Application of the broad acting HTR antagonist methiothepin
Application of the broad-acting 5-HTR antagonist methiothepin (Bard et al., 1996, Hoyer et al., 1994, Peroutka, 1990) converted the TBS-induced response of both thalamocortical and intracortical A1 synapses from LTP to LTD, an effect that was mimicked by the selective 5-HT2R antagonist ketanserin (Leysen et al., 1982, Peroutka, 1990). Thus, 5-HT2Rs appear to play a critical role in LTP induction and determining the direction of plasticity induction at neocortical synapses. Prior work is consistent with this interpretation, in that 5-HT2R stimulation has been shown to enhance LTP induction at synapses in the basolateral amygdala and hippocampal CA1 field in vitro (Chen et al., 2003, Rozas et al., 2012). 5-HT2Rs are widely distributed throughout the neocortex and most cortical layers (layers II to VI; Hamada et al., 1998, Jakab and Goldman-Rakic, 2000). Interestingly, some types of 5-HT2Rs (e.g., 5-HT2ARs) are located on both cortical pyramidal cells and GABAergic interneurons (Jakab and Goldman-Rakic, 2000), as well as presynaptically on terminals of thalamocortical atm kinase inhibitor (Barre et al., 2016). Thus, there are a number of complementary mechanisms by which these receptors can modulate cellular excitability and plasticity induction in cortical networks. Activation of 5-HT2Rs in neocortical pyramidal cells often elicits excitatory effects, including increases in spontaneous excitatory postsynaptic currents, cellular depolarization, and inhibition of the after-hyperpolarization current that follows neuronal discharge (Aghajanian and Marek, 1997, Araneda and Andrade, 1991, Celada et al., 2013, Davies et al., 1987, Newberry et al., 1999). Blockade of 5-HT2Rs by ketanserin can result in a facilitation of inhibitory responses to 5-HT application (Lakoski and Aghajanian, 1985), which may be related to the conversion of LTP to LTD noted in the present study. Recent work has shown that activation of presynaptic 5-HT2ARs on thalamocortical fibers terminating in the prefrontal cortex acts to facilitate NMDAR-mediated transmission (Barre et al., 2016). The precise mechanisms mediating the LTP-to-LTD conversion seen during methiothepin or ketanserin application remain to be elucidated; however, the observation that LTP at thalamocortical A1 synapses involves a presynaptic component (as measured by changes in paired-pulse ratios; Soutar et al., 2016) is consistent with at least a partial presynaptic locus of this effect. Application of granisetron, a potent antagonist of 5-HT3Rs (Plosker and Goa, 1991, Sanger and Nelson, 1989) exerted a surprising, selective effect on plasticity induction by blocking LTP of the first, but not the second peak of cortical fPSPs. Thus, it appears that 5-HT3Rs play an important role in the gating of plasticity at thalamocortical, but not intracortical synapses in A1. 5-HT3Rs are ligand-gated ion channels that mediate rapid neuronal depolarization (Barnes and Sharp, 1999, Celada et al., 2013) and are expressed widely throughout the forebrain, including the neocortex (Barnes and Sharp, 1999, Kilpatrick et al., 1987). Since many 5-HT3Rs are located on GABAergic interneurons in the hippocampus and neocortex (Jakab and Goldman-Rakic, 2000, Morales et al., 1996), they are able to profoundly shift the excitatory-inhibitory balance of cortical circuits. At present, it is not known if the impairment of LTP induction by granisetron involves changes in inhibitory tone in A1. It would appear that a blockade of 5-HT3Rs on GABAergic cells should result in cortical disinhibition and enhanced plasticity induction, rather than the LTP impairment observed in our experiments. Future experiments measuring both excitatory and inhibitory currents and LTP in principal cells and different classes of interneurons are required to more fully elucidate the mechanisms involved in LTP gating by 5-HT3Rs in the rodent A1. We also noted that application of granisetron resulted in a clear enhancement of baseline (i.e., non-potentiated) fPSP amplitude, which reached 126% and 187% of baseline levels for the first and second fPSP peak, respectively (it is noteworthy that these levels of facilitation are greater than those seen during the induction of LTP in A1). Prior work has demonstrated the involvement of 5-HT1ARs and 5-HT2ARs in modulating inhibitory transmission in layers II/III of A1 (García-Oscos et al., 2015). The present data extend these findings by showing that 5-HT3Rs also exert a powerful role in controlling A1 synaptic activity by inhibiting thalamocortical and intracortical A1 synapses, an effect that was relieved during granisetron application. It is possible that this inhibitory role of 5-HT3Rs is directly related to the presence of these receptors on GABA-containing interneurons in A1, as discussed previously.