Understanding how the complexity of connections among the neurons in the brain is
established and modified in an experience- and activity-dependent way is a challenging
task of Neuroscience. Although in the last decades many progresses have been made in
characterising the basic mechanisms of synaptic transmission, a full comprehension of
how information is transferred and processed by neurons has not been fully achieved.
In the present study, theoretical tools and patch clamp experiments were used to further
investigate synaptic transmission, focusing on quantal transmission at single synapses
and on different types of signalling at the level of a particular interneuronal network in
the CA1 area of the rodent hippocampus.
The simultaneous release of more than one vesicle from an individual presynaptic active
zone is a typical mechanism that can affect the strength and reliability of synaptic
transmission. At many central synapses, however, release caused by a single presynaptic
action potential is limited to one vesicle (univesicular release). The likelihood of
multivesicular release at a particular synapse has been tied to release probability (Pr), and
whether it can occur at Schaffer collateral\u2013CA1 synapses, at which Pr ranges widely, is
controversial. In contrast with previous findings, proofs of multivesicular release at this
synapse have been recently obtained at late developmental stages; however, in the case of
newborn hippocampus, it is still difficult to find strong evidence in one direction or
another.
In order to address this point, in the first part of this study a simple and general stochastic
model of synaptic release has been developed and analytically solved. The model
solution gives analytical mathematical expressions relating basic quantal parameters with
average values of quantities that can be measured experimentally. Comparison of these
quantities with the experimental measures allows to determine the most probable values
of the quantal parameters and to discriminate the univesicular from the multivesicular
mode of glutamate release. The model has been validated with data previously collected
at glutamatergic CA3-CA1 synapses in the hippocampus from newborn (P1-P5 old) rats.
The results strongly support a multivesicular type of release process requiring a variable
pool of immediately releasable vesicles. Moreover, computing quantities that are
functions of the model parameters, the mean amplitude of the synaptic response to the release of a single vesicle (Q) was estimated to be 5-10 pA, in very good agreement with
experimental findings. In addition, a multivesicular type of release was supported by
various experimental evidences: a high variability of the amplitude of successes, with a
coefficient of variation ranging from 0.12 to 0.73; an average potency ratio a2/a1 between
the second and first response to a pair of stimuli bigger than 1; and changes in the
potency of the synaptic response to the first stimulus when the release probability was
modified by increasing or decreasing the extracellular calcium concentration. This work
indicates that at glutamatergic CA3-CA1 synapses of the neonatal rat hippocampus a
single action potential may induce the release of more than one vesicle from the same
release site.
In a more systemic approach to the analysis of communication between neurons, it is
interesting to investigate more complex, network interactions. GABAergic interneurons
constitute a heterogeneous group of cells which exert a powerful control on network
excitability and are responsible for the oscillatory behaviour crucial for information
processing in the brain. They have been differently classified according to their
morphological, neurochemical and physiological characteristics.
In the second part of this study, whole cell patch clamp recordings were used to further
characterize, in transgenic mice expressing EGFP in a subpopulation of GABAergic
interneurons containing somatostatin (GIN mice), the functional properties of EGFPpositive
cells in stratum oriens of the CA1 region of the hippocampus, in slice cultures
obtained from P8 old animals. These cells showed passive and active membrane
properties similar to those found in stratum oriens interneurons projecting to stratum
lacunosum-moleculare. Moreover, they exhibited different firing patterns which were
maintained upon membrane depolarization: irregular (48%), regular (30%) and clustered
(22%). Paired recordings from EGFP-positive cells often revealed electrical coupling
(47% of the cases), which was abolished by carbenoxolone (200 mM). On average, the
coupling coefficient was 0.21 \ub1 0.07. When electrical coupling was particularly strong it
acted as a powerful low-pass filter, thus contributing to alter the output of individual
cells. The dynamic interaction between cells with various firing patterns may differently
control GABAergic signalling, leading, as suggested by simulation data, to a wide range
of interneuronal communication. In additional paired recordings of a presynaptic EGFP positive interneuron and a postsynaptic principal cell, trains of action potentials in
interneurons rarely evoked GABAergic postsynaptic currents (3/45 pairs) with small
amplitude and slow kinetics, and that at 20 Hz exhibited short-term depression. In
contrast, excitatory connections between principal cells and EGFP-positive interneurons
were found more often (17/55 pairs) and exhibited a frequency and use-dependent
facilitation, particularly in the gamma band. In conclusion, it appears that EGFP-positive
interneurons in stratum oriens of GIN mice constitute a heterogeneous population of cells
interconnected via electrical synapses, exhibiting particular features in their chemical and
electrical synaptic signalling. Moreover, the dynamic interaction between these
interneurons may differentially affect target cells and neuronal communication within the
hippocampal network