Matrix metalloproteinases MMP2 and MMP9 are upregulated by noradrenaline in the mouse neuroendocrine hypothalamus.

Magnocellular neurons of the hypothalamic supraoptic nuclei (SON) are involved in the synthesis and release of two major neuropeptides: oxytocin (OT) and arginine-vassopressin (AVP). Neurochemical plasticity in this system is induced by physiological conditions such as lactation, parturition and dehydration, and may be accompanied by reversible structural plasticity affecting neurons, astrocytes and the extracellular matrix (ECM). The noradrenergic system plays a critical role in triggering this chemical plasticity associated with structural plasticity. Matrix metalloproteinases (MMPs) are good candidates for involvement in the ECM remodelling observed in structural plasticity. We investigated the possible regulation of the two gelatinases, MMP2 and MMP9, by noradrenaline (NA) in the mouse neuroendocrine hypothalamus. We looked for the presence, location and activity of MMP2 and MMP9 in the SON, using an ex vivo experimental model of mouse hypothalamic slices incubated for 4 h with 10(-4) m NA. We showed that: (i) immunoreactivity for MMP2 and MMP9 was detected not only in AVP-positive and OT-positive magnocellular neurons, but also in astrocyte processes in control and NA-treated slices; (ii) the number of MMP2- and MMP9-positive cells increased after incubation with NA; (iii) MMP2 and MMP9 displayed markedly higher levels of gelatinolytic activity after NA treatment. These results suggest that both MMP2 and MMP9 are regulated by NA, and could therefore also be involved in structural plasticity within the SON.

Seizure-related opening of the blood-brain barrier produced by the anticholinesterase compound, soman: new ultrastructural observations.

Previous macroscopic and light microscopic observations established that the organophosphate soman, an irreversible inhibitor of cholinesterases, produces seizure-related opening of the blood-brain barrier (BBB) to proteins. In Wistar rats, this BBB alteration was found to be reversible. This alteration was greatest during the first hour of seizures, and was topographically limited to sensitive areas such as the thalamus. In contrast, the hippocampus remained free of any vascular leakage. The present study is an attempt to elucidate, in rat thalamus, the subcellular mechanisms involved in soman-induced BBB alteration. A combination of three ultrastructural approaches was used: examination of ultra-thin sections, freeze-fracture, and post-embedding protein A-gold immunocytochemistry of the endogenous, normally exclusively blood-borne, albumin. Our findings show that soman-induced seizure activity produced no discernible structural change in the endothelial tight junctions, whereas it unambiguously increased the number of endothelial vesicles. Finally, immunolabelled albumin clearly crossed the endothelium, but was not systematically found inside the endothelial vesicles. Altogether, the present ultrastructural study confirms that soman can alter the integrity of the BBB, and demonstrates that the blood-to-brain passage of proteins does not mainly derive from the opening of tight junctions. Although transcytosis is clearly increased through the cerebral endothelium, there is little evidence that blood-borne proteins penetrate the brain in this way. The actual mechanisms of transport thus remain to be clarified.

Compared effects of extracellular K+ ions and soman, a neurotoxic, on cerebral astrocyte morphology. An in vitro study.

One of the roles of astrocytes in the maintenance of perineuronal ionic balance during intense neuronal activity occurring after injection of convulsant agents like soman. Soman is an irreversible cholinesterase inhibitor and induces brain damage with early swelling of astrocytic perivascular processes. Mature astrocytes are easily characterized on freeze-fracture replicas owing to the presence of regular geometric aggregates of intramembranous particles: the 'orthogonal arrays' (OAs). In primary cultures of astrocytes OA distribution is homogeneous throughout the plasma membrane. A present hypothesis (see review in Risau and Wolburg, 1990) considers that these OAs are associated with channels controlling potassium ion concentration in the cerebral parenchyma. We have investigated the effects of extracellular concentrations of K+ ions identical to those observed during neuronal activity on primary cultures of astrocytes and effects induced by soman. High concentrations of K+ ions (60 mM) as well as soman exerted direct effects on astrocytic plasma membranes: K+ ion influx within astrocytes induces a partial disaggregation of OAs and more acutely than soman. Neither K+ ions nor soman induce swelling of astrocytic end-feet.

Effects of soman on cerebral astrocyte plasma membranes: a freeze-fracture study.

Astrocytes are identifiable on freeze-fracture by regular geometric aggregates of uniform intramembranous particles (IMP), the 'orthogonal arrays' (OA). These OAs are considered to be potassium channels that maintain the ionic balance around neurons. After subcutaneous administration of a single near LD50-dose of soman in rats, neurotoxic epileptogenic irreversible cholinesterase inhibitor, replicas showed a significant decrease in OA density in the convulsed animals. This dissociation by soman confirms that OA are a particularly labile membranes specialization. Frequent clumping of IMP and numerous cleavage planes were observed; this could be the result of an interaction between the liposoluble poison and membrane phospholipid fluidity.