Cholinergic Septohippocampal Pathway Research Articles

Dopamine-GABA interactions in the nucleus accumbens and lateral septum of the rat

The relationships between dopaminergic afferents and GABAergic neurones were studied at the electron microscopic level in the rat lateral septum and nucleus accumbens by coupling 6-hydroxydopamine degeneration and γ-aminobutyric acid (GABA) immunocytochemistry. Degenerating fibres were observed in the two regions making synaptic contact with GABA-immunoreactive and non-labelled cell bodies and dendrites. It is concluded that dopaminergic afferents to the septum and the nucleus accumbens contact, among others, a population of GABAergic cells. A similar route of regulation of the basalo-cortical and septo-hippocampal cholinergic pathways by dopaminergic afferents is proposed.

Hippocampal formation neurons code the level of activation of the cholinergic septohippocampal pathway

In these experiments, differing levels of activation of the cholinergic septohippocampal pathway, as reflected by shifts in type 2 slow wave theta frequency recorded in the hippocampal formation, were studied in urethane-anesthetized rats. The purpose was to determine whether the discharge rates of cells in the hippocampal formation varied in any systematic way with the frequency shifts produced during spontaneous theta and by stimulation of the dorsomedial-posterior hypothalamic region. Linear regression analysis revealed that a class of neurons called theta-on cells precisely coded the level of activation of the septohippocampal pathway through their discharge rates. Administration of atropine sulfate abolished this coding. A second class of neurons called theta-off cells were shown to have reciprocal relationships with theta-on cells. Implications for the sensorimotor model of hippocampal formation theta subsystems were discussed.

Differential effects of selective lesions of cholinergic and dopaminergic neurons on serotonin-type 1 receptors in rat brain.

Serotonin (5-HT)-type1 (abbreviated as a subscript 1) receptor binding sites are discretely distributed in rat brain. High densities of [3H]5-HT1 binding sites are especially located in areas enriched with cholinergic and dopaminergic innervation, such as the substantia innominata/ventral pallidum, striatum, septal nuclei, hippocampus and substantia nigra. The possible association of [3H]5-HT1 binding sites with cholinergic or dopaminergic cell bodies and/or nerve fiber terminals was investigated by selective lesions of the substantia innominata/ventral pallidum-cortical and septohippocampal cholinergic pathways and the nigrostriatal dopaminergic projection. [3H]5-HT1 receptor binding sites are possibly located on cholinergic cell bodies in the ventral pallidum-cortical pathway since [3H]5-HT1 binding in the substantia innominata/ventral pallidal area was markedly decreased following kainic acid lesions. Fimbriaectomies markedly decreased [3H]5-HT1 binding in the hippocampus, suggesting the presence of 5-HT1 binding sites on cholinergic nerve fiber terminals in the septohippocampal pathway. Lesions of the nigrostriatal dopaminergic projection did not modify [3H]5-HT1 binding in the substantia nigra and the striatum, suggesting that 5-HT1 receptors are not closely associated with dopaminergic cell bodies and nerve terminals in this pathway. These results demonstrate differential association between 5-HT1 receptors and cholinergic and dopaminergic innervation in rat brain.

Cholinergic neurons, learning, and recovery of function.

Two experiments were designed to examine the role of the cholinergic septo-hippocampal projection in spatial and nonspatial learning processes. In Experiment 1 the interaction of individual learning strategies and recovery of function was investigated following medial septal lesions, by assessing individual learning styles and number of trials to recovery on a standard radial eight-arm-maze task. Experiment 2 addressed the relevance of the cholinergic septo-hippocampal pathway in the acquisition of spatially mediated behavior, by employing medial septal lesions, prior to acquisition learning of a modified version of the radial eight-arm-maze task. The data from these two experiments demonstrated that (a) the cholinergic septo-hippocampal projection is important for the acquisition and maintenance of spatial learning strategies, (b) loss of cholinergic function leads to alteration of learning strategies, (c) different learning strategies can be employed to acquire the same behaviors, and (d) the rate of recovery following brain injury can be influenced by preoperative learning strategies.

Reversal of scopolamine-induced amnesia of passive avoidance by pre- and post-training naloxone.

In a series of five experiments, the modulating role of naloxone on a scopolamine-induced retention deficit in a passive avoidance paradigm was investigated in mice. Scopolamine, but not methyl scopolamine (1 and 3 mg/kg), induced an amnesia as measured by latency and duration parameters. Naloxone (0.3, 1, 3, and 10 mg/kg) injected prior to training attenuated the retention deficit with a peak of activity at 3 mg/kg. The effect of naloxone could be antagonized with morphine (1, 3, and 10 mg/kg), demonstrating the opioid specificity of the naloxone effect. Post-training administration of naloxone (3 mg/kg) as a single or as a split dose also attenuated the scopolamine-induced amnesia. Control experiments indicated that neither an increase in pain sensitivity (pre-training naloxone) nor an induced aversive state (post-training naloxone) appear to be responsible for the influence of naloxone on the scopolamine-induced retention deficit. These results extend previous findings implicating a cholinergic-opioid interaction in memory processes. A possible mechanism for this interaction involving the septo-hippocampal cholinergic pathway is discussed.

Cerebral embolization impairs memory function and reduces cholinergic marker enzyme activities in various brain regions in rats.

Cerebral embolization was produced by injecting microspheres into the left internal carotid artery in rats. In these embolized rats, passive avoidance response was impaired at early stages after the embolization (3 days, 1 and 2 weeks), but 4 to 8 weeks later, recovery was achieved. A reduction in the activities of choline acetyltransferase (CAT) in the left side of the occipital cortex and hippocampus was observed in parallel with the impairment of learning behavior. These results suggest that impairments in the septo-hippocampal cholinergic pathways are likely to be closely related with the behavioral impairment in these rats.

Effect of Oxiracetam and Piracetam on Central Cholinergic Mechanisms and Active-Avoidance Acquisition

Oxiracetam at 100 and 300 mg/kg i.p. dose levels increased acetylcholine (ACh) utilization in the rat cerebral cortex and hippocampus. ACh utilization was assessed by measuring, with a gas chromatographic method, the decrease in ACh level after inhibiting its synthesis by 15 micrograms intracerebroventricularly (i.c.v.) injection of hemicholinium (HC-3). ACh steady state levels were not affected. Piracetam (300 mg/kg i.p.) also increased ACh utilization in the hippocampus. Repeated daily administration of oxiracetam 100 mg/kg i.p. caused a 31% increase in high-affinity choline uptake (HACU) in the hippocampus. A single administration of 300 mg/kg i.p. of oxiracetam and piracetam also increased HACU rate in the hippocampus. However, the effect of piracetam was over within 3 h, while 3 h after its administration oxiracetam still caused a 40% increase in HACU rate. Oxiracetam (100 mg/kg i.p.) significantly antagonized the impairment in the acquisition of an active-avoidance conditioned response (pole climbing) associated with the inhibition of ACh synthesis by HC-3. These results indicate that oxiracetam enhances the activity of the septohippocampal cholinergic pathways, and to a lesser extent, of the cortical cholinergic network.

Gangliosides alter morphology and growth of astrocytes and increase the activity of choline acetyltransferase in cultures of dissociated septal cells

Administration of gangliosides has been reported to stimulate regeneration of motoneurons and of central dopaminergic and cholinergic neurons. To shed light on the mechanism by which gangliosides mediate the effects on cholinergic neurons, we studied their actions on cultures of cells dissociated from the septal area of fetal rat brains. These cultures contain cholinergic neurons, which, in vivo, give rise to the cholinergic septo-hippocampal pathway. Gangliosides produced prominent changes in the morphological appearance of the cultures. In contrast to control cultures, which contained many process-bearing cells and a confluent layer of flat cells, there were no flat cells in cultures grown in the presence of gangliosides (0.2 to 0.8 mg/ml of medium). Using immunocytochemical visualization of the astrocytic marker glial fibrillary acid protein, it was shown that all astrocytes in cultures grown in the presence of gangliosides exhibited the morphology of process-bearing cells, whereas in control cultures astrocytes represented the majority of the flat cells. Furthermore, gangliosides attenuated astrocytic proliferation. The effects of gangliosides apparently were not mediated by cAMP, since they could be differentiated from actions of forskolin, an activator of adenylate cyclase. Astrocytic growth and morphology were affected by ganglioside mixtures of various sources and composition and also by the pure gangliosides GM1 and GD1a, whereas lipid and carbohydrate components of gangliosides were ineffective. In contrast to the prominent effects on astrocytes, gangliosides failed to significantly alter survival or fiber growth of cholinergic neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of inhibition in the hippocampus in vivo.

The nature and mechanisms of septohippocampal transmission have been elucidated by taking advantage of an in situ preparation in experiments with Sprague-Dawley rats under urethane. Both extracellular field potentials and intracellular recordings were made in CA1-3 regions of the hippocampus; and the hippocampal commissure and medial septum stimulated to evoke synaptic activity. Using muscarinic and nicotinic agonists and antagonists it was shown that both acetylcholine and medial septal activity can increase the excitability of pyramidal cells, mainly through muscarinic receptors. The effect of septal stimulation was enhanced by local application of physostigmine and reduced by intraventricular injections of hemicholinium. It was also shown that acetylcholine, when applied in the stratum pyramidale, can reduce the voltage and conductance changes observed during evoked inhibitory postsynaptic potentials (IPSP) without affecting the action of gamma-aminobutyric acid on membrane conductance and voltage. It is therefore proposed that acetylcholine can reduce evoked IPSPs through presynaptic inhibition. Evidence is also presented that medial septal stimulation can reduce the efficacy of evoked IPSPs. These observations provide further support for the existence of a cholinergic septohippocampal pathway.

Activation of identified septo-hippocampal neurons by noxious peripheral stimulation

Septo-hippocampal neurons (SHNs) were recorded from the medial septum-diagonal band area of rats anaesthetized with either urethane or fluothane. They were identified by their antidromic response to the electrical stimulation of the fimbria. Their responses to peripheral somatic noxious and non-noxious stimulation were studied. Non-noxious natural stimulations were relatively ineffective. In contrast, 68% of the SHNs were driven by noxious stimulation. The SHNs could be driven either by mechanical or thermal stimulation. Intraperitoneal injection of bradykinin excited about half of the SHNs. Some neurons were able to encode stimulus intensity (strength of the mechanical stimulation and/or temperature of the thermal stimulation). The receptive fields of the SHNs were large, usually involving half of the body or the whole body surface. These results suggest that SHNs, which are at the origin of the cholinergic septo-hippocampal pathway, might be involved in cerebral mechanisms related to nociception.

Septo-hippocampal and other medial septum-diagonal band neurons: electrophysiological and pharmacological properties

Neurons located in the medial septum-nucleus of the diagonal band (vertical limb) area and antidromically activated by electrical stimulation of the fimbria were recorded in urethane anesthetized rats. Forty-five percent of these septo-hippocampal neurons (SHNs) discharged rhythmically in short bursts (mean burst frequency, 4 Hz). They were antidromically driven at short latencies from the fimbria. SHNs driven at long latencies (above 5 ms) were never bursting neurons. Fimbria stimulation also had a powerful inhibitory effect on the spontaneous activity of SHNs. The vast majority of the septo-hippocampal neurons were excited by the iontophoretic application of acetylcholine or cholinergic agonists, carbachol being the most effective. The acetylcholine-induced excitations were readily abolished by atropine. In contrast hexamethonium and mecamylamine were less effective. The rhythmic bursting activity could not be consistently altered by the iontophoretic application of cholonergic agonists or antagonists or of divalent cations. SHNs were also sensitive to various othe putative neurotransmitters (substance P, GABA) known to play a role in the medial septal area. Bursting neurons and ACh-sensitive neurons were less frequent among unidentified medial septal neurons. The regulation of the septo-hippocampal cholinergic pathway is therefore not likely to be due to a direct feedback inhibition by locally released acetylcholine. However a strong inhibitory feedback could be exerted by the hippocampo-septal pathway impinging directly or indirectly on septo-hippocampal neurons.

Cells of origin of the ventral cholinergic septohippocampal pathway undergoing compensatory collateral sprouting following fimbria-fornix transection

The axons of the septohippocampal pathway reach the hippocampal formation via the fimbria, dorsal fornix and supracallosal striae. Complete lesions of these pathways denervate most of the hippocampal formation but a small, residual innervation occurs via a different ventral route. The cells of origin for this additional route for cholinergic innervation of the hippocampal formation have been identified in the present study with a combined technique using a fluorescent retrograde tracer and acetylcholinesterase histochemistry. These cells were localized to the diagonal band of Broca. In a subsequent experiment it was demonstrated, by destroying these cells unilaterally, that they were responsible to a large extent for the cholinergic compensatory collateral sprouting in the hippocampus that has previously been shown to develop following complete fimbria-fornix transection.

Chronic intraventricular injections of nerve growth factor elevate hippocampal choline acetyltransferase activity in adult rats with partial septo-hippocampal lesions

Nerve growth factor (NGF) was injected intraventricularly during 4 weeks into adult rats with unilateral partial lessions of the cholinergic septo-hippocampal pathway. On the lesioned side, NGF treatment elevated choline acetyltransferase (ChAT) activity up to 60% above the activity measured on the lesioned side of cytochrome c-treated controls. On the unlesioned side, NGF treatment increased ChAT activity only to an insignificant degree. ChAT activity in the septum of NGF-treated animals was increased by 60% as compared to controls. The NGF-induced increases on the lesioned side and in the septum were not accompanied by elevations in acetylcholinesterate (AChE) activity. Furthermore, histochemical analysis revealed no difference in AChE staining pattern or intensity between NGF-treated and control animals. The lack of effect on AChE strongly suggests that the increases in ChAT activity in hippocampus and septum, are due to an elevation of ChAT activity within cholinergic neurpns surviving the lesion rather than to a promotion of sprouting of cholinergic fibers.

Behavior during hippocampal microinfusions: Anticholinesterase-induced locomotor activation

Awake, unrestrained rats received direct bilateral infusions of the cholinesterase inhibitor, echothiophate, into the dentate gyrus during a 40-min experimental session in a holeboard/activity chamber. Additional animals were similarly tested during intrahippocampal infusions of a novel cholinesterase inhibitor, NBD-AP-MPF. The computerized behavioral pattern monitor recorded the animals' locomotor activity, holepoking, and rearing in a manner that permitted the reconstruction and analysis of their sequential patterns of movement. Both echothiophate and NBD-AP-MPF infusions produced dose-dependent increases in locomotor activity that were accompanied by increased holepoking and rearing. The hyperactivity induced by anticholinesterase infusions was qualitatively similar to the atropine-sensitive effects of infusions of the cholinergic agonist, carbachol, as previously reported. Dye infusions revealed that spread of the infusate was restricted to the dentate gyrus of the anterodorsal hippocampal formation. The locomotor activation caused by the echothiophate infusions into the dentate gyrus were interpreted as indicative of a role for the cholinergic septo-hippocampal pathway in the release of motor responding.

Sustained elevation of hippocampal cyclic 3'-5' adenosine monophosphate levels after medial septal lesions.

The cyclic 3'-5' adenosine monophosphate (cyclic AMP) content of the rat hippocampal formation doubles during the week following a medial septal lesion and remains elevated for at least 1 month, the longest time period studied. This elevation in cyclic AMP does not result from sympathetic ingrowth, as neither superior cervical ganglion stimulation nor ganglionectomy influences hippocampal cyclic AMP content after lesions. Interruption of the cholinergic septohippocampal pathway in the fornix did not elevate hippocampal cyclic AMP content. Further, treatment of septal-lesioned animals with oxotremorine or of normal animals with atropine did not influence hippocampal cyclic AMP content. Finally, neither locus ceruleus lesions nor treatment with propranolol affected hippocampal cyclic AMP content. We believe this to be the first report of a sustained elevation in hippocampal cyclic AMP content. Like other long-term events, it is likely to have profound effects on hippocampal function and represents a remarkable brain adaptation to remote injury.

Septal modulation of the population spike in the fascia dentata produced by perforant path stimulation in the rat

Electrical stimulation of the perforant path, which originates in the entorhinal cortex, produces a characteristic excitatory postsynaptic field potential (extracellular EPSP) which can be recorded in the fascia dentata This evoked response may include a population spike, if stimulation is sufficient. In the anaesthetized rat, stimulation of the medial septum, when paired with stimulation, was found to augment the population spike component of the evoked field potential. Stimulation of the septum alone produced no apparent field potential. The augmentation effect was found to have a rapid onset (4 ms), which is sufficient for the participation of interneurons, and a relatively long time course (150 ms). Presynaptic mechanisms of facilitation were ruled out as there was no concurrent af the extracellular EPSP. A change in population spike threshold, compatible with a postsynaptic mechanism, was obserered and some possible models of action discussed. Augmentation survived depletion of hippocampal norepinephrine caused by injections of 6-hydroxydopamine into the dorsal noradrenergic bundle, indicating that the facilitation was not due to an activation of the ascending noradrenergic fibres of passage originating from the locus coeruleus. The cholinergic septo-hippocampal pathway was ruled out as a likely candidate for the modulation as the augmentation survived injections of the muscarinic antagonists atropine and scopolamine and the nicotinic antagonists tubocurarine and dihydro-ß-erythroidine. A relationship between the septal modulation and hippocampal theta was suggested.

Heart conditioned medium promotes central cholinergic regeneration in vivo

The aim of the present study was to evaluate the effect of a proposed cholinergic growth factor, heart conditioned medium (HCM), on the regeneration of the cholinergic septo-hippocampal pathway in rats. At 0, 4, 8 and 16 days after mechanical injury to septo-hippocampal axons, animals received either repeated intraseptal injections of HCM or control vehicle. Recovery of choline acetyltransferase (CAT) activity in iris implants placed in the hippocampus was used as an index of cholinergic regeneration. The results indicate that CAT activity in iris implants from animals receiving HCM was significantly greater than in control implants. Thus, this is the first report of the in vivo activity of heart conditioned medium as a cholinergic growth factor.

A substance P projection to the hippocampus

Electrolytic lesions of the septal area in the rat resulted in a major decrease in the activity of choline acetyltransferase in the hippocampus, consistent with the interruption of the septo-hippocampal cholinergic pathway. These lesions also significantly decreased the levels of substance P in the hippocampus. It is suggested that a substance P projection to the hippocampus arises in or passes through the septal area.

An investigation of whether septal γ-aminobutyrate-containing interneurons are involved in the reduction in the turnover rate of acetylcholine elicited by substance P and β-endorphin in the hippocampus

Afferents to the septum that contain substance P or β-endorphin probably have an inhibitory action upon the acetylcholine metabolism of the cholinergic septohippocampal pathway because intraseptal administration of these peptides decreases the turnover rate of acetylcholine in the hippocampus. Bicuculline, an antagonist of γ-aminobutyric acid, blocks the inhibitory action of β-endorphin on acetylcholine turnover suggesting that it is mediated via the septal γ-aminobutyric acid-containing interneurons. Inhibition by substance P of the turnover of acetycholine in the hippocampus is independent of this type of interneuron. Dopamine, like β-endorphin, acts on γ-aminobutyrie acid-containing interneurons of the septum. However, lesions of dopaminergic cell bodies located in area A 10 have established that the dopaminergie and β-endorphin-containing inputs to the septal γ-aminobutyric acidcontaining interneurons are independent of each other.

The effects of muscarinic receptor blockers on the turnover rate of acetylcholine in various regions of the rat brain

The actions of antimuscarinic agents (benztropine, trihexyphenidyl, and scopolamine) on the dynamics of acetylcholine (ACh) in central cholinergic neurons were examined in various rat brain areas. It was found that the pattern of changes in ACh turnover (TRACh) elicited by these drugs exhibited marked regional variations. After administration of the anticholinergic drugs, the TRACh in hippocampus and thalamus was increased, in cortex it was decreased, and in striatum it was unchanged. ACh concentration in the cortex and striatum was decreased while in hippocampus and thalamus ACh levels were unaltered. Further analysis of the cholinergic septo-hippocampal pathway using lesions of the fimbria-fornix and local drug injections into the septum argue against an in vivo action of these drugs on presynaptic or cell body muscarinic autoreceptors. Moreover, the data suggest that muscarinic receptor blockers cause an increased TRACh only in those areas where a feedback loop is operative, possibly by inhibiting a neuronal feedback loop involving at least one noncholinergic interneuron.