In the mammalian brain, many bioactive molecules function, and their expressions are spatiotemporally regulated. Among them, brain-derived neurotrophic factor (BDNF) and its specific receptor, TrkB, are highly expressed in the central nervous system (CNS) (Masana et al., 1993; Nawa et al., 1995; Ohira et al., 1999; Yan et al., 1997). The trkB locus encodes at least three subtypes; full-length form (TrkB-FL) and two truncated forms (TrkB-T1 and TrkB-T2) (Klein et al., 1990a, Klein et al., 1990b). TrkB-FL has been shown to be involved in signal transduction with its tyrosine kinases in the intracellular domain. Both TrkB-T1 and TrkB-T2 lack the tyrosine kinase domains. BDNF, TrkB-FL, and truncated TrkB exhibit many various physiological functions of the CNS, such as neuronal survival, dendrites growth, and synaptic plasticity (Allendoerfer et al., 1994; Baxter et al., 1997; Beck et al., 1993; Biffo et al., 1995; Eide et al., 1996; Fryer et al., 1997; Haapasalo et al., 1999). BDNF is localized to the pre- and postsynaptic terminals (Aloyz et al., 1999; Fawcett et al., 1997; Lin et al., 1998), secreted in an activity-dependent fashion (Haubensak et al., 1998), and induces the autophosphorylation of TrkB-FL (Aloyz et al., 1999). This BDNF-induced activation of TrkB-FL is essential to the neuronal plasticity (Akaneya et al., 1997; Figurov et al., 1996; Tessarollo and Yanpallewar, 2022). On the other hand, although the truncated TrkB, namely TrkB-T1, is a predominant form in the adult mammalian brain, almost all of BDNF functions seemed to be due to the TrkB-FL signaling. TrkB-T1 has been hypothesized to be a dominant-negative form of TrkB-FL, because of a lack of the tyrosine kinase domain, and to be involved in negative functions against TrkB-FL, such as TrkB-FL phosphorylation (Knüsel et al., 1994), calcium efflux (Eide et al., 1996), dendritic growth (Yacoubian and Lo, 2000), cell survival activity (Haapasalo et al., 2001), and gene expression by BDNF (Offenhäuser et al., 2002). However, it is becoming clear that TrkB-T1 has unique functions. For examples, TrkB-T1 has been found to be involved in the contractile force of the heart muscle by the control of calcium inflow (Fulgenzi et al., 2015). TrkB-T1 in the hypothalamus regulates energy homeostasis via astrocytes (Ameroso et al., 2022). Importantly, there are evidence that TrkB-T1 has its own signaling pathway, including calcium influx (Rose et al., 2003), morphological control of astrocytes (Ohira et al., 2005a, Ohira et al., 2007), induction for liver metastasis of pancreatic cancer (Li et al., 2009), neuroprotective effect (Gomes et al., 2012).

TrkB-T1 seems to play important roles in 1) dominant-negative regulation of TrkB-FL, 2) neural plasticity via its own signaling pathway, and/or 3) the combination of 1) and 2), but its precise mechanism remains to be unclear. One of the reasons why the mechanism is unclear is that the localization of TrkB-T1 proteins at the cellular level is not well understood.

Currently, there are few TrkB-T1 antibodies available for immunostaining, except for one antibody. However, the antibody for TrkB-T1 requires treatment with 6 M guanidine chloride for antigen activation for immunostaining (Ohira et al., 2005b; Ohira and Hayashi, 2003). The use of 6 M guanidine chloride often causes loss of antigenicity of other antigens, making double staining difficult. In this study, we produced an anti-TrkB-T1 antibody that does not require antigen retrieval, and investigated the localization of TrkB-T1 in the cerebral cortex of adult mice, using the antibody.