Elsevier

Brain Research Reviews

Volume 24, Issues 2–3, 19 September 1997, Pages 115-195
Brain Research Reviews

Full-length review
Connections of the rat lateral septal complex1

https://doi.org/10.1016/S0165-0173(97)00009-XGet rights and content

Abstract

The organization of lateral septal connections has been re-examined with respect to its newly defined subdivisions, using anterograde (PHAL) and retrograde (fluorogold) axonal tracer methods. The results confirm that progressively more ventral transverse bands in the hippocampus (defined by the orientation of the trisynaptic circuit) innervate progressively more ventral, transversely oriented sheets in the lateral septum. In addition, hippocampal field CA3 projects selectively to the caudal part of the lateral septal nucleus, which occupies topologically lateral regions of the transverse sheets, whereas field CA1 and the subiculum project selectively to the rostral and ventral parts of the lateral septal nucleus, which occupy topologically medial regions of the transverse sheets. Finally, the evidence suggests that progressively more ventral hippocampal bands innervate progressively thicker lateral septal sheets. In contrast, ascending inputs to the lateral septum appear to define at least 20 vertically oriented bands or subdivisions arranged orthogonal to the hippocampal input (Risold, P.Y. and Swanson, L.W., Chemoarchitecture of the rat lateral septal nucleus, Brain Res. Rev., 24 (1997) 91–113). Hypothalamic nuclei forming parts of behavior-specific subsystems share bidirectional connections with specific subdivisions of the lateral septal nucleus (especially the rostral part), suggesting that specific domains in the hippocampus may influence specific hypothalamic behavioral systems. In contrast, the caudal part of the lateral septal nuceus projects to the lateral hypothalamus and to the supramammillary nucleus, which projects back to the hippocampus and receives its major inputs from brainstem cell groups thought to regulate behavioral state. The neural system mediating defensive behavior shows these features rather clearly, and what is known about its organization is discussed in some detail.

Introduction

The connections of the septal region have been of special interest because it shares bi-directional interactions with the hippocampus. However, a massive projection to the diencephalon from the septum, which courses through the medial forebrain bundle, was also characterized during the 1950's and 1960's with degeneration techniques (for a review, see [110]). Unfortunately, it was unclear from this work which of these projections arise in the medial septal complex and which arise in the lateral septal nucleus (LS). In fact, even septal and hippocampal projections to the diencephalon were difficult to distinguish because experimental lesions interrupted various components of the fornix system arising in the subicular complex, caudal to both the septum and hippocampus.

The introduction of the autoradiographic technique during the 1970's provided new insight into the organization of projections from the LS. In particular, Swanson and Cowan [156]distinguished efferents arising in the LS from those arising in the medial septal nucleus and in the various components of the hippocampal formation. Furthermore, because small injections were placed within the boundaries of cytoarchitectonically defined parts of the LS, it became clear that projections to the hypothalamus are topographically organized such that a dorsal part innervates the lateral hypothalamus and a ventral part preferentially innervates the medial zone nuclei.

No systematic analyses of the anatomical organization of the LS have since been carried out in the rat. In 1991, however, Staiger and Nürnberger 140, 141used the PHAL method to analyze LS projections in the guinea pig. By and large, their results confirmed those of Swanson and Cowan [156]. The main differences can be explained by the advantages of the PHAL method: greater sensitivity, smaller injection sites, and the ability to visualize boutons-of-passage. One major difference was the observation by Staiger and Nürnberger that rostral regions of the intermediate LS project in a way similar to that of the ventral LS, whereas caudal regions of the intermediate LS display projections similar to those of the dorsal LS. This suggested that a rostral to caudal gradient of projections arising in the intermediate LS is superimposed on the dorsoventral gradient previously described by Swanson and Cowan for the nucleus as a whole. These observations, combined with the finding of Swanson and Cowan 154, 155that the rostral third of the LS receives cortical inputs from only hippocampal field CA1 and the subiculum (while the rest of the LS also receives cortical inputs from field CA3), clearly suggest a more complex organization of LS connections.

In the accompanying paper [117]on the chemoarchitecture of the LS, a new parcellation of the LS is presented in detail. In agreement with a previous short report [116], we now suggest that the LS can be divided into three major parts: a ventral part (LSv) characterized by abundant expression of estrogen receptor mRNA and connected with the medial preoptic nucleus-periventricular zone of the hypothalamus; a rostral part (LSr) characterized by the expression of enkephalin and neurotensin mRNAs and connected mainly with hypothalamic medial zone nuclei (especially the anterior hypothalamic nucleus); and a caudal part (LSc) characterized by the expression of somatostatin mRNA and connected with the hypothalamic lateral zone and lateral supramammillary nucleus. This paper is the second part of a review of the anatomical organization of the LS, and is concerned with the connections of the chemoarchitectonically defined parts of the nucleus as currently understood. Projections from the septohippocampal and septofimbrial nuclei, which may form part of the same complex, will also be considered.

Section snippets

Materials and methods

Adult male rats (300–350 g; Harlan Sprague–Dawley) were anesthetized with a mixture of ketamine and xylazine (v/v; 1 ml/kg body weight). For PHAL experiments (n=12), a 2.5% solution of the lectin (Vector laboratories) was prepared in 0.01 M NaPBS, and was injected iontophoretically through a glass micropipette (tip diameter: 10–15 μm) into the LS and surrounding areas. For injections, a positive current (5 μA, 7 s off/7 s on) was applied for 15–20 min. Similar procedures were used for the

General organization of projections from LS to hypothalamus

As outlined in the Introduction, the LS sends topographically organized projections to the hypothalamus, with the neurons generating these projections reportedly arranged in dorsoventral and rostrocaudal gradients. Furthermore, recent preliminary evidence based on PHAL experiments [116]suggests that LS projections may follow pathways to the hypothalamus that are organized from medial to lateral (see also 140, 141). The first set of experiments was designed to examine this possibility in more

General organization of LS connections, possible neurotransmitters, and functional considerations

Expanding on previous autoradiographic and PHAL anterograde tracing studies 47, 74, 90, 140, 141, 156, the present results indicate that the LS (along with the septofimbrial and septohippocampal nuclei) sends the vast majority of its projections in a topographically organized way to ventral pallidal regions (medial septal/nucleus of the diagonal band complex and substantia innominata), most of the hypothalamus, parts of the midline thalamus, and limited parts of the brainstem. In turn, the LS

Abbreviations

AAAanterior amygdaloid area
acanterior commissure
ACBnucleus accumbens
acoanterior commissure, olfactory limb
actanterior commissure, temporal limb
ADanterodorsal nucleus thalamus
ADPanterodorsal preoptic nucleus
AHAmmon's horn
AHAanterior hypothalamic area
AHNanterior hypothalamic nucleus
AHNaanterior hypothalamic nucleus, anterior part
AHNcanterior hypothalamic nucleus, central part
AHNdanterior hypothalamic nucleus, dorsal part
AHNpanterior hypothalamic nucleus, posterior part
alvalveus
AManteromedial

Acknowledgements

This work was supported in part by National Institutes of Health Grant NS16686.

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