Feedforward inhibition regulates perirhinal transmission of neocortical inputs to the entorhinal cortex: ultrastructural study in guinea pigs.

The perirhinal cortex is an elongated strip of cortex located in the fundus (area 35) and lateral bank (area 36) of the rhinal sulcus and is believed to play a critical role in recognition memory (reviewed in Brown and Aggleton, 2001). Medially, the perirhinal cortex abuts on the entorhinal area, with which it is reciprocally connected. Together, these cortical regions occupy a strategic position in the temporal lobe because they relay most sensory inputs from the neocortex to the hippocampus. Moreover, the rhinal cortices represent the main return path for hippocampal efferents to the neocortex (reviewed in Witter et al., 2000). Indeed, tract-tracing studies suggest that information transfer between the neocortex and hippocampus depends on the sequential, stepwise activation of longitudinal bands of cortex within the rhinal cortices (neocortex to perirhinal area 36 to area 35 to EC to hippocampus and conversely). This stepwise progression of impulses is not absolute however as some neurons project beyond the adjoining area in the lateromedial or mediolateral directions in primates (Van Hosen and Pandya, 1975; Insausti et al., 1987; Suzuki and Amaral, 1994a,b), felines (Room and Groenewegen, 1986a; Witter and Groenewegen, 1986) and rodents (Deacon et al., 1983; Saleem and Tanaka, 1996; McIntyre et al., 1996; Burwell and Amaral, 1998a,b). In general, medially directed projections (from the neocortex and perirhinal cortex toward the entorhinal cortex) primarily originate and end in superficial layers whereas return projections toward the neocortex originate in deep layers (Deacon et al., 1983; Witter and Groenewegen, 1986; Room and Groenewegen, 1986a; Insausti et al., 1987; Burwell and Amaral, 1998a,b). At odds with this evidence of profuse reciprocal connections however, physiological studies in the whole guinea pig brain kept in vitro have revealed that perirhinal transmission of neocortical and entorhinal inputs occurs with an extremely low probability. For instance, electrical stimulation of the lateral olfactory tract evokes massive neuronal excitation in the entorhinal cortex but no responses in area 36 (Biella et al., 2003). Similarly, stimulation of the temporal neocortex or area 36 evokes no local field responses in the entorhinal cortex (Biella et al., 2002) and analogous results were obtained in imaging studies of intrinsic (Frederico et al., 1994) and voltage-sensitive signals (de Curtis et al., 1999; Biella et al., 2003). Importantly, these results are not an artifact of the whole brain in vitro preparation since recent in vivo studies, using multisite extracellular recordings of neocortical, perirhinal, and entorhinal neurons, reached the same conclusions (Pelletier et al., 2004, 2005). Thus, there seems to be a discrepancy between tract-tracing data, showing strong reciprocal connections between the perirhinal and entorhinal cortices, and physiological data about this pathway. Given the important role played by the rhinal cortices in memory (reviewed in Brown and Aggleton, 2001), understanding the factors that regulate neuronal excitability in this pathway emerges as an issue of critical importance. Valuable insights into this matter might be gained by considering the ultrastructural organization of the pathways linking the rhinal cortices and neocortex. However, the available studies have focused on presubicular and olfactory afferents to the entorhinal cortex as well as on its intrinsic network (for instance see Wouterlood and Nederlof, 1983; Wouterlood et al., 1985, 1995, 2000, 2004; van Haeften et al., 2003). Accordingly, the present study was undertaken to shed light on the inhibitory mechanisms that control impulse traffic in the pathway linking the neocortex to the entorhinal cortex by way of the perirhinal cortices. To this end, we have combined iontophoretic injections of the anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHAL) into the temporal neocortex or perirhinal areas 36 and 35, with pre-embedding GABA immunocytochemistry, and electron microscopic observations of anterogradely-labeled axon terminals. Our results indicate that impulse propagation from the neocortex to the entorhinal cortex is restricted by strong inhibitory influences, accounting for previous physiological observations.