Functional morphology on the synaptic plasticity : Rapid remodeling of the synapse during long-term potentiation.

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Long-term potentiation (LTP) following a conditioning stimulation is considered to express synaptic plasticity and to provide elementary mechanisms of learning processes and memory formation at the synaptic level in the central nervous system (CNS) (Bliss and Lomo, 1973). Similar phenomenon of LTP is observed not only in CNS but also in the sympathetic ganglion of the peripheral nervous system. In the present investigation synaptic plasticity following a conditioning stimulation was electrophysiologically and ultrastructurally studied in the cat superior cervical ganglion in situ under intact blood supply. The preganglionic nerve fiber was tetanically stimulated at 10 Hz for 50 seconds for conditioning and then at 1 Hz for 1 hour for monitoring. The postganglionic compound action potential (PGP) in response to preganglionic nerve stimulation was recorded for detecting increase in synaptic efficacy. Long-term potentiation (LTP) was in many cases found to be a persistent potentiation of … More PGP, around 120% of the control, which lasted for 1 hour as long as monitored. The ganglia were rapidly fixed for electron microscopy via the lingual artery before or after the tetanus. The synapse between the preganglionic nerve terminal and the principal cell was exclusively occupied by the axodendritic synapse of Gray's type I, which contacted the dendritic shaft lacking the dendritic spine. The axodendritic synapse was characterized by increase of the perforated synapse and synaptic spinule in population following the tetanus. The perforated synapse accompanying a synaptic spinule was termed here as a 'spinule-synapse'. The population of perforated synapse augmented from 5% in control to ca. 30% after the tetanus. The spinule-synapse, few in number in control, increased to ca. 9% during LTP period. The perforation site of the perforated synapse was small in width at earlier stages of LTP and became larger later. In summary, the conditioning tetanus, inducing LTP, brought about remodeling of axodendritic synapses, which resulted in an increase in number of synaptic contacts. Less