The distribution of septal projections to the hippocampus of the rat

Response of septal cholinergic neurons to axotomy

In the present study we employed quantitative morphometric techniques to assay the response of septal cholinergic neurons following unilateral transection of the fimbria/fornix and supracallosal stria. Analysis of 50-micron-thick tissue sections with a Quantimet 920 image analysis system demonstrated a reduction in ChAT immunoreactivity as early as 1 day following denervation. This decrease was associated with a drop in the number of labeled cells ipsilateral to the lesion and a decrease in the area of cholinergic perikarya on the lesioned and nonlesioned side of the septum. The response at 1 day, however, was transient, and at 4 days the number of labeled neurons was not significantly different from controls. By 8 days we observed a dramatic reduction in the number and size of ChAT-positive cells ipsilateral to the lesion and a reduction in the size of cholinergic perikarya on the contralateral (i.e., nonlesioned) side. These values persisted throughout the remainder of the study. To assess more completely the morphologic response of neurons to axotomy than can be determined in 50-micron-thick tissue sections, we embedded the adjacent immunolabeled tissue section in Epon and then serially sectioned it to a thickness of 0.75-1.0 micron. By using this method, we were able to measure the area, length, and width of the cell, the area of the nucleus and nucleolus, and the position of the nucleus (i.e., eccentricity). Measurements were performed on ChAT-labeled and nonlabeled cells. The results of our studies demonstrate that cholinergic and noncholinergic cells responded to axotomy in a characteristic yet different fashion from each other and that this response could be quantitatively assayed. In general, labeled and nonlabeled cells on the lesioned side of the septum shrink in response to denervation. This shrunken state was reflected in measurements of cellular area, length, width, and nuclear area. Moreover, other measurements of cellular morphology (i.e., area of the nucleolus, position of the nucleus) indicate that none of the neuronal populations examined in the present study displayed morphologic evidence of regeneration. Our results indicate a dramatic loss of cholinergic perikarya ipsilateral to the lesion. Moreover, although a few neurons do persist they do so in a shrunken state. These data provide an essential baseline for the second study in this series, which will evaluate the effect of nerve growth factor on the survival of denervated septal neurons.

Modulation of the excitability of septohippocampal terminals in the rat: relation to neuronal discharge rate

The excitability of the axonal terminals of medial septal neurons projecting to the dentate gyrus has been studied in the anesthetized rat under various experimental conditions: spontaneous or drug-induced variations in neuronal soma discharge rate, conditioning stimulation of afferent pathways (perforant path, commissural pathway, fimbria-fornix). It has been observed that terminals excitability is inversely correlated to the level of neuronal ongoing activity. These effects were observed on virtually all septal neurons projecting to the dentate gyrus. Since about one half of the septohippocampal neurons are likely to be cholinergic, it follows that such a phenomenon is not transmitter specific.

Hippocampus of the seizure-sensitive gerbil is a specific site for anatomical changes in the GABAergic system

The brains of seizure-sensitive (SS) and seizure-resistant (SR) gerbils were studied with an immunocytochemical method to localize glutamic acid decarboxylase (GAD) to determine whether a defect existed in the inhibitory GABAergic system similar to that which has been reported in animal models of focal epilepsy in which GABAergic cell bodies and terminals are decreased in number. A major difference between the two strains of gerbils was found in the number of GABAergic neurons in the hippocampal formation. Specifically, a paradoxical increase occurred in the number of glutamate decarboxylase GAD-immunoreactive neurons: there were approximately 65% more GABAergic cells within the dentate gyrus and the CA3 region of the hippocampus in the SS gerbils. Furthermore, the density of GAD-immunoreactive puncta, the light microscopic correlates of synaptic boutons, was greater in the SS animals. Other histological methods were used to determine if the difference between SS and SR gerbils was specific for the GABAergic system. Nissl-stained preparations showed that the number of granule cells in the dentate gyrus was 20% greater in SS gerbils than in SR gerbils. An examination of some hippocampal afferents, efferents, and intrinsic connections with acetylcholinesterase histochemistry and the Timm's stain for heavy metals demonstrated no differences between the two strains. In addition, Golgi-stained preparations of the dentate gyrus indicated that the morphology of basket cells did not differ between the two strains nor between the gerbil and the rat. Several brain regions in addition to the hippocampus were studied to determine whether or not the increased number of GAD-immunoreactive neurons was specific for the hippocampal formation. These regions included the substantia nigra, motor cortex, and nucleus reticularis thalami and were selected because they contain large populations of GABAergic neurons and have been implicated in seizure activity. No differences between the two strains were detected in any of these regions. Therefore, a major morphological difference between the brains of SS and SR gerbils exists in the hippocampal formation of SS gerbils in which an increase occurs in the number of GABAergic neurons and granule cells. If these additional inhibitory neurons act mainly to inhibit other inhibitory neurons, the net effect would be increased disinhibition of the principal excitatory neurons of the hippocampal formation. This could lead to seizure activity within the hippocampal formation and at distant sites through multiple synaptic connections.

The septo-hippocampal pathway in patients suffering from senile dementia of Alzheimer's type. Evidence for neuronal plasticity?

In 7 cases of senile dementia of Alzheimer's type (SDAT) and in 7 age-matched controls, nerve cells of the medial septal nuclei (area CH1) and of the vertical limb of the diagonal band of Broca (area CH2) of the right hemisphere were counted in Cresyl-fast-violet-stained serial sections. The granular cells of the ipsilateral fascia dentata were Golgi-stained and dendritic spine density was quantified in 10-micron segments. There was a significant loss of neurons in areas CH1 and CH2 in SDAT cases compared to controls. The spine density of the granular cell dendrites was significantly reduced in the distal parts of the dendrites. In the most proximal part, where cholinergic septal fibers form synapses, the spine density was not significantly different between the two groups. We assume that collateral sprouting of undamaged inputs occurs maintaining a constant number of spines in the proximal segments despite the loss of source neurons within CH1 and CH2.

Detailed projection patterns of septal and diagonal band efferents to the hippocampus in the rat with emphasis on innervation of CA1 and dentate gyrus

The detailed patterns of afferentation to the ammon's horn and dentate gyrus of the hippocampus in the rat were investigated employing the anterograde tracer Phaseolus vulgaris leuco-agglutinin (PHA-L) after punctate iontophoretic injections in the medial septum (MS) and vertical limb of the diagonal band of Broca (VDB). The topographically ordered innervation pattern was different in the regio superior (or CA1) vs. the regio inferior (or CA3) and in the dorsal vs. ventral aspects of ammon's horn and dentate gyrus. The CA1 pyramidal and dentate granule cell layers in the dorsal hippocampus received afferent input almost exclusively from the VDB, whereas those cell layers in ventral hippocampus were supplied from both VDB and MS. The PHA-L labeled projecting fibers could be differentiated into two distinct fiber systems. One class of thick and coarse axons (tentatively called type I fibers) carried fewer but larger terminal boutons and were found to infiltrate the entire stratum oriens, dentate hilus, all layers of the regio inferior and the CA1 str. moleculare. A second, delicate thin (type II) fiber system provided with numerous and passant varicosities showed a much more restricted laminar innervation pattern and appeared to originate from areas in MS-VDB which are rich in AChE-positive neurons. The densest type II fiber networks could be observed in the CA1 subpyramidal and dentate supragranular zones, in the CA1 stratum lacunosum-moleculare and in the dentate middle third molecular layer. This laminar type II innervation pattern showed a remarkable coincidence with the reported distribution of cholinergic marker enzymes. The topographic and spatial organization of the projections described above will be discussed in relation to their possible functional significance.

Septohippocampal and commissural pathways antagonistically control inhibitory interneurons in the dentate gyrus

Available evidence suggests that a portion of the septohippocampal pathway may form inhibitory synapses on inhibitory interneurons in the dentate gyrus. In contrast, a portion of the commissural input from the contralateral hilus may form excitatory synapses on inhibitory interneurons. To ascertain whether these pathways synapse onto a common population of interneurons, a series of pulses were applied and their effects on perforant path evoked, granule cell population spikes were measured. The population spike was markedly reduced when the perforant path pulse was preceded by a pulse to the contralateral hilus. This inhibition was markedly reduced, however, when a medial septal pulse was applied either prior to, or within 3 ms after the commissural pulse. The disinhibition was critically dependent on the temporal relationship between the medial septal and commissural pulse, and not on the medial septal-perforant path relationship. This finding suggests that the septohippocampal pathway inhibits the interneurons through which the commissural pathway is able to inhibit granule cells.

An immunocytochemical study of choline acetyltransferase-containing neurons and axon terminals in normal and partially deafferented hippocampal formation

Monoclonal antibodies to the acetylcholine synthesizing enzyme, choline acetyltransferase (ChAT), have been used to study putative cholinergic structures in immunocytochemical preparations of normal rat hippocampal formation and of hippocampal formation deprived of its septal innervation. Small numbers of ChAT-positive (ChAT+) neuronal somata were observed scattered throughout the septotemporal extent of the normal hippocampal formation. They were most common in stratum lacunosum-moleculare of regio superior, but were also found in various layers of the dentate gyrus and occasionally in the remaining hippocampal laminae. In addition, light microscopy demonstrated that ChAT+ terminal fields in normal hippocampal formation were organized in discrete bands and laminae. Pronounced dense bands were observed: immediately superficial to stratum granulosum; deep to stratum pyramidale; and at the border between stratum radiatum and stratum lacunosum-moleculare. In the dentate gyrus, ChAT+ staining was pronounced in the hilus at temporal levels, but only moderate staining occurred in the anterior hilus and throughout the molecular layer. A close correspondence was observed in the density and distribution of ChAT+ immunoreactivity and acetylcholinesterase staining. Electrolytic lesions of the medial septal nucleus/diagonal band complex had no effect on the occurrence of ChAT+ somata, but virtually abolished the ChAT+ laminar staining pattern and eliminated all but occasional small patches of ChAT+ terminals. These results confirm that the vast majority of hippocampal cholinergic terminals originate either from neurons of the medial septum/diagonal band complex or from fibers of passage. The newly observed intrinsic hippocampal neurons can account for at least some of the ChAT activity remaining after septal lesions, and they apparently contribute to the cholinergic innervation of the hippocampal formation.

Effects of selective lesions of fimbria-fornix on learning set in the rat

The effects of selective partial lesions of the Fimbria-Fornix (FiFx) on reversal and place learning sets were investigated in rats by using a T-maze and a semi-circular multiple discrimination apparatus. Lesions restricted to the Fimbria (Fi) produced a significant deficit in reversal and place learning set, whereas lesions to the Fornix (Fx) only disturbed the learning set based on a reversal procedure. Combined Fi + Fx lesions resulted in impairment in the retention of spatial discrimination tested in the two mazes. Ventral Hippocampal Commissure (vhc) had no significant effect on reversal learning set. These results confirm previous data that the hippocampal formation is involved in learning transfer, and suggest that the Fi and the Fx may play a role in learning set. Our data also confirm previous demonstrations of the ability of rats to rapidly acquire place learning set.

Quantitative light microscopic autoradiographic localization of cholinergic muscarinic receptors in the human brain: forebrain

The distribution of muscarinic cholinergic receptors in the human forebrain and cerebellum was studied in detail by quantitative autoradiography using N-[3H]methylscopolamine as a ligand. Only postmortem tissue from patients free of neurological diseases was used in this study. The highest densities of muscarinic cholinergic receptors were found in the striatum, olfactory tubercle and tuberal nuclei of the hypothalamus. Intermediate to high densities were observed in the amygdala, hippocampal formation and cerebral cortex. In the thalamus muscarinic cholinergic receptors were heterogeneously distributed, with densities ranging from very low to intermediate or high. N-[3H]Methylscopolamine binding was low in the hypothalamus, globus pallidus and basal forebrain nuclei, and very low in the cerebellum and white matter tracts. The localization of the putative muscarinic cholinergic receptors subtypes M1 and M2 was analysed in parallel using carbachol and pirenzepine at a single concentration to partially inhibit N-[3H]methylscopolamine binding. Mixed populations of both subtypes were found in all regions. M1 sites were largely predominant in the basal ganglia, amygdala and hippocampus, and constituted the majority of muscarinic cholinergic receptors in the cerebral cortex. M2 sites were preferentially localized in the diencephalon, basal forebrain and cerebellum. In some areas such as the striatum and substantia innominata there was a tendency to lower densities of muscarinic cholinergic receptors with increasing age. In general, we observed a slight decrease in M2 sites in elderly cases. Muscarinic cholinergic receptor concentrations seemed to be reduced following longer postmortem periods. The distribution of acetylcholinesterase was also studied using histochemical methods, and compared with the localization of muscarinic cholinergic receptors and other cholinergic markers. The correlation between the presence of muscarinic cholinergic receptors and the involvement of cholinergic mechanisms in the function of specific brain areas is discussed. Their implication in neurological diseases is also reviewed.

Reactive synaptogenesis in hippocampal area CA1 of aged and young adult rats

Selective lesions that result in a partial loss of neuronal input appear to signal residual, undamaged inputs to sprout and replace synaptic connections that have been lost. Previous investigations have compared this process of reactive synaptogenesis in young and old animals in the hippocampal dentate gyrus and have demonstrated that the aged brain has a diminished capacity for reinnervation following massive denervation of a target area. This investigation has focused on the lesion-induced plasticity of an adjacent area of the hippocampal formation, area CA1 of regio superior, in young adult and aged rats. Young adult aged Fischer 344 rates were subjected to a unilateral, intraventricular injection of kainic acid that selectively destroyed the CA3-CA4 hippocampal pyramidal neurons. Following a 2-day interoperative interval, the rats sustained an ipsilateral transection of the fimbria-fornix. Animals were killed at 4, 10, 30, and 60 days following the second transection and processed for electron microscopic analysis. Photographic montages were constructed of area CA1 extending from the alveus to the hippocampal fissure. The density of synapses, both intact and degenerating, was determined and analyzed as a function of age, days postlesion, and zone of analysis. Synaptic density decreased 30-40% contralaterally and 60-70% ipsilaterally in both aged and young adult rats. While both age groups restored synaptic density to preoperative levels, aged subjects required significantly more time. Aged rats appeared to be retarded in the initial phases of synaptic replacement. The delay in the aged animals' reactive response was not due to any differences in degeneration clearance between the age groups.

Autoradiographic localization of M1 and M2 muscarine receptors in the rat brain

The distribution of M1 and M2 muscarine receptors in the rat brain was investigated by in vitro autoradiography. Muscarine receptors were visualized after complete receptor uncoupling in ethylenediaminetetraacetic acid buffer containing 1 mM N-ethyl maleimide and saturation with the ligand [3H]quinuclidinyl benzilate. Pirenzepine, an M1-selective antagonist, was used in our assays as a counter ligand to occlude M1 sites, allowing the primary ligand, [3H]quinuclidinyl benzilate, to label the remaining M2 muscarine receptors. In adjacent section, M1 muscarine receptors were labelled with [3H]quinuclidinyl benzilate in the presence of sufficient carbachol, and M2-selective agonist, to inhibit the binding to M2 sites. Our results reveal a heterogeneous distribution of M1 and M2 receptors. Increased densities of carbachol-resistant and pirenzepine-sensitive sites (M1 receptor subtype) were apparent over many forebrain structures including the olfactory tubercle, caudate-putamen, nucleus accumbens, hippocampus, amygdala and cerebral cortex. In contrast, pirenzepine-resistant and carbachol-sensitive sites (M2 receptor subtype) were distributed throughout the brain with increased densities apparent over regions known to contain large numbers of cholinergic cell bodies. M2 receptor localization patterns were largely coincident with the regional distribution and intensity of acetylcholinesterase positive sites. Since the M2 receptor pattern appears to parallel regional innervation densities, we conclude that the M2 receptor may serve as a marker for cholinergic pathways. The findings also suggest that M1 muscarine receptors are involved in the presumptive postsynaptic actions of acetylcholine in many forebrain structures.

Enhanced long-term potentiation induced in rat dentate gyrus by coactivation of septal and entorhinal inputs: temporal constraints

High-frequency activation of the entorhinal cortical (perforant path) inputs to the rat dentate gyrus can produce a long-term potentiation (LTP) of perforant path-dentate evoked responses. In this paper we examined the enhanced LTP effects produced by coactivation of septal and entorhinal inputs to the dentate gyrus. Trains of electrical stimulation applied to the two inputs were found to increase the magnitude of LTP to a level above that produced by trains applied to the perforant path alone. The largest LTP increments were observed when the septal trains were applied less than 100 ms prior to the perforant path trains. If the septal trains followed the perforant path trains there was no additional increment in LTP magnitude, regardless of the intertrain interval. The relationship of this cooperativity effect to mechanisms of associative learning is discussed.

Interactions between septal and entorhinal inputs to the rat dentate gyrus: facilitation effects

We examined facilitation effects between the medial septum and perforant path inputs to the dentate gyrus for the four possible combinations of paired-pulse activation. Facilitation effects occurred in all cases. The largest facilitation effects occurred when the septal pulse served as the conditioning pulse for the population spike subsequently evoked by a perforant path pulse. Using 3 pulses, we also examined the influence of septal activation on paired-pulse facilitation of the perforant path-granule cell population spike. A septal stimulation pulse, applied 6-10 ms prior to the onset of the population spike evoked by a perforant path conditioning pulse, did not affect the perforant path-dentate test response at any interpulse interval. If the septal pulse occurred immediately prior to population spike onset, however, there was a significantly greater depression of the test response from 70-3000 ms, but no effect at early intervals (20-50 ms). The effect of the septal pulse appears more consistent with a direct action of the septal terminals on granule cells than with an indirect action via the recurrent inhibitory interneurons.

Ultrastructural characterization of identified cholinergic neurons transplanted to the hippocampal formation of the rat

The ultrastructural features of cholinergic neurons transplanted to the rat hippocampal formation were studied by using a monoclonal antibody to choline acetyltransferase (ChAT). Septal cell suspensions were prepared from E-18 rat embryos and injected into the hippocampus of host rats that had been previously subjected to a bilateral transection of the fimbria-fornix. Rats with fimbria-fornix lesions alone and unoperated rats served as controls and were examined to characterize the native hippocampal cholinergic system. Both unoperated controls and rats with fimbria-fornix lesions showed a sparse population of intrinsic ChAT-immunoreactive neurons that were most numerous in the subgranular zone, the hilus fascia dentata, and near the hippocampal fissure. ChAT-positive terminals from controls formed synapses on dendritic structures that were primarily symmetrical. ChAT-positive dendrites in controls received synaptic input from nonimmunoreactive axon terminals. In rats with septal transplants, ChAT-immunoreactive transplant neurons were found that were either bipolar or multipolar. Axons of transplanted neurons were unmyelinated and arose either from the cell body or a primary dendritic process where they gave off numerous collaterals. Terminals from transplant neurons formed synapses with many nonimmunoreactive neurons. In transplant animals, two main targets of ChAT-immunoreactive terminals were identified: The great majority of synapses were symmetrical junctions with dendritic spines and shafts. A number of terminals were found that appeared to be juxtaposed to nonimmunoreactive axon terminals, possibly forming symmetrical axo-axonic connections. In contrast, such axo-axonic contacts were not observed in the controls. It is concluded that transplanted cholinergic neurons may reinnervate the host hippocampus; however, this reinnervation is different from what is seen in the intact hippocampal formation.

Formation of cholinergic synapses by intrahippocampal septal grafts as revealed by choline acetyltransferase immunocytochemistry

The ultrastructural features of the contacts established by intrahippocampal grafts of foetal septal/diagonal band neurones in the dentate gyrus and the CA1 region of the previously denervated host hippocampus have been analysed with electron microscopic immunocytochemistry using a monoclonal antibody to choline acetyltransferase (ChAT). The results show that the grafted ChAT-positive neurones are capable of forming extensive synaptic contacts with neuronal targets in areas of the dentate gyrus and CA1 which normally receive such innervation. While all types of contacts normally found in association with the granule and pyramidal cell layers were also present in the graft-reinnervated specimens, the quantitative relationship between somatic and dendritic synapses was abnormal. Thus, the ChAT-immunoreactive synapses on cell bodies, which amounted to only a few percent in the normal animal, constituted over 60% in the grafted animals. Conversely, synapses on dendrites which constituted over 90% in the normal dentate were reduced to less than 40% in the grafted animals. The postsynaptic targets of the graft-derived cholinergic synapses included dendrites and cell bodies of dentate granule cells and CA1 pyramidal cells. This supports previous electrophysiological studies and indicates that the septal grafts may be able to modulate host hippocampal function via direct efferent connections onto the granule and pyramidal neurons in the host hippocampal formation.

Histamine in dorsal and ventral hippocampus. II. Effects of H1 and H2 histamine antagonists on exploratory behavior in male rats

The effects on Hole-Board behavior of histamine (HA) microinjected into different parts of the hippocampus and the effects of pyrilamine (PYR, an H1-histamine antagonist), ranitidine (RAN, an H2-histamine antagonist) or alpha-fluoromethyl-histidine (alpha-FMH, an irreversible inhibitor of the HA synthetizing enzyme) injected into the hippocampus on behavior were studied. Forty five nMol of HA were injected stereotaxically into the dorsal or ventral hippocampus. Five min later, Hole-board behavior was measured. It was observed that HA inhibited locomotion and rearing only in the rats injected into the ventral hippocampus. In other experiments, animals were microinjected into the ventral hippocampus with 135 nMol of PYR or RAN in 1 microliter of saline solution. Ten min later, they were microinjected with 45 nMol of HA. Hole-board exploratory activity was measured 5 min thereafter. Results showed that both PYR and RAN were effective in counteracting the inhibitory effect of HA on locomotor activity, but only RAN was able to block the inhibitory action of HA on rearing behavior. Head-dipping frequency was not affected by these treatments. In rats microinjected with 20 nMol of alpha-FMH, increased scores of locomotion were observed but the other behaviors (head-dipping frequency, grooming and rearing) were not affected. The present results support the hypothesis that HA in hippocampus may be exerting a regulatory role on behavior by interaction with H1 and H2 receptors.

Medial septal facilitation of hippocampal granule cell activity is mediated by inhibition of inhibitory interneurones

Rats under barbiturate anaesthesia were implanted with stimulating electrodes in the medial septal nucleus and the medial perforant path. A recording electrode and cannula were implanted in the hilus of the dentate gyrus. Electrodes were positioned so that a conditioning pulse to the medial septum, although eliciting no field potential of its own, facilitated the granule cell population spike evoked by medial perforant path stimulation. In the first experiment, the infusion into the hilus of the GABA antagonist picrotoxin was found to block the facilitation. In a second experiment, it was found that a medial septal conditioning pulse blocked recurrent inhibition of the granule cells, only if it was timed to coincide with their initial activation. We suggest that these effects are mediated through an inhibitory connection from the medial septum onto inhibitory interneurones in the dentate gyrus, and that this connection may utilize the neurotransmitter GABA.

Cholinergic innervation of the rat dentate gyrus: an immunocytochemical and electron microscopical study

Immunocytochemical studies using a monoclonal antibody to choline acetyltransferase (ChAT) were performed on sections of rat dentate gyrus. Light microscopical analysis of the immunoreactivity revealed dense fiber networks and many punctate structures predominantly located at the interface of the granule cell layer and molecular layer. In the electron microscope, the immunostained punctate structures were identified as synaptic boutons which formed mainly symmetrical contacts onto dendritic elements. Few ChAT-immunoreactive boutons formed axosomatic contacts.

A [14C]2-deoxyglucose analysis of the functional neural pathways of the limbic forebrain in the rat. V. The septal area

The [14C]2-deoxyglucose (2-DG) metabolic mapping technique has been used to identify the regions responding with an augmented rate of metabolism following focal electrical stimulation of various sites within the lateral septal nucleus and medial septal nucleus/diagonal band (MSN/DB) complex in the rat. Since 2-DG uptake has been correlated with rates of functional activity, it was the intention of this study to suggest the anatomical substrates underlying various physiological and behavioral responses elicited by stimulation of the septal area. The results show that stimulation of any region within the lateral septal nucleus produced a profound bilateral activation of both the lateral septal nucleus, as well as the hippocampal formation. While stimulation of a number of different fiber systems associated with the lateral septum could contribute to the observed pattern of labeling, the data suggest that, functionally, a major consequence of such stimulation is the antidromic activation of CA3----lateral septum fibers to axonal branch points, beyond which, orthodromic propagation of the impulse produces activation in CA3 target regions, including subfields CA1 and CA3, as well as the lateral septal nucleus, bilaterally. In addition, regions typically manifesting metabolic activation following stimulation of the lateral septal nucleus included the ipsilateral diagonal band of Broca, nucleus accumbens, lateral preoptic area and lateral hypothalamus, posteriorly, and the prelimbic cortex, anteriorly. Occasionally, target regions of the postcommissural fornix, including the medial mammillary nucleus and anterior thalamic nuclei were also activated following stimulation of the lateral septal nucleus. In contrast to the widespread pattern of activation resulting from stimulation of the lateral septal nucleus, stimulation of the MSN/DB complex produced activation which was largely confined to the medial forebrain bundle. In a final phase of the experiment, afterdischarge activity was elicited by sodium penicillin injection into the lateral septal nucleus. Such treatment produced more widespread 2-DG uptake, including more extensive activation within the lateral septal nucleus, hippocampal formation, amygdala, and thalamus. Additionally, the prefrontal cortex and temporal neocortex were activated.

Cholinergic innervation of the rat hippocampus as revealed by choline acetyltransferase immunocytochemistry: a combined light and electron microscopic study

The cholinergic innervation of the rat hippocampus proper and fascia dentata was investigated by using a monoclonal antibody against choline acetyltransferase (ChAT). At the light microscopic level, thin varicose ChAT-immunoreactive fibers were observed mainly in the vicinity of the pyramidal and granular layers where they formed a fine network around proximal dendrites of pyramidal and granule cells. In addition, many ChAT-immuno-reactive fibers were found in the hilar region and in stratum oriens, radiatum, and lacunosum-moleculare of all hippocampal sectors. Electron microscopic analysis revealed ChAT immunoreactivity in thin unmyelinated varicose axons and terminals which established synaptic contacts. Asymmetric contacts of ChAT-immunoreactive terminals were found on small spines in the dendritic layers of the hippocampus proper and in the molecular layer of the fascia dentata. Symmetric synaptic contacts were formed on the cell bodies of pyramidal and granule cells. Both symmetric and asymmetric synaptic contacts occurred on dendritic shafts. The analysis of serial thin sections, which allows identification of postsynaptic elements, suggests that pyramidal cells, granule cells, and nonpyramidal neurons of the hippocampus receive a cholinergic input.

Perinatal glucocorticoids alter dentate gyrus electrophysiology

Perinatal glucocorticoid administration produces permanent spatial discrimination learning deficits in rats, presumably referable to changes in the development of neural systems subserving such functions. Because the hippocampal dentate gyrus and its afferent/efferent circuitry appear selectively vulnerable to neonatal steroid treatments, we have examined adult rats treated with neonatally administered glucocorticoids using electrophysiological methods. The techniques were chosen to reveal the topographic and neurophysiologic responsiveness of the major afferent supply to the dentate gyrus. Rats of both sexes received either a high dose (100 mg/kg) or low dose (1 mg/kg) of the synthetic glucocorticoid dexamethasone on postnatal day four, with control subjects receiving an injection of saline. These dosages have been shown to disrupt hippocampal dependent learning [6,38]. Laminar depth profile analyses of entorhinal cortex-dentate gyrus afferents revealed a significant shift in the spatial distribution of evoked extracellular population synaptic potentials (EPSPs) in glucocorticoid treated subjects. Stimulus-response functions also differed between glucocorticoid treated and control subjects. While response amplitudes at threshold stimulus intensities did not differ between groups, at higher stimulus intensities population spike potentials and associated EPSPs differed in glucocorticoid versus control subjects.

Selective lesions of the fimbria and the fornix in the rat: differential effects on CA1 and dentate theta

The effects of electrolytic lesions of the dorsal fornix and the dorsomedial fimbria on the CA1 and the dentate theta rhythms (theta s) recorded from the dorsal hippocampal formation were investigated in the ether-anesthetized rat. The results showed that (i) fornix lesions mainly affected CA1 theta, (ii) fimbrial lesions mainly affected dentate theta, and (iii) combined fornix-fimbria lesions suppressed both CA1 and dentate theta s. When considered in connection with other observations, these data suggest that the septal projections pacing the CA1 theta may course essentially within the dorsomedial fornix whereas those pacing the dentate theta may pass essentially within the dorsomedial fimbria. Moreover, our data provide new support for the hypothesis that at least two septohippocampal neural systems are anatomically and functionally independent and capable of controlling the theta activity of the dorsal hippocampal formation of the anesthetized rat.

Mediation of passive avoidance learning by nicotinic hippocampo-entorhinal components in young rats

Young rats, 11, 16, and 20 days of age, received bilateral injections of three antinicotinic agents into the posteroventral hippocampo-subiculo-entorhinal area, and were trained to learn a cool-draft-stimulus, passive-avoidance task shortly after (17 min). Gallamine triethiodide had no action at low doses and provoked convulsions at higher concentrations. Pempidine tartrate produced age- and dose-dependent impairments of the passive avoidance, and was much more effective in younger groups (11 and 16 days) than at 20 days. alpha-bungarotoxin also induced dose-dependent deficits. These results, together with the mecamylamine-induced deficits already reported, suggest that nicotinic cholinergic synapses located in the posteroventral part of the hippocampal complex play a role in passive-avoidance learning in the young rat as soon as this type of conditioning is possible, but become relatively less important at older ages, when muscarinic mechanisms also become involved.

Cholinergic influences on hippocampal glucose metabolism

2-Deoxy-D-[3H]glucose autoradiography was employed to investigate the effects of acute cholinergic manipulations on hippocampal glucose metabolism. In general, manipulations designed to reduce cholinergic activity (medial septal ablation, atropine treatment) reduced hippocampal glucose metabolism. Maximal decrements were found in the terminal fields of the septohippocampal projection after medial septal lesions, while maximal deficits after atropine treatment correlated with muscarinic receptor binding. Electrical stimulation of the medial septum resulted in increased glucose utilization in some terminal fields of the septohippocampal projection and decreased utilization in the terminal fields of the perforant pathway. Our data clearly indicate that acute alterations in cholinergic activity can affect hippocampal glucose metabolism but the distribution, direction and degree of these changes is dependent on the specific treatment.

Loss of M2 muscarine receptors in the cerebral cortex in Alzheimer's disease and experimental cholinergic denervation

Cerebral cortex samples from patients with Alzheimer's disease and from rats after experimental cholinergic denervation of the cerebral cortex exhibited reductions in the presynaptic marker choline acetyltransferase activity and in the number of M2 muscarine receptors, with no change in the number of M1 receptors. These results are in keeping with evidence that M2 receptors function in cholinergic nerve terminals to regulate the release of acetylcholine, whereas M1 receptors are located on postsynaptic cells and facilitate cellular excitation. New M1-selective agonists and M2-selective antagonists directed at post- or presynaptic sites deserve consideration as potential agents for the treatment of the disease.

Enhanced acetylcholinesterase staining in hippocampal area CA3 after lesion of granule cells by infusion of colchicine

Unilateral infusion of colchicine into the lateral ventricle produced relatively selective destruction of dentate granule cells in the ipsilateral dorsal hippocampal formation of the rat. Timms silver sulfide stain is markedly reduced in the mossy fiber layer on the colchicine treated side but is normal contralaterally. After colchicine treatment, an increase in acetylcholinesterase staining is apparent in the apical dendritic zone of CA3 pyramidal cells. This enhanced staining is localized in the proximal apical dendritic layer of CA3, a region normally occupied by the mossy fiber terminals of dentate granule cells. These results suggest that cholinergic fibers proliferate in CA3 after granule cell lesion and may participate in reinnervation of the denervated area.

Cross-species septal transplants: recovery of choline acetyltransferase activity

Following interruption of the fornix-fimbria pathway, hippocampal choline acetyltransferase (ChAT) activity was restored gradually by cross-species cell suspension transplants of embryonic septum. The hippocampal segment closest to the implant reached 35% of normal 17 weeks after transplantation. The overall restoration of ChAT by xenogenic cell suspension had many similarities to that reported for homogenic solid and cell suspension septal grafts. The time course of the recovery of ChAT activity was different from the time course of the ingrowth of acetylcholinesterase stained fibers reported previously.

Recovery of enzyme markers for cholinergic terminals in septo-temporal regions of the hippocampus following selective fimbrial lesions in adult rats

The activities of choline actyltransferase and acetylcholinesterase were determined in five consecutive septo-temporal regions of the ipsilateral and contralateral hippocampus from unlesioned controls and lesioned animals at various times following lateral, medial or complete unilateral transection of the fimbrial bundle in rats. In control animals distribution of cholinergic enzymes suggests a relatively heavier innervation of the ventral hippocampus. In lesioned animals depletion of enzyme activities in septo-temporal regions of the ipsilateral hippocampus was consonant with the known topography of cholinergic innervation of the hippocampus via the dorsal and ventral pathways. After 4 and 8 week post-lesion survival, a substantial recovery of both enzyme activities was evident following either of the lesion paradigms employed. However, the extent and the pattern of enzyme restitution depended on the type of fimbrial transection and the hippocampal region under consideration. Significant enzyme alterations were also observed in the contralateral hippocampus following all three lesion types. We interpret the lesion-induced temporal consequences in cholinergic enzymes to indicate initial degeneration and subsequent regeneration of cholinergic terminals in the hippocampus. The present findings also suggest that homologous fimbrial fibres spared by the partial lesions are responsible for the ensuing recovery. Thus, partial lesions of well-defined efferents constitute a suitable experimental paradigm to demonstrate homotypic reconstruction in the adult mammalian central nervous system.

Effects of physostigmine on septo-hippocampal averaged evoked field potentials

Field potentials were elicited in the CA3 field of freely-moving animals by paired-pulse stimulation of the ventral portion of the medial septum before and after exposure to physotigmine. Physostigmine had no effect on response components elicited by a single stimulus or by the first stimulus of a pulse pair. However, the amplitude of a late positive component elicited by the second stimulus of a pair was increased significantly following physostigmine. These results suggest that activity in a cholinergic portion of the septo-hippocampal pathway may serve to modulate the responsiveness of neurons in the CA3 field to subsequent stimulation.

Guidance of acetylcholinesterase-containing fibres by target tissue in co-cultured brain slices

Slices of various brain regions were prepared from newborn and from 7-day old rats and co-cultured in different combinations. In the majority of co-cultures of septal and hippocampal slices, acetylcholinesterase-positive fibres originating in the septal nuclei invaded the adjacent hippocampal slice. A similar pattern of hippocampal ingrowth by acetylcholinesterase-positive fibres occurred with slices prepared from the nucleus basalis of Meynert and from spinal cord. Septal neurones also projected to cortical slices, an effect which even occurred in the presence of their natural target tissue. In contrast to these massive projections to brain areas which in situ receive cholinergic inputs, no significant acetylcholinesterase-positive fibre ingrowth was observed in tissues which lack major cholinergic afferents in situ (hypothalamus, substantia nigra and cerebellum). These results indicate that under our culture conditions, acetylcholinesterase-positive fibres selectively invade cholinergic target areas. This effect is independent of the brain area from which the cholinergic neurones were derived.

Induced homotypic collateral sprouting of serotonergic fibers in the hippocampus of rat

The organization of the raphe cells efferent to the dorsal hippocampus (DHipp) has been shown to consist of two homologous groups of cells in the median raphe nucleus (MRN). Their axons project by way of the cingulum bundle (CB) and the fornix-fimbria (FF)58. A group of neurons located in the interfascicular part of the dorsal raphe nucleus (IFN) was found to use the CB route exclusively. Quantitation of the HRP-labeled cells in the MRN neurons projecting to the DHipp decreased by 55% and 92% 3 days following CB and CB-FF 5,7-DHT injections respectively. There was a significant increase in HRP-labeled neurons in the MRN at 21 and 42 days compared to 3 days after CB injection (a 2% decrease at 21 days and 38% increase at 42 days compared to sham injected). No change in the number of labeled MRN cells was seen at these long-term time points after combined CB-FF 5,7-DHT microinjections. The labeled cells in the MRN increased in size in the long survival group of CB lesioned animals. The labeled cells localized in the IFN which were lost after CB 5,7-DHT injections did not reappear after long-term survival. These results indicate that CB 5-HT fibers do not regenerate into the DHipp, but FF 5-HT fibers increase their terminal territory by collateral sprouting in response to homotypic denervation.

Characterization of a slow cholinergic post-synaptic potential recorded in vitro from rat hippocampal pyramidal cells

Intracellular recording from CA1 pyramidal cells in the hippocampal slice preparation was used to compare the action of exogenously applied acetylcholine (ACh) and cholinomimetics to the effect of electrically stimulating sites in the slice known to contain cholinergic fibres. ACh depolarized pyramidal cells with an associated increase in input resistance, blocked a calcium-activated potassium conductance (GK(Ca], and blocked accommodation of action potential discharge. All of these actions were blocked by the muscarinic antagonist, atropine. Repetitive electrical stimulation of stratum (s.) oriens evoked a series of fast excitatory post-synaptic potentials (e.p.s.p.s) followed by an inhibitory post-synaptic potential. These potentials were followed by a slow e.p.s.p. that lasted 20-30 s. The slow e.p.s.p. was selectively enhanced by eserine and blocked by atropine. Ionophoretic application of ACh closely mimicked the time course of the slow e.p.s.p. The slow e.p.s.p. was blocked by tetrodotoxin and cadmium, indicating that it was dependent on propagated action potentials and on calcium. Considerably higher stimulus strengths were needed to elicit a slow e.p.s.p. than to elicit the earlier synaptic potentials. The size of the slow e.p.s.p. was markedly increased by repetitive stimulation. Stimulation of the alveus, s. oriens, s. pyramidale and fimbria all evoked a slow e.p.s.p., while stimulation of s. radiatum was relatively ineffective. The input resistance of the cell increased during the slow e.p.s.p. Hyperpolarizing the cell decreased the size of the slow e.p.s.p. and at membrane potentials of -70 mV or greater, little response was recorded. Stimulation of s. oriens blocked GK(Ca) and accommodation of action potential discharge. These effects, which could be seen in the absence of any change in membrane potential, were enhanced by eserine and blocked by atropine. The present electrophysiological results establish that CA1 pyramidal cells receive a cholinergic input and demonstrate that this input can dramatically alter the firing properties of these neurones for tens of seconds in the absence of any marked effect on membrane potential. Such an action contrasts with previously characterized synaptic potentials in this region of the brain.

Cholinergic denervation of the hippocampal formation does not produce long-term changes in glucose metabolism

Decreased glucose metabolism is found in Alzheimer's disease associated with a loss of cholinergic neurons. The relationship between the chronic cholinergic denervation produced by medial septal lesions and glucose metabolism was studied using 2-deoxy-D-[3H]glucose in the rat hippocampal formation. Hippocampal glucose metabolism was increased 1 week after medial septal lesions. Three weeks after lesions, glucose metabolism was profoundly suppressed in all regions. By 3 months, intraregional hippocampal glucose metabolism had returned to control values. Our results demonstrate that chronic cholinergic denervation of the hippocampal formation does not result in permanent alterations of metabolic activity.

Neural substrates of two different rhythmical vibrissal movements in the rat

Normal rats show two types of rhythmical vibrissal movement, one synchronized precisely with alpha waves (about 9 Hz) of the thalamocortical system, and the other often synchronized with theta waves (about 7 Hz) of the septohippocampal system. The alpha-synchronized vibrissal movements appear while the rat stands still with a slow respiratory pattern, and are of small amplitude (a fine tremor). The theta-synchronized vibrissal movements appear during exploratory sniffing behavior, and are of large amplitude. Thus, a group of facial motoneurons which constitute the final common pathway for vibrissal movement apparently receive input convergently from these two neural systems. In the present study, we observed the following: (1) the two types of movement rarely, if ever, appeared simultaneously and the same was true of the two brain wave patterns. Topographically, the predominant appearance of the alpha waves was in the frontal (sensorimotor) cortex, whereas that of the theta waves was in the occipital cortex and hippocampus. (2) Bilateral ablation of either the entire neocortex or just its anterior (but not posterior) half eliminated the vibrissal alpha-tremor movement while leaving vibrissal theta-sniffing movement normal. In anterior decorticate rats, the tremor movements started to recover by 1-4 months, and were abolished again by the removal of the remaining posterior cortex. (3) Lesions of the medial septum or the fornix eliminated hippocampal theta waves, but had no effect on vibrissal sniffing movement or alpha wave-vibrissal tremor. (4) Cerebellectomy and, to a lesser extent, pharmacological lesions of the inferior olive slowed, but did not block, alpha waves. In addition, vibrissal tremor movement became intermittent and less vigorous. The same manipulations, however, did not affect theta wave-vibrissal sniffing movement. (5) Harmaline (30 mg/kg, i.p.) did not induce alpha-tremor, which, in combination with the results with cerebellectomy and inferior olive lesions, indicates that alpha-tremor is generated by a neural mechanism that is different from that for harmaline-induced generalized tremor of 10 Hz. These findings confirmed that there exist two patterns of synchrony between vibrissal movement and rhythmical brain activity in the rat, i.e. alpha wave-vibrissal tremor movement and theta wave-vibrissal sniffing movement, and suggest that the two patterns reflect a rhythmical mode of functioning of two different neural systems, probably the thalamocortical and the septohippocampal system, respectively.

Genetically related rats with differences in hippocampal uptake of norepinephrine and maze performance

The spontaneously hypertensive rat (SHR) and its progenitor strain, the Wistar-Kyoto (WKY) display marked differences in brain catecholamines and behavior. The behavioral differences are suggestive of alterations in hippocampal function and, in particular, the noradrenergic input to the hippocampus. To test these hypotheses we have analyzed the performance of the SHR and WKY in a spatial memory maze task that is specific to hippocampal function and determined the kinetics of norepinephrine (NE) uptake in synaptosomal preparations of the hippocampus. We have found that WKYs exhibit an abnormally strong bias tendency in T-maze arm preference that influences the rate of acquisition and the final level of maze performance. We have also found differences in noradrenergic uptake in hippocampal synaptosomes. WKYs exhibit higher NE uptake rates and higher kinetic constants for NE uptake when compared with SHRs, suggesting that strain differences in noradrenergic function may contribute to the observed behavioral differences.

Interactions between hippocampal penicillin spikes and theta rhythm

The relationships between theta rhythm and epileptic spikes, evoked by penicillin, were studied in the rat hippocampus. Records were taped and processed off-line, and autocorrelation functions, averages and power spectra of the EEG and frequency histograms of the epileptic spikes were calculated. Results showed that: (1) epileptic spikes tend to occur in a preferred phase of theta rhythm; (2) they provoked a reset of the phase of theta rhythm, acting as internal stimuli; (3) epileptic spikes decreased in frequency and were often abolished when theta rhythm was evoked; (4) these effects appeared to be dependent on the medial septal pathway. These findings indicate the existence of an antagonism between two hippocampal phenomena (epilepsy and theta rhythm). On the other hand, they also seem to be interrelated since the generation of one is accompanied by a reciprocal decrease in the other.

A study of the reciprocal connections between the septum and the entorhinal area using anterograde and retrograde axonal transport methods in the rat brain

The reciprocal connections between the septum and the entorhinal area (EA) was studied in the rat brain using antero- and retrograde axonal transport methods. After injections of large volumes (2 X 100 nl) of horseradish peroxidase (HRP) conjugated to wheat-germ agglutinin (WGA) into the medial septum (MS) and the diagonal band of Broca (dbB), anterogradely transported HRP-WGA was found primarily in layers II and IV of the medial and lateral EA. Injections of HRP-WGA (50-100 nl) or fluorescent dyes (50-100 nl) into different parts of the retrohippocampal region resulted in labeling, by retrograde axonal transport, of cells in the MS and dbB, both ipsi- and contralateral to the injected hemisphere. The labeled cells were either small (long axis of soma: 10-15 micron), round, and oval, or medium (15-25 micron) to large (25-35 micron) of fusiform or multipolar shape. By using the method of retrograde fluorescent double labeling, the septal afferents to the EA were found to give off collaterals to other parts of the hippocampal region. A much smaller number of septal cells appeared to send bilateral projections to the EA of both hemispheres. Studies employing retrograde transport of HRP in combination with acetylcholinesterase (AChE) histochemistry on the same tissue section showed that, while a large number of cells projecting to the EA contain AChE, many projecting cells are devoid of AChE reaction products. These findings suggest that the septo-entorhinal projection consists of a cholinergic as well as a noncholinergic component. The entorhinal efferents to the septum were studied after injections of HRP-WGA into different parts of the retrohippocampal region. Labeled fibers could be traced through the fimbria to their terminal fields in the intermediate parts of the lateral septal nucleus and to the most lateral aspect of the vertical limb of the dbB. The cells giving rise to this projection were situated in layer IV of the medial and layers II through V of the lateral EA. Taken together, the present findings demonstrate a close anatomical relationship between the septum and the entorhinal area, in addition to the better known connections between the septum and the Ammon's horn.

Organization of cerebral cortical afferent systems in the rat. II. Magnocellular basal nucleus

The organization of the magnocellular basal nucleus (MBN) projection to cerebral cortex in the rat has been studied by using cytoarchitectonic, immunohistochemical, and retrograde and anterograde transport methods. The distribution of retrogradely labeled basal forebrain neurons after cortical injections of wheat germ agglutinin-horseradish peroxidase conjugate was essentially identical to that of neurons staining immunohistochemically for choline acetyltransferase. These large (20-30 micrometers perikaryon diameter) multipolar neurons were found scattered through a number of basal forebrain cell groups: medial septal nucleus, nucleus of the diagonal band of Broca, magnocellular preoptic nucleus, substantia innominata, and globus pallidus. This peculiar distribution mimics the locations of pathways by which descending cortical fibers enter the diencephalon. Each cortical area was innervated by a characteristic subset of MBN neurons, always located in close association with descending cortical fibers. In many instances anterogradely labeled descending cortical fibers appeared to ramify into diffuse terminal fields among MBN neurons which were retrogradely labeled by the same cortical injection. Double label experiments using retrograde transport of fluorescent dyes confirmed that MBN neurons innervate restricted cortical fields. Anterograde autoradiographic transport studies after injections of 3H-amino acids into MBN revealed that MBN axons reach cerebral cortex primarily via two pathways: (1) The medial pathway, arising from the medial septal nucleus, nucleus of the diagonal band, and medial substantia innominata and globus pallidus MBN neurons, curves dorsally rostral to the diagonal band nucleus, up to the genu of the corpus callosum. Most of the fibers either directly enter medial frontal cortex or turn back over the genu of the corpus callosum into the superficial medial cingulate bundle. Many of these fibers enter anterior cigulate or retrosplenial cortex, but some can be traced back to the splenium of the corpus callosum, where a few enter visual cortex but most turn ventrally and sweep into the hippocampal formation. Here they are joined by other fibers which, at the genu of the corpus callosum, remain ventrally located and run caudally through the dorsal fornix into the hippocampus. (2) The lateral pathway arises in part from medial septal, diagonal band, and magnocellular preoptic neurons whose axons sweep laterally through the substantia innominata to innervate primarily piriform, perirhinal, and endorhinal cortex. Some of these fibers may also enter the hippocampal formation from the entorhinal cortex via the ventral subiculum.(ABSTRACT TRUNCATED AT 400 WORDS)

Cholinomimetics produce seizures and brain damage in rats

Microinjections of the cholinergic agonists, carbachol and bethanechol, either into the amygdala or into the dorsal hippocampus produced sustained limbic seizures and brain damage in rats. Systemic administration of pilocarpine in rats resulted in a sequence of convulsive disorders and widespread brain damage as well. Scopolamine prevented the development of convulsive activity and brain damage produced by cholinomimetics. These results suggest that the excessive stimulation of cholinergic muscarinic receptors can lead to limbic seizures and brain damage. It is postulated that muscarinic cholinergic mechanisms are linked to the etiology of temporal lobe epilepsy and epileptic brain damage.

Postnatal developments of AChE activity in the hippocampus of the reeler mutant mouse

The acetylcholinesterase (AChE) activity of the hippocampus and the dentate gyrus in the reeler mutant mouse was studied histochemically by the method of Karnovsky and Roots. Developmental studies were done on days 0,3,6,9,12,15,18 and 21 postnatally, and in adults. The adult reeler mouse differs from the normal mouse in that there is no accumulation of the activity at the junction between the stratum lacunosum-moleculare and the stratum radiatum, weaker and divided activity in CA1, and the translocation of the activity to the granule cell zone of the dentate gyrus. The results obtained are considered to be due to the cells' ectopia. But also, other factors such as genesis of the cells in regard to constructing the cytoarchitecture, may influence them. According to the postnatal observations, the developmental pattern of AChE activity in the reeler mouse is about the same as in the normal mouse. AChE-rich cells in the hippocampus showed maximum activity and number from days 12 to 15. The neuropile reactions increased after day 9 in both kinds of mice. The relationship between the movement of the AChE positive neuropiles and cells and their functions are also discussed.

Topographic organization of septal cells innervating the dorsal hippocampal formation of the rat: special reference to both the CA1 and dentate theta generators

We determined the topographic organization of septal cells innervating monosynaptically the two generators of theta rhythm of the rostral hippocampal formation of the rat. Under ether anesthesia, horseradish peroxidase (HRP) was injected into the dorsal CA 1 region close to the corpus callosum, the ventral part of CA 1 region close to the hippocampal fissure, and the dentate hilus. The HRP micropipet tip was positioned in these hippocampal-dentate sites by recording the hippocampal EEG through the micropipet itself before HRP injection. The major findings were: (i) Injection of HRP in the superficial part of the dorsal CA 1 region (i.e., the septal pole) resulted in labeling of neurons situated mainly in the diagonal band of Broca, ipsilateral to the injection site. (ii) Injection of HRP in the deep part of the dorsal CA 1 region resulted in the labeling of a few cells scattered in the septal region ipsilateral to the injection site. (iii) After injection of HRP in the dentate hilus, labeled cells were selectively detected in the rostral half of the medial septal nucleus mainly ipsilateral to the injection site. A high degree of organization was revealed in the connections between the septal region and the rostral hippocampal formation of the rat, two cerebral regions critically involved in theta production. The results are relevant to the two-generator hypothesis of theta.

The septohippocampal projection in the rat: an electron microscopic horseradish peroxidase study

In order to identify the specific targets of the septohippocampal projection in the rat, horseradish peroxidase localization at the electron microscopic level was used. Following injections of free horseradish peroxidase into the medial septum, sections of the dorsal hippocampal formation were reacted with diaminobenzidine and processed for electron microscopy by routine methods. Sections were viewed unstained. Horseradish peroxidase labeling in the dentate gyrus was predominantly in the supra- and infragranular layers. All postsynaptic elements were neuronal. They included granule cell somata and somata and dendrites of hilar cells; these may include pyramidal basket cells. No synaptic contacts with vascular or glial elements were found. These results provide a basis for comparing the specific targets of the septohippocampal projection with those of the sympathohippocampal pathway, which innervates the dentate following lesions of the septohippocampal projection.