Primary Afferent Synapses on Developing and Adult Renshaw Cells

Target selection of proprioceptive and motor axon synapses on neonatal V1‐derived Ia inhibitory interneurons and Renshaw cells

“…This antibody labels a single band of appropriate molecular weight (≈62 kDa) in Western blots of synaptic vesicle fractions of rat brains (Synaptic Systems, Goettingen, Germany). In our spinal cord sections the laminar distribution of immunolabeled puncta (high densities in LIII-LV, LVII, and LIX; weaker densities in LI and LII and LVIII) is identical to that previously published by us Mentis et al, 2006) and other authors using this or similar antibodies raised in different species (Todd et al, 2003;Oliveira et al, 2003).Vesicular acetylcholine transporter (VAChT) antibodies: We used two polyclonal antibodies raised in guinea pig and goat, respectively, each against a similar, but not identical, sequence in the C-terminus (see Table 1). By Western blot the guinea pig antibody reacted with a single band in rat brain homogenates that corresponded to the size predicted for VAChT (Chemicon, Temecula, CA).…”

“…In contrast, similar studies in the cat concluded that adult RCs lack monosynaptic activation by Ia or any other dorsal root afferent (Renshaw, 1946;Curtis and Ryall, 1966;Ryall et al, 1972; but see also Frank and Fuortes, 1956). Thus, monosynaptic activation of RCs by sensory afferents appears to be restricted to neonates (Mentis et al, 2006) and at this stage this input already shows quantitative differences in bouton density compared to IaINs.In The results therefore argue against the existence of "generic" embryonic V1 cells competent to receive similar complements of synaptic inputs, which are thereafter selected or deselected on individual interneuron subclasses. On the contrary, the results favor a model of significant input selectivity and organization before birth on different interneurons or even neuronal compartments (somata, proximal, or distal dendrites).…”

“…The different sources of excitatory drive places each interneuron in a different functional position within the motor network and also serves as a method of identification. However, we recently reported that RCs also received functional Ia inputs in the neonate which appear to be reduced in density during late postnatal development (Mentis et al, 2006). In contrast, Ia afferent synapses strengthen on motoneurons postnatally in part by the addition of new synaptic boutons through mechanisms that are distinct from those controlling synaptic targeting (Shneider et al, 2009; reviewed in Mentis et al, 2010).…”

“…7D). We attempted to study V1-derived IaINs at P0/P1 but consistent VGLUT1-IR at this age was only obtained with an alternative antibody raised in rabbit (see Mentis et al, 2006). The most likely explanation is sensitivity differences between both antibodies being VGLUT1 expressed at low levels in the spinal cord of newborns and dramatically upregulated after birth (Mentis et al, 2006).…”

“…In contrast, only 6.9% of VAChT-IR boutons (n = 162 boutons) on V1-IaINs were labeled. These analyses were not repeated at more mature ages because the efficiency of retrograde labeling dramatically decreases after P10 (discussed in Mentis et al, 2006). To investigate whether cholinergic terminals on IaINs arise from cholinergic Pitx2 (V0c) interneurons, which also give raise to the C-terminals associated with motoneurons (Zagoraiou et al, 2009), we analyzed sections from P15 dual pitx2::cre and En1::cre animals crossed to the Tau-lacZ-mEGFP reporter line.…”