Septohippocampal Neurons Research Articles

In vivo acetylcholine release as measured by microdialysis is unaltered in the hippocampus of cognitively impaired aged rats with degenerative changes in the basal forebrain

Acetylcholine (ACh) release was studied in awake, freely moving animals in vivo microdialysis in the hippocampus of young (3-month-old) and aged (24-month-old) female Sprague-Dawley rats. Two groups of aged rats were selected on basis of their spatial learning performance in the Morris water maze: non-impaired aged rats which performed as well as the young control animals, and impaired aged rats which learnt the task very poorly. Baseline ACh overflow (in the presence of 5 μM neostigmine) was 1.9 ± 0.3 ±pmol/15min in the young animals and 1.6 ± 0.4pmol/15min in both the impaired and the non-impaired aged rats; these levels did not differ from each other. Depolarization by KCl (100 mM) or muscarinic receptor blockade by atropine (3 μM) added to the perfusion fluid produced dramatic, 4–6-fold, increases in ACh overflow that was similar in magnitude in both the young and the aged impaired and non-impaired rats. Behavioral activation by either handling or electrical stimulation of the lateral habenula produced 2–3-fold increases in extracellular ACh-levels in the hippocampus similarly in all three groups. The results indicate that hippocampal ACh release is maintained in aged rats that exhibit severe spatial learning and memory impairments and that the septo-hippocampal cholinergic system retains its capacity to increase its ACh release in response to both K +-induced depolarization and behavioral activation in the aged rat. In the subsequent morphometric analysis the cholinergic neurons in the septal-diagonal band area showed the expected reduction in both number (−20–30%) and size (−8%) in the aged impaired animals. This suggests that the ageing septo-hippocampal cholinergic system can compensate functionally for ongoing degeneration or atrophic processes also in the most severely affected aged animals. The present data do not exclude, however, that the behaviorally impaired aged rats may exhibit functionally important deficits in other dynamic aspects of cholinergic neurotransmission, or in the functional properties of the septo-hippocampal neurons not picked up by the microdialysis method.

Colchicine-induced cholinergic denervation of the hippocampus elicits sympathetic ingrowth

Sympathetic neurons from the peripheral nervous system invade the hippocampus following destruction of its septal inputs. It is thought that sympathetic ingrowth is due to the loss of cholinergic innervation since damage to the medial septum-diagonal band complex (MSDB) or its major efferent bundle, the fimbria-fornix, is required to induce ingrowth. The MSDB provides the major source of cholinergic fibers projecting to the hippocampus; however, non-cholinergic (e.g. GABAergic) neurons are also present in the MSDB and project to the hippocampus. Thus, the role of cholinergic denervation in sympatho-hippocampal sprouting cannot be directly tested by non-specific lesion techniques. In the present study, colchicine, which has been demonstrated to be specifically toxic to cholinergic neurons in the medial septum, was injected into each lateral ventricle of female Sprague-Dawley rats. Following colchicine-induced degeneration of cholinergic septohippocampal neurons, coarse, branched fibers immunoreactive for dopamine-beta-hydroxylase were observed predominantly in the dentate gyrus, on both sides of the granule cell layer, with increasing density as survival time increased. These results support the hypothesis that the invasion of the hippocampal formation by sympathetic fibers results from cholinergic denervation.

Survival-promoting effect of NGF on in vitro septohippocampal neurons with cholinergic and GABAergic phenotypes

Recently, we demonstrated a survival-promoting effect of nerve growth factor (NGF) on cultured hippocampus-projecting neurons from developing septum/diagonal band region using fluorescent latex microspheres as a retrograde neuronal marker (Arimatsu et al., 1989). In the present study, we characterized these projection neurons by combining the retrograde cell labeling and histochemical stainings for acetylcholinesterase (AChE) activity and NGF receptor-, choline acetyltransferase- (ChAT-) and γ-aminobutyric acid- (GABA-) immunoreactivities. The surviving microsphere-labeled neurons were, for the most part, immunoreactive for NGF receptor in the culture. A great majority (about 90%) of the microsphere-labeled neurons showed strong AChE activity and ChAT-immunoreactivity. The number of strongly AChE-positive neurons and that of ChAT-immunoreactive neurons in the culture supplemented with NGF was much greater with than without exogenous NGF. In addition, a major part (about 70%) of the microsphere-labeled neurons exhibited GABA-immunoreactivity in the presence of NGF. The number was also much greater than that without NGF. A considerable portion of cultured septal cholinergic neurons were shown to express GABA-immunoreactivity by a two-color immunofluorescence labeling experiment for ChAT and GABA. These findings are consistent with the assumption that NGF plays an important role in the development and organization of the cholinergic and GABAergic septohippocampal systems by supporting the neuronal survival, and raise a possibility that cholinergic and GABAergic fractions of the septohippocampal neurons may be developmentally correlated.

A comparison of the working memory performances of young and aged mice combined with parallel measures of testing and drug-induced activations of septo-hippocampal and nbm-cortical cholinergic neurones

The spatial working memory performances of young (2 months) and aged (24–26 months) mice of the C57BL/6 strain were compared using a delayed nonmatching to place (DNMTP) protocol in an automated 8-arm radial maze. The aged mice were observed to exhibit a selective and interference-related memory deficit. Parallel neurochemical analysis of the activity of septo-hippocampal and nbm-cortical cholinergic neurones in vivo was conducted using measures of sodium-dependent high-affinity choline uptake. Results showed that whereas the level of cholinergic activity in both brain regions varied less than 10% between young and aged mice in quiet conditions (basal) the activation usually observed at 30-sec posttest (+20–25%) in young mice was greatly attenuated in the frontal cortex and almost totally absent in the hippocampus of aged mice. In view of these results a complementary experiment was carried out in order to test the intrinsic ability of septo-hippocampal cholinergic neurones to activate using acute injection of scopolamine (1 mg/kg IP 20 min) to both young and aged mice in quiet conditions. The drug injection resulted in a very large (+70%) increase in hippocampal high-affinity choline uptake and with amplitudes which did not vary significantly between young and aged subjects. These observations attest to a relatively well-preserved state of central cholinergic neurones and an intact capacity to activate normally when challenged pharmacologically in aged mice. The results strongly suggest that the loss of cholinergic activation and associated memory deficit in aged mice might rather be related to a hypofunction of phasically active transsynaptic processes which normally mediate the activation of these cholinergic pathways during memory testing.

Distribution of septohippocampal neurons containing parvalbumin or choline acetyltransferase in the rat brain

A combination of retrograde transport of horseradish peroxidase or wheat germ agglutinin-colloidal gold with either single or double-label immunohistochemistry is used to describe the comparative topographic distribution of parvalbumin- and choline acetyltransferase-immunoreactive septal neurons that project to the hippocampal formation of the rat. The morphometric parameters of the retrogradely labelled, parvalbumin-containing neurons were very similar, if not identical, to those neurons of the midline and medial part of the medial septum and the diagonal band regions that had previously been shown to be immunoreactive for gamma-aminobutyric acid or for glutamate decarboxylase following colchicine treatment. The total number of parvalbumin-immunoreactive and choline acetyltransferase-positive retrogradely labelled cells was counted at 9 representative levels through the rostrocaudal extension (from 2.4 mm anterior to the level of bregma) of the medial septal-diagonal band complex. In the whole medial septum-vertical limb of the diagonal band region, about 33% of the total retrogradely labelled neurons showed immunoreactivity to parvalbumin, whereas the parvalbumin-negative cells were mainly choline acetyltransferase-immunopositive. In comparison with the average figure, the proportion of the retrogradely labelled parvalbumin-containing neurons was higher in the middle part (around 1.5 mm anterior to the bregma) than in either the rostral or caudal ends. The reverse was true for the distribution of the cholinergic septohippocampal neurons. At the maximum levels the parvalbumin-immunoreactive neurons accounted for more than half of the total retrogradely labelled cells in 4 out of 6 rats. Moreover, within the complexity of the septal neurons, a marked regularity of topographic organisation was observed in the distribution of retrogradely labelled parvalbumin-containing GABAergic and choline acetyltransferase-positive cholinergic neurons as if they were subdivided cytoarchitectonically.

Fate of septohippocampal neurons following fimbria-fornix transection: A time course analysis

Neurons in the medial septum (MS) and vertical limb of the diagonal band (vDB) undergo degenerative changes following transection of their axons. These changes have been well studied by histological techniques such as Nissl stains and immunocytochemistry. A dramatic loss of stained neurons occurs following axotomy and this has been interpreted as indicative of neuronal death. However, since the staining intensity and the size of affected neurons may be reduced by axotomy, it is possible that the apparent neuronal death may actually be due to a decrease in somal size or the ability to detect neurons by routine histological methods. The present study describes the effects of axotomy on MS and vDB neurons which have been labeled by hippocampal injections of the retrograde tracer, Fluoro-Gold (FG), prior to transection of the fimbria-fornix and supracallosal stria. The number of FG-labeled neurons in the MS decreased by 21% at three weeks, 36% at six weeks, and 31% at ten weeks after fimbria-fornix transection. The reduction was statistically significant at 6 and 10 weeks. The number of FG-labeled neurons in the vDB showed no reduction at three weeks but was decreased by 31% and 37% at six and ten weeks, respectively. This was statistically significant only at 10 weeks. By comparison, the number of neurons immunoreactive for choline acetyltransferase (ChAT) was reduced by 75–80% at these time points. The size (area and diameter) of FG-labeled somata decreased in both the MS and vDB within three weeks following fimbria-fornix transection and remained relatively constant at the six- and ten-week time points. These data suggest that the majority of MS and vDB neurons which project to the hippocampus do not die within 10 weeks following axotomy but rather shrink and become undetectable by routine histological methods.

Intrafimbrial colchicine produces transient impairment of radial-arm maze performance correlated with morphologic abnormalities of septohippocampal neurons expressing cholinergic markers and nerve growth factor receptor

The microtubule disrupting agent, colchinine, was infused both bilaterally and unilaterally into the fimbria of the rat brain. Such infusions produced a transient impairment in radial-arm maze performance during the first week following surgery but only in bilaterally injected animals. This behavioral finding was correlated with a reduction in the number of neurons expressing choline acetyltransferase and nerve growth factor receptor in the septum and vertical limb of the diagonal band but not in other regions of the basal nuclear complex. The altered expression of the two neurochemical markers was not due to cellular degeneration because numbers of neurons demonstrated by Nissl staining were unchanged. Putative cholinergic fibers in the fimbria demonstrating acetylcholinesterase and nerve growth factor receptor also showed aberrations in caliber, shape, and course.

Release of acetylcholine from the medial septum/diagonal band of rat brain

N-Methyl-D-aspartate (NMDA)-evoked release of [3H]acetylcholine (ACh) from slices of rat brain medial septum/vertical limb of the diagonal band (ms/vdB) was examined. NMDA increased the release of tritium in a concentration-dependent manner and the specific non-competitive NMDA antagonist, MK-801, and the competitive NMDA antagonist kynurenic acid inhibited this release. Tetrodotoxin inhibited the NMDA-evoked release suggesting the [3H]ACh released arises from collaterals of the cholinergic septohippocampal neurons. Basal release of tritium was significantly increased by glycine alone and strychnine inhibited this response while having no effect on NMDA-evoked release. However, glycine, although not affecting the NMDA-evoked release, did enhance release of tritium in the presence of NMDA and blocking concentrations of Kynurenic acid. Together, these findings suggest that under the conditions of these experiments sufficient concentrations of glycine permit the full expression of NMDA-evoked modulation of [3H]ACh release, and that the predominant actions of glycine were mediated by a specific, strychnine-sensitive receptor.

Aging and stress-induced changes in choline and glutamate uptake in hippocampus and septum of two rat strains differing in longevity and reactivity to stressors

Stress induced changes in neurochemical indices of neurotransmission are more pronounced in the septohippocampal cholinergic system of Wistar Kyoto rats, which are behaviorally more reactive to Stressors and have a shorter life span, than in Brown Norway rats. Moreover, pronounced degeneration of septohippocampal cholinergic neurons occurs earlier in life in Wistar Kyoto rats. In the present study the high affinity synaptosomal uptakes of choline and glutamate were used as indices for cholinergic and glutamatergic systems respectively. Following 2 hr of mild restraint stress increases in both uptake systems were observed in all regions examined (hippocampus, septum and frontal cortex). The stress-induced increases were generally similar in young (3 months) and aged (20 months) rats of both strains. The noted exception was that choline uptake levels, which were reduced in the hippocampus of unhandled aged WKY rats, remained unchanged after stress. The results confirm the involvement of the septohippocampal cholinergic system in the response to acute stress and extend the findings to include the hippocamposeptal glutamatergic system activation as well. It is suggested that in spite of neuronal degeneration during aging, these responses to stress can be maintained by compensatory efforts of neurons that remain intact.

Septal α-noradrenergic antagonism in vivo blocks the testing-induced activation of septo-hippocampal cholinergic neurones and produces a concomitant deficit in working memory performance of mice

In order to test the hypothesis that alpha-noradrenergic receptors in the septum 1) play an important functional role in the mediation of trans-synaptic control of the neurones of the cholinergic septo-hippocampal pathway and 2) produce resultant modulation of working memory performance, we have investigated the effects in vivo of the acute intraseptal injection of an alpha-antagonist, phenoxybenzamine, in mice. Neurochemical analysis was performed using measures of the kinetics of sodium-dependent high-affinity choline uptake in samples of hippocampus from injected mice and their relevant controls in both quiet conditions and immediately following selective working memory testing in an 8-arm radial maze. Results show that whereas the injection of phenoxybenzamine produces no significant alteration of the activity of the cholinergic septo-hippocampal neurones in quiet conditions, the pretrial (20 min) administration of this drug almost totally abolished the usually observed increase in hippocampal cholinergic activity induced by testing. This inhibition of cholinergic activation was associated with a parallel working memory deficit. The results provide further direct support for the hypothesis that septal noradrenergic afferents via alpha-receptors mediate a phasic and net excitatory trans-synaptic influence on the cholinergic septo-hippocampal pathway during working memory testing and thereby significantly contribute to the modulation of the level of working memory performance.

Age-related loss of galanin-immunoreactive cells in the rat septal area

We have examined the distribution of galanin-like immunoreactive (LI) cell bodies in the medial septal nucleus (MS) and the nucleus of the diagonal band of Broca (nDBB) of young (3 months) and aged (25–30 months) rats, and assessed their respective contribution to the septohippocampal pathway. Immunohistochemical techniques were used alone or combined with the retrograde transport of a protein-gold complex injected into the dorsal hippocampus. In both groups, galanin-LI cells were observed in the MS and the nDBB. In aged rats, a significant decrease in both the staining intensity and the number of galanin-LI perikarya throughout the MS-nDBB complex was observed. Some immunoreactive cells appeared shrunken. The reduction in cell number ranged from 30 to 85%. There was also a decrease in the proportion of septohippocampal neurons containing galanin in aged rats (13% vs 20% in young animals) which however did not reach statistical significance. These results suggest that galanin-positive cells in the medial septal area undergo alterations with aging in the rat.

A study of NADPH-diaphorase positive septohippocampal neurons in rat

Retrograde transport of fluorescent tracers and nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) histochemical techniques were combined in a study of septohippocampal projections in the rat. The dorsal (DH) and ventral (VH) hippocampus were simultaneously injected with different tracers (Fast Blue or Fluoro-Gold). Histochemical procedures revealed many NADPH-d positive cells located in the medial septum and the horizontal limb of the diagonal band. In the medial septum, NADPH-d positive neurons were mostly located lateral to the midline region and some of these were double-labeled by the tracer injected into the VH. Also, NADPH-d positive cells were found in the horizontal diagonal band and some of these were double-labeled following injections into the DH. No fluorescence/NADPH-d double-labeled neurons were observed in other structures known to project to the hippocampus.

Detection of an atropine-resistant component of the hippocampal theta rhythm in urethane-anesthetized rats

An important pharmacological feature of the hippocampal θ rhythm in urethane-anesthetized animals is its apparent sensitivity to antimuscarinic drugs. This sensitivity may be partly due to a masking of the θ frequency by increases in both higher and lower frequency EEG components that are unrelated to any residual θ rhythm. The discovery of atropine-resistant, rhythmic medial septal neurons has provided a physiological trigger for averaging EEG and unit activity after large atropine doses. Such averaging has permitted the detection of an atropine-resistant component of the hippocampal θ rhythm in urethane-anesthetized rats. The postatropine θ activity recorded from both CA1 (superficial to the pyramidal cell layer) and dentate (near the hippocampal fissure) in 15 rats was typically reduced in amplitude, but the recordings from the two locations maintained their phase relations to the septal units and to each other. The presence of this residual θ component after doses as large as 100 mg/kg indicates that it cannot be mediated by muscarinic cholinergic receptors. The coupling of the signal to the atropine-resistant septal cells strengthens our previous suggestion that these septo-hippocampal neurons are not cholinergic, and are therefore probably GABAergic.

Rhythmical bursting activity and GABAergic mechanisms in the medial septum of normal and pertussis toxin-pretreated rats.

The possible involvement of GABA in the control of the rhythmical bursting activity (RBA) of septo-hippocampal neurons (SHNs) has been studied in the rat in vivo. The discharge frequency of SHNs was modified by the iontophoretic application of a GABA agonist and antagonist as well as by the application of the GABA uptake blocker, nipecotic acid. The GABAB agonist baclofen inhibited the SHNs' activity, this effect being antagonized by the GABAB antagonist phaclofen. However, these different pharmacological manipulations did not modify the RBA frequency. Pretreatment of the rats with pertussis toxin, a substance which is known to block the events mediated by G-proteins (Gi or Go), decreased the RBA frequency. Neither agonists nor antagonists of GABAA or GABAB types had significant effects on the rhythmical bursting activity of SHNs. The effect of pertussis toxin suggests that other neurotransmitters or intrinsic mechanisms involving a G-protein influence this rhythm.

Galanin blocks the slow cholinergic EPSP in CA1 pyramidal neurons from ventral hippocampus

Using intracellular recordings from slice preparation, we studied the effects of the peptide galanin on the properties of CA1 pyramidal neurons from rat ventral hippocampus. Galanin, applied in the superfusing medium, had a weak and inconsistent effect on the membrane potential or on the afterhyperpolarization which follows a train of spikes. Galanin, which is localized in some cholinergic neurons of the septo-hippocampal pathway, did not affect the action of acetylcholine or carbachol on CA1 pyramidal neurons. However, it did have a presynaptic inhibitory effect on the cholinergic terminals, blocking the slow cholinergic excitatory post-synaptic potential (EPSP) induced by the release of endogenous acetylcholine on the pyramidal neurons. This effect was reversible and mimicked by atropine. These results suggest that the peptide galanin, colocalized with acetylcholine in some septo-hippocampal neurons might play a role in the control of acetylcholine release.

Involvement of a pertussis toxin‐sensitive G‐protein in the pharmacological properties of septo‐hippocampal neurones

1. The physiological and pharmacological properties of identified septo-hippocampal neurones (SHNs) have been studied in rats pretreated with the bacterial toxin, pertussis toxin (PTX). 2. In rats anaesthetized with urethane and pretreated with PTX, the axonal conduction velocity was unchanged while the mean spontaneous activity was significantly increased. 3. PTX pretreatment had no effect on responses of SHNs to the iontophoretic application of gamma-aminobutyric acid (GABA) and cholinoceptor agonists (acetylcholine or carbachol). 4. Baclofen and 5-hydroxytryptamine (5-HT), almost exclusively inhibitory in control rats, had little effect or an excitatory effect in PTX pretreated rats. 5. These results suggest the involvement of a pertussis toxin-sensitive G-protein in responses medicated by 5-HT and GABAB-receptors but not in responses mediated by cholinoceptors and GABAA-receptors in medial septum neurones projecting into the hippocampus.

Septo-hippocampal neurons in the aged rat: Relation between their electrophysiological and pharmacological properties and behavioral performances

The performances of aged (24–26 months) rats in two behavioral tasks (passive avoidance and spontaneous alternation) have been studied. Subpopulations of old animals were found to be impaired in these tasks. Most of the impaired animals, however, were not impaired in both tasks. The properties of the septo-hippocampal neurons (SHNs) were subsequently studied in the same group of experimental animals, anesthetized with urethane, using electrophysiological techniques. The spontaneous activity of SHNs displaying a rhythmically bursting activity (RBA) was significantly higher in animals impaired in the spontaneous alternation task. The proportion of SHNs with RBA was significantly lower and the frequency of the RBA was higher in animals impaired in the passive avoidance task. The pharmacological properties of the SHNs were not significantly different in the various groups. The significance of these complex correlations with regard to the age-related alterations of SHNs properties is discussed.

Hemicholinium-3 selectively alters the rhythmically bursting activity of septo-hippocampal neurons in the rat

The medial septal area contains neurons which project to the hippocampal formation. A sizeable proportion of these septo-hippocampal neurons (SHNs) are cholinergic. About 40% of them also display a characteristic discharge pattern in rhythmic bursts. We hypothesized that SHNs with a rhythmically bursting activity (RBA) are the cholinergic ones. To test this hypothesis we studied the effects of acetylcholine synthesis blockade by hemicholinium-3 (HC-3) on the properties of the SHNs. HC-3 (16, 32 or 64 μg total dose) or saline were injected in the lateral ventricles of adult male Sprague-Dawley rats anesthetized with urethane. Extracellular recordings from SHNs in the medial septal area were obtained within hours after HC-3 injections ( n = 24 animals). SHNs were identified by their antidromic response following electrical stimulation of the fimbria-fornix. The pharmacological properties of SHNs were studied in some animals using microiontophoretic applications from multibarreled electrodes filled with various neurotransmitters. The hippocampal rhythmic slow activity (RSA or theta) was abolished even after the lowest dose of HC-3 tested (16 μg). No significant change in SHNs conduction velocity or spontaneous activity was observed at any dose of HC-3. The percentage of SHNs with RBA was unchanged. In contrast the mean frequency of the RBA was decreased by HC-3 in a dose-dependent fashion. The mean frequency was lowest within the first 3 h after injection. Although the mean spontaneous activity was unchanged SHNs tended to have more spikes per burst. The effects of various neurotransmitters on SHNs were qualitatively unchanged after HC-3 injection. These results suggest that acetylcholine synthesis blockade by HC-3 leads not only to the disappearance of the hippocampal RSA in urethane-anesthetized animals, but also to a decrease in the frequency of the rhythmically bursting activity of the SHNs. Since the 4-Hz hippocampal theta is atropine-sensitive, the results provide indirect evidence that the SHNs with rhythmically bursting activity are the cholinergic SHNs.

Cholinergic and peptidergic projections from the medial septum and the nucleus of the diagonal band of broca to dorsal hippocampus, cingulate cortex and olfactory bulb: A combined wheatgerm agglutinin-apohorseradish peroxidase-gold immunohistochemical study

We have examined the distribution pattern and the density of various neuropeptide, neurotransmitter and enzyme containing neurons in the rat medial septum and the nucleus of the diagonal band of Broca to assess their possible involvement in the septohippocampal, septocortical and septobulbar pathways. Immunohistochemical methods were combined with the retrograde transport of a protein-gold complex injected in the hippocampus, the cingulate cortex or the olfactory bulb. Cholinergic neurons were the most numerous. Galanin-positive neurons were about two or three times less numerous than cholinergic cells. Both these cell types had a similar location though the choline acetyl transferase-like immunoreactive cells extended more caudally in the horizontal limb of the nucleus of the diagonal band of Broca. Immunoreactive cells for other neuroactive substances were few (calcitonin gene-related peptide, luteinizing hormone releasing hormone, [Met]enkephalin-arg-gly-leu) or occasional (dynorphin B, vasoactive intestinal polypeptide, somatostatin, neurotensin, cholecystokinin, neuropeptide Y and substance P). No immunoreactive cells for bombesin, α atrial natriuretic factor, corticotropin releasing factor, 5-hydroxytryptamine, melanocyte stimulating hormone, oxytocin, prolactin, tyrosine hydroxylase or arg-vasopressin were present. Choline acetyltransferase- and galanin-like immunoreactive cells densely participate to septal efferents. Cholinergic neurons constituted the bulk of septal efferent neurons. Galanin-positive cells were 22% of septohippocampal, 8% of septocortical, and 9% of septobulbar neurons. Galanin containing septohippocampal neurons were found in the medial septum and the nucleus of the diagonal band of Broca; galanin-positive septobulbar and septocortical cells were limited to the nucleus of the diagonal band of Broca. Occasional double-labellings were noticed with some peptides other than galanin. Luteinizing hormone-releasing hormone, calcitonin gene-related peptide and enkephalin were the most often observed; some other projecting cells stained for vasoactive intestinal polypeptide or dynorphin B. Luteinizing hormone-releasing hormone, calcitonin gene-related peptide and enkephalin were observed in septohippocampal neurons; luteinizing hormone-releasing hormone and vasoactive intestinal peptide were observed in septocortical neurons and calcitonin gene-related peptide, luteinizing hormone-releasing hormone and dynorphin B were observed in septo-bulbar cells. These results show that, in addition to acetylcholine, galanin is a major cellular neuroactive substance in septal projections to the hippocampus, the cingulate cortex and the olfactory bulb. The presence of septal projecting neurons immunoreactive for other peptides shows that a variety of distinct peptides may also participate, but in a smaller number, to septal efferent pathways.

Effect of psychotropic drugs on identified septohippocampal neurons

The effects of various psychotropic drugs (benzodiazepines, antidepressants, neuroleptics and nootropic drugs, a family of cognition activator agents) on firing rates of septohippocampal neurons, identified by electrical antidromic stimulation, were studied in the medial septum-nucleus of the diagonal band of Broca of rats anaesthetized with urethan. Extracellular potentials from single septohippocampal neurons were recorded using glass pipettes. Drugs were applied by either microiontophoresis or intravenous injections (i.V.). Benzodiazepines produced a marked depression of spontaneous firing rates of septohippocampal neurons whether applied i.v. (diazepam) or iontophoretically (flurazepam, midazolam). In addition, diazepam had a potent depressant effect on the rhythmically bursting activity of the septohippocampal neurons. Baclofen also had an inhibitory effect. Antidepressant drugs (applied by iontophoresis) as well as amphetamine, had a depressant effect on spontaneous firing rates. Neuroleptics (i.v.) had less significant or consistent effects on septohippocampal neurons, although the effects of haloperidol were usually inhibitory. Nootropic drugs were generally ineffective. These data indicate that most psychotropic drugs tested (with the exception of nootropic drugs) have an inhibitory effect on the spontaneous activity of septohippocampal neurons. However, benzodiazepines seem to be more active than antidepressants or neuroleptics. Oxotremorine (i.v.) had a potent excitatory effect on septohippocampal neurons. Atropine (i.v.) increased the septohippocampal neurons' firing rate in some cases. These results are discussed in view of the possible implication of the involvement of septohippocampal neurons in the mediation of the effects of psychotropic drugs on the central nervous system and, more specifically, on the cholinergic systems.