Catecholaminergic, GABAergic, and hippocamposeptal innervation of GABAergic "somatospiny" neurons in the rat lateral septal area

A hypothalamic dopamine locus for psychostimulant-induced hyperlocomotion in mice

The lateral septum (LS) has been implicated in the regulation of locomotion. Nevertheless, the neurons synchronizing LS activity with the brain’s clock in the suprachiasmatic nucleus (SCN) remain unknown. By interrogating the molecular, anatomical and physiological heterogeneity of dopamine neurons of the periventricular nucleus (PeVN; A14 catecholaminergic group), we find that Th⁺/Dat1⁺ cells from its anterior subdivision innervate the LS in mice. These dopamine neurons receive dense neuropeptidergic innervation from the SCN. Reciprocal viral tracing in combination with optogenetic stimulation ex vivo identified somatostatin-containing neurons in the LS as preferred synaptic targets of extrahypothalamic A14 efferents. In vivo chemogenetic manipulation of anterior A14 neurons impacted locomotion. Moreover, chemogenetic inhibition of dopamine output from the anterior PeVN normalized amphetamine-induced hyperlocomotion, particularly during sedentary periods. Cumulatively, our findings identify a hypothalamic locus for the diurnal control of locomotion and pinpoint a midbrain-independent cellular target of psychostimulants.

The psychostimulant-sensitive neural mechanism linking the circadian clock to locomotion is unknown. Here, hypothalamic A14 neurons are shown to time diurnal activity by entraining the lateral septum, and their activity is shown to be sensitive to amphetamine.

A limbic circuit selectively links active escape to food suppression

Stress has pleiotropic physiologic effects, but the neural circuits linking stress to these responses are not well understood. Here, we describe a novel population of lateral septum neurons expressing neurotensin (LS^Nts) in mice that are selectively tuned to specific types of stress. LS^Nts neurons increase their activity during active escape, responding to stress when flight is a viable option, but not when associated with freezing or immobility. Chemogenetic activation of LS^Nts neurons decreases food intake and body weight, without altering locomotion and anxiety. LS^Nts neurons co-express several molecules including Glp1r (glucagon-like peptide one receptor) and manipulations of Glp1r signaling in the LS recapitulates the behavioral effects of LS^Nts activation. Activation of LS^Nts terminals in the lateral hypothalamus (LH) also decreases food intake. These results show that LS^Nts neurons are selectively tuned to active escape stress and can reduce food consumption via effects on hypothalamic pathways.

The lateral preoptic area and ventral pallidum embolden behavior

While recently completing a study of the effects of stimulating the lateral preoptic area (LPO) and ventral pallidum (VP) on locomotion and other movements, we also noticed LPO and VP effects on motivational drive and threat tolerance. Here, we have investigated these latter effects by testing conditioned place preference (CPP), behavior on the elevated plus maze (EPM) and the willingness of sated rats to occupy a harshly lit open field center to acquire sweet pellets, a measure of threat tolerance, following infusions of vehicle or bicuculline (bic) into the LPO and VP. LPO-bic infusions robustly increased total locomotion, and, in direct proportion, occupancy of both the harshly lit field center and open arms of the EPM. LPO bic also generated CPP, but did not increase sweet pellet ingestion. These effects were attenuated by dopamine D1 and D2 receptor antagonists, whether given individually or as a cocktail and systemically or infused bilaterally into the nucleus accumbens. VP-bic infusions did not increase total locomotion, but preferentially increased field center occupancy. VP-bic-infused rats compulsively ingested sweet pellets and did so even under the spotlight, whereas harsh illumination suppressed pellet ingestion in the control groups. VP bic produced CPP and increased open arm occupancy on the EPM. These effects were attenuated by pretreatment with dopamine receptor antagonists given systemically or as bilateral infusions into the VP, except for % distance in the field center (by D1 or D2 antagonists) and pellet ingestion (by D1 antagonist). Thus, boldness generated in association with LPO activation is tightly tied to locomotor activation and, as is locomotion itself, strongly DA dependent, whereas that accompanying stimulation of the VP is independent of locomotor activation and, at least in part, DA signaling. Furthermore, respective emboldened behaviors elicited from neither LPO nor VP could clearly be attributed to goal pursuit. Rather, emboldening of behavior seems more to be a fixed action response not fundamentally different than previously for reported locomotion, pivoting, backing, gnawing, and eating elicited by basal forebrain stimulation.

Amphetamine treatment affects the extra-hypothalamic vasopressinergic system in a sex- and nucleus-dependent manner

The lateral septum (LS), a brain structure implicated in addictive behaviours, regulates the activation of dopaminergic neurones in the ventral tegmental area. Vasopressinergic projections from the extended amygdala to the LS, which are sexually dimorphic, could be responsible for the vulnerability to addiction in a sex-dependent manner. The present study aimed to investigate the modulatory effects of amphetamine (AMPH) on the expression of vasopressin (AVP) in the vasopressinergic extra-hypothalamic system in sensitised male and female rats. Adult male and female Sprague-Dawley rats underwent an AMPH-locomotor sensitisation protocol. Acute AMPH increased AVP mRNA expression in the medial amygdala (MeA), whereas AMPH-induced sensitisation increased AVP mRNA expression in the bed nucleus of the stria terminalis (BNST) only in females. Interestingly, the increase in AVP expression in BNST was higher in oestrus females compared to dioestrus females and acute AMPH resulted in a decrease in AVP levels in the LS, only in males. Thus, there are complex and region-specific interactions between AMPH and the extra-hypothalamic vasopressinergic system in the brain, underlying possible alterations in different behaviours caused by acute and chronic AMPH exposure.

Dynamic changes in extracellular release of GABA and glutamate in the lateral septum during social play behavior in juvenile rats: Implications for sex-specific regulation of social play behavior

Social play is a motivated and rewarding behavior that is displayed by nearly all mammals and peaks in the juvenile period. Moreover, social play is essential for the development of social skills and is impaired in social disorders like autism. We recently showed that the lateral septum (LS) is involved in the regulation of social play behavior in juvenile male and female rats. The LS is largely modulated by GABA and glutamate neurotransmission, but their role in social play behavior is unknown. Here, we determined whether social play behavior is associated with changes in the extracellular release of GABA and glutamate in the LS and to what extent such changes modulate social play behavior in male and female juvenile rats. Using intracerebral microdialysis in freely behaving rats, we found no sex difference in extracellular GABA concentrations, but extracellular glutamate concentrations are higher in males than in females under baseline conditions and during social play. This resulted in a higher glutamate/GABA concentration ratio in males vs. females and thus, an excitatory predominance in the LS of males. Furthermore, social play behavior in both sexes is associated with significant increases in extracellular release of GABA and glutamate in the LS. Pharmacological blockade of GABA-A receptors in the LS with bicuculline (100 ng/0.5 μl, 250 ng/0.5 μl) dose-dependently decreased the duration of social play behavior in both sexes. In contrast, pharmacological blockade of ionotropic glutamate receptors (NMDA and AMPA/kainate receptors) in the LS with AP-5+CNQX (2mM+0.4mM/0.5 μl, 30 mM+3mM/0.5 μl) dose-dependently decreased the duration of social play behavior in females, but did not alter social play behavior in males. Together, these data suggest a role for GABA neurotransmission in the LS in the regulation of juvenile social play behavior in both sexes, while glutamate neurotransmission in the LS is involved in the sex-specific regulation of juvenile social play behavior.

An update on the connections of the ventral mesencephalic dopaminergic complex

This review covers the intrinsic organization and afferent and efferent connections of the midbrain dopaminergic complex, comprising the substantia nigra, ventral tegmental area and retrorubral field, which house, respectively, the A9, A10 and A8 groups of nigrostriatal, mesolimbic and mesocortical dopaminergic neurons. In addition, A10dc (dorsal, caudal) and A10rv (rostroventral) extensions into, respectively, the ventrolateral periaqueductal gray and supramammillary nucleus are discussed. Associated intrinsic and extrinsic connections of the midbrain dopaminergic complex that utilize gamma-aminobutyric acid (GABA), glutamate and neuropeptides and various co-expressed combinations of these compounds are considered in conjunction with the dopamine-containing systems. A framework is provided for understanding the organization of massive afferent systems descending and ascending to the midbrain dopaminergic complex from the telencephalon and brainstem, respectively. Within the context of this framework, the basal ganglia direct and indirect output pathways are treated in some detail. Findings from rodent brain are briefly compared with those from primates, including humans. Recent literature is emphasized, including traditional experimental neuroanatomical and modern gene transfer and optogenetic studies. An attempt was made to provide sufficient background and cite a representative sampling of earlier primary papers and reviews so that people new to the field may find this to be a relatively comprehensive treatment of the subject.

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.

Newly identified patterns of Pax2 expression in the developing mouse forebrain

Background

The availability of specific markers expressed in different regions of the developing nervous system provides a useful tool for the study of mouse mutants. One such marker, the transcription factor Pax2, is expressed at the midbrain-hindbrain boundary and in the cerebellum, spinal cord, retina, optic stalk, and optic chiasm. We recently described a group of diencephalic cells that express Pax2 as early as embryonic day (E) 10.5, and become part of the eminentia thalami by E11.5. The discovery of this previously undescribed cell population prompted us to examine Pax2 protein expression in the developing mouse forebrain in more detail.

Results

We determined the expression pattern of Pax2 in the forebrain of wild type mouse embryos between E10.5 and postnatal day (P) 15. Pax2 expression was detected in the septum of the basal forebrain, hypothalamus, eminentia thalami and in the subfornical organ. To evaluate Pax2 as a marker for septal cells, we examined Pax2 expression in Pax6^Sey/Sey mutants, which have an enlarged septum. We found that Pax2 clearly marks a population of septal cells equivalent to that seen in wild types, indicating its utility as a marker of septal identity. These cells did not express the GABAergic marker calbindin nor the cholinergic marker choline acetyltransferase and were not detectable after P15.

Conclusion

Pax2 is expressed in populations of cells within the developing septum, hypothalamus, and eminentia thalami. It seems especially useful as a marker of the telencephalic septum, because of its early, strong and characteristic expression in this structure. Further, its expression is maintained in the enlarged septum of Pax6^Sey/Sey mutants.

Vesicular glutamate transporter 3-immunoreactive pericellular baskets ensheath a distinct population of neurons in the lateral septum

The lateral septum (LS) plays a role in the adjustment of behavioral responses according to environmental demands. This is a complex integrative process wherein a variety of modulatory systems, i.e. cholinergic, dopaminergic and serotonergic projections forming pericellular baskets around LS neurons, are involved. Recently, vesicular glutamate transporter 3 (VGLUT3)-immunoreactive (-ir) structures outlining unlabeled somata and their proximal dendrites were described in the LS. However, the vesicular transporters for acetylcholine and GABA were not or only rarely co-expressed with VGLUT3. In this study, the morphology and distribution of these VGLUT3-ir structures were systematically analyzed revealing that (1) they form distinct pericellular baskets (PBs) displaying variable shapes, (2) they are arranged in a layer-like pattern similar to the terminals of other modulatory systems, (3) beside a few exceptions (e.g., choline acetyltransferase), they are generally not or very sparsely co-localized with other neurochemical markers characterizing major neuron populations or afferent systems of the LS, i.e. calcium-binding proteins, tyrosine hydroxylase, tryptophan hydroxylase, vesicular glutamate transporters 1 (VGLUT1) and 2 (VGLUT2) and the vesicular GABA transporter. Thus, in the LS, a separate population of neurons is covered by VGLUT3-ir PBs. The distribution pattern and the lack of co-localization indicate that the VGLUT3-expressing cells of origin are located in the brainstem and that they could be pure glutamatergic projection neurons-different from the well-defined canonical VGLUT1- and VGLUT2-expressing neurons. Alternatively, they could simultaneously express VGLUT3 and second transmitter, but use different release sites inside the LS for both.

Presynaptic modulation of 5-HT release in the rat septal region

5-HT released from serotonergic axon terminals in the septal nuclei modulates the activity of septal output neurons (e.g. septohippocampal cholinergic neurons) bearing somatodendritic 5-HT receptors. Therefore, we studied the mechanisms involved in the presynaptic modulation of 5-HT release in the lateral (LS) and medial septum (MS), and the diagonal band of Broca (DB). HPLC analysis showed that tissue concentrations of noradrenaline, dopamine and 5-HT were highest in DB (DB>MS>LS). Slices prepared from LS, MS and DB regions were preincubated with [(3)H]5-HT, superfused in the presence of 6-nitro-2-(1-piperazinyl)-quinoline (6-nitroquipazine) and electrically stimulated up to three times (first electrical stimulation period (S(1)), S(2), S(3); 360 pulses, 3 Hz, 2 ms, 26-28 mA). In all septal regions the Ca(2+)-dependent and tetrodotoxin-sensitive electrically-evoked overflow of [(3)H] was inhibited by the 5-HT(1B) agonist CP-93,129 and the alpha(2)-adrenoceptor agonist 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline tartrate (UK-14,304). Also the mu- and kappa-opioid receptor agonists (d-Ala(2), N-Me-Phe(4), glycinol(5))-enkephalin (DAMGO) and [trans-(1S,2S(-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl) cyclohexyl]-benzenacetamide hydro-chloride] (U-50,488H), respectively, acted inhibitory (although less potently), whereas the delta-opioid receptor agonist (d-Pen(2), d-Pen(5))-enkephalin (DPDPE), the dopamine D(2) receptor agonist quinpirole and the adenosine A(1) receptor agonist N(6)-cyclopentyladenosine were all ineffective; the GABA(B) receptor agonist baclofen had weak effects. All inhibitory effects of the agonists were antagonized by the corresponding antagonists (3-[3-(dimethylamino)propyl]-4-hydroxy-N-[4-(4-pyridinyl)phenyl]benzamide dihydrochloride (GR-55,562), idazoxan, naloxone, nor-binaltorphimine), which also significantly enhanced the evoked release of 5-HT at S(1). It is concluded that 5-HT release in septal nuclei of the rat is modulated by presynaptic 5-HT(1B) autoreceptors, as well as by alpha(2)-, mu- and kappa-opioid heteroreceptors. All of these receptors seem to be under a tonic inhibitory influence of the corresponding endogenous agonists and show qualitatively comparable modulatory properties along the dorso-ventral distribution of the 5-HT terminals.

Evidence for a distinct group of nestin-immunoreactive neurons within the basal forebrain of adult rats

Nestin is an intermediate filament protein serving as a marker for neuroprogenitor and stem cells. Here we report that a cluster of previously unrecognized nestin immunoreactive (nestin-ir) neurons was located in the medial septum-diagonal band of Broca (MS-DBB) of the basal forebrain in adult rats. Nestin-ir neurons were exclusively located in the MS-DBB and intermingled with choline acetyltransferase-ir (ChAT-ir), parvalbumin-ir (PV-ir), or nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase reactive (NADPHd-reactive) neurons. However, there was no colocalization between nestin-ir and PV-ir in single neurons in MS-DBB; only about 35% of nestin-ir neurons were ChAT-ir, and 8%-12% of nestin-ir neurons were NADPHd-reactive. Morphologically, nestin-ir neurons showed a larger size of somata than that of ChAT-ir or PV-ir neurons and the distribution of nestin-ir neurons spread across the rostro-caudal extent of the MS-DBB. Moreover, retrograde tracing revealed that a significant portion of these nestin-ir neurons projected to the thalamus and hippocampus. These results, for the first time, provide strong evidence that there exists a cluster of previously unrecognized nestin-ir neurons in MS-DBB of the basal forebrain in adult rats and that these nestin-ir neurons are distinguishable from ChAT-ir, PV-ir, and NADPHd-reactive neurons.

Particular subpopulations of midbrain and hypothalamic dopamine neurons express vesicular glutamate transporter 2 in the rat brain

Vesicular glutamate transporters (VGLUT1, -2, and -3) mediate the accumulation of transmitter glutamate into synaptic vesicles in glutamatergic neurons. VGLUT1 and VGLUT2 are more reliable glutamatergic neuron markers, since VGLUT3 also exists in other neuron types. To study whether the dopaminergic neuron uses glutamate as a cotransmitter, we analyzed VGLUTs expression in dopamine neurons of adult male rats by in situ hybridization and immunohistochemistry. In the ventral midbrain, in situ hybridization analysis revealed no VGLUT1 mRNA expression, a widespread but discrete pattern of VGLUT2 mRNA expression, and a highly limited expression of VGLUT3 mRNA. Reverse-transcriptase polymerase chain reaction analysis detected full-length VGLUT2 gene transcripts in the ventral midbrain. Using in situ hybridization combined with tyrosine hydroxylase (TH) immunostaining, only VGLUT2 signals were detectable in some TH-labeled neurons of A10 dopamine neuron groups, with the highest incidence (20%) in the rostral linear nucleus of the ventral tegmental area. In the forebrain, VGLUT2 signals were demonstrated in half of the A11 TH-labeled neurons in the hypothalamus. Double-label immunostaining for VGLUT2 and vesicular monoamine transporter 2 or TH showed that double-labeled varicosities are rarely observed in any target regions examined of A10 and A11 dopamine neuron groups. These results indicate that VGLUT2 is expressed in subsets of A10 and A11 dopamine neurons, which might release dopamine and glutamate separately from different varicosities in the majority of their single axons.

Forebrain projections to the hypothalamus are topographically organized in anurans: conservative traits as compared with amniotes

The organization of the forebrain in amphibians (anamniotes) is currently being re-evaluated in terms of evolution and several evidences have corroborated numerous traits shared by amphibians and amniotes, such as the organization of the basal ganglia and the amygdaloid complex. In the present study we have analysed the organization of forebrain afferent systems to the hypothalamus of the frog Rana perezi. In vivo and in vitro tract-tracing techniques with dextran amines and immunohistochemistry for localizing nitric oxide synthase (NOS) in a series of single or combined experiments were used as NOS labelling reveals hypothalamic afferents arising from the lateral amygdala and the combination allowed analysis of the relationship between fibers of different origins in the same section. The results showed a large segregation of afferents in the hypothalamic region depending on their site of origin in the forebrain. Four highly topographically organized prosencephalic tracts reaching the anuran hypothalamus were observed: (i) the medial forebrain bundle, from the medial pallium and septal complex; (ii) the caudal branch of the stria terminalis formed by fibers arising in the lateral and medial amygdala; (iii) part of the lateral forebrain bundle with fibers from the central amygdala and (iv) the dorsal thalamo-hypothalamic tract. Fibers coursing in each tract reach the hypothalamus and terminate in distinct fields. The resemblance in pattern of forebrain-hypothalamic organization between amphibians and amniotes suggests that this feature represents an important trait conserved in the evolution of all tetrapods and therefore essential for the hypothalamic function.

Regulation of affect by the lateral septum: implications for neuropsychiatry

Substantial evidence indicates that the lateral septum (LS) plays a critical role in regulating processes related to mood and motivation. This review presents findings from the basic neuroscience literature and from some clinically oriented research, drawing from behavioral, neuroanatomical, electrophysiological, and molecular studies in support of such a role, and articulates models and hypotheses intended to advance our understanding of these functions. Neuroanatomically, the LS is connected with numerous regions known to regulate affect, such as the hippocampus, amygdala, and hypothalamus. Through its connections with the mesocorticolimbic dopamine system, the LS regulates motivation, both by stimulating the activity of midbrain dopamine neurons and regulating the consequences of this activity on the ventral striatum. Evidence that LS function could impact processes related to schizophrenia and other psychotic spectrum disorders, such as alterations in LS function following administration of antipsychotics and psychotomimetics in animals, will also be presented. The LS can also diminish or enable fear responding when its neural activity is stimulated or inhibited, respectively, perhaps through its projections to the hypothalamus. It also regulates behavioral manifestations of depression, with antidepressants stimulating the activity of LS neurons, and depression-like phenotypes corresponding to blunted activity of LS neurons; serotonin likely plays a key role in modulating these functions by influencing the responsiveness of the LS to hippocampal input. In conclusion, a better understanding of the LS may provide important and useful information in the pursuit of better treatments for a wide range of psychiatric conditions typified by disregulation of affective functions.

Noradrenergic innervation of the developing and mature septal area of the rat

The noradrenergic innervation of the developing and mature septal area of the rat was examined with light and electron microscopic immunocytochemistry using an antibody against dopamine-beta-hydroxylase. At birth, a small number of relatively thick noradrenergic fibers were found to innervate the lateral septum (mainly its intermediate part) and the nuclei of the vertical and horizontal limbs of the diagonal band of Broca. By postnatal day 7, a substantial increase in their density was observed. At this age some labeled fibers left the medial forebrain bundle and invaded the nucleus of the horizontal limb of the diagonal band. These fibers then ran in a ventrodorsal direction and innervated the nucleus of the vertical limb before entering the medial septum. Immunoreactive fibers were finer and more varicose than at birth. In the subsequent 2 weeks, the density of labeled fibers in the septal area was further increased. By postnatal day 21, the distribution pattern and density of the noradrenergic innervation appeared similar to the adult. In the adult, noradrenergic fibers exhibited more varicosities than in younger rats. Electron microscopic analysis revealed a low proportion (peaked at P7) of noradrenergic varicosities engaged in synaptic contacts throughout development. The overwhelming majority of these synapses were symmetrical, predominantly with small or medium-sized dendrites. The present findings provide the morphological basis for the functional interactions between noradrenergic afferents and neuronal elements in the septal area. The low proportion of synaptic contacts found in this study suggests that noradrenaline may exert its action in the septal area mainly through transmission by diffusion (volume transmission), as has been suggested for other areas of the developing and adult brain.

A neural network underlying individual differences in emotion and aggression in male golden hamsters

In rodents, aggressive behavior can be altered by experimental manipulations of emotional responsiveness. The goal of this study was to identify characteristics of emotional reactivity associated with individual differences in aggressive behavior and their integration within a common neural network. Male golden hamsters were first screened for offensive aggression. Then, the animals were trained through immediate reinforcement and tested for their adaptation to a delayed reward. Similar protocols have been used to test behaviors associated with frustration. At first, all hamsters showed increased frequency of bar pressing per reward during delayed reinforcement. However, Low-Aggression animals were able to adapt to the delay and showed a decreased rate of bar pressing per reward within 5 days. In contrast, High-Aggression animals maintained a high rate of bar pressing per reward. In addition, brains were collected after immediate reward training or delayed reward testing, and labeled for pCREB-immunoreactivity as a marker of trans-synaptic activity. In High-Aggression individuals, elevated density of cyclic AMP response element binding protein, phosphorylated (pCREB) immunostaining was found within the anterior hypothalamus, an area critical to the control of aggression. Delayed reinforcement was associated with enhanced pCREB immunostaining within the central amygdala, medial amygdala and preoptic area/hypothalamus continuum. Further analysis of the data also showed a positive correlation in labeling density between the lateral septum and the anterior hypothalamus, specifically in Low-Aggression animals exposed to delayed reward. Therefore, as High-Aggression individuals lack control of their emotional reactivity, they are also characterized by a de-synchronization between the inhibitory output of the septum and the aggression areas of the hypothalamus. Finally, our data also show that frustration is associated with an extensive activation of the preoptic area/hypothalamus continuum and amygdala.

Catecholaminergic innervation of the septum in the frog: a combined immunohistochemical and tract-tracing study

In the present study, we have investigated the distribution and the origin of the catecholaminergic innervation of the septal region in the frog Rana perezi. Immunohistochemistry for dopamine and two enzymes required for the synthesis of catecholamines, tyrosine hydroxylase (TH) and dopamine beta-hydroxylase (DBH) revealed a complex pattern of catecholaminergic (CA) innervation in the anuran septum. Dopaminergic fibers were primarily present in the dorsal portion of the lateral septum, whereas noradrenergic (DBH immunoreactive) fibers predominated in the medial septum/diagonal band complex. Catecholaminergic cell bodies were never observed within the septum. To determine the origin of this innervation, applications of dextran amines, both under in vivo and in vitro conditions, into the septum were combined with immunohistochemistry for TH. Results from these experiments demonstrated that four catecholaminergic cell groups project to the septum: (1) the group related to the zona incerta in the ventral thalamus, (2) the posterior tubercle/mesencephalic group, (3) the locus coeruleus, and (4) the nucleus of the solitary tract. While the two first groups provide dopaminergic innervation to the septum, the locus coeruleus provides the major noradrenergic projection. Noradrenergic fibers most likely arise also in the nucleus of the solitary tract. The results obtained in Rana perezi are readily comparable to those in mammals suggesting that the role of catecholamines in the septum is well conserved through phylogeny and that the CA innervation of the amphibian septum may be involved in functional circuits similar to those in mammals.

The lateral septal area is involved in mediation of immobilization stress-induced blood pressure increase in rats

Immobilization stress increased the number of neurons with Fos immunoreactivity, mainly in the ventral zone of the rostral part of the lateral septal nucleus (LSV) in rats. Immobilization stress caused an increase in blood pressure, and the stress-induced pressor response was inhibited by the GABA(A) receptor agonist, muscimol (8 and 80 pmol), injected bilaterally into the rostral part of the LSV in a dose-dependent manner. Intracerebroventricular injection of muscimol (16 pmol) did not affect the immobilization stress-induced pressor response. These findings suggest that the rostral part of the LSV is involved in mediation of the stress-induced pressor response.

Noradrenaline modulates GABA-mediated synaptic transmission in neurones of the mediolateral part of the guinea pig lateral septum via local circuits

The response to bath-applied noradrenaline (NA, 50 microM) was tested with conventional intracellular recordings in neurones of the mediolateral part of the lateral septum (LSml) by using guinea-pig brain slices. NA elicited direct hyperpolarizations and depolarizations and affected the frequency of tetrodotoxin (TTX)- and bicuculline-sensitive inhibitory post-synaptic potentials (IPSPs). Decrease or increase in IPSPs frequency was observed in 37.6 and 18.2% neurones respectively. Frequency of bicuculline-sensitive miniature IPSPs recorded under TTX was unchanged. Increase in IPSPs frequency was blocked by TTX and was observed during iontophoretic application of NA in the LSml. Taken together, these data suggest that NA hyperpolarizes or depolarizes local presynaptic GABAergic neurones to cause alterations in action potential-dependent transmitter release. NA-induced decrease or increase in IPSPs frequency were respectively mimicked by clonidine and L-phenylephrine and blocked by yohimbine and prazosin, indicating that NA modifies release of GABA by activation of alpha(2) and alpha(1) adrenoceptors at the somato-dendritic region of presynaptic GABAergic neurones. These findings show that, in addition to their direct input from NA afferents, LSml neurones receive a indirect input by way of GABAergic interneurones. These interneurones may serve as inhibitory relays for noradrenergic afferents originating from the locus coeruleus and may be involved in the NA control of LSml functions.

Evidence that brain-derived neurotrophic factor from presynaptic nerve terminals regulates the phenotype of calbindin-containing neurons in the lateral septum

Brain-derived neurotrophic factor (BDNF) is transported anterogradely in neurons of the CNS and can be released by activity-dependent mechanisms to regulate synaptic plasticity. However, few neural networks have been identified in which the production, transport, and effects of BDNF on postsynaptic neurons can be analyzed in detail. In this study, we have identified such a network. BDNF has been colocalized by immunocytochemistry with tyrosine hydroxylase (TH) in nerve fibers and nerve terminals within the lateral septum of rats. BDNF-containing nerve fibers terminate on a population of calbindin-containing neurons in lateral septum that contain TrkB, the high-affinity receptor for BDNF. Overexpression of BDNF in noradrenergic neurons increased levels of calbindin in septum, as well as in whole-brain lysates. Septal levels of calbindin and BDNF partially decreased after unilateral lesions of the medial forebrain bundle (MFB), induced with 6-hydroxydopamine, a treatment that abolished TH staining. These data suggest that BDNF is anterogradely transported within the MFB in catecholaminergic neurons arising from brainstem nuclei. To determine whether BDNF affects the production of calbindin in lateral septal neurons directly, we tested the effects of BDNF on cultures of septal neurons from embryonic day 16-17 rats. BDNF promoted the expression of calbindin, as well as the arborization of calbindin-containing neurons, but BDNF had no effect on cell division or survival. Together, these results suggest that BDNF, anterogradely transported in catecholaminergic neurons, regulates calbindin expression within the lateral septum.

Vasopressin selectively modulates the release of taurine within the septum of the rat brain

Previous experiments have shown that arginine vasopressin (AVP) released within the septal brain area of adult male rats in response to defined stressor exposure is involved in emotionality-related behavior. We report here that a 10-min forced swimming session stimulated the release of glutamate, aspartate, arginine, gamma-aminobutyric acid (GABA) and taurine but not alanine and serine in the medio-lateral part of this brain structure. Local administration of the AVP V1 receptor antagonist d(CH2)5Tyr(Me)AVP by inverse microdialysis caused a significant increase in the concentration of taurine in microdialysates under resting conditions that was further potentiated during forced swimming. In contrast, the release of alanine, arginine, GABA and serine was unaffected by antagonist treatment. Taken together with previous data, our results suggest that the effects of intraseptally released AVP on stress-coping strategies might be mediated at least in part via its influence on the local release of taurine.

Noradrenergic responses of neurones in the mediolateral part of the lateral septum: alpha1-adrenergic depolarization and rhythmic bursting activities, and alpha2-adrenergic hyperpolarization from guinea pig brain slices

Bath application of noradrenaline on neurones of class A, B and C within the mediolateral part of the lateral septum from guinea pig brain slices elicited depolarization (through alpha1-adrenoceptors) or hyperpolarization (through alpha2-adrenoceptors) and rhythmic bursting activities (through alpha1-adrenoceptors). A co-existence of these two types of adrenoceptors (alpha1 and alpha2) mediating opposite effects on membrane excitability was sometimes observed on the same neurone. Three types (I, II and III) of neurones were identified on the basis of their bursting properties during noradrenaline application. With the bursting activities persisting under tetrodotoxin and their frequency being sensitive to changes in membrane potential, these neurones could be considered as conditional bursters. An afterdepolarizing potential could promote burst initiation in the three types of neurones. Neuronal input resistance decreased at the afterdepolarizing potential peak. Application of low-Na+ medium blocked the generation of bursts and afterdepolarizing potentials induced by noradrenaline in the three types of neurones. Furthermore, the bursts were dependent on the presence of Ca(2+) in the medium in a subpopulation of neurones. We conclude that afterdepolarizing potentials and bursts induced by noradrenaline are generated by a cationic conductance largely permeable to Na+ in neurones of the mediolateral part of the lateral septum.

Norepinephrine inhibits neurons of the intermediate subnucleus of the lateral septum via alpha2-adrenoceptors

The physiological and pharmacological actions of norepinephrine (NE) on neurons of the intermediate subnucleus of the lateral septum (LSI) were examined using intracellular recordings in rat brain-slices. Bath-applied NE inhibited 72.5%, excited 5.5% and had no effect on 22% of LSI neurons tested; this study focused on the inhibitory effects of NE. In current clamp recordings, 100 microM NE produced a hyperpolarization of 10.82+/-0.72 mV (n=84) with a decrease in input resistance. In voltage-clamp, NE produced a direct, post-synaptic outward current of 206.8+/-22 pA (n=37) with a 64. 3+/-4.9% increase in input conductance (IC50-17.7+/-4 microM). The NE-induced inhibition was mimicked by the alpha2-agonist, UK14,304, but not by the alpha1- or beta-adrenoceptor agonists. The alpha2-agonist, clonidine, had a weak effect in LSI neurons. Interestingly, the magnitude of the UK14,304-induced response varied between cells (ranging from 29.5 to 320% of the maximal NE inhibition), possibly suggesting the involvement of alpha2A-(high affinity for UK14,304) and non-alpha2A (low affinity for UK14,304) adrenoceptor subtypes. While the alpha2-antagonists, yohimbine, rauwolscine and idazoxan blocked NE-induced inhibition in all neurons tested, the prototypical alpha1-antagonist, prazosin produced a variable degree of block (9-58%), further indicating the possible involvement of alpha2A (prazosin-insensitive) and non-alpha2A (prazosin-sensitive) receptors. However a lack of more selective pharmacological tools precludes definitive classification of the alpha2-receptor-mediated responses into different subtypes. The alpha2-receptor-mediated current in LSI neurons displayed Ba2+-sensitive inward rectification, reversed polarity near EK and was sensitive to external K+. In conclusion, NE inhibits LSI neurons via alpha2-adrenoceptor subtypes.

Axonal expression sites of tyrosine hydroxylase, calretinin- and calbindin-immunoreactivity in striato-pallidal and septal nuclei of the rat brain: a double-immunolabelling study

Besides the dopaminergic afferent projection system, calbindin (CALB)- and calretinin (CR)-immunoreactive fibres of intrinsic and extrinsic origin represent the most abundant axonal categories in the rat striatal and lateral septal areas. The question arises whether or not they may represent separate populations, or whether they form subgroups which co-express more than one of these antigens. Therefore, the present study is focused on the distribution patterns of the axons single-immunolabelled by the catecholaminergic marker tyrosine hydroxylase (TH), and on TH-immunoreactive axons displaying also CR- and/or CALB-immunoreactivity in double-immunostained sections. Striking differences were found between the patch and matrix compartments of the caudate-putamen (CP). Whereas the vast majority of TH-immunoreactive fibres in the patches and a patch-associated subcallosal layer co-expressed CR but not CALB, fibres mono-labelled by the TH-immunoreactivity were predominant in the matrix. The matrix-like regions of the core of nucleus accumbens (CACC), fundus striati (FS), the striatal cell bridges (CB) and the striatal part of olfactory tubercle (OTU) coincided in this respect with the matrix in CP. The absence of CR-immunoreactivity was also characteristic of the TH-immunoreactive fibres in the patch-like areas of the accumbal core, although a high number of separate CR-immunoreactive axons were present. In the shell of nucleus accumbens (SACC) which receives a rich catecholaminergic innervation, fibres co-expressing either one of the calcium-binding proteins were absent. The islands of Calleja (CJI) displaying a strongly TH-immunoreactive centre and a periphery of lower staining intensity, showed only a low number of TH-immunoreactive fibres co-expressing CR or CALB. The broad shell-like band of TH-immunoreactive axons between medial and lateral part of the septum was single-stained with the TH-immunoreactivity. In contrast, the TH-positive fibres forming basket-like arrangements around some neurons in the dorsal lateral septal nucleus co-expressed also CR, but not CALB. The results are discussed in view of the recent concepts of basal forebrain organization and the cytochemical characteristics of mesencephalic dopaminergic nuclei giving rise to the vast majority of the striatal and septal TH-immunoreactive fibre supply, in order to correlate the known projection patterns with the content of calcium-binding proteins in TH-immunolabelled fibres and presumed cells of origin. The TH-immunoreactive fibres in the striatal patches displaying CR- but not CALB-immunoreactivity may originate mainly from neurons in the ventral tier of pars compacta (SNC) and from the pars reticulata of substantia nigra (SNR) which show identical cytochemical properties. Axons in the matrix of CP and the accumbal core as well as in the islands of Calleja single-labelled by the TH-immunoreactivity or additionally containing CALB and CR may originate from neurons in the dorsal tier of mesencephalic nuclei like SN, pars compacta and ventral tegmental area. CR-containing TH-immunoreactive basket-like axon terminations in the dorsal lateral septal nucleus are likely to originate either from mesencephalic nuclei or from the supramammillary region.

Differential patterns of c-fos mRNA expression in amygdala during successive stages of odor discrimination learning

Expression of the activity-dependent gene c-fos was used to assess relative levels of neuronal activation in the amygdala and related structures of rats at different stages of odor discrimination learning. In situ hybridization was used to evaluate c-fos mRNA content within the amygdalar subdivisions, the bed nucleus of the stria terminalis, and the hippocampus. After initial exploration of the test apparatus, c-fos mRNA levels were increased in the medial and, to lesser extent, basolateral subdivisions and remained low in the central division. The balance of amygdala to hippocampal labeling favored hippocampus. Rats engaged in familiar nose-poke responses had comparably elevated labeling in the medial and basolateral divisions and low labeling densities in the central division. The ratio of hippocampal to amygdala labeling was at control levels. Rats required to switch from ad libitum responding to cued responding to odors had high basolateral to medial labeling ratios. This was in marked contrast to the medial dominance found in control and exploration rats. Hybridization was substantially more dense in basolateral amygdala than in hippocampal CA1; this imbalance was unique to the group required to form first associations between odors and rewards. Rats performing an overtrained odor discrimination had the least differentiation between amygdalar subdivisions of any behavioral group. The hippocampus-to-amygdala labeling ratio favored hippocampus and was nearly identical to the ratio in exploration rats. These results demonstrate that the balance of activity within the between limbic structures shifts according to behavioral demands. It is suggested that the balances reflect the availability of pertinent afferent cues, interactions between hippocampus and the extended amygdala, and relative levels of activity in the diffuse projections to the limbic system.

Distribution and synaptology of dopaminergic fibers in the mature and developing lateral septum of the rat

The dopamine innervation of the adult and developing lateral septum of the rat was investigated with light and electron microscope immunocytochemistry using anti-dopamine antibodies. Light microscopic analysis showed that the pattern of innervation of the lateral septum exhibited a marked reorganization during the first 2 postnatal weeks, when it acquired features comparable to the adult. Ultrastructural analysis suggested that there may be two different dopamine inputs in the lateral septum. The first develops earlier in life and, through symmetrical axodendritic synapses, affects remote parts of neurons and may cause inhibition. The second develops later and, through asymmetrical axosomatic synapses, affects neuronal somata and may cause excitation. These findings may explain the reported contradictory results concerning the physiological role of dopamine in neurons of the lateral septum.

Glutamatergic modulation of cortical acetylcholine release in the rat: a combined in vivo microdialysis, retrograde tracing and immunohistochemical study

The microdialysis technique with one or two probes was used to investigate the modulation of cortically projecting cholinergic neurons by glutamatergic input in the rat in vivo. Male albino Wistar rats (250-300 g) were used. Under chloral hydrate anaesthesia microdialysis membranes were positioned in the parietal cortex, nucleus basalis magnocellularis (NBM) or medial septum. Acetylcholine was assayed using high-performance liquid chromatography (HPLC) with electrochemical detection while GABA was detected using HPLC with fluorimetric detection after derivatization of the amino acid with o-phthalaldehyde. Septo-cortical neurons were retrogradely labelled with fluoro-gold. Double labelling with choline acetyltransferase (ChAT) immunoreactivity was performed to identify these neurons. Our main findings were that: (i) i.c.v. administration of the NMDA antagonist 3-((R)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP, 1-5 nmol) increased cortical acetylcholine outflow; (ii) local administration of CPP (100 microM) to the cortex had no effect on cortical acetylcholine outflow; (iii) local administration of CPP (100 microM) to the NBM decreased cortical acetylcholine outflow; (iv) local administration of CPP (100-200 microM) to the septum increased cortical GABA and acetylcholine outflow; (v) administration of muscimol to the septum prevented the effect of CPP on cortical acetylcholine outflow; (vi) retrograde tracing with fluoro-gold labelled cell bodies in the medial septum; (vii) septal fluoro-gold-positive neurons were not ChAT-immunoreactive. Our in vivo neurochemical results, in combination with retrograde tracing and immunohistochemistry, indicate that the cortically projecting cholinergic system is indirectly regulated by a glutamatergic input via a polysynaptic GABAergic circuitry located in the septum.

Distribution of calretinin-containing neurons relative to other neurochemically identified cell types in the medial septum of the rat

The topographic distribution of calretinin-immunoreactive neurons was studied in the medial septum diagonal band of Broca complex of the rat, in relation to the localization of other neurochemically identified cell groups containing choline acetyltransferase, parvalbumin or calbindin D28k. Double-labelling experiments revealed that these four antigen-containing cells formed distinct dorsoventrally running lamellae overlayed on top of each other similar to onion leaves. There was only a slight overlapping of the various cell groups. None of the four antigens were co-localized in the same cells. The lamella occupied by calretinin-positive neurons is situated at the border of the medial septum and the intermediolateral septal nucleus, and shows some overlap with the area occupied by cholinergic neurons. Retrograde transport of horseradish peroxidase from the hippocampus combined with immunostaining for calretinin revealed that calretinin-containing neurons do not participate in the septohippocampal projection. The lack of projection to the amygdala was also confirmed. Thus, calretinin-containing neurons represent a distinct cell group in the medial septal region, which either projects to subcortical areas, or may function as interneurons relaying hippocampal feedback to the medial septal projection neurons.

Modulation of cognitive processes by transsynaptic activation of the basal forebrain

Each of the neurotransmitter-specific afferents to the basal forebrain (BF) carry different types of information which converge to regulate the activity of cholinergic projections to telencephalic areas. Brainstem monoaminergic and cholinergic inputs are critical for context-dependent arousal. GABAergic afferents are gated by a variety of ascending and descending systems, and in addition provide an intrinsic control of BF output excitability. Corticofugal glutamatergic inputs represent reciprocal connections from sites to which BF afferents project, and carry information about the current level of cortical processing intensity and capacity. Peptidergic inputs arise from hypothalamic sources and locally modulate BF output as a function of motivational and homeostatic processes. The significance of these afferent systems can be studied by examining the behavioral consequences of infusion into the BF of drugs that act on the specific receptor systems. Although traditional analyses suggest that the BF has many behavioral functions that can be subdivided regionally, an analysis of studies employing transsynaptic approaches lead to the conceptualization of the BF as having a uniform function, that of maximizing cortical processing efficiency. The BF is conditionally active during specific episodes of acquisition and processing of behaviorally significant, externally-derived information, and drives cortical targets into a state of readiness by reducing interference and amplifying the processing of relevant stimuli and associations, thus allowing for more efficient processing. This paper describes the transsynaptic approach to studying BF function, reviews the neurobiological and behavioral consequences of altering neurotransmitter-specific inputs to the BF, and explores the functional significance of the BF.

Parvalbumin-containing cells of the angular portion of the vertical limb terminate on calbindin-immunoreactive neurons located at the border between the lateral and medial septum of the rat

In the septal complex, both parvalbumin and calbindin neurons cocontain GABA. In the same area, a large number of GABA-GABA synaptic connections can be observed. In order to further characterize their neurochemical nature, as well as the extrinsic and/or intrinsic origin of these GABA terminals, the following experiments were performed: (1) correlated light- and electron-microscopic double immunostaining for calbindin and parvalbumin on septal sections of control rats: (2) light microscopic parvalbumin immunostaining of septal sections after surgical isolation (5 days) of the septum from its telencephalic or (3) hypothalamic afferents; and (4) parvalbumin immunostaining of sections prepared from the entire brain 2 days following horseradish peroxidase injection into the border between the lateral and medial septum. The results demonstrated that: (1) in a well-circumscribed, vertically longitudinal area located between the lateral and medial septum, 0.1-0.6 mm anterior to the bregma, a group of calbindin-containing, nonsomatospiny neurons are surrounded by parvalbumin-immunoreactive baskets; (2) these basket-forming axon terminals establish symmetric synaptic contacts with their targets; and (3) their cells of origin are not in the medial septum, but in the angular portion of the vertical limb. These observations indicate that a portion of the septal complex GABA-GABA synaptic connections represent functional interaction between two different types of GABAergic neurons. The presynaptic GABAergic neurons contain parvalbumin, and the postsynaptic GABAergic cells are immunoreactive for calbindin. Furthermore, a population of the medial septum/diagonal band parvalbumin neurons project only to the hippocampus, while others, which may also send axons to the hippocampus, terminate on lateral septum calbindin cells as well.

Raphe-septal neurons changes in sensitivity to desipramine following an early septal lesion in the rat

1. The electrophysiological responsivity to desipramine (DMI) applied by systemic or local route was tested in Wistar female rats submitted to a wide lesion in the lateral septal area on the 8th day after birth. 2. One year after a septal lesion, the dorsal raphe nucleus (DRN) stimulation produced an initial brief response in lateral septal and CA1/CA3 neurons. In the control and sham-lesion groups, most recordings showed only this initial brief response (non-late responding neurons); however, in the lesion group most of the recorded neurons showed an afterdischarge. 3. DMI (2.14 mg/kg, 21 days, i.p.) increased the firing rate in septal and CA1/CA3 non-late responding neurons. In the equivalent septal neurons from lesion group, DMI produced the inverse effect, i.e., a decreased firing rate. 4. In septal non-late responding neurons, DMI (2 mM; 10-15 nAmps, 0.5 sec) applied by microiontophoresis increased the firing rate only after long-term systemic DMI impregnation. The response to locally applied DMI did not occur in the lesion group even after long-term DMI treatment. 5. In conclusion, an early lateral septal lesion canceled the response of intermediate-dorsal septal neurons to DMI applied by systemic and local routes.

Distribution of NADPHdiaphorase and calbindin-D28k neurons in the lateral septal area of the guinea pig, with special reference to the enkephalinergic hypothalamo-septal tract

Nicotinamide adenine dinucleotide phosphate-diaphorase (NADPHd) histochemical techniques were used to identify neurons synthesizing nitric oxide in the lateral septum of the guinea pig. Double immunocytochemical procedures were used to detect neurons immunoreactive for calbindin-D28k and enkephalinergic fibers which project to the lateral septum. The present data demonstrate that (1) the neurons containing NADPH diaphorase and the neurons immunoreactive for calbindin-D28k are observed in discrete regions of the lateral septum; (2) these populations overlap in various areas of the lateral septum including its dorsal and mediolateral parts; (3) NADPH diaphorase and calbindin-D28k are colocalized in neurons located in the overlapping areas; (4) neurons identified by the presence of calbindin-D28k, NADPH diaphorase or both substances, are surrounded by enkephalinergic fibers. These observations indicate the chemical heterogeneity of the lateral septum and suggest that the enkephalinergic hypothalamo-septal tract does not preferentially contact a subpopulation of neurons.

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

The electrophysiological observations that substance P administration to the lateral septal area elicits both excitatory and inhibitory responses, together with earlier reports on the multiple sources of substance P innervation of the septum, implies that these axons with distinct origins have different functions. This prompted us to examine the origin and neurochemical character of substance P afferents to the lateral septal area. Chronic surgical isolation of the septum from its ventral afferents and retrograde tracer experiments using wheat germ agglutinin-conjugated horseradish peroxidase, both followed by an immunostaining for substance P, were employed to elucidate the origin of these axon terminals. In order to assess the possible co-existence of substance P with other neurotransmitter substances in the parent cells of the septopetal projections, co-localization studies for substance P and choline acetyltransferase, as well as substance P and GABA, were performed. The comparative distribution of substance P fibers and septal calbindin-containing neurons was also investigated using correlated light and electron microscopic double immunostaining. The results are summarized as follows: (i) the substance P innervation of the lateral septal area derives from several hypothalamic nuclei (including the lateral and lateroanterior hypothalamic area, tuber cinereum and ventromedial hypothalamic nucleus) and tegmental nuclei (the majority of fibers from the laterodorsal and a few from the pedunculopontine tegmental nucleus), as well as intrinsic septal cells; (ii) the septopetal substance P fibers of tegmental origin are cholinergic; intraseptal substance P neurons located in the dorsolateral part of the lateral septum also contain GABA, while substance P neurons seen on the border between the medial and lateral septal area and septopetal hypothalamic substance P cells do not contain GABA or acetylcholine; (iii) substance P fibers from pericellular baskets around calbindin-containing lateral septal neurons with a high degree of selectivity; (iv) approximately 90% of the entire calbindin cell population are postsynaptic targets of substance P axons; (v) their terminals contact the soma and the dendrites of these cells, among them the somatospiny neurons; and (vi) the extrinsic substance P boutons establish asymmetric, while the intrinsic substance P axon terminals form symmetric membrane specializations. Because neurons in the lateral septal area receive hippocampal input and project massively to hypothalamic areas, the different types of substance P input on these neurons can modify the information flow arriving from the hippocampus to diencephalic brain structures at the level of the lateral septal area.

Regional and cellular expression sites of the alpha 1 subunit of GABAA receptors in the rat basal forebrain: a cytochemical study with glutamic acid decarboxylase, choline acetyltransferase, calcium-binding proteins and nitric oxide synthase as second markers

Forebrain sections of adult male Wistar rats were processed for the immunohistochemical detection of the GABAA receptor alpha 1 subunit. Alternate sections were used for double-staining with antibodies to glutamic acid decarboxylase (GAD), choline acetyltransferase (ChAT), the calcium binding proteins parvalbumin (PARV), calbindin (CALB) and calretinin (CR) as well as to nitric oxide synthase (NOS). alpha 1 receptor subunit-immunoreactive neurons were found to be inhomogeneously distributed in the rat basal forebrain. Numerous alpha 1 subunit-immunostained neurons occupied the central part of medial septum and diagonal band, the whole ventral pallidum and the globus pallidus. A moderate number was found in the lateral septum, and only very few in the striatum and nucleus accumbens. Double-immunofluorescence labelling revealed an apparently complete co-expression of GAD-immunoreactivity in alpha 1 subunit-immunoreactive cells of rat basal forebrain, but only a region-dependent proportion of GAD-immunoreactive cells showed alpha 1 subunit-immunoreactivity. Co-expression of PARV-immunoreactivity characterized the vast majority of the alpha 1 subunit-immunoreactive cells in the medial septum, diagonal band, ventral pallidum and globus pallidus. Striatal alpha 1 subunit-immunopositive neurons appeared PARV-immunonegative and did also not react with the other immunoreagents used in this study, except the GAD-antibody. CR-immunoreactivity was co-expressed in alpha 1 subunit-immunopositive cells of the ventral lateral septal nucleus and only exceptionally in the ventral pallidum, where the vast majority of CR-positive cells was monolabelled. A small minority of ChAT-immunoreactive, but in no case CALB- and NOS-immunoreactive cells were found to express the alpha 1 subunit-immunoreactivity. These findings confirm the data obtained by analyses of other brain regions suggesting a preferred co-existence of this GABAA receptor subunit with PARV and to a lesser degree with CR.

Hypothalamic Leu-enkephalin-immunoreactive fibers terminate on calbindin-containing somatospiny cells in the lateral septal area of the rat

Correlated light and electron microscopic double-immunostaining experiments for Leu-enkephalin and calbindin were employed to determine the postsynaptic targets in the septal complex of Leu-enkephalin fibers. Chronic surgical isolation of the septal complex from its hypothalamic afferents and retrograde tracer studies using wheat germ agglutinin-conjugated horseradish peroxidase, both followed by an immunostaining for Leu-enkephalin, were performed to elucidate the location of the origin of these axon terminals. Furthermore, a colocalization study for glutamic acid decarboxylase and Leu-enkephalin was carried out on hypothalamic sections to determine their possible coexistence in cells projecting to the lateral septum. These studies revealed that 1) Leu-enkephalin-immunoreactive axons form pericellular baskets around a population of lateral septal area neurons; 2) they establish exclusively asymmetric synaptic contacts on their soma and initial dendritic segments; 3) 10% of the lateral septal area calbindin-containing cells, which are all of the gamma-aminobutyric acid (GABA)-ergic somatospiny type, are innervated by Leu-enkephalin-immunoreactive baskets; 4) only 40% of the Leu-enkephalin target neurons are calbindin immunopositive; 5) the septopetal Leu-enkephalin fibers derive from neurons located in the ipsilateral perifornical area and anterior hypothalamus; and 6) none of their cells of origin cocontains the inhibitory transmitter GABA. These observations indicate that hypothalamic Leu-enkephalin-containing neurons are non-GABAergic excitatory cells. Hence, they can effectively stimulate a population of lateral septal area neurons, including the somatospiny cells, which are all GABAergic. Therefore, after stimulatory Leu-enkephalin action, these neurons can inhibit their postsynaptic targets, including other projective lateral septal neurons.

Electrophysiology of the neurons in the area of the enkephalinergic magnocellular dorsal nucleus of the guinea-pig hypothalamus, studied by intracellular and whole-cell recordings

The electrophysiological characteristics of 103 hypothalamic neurons in the area of the guinea-pig enkephalinergic magnocellular dorsal nucleus were studied in a thick slice preparation with sharp microelectrodes (63 neurons) and patch pipettes for whole-cell recordings (40 neurons). Of the sampled cells, 79.6% displayed tetrodotoxin-resistant, calcium-dependent slow-depolarizing potentials when the membrane potential was hyperpolarized to approximately -70 mV (type I neurons). Half of them showed robust slow depolarizing potentials, generating bursts of fast action potentials. In the remaining neurons, the slow-depolarizing potentials did not cause burst-firing action potentials but triggered single action potentials. The other class of neurons (20.4% of the sample: type II neurons) did not exhibit calcium-dependent slow-depolarizing potentials. Resting potential, input resistance and the membrane time constant did not distinguish among the two classes of neurons. Current-voltage relationships were heterogeneous. A transient outward rectification was observed in the two classes. This was not totally blocked by 2 mM 4-aminopyridine but was abolished when using perfusion with cobalt instead of calcium. Input resistance and the time constant were higher when measured in the whole-cell mode but the other electrical parameters and the sampling of the recorded neurons were strikingly similar between the two methods of recording. Intracellular staining of 22 neurons retrogradely labelled from the lateral septum allowed confirmation of their location within the magnocellular dorsal nucleus. The study indicates that the electrical properties of these neurons did not differ from those of neurons found throughout the area explored. It also indicates the presence of distinct electrophysiological types of cells in the magnocellular dorsal nucleus, although the nucleus is composed of a single type of enkephalinergic neuron. It provides a basis for the study of the regulation of activity of the neurons at the origin of an enkephalinergic tractus which is involved in neuroendocrine, psychoneuroendocrine and immune processes.

Aromatase- (estrogen synthetase) immunoreactive neurons in the rat septal area. A light and electron microscopic study

The aromatase enzyme (estrogen synthetase) catalyzes the conversion of testosterone to estrogen in peripheral and central nervous tissue. Light and electron microscopic immunocytochemistry was used to study the localization of this enzyme in the septal area of adult male and female albino rats. Aromatase-immunoreactivity was found restricted to neuronal somata and dendritic arbors, and no sex differences were detected in its distribution or intensity. Most aromatase-immunoreactive neurons formed two oblique bands in the lateral and the medial zones of the lateral septum; in addition, labeled cells were present in the septohippocampal nucleus and the laterodorsal portion of the bed nucleus of the stria terminalis. Electron microscopy revealed that the majority of aromatase-positive neurons in the lateral septum exhibit somatic spines, a characteristic marker of a neuron population that is known to contribute to local and extraseptal projections. The presence of aromatase in lateral septal somatospiny neurons suggests that estrogen formed by these neurons may be critically involved in the septal control of steroid-dependent behaviors.

Effects of intraseptally injected glutamatergic drugs on hippocampal sodium-dependent high-affinity choline uptake in "naive" and "trained" mice

We have previously reported that spatial reference memory (RM) training-induced alterations in hippocampal cholinergic activity as measured by sodium-dependent high-affinity choline uptake (SDHACU). Each training session was found to induce an immediate (30 s) increase in SDHACU followed (30 s to 15 min posttest) by a deactivation and long-lasting inhibition (15 min to 24 h) of this cholinergic marker. The present experiments were designed to assess the role of septal glutamatergic receptors in this posttraining cholinergic deactivation. In the first experiment, the effects of intraseptal injections of different doses of glutamic acid and glutamatergic antagonists (kynurenic acid, KYN, and AP5) on hippocampal SDHACU were studied in awake but otherwise resting (i.e., naive) mice. The results showed that glutamic acid at the lowest dose used (5 ng) produced a decrease in SDHACU, whereas both glutamatergic antagonists produced a dose-related increase in this cholinergic marker. It was concluded that septal glutamatergic receptors mediate a tonic inhibitory input on the cholinergic cells. Hence, in a second experiment the effect of intraseptal injections of KYN (5 ng) on the training-induced changes in hippocampal cholinergic activity were assessed following variable amounts of radial maze RM training. Trained mice were injected 20 min before the first or the ninth training session and killed 30 s or 15 min posttraining for determination of SDHACU. KYN slowed the posttesting cholinergic deactivation (disinhibition), this effect being more marked in good learners than in bad learners. The present findings suggest that septal glutamatergic receptors mediate an inhibitory input on the cholinergic cells, and that this input could play a role in memory consolidation.

Light- and electron-microscopic study of electrophysiologically characterized neurons in the mediolateral part of the lateral spetum of the guinea-pig

In slices of guinea-pig brains, 36 neurons located in the mediolateral part of the lateral septum were stained intracellularly with horseradish peroxidase (n = 28) or biocytin (n = 8) after electrophysiological characterization. These neurons belonged to class A neurons (n = 23), which generated pronounced Ca(++)-dependent high-threshold spikes in control medium, or to class C neurons (n = 9), which were recognized by the occurrence of small-amplitude sodic spikes followed by slower larger calcic spikes. The present results demonstrate that, despite the variety of individual cell types, the major morphological population (30/36 cells) was composed of a homogeneous class of large-sized neurons that displayed thick primary dendrites and abundant dendritic appendages. The remaining 6 cells were small-sized, poorly-spiny neurons. Somatic spines were observed on 5 out of the 30 large cells and on one out of the six smaller cells. Labeled axons were mainly oriented to the anterior commissure. The axons of nine cells richly collateralized near the perikaryon. Ultrastructural examination of 3 horseradish peroxidase-injected cells showed indented nuclei, classic organelles and somatic spines. Terminal boutons established symmetric synapses with the injected cells. These results describe the morphological features of electrophysiologically identified neurons and indicate that class A and class C neurons are distributed among morphological populations differing in perikaryal size. This suggests that the different electrical properties of class A and class C neurons reflect recordings from different parts of the neuron rather than from neurons of different types. Furthermore, the present findings demonstrate that, in the guinea-pig, electrical and morphological characteristics of somatospiny neurons are comparable with those of non-somatospiny neurons. Somatospiny neurons have a recognized integrative role in the hippocampo-septo-hypothalamic complex.

A theoretical and neurophysiological consideration on the pathogenesis of positive symptoms of schizophrenia: implications of dopaminergic function in the emotional circuit

The implications of the emotional circuit and the gating mechanism by dopamine (DA) proposed by Maeda in the pathogenesis of positive symptoms of schizophrenia were reconsidered based upon recent advances and findings in the fields of neurophysiology and neuropharmacology and in biological studies of schizophrenia. The gating mechanism by DA was partly supported by new evidence that glutamatergic or GABAergic neurotransmission, which mediates the hippocampo-lateral septal or the piriform cortico-amygdaloid neuronal connections, is likely to be modulated by DA. The compensation-facilitating or gating functions of DA was considered again to play an important role in producing positive symptoms in schizophrenics, who have been suggested to have morphological abnormalities in the limbic system or in the prefrontal cortex prior to the appearance of positive symptoms.

A comparative study of age-related changes in inhibitory processes and long-term potentiation in the lateral septum of mice

Anaesthetized C57 BL/6 mice of different ages (young: 5 months; middle-aged: 15 months; and old: 21 months) were used to determine whether aging alters the efficiency of synaptic inhibition and long-term potentiation (LTP) in the lateral septum (LS). Electrical stimulation of the fimbria induced field potentials in the ipsilateral LS comprising two initial negative components (N2 and N3) followed by a positive wave of low amplitude. Paired-pulse experiments showed a facilitation of the N2 component and a concomitant depression of the N3 components. Facilitation of the N2 component was stronger in both middle-aged and old mice as compared to young mice, whereas an inverse pattern of changes was observed for inhibition of the N3 component. High-frequency stimulation of the fimbria produced a persistent increase in the N3 amplitude. This LTP was of significantly higher amplitude in both young and middle-aged mice as compared to old mice. These results suggest that aging impairs both inhibitory processes and synaptic plasticity in the mouse LS, but that inhibitory processes appear to be affected earlier.

Comparative distribution of calbindin and Met-enkephalin immunoreactivities in the guinea-pig lateral septum, with reference to electrophysiologically characterized neurons in the mediolateral part

The distribution both of Met-enkephalinergic nerve terminals and of calbindin-containing neurons was investigated in the guinea-pig lateral septum, using a double-immunostaining technique. The findings show that the two immunoreactivities overlapped for neurons located in areas of the dorsal part and of the mediolateral part of the lateral septum. Nine cells of the mediolateral part which were electrophysiologically characterized and intracellularly labelled were subjected to the double-immunostaining protocol. All these cells displayed characteristic discharges due to the activation of high-threshold Ca2+ conductances. Two of them contained calbindin and were the target of enkephalinergic inputs; they possessed somatic spines. These data demonstrate: (1) that the guinea-pig lateral septum contains subpopulations of calbindin neurons which are postsynaptic to enkephalinergic inputs; (2) that Ca2+ conductances are not related to the presence of calbindin; (3) that somatospiny neurons, which are involved in the regulation of the hippocampo-septo-hypothalamic complex, contain calbindin and are the target of enkephalinergic endings.

Serotonergic innervation of the mature and developing lateral septum of the rat: a light and electron microscopic immunocytochemical analysis

The serotonergic innervation of the adult and developing lateral septum of the rat was studied with immunocytochemical techniques at the light and electron microscopic levels. A few, relatively thick serotonergic fibres are found in the lateral septum at the time of birth, but they are restricted to its medial part. They subsequently extend towards the lateral ventricle, increase in number and attain their final distribution pattern by the end of the first postnatal week. Thereafter they become finer, with regularly spaced varicosities, show a higher density, and generally exhibit features, density, and pattern of innervation comparable to the adult at the end of the third postnatal week. In the dorsal portion of the lateral septum, serotonergic fibres form characteristic pericellular basket-like arrangements around cell somata and their primary dendrites. These baskets are first observed at P7, and subsequently increase both in number and in terms of the number of serotonergic terminals which they comprise. The present findings suggest that the development of serotonergic innervation of the lateral septum parallels the neuronal differentiation in this area. Ultrastructural analysis has shown that the vast majority (congruent to 95%) of serotonin varicosities make symmetrical synapses with somata, dendritic shafts and spines. These varicosities in new-born animals are in close association with neuronal elements, without any intervening neuroglial processes, but towards the end of the first postnatal week they exhibit well-defined synaptic specializations. The mean diameter of serotonergic varicosities making synapses does not change substantially with age. Serotonin-receptive neurons have several morphological features in common with the type I cells described in a previous Golgi study of the lateral septum [Alonso and Frotscher (1989) J. comp. Neurol. 286, 472-487]. Some speculations on the chemical identity of the serotonin-receptive cells have been put forward in the present study but double-labelling studies will certainly shed more light on the organization of the serotonergic innervation of the lateral septum.

Aromatase immunoreactivity in the rat brain: gonadectomy-sensitive hypothalamic neurons and an unresponsive "limbic ring" of the lateral septum-bed nucleus-amygdala complex

The aromatase (estrogen synthetase) enzyme catalyzes the conversion of androgens to estrogens in peripheral tissues, as well as in the brain. Our study aimed at comparing the brain distribution of aromatase-immunoreactive neurons in male and female, normal and gonadectomized rats. Light microscopic immunostaining was employed using a purified polyclonal antiserum raised against human placental aromatase. Two anatomically separate aromatase-immunoreactive neuronal systems were detected in the rat brain: A "limbic telencephalic" aromatase system was composed by a large population of labeled neurons in the lateral septal area, and by a continuous "ring" of neurons of the laterodorsal division of the bed nucleus of stria terminalis, central amygdaloid nucleus, stria terminalis, and the substantia inominata-ventral pallidum-fundus striati region. The other, "hypothalamic" aromatase system consisted of neurons scattered in a dorsolateral hypothalamic area including the paraventricular, lateral and dorsomedial hypothalamic nuclei, the subincertal nucleus as well as the zona incerta. In addition, a few axon-like processes (unresponsive to gonadectomy) were present in the preoptic-anterior hypothalamic complex, the ventral striatum, and midline thalamic regions. No sexual dimorphism was observed in the distribution or intensity of aromatase-immunostaining. However, 3 days, 2, 3, 8, 16, or 32 weeks after gonadectomy, aromatase-immunoreactive neurons disappeared from the hypothalamus, whereas they were still present in the limbic areas of both sexes. The results indicate the existence of two distinct estrogen-producing neuron systems in the rat brain: (1) a "limbic ring" of aromatase-labeled neurons of the lateral septum-bed nucleus-amygdala complex unresponsive to gonadectomy; and (2) a sex hormone-sensitive "hypothalamic" aromatase neuron system.

PHA-L analysis of projections from the supramammillary nucleus in the rat

The projections of the supramammillary nucleus (SUM) were examined in the rat by the anterograde anatomical tracer Phaseolus vulgaris leucoagglutinin (PHA-L). The majority of labeled fibers from SUM ascended through the forebrain within the medial forebrain bundle. SUM fibers were found to terminate heavily in the hippocampal formation, specifically within the granule cell layer and immediately adjoining molecular layer of the dentate gyrus. In addition, SUM fibers were shown to distribute densely to several structures with strong connections with the hippocampus, namely, the nucleus reunions of the thalamus, the medial and lateral septum, the entorhinal cortex, and the endopiriform nucleus. SUM fibers were also shown to project significantly to several additional subcortical and cortical sites. The subcortical sites were the dorsal raphe nucleus, the midbrain central gray, the fields of Forel/zona incerta, the dorsomedial hypothalamic area, midline/intralaminar nuclei of the thalamus (posterior paraventricular, rhomboid, central medial, intermediodorsal, and mediodorsal), the medial and lateral preoptic areas, the bed nucleus of the stria terminalis, the substantia innominata, the vertical limb of the diagonal band nucleus, and the claustrum. The cortical sites were the occipital, temporal, parietal, and frontal cortices. Some notable differences were observed in projections from the lateral as compared to the medial SUM. For example, fibers originating from the lateral SUM distributed heavily to the hippocampal formation and parts of the cortex, whereas those from the medial SUM projected sparsely to these two regions. The SUM projections to the hippocampal formation and associated structures may serve as the substrate for a SUM involvement in the generation of the theta rhythm of the hippocampus and the gating of information flow through the hippocampal formation.

Septo-hippocampal and nBM-cortical cholinergic neurones exhibit differential time-courses of activation as a function of both type and duration of spatial memory testing in mice

We previously showed that the initial acquisition session of a spatial discrimination (mixed reference/working memory) test in an 8-arm radial maze induced differential activations in the ascending cholinergic septo-hippocampal and nBM-cortical pathways in mice. This data showed that the duration of post-test cholinergic activation was longer in the nBM-cortical pathway than in the septo-hippocampal projection. Moreover, the post-test durations but not the immediate post-test amplitudes of activation in each pathway decreased progressively as a function of repeated daily acquisition sessions. In the present study we have thus tested the hypotheses that the time-courses of post-test cholinergic activation in the septo-hippocampal and nBM-cortical pathways may vary both as a function of the type of memory used (working vs. reference) and according to the duration of repeated daily testing. Cholinergic activity in vivo in the hippocampus or frontal cortex of mice was quantified using measures of sodium-dependent high-affinity choline uptake at two different times (30 s and 15 min) following specific spatial working or reference memory testing in an 8-arm radial maze. The memory tests were administered daily over a 13-day period to attain high levels of performance in each type of task. In comparison to control groups both types of memory testing induced significant post-test cholinergic activations in each brain region on Day 15. However, cholinergic activity remained elevated in frontal cortex at 15 min post-test following reference memory testing, whereas significantly shorter durations of cortical and hippocampal cholinergic activation were observed following working memory testing using short (1 min) retention intervals. The possible significance of these differential modifications to the time-course of the post-test activations in these cholinergic pathways in working and reference memory processes and the putative transsynaptic mechanisms involved are discussed.

GABAergic mediation of indirect transsynaptic control over basal and spatial memory testing-induced activation of septo-hippocampal cholinergic activity in mice

A neurochemical study of the transsynaptic interactions established between septal GABAergic interneurones and cholinergic septo-hippocampal neurones was conducted using mice. The effects of acute in vivo injections of either muscimol (20-500 ng/0.2 microliter), bicuculline (100 ng-1 micrograms/0.2 microliter) or saline vehicle (0.2 microliter) into the medial septum on septo-hippocampal cholinergic activity were evaluated using measures of hippocampal high affinity choline uptake at 30 min post-injection in two main groups of mice. The first (quiet control) remained in their home cages during the post-injection period whereas the second (active) were submitted, 10 min following injection to a 20-min period of spatial working memory testing in an 8-arm radial maze. Intraseptal injections of either muscimol or bicuculline produced significant (25-50%) inhibition of hippocampal cholinergic activity in quiet conditions (basal) as compared to intact or saline-injected mice. In the active groups, whereas memory testing induced significant cholinergic activation (+15-20%) in intact and saline injected mice at 30 s post-test no significant memory testing-induced activation was observed in either muscimol or bicuculline-injected mice at any dose. The role of septal GABAergic interneurones in the indirect transsynaptic control over the basal and activated states of septo-hippocampal cholinergic activity is discussed with respect to the concept that these complex neuronal interactions contribute to the physiological mechanisms involved in the modulation of working memory performance.