Synaptogenesis and distribution of presynaptic axonal varicosities in low density primary cultures of neocortex: an immunocytochemical study utilizing synaptic vesicle-specific antibodies, and an electrophysiological examination utilizing whole cell recording.

Low-density primary cultures of neocortical neurons were utilized to examine: (i) early interactions of growing neurites with morphological characteristics of axons with other neuronal elements, and (ii) the distribution of presynaptic axonal varicosities closely apposed to MAP-2 immunoreactive, putatively postsynaptic, dendrites. At the light microscopical level axonal varicosities, presumably presynaptic terminals, were identified using immunocytochemistry incorporating antibodies specific for the synaptic vesicle antigens synaptophysin and synapsin. The presence of synaptophysin- and synapsin-immunoreactive swellings along axonal processes was first detected at 5 days post-plating and was also apparent in axons growing in isolation. At 5-7 days in vitro, immunolabelled axonal varicosities in close apposition to putative postsynaptic dendrites (MAP-2 immunoreactive) dendrites were detected. Electrophysiologically active synaptic contacts can also readily be detected at this stage. After 3 weeks in vitro presynaptic contacts do appear to be distributed heterogeneously along postsynaptic dendrites of many neurons in culture. As the culture matures a higher number of presynaptic profiles can be seen along dendrites, with a centrifugal distribution, e.g. a higher density of presynaptic axonal terminals in close apposition to more distal regions of larger dendrites, putatively considered to be apical dendrites of pyramidal-like neurons. In our cultures, the overall increase in the density and the pattern of distribution of presynaptic axon terminals immunoreactive for synaptic vesicle antigens closely apposed to putative post-synaptic structures mimics the general postnatal increase of synaptic density in the neocortex in vivo. Thus, low density primary cultures of neocortical neurons offer a valuable system to explore and manipulate (i) the molecular and cellular basis of neocortical synaptogenesis, and (ii) the pharmacology of neocortical synaptic transmission.