There is ample evidence for localization of messenger RNAs (mRNAs) and protein synthesis in neuronal dendrites; however, demonstrations of these processes in presynaptic terminals are limited. We used expansion microscopy to resolve pre- and postsynaptic compartments in rodent neurons. Most presynaptic terminals in the hippocampus and forebrain contained mRNA and ribosomes. We sorted fluorescently labeled mouse brain synaptosomes and then sequenced hundreds of mRNA species present within excitatory boutons. After brief metabolic labeling, >30% of all presynaptic terminals exhibited a signal, providing evidence for ongoing protein synthesis. We tested different classic plasticity paradigms and observed distinct patterns of rapid pre- and/or postsynaptic translation. Thus, presynaptic terminals are translationally competent, and local protein synthesis is differentially recruited to drive compartment-specific phenotypes that underlie different forms of plasticity.

Local protein synthesis is a ubiquitous feature of neuronal pre-and postsynaptic compartments.One sentence summary: Protein synthesis occurs in all synaptic compartments, including excitatory and inhibitory axon terminals. Presynaptic terminals from mouse cortex and hippocampus contain translation machineryEfforts to localize molecules or cell biological events to neuronal pre-or postsynaptic compartments using fluorescence microscopy are limited by the tight association of the axonal bouton and the dendritic spine or synapse; the synaptic cleft, the only clear region of separation, is only ~ 20 nm wide. Here, in order to increase the resolving power to visualize mRNA molecules in pre-and postsynaptic compartments, we optimized fluorescence in situ hybridization (FISH) and nascent protein detection methods for use with expansion microscopy (22) ( Fig. 1A; see Methods). We used adult mouse brain slices or rat cultured hippocampal neurons and found that expansion resulted in an enlargement of both pre-and postsynaptic compartments, with an average expansion of ~3.4 fold. This yielded a clear separation between the pre-and postsynaptic compartments. To evaluate whether ribosomes and mRNA species are present in defined presynaptic compartments, we used immunolabelling for either excitatory (vGLUT1; (23, 24) or inhibitory (vGAT; (25, 26)) nerve terminals in expanded mouse brain sections (both cortex and hippocampus) ( Fig. 1B-E) or rat cultured hippocampal neurons. We took care to identify the molecules-of-interest within individual z-sections positively labeled for excitatory or inhibitory terminals. We noted that signal detected outside of immunolabeled compartments corresponded to signal arising from nearby unlabeled cells. We detected ribosomes in a large majority (>75%) of both excitatory and inhibitory presynaptic nerve terminals, using antibodies directed against either a small (RPS11) or a large (RPL26) ribosomal protein ( Fig. 1 B-E). Next, we used FISH probes to detect 18s and 28s rRNA as well as polyadenylated mRNA (detected with a poly d(T) probe) in expanded samples ( Fig. 1B-E). Consistent with the abundance of ribosomal proteins, we detected rRNA in over 80% of both excitatory and inhibitory nerve terminals ( Fig. 1B-E). RNase treatment effectively reduced all rRNA signal. In cultured neurons, we also noted that poly(A) mRNA was abundant, as expected, in dendritic spines. In addition, we used an anti-tau antibody to label axons and detected both 18s and 28s rRNA in axonal segments. Thus, mRNAs and ribosomes were abundant in excitatory and inhibitory presynaptic nerve terminals from both mouse brain slices and rat hippocampal cultured neurons.

To help elucidate the role of inhibitory feedback in the genesis of odour-evoked synchronization of neural activity, we investigated the distribution of gamma-aminobutyric acid (GABA)ergic synaptic terminals in the antennal lobes (AL) and mushroom bodies (MB) of the locust olfactory system. Electron-microscopy, intracellular horseradish peroxidase labelling, and immunocytochemistry were combined to assess the distribution of GABAergic synapses, using established methods (Leitch and Laurent [1993] J. Comp. Neurol. 337:461-470). In the AL, GABA-immunoreactive presynaptic terminals contacted both immunoreactive and immunonegative profiles. Conversely, GABA-immunoreactive profiles received direct input from both reactive and negative terminals. The tract containing the axons of the projection neurons that run from the AL to the MB contained about 830 axons of fairly uniform size, none of which was immunoreactive for GABA. In the calyx of the MB, large immunoreactive terminals contacted very-small-diameter profiles thought to belong to the Kenyon cells (KCs). This was confirmed by combining immunocytochemistry with intracellular HRP-labelling of KCs. KCs were not immunoreactive for GABA. Although some GABAergic contacts were made onto the spiny profiles of KCs, others were made onto their dendritic shafts. Large GABA-immunoreactive profiles were also found to contact large negative profiles that were presynaptic to KC terminals. This suggests that KC dendrites can be both pre- and post-synaptically inhibited in the calyx. The MB pedunculus contained ca. 50,000 tightly packed KC axons, showing conspicuous en passant and often reciprocal synaptic contacts between neighbouring axons. KC axons were immunonegative, but received direct input from, and contacted directly, large immunoreactive profiles running across or along the KC axons. In the alpha- and beta-lobes of the MB, connections similar to those in the pedunculus were seen with two main differences: (1) The density of synaptic profiles was higher, giving on occasion numerous serially connected profiles in a single section; (2) large immunonegative profiles with dense-core vesicles were abundant and were frequently presynaptic to GABAergic processes and to very-small-diameter profiles which possibly belong to KCs. These results are discussed in the context of the known physiological data on olfactory processing in these complex circuits.

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