Regular theta‐firing neurons in the nucleus incertus during sustained hippocampal activation

Abstract

This paper describes the existence of theta-coupled neuronal activity in the nucleus incertus (NI). Theta rhythm is relevant for cognitive processes such as spatial navigation and memory processing, and can be recorded in a number of structures related to the hippocampal activation including the NI. Strong evidence supports the role of this tegmental nucleus in neural circuits integrating behavioural activation with the hippocampal theta rhythm. Theta oscillations have been recorded in the local field potential of the NI, highly coupled to the hippocampal waves, although no rhythmical activity has been reported in neurons of this nucleus. The present work analyses the neuronal activity in the NI in conditions leading to sustained hippocampal theta in the urethane-anaesthetised rat, in order to test whether such activation elicits a differential firing pattern. Wavelet analysis has been used to better define the neuronal activity already described in the nucleus, i.e., non-rhythmical neurons firing at theta frequency (type I neurons) and fast-firing rhythmical neurons (type II). However, the most remarkable finding was that sustained stimulation activated regular-theta neurons (type III), which were almost silent in baseline conditions and have not previously been reported. Thus, we describe the electrophysiological properties of type III neurons, focusing on their coupling to the hippocampal theta. Their spike rate, regularity and phase locking to the oscillations increased at the beginning of the stimulation, suggesting a role in the activation or reset of the oscillation. Further research is needed to address the specific contribution of these neurons to the entire circuit.

Afferent and efferent connections of the interpeduncular nucleus with special reference to circuits involving the habenula and raphe nuclei

“…Importantly, our findings underscore that the main theme of IP connections are robust bidirectional connections with structures traditionally considered as key regulators of hippocampal theta activity, such as the SuM, MS/DB, and MnR. In addition, we further detailed reciprocal connections between IP and structures only recently acknowledged as major modulators of theta rhythm, such as the NI (Ma et al, ; Martínez‐Bellver et al, ; Nuñez, Cervera‐Ferri, Olucha‐Bordonau, Ruiz‐Torner, & Teruel, ) and LHb (Aizawa et al, ; Goutagny et al, ). Overall, this suggests a much broader role of IP in modulating theta rhythm than formerly assumed, which awaits further profound investigations by pharmacological/electrophysiological techniques.…”

“…Combined electrophysiological/lesion (Nuñez et al, ) and tract tracing (Teruel‐Martí et al, ) studies have characterized NI as a hub between PnO and MS/DB. Due to this unique set of connections, NI is considered a node that integrates behavioral activation with the hippocampal theta rhythm (Ma & Gundlach, ; Martínez‐Bellver et al, ; Ryan et al, ). However, it has been proposed that NI is not only involved in behavioral activation but also in active avoidance, with inputs from IP and MnR exerting inhibitory influence over NI (Goto et al, ).…”

“…Although, in the case of the septum, IP projections to this structure are differential from those originating from NI and MnR, since IP axons mainly innervate the LSV, whereas MnR (Vertes et al, 1999) and NI (Goto et al, 2001) most prominently target the MS. Combined electrophysiological/lesion (Nuñez et al, 2006) (Ma & Gundlach, 2015;Martínez-Bellver et al, 2015;Ryan et al, 2011). However, it has been proposed that NI is not only involved in behavioral activation but also in active avoidance, with inputs from IP and MnR exerting inhibitory influence over NI (Goto et al, 2001).…”

GABAergic Neurons in the Rat Medial Septal Complex Express Relaxin-3 Receptor (RXFP3) mRNA

“…Moreover, infusion of a Rln3 analog into the MS increased hippocampal theta rhythm, whereas infusion of a Rln3 receptor antagonist impaired the theta rhythm produced by novel environment exploration or RPO stimulation ( Ma et al, 2009 ). Different approaches in recent years have confirmed and extended these observations regarding the role of the NI and its associated peptide Rln3 in subcortical modulation of hippocampal theta rhythm, with an observed synchrony between the firing of NI neurons and different phases of hippocampal theta rhythm ( Ma et al, 2013 ; Martínez-Bellver et al, 2015 , 2017 ).…”

Conexôes aferentes e eferentes do núcleo interpeduncular com enfoque especial para os circuitos entre a habênula, o núcleo interpeduncular e os núcleos da rafe.

“…Em conjunto, esses estudos destacaram que, 1977;METZGER, 2012;SEGO et al, 2014). (NUÑEZ et al, 2006;MA et al, 2009;MARTÍNEZ-BELLVER et al, 2015) e a LHb (AIZAWA et al, 2013;GOUTAGNY et al, 2013). Ao todo, isso sugere um papel mais amplo do IP na modulação do ritmo teta do que anteriormente relatado que deve em estudos futuros também sendo investigado através de técnicas farmacológicas e eletrofisiológicas.…”