Biology subjects

Lipovsek, M.

Publications and source records attributed to Lipovsek, M..

6 recordsLinked to original sources

bioRxiv2024

Functional maturation and experience-dependent plasticity in adult-born olfactory bulb dopaminergic neurons

Continued integration of new neurons persists in only a few areas of the adult mouse brain. In the olfactory bulb (OB), immature adult-born neurons respond differently to olfactory stimuli compared to their more mature counterparts, and have heightened levels of activity-dependent plasticity. These distinct functional features are thought to bestow unique properties onto existing circuitry. OB interneurons, including those generated through adult neurogenesis, consist of a set of highly distinct subtypes. However, we do not currently know the different cell-type-specific mechanisms underlying their functional development and plastic potential. Here, we specifically characterised electrophysiological maturation and experience-dependent plasticity in a single, defined subtype of adult-born OB neuron: dopaminergic cells. We selectively live-labelled both adult-born and resident dopaminergic cells, and targeted them for whole-cell patch-clamp recordings in acute mouse OB slices. Surprisingly, we found that from the time - at [~]1 month of cell age - that live adult-born dopaminergic neurons could first be reliably identified, they already possessed almost fully mature intrinsic firing properties. We saw significant maturation only in increased spontaneous activity and decreased medium afterhyperpolarisation amplitude. Nor were adult-born dopaminergic cells especially plastic. In response to brief sensory deprivation via unilateral naris occlusion we observed no maturation-specific plastic alterations in intrinsic properties, although we did see deprivation-associated increases in spike speed and amplitude across all adult-born and resident neurons. Our results not only show that adult-born OB dopaminergic cells rapidly functionally resemble their pre-existing counterparts, but also underscore the importance of subtype identity when describing neuronal maturation and plasticity.

neuroscience↗

bioRxiv2024

Strikingly different neurotransmitter release strategies in dopaminergic subclasses

Neuronal function is intimately tied to axodendritic polarity. Neurotransmitter release, for example, is usually the role of the axon. There are widespread exceptions to this rule, however, including many mammalian neuronal types that can release neurotransmitter from their dendrites. In the mouse olfactory bulb, closely related subclasses of dopaminergic interneuron differ markedly in their polarity, with one subtype lacking an axon entirely. These axon-bearing and anaxonic dopaminergic subclasses have distinct developmental profiles and sensory responses, but how their fundamental polarity differences translate to functional outputs remains entirely unknown. Here, we provide anatomical evidence for distinct neurotransmitter release strategies among these closely related dopaminergic subtypes: anaxonic cells release from their dendrites, while axon-bearing neurons release exclusively from their intermittently myelinated axon. These structural differences are linked to a clear functional distinction: anaxonic, but not axon-bearing dopaminergic neurons are capable of self-inhibition. Our findings suggest that variations in polarity can produce striking distinctions in neuronal outputs, and that even closely related neuronal subclasses may play entirely separate roles in sensory information processing.

neuroscience↗

bioRxiv2020

Loss of choline agonism in the inner ear hair cell nicotinic acetylcholine receptor linked to the α10 subunit

The 910 nicotinic acetylcholine receptor (nAChR) plays a fundamental role in inner ear physiology. It mediates synaptic transmission between efferent olivocochlear fibers that descend from the brainstem and hair cells of the auditory sensory epithelium. The 9 and 10 subunits have undergone a distinct evolutionary history within the family of nAChRs. Predominantly in mammalian vertebrates, the 910 receptor has accumulated changes at the protein level that may ultimately relate to the evolutionary history of the mammalian hearing organ. In the present work we investigated the responses of 910 nAChRs to choline, the metabolite of acetylcholine degradation at the synaptic cleft. Whereas choline is a full agonist of chicken 910 receptors it is a partial agonist of the rat receptor. Making use of the expression of 910 heterologous receptors, encompassing wild-type, heteromeric, homomeric, mutant, chimeric and hybrid receptors, and in silico molecular docking, we establish that the mammalian (rat) 10 nAChR subunit underscores the reduced efficacy of choline. Moreover, we show that whereas the complementary face of the 10 subunit does not play an important role in the activation of the receptor by ACh, it is strictly required for choline responses. Thus, we propose that the evolutionary changes acquired in the mammalian 910 nAChR resulted in the loss of choline acting as a full agonist at the efferent synapse, without affecting the triggering of ACh responses. This may have accompanied the fine-tuning of hair cell post-synaptic responses to the high frequency activity of efferent medial olivocochlear fibers that modulate the cochlear amplifier. Contribution to the Field StatementIn the inner ear of mammals, several evolutionary changes have occurred resulting in an expansion of the hearing range to higher sound frequencies. Fine tuning of cochlear synapses is required for sound enconding. The synapse between efferent olivocochlear fibers, that descend from the hindbrain, and sensory hair cells modulates sound amplification at the periphery and it has been proposed as a major player in the expansion of the hearing range. The 910 nicotinic acetylcholine receptor, which mediates synaptic neurotransmission at the efferent fiber-hair cell synapses, has accumulated a high number of amino acid substitutions in the mammalian lineage. We now show that these evolutionary acquired changes have led to a mammalian receptor with a lower efficacy for choline, the metabolite produced at the synaptic cleft by acetylcholine degradation. Making use of molecular, electrophysiological and in silico simulations techniques we show that it is the 10 subunit the one responsible for the loss of full choline agonism on the efferent receptor in mammals. This functional change may prove fundamental to faithfully reproduce the high frequency activity of efferent medial olivocochlear fibers and the modulation of the cochlear amplifier.

neuroscience↗

bioRxiv2020

Brief sensory deprivation triggers plasticity of neurotransmitter-synthesising enzyme expression in genetically labelled olfactory bulb dopaminergic neurons

In the glomerular layer of the olfactory bulb, local dopaminergic interneurons play a key role in regulating the flow of sensory information from nose to cortex. These dual dopamine- and GABA-releasing cells are capable of marked experience-dependent changes in the expression of neurotransmitter-synthesising enzymes, including tyrosine hydroxylase (TH). However, such plasticity has most commonly been studied in cell populations identified by their expression of the enzyme being studied, and after long periods of sensory deprivation. Here, instead, we used brief 1- or 3-day manipulations of olfactory experience in juvenile mice, coupled with a conditional genetic approach that labelled neurons contingent upon their expression of the dopamine transporter (DAT-tdTomato). This enabled us to evaluate the potential for rapid changes in neurotransmitter-synthesising enzyme expression in an independently identified neuronal population. Our labelling strategy showed good specificity for olfactory bulb dopaminergic neurons, whilst revealing a minority sub-population of non-dopaminergic DAT-tdTomato cells that expressed the calcium-binding protein calretinin. Crucially, the proportions of these neuronal subtypes were not affected by brief alterations in sensory experience. Short-term olfactory manipulations also produced no significant changes in immunofluorescence or whole-bulb mRNA for the GABA-synthesising enzyme GAD67/Gad1. However, in bulbar DAT-tdTomato neurons brief sensory deprivation was accompanied by a transient, small drop in immunofluorescence for the dopamine-synthesising enzyme dopa decarboxylase (DDC), and a sustained decrease for TH. Deprivation also produced a sustained decrease in whole-bulb Th mRNA. Careful characterisation of an independently identified, genetically labelled neuronal population therefore enabled us to uncover rapid experience-dependent changes in dopamine-synthesising enzyme expression.

neuroscience↗

bioRxiv2019

Distinct evolutionary trajectories of neuronal and hair cell nicotinic acetylcholine receptors

The expansion and pruning of ion channel families has played a crucial role in the evolution of nervous systems. Remarkably, with a highly conserved vertebrate complement, nicotinic acetylcholine receptors (nAChRs) are unique among ligand-gated ion channels in that members of the family have distinct roles in synaptic transmission in non-overlapping domains, either in the nervous system, the inner ear hair cells or the neuromuscular junction. Here, we performed a comprehensive analysis of vertebrate nAChRs sequences, single cell expression patterns and comparative functional properties of receptors from three representative tetrapod species. We show that hair cell nAChRs underwent a distinct evolutionary trajectory to that of neuronal receptors. These were most likely shaped by different co-expression patterns and co-assembly rules of component subunits. Thus, neuronal nAChRs showed high degree of coding sequence conservation, coupled to greater co-expression variance and conservation of functional properties across tetrapod clades. In contrast, hair cell 910 nAChRs exhibited greater sequence divergence, narrow co-expression pattern and great variability of functional properties across species. These results point to differential substrates for random change within the family of gene paralogs that relate to the segregated roles of nAChRs in synaptic transmission.\n\nSignificance statementOur work exploits several peculiarities of the family of vertebrate nicotinic acetylcholine receptors (nAChRs) to explore the evolutionary trajectories of a ligand-gated ion channel family. By performing a comprehensive comparative analysis of nAChR subunits coding sequences, single cell expression patterns and functional properties we found a contrasting evolutionary history between nAChRs with widespread expression in the nervous system compared to those with isolated expression in the inner ear. Evolutionary changes were focused on differences in co-expression and co-assembly patterns for the former and coding sequences in the latter. This multidisciplinary approach provides further insight into the evolutionary processes that shaped the nervous and sensory systems of extant animals.

neuroscience↗

bioRxiv2018

Conserved and divergent development of brainstem vestibuloacoustic nuclei

Vestibular function was established early in vertebrates and has remained, for the most part, unchanged. In contrast, tetrapods underwent independent evolutionary processes to solve the problem of hearing on land. Thus, the vestibuloacoustic nuclei of the hindbrain provide an ideal framework on which to address the participation of developmental processes to the evolution of neuronal circuits.\n\nWe employed an electroporation strategy to unravel the contribution of dorsoventral and axial lineages to the development of the chick hindbrain vestibular and auditory nuclei. We compare the chick developmental map with recently stablished genetic fate-maps of the mouse hindbrain. Overall, we find considerable conservation of developmental origin for the vestibular nuclei. In contrast, auditory hindbrain development echoes the complex evolutionary history of the auditory system. In particular, we find that the developmental origin of a chick sound localisation circuit supports its emergence from an ancient vestibular network, unrelated to the analogous mammalian counterpart.

developmental biology↗