Development of structural and functional connectivity in the thalamocortical somatosensory pathway in the wallaby

Abstract

Neuronal activity is implicated as a driving force in the development of sensory systems. In order for it to play a developmental role, however, the pathways involved must be capable of transmitting this activity. The relationship between afferent arrival, synapse formation and the onset of chemical neurotransmission has been examined using the advantageous model of a marsupial mammal, the wallaby (Macropus eugenii), to determine at what stage activity has the capacity to influence cortical development. It is known that thalamocortical afferents arrive in the somatosensory cortex on postnatal day (P)15 and that their growth cones reach to the base of the compact cell zone of the cortical plate. However, electronmicroscopy showed that thalamocortical synapses were absent at this stage. Glutamatergic responses were recorded in the cortex following stimulation of the thalamus in slices at this time but only in magnesium-free conditions. The responses were mediated entirely by N-methyl-d-aspartate (NMDA) receptors. From P28, responses could be recorded in normal magnesium and comprised a dominant NMDA-mediated component and a non-NMDA mediated component. At this time thalamocortical synapses were first identified and they were in the cortical plate. By P63 the non-NMDA-mediated component had increased relative to the NMDA-mediated component, and by P70 layer IV began to emerge and contained thalamocortical synapses. By P76 a fast non-NMDA-mediated peak dominated the response. This coincides with the appearance of cortical whisker-related patches and the onset in vivo of responses to peripheral stimulation of the whiskers.

Development of the Olfactory System in a Wallaby <i>(Macropus eugenii)</i>

“…However, whether this indicates a lack of functional connections in the olfactory bulb at birth is not clear. Discrepancies have been reported between the onset of synaptic activity in in vivo (i.e., anaesthetized intact animals) versus in vitro conditions (i.e., dissected brain tissue maintained in culture conditions) in the tammar in other sensory pathways, the former being detected much later in development [Mark et al, 2002;Flett et al, 2006;Leamey et al, 2007]. Furthermore, it is not known whether more central components of the olfactory pathway are sufficiently mature at birth to contribute to the control of pouch or nipple finding behavior.…”

Quantitative analysis of somatosensory cortex development in eutherians, with a comparison with metatherians and monotremes

“…These might include: (1) the rate of expansion of the pallial proliferative compartment; (2) the release of neuronal daughter cells to the cortical plate; (3) the duration of proliferative activity in the pallial and ganglionic proliferative compartments; (4) the degree of neurite growth; and (5) the extent of connection formation . Although each of these has been studied intensively in a limited number of domestic eutherians (Darlington et al 1999;Carney et al 2007;Jones 2009;Cheung et al 2010) and some metatherians (Reynolds et al 1985;Saunders et al 1989;Sheng et al 1991;Krause and Saunders 1994;Marotte et al 1997;Molnár et al 1998;Darlington et al 1999;Leamey et al 2007;Wang et al 2011), and deductions have been made from the study of adult brain structure in diverse taxa (Herculano-Houzel 2011, 2012, 2014Herculano-Houzel et al 2014), no study has directly compared these features in embryos across diverse eutherian taxa.…”

Ten‐m4 plays a unique role in the establishment of binocular visual circuits

“…The expression of Ten-m4 in the mouse visual system provides a unique opportunity to gain insight into its function by examining its role in the development and function of this well-characterized and accessible neural pathway. Given the high degree of homology between Ten-m members (Oohashi et al, 1999), their expression patterns, and importance in regulating neural connectivity in vertebrate (Antinucci et al, 2013(Antinucci et al, , 2016Berns et al, 2018;Cheung et al, 2019;Dharmaratne et al, 2012;Kenzelmann et al, 2008;Leamey, Flett, et al, 2007;Leamey, Merlin, et al, 2007;Merlin et al, 2013;Pederick et al, 2021;Rubin et al, 2002Rubin et al, , 1999Tran et al, 2015;Young et al, 2013;Zhang et al, 2022) and invertebrate (Hong et al, 2012;Mosca et al, 2012) species, we hypothesized that Ten-m4 may have a specific role in the formation and function of binocular visual circuits.…”