Extrinsic and intrinsic substance P innervation of the rat lateral septal area calbindin cells

Neurokinin receptor mechanisms in forebrain medial septum modulate nociception in the formalin model of inflammatory pain

The present study has explored the hypothesis that neurokinin1 receptors (NK1Rs) in medial septum (MS) modulate nociception evoked on hind paw injection of formalin. Indeed, the NK1Rs in MS are localized on cholinergic neurons which have been implicated in nociception. In anaesthetized rat, microinjection of L-733,060, an antagonist at NK1Rs, into MS antagonized the suppression of CA1 population spike (PS) evoked on peripheral injection of formalin or on intraseptal microinjection of substance P (SP), an agonist at NK1Rs. The CA1 PS reflects the synaptic excitability of pyramidal cells in the region. Furthermore, microinjection of L-733,060 into MS, but not LS, attenuated formalin-induced theta activation in both anaesthetized and awake rat, where theta reflects an oscillatory information processing by hippocampal neurons. The effects of L-733,060 on microinjection into MS were nociceptive selective as the antagonist did not block septo-hippocampal response to direct MS stimulation by the cholinergic receptor agonist, carbachol, in anaesthetized animal or on exploration in awake animal. Interestingly, microinjection of L-733,060 into both MS and LS attenuated formalin-induced nociceptive flinches. Collectively, the foregoing novel findings highlight that transmission at NK1R provide an affective valence to septo-hippocampal information processing and that peptidergic transmission in the septum modulates nociceptive behaviours.

Substance P: A neuropeptide involved in the psychopathology of anxiety disorders

Substance P (SP) is the most widely distributed neuropeptide in central nervous system (CNS) where it participates in numerous physiological and pathophysiological processes including stress and anxiety related behaviors. In line with this notion, brain areas that are thought to be involved in anxiety regulation contains SP and its specific NK1 receptors. SP concentration in different brain regions alters with the exposure of stressful stimulus and affected NK1 receptor binding is observed. SP is released in response to a stressor, which produces anxiogenic effects via activation of hypothalamic-pituitary-adrenal (HPA) axis, resulting in the liberation of cortisol. Moreover, SP is also involved in the activation of the sympathetic nervous system via stimulation of locus coeruleus (LC). This sympathetic surge initiates cortisol discharge by activation of HPA axis, representing the indirect anxiogenic effect of SP. Besides the aforementioned regions, SP also has an impact on other brain regions known to be involved in stress and anxiety mechanisms, including amygdala, lateral septum (LS), periaqueductal gray (PAG), ventromedial nucleus of the hypothalamus (VMH), and bed nucleus of stria terminalis (BNST). Thus, SP acts as an important neuromodulator in various brain regions in stress and anxiety response. Consistent with the above statement, SP makes a robust link in the psychopathology of anxiety disorders. As SP concentration is found elevated in stressed conditions, several studies have reported that the pharmacological antagonism or genetic depletion of NK-1 receptors results in the anxiolytic response making them a suitable therapeutic target for the treatment of stress and anxiety related disorders.

Anxiolytic-like effects of the neurokinin 1 receptor antagonist GR-205171 in the elevated plus maze and contextual fear-potentiated startle model of anxiety in gerbils

Gerbils show a neurokinin (NK)1 receptor pharmacological profile, which is similar to that observed in humans, and thus have become a commonly used species to test efficacy of NK1 receptor antagonists. The aim of this study was to determine whether systemic administration of the NK1 receptor antagonist GR-205171 produced anxiolytic-like effects in the elevated plus maze and in a novel contextual conditioned fear test using fear-potentiated startle (FPS). On the elevated plus maze, treatment with GR-205171 at 0, 0.3, 1.0, and 5.0 mg/kg doses, 30 min before testing produced anxiolytic-like effects in an increasing dose-response manner as measured by the percentage of open arm time and percentage of open arm entries. For contextual fear conditioning, gerbils were given 10 unsignaled footshocks (0.6 mA) at a 2-min variable interstimulus interval in a distinctive training context. Twenty-four hours after training, gerbils received treatment of GR-205171 at 0, 0.3, 1.0, and 5.0 mg/kg doses, 30 min before testing in which startle was elicited in the same context in which they were trained. Contextual FPS was defined as an increase in startle over pretraining baseline values. All drug dose levels (0.3, 1.0, and 5.0 mg/kg) significantly attenuated contextual FPS when compared with the vehicle control group. A control group, which received testing in a different context, showed little FPS. These findings support other evidence for anxiolytic activity of NK1 receptor antagonists and provide a novel conditioned fear test that may be an appropriate procedure to test other NK1 antagonists for preclinical anxiolytic activity in gerbils.

Topographical and quantitative distribution of the projecting neurons to main divisions of the septal area

OBJECTIVE

Septal area is a limbic structure that is involved in the regulation of several autonomic, learning-related and behavioral functions. Participation of this area in various physiologic functions is indicative of its extensive connections with different brain areas. It contains two major divisions: lateral septum (LS) and medial septum/diagonal band of Broca (MS/DBB). In the present work, we examined topographical distribution of projecting neurons to these divisions and quantitatively verified them.

METHODS

Horseradish peroxidase (HRP) retrograde tract tracing was performed.

RESULTS

Our results show that about two-thirds of projections to the septal area terminate in the LS. They mostly originate ipsilaterally from the septal area itself (8%), hippocampal formation (38%), non-specific thalamic nuclei (23%), lateral pre-optic area, lateral hypothalamus, perifornical area and mammillary complex in hypothalamus (20%), ventral tegmental area, raphe and tegmental nuclei, and also locus coeruleus in brainstem (10%). Most afferents to the MS come ipsilaterally from the septal area itself (18%), hippocampal formation (12%), lateral pre-optic area, lateral hypothalamus and mammillary complex in hypothalamus (42%), ventral tegmental area, raphe and tegmental nuclei, central gray matter and also locus coeruleus in brainstem (20%). Some afferents to the septal area originate contralaterally from the lateral hypothalamus, supramammillary area, raphe nuclei and locus coeruleus.

DISCUSSION

Afferents from the interanterodorsal and mediodorsal thalamic nuclei, which increase the role of the septal area in arousal and awareness, are reported for the first time. Projecting cells to the MS support the learning-related function of this area. Projecting cells to the LS that are more scattered throughout the brain indicate its involvement in more diverse functions.

Neurokinin1 antagonists potentiate antidepressant properties of serotonin reuptake inhibitors, yet blunt their anxiogenic actions: a neurochemical, electrophysiological, and behavioral characterization

Though neurokinin(1) (NK(1)) receptor antagonists are active in experimental models of depression, clinical efficacy has proven disappointing. This encourages interest in association of NK(1) receptor blockade with inhibition of serotonin (5-HT) reuptake. The selective NK(1) antagonist, GR205171, dose-dependently enhanced citalopram-induced elevations of extracellular levels of 5-HT in frontal cortex, an action expressed stereospecifically vs its less active distomer, GR226206. Further, increases in 5-HT levels in dorsal hippocampus, basolateral amygdala, nucleus accumbens, and striatum were likewise potentiated, and GR205171 similarly facilitated the influence of fluoxetine upon levels of 5-HT, as well as dopamine and noradrenaline. In parallel electrophysiological studies, the inhibitory influence of citalopram and fluoxetine upon raphe-localized serotonergic neurones was stereospecifically blunted by GR205171. Antidepressant actions of citalopram in a forced-swim test in mice were stereospecifically potentiated by GR205171, and it also enhanced attenuation by citalopram of stress-related ultrasonic vocalizations in rats. Further, GR205171 and citalopram additively abrogated the advance in circadian rhythms provoked by exposure to light in hamsters. By contrast, GR205171 stereospecifically blocked anxiogenic actions of citalopram in social interaction procedures in rats and gerbils, and stereospecifically abolished facilitation of fear-induced foot tapping by fluoxetine in gerbils. By analogy to GR205171, a further NK(1) antagonist, RP67580, enhanced the influence of citalopram upon frontocortical levels of 5-HT and potentiated its actions in the forced swim test. In conclusion, NK(1)receptor blockade differentially modulates functional actions of SSRIs: antidepressant properties are reinforced, whereas anxiogenic effects are attenuated. Combined NK(1) receptor antagonism/5-HT reuptake inhibition may offer advantages in the management of depressed and anxious states.

Neurokinin 1 receptor antagonism promotes active stress coping via enhanced septal 5-HT transmission

Antagonists of the substance P (SP) preferring neurokinin 1 receptor (NK1R) represent a promising novel class of drugs for the treatment of stress-related disorders such as depression and anxiety disorders; however, the involved neuronal pathways releasing SP in response to stressors are ill defined. By using in vivo microdialysis in combination with a highly sensitive and selective radioimmunoassay we found that exposure to forced swim stress increased SP release in the rat lateral septum (LS), a key area in processing emotions and stress responses. Acute administration of the selective NK1R antagonist L-822429 injected either systemically or locally into the LS reduced passive and facilitated active stress-coping strategies in the forced swim test. This effect seems to be mediated by enhanced intraseptal serotonergic transmission via serotonin (5-HT)1A receptors since NK1R blockade reversed the swim stress-induced decrease to an increase in extracellular 5-HT efflux, and furthermore the behavioral effects of L-822429 were blocked by intraseptal 5-HT1A receptor antagonism. A direct heterosynaptic regulation by NK1R on 5-HT release from serotonergic fibers was ruled out by immunocytochemistry at the light and electron microscopic level indicating involvement of GABAergic interneuron(s) in this interaction. Taken together, our data identify the LS as a critical brain area for the involvement of SP transmission in the modulation of stress responses and demonstrate that NK1R blockade can elicit a functionally significant facilitatory effect on 5-HT transmission, which does not necessarily involve the previously proposed interaction with neuronal firing at the cell body level of raphe neurons.

Neurokinin-1 receptor antagonism by SR140333: enhanced in vivo ACh in the hippocampus and promnestic post-trial effects

Substance P (SP) has memory-promoting, reinforcing and anxiolytic-like effects when applied systemically or centrally. Such effects may be mediated by the neurokinin-1 (NK-1) receptor, since SP preferentially binds to this receptor. We measured the effects of a selective non-peptide NK-1 receptor antagonist, SR140333 (1, 3 and 9 mg/kg i.p.) on ACh levels in frontal cortex, amygdala and hippocampus by microdialysis and HPLC. Levels of ACh in the hippocampus increased dose-dependently immediately after treatment. The same doses of SR140333 given post-trial had minor facilitative effects on inhibitory avoidance learning and open-field habituation, but did not have reinforcing effects in a conditioned place preference (CPP) task. The selective action of NK-1 receptor antagonism on hippocampal ACh may be related to its positive influence on learning.

Efferent connections of septal nuclei of the domestic chick (Gallus domesticus): An anterograde pathway tracing study with a bearing on functional circuits

Small iontophoretic injections of the anterograde tracer Phaseolus vulgaris leucoagglutinin were placed in different subregions of the septum of domestic chicks. The main targets of septal projections comprised the ipsi- and contralateral septal nuclei, including the nucleus of the diagonal band, basal ganglia, including the ventral paleostriatum, lobus parolfactorius, nucleus accumbens, and olfactory tubercle, archistriatum, piriform cortex, and anterior neostriatum. Further diencephalic and mesencephalic septal projections were observed in the ipsilateral preoptic region, hypothalamus (the main regions of afferentation comprising the lateral hypothalamic nuclei, ventromedial, paraventricular and periventricular nuclei, and the mammillary region), dorsal thalamus, medial habenular and subhabenular nuclei, midbrain central gray, and ventral tegmental area. Contralateral projections were also encountered in the septal nuclei, ventral paleostriatum, periventricular and anteromedial hypothalamic nuclei, suprachiasmatic nucleus, and the lateral hypothalamic area. Avian septal efferents are largely similar to those of mammals, the main differences being a relatively modest hippocampal projection arising mainly from the nucleus of the diagonal band (as confirmed by a specific experiment with the retrograde pathway tracer True blue), the lack of interpeduncular projection, and a greater contingent of amygdalar efferents arising from the lateral septum rather than the nucleus of the diagonal band. This pattern of connectivity is likely to reflect an important role of the avian septal nuclei in the coordination of limbic circuits and the integration of a wide variety of information sources modulating the appropriate behavioral responses: attention and arousal level, memory formation, hormonally mediated behaviors, and their affective components (such as ingestive, reproductive, and parental behaviors), social interaction, locomotor modulation, and circadian rhythm.

Intrinsic vesicular glutamate transporter 2-immunoreactive input to septohippocampal parvalbumin-containing neurons: Novel glutamatergic local circuit cells

Glutamatergic influence on the medial septum diagonal band of Broca complex (MSDB) is a crucial and powerful driver of hippocampal theta rhythm and associated memory processes, in the rat. The recent discovery of vesicular glutamate transporters (VGLUT) provided a specific marker for glutamatergic neuronal elements. Therefore, this study aimed to address two specific questions: (1) do glutamatergic axons innervate MSDB gamma-aminobutyric acid (GABA)ergic, parvalbumin (PV)-containing septohippocampal neurons that are known to have a great influence on the electric activity of the hippocampus; and (2) is the origin of these glutamatergic axons extrinsic and/or intrinsic to the septum. The results of the correlated light and electron microscopic double-labeling immunohistochemistry for VGLUT2 and PV, and single immunostaining for VGLUT2 in colchicine-treated animals, showed that (1) VGLUT2-containing boutons establish asymmetric synaptic contacts with PV-positive perikarya and dendrites; (2) a large population of VGLUT2-immunoreactive neurons is located primarily in the posterior division of the septum; and (3) following surgical fimbria/fornix transection and septal undercut, most VGLUT2-containing axons, including those terminating on MSDB PV cells, remains intact. The latter two observations suggest that the major portion of MSDB glutamate axons have an intraseptal origin and raise a novel functional aspect of glutamatergic cells as local circuit neurons. A constant impulse flow in the septohippocampal GABA pathway is essential for the generation of theta rhythm. Thus, the heavy glutamatergic innervation of these septohippocampal GABA cells establishes the morphological basis for the powerful glutamatergic influence upon theta rhythm and hippocampus-associated memory processes.

Control by tachykinin NK(2) receptors of CRF(1) receptor-mediated activation of hippocampal acetylcholine release in the rat and guinea-pig

In vivo microdialysis was employed to explore the effects of different selective non-peptides NK(1),NK(2) and NK(3) receptor antagonists on the corticotropin releasing factor (CRF)-induced release of acetylcholine (ACh) in the hippocampus of rats and guinea-pigs. In both species, the intracerebroventricular (i.c.v.) administration of CRF produced a time- and dose-dependent increase in hippocampal ACh release that was totally suppressed by an intraperitoneally (i.p.) pretreatment with the selective non-peptide CRF(1) receptor antagonist antalarmin (30 mg/kg). Pretreatment with the selective NK(2) receptor antagonist SR48968 (1mg/kg, i.p.) significantly reduced the increase of ACh induced by CRF. In contrast, its low-affinity enantiomer SR48965 (1mg/kg, i.p.) or the NK(1) receptor antagonist, GR205171 (1mg/kg, i.p.) did not exert any antagonist effect. Moreover, administration of the selective NK(3) receptor antagonist SR142801 (1mg/kg, i.p.) did not significantly reduce the CRF-induced hippocampal ACh release in guinea-pigs (the only species studied). The selective activity of SR48968 versus GR205171 or SR142801 indicates that NK(2) receptors play a major role in the control of CRF-induced hippocampal ACh release. Moreover, in freely moving rats, two sessions of stroking of the neck and back of the rat for 30 min, at 90 min intervals, known to be a stressful stimulus, produced a marked and reproducible increase in hippocampal ACh release. This effect was prevented by the administration of the two selective non-peptide CRF1 and NK(2) receptor antagonists antalarmin (30 mg/kg, i.p.) and SR48968 (1mg/kg, i.p.), respectively. This suggests that stress-induced activation of the hippocampal ACh system may be under the control of both endogenously released CRF and NKA, and opens the possibility of the existence of a functional interplay between the pathways containing these peptides as we observed in our experiments on anaesthetized animals.

Principles of rat subcortical forebrain organization: a study using histological techniques and multiple fluorescence labeling

In the present study, we introduce new views on neuro- and chemoarchitectonics of the rat forebrain subcortex deduced from traditional and current concepts of anatomical organization and from our own results. It is based on double and triple immunofluorescence of markers for transmitter-related enzymes, calcium-binding proteins, receptor proteins, myelin basic protein (MBP) and neuropeptides, and on histological cell/myelin stains. The main findings can be summarized as follows: (i) the dorsal striatum of rat and other myomorph rodents reveals a small caudate equivalent homotopic to the caudate nucleus (C) of other mammals, and a large putamen (Pu). (ii) Shell and core can be distinguished also in the 'rostral pole' of nucleus accumbens (ACC) with the calretinin/calbindin and neuropeptide Y (NPY) immunostaining. The shell reveals characteristics of a genuine striatal but not of an extended amygdala (EA) subunit. (iii) EA and lateral septum show striking similarities in structure and fiber connections and may therefore represent a separate parastriatal complex. (iv) The meandering dense layer (DL) of olfactory tubercle (OT) forms longitudinal gyrus- and sulcus-like structures converging in its rostral pole. (v) The core regions of the islands of Calleja that border the ventral pallidum (VP) sharing some of its features are invaded by myelinated fibers of the medial forebrain bundle (MFB). The island of Calleja magna is also apposed to an inconspicuous, slender dorsal appendage of VP. (vi) The VP is composed of a large dorsal reticulated part traversed by the myelinated GABAergic parvalbumin-immunoreactive axons of the MFB and a slender ventral non-reticulate part close to the islands of Calleja. (vii) Considering their close association to the limbic system, ventral striatum (VS) and VP may represent the oldest part of basal ganglia, whereas dorsal striatopallidal subunits were progressively developed in parallel to the growing neocortical influence on motor behavior.

Characterization of the extent of pontomesencephalic cholinergic neurons' projections to the thalamus: comparison with projections to midbrain dopaminergic groups

We sought to determine whether pontomesencephalic cholinergic neurons which we have been shown previously to project to the substantia nigra and ventral tegmental area also contribute to the thalamic activation projection from the pedunculopontine and laterodorsal tegmental nuclei. Retrograde tracing, immunohistochemical localization of choline acetyltransferase and statistical methods were used to determine the full extent of the cholinergic projection from the pedunculopontine and laterodorsal tegmental nuclei to the thalamus. Progressively larger Fluoro-Gold injections in to the thalamus proportionally labeled increasing numbers of pontomesencephalic cholinergic cells both ipsi- and contralaterally in the pedunculopontine and laterodorsal tegmental nuclei. Multiple large thalamic injections left only a small fraction of the ipsilateral pontomesencephalic cholinergic group unlabeled. This small remainder did not correspond to the populations which project to the substantia nigra and ventral tegmental area, thereby indicating that substantia nigra- and ventral tegmental area-projecting cholinergic neurons must also project to the thalamus. We examined whether there existed any set of cholinergic neurons in the pedunculopontine and laterodorsal tegmental nuclei which did not innervate a thalamic target. The distribution of descending projections of the pedunculopontine and laterodorsal tegmental nuclei demonstrated that the unlabeled remainder cannot correspond to a purely descending group. We also show that substance P-positive cholinergic cells in the laterodorsal tegmental nucleus project to the thalamus. Further studies demonstrated that the small population of cholinergic cells left unlabeled from the thalamus were the smallest sized cholinergic cells, and included two groups of small, light-staining cholinergic cells located in the parabrachial area and central gray, adjacent to the main pedunculopontine and laterodorsal tegmental nuclei cholinergic groups. These small cells, in contrast to thalamic-projecting cholinergic cells, did not stain positively for reduced nicotinamide adenine dinucleotide phosphate-diaphorase. Taken together, these results indicated that all of the reduced nicotinamide adenine dinucleotide phosphate diaphorase-positive/choline acetyltransferase-positive neurons of the pedunculopontine/laterodorsal tegmental nuclei ascend to innervate some portion of the thalamus, in addition to the other targets they innervate. These findings indicate that the diverse physiological and behavioral effects attributed to the activity of pontomesencephalic cholinergic neurons should not be dissociated from their activating effects in the thalamus.

Ultrastructural evidence that substance P neurons form synapses with noradrenergic neurons in the A7 catecholamine cell group that modulate nociception

Potent antinociception can be produced by electrical stimulation of spinally projecting noradrenergic neurons in the A7 catecholamine cell group and this effect is blocked by intrathecal injection of alpha2-adrenoceptor antagonists. Microinjection of substance P near A7 neurons also produces antinociception that is blocked by intrathecal injection of alpha2-adrenoceptor antagonists. These observations suggest that substance P produces antinociception by activating noradrenergic A7 neurons. However, it is not known whether this effect of substance P is produced by a direct or an indirect action on A7 neurons. Although light microscopic studies have demonstrated the existence of both substance P-containing axon terminals and neurokinin-1 receptors in the region of the A7 cell group, it is not known whether substance P terminals form synapses with noradrenergic A7 neurons. These experiments used double-labeling immunocytochemical methods and electron microscopic analysis to determine whether substance P-containing axons form synapses with noradrenergic neurons in the A7 cell group. Pre-embedding immunocytochemistry, combined with light and electron microscopic analysis, was used to provide ultrastructural evidence for synaptic connections between substance P-immunoreactive terminals labeled with immunoperoxidase and tyrosine hydroxylase-immunoreactive A7 neurons labeled with silver-enhanced immunogold. Tyrosine hydroxylase labeling was found in perikarya and dendrites in the A7 region, and substance P labeling was found in axons and synaptic terminals. Substance P-labeled terminals formed asymmetric synapses with tyrosine hydroxylase-labeled dendrites, but only a few of these were present on tyrosine hydroxylase-labeled somata. Substance P-labeled terminals also formed asymmetric synapses with unlabeled dendrites, and many unlabeled terminals formed both symmetric and asymmetric synapses with tyrosine hydroxylase-labeled dendrites. These results demonstrate that substance P neurons form a significant number of synapses with the dendrites of noradrenergic A7 neurons and support the conclusion that microinjection of substance P in the A7 cell group produces antinociception by direct activation of spinally projecting noradrenergic neurons.

A tachykinin NK1 receptor antagonist, CP-122,721-1, attenuates kainic acid-induced seizure activity

Substance P (SP) can play an important role in neuronal survival. To analyze the role of SP in excitotoxicity, kainic acid (KA) was administered to rats and in situ hybridization was used to analyze the levels of the SP encoding preprotachykinin-A (PPT-A) mRNA in striatal and hippocampal subregions 1, 4, and 24 h and 7 days after KA. In striatum and piriform cortex, PPT-A mRNA peaked 4 h after KA while in hippocampus, levels peaked after 24 h. KA caused seizures and neuronal toxicity as indicated by a reduction of the number of neurons in the hippocampal CA1 subregion after 7 days. KA was later administered alone or following pretreatment with the tachykinin NK1 receptor antagonist CP-122,721-1 (0.3 mg/kg). The pretreatment decreased seizure activity and a negative correlation was found between seizure activity and survival of CA1 neurons. Conclusively, treatment with CP-122,721-1 has a seizure inhibiting property and may possibly counteract KA-induced nerve cell death in CA1.

Expression and presence of septal neurokinin-2 receptors controlling hippocampal acetylcholine release during sensory stimulation in rat

We examined the expression and presence of NK2 receptors in the septal area of rat brain, and investigated their functional role in the regulation of the septohippocampal cholinergic system. Using reverse transcription-polymerase chain reaction (RT-PCR) analysis, we showed the presence of NK2 receptor mRNA expression in the septal area, and detected septal NK2 binding sites by using a fluorescent-tagged neurokinin A (NKA) derivative. In vivo microdialysis was employed to explore the functional role of NK2 receptors in the release of hippocampal acetylcholine evoked by tactile stimulation in freely moving rats. Two sessions of stroking of the neck and back of the rat for 30 min, at 90 min intervals, produced a marked and reproducible increase in hippocampal acetylcholine release. This effect was dose-dependently prevented by intraperitoneal administration of the two selective non-peptide tachykinin NK2 receptor antagonists SR144190 (0.03-0.3 mg/kg, i.p.) and SR48968 (0.3 and 1 mg/kg, i.p.), but not by the inactive enantiomer of SR48968 (SR48965, 1 mg/kg) nor by the two non-peptide NK1 receptor antagonists SR140333 (3 mg/kg, i.p.) and GR205171 (1 mg/kg, i.p.). Furthermore, the intraseptal application of SR144190 (10(-8) M) reduced the sensory response. Finally, intraseptal perfusion of neurokinin A (0.01-10 microM) in anaesthetized rats produced a concentration-dependent increase in hippocampal acetylcholine release. The response to neurokinin A (0.1 microM) was prevented by SR144190 (0.03-0.3 mg/kg, i.p.) and SR48968 (0.3-1 mg/kg, i.p.). In conclusion, this study provides direct evidence for the role of endogenous NKA/substance P, through the activation of NK2 receptors, in regulating the septohippocampal cholinergic function.

Connections of the rat lateral septal complex

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 nucleus 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.

Chemoarchitecture of the rat lateral septal nucleus

The distribution of neurons and terminal fields that contain a variety of neurotransmitters and steroid hormone receptors has been examined with in situ hybridization and immunohistochemistry in closely spaced series of sections throughout the rostrocaudal extent of the rat lateral septal nucleus, as well as the adjacent septohippocampal and septofimbrial nuclei. The results indicate that the lateral septal nucleus is divided into major rostral, caudal, and ventral parts that differ from the widely used cytoarchitectonic parcellation into dorsal, intermediate, and ventral parts. Furthermore, the rostral, caudal, and ventral parts are turn divided into about 20 zones, regions, and domains on the basis of differential terminal fields and neurons that express particular neuropeptides and steroid hormone receptors. In general, the small zones and regions form dorsoventrally oriented sheets or bands that are arranged in a complex way. Differential connections of these lateral septal components are analyzed in the accompanying paper (Risold, P. Y. and Swanson, L. W., Connections of the rat lateral septal complex, Brain Res. Rev., 24 (1997) 115-195).

A population of supramammillary area calretinin neurons terminating on medial septal area cholinergic and lateral septal area calbindin-containing cells are aspartate/glutamatergic

The excitatory amino acid, aspartate/glutamate content of septal complex calretinin (CR)-, choline acetyltransferase plus substance P-, and Leu-enkephalin (Leu-enk)-containing extrinsic afferents was examined. Experiments were carried out using the transmitter-specific [3H]-D-aspartate retrograde tracer technique in combination with immunostaining for CR, choline acetyltransferase, and Leu-enk. The extrinsic and intrinsic CR innervation of the same brain areas were elucidated on control rats and on animals in which the septum was surgically separated from its ventral afferents. Correlated light and electron microscopic double-immunostaining experiments were used to determine the synaptic connections between CR axon terminals and lateral septal area calbindin (CB)- and medial septal area choline acetyltransferase-immunoreactive neurons. Furthermore, to determine the synaptic power of supramammilloseptal aspartate/glutamatergic neurons on the septal complex, semiquantitative analyses were performed in the supramammillary area on retrogradely (1) [3H]-D-aspartate-radiolabeled and (2) HRP-labeled material. The results demonstrated that a population of the extrinsic CR axons originating in the supramammillary area are aspartate/glutamatergic. These fibers forming asymmetric synaptic contacts terminate on both CB and cholinergic neurons. Intraseptal CR neurons, which establish symmetric synapses, innervate only lateral septal area neurons, including the CB-containing cells. These observations, together with other published data, raise the possibility of a hippocampus-lateral septal (GABAergic CB-containing neurons)-supramammillary area (aspartate/glutamatergic cells)-medial septal (cholinergic neurons)-hippocampus signal loop, which might be involved in the generation and regulation of hippocampal theta rhythm activity.