Long-Term Posttetanic Changes of Synaptic Transmission and Synapse Morphology of the Pike Esox lucius Olfactory Bulb

Abstract

In the olfactory bulb (OB) of the pike, phenomena of long posttetanic potentiation (LPTP) and long posttetanic depression (LPTD) are reproduced. Probability of induction of LPTP and level of its expression are shown to depend direct on parameters of density of axo-dendritic synapses in the glomerular neuropil and size of its active zones (AZ). In pikes with the high level of these parameters (the synapse density 5.2 per 20 µm2, the mean length of AZ cross-section 293 nm), potentiation appear in 95% of cases, while in pikes with low values (the density 3.1, the AZ length 239 nm) in as low as 30%, with LPTD developing rather often instead of LPTP. The LPTP in the phase of its maximal development (60 min after tetanus) corresponds to an increase of the mean length of AZ cross-section of axodendritic synapses by 13.6%, while the LPTD—to its decrease by 16.5% as compared with control. The LPTP induction phase (5–15 min after tetanus) is not associated with any ultrastructural transformations of postsynaptic AZ of axo-dendritic synapses. By the morphometric method, it has been established that LPTP in the pike OB is of the homosynaptic nature, i.e., is due to an enhancement of synaptic efficiency of the excitatory afferent input just subjected to tetanization. On the whole, by using axo-dendritic synapses of the pike OB as an example it was shown that in simple, flat, asymmetric excitatory synapses a steady enhancement of their effectiveness was accompanied by an increase of length of AZ cross-sections, while a reduction of their efficiency—by its decrease.