GABAergic neurons in the rat dentate gyrus are innervated by subcortical calretinin-containing afferents

GABA-glutamate supramammillary neurons control theta and gamma oscillations in the dentate gyrus during paradoxical (REM) sleep

Several studies suggest that neurons from the lateral region of the SuM (SuML) innervating the dorsal dentate gyrus (DG) display a dual GABAergic and glutamatergic transmission and are specifically activated during paradoxical (REM) sleep (PS). The objective of the present study is to characterize the anatomical, neurochemical and electrophysiological properties of the SuML-DG projection neurons and to determine how they control DG oscillations and neuronal activation during PS and other vigilance states. For this purpose, we combine structural connectivity techniques using neurotropic viral vectors (rabies virus, AAV), neurochemical anatomy (immunohistochemistry, in situ hybridization) and imaging (light, electron and confocal microscopy) with in vitro (patch clamp) and in vivo (LFP, EEG) optogenetic and electrophysiological recordings performed in transgenic VGLUT2-cre male mice. At the cellular level, we show that the SuML-DG neurons co-release GABA and glutamate on dentate granule cells and increase the activity of a subset of DG granule cells. At the network level, we show that activation of the SuML-DG pathway increases theta power and frequency during PS as well as gamma power during PS and waking in the DG. At the behavioral level, we show that the activation of this pathway does not change animal behavior during PS, induces awakening during slow wave sleep and increases motor activity during waking. These results suggest that the SuML-DG pathway is capable of supporting the increase of theta and gamma power in the DG observed during PS and plays an important modulatory role of DG network activity during this state.

Supramammillary Nucleus Afferents to the Dentate Gyrus Co-release Glutamate and GABA and Potentiate Granule Cell Output

The supramammillary nucleus (SuM) of the hypothalamus projects to the dentate gyrus (DG) and the CA2 region of the hippocampus. Although the SuM-to-hippocampus circuits have been implicated in spatial and emotional memory formation, little is known about precise neural connections between the SuM and hippocampus. Here, we report that axons of SuM neurons make monosynaptic connections to granule cells (GCs) and GABAergic interneurons, but not to hilar mossy cells, in the DG and co-release glutamate and γ-aminobutyric acid (GABA) at these synapses. Although inputs from the SuM can excite some interneurons, the inputs alone fail to generate spikes in GCs. However, despite the insufficient excitatory drive and GABAergic co-transmission, SuM inputs have net excitatory effects on GCs and can potentiate GC firing when temporally associated with perforant path inputs. Our results indicate that the SuM influences DG information processing by modulating GC outputs.

Supramammillary serotonin reduction alters place learning and concomitant hippocampal, septal, and supramammillar theta activity in a Morris water maze

Hippocampal theta activity is related to spatial information processing, and high-frequency theta activity, in particular, has been linked to efficient spatial memory performance. Theta activity is regulated by the synchronizing ascending system (SAS), which includes mesencephalic and diencephalic relays. The supramamillary nucleus (SUMn) is located between the reticularis pontis oralis and the medial septum (MS), in close relation with the posterior hypothalamic nucleus (PHn), all of which are part of this ascending system. It has been proposed that the SUMn plays a role in the modulation of hippocampal theta-frequency; this could occur through direct connections between the SUMn and the hippocampus or through the influence of the SUMn on the MS. Serotonergic raphe neurons prominently innervate the hippocampus and several components of the SAS, including the SUMn. Serotonin desynchronizes hippocampal theta activity, and it has been proposed that serotonin may regulate learning through the modulation of hippocampal synchrony. In agreement with this hypothesis, serotonin depletion in the SUMn/PHn results in deficient spatial learning and alterations in CA1 theta activity-related learning in a Morris water maze. Because it has been reported that SUMn inactivation with lidocaine impairs the consolidation of reference memory, we asked whether changes in hippocampal theta activity related to learning would occur through serotonin depletion in the SUMn, together with deficiencies in memory. We infused 5,7-DHT bilaterally into the SUMn in rats and evaluated place learning in the standard Morris water maze task. Hippocampal (CA1 and dentate gyrus), septal and SUMn EEG were recorded during training of the test. The EEG power in each region and the coherence between the different regions were evaluated. Serotonin depletion in the SUMn induced deficient spatial learning and altered the expression of hippocampal high-frequency theta activity. These results provide evidence in support of a role for serotonin as a modulator of hippocampal learning, acting through changes in the synchronicity evoked in several relays of the SAS.

Major diencephalic inputs to the hippocampus: supramammillary nucleus and nucleus reuniens. Circuitry and function

The hippocampus receives two major external inputs from the diencephalon, that is, from the supramammillary nucleus (SUM) and nucleus reuniens (RE) of the midline thalamus. These two afferents systems project to separate, nonoverlapping, regions of the hippocampus. Specifically, the SUM distributes to the dentate gyrus (DG) and to CA2 of the dorsal and ventral hippocampus, whereas RE projects to CA1 of the dorsal and ventral hippocampus and to the subiculum. SUM and RE fibers to the hippocampus participate in common as well as in separate functions. Both systems would appear to amplify signals from other sources to their respective hippocampal targets. SUM amplifies signals from the entorhinal cortex (EC) to DG, whereas RE may amplify them from CA3 (and EC) to CA1 of the hippocampus. This "amplification" may serve to promote the transfer, encoding, and possibly storage of information from EC to DG and from CA3 and EC to CA1. Regarding their unique actions on the hippocampus, the SUM is a vital part of an ascending brainstem to hippocampal system generating the theta rhythm of the hippocampus, whereas RE importantly routes information from the medial prefrontal cortex to the hippocampus to thereby mediate functions involving both structures. In summary, although, to date, SUM and RE afferents to the hippocampus have not been extensively explored, the SUM and RE exert a profound influence on the hippocampus in processes of learning and memory.

Immunocytochemical localization of calretinin-containing neurons in the rat periaqueductal gray and colocalization with enzymes producing nitric oxide: a double, double-labeling study

The pattern of distribution and colocalization of the calcium-binding protein calretinin (Cal) and of enzymes producing nitric oxide (NO) was examined in the rat periaqueductal gray matter (PAG) using two different experimental approaches, by combining Cal immunocytochemistry with NADPH-diaphorase (NADPH-d) histochemistry and with NOS immunocytochemistry, respectively. Cal-immunopositive neurons were found throughout the rostrocaudal extension of both dorsolateral (PAG-dl) and ventrolateral PAG (PAG-vl). Double-labeled neurons were found only in PAG-dl. The first experimental approach indicated that 33-41% of the NADPH-d-positive (Nadph+) cells were immunoreactive for Cal, whereas NADPH-d activity appeared in 19-26% of the Cal-immunopositive (Cal(IP) ) neurons. Two-color immunofluorescence revealed that ∼39-43% of NOS-immunoreactive (NOS(IR) ) neurons were double-labeled with Cal and ∼23% of Cal(IP) neurons expressed NOS immunoreactivity. Measurement in semithin sections of the size of the three neuronal populations found in PAG-dl, showed that Cal(IP) neurons had a cross-sectional area of 94.7 μm², whereas Nadph+ neurons and double-labeled neurons were slightly smaller, having a cross-sectional area of 90.5 and 91.4 μm², respectively. On electron microscopy, Cal(IP) axon terminals formed either symmetric or asymmetric synapses; although the latter synapses were more numerous, both types contacted preferentially Cal(IP) dendrites. These experiments suggest that PAG-dl is characterized by a high degree of heterogeneity.

Glutamatergic neurons of the mouse medial septum and diagonal band of Broca synaptically drive hippocampal pyramidal cells: relevance for hippocampal theta rhythm

Neurons of the medial septum and diagonal band of Broca (MS-DBB) provide an important input to the hippocampus and are critically involved in learning and memory. Although cholinergic and GABAergic MS-DBB neurons are known to modulate hippocampal activity, the role of recently described glutamatergic MS-DBB neurons is unknown. Here, we examined the electrophysiological properties of glutamatergic MS-DBB neurons and tested whether they provide a functional synaptic input to the hippocampus. To visualize the glutamatergic neurons, we used MS-DBB slices from transgenic mice in which the green fluorescent protein is expressed specifically by vesicular glutamate transporter 2-positive neurons and characterized their properties using whole-cell patch-clamp technique. For assessing the function of the glutamatergic projection, we used an in vitro septohippocampal preparation, electrically stimulated the fornix or chemically activated the MS-DBB using NMDA microinfusions and recorded postsynaptic responses in CA3 pyramidal cells. We found that glutamatergic MS-DBB neurons as a population display a highly heterogeneous set of firing patterns including fast-, cluster-, burst-, and slow-firing. Remarkably, a significant proportion exhibited fast-firing properties, prominent I(h), and rhythmic spontaneous firing at theta frequencies similar to those found in GABAergic MS-DBB neurons. Activation of the MS-DBB led to fast, AMPA receptor-mediated glutamatergic responses in CA3 pyramidal cells. These results describe for the first time the electrophysiological signatures of glutamatergic MS-DBB neurons, their rhythmic firing properties, and their capacity to drive hippocampal principal neurons. Our findings suggest that the glutamatergic septohippocampal pathway may play an important role in hippocampal theta oscillations and relevant cognitive functions.

Heterogeneity of the supramammillary-hippocampal pathways: evidence for a unique GABAergic neurotransmitter phenotype and regional differences

The supramammillary nucleus (SuM) provides substantial projections to the hippocampal formation. This hypothalamic structure is involved in the regulation of hippocampal theta rhythm and therefore the control of hippocampal-dependent cognitive functions as well as emotional behavior. A major goal of this study was to characterize the neurotransmitter identity of the SuM-hippocampal pathways. Our findings demonstrate two distinct neurochemical pathways in rat. The first pathway originates from neurons in the lateral region of the SuM and innervates the supragranular layer of the dorsal dentate gyrus and, to a much lesser extent, the ventral dentate gyrus. This pathway displays a unique dual phenotype for GABAergic and glutamatergic neurotransmission. Axon terminals contain markers of GABAergic neurotransmission, including the synthesizing enzyme of GABA, glutamate decarboxylase 65, and the vesicular GABA transporter and also a marker of glutamatergic neurotransmission, the vesicular glutamate transporter 2. The second pathway originates from neurons in the most posterior and medial part of the SuM and innervates exclusively the inner molecular layer of the ventral dentate gyrus and the CA2/CA3a pyramidal cell layer of the hippocampus. The axon terminals from the medial part of the SuM contain the vesicular glutamate transporter 2 only. These data demonstrate for the first time the heterogeneity of the SuM-hippocampal pathways, not only from an anatomical but also a neurochemical point of view. These pathways, implicated in different neuronal networks, could modulate different hippocampal activities. They are likely to be involved differently in the regulation of hippocampal theta rhythm and associated cognitive functions as well as emotional behavior.

Origin of calretinin-containing, vesicular glutamate transporter 2-coexpressing fiber terminals in the entorhinal cortex of the rat

The entorhinal cortex of the rat (EC) contains a dense fiber plexus that expresses the calcium-binding protein calretinin (CR). Some CR fibers contain vesicular glutamate transporter 2 (VGluT2, associated with glutamatergic neurotransmission). CR-VGluT2 coexpressing fibers may have an extrinsic origin, for instance, the midline thalamic nucleus reuniens. Alternatively, they may belong to cortical interneurons. We studied the first possibility with anterograde and retrograde neuroanatomical tracing methods combined with CR and VGluT2 immunofluorescence and confocal laser scanning. The alternative possibility was studied with in situ hybridization fluorescence histochemistry for VGluT2 mRNA combined with CR immunofluorescence. In the anterograde tracing experiments, we observed many labeled reuniens fibers in EC expressing CR. Some of these labeled fibers contained immunoreactivity for VGluT2 and CR. In the complementary retrograde tracing experiments, we found retrogradely labeled cell bodies in nucleus reuniens of the thalamus that coexpressed CR. We also examined the colocalization of VGluT2 and CR in the entorhinal cortex by using in situ hybridization and CR immunofluorescence. In these experiments, we observed CR-immunopositive cortical neurons that coexpressed VGluT2. For the same sections, with CR as the principal marker and parvalbumin as a control marker, we found that parvalbumin neurons were negative for VGluT2 mRNA. Thus, CR-VGluT2-expressing axon terminals in EC belong to two sources: projection fibers from the thalamus and axon collaterals of local interneurons. VGluT2 expression is linked to the synaptic transmission of the excitatory neurotransmitter glutamate, so these thalamic CR-VGluT2 projection neurons and entorhinal CR-VGluT2 interneurons should be regarded as excitatory.

Ectopic granule cells of the rat dentate gyrus

Granule cells of the mammalian dentate gyrus normally form a discrete layer, and virtually all granule cells migrate to this location. Exceptional granule cells that are positioned incorrectly, in 'ectopic' locations, are rare. Although the characteristics of such ectopic granule cells appear similar in many respects to granule cells located in the granule cell layer, their rare occurrence has limited a full evaluation of their structure and function. More information about ectopic granule cells has been obtained by studying those that develop after experimental manipulations that increase their number. For example, after severe seizures, the number of ectopic granule cells located in the hilus increases dramatically. These experimentally-induced ectopic granule cells may not be equivalent to normal ectopic granule cells necessarily, but the vastly increased numbers have allowed much more information to be obtained. Remarkably, the granule cells that are positioned ectopically develop intrinsic properties and an axonal projection that are similar to granule cells that are located normally, i.e., in the granule cell layer. However, dendritic structure and synaptic structure/function appear to differ. These studies have provided new insight into a rare type of granule cell in the dentate gyrus, and the plastic characteristics of dentate granule cells that appear to depend on the location of the cell body.

Extrinsic afferent systems to the dentate gyrus

The dentate gyrus is the first stage of the intrahippocampal, excitatory, trisynaptic loop, and a primary target of the majority of entorhinal afferents that terminate in a laminar fashion on granule cell dendrites and carry sensory information of multiple modalities about the external world. The electric activity of the trisynaptic pathway is controlled mainly by different types of local, GABAergic interneurons, and subcortical and commissural afferents. In this chapter we will outline the origin and postsynaptic targets in the dentate gyrus of chemically identified subcortical inputs. These systems are afferents originating from the medial septum/diagonal band of Broca GABAergic and cholinergic neurons, neurochemically distinct types of neurons located in the supramammillary area, serotonergic fibers from the median raphe, noradrenergic afferents from the pontine nucleus, locus ceruleus, dopamine axons originating in the ventral tegmental area, and the commissural projection system. Because of the physiological implications, these afferents are discussed in the context of the glutamatergic innervation of the dentate gyrus. One common feature of the extrinsic dentate afferent systems is that they originate from a relatively small number of neurons. However, the majority of these afferents are able to exert a powerful control over the electrical activity of the hippocampus. This strong influence is due to the fact that the majority of the extrinsic afferents terminate on a relatively small, but specific, populations of neurons that are able to control large areas of the hippocampal formation.

The supramammillary area: its organization, functions and relationship to the hippocampus

The supramammillary area of the hypothalamus, although small in size, can have profound modulatory effects on the hippocampal formation and related temporal cortex. It can control hippocampal plasticity and also has recently been shown to contain cells that determine the frequency of hippocampal rhythmical slow activity (theta rhythm). We review here its organization and anatomical connections providing an atlas and a new nomenclature. We then review its functions particularly in relation to its links with the hippocampus. Much of its control of behaviour and its differential activation by specific classes of stimuli is consistent with a tight relationship with the hippocampus. However, its ascending connections involve not only caudal areas of the cortex with close links to the hippocampus but also reciprocal connections with more rostral areas such as the infralimbic and anterior cingulate cortices. These latter areas appear to be the most rostral part of a network that, via the medial septum, hippocampus and lateral septum, is topographically mapped into the hypothalamus. The supramammillary area is thus diffusely connected with areas that control emotion and cognition and receives more topographically specific return information from areas that control cognition while also receiving ascending information from brain stem areas involved in emotion. We suggest that it is a key part of a network that recursively transforms information to achieve integration of cognitive and emotional aspects of goal-directed behaviour.

A novel population of calretinin-positive neurons comprises reelin-positive Cajal-Retzius cells in the hippocampal formation of the adult domestic pig

Calretinin-containing neurons in the hippocampal formation, including the subiculum, presubiculum, parasubiculum, and entorhinal cortex, were visualized with immunocytochemistry. Calretinin immunoreactivity was present exclusively in non-principal cells. The largest immunoreactive cell population was found in the outer half of the molecular layer of the dentate gyrus and in the stratum lacunosum-moleculare of Ammon's horn. A proportion of these cells were also immunoreactive for reelin, a Cajal-Retzius cell marker. Similar calretinin-positive cells were found in the molecular layer of the subicular complex and entorhinal cortex. In the parasubiculum, a few immunoreactive bipolar and multipolar cells could be observed in the superficial and deep pyramidal cell layers. In the entorhinal cortex, bipolar and multipolar calretinin-positive cells were frequent in layer II, and large numbers of multipolar cells in layer V were immunoreactive. Electron microscopic analysis showed that somata of calretinin-positive cells contained either round nuclei with smooth nuclear envelopes or nuclei with multiple deep infoldings. Immunoreactive dendrites were smooth varicose, and the apposing axon terminals formed both symmetric and asymmetric synapses. Zonula adherentia were observed between calretinin-positive dendrites. Calretinin-positive axon terminals formed two types of synapses. Axon terminals with asymmetric synapses were found close to the hippocampal fissure, whereas axon terminals forming symmetric synapses innervated spiny dendrites in both the molecular layer of the dentate gyrus and in stratum lacunosum-moleculare of Ammon's horn. Calretinin-positive axon terminals formed both symmetric and asymmetric synapses with calretinin-positive dendrites. In conclusion, calretinin-positive neurons form two major subpopulations in the adult domestic pig hippocampus: (1) a gamma-aminobutyric acid (GABA)ergic subpopulation of local circuit neurons that innervates distal dendrites of principal cells in both the dentate gyrus and in Ammon's horn; and (2) Cajal-Retzius type cells close to the hippocampal fissure, as well as in the molecular layer of the subicular complex and entorhinal cortex.

Structural and functional asymmetry in the normal and epileptic rat dentate gyrus

The rat dentate gyrus is usually described as relatively homogeneous. Here, we present anatomic and physiological data which demonstrate that there are striking differences between the supra- and infrapyramidal blades after status epilepticus and recurrent seizures. These differences appear to be an accentuation of a subtle asymmetry present in normal rats. In both pilocarpine and kainic acid models, there was greater mossy fiber sprouting in the infrapyramidal blade. This occurred primarily in the middle third of the hippocampus. Asymmetric sprouting was evident both with Timm stain as well as antisera to brain-derived neurotrophic factor (BDNF) or neuropeptide Y (NPY). In addition, surviving NPY-immunoreactive hilar neurons were distributed preferentially in the suprapyramidal region of the hilus. Extracellular recordings from infrapyramidal sites in hippocampal slices of pilocarpine-treated rats showed larger population spikes and weaker paired-pulse inhibition in response to perforant path stimulation relative to suprapyramidal recordings. A single stimulus could evoke burst discharges in infrapyramidal granule cells but not suprapyramidal blade neurons. BDNF exposure led to spontaneous epileptiform discharges that were larger in amplitude and longer lasting in the infrapyramidal blade. Stimulation of the infrapyramidal molecular layer evoked larger responses in area CA3 than suprapyramidal stimulation. In slices from the temporal pole, in which anatomic evidence of asymmetry waned, there was little evidence of physiological asymmetry either. Of interest, some normal rats also showed signs of greater evoked responses in the infrapyramidal blade, and this could be detected with both microelectrode recording and optical imaging techniques. Although there were no signs of hyperexcitability in normal rats, the data suggest that there is some asymmetry in the normal dentate gyrus and this asymmetry is enhanced by seizures. Taken together, the results suggest that supra- and infrapyramidal blades of the dentate gyrus could have different circuit functions and that the infrapyramidal blade may play a greater role in activating the hippocampus.

Synaptotagmin I hypothalamic knockdown prevents amygdaloid seizure-induced damage of hippocampal neurons but not of entorhinal neurons

We have previously demonstrated that an acute pharmacological interruption of the afferent inputs from the hypothalamus to the hippocampus resulted in the blockade of the genesis and spread of intra-amygdala kainate-induced seizure activity in the hippocampus. This finding suggests that a sustained interruption of the hypothalamic stimulative influences may completely prevent amygdaloid seizure-induced hippocampal neuron damage. To test this assumption, we delivered antisense oligodeoxynucleotides (ODNs) against synaptotagmin I, a regulatory protein of the transmitter release machinery, into the hypothalamus by using a Hemagglutinating virus of Japan (HVJ)-liposome-mediated gene transfer technique. Four days prior to the induction of status epilepticus by intra-amygdala injection of kainate, the synaptotagmin I antisense was injected into the supramammillary nucleus (SuM) of the hypothalamus to chronically suppress the stimulative influences to the hippocampus via the reduction of transmitter release. The synaptotagmin I hypothalamic knockdown resulted in the almost complete prevention of seizure-induced damage of hippocampal neurons but not of entorhinal neurons following the kainate-induced amygdaloid seizures. This result suggests that the hypothalamic stimulative influences to the hippocampus have a major contribution to the amygdaloid seizure-induced hippocampal sclerosis, probably via disinhibition mechanism.

Increased extracellular release of hippocampal NE is associated with tetanization of the medial perforant pathway in the freely moving adult male rat

The induction of long-term potentiation (LTP) within the dentate gyrus of the hippocampal formation is modulated by many afferent influences from a number of subcortical structures known to be intimately involved in hippocampal-dependent learning and memory. It has been demonstrated in slice and anesthetized preparations that norepinephrine (NE) is one of these major neuromodulators involved in the induction of LTP. However, the majority of these studies have not been conducted in the freely moving animal. Recently, we developed surgical procedures and instrumentation techniques to simultaneously record electrophysiological and neurochemical data from the hippocampal formation. The present study uses these techniques to examine the underlying neurochemical changes in the hippocampus associated with the induction of hippocampal dentate LTP in the freely moving adult rat. These findings establish baseline levels of NE that can be used to evaluate the impact of various tetanization paradigms as well as the effect of a variety of insults on hippocampal plasticity.

Hormonal regulation of hippocampal spine synapse density involves subcortical mediation

It is well established that estrogen has positive effects on the density of pyramidal cell spines in the hippocampal CA1 subfield. This study explored whether afferent connections of the hippocampus that come from estrogen-sensitive subcortical structures, including the septal complex, median raphe and supramammillary area, play a role in this estrogen-induced hippocampal synaptic plasticity. These particular subcortical structures have major influences on hippocampal activity, including theta rhythm and long-term potentiation. The latter also promotes the formation of new synapses. All of the rats were ovariectomized; the fimbria/fornix, which contains the majority of subcortical efferents to the hippocampus, was transected unilaterally in each, and half of the animals received estrogen replacement. Using unbiased electron microscopic stereological methods, the CA1 pyramidal cell spine synapse density was calculated. In the estrogen-treated rats, contralateral to the fimbria/fornix transection, the spine density of CA1 pyramidal cells increased dramatically, compared to the spine density values of both the ipsilateral and contralateral hippocampi of non-estrogen-treated animals and to that of the ipsilateral hippocampus of the estrogen replaced rats. These observations indicate that fimbria/fornix transection itself does not considerably influence CA1 area pyramidal cell spine density and, most importantly, that the estrogenic effect on hippocampal morphology, in addition to directly affecting the hippocampus, involves subcortical mediation.

The supramammillary nucleus contributes to associative EPSP-spike potentiation in the rat dentate gyrus in vivo

The supramammillary nucleus (SUM) of the hypothalamus sends neural projections to the hippocampus and is supposed to be involved in learning and memory. To test the possibility that SUM afferents modulate hippocampal functions, we investigated the effect of electrical stimulation of the SUM on the induction of long-term potentiation (LTP) at medial perforant path (PP)--granule cell synapses in the dentate gyrus (DG) of anaesthetized rats. High-frequency stimulation of the SUM (100 pulses at 100 Hz) alone did not change PP--DG field potentials. However, when the SUM stimulation was applied simultaneously with weak tetanic stimulation of the PP (20 pulses at 20 Hz) which alone did not induce any potentiation, it produced a long-lasting potentiation of the population spike, without an accompanying increase in the population excitatory postsynaptic potential (EPSP). The EPSP-spike (E-S) potentiation induced by pairing SUM and PP stimulation was abolished by lesions of the fimbria--fornix, a major pathway of SUM afferents. SUM stimulation applied 1 s before or after PP stimulation failed to produce E-S potentiation, and SUM stimulation augmented PP--DG field potentials during tetanic stimulation. Furthermore, the E-S potentiation was abolished by blocking GABAergic neurotransmission with picrotoxin. These results suggest that coactivation of SUM and PP inputs produces a long-lasting increase of granule cell excitability by modulating GABAergic inhibition. SUM afferents may contribute to associative memory processing by modulating hippocampal excitability.

Supramammillary area mediates subcortical estrogenic action on hippocampal synaptic plasticity

It is well established that systemically administered estrogen to ovariectomized rats positively affects the density of pyramidal cell spines in the hippocampal CA1 subfield and intact subcortical connections of the hippocampus are essential in this hormonal action. This study explored whether local estrogen administration into the supramammillary area influences the density of CA1 pyramidal cell spine synapses in ovariectomized rats. The first group of experiments using a combination of retrograde tracer technique and immunostaining for estrogen receptor-alpha demonstrated that a large population of supramammillary area estrogen receptor-alpha-containing neurons projects to the hippocampus. Animals belonging to the second experimental group were ovariectomized and received cannulae filled with 0.4% 17 beta-estradiol placed unilaterally into the supramammillary area. Control animals received a cholesterol-containing cannula into the supramammillary area or an estrogen-filled cannula implanted into the head of the caudate nucleus. One week later, rats were killed and CA1 pyramidal cell spine synapse density was determined using electron microscopic unbiased stereological procedures. Animals that received an estrogen-filled cannula into the supramammillary area exhibited a significantly higher (37%) density of CA1 pyramidal cell spine synapses than both other control groups. These observations indicate that the supramammillary area is involved in mediating synaptoplastic, estrogenic effects to the hippocampus.

Collateral projections from the supramammillary nucleus to the medial septum and hippocampus

Previous reports have shown that the supramammillary nucleus projects to the medial septum and to the hippocampus, and specifically to the dentate gyrus and the CA2/CA3a region of the hippocampus. The aim of the present study was to examine collateral projections from the supramammillary nucleus to the septum and hippocampus. The fluorescent retrograde tracers, Fluororuby and Fluorogold, were injected into regions of the septum and hippocampus, respectively, and the supramammillary nucleus was examined for the presence of single- and double-labeled neurons. The main findings were: 1) pronounced numbers of single-labeled cells (about 40-60/section) were present in the supramammillary nucleus following retrograde tracer injections in either the septum or hippocampus; 2) single and double retrogradely labeled neurons were intermingled within the supramammillary nucleus and mainly localized to the lateral two-thirds of the supramammillary nucleus; 3) approximately 5-10% of supramammillary cells were double-labeled, ipsilaterally, and 2-4%, contralaterally, with injections in medial or lateral parts of the medial septum and the dentate gyrus of the hippocampus; and 4) approximately 3-5% of supramammillary cells were double-labeled, ipsilaterally, and 1-2%, contralaterally, with injections in the medial septum and CA2/CA3a of the dorsal hippocampus. Cells of the supramammillary nucleus have been shown to fire rhythmically in bursts synchronous with the hippocampal theta rhythm and have been implicated in the generation of the theta rhythm. The supramammillary cells that we identified with collateral projections to the septum and hippocampus may be directly involved in generation of the theta rhythm.

The supramammillo-hippocampal and supramammillo-septal glutamatergic/aspartatergic projections in the rat: a combined [3H]D-aspartate autoradiographic and immunohistochemical study

It is well established that the supramammillary nucleus plays a critical role in hippocampal theta rhythm generation/regulation by its direct and indirect (via the septal complex) connections to the hippocampus. Previous morphological and electrophysiological studies indicate that both the supramammillo-hippocampal and supramammillo-septal efferents contain excitatory transmitter. To test the validity of this assumption, transmitter specific retrograde tracer experiments were performed. [3H]D-aspartate was injected into different locations of the hippocampus (granular and supragranular layers of the dentate gyrus and CA2 and CA3a areas of the Ammon's horn) and septal complex (medial septum and the area between the medial and lateral septum) that are known targets of the supramammillary projection. Consecutive vibratome sections prepared from the entire length of the posterior hypothalamus, including the supramammillary area, were immunostained for calretinin, tyrosine hydroxylase, or calbindin, and further processed for autoradiography. Radiolabeled, radiolabeled plus calretinin-containing, and calretinin-immunoreactive neurons were plotted at six different oro-caudal levels of the supramammillary area. The results demonstrated that following both hippocampal and septal injection of the tracer, the majority of the retrogradely radiolabeled (glutamatergic/aspartatergic) cells are immunoreactive for calretinin. However, non-radiolabeled calretinin-containing neurons and radiolabeled calretinin-immunonegative cells were also seen, albeit at a much lower density. These observations clearly indicate the presence of glutamatergic/aspartatergic projections to both the hippocampus and septal complex. It may be assumed that this transmitter could play a role in hippocampal theta rhythm generation/regulation.

Laminar boundaries persist in the hippocampal dentate molecular layer of the mutant Shaking Rat Kawasaki despite aberrant granule cell migration

The present report provides the first detailed description of the hippocampus in the Shaking Rat Kawasaki (SRK) mutant by using a panel of antibody markers to delineate its laminar organization. The mutant was characterised at postnatal day 21 by severe malformations of both neuronal position and orientation, the most striking of which was the presence of a rounded central granule cell mass in the dentate gyrus rather than the normal V-shaped granule cell layer. Despite this finding, the SRK dentate gyrus not only retained a cell-sparse molecular layer (thinner but similar in gross appearance to that of control littermates), but the sharp laminar boundary between its inner and outer parts was as clearly marked by IM1 and OM4 antibody staining as it was in the normal dentate gyrus. These immunocytochemical data suggest that the entorhinal terminal field of the dentate gyrus may be relatively normal in the mutant, despite entorhinal afferents appearing to take an abnormal trajectory after they fail to cross the hippocampal fissure. Laminar malformations included disruption of the SRK pyramidal cell layer, with spreading of the CA3 mossy fibre projection to an ectopic infrapyramidal position, radial displacement of CA1 pyramids, and transposition of a hitherto unremarked longitudinal fibre bundle immunoreactive for calretinin from its normal position in the stratum lacunosum-moleculare of field CA2 to an alvear position in SRK. The SRK malformations were very like but not identical to those seen in the mouse reeler mutant, suggesting similar underlying developmental mechanisms.

The entorhino-septo-supramammillary nucleus connection in the rat: morphological basis of a feedback mechanism regulating hippocampal theta rhythm

Recent electrophysiological observations suggest that, in addition to the medial septal area pacemaker system, several alternative or additional mechanisms are involved in the generation/regulation of hippocampal theta activity. Discharging neurons phase-locked to hippocampal theta waves have been observed in the dorsal raphe, nucleus reticularis pontis oralis and especially in the supramammillary region of rats. Since these areas are reciprocally interconnected with the hippocampal formation, including the entorhinal cortex, it would aid our understanding of limbic function to elucidate the location and neurochemical content of the entorhino-septal and septo-supramammillary projection neurons, as well as that of their postsynaptic targets. Light and electron microscopic immunostaining for calretinin, in combination with antero- and retrograde tracer techniques, postembedding immunostaining for GABA and the transmitter specific [3H]D-aspartate retrograde radiolabeling, as well as a co-localization experiment for calretinin and glutamate decarboxylase in rat supramammillary and septal neurons, demonstrated that: (i) a large population of entorhinal cells that forms asymmetric synaptic contacts on calretinin-containing neurons located at the border between the medial and lateral septal areas contains calretinin and are aspartate/glutamatergic; (ii) the overwhelming majority of calretinin-immunoreactive cells located at the border between the lateral and medial septal area are GABAergic; (iii) these neurons can be retrogradely labeled from the supramammillary area; (iv) anterogradely labeled axons originating in the border between the medial and lateral septum are GABAergic and (v) terminate on supramammillary area non-GABAergic, calretinin-containing neurons, which are known to project to the septal complex and hippocampus. These observations indicate that a large population of cells participating in the hippocampal feedback regulation of theta regulation/generation contain the same calcium-binding protein. Furthermore, entorhinal excitatory transmitter-containing neurons can depress the activity of supramammillary theta generating/regulating cells via septal inhibitory neurons.

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

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