Modulation of hippocampal theta oscillations and spatial memory by relaxin-3 neurons of the nucleus incertus

Abstract

Hippocampal theta rhythm is thought to underlie learning and memory, and it is well established that ''pacemaker'' neurons in medial septum (MS) modulate theta activity. Recent studies in the rat demonstrated that brainstem-generated theta rhythm occurs through a multisynaptic pathway via the nucleus incertus (NI), which is the primary source of the neuropeptide relaxin-3 (RLN3). Therefore, this study examined the possible contribution of RLN3 to MS activity, and associated hippocampal theta activity and spatial memory. In anesthetized and conscious rats, we identified the ability of intraseptal RLN3 signaling to modulate neuronal activity in the MS and hippocampus and promote hippocampal theta rhythm. Behavioral studies in a spontaneous alternation task indicated that endogenous RLN3 signaling within MS promoted spatial memory and exploratory activity significantly increased c-Fos immunoreactivity in RLN3-producing NI neurons. Anatomical studies demonstrated axons/terminals from NI/RLN3 neurons make close contact with septal GABAergic (and cholinergic) neurons, including those that project to the hippocampus. In summary, RLN3 neurons of the NI can modulate spatial memory and underlying hippocampal theta activity through axonal projections to pacemaker neurons of the MS. NI/RLN3 neurons are highly responsive to stress and express corticotropin-releasing factor type-1 receptors, suggesting that the effects observed could be an important component of memory processing associated with stress responses.

Relaxin-3/RXFP3 system regulates alcohol-seeking

“…RXFP3 antagonism at equivalent doses did not impact general arousal or activity or procedural memory for lever pressing, which is an important possibility to exclude, given that previous studies have suggested a role for relaxin-3 in arousal, learning, and memory (28,29,44). RXFP3 antagonism produced no effect on food intake in rats following overnight restricted food access, suggesting no major impact of the relaxin-3/RXFP3 system on appetitive drive stimulated by the need for food in this paradigm.…”

“…However, there was a significant difference between vehicle-injected rats and lever pressing during extinction (P < 0.01); n = 23 rats. (B) There was no significant effect on inactive lever presses for cue-induced reinstatement in sucrose-trained rats (F (2,44) = 0.68, P = 0.51). (C) Although there was a significant effect of treatment on active lever presses for stress-induced reinstatement in sucrose-trained rats (F (2,22) = 16.60, P < 0.0001), post hoc tests indicated that the lever pressing for vehicleinjected rats was not significantly different from lever pressing of R3(B1-22) R-treated rats.…”

Distribution and targets of the relaxin‐3 innervation of the septal area in the rat

“…Furthermore, because the nucleus incertus is a RLN3 locus in the brain, we hypothesized that RLN3 might contribute to these effects. Consistent with the presence of RLN3 and RXFP3 in the medial septum, injections of a selective RXFP3 agonist peptide (R3/I5; Lui et al, 2005) into this area increased theta activity of the hippocampal field potential in urethane-anesthetized rats, which was significantly attenuated by prior injection of a selective RXFP3 antagonist peptide, R3(B∆23-27)R/I5 (Kuei et al, 2007) (Ma et al, 2009b). R3/I5 infusion into the medial septum also increased hippocampal theta in rats in a familiar home cage environment, whereas R3(B∆23-27)R/I5 decreased hippocampal theta in rats exploring a novel enriched context (Ma et al, 2009b).…”

“…Consistent with the presence of RLN3 and RXFP3 in the medial septum, injections of a selective RXFP3 agonist peptide (R3/I5; Lui et al, 2005) into this area increased theta activity of the hippocampal field potential in urethane-anesthetized rats, which was significantly attenuated by prior injection of a selective RXFP3 antagonist peptide, R3(B∆23-27)R/I5 (Kuei et al, 2007) (Ma et al, 2009b). R3/I5 infusion into the medial septum also increased hippocampal theta in rats in a familiar home cage environment, whereas R3(B∆23-27)R/I5 decreased hippocampal theta in rats exploring a novel enriched context (Ma et al, 2009b). These data support a significant contribution of nucleus incertus and RLN3 inputs to the septum in regulating a fundamental brain activity and associated complex behaviors, and therefore characterization of the anatomical and cellular interactions between these inputs and their targets is required.…”

“…Earlier we described the ultrastructure of anterogradely-labeled terminals in the medial septum following mR injections into the nucleus incertus (Ma et al, 2009b). In this study, we examined RLN3-immunostaining in ultrathin sections of the medial septum using electron microscopy.…”