in astrocytes, their ability to synthesize vasoactive products, and close spatial relationship of these cells both with LHRH neurons and cells of the vasculature implicate them in bi-directional signaling processes in the CNS. Signals, in turn, originating from the LHRH neurons could initiate important intracellular changes in astrocytes. The resulting release of prostanoids, and nitrosyl compounds could have profound modulatory effects on the activity of the adjacent (astrocyte/neuronal) cell (Fig. 6).