Developmental Determinants of Synapse-specificRetrograde Signalling in Cortical Neuronal Networks

Retrograde synaptic signalling is a mechanism through which a retrograde messenger is produced and released from postsynaptic neurons, commutes to the presynaptic terminal where it engages its cognate receptor(s) to modulate the probability of neurotransmitter release. Retrograde signalling is recognized as a fundamental form of feedback regulation at many cortical synapses with a broad array of diffusible messenger molecules implicated in initiating the cascade of presynaptic molecular events. An accumulating body of evidence suggests that the molecules acting as retrograde messengers in adult brain all have discrete functions during brain development, and, most notably, there is an overarching continuum of regulatory functions these messengers execute from early events of presynaptic neurotransmitter to the maintenance of synaptic pasticity in adulthood. Here, developmental roles of signal molecules (and, in a broader context, the underlying signalling cassettes) that otherwise act as retrograde messengers at mature synapses have been studied with emphasis on how the synapse specificity of retrograde signalling is achieved and translated into specific molecular pathways subserving synaptic information processing. Thematically, this thesis first summarizes current concepts on the organization of synapsespecific retrograde signalling networks operated by endocannabinoids, brain-derived neurotrophic factor (BDNF) or glutamate as retrograde messengers in the primate and rodent telencephalon emphasizing how cholinergic projection neurons in basal forebrain territories modulate the plasticity of their afferents. We demonstrate a high degree of evolutionary conservation in neuronal diversity and neurochemical specificity in basal forebrain nuclei between rodents and prosimian primates. We find that the areal expansion and cytoarchitectonic differentiation of neocortical subfields in primates is associated with differential cortical patterning of CB1 cannabinoid receptor-containing subcortical and intracortical afferents and these connectivity patterns are maintained during senescence. Next we asked whether basal forebrain cholinergic neurons have a preferred means to tune the activity of afferent synapses or instead they possess a capacity to recruit molecularly distinct signalling networks at non-overlapping subsynaptic dendrite domains receiving functionally segregated synaptic inputs. Our studies revealed a striking dichotomy of CB1 cannabinoid receptor or vesicular glutamate transporter 3 (VGLUT3)-containing afferents – defining components of endocannabinoid and glutamate-mdiated retrograde signalling, respectively – in the rodent basal forebrain. In contrast, we find that CB1 cannabinoid receptors and VGLUT3 co-exist at nerve terminals of cholecystokinin-containing interneurons impinging layer 2/3 pyramidal cells synapses in the cerebral cortex. Based on the above anatomical indices we hypothesized that (i) different forms of retrograde synaptic communication may co-exist in non-overlapping, functionally discrete domains along the elaborate dendritic arbor of a neuron and (ii) the molecular identity of a presynaptic neuron drives the developmental assembly of retrograde signalling mechanisms operated at its synapses. We uncovered that synapses of parvalbumin-containing fast-spiking (FS) basket cells in layer 2/3 of the neocortex undergo endocannabinoid independent retrograde modifications in response to high-frequency burst firing of postsynaptic pyramidal cells in the absence of presynaptic CB1 cannabinoid receptors. We identified VGLUT3-dependent glutamate release from subsynaptic pyramidal cell dendrites as a substitute negative feedback loop. Interneurons of the cerebral cortex are neurochemically and functionally heterogeneous. This limits our ability to study the development of synaptic communication en masse at select subsets of inhibitory terminals. We have challenged this status quo by introducing a novel technology, target specific isolation (TSI), to enrich neurochemically-defined cortical and hippocampal interneuron progenies for large scale, high-throughput developmental, cell biology, and gene expression profiling studies. We have successfully utilized TSI to characterize the temporal dynamics of FS cell synaptogenesis in vitro, and to describe that calpain, a Ca2+-dependent protease, cleaves synaptosomal-associated protein of 25 kDa (SNAP-25) in vivo and reduces SNAP-25 levels during GABAergic interneuron development. We have also explored whether endocannabinoids drive particular aspects of interneuron development and found that these bioactive eicosanoids act as short-range diffusible chemoacttractants to facilitate radial (interlaminar) interneuron migration. Our studies also demonstrate that endocannabinoids are negative regulators of interneuron morphogenesis. Finally, we have uncovered that a branch of neurodevelopmental endocannabinoid actions is mediated by a novel interplay between endocannabinoid and BDNF-mediated signalling networks because endocannabinoids can increase TrkB receptor phosphorylation through a signalling mechanism involving src kinase activation in the absence of BDNF. Overall, a coherent set of experimental evidence is collated in this thesis in support of the concept that molecules acting as retrograde messengers in adult brain play fundamental roles during neurodevelopment.