Neuroscience Letters 517 (2012) 71–76 Contents lists available at SciVerse ScienceDirect Neuroscience Letters jou rn al h om epage: www.elsevier.com/locate/neulet Glutamatergic projection from the nucleus incertus to the septohippocampal system Ana Cervera-Ferri ∗ , Yasamin Rahmani, Sergio Martínez-Bellver, Vicent Teruel-Martí, Joana Martínez-Ricós Dept. Anatomia y Embriología Humana, Facultad de Medicina, Universidad de Valencia, Avd. Blasco Iba˜ nez, 15, 46010 Valencia, Spain a r t i c l e i n f o Article history: Received 2 January 2012 Received in revised form 3 April 2012 Accepted 5 April 2012 Keywords: Glutamate Septohippocampal Brainstem Theta rhythm Nucleus incertus a b s t r a c t Recent findings support a relevant role of the nucleus incertus in the control of the hippocampal activity through the modulation of theta rhythm. Previous studies from our group have shown that this nucleus is a critical relay between reticularis pontis oralis and the medial septum/diagonal band, regarded as the main activator and the pacemaker of the hippocampal oscillations, respectively. Besides, the nucleus incertus is highly linked to activated states related to the arousal response. The neurotransmission of the nucleus incertus, however, remains uncertain. Only GABA and the neu- romodulator relaxin 3 are usually considered to be involved in its contribution to the septohippocampal system. In this work, we have analyzed the existence of an excitatory projection from the nucleus incertus to the medial septum. We have found a group of glutamatergic neurons in the nucleus incertus project- ing to the medial septum. Moreover, we were able to describe a segregated distribution of calbindin and calretinin neurons. While calretinin expression was restricted to the nucleus incertus pars com- pacta, calbindin positive neurons where observed both in the pars dissipata and the pars compacta of the nucleus. The present work provides innovative data supporting an excitatory component in the pontoseptal pathway. © 2012 Elsevier Ireland Ltd. All rights reserved. 1. Introduction Hippocampal theta oscillation is involved in spatial cognition, memory processes, and sensorimotor integration [3,6]. Despite theta rhythmicity is the most prominent hippocampal field pattern, its function and generation are not fully understood [6,37]. A distributed neuronal network is involved in the generation of theta rhythmicity. The medial septum (MS) and vertical limb of the diagonal band of Broca (DBv) complex (MS/DBv) is an ultimate generator of theta waves [16]. The current theta model suggests that the septal pacemaker system has a crucial role, with choliner- gic and GABAergic populations projecting to the hippocampus [15]. More recently, glutamatergic neurons have been described as a third component of the neurochemical basis of the septohippocam- pal system, although its precise role still remains undetermined [12,27]. It is well documented that the brainstem reticular formation is directly involved in the control of the hippocampal theta [39,40]. The nucleus incertus (NI) constitutes one of the major inputs to ∗ Corresponding author. E-mail address: apicerfe@uv.es (A. Cervera-Ferri). the septohippocampal system via the ascending pathway through the medial longitudinal fasciculus (mlf). Tract-tracing studies sug- gest that this nucleus is a component of the extensive circuitry implied in the physiological processes related to cognitive process- ing [18,31]. Among its widespread connections, NI constitutes the main relay from the most efficient reticular activator of theta oscil- lation to the MS/DBv, the reticularis pontis oralis nucleus (RPO) [38]. Our group has also evidenced that NI is necessary for the reticular activation of hippocampal theta rhythm in anesthetized rats [9,30]. This effect depends, at least in part, on its projection over the MS/DBv [25]. However, the mechanisms underlying the functional relationship between the NI and the theta rhythm are virtually unknown. The NI is located in the mesopontine tegmentum, caudal to the dorsal raphe nucleus and adjacent to dorsal (DT) and laterodorsal (LDT) tegmental nuclei [18,28,31]. Two subregions can be distin- guished based on the cytoarchitecture and neuronal distribution: the medial NI pars compacta (NIc) and the lateral NI pars dissipata (NId) [18,31]. Until now, the characterization of the neurochemical phenotype of the NI neurons has led to include several neurotransmitters and neuromodulators [36]. Several neuropeptides, including cholecys- tokinin, neurotensin and ranatensin, have been immuno-localized 0304-3940/$ – see front matter © 2012 Elsevier Ireland Ltd. All rights reserved. http://dx.doi.org/10.1016/j.neulet.2012.04.014