Heterochromatin characterization and distribution in the chromosomes of two populations ofIdotea baltica basteri Audouin, 1826 (Isopoda,Valvifera)

Two mediterranean populations ofIdotea baltica basteri from Messina and Naples showed a set of chromosomes composed by 58 all-biarmed chromosomes. The heterochromatin was located in the pericentromeric region of the chromosomes, and its composition appeared heterogeneous. In fact, not all the homologs showed heterochromatin resistant to digestion with three restriction enzymes (Alu I, Hae III and Sau 3A). Moreover, the two populations showed polymorphism in a band of G+C-rich telomeric heterochromatin, which was present only in the population from Messina. It is hypothesized that chromosomal polymorphism might reflect the geographical isolation of the two populations. It is also suggested that a process of diversification is taking place.     

Neuronal cell-cell adhesion depends on interactions of N-CAM with heparin-like molecules

Cell-cell interactions are of critical importance during neural development, particularly since the migration of neural cells and the establishment of functional interactions between growing axons and their target cells has been suggested to depend upon cell recognition processes. Neurone-neurone adhesion has been well studied in vitro, and is mediated in part by the neural cell adhesion molecule N-CAM. N-CAM-mediated cell-cell adhesion has been postulated to occur by a homophilic binding mechanism, in which N-CAM on the surface of one cell binds to N-CAM on a neighbouring cell. Studies in our laboratory have identified a cell surface glycoprotein, now known to be N-CAM, which participates in cell-substratum interactions in the developing chicken nervous system. Although this adhesion involves a homophilic binding mechanism, the binding of the cell surface proteoglycan heparan sulphate to the glycoprotein is also required. This raises the question of whether the binding of heparan sulphate to N-CAM is also required for cell-cell adhesion. Here we show that the binding of retinal probe cells to retinal cell monolayers is inhibited by heparin, a functional analogue of heparan sulphate, but not by chondroitin sulphate. Monoclonal antibodies that recognize two different domains on N-CAM, the homophilic-binding and heparin-binding domains, inhibit cell-cell adhesion. The heparin-binding domain isolated from N-CAM by selective proteolysis also inhibits cell-cell adhesion when bound to the probe cells.     

Possible role of acetylcholinesterase in regulation of postsynaptic receptor efficacy at a central inhibitory synapse of Aplysia

Most of the effects of acetylcholinesterase (AChE) on synaptic transmission are considered to be related to its acetylcholine (ACh) hydrolysing properties. This is clearly apparent from changes which occur in the characteristics of the miniature endplate potential and of the endplate potential at neuromuscular junctions when AChE is inhibited1-4 and during the development of enzymatic AChE activity at maturing synapses5. However, we report here that after inhibiting AChE in a cholinergic synapse in Aplysia, we found an increase not only in postsynaptic responses to presynaptic stimulation and to ionophoretic application of ACh on postsynaptic receptors, but also to ionophoretic application of carbachol. This could not be explained by the inhibition of the ACh hydrolysing function of the enzyme, as carbachol is not hydrolysed by AChE. A possible explanation of these observations is that inhibition of the enzyme affects a property of the ACh receptor (AChR) itself.