The rat hippocampal formation is an ideal system to assess the reactive cellular and molecular changes that follow neural injury. The effects of neural injury in the hippocampal formation are of clinical relevance as Alzheimer’s disease (AD) is characterized by a massive loss of cerebral neurons within and connected to this structure, particularly those that project to the hippocampal formation from the entorhinal cortex (EC; Hyman et al., 1984) and the basal forebrain (Grothe et al., 2010, Grothe et al., 2012). Synaptic remodeling following disconnection of the hippocampus from the entorhinal area has been extensively studied (reviewed in Ramirez, 2001, Perederiy and Westbrook, 2013). In rats, the entorhinal cortex transmits a majority of its fibers to the ipsilateral dentate gyrus (DG) via the perforant pathway (PP) while a small number of fibers cross the midline and terminate in the contralateral dentate via the crossed temporodentate (CTD) pathway (Steward, 1976; Steward and Scoville, 1976). Although a unilateral entorhinal lesion denervates the dentate outer molecular layer by as much as 90% (Steward and Vinsant, 1983), the CTD undergoes extensive axonal sprouting from 6-12 days postlesion. This normally sparse projection in intact animals continues to proliferate approximately 6-fold for at least 60 days following unilateral EC lesion (Steward, Cotman, and Lynch, 1976) and can enhance responses in DG granule cells by as much as 500-800% (Steward et al., 1976a; Steward and Loesche, 1977). The proliferated CTD exhibits similar functional capacity to the homologous PP in that it is capable of eliciting granule cell discharge and potentiating the field excitatory postsynaptic potentials (fEPSP) of the granule cells. Importantly, the recovery of spatial memory function after unilateral entorhinal lesions parallels the time course of morphological and electrophysiological changes in the CTD (Loesche and Steward, 1977, Reeves and Smith, 1987, Ramirez et al., 1996). Dentate afferents arising from basal forebrain, in particular the medial septum, also exhibit extensive proliferation shortly following EC lesion (Fass and Ramirez, 1984) and continue to exhibit reactive changes at least 30 days after injury (reviewed in Ramirez, 2001). Unlike the reactive changes in the CTD, heterotypic changes in the cholinergic, acetylcholinesterase-containing septodentate (SD) fibers differ from the damaged PP in type of neurotransmitter released and in neuroanatomical origin. The significance of their proliferation and influence on granule cell activity following EC lesion remains unknown.

Paired-pulse stimulation has been used to assess the capacity for a single pathway to homosynaptically alter the excitability of its target structure or evaluate an interaction of two dissimilar pathways capacity to heterosynaptically alter their target (Bilkey and Goddard, 1985, Reeves et al., 1997; Kleschenvnikov and Routtenberg, 2003; Dolleman-van der Weel et al., 2017). The dentate granule cell response is potentiated by homosynaptic paired-pulse stimulation of the intact PP, whereas it is only weakly potentiated by the CTD in intact animals (Steward et al., 1976b). Homosynaptic paired-pulse potentiation of dentate granule cell response is more readily observed in proliferated CTD following a unilateral EC lesion, although still not to the same degree as in the ipsilateral fibers of the PP (Steward et al., 1976b). Similarly, initial stimulation of the SD fibers has been observed to heterosynaptically potentiate the capacity for subsequent entorhinal stimulation to enhance dentate granule cell excitability via the intact PP (Fantie and Goddard, 1982, McNaughton and Miller, 1984).

The current study addressed whether the sprouted SD pathway modulates CTD-evoked responses in the DG of rats with unilateral entorhinal lesion. The functional interaction of SD and CTD sprouting was assessed at 4, 15, and 90 days following entorhinal lesion. Using a paired-pulse paradigm, we systematically varied the interpulse interval (IPI) between SD and CTD stimulation to further assess if facilitation or inhibition of granule cell activity was dependent upon the temporal relationship of paired SD-CTD stimulation. We hypothesized that proliferating SD fibers following a unilateral entorhinal lesion would synergistically affect the excitability of the granule cells by the sprouted crossed entorhinal afferents (Ramirez, 1997, Ramirez, 2001).