The cholinergic transmission represents one of the fundamental modulatory systems in the nervous system (Hasselmo and Giocomo, 2006; Picciotto et al., 2012). Cholinergic neurons in the central nervous system originate from basal forebrain and brainstem nuclei (Ballinger et al., 2016; Smythies, 2005) and support various brain functions including sensory processing, attention, learning - memory and sleep-wake cycle (Ballinger et al., 2016; Sarter and Bruno, 2000; Schmitz and Duncan, 2018). Cholinergic transmission is mediated through G protein-coupled metabotropic muscarinic receptors (mAChRs) and ionotropic nicotinic receptors (Dani, 2001; Thiele, 2013). The five main subtypes of mAChRs (M1-M5 receptors) form two classes of receptors, the M1-type comprising M1, M3, and M5 receptors, and the M2-type that comprises M2 and M4 receptors (Thiele, 2013; Wess, 2004). Generally, M1/5 receptors are preferentially coupled with Gq/11 family of G proteins and exert excitatory actions, while M2/4 receptors preferentially couple to Gi/o proteins, inhibit adenylate cyclase and protein kinase A (PKA), thereby exerting inhibitory actions (Brown, 2010; Wess et al., 1996). Furthermore, M2 receptors activate also G protein-coupled inwardly rectifying potassium channels (GIRKs) to reduce excitatory synaptic transmission (Wickman and Clapham, 1995).

The hippocampus, a brain area crucially involved in learning and memory (Eichenbaum et al., 2016), receives cholinergic projections from the medial septal nuclei and the nucleus of the diagonal band (Dutar et al., 1995). Previous studies have suggested that cholinergic transmission is not homogenous along the longitudinal, i.e., septotemporal or dorsoventral axis of the hippocampus. Notably, the levels of choline acetyltransferase, the enzyme that synthetize acetylcholine are higher in the ventral than the dorsal hippocampus (Dohanich and McEwen, 1986; Dravid and Van Deusen, 1984; Hoover et al., 1978; Hortnagl et al., 1991), and the levels of acetylcholine are also slightly higher in the ventral hippocampus (Hoover et al., 1978). Furthermore, it has previously been reported that the efficacy of muscarinic receptor agonist carbachol in stimulating inositol lipid turnover is higher in the ventral than dorsal hippocampus (Garcia Ruiz et al., 1993). These data suggested that muscarinic transmission may be differentiated along the dorsoventral axis of the hippocampus.

In the neocortex, acetylcholine suppresses glutamate release through M4 receptors (Eggermann and Feldmeyer, 2009; Gigout et al., 2012; Yang et al., 2020), and suppresses inhibitory synapses through M2 receptors (Gigout et al., 2012; Seeger et al., 2004). In the dorsal hippocampus, M4 muscarinic receptors are mainly expressed in CA1 pyramidal cells (Levey et al., 1995) and activation of presynaptic M4 receptors leads to a suppression of transmission at the Schaffer collateral synaptic pathway (Dasari and Gulledge, 2011; Hasselmo and Schnell, 1994; Thorn et al., 2017). Furthermore, previous studies have shown that M2 receptors in the CA1 hippocampal field are expressed mainly in interneurons (Hájos et al., 1998; Levey et al., 1995; Seeger et al., 2004), but their role at Schaffer collaterals is still uncertain. Yet, the effects of muscarinic receptors on basal excitatory synaptic transmission in the CA1 field have never been examined along the long axis of the hippocampus.

Modulation of transmitter release in a synapse can result in modifications of short-term plasticity properties of that synapse (Ito and Schuman, 2008; Manabe et al., 1993; Zucker and Regehr, 2002). In analogy, short-term synaptic plasticity is profoundly modulated by changes in the probability of transmitter release from presynaptic terminals (Mallart and Martin, 1968; Manabe et al., 1993; Zucker and Regehr, 2002). Short-term synaptic plasticity represents a fundamental mechanism in neural information processing critically influencing input-output properties at synapses (Abbott and Regehr, 2004; Silver, 2010). Indeed, many neuromodulators control information flow in neuronal circuits by modulating the properties of STSP through actions at presynaptic terminals (Cheng et al., 2018; Lee et al., 1983; Manabe et al., 1993). In the hippocampus, muscarinic cholinergic transmission has been shown to enhance paired-pulse facilitation (Fernández de Sevilla and Buño, 2003; Mann et al., 2005; Tang et al., 2009; Wang et al., 2007; Yun et al., 2000), a simple form of STSP. However, it is still not known how muscarinic cholinergic transmission modulates paired-pulse facilitation and other forms of STSP along the dorsoventral axis of the hippocampus.

Considering the proposed role of STSP in working memory (Devaraju et al., 2017; Pals et al., 2020) which has been shown to involve cholinergic activity (Bava et al., 2023; Frielingsdorf et al., 2006), it would be hypothesized that cholinergic transmission significantly modulates STSP in the hippocampus. Interestingly, the function of working memory is segregated along the dorsoventral axis of the hippocampus (Hauser et al., 2020), and the dorsal and ventral hippocampus display very different properties of STSP between each other, with the dorsal hippocampus displaying mostly facilitation and the ventral hippocampus displaying mostly depression (Babiec et al., 2017; Dubovyk and Manahan-Vaughan, 2018; Koutsoumpa and Papatheodoropoulos, 2019; Maruki et al., 2001; Papatheodoropoulos, 2015; Papatheodoropoulos, 2017; Papatheodoropoulos and Kostopoulos, 2000).

In this study, we comparatively investigated the effects of muscarinic transmission on basal excitatory transmission and STSP at the CA3-to-CA1 synapses in the dorsal and ventral hippocampus. We found that M4 muscarinic receptors are involved in controlling fEPSP and modulating STSP in both segments of the hippocampus, while M2 receptors participate in controlling fEPSP and increasing STSP in the ventral but not the dorsal hippocampus. These results are corroborated by the increased expression of M2 receptors in the ventral hippocampus and a similar expression of M4 receptors in the two hippocampal segments.