Modulation of septal influences on hippocampal neurons by cholinergic substances

The effects of electrical stimulation of the medial septal area (MS-DB) for the purpose of distinguishing and assessing the cholinergic component of the septohippocampal input were investigated in awake rabbits in chronic experiments. Initial inhibitory effects of a standard duration of 40–140 msec (54%) predominated in the intact rabbits. In animals with chronic basal undercutting of the MS-DB, initial inhibitory reactions predominated absolutely (90%). An increase in the level of endogenous acetylcholine by administration of eserine led to a partial or complete suppression of all effects of stimulation in 78% of the hippocampal neurons of the intact rabbits against the background of intensification of the theta modulation of the activity of hippocampal neurons. Scopolamine removed theta modulation and restored the reactivity of neurons to stimulation of the MS-DB. These influences of cholinergic substances were maintained in the animals with basal undercutting of the MS-DB. It is inferred that the general initial influence of septal input on neurons of the hippocampus is expressed in the suppression of their activity (“reset”), which depends on the noncholinergic (GABAergic) component of the septohippocampal connections. The cholinergic component limits the effectiveness of both extraseptal (brainstem) and primary inhibitory septal influences on hippocampal neurons. This study was supported by the Russian Basic Research Fund (project No. 93-04-21907). Institute of Theoretical and Experimental Biophysics, Russian Academy of Sciences, Pushchino. Translated from Zhurnal Vysshei Nervnoi Deyatel'nosti imeni I. P. Pavlova, Vol. 44, No. 4–5, pp. 751–761, July–October, 1994.     

The spontaneous activity of hippocampal neurons during the modulation of theta rhythm by cholinergic substances

A statistical analysis of the baseline activity of neurons, recorded intracellularly in the hippocampus of awake, nonimmobilized rabbits in three states, control and during the systemic administration of eserine and scopolamine, was carried out. Neurons of the hippocampus were additionally tested in a similar manner following the chronic basal undercutting of the septum, removing stem influences. The cholinergic substances regulate the number of neurons of the hippocampus having theta modulation and the degree of its stability, but do not influence its frequency. When the cholinergic theta rhythm is activated, regularization of the activity takes place with the suppression of delta modulation and of complex spikes; its blockade is accompanied by the opposite changes. Both substances stably alter the level of the baseline frequency of discharges of the majority of neurons, although the total average frequency remains constant. Regression analysis shows the predominance of a decrease in the activity in highfrequency (> 25 spikes/sec) and an increase in the lowf-requency (< 25 spikes/sec) neurons during the effect of both substances. The constancy of the total average frequency and the unidirectionality of the shifts in the level of discharges of the neurons during the intensification (eserine) and blockade (scopolamine) of the cholinergic component of the theta rhythm points to the fact that the cholinergic septal input directly influences mainly the structure but not the level of the activity of the hippocampal neurons.Translated from Zhurnal Vysshei Nervnoi Deyatel'nosti imeni I. P. Pavlova, Vol. 42, No. 5, pp. 944–954, September–October, 1992.     

Modulation of long-term potentiation in rat hippocampal pyramidal neurons by zinc

The phenomenon of long-term potentiation is frequently promulgated as an example of learning and memory mechanisms at the synaptic level in the mammalian central nervous system. In the CA3 region of the hippocampus there is an abundance of zinc, which is located in presynaptic mossy fibre nerve terminals. Stimulation of these fibres can cause the release of zinc, which interacts with excitatory amino acid receptors and may therefore modulate long-term potentiation. We now demonstrate in CA1 and CA3 neurons that zinc (100–300 M) enhances non-N-methyl-d-aspartate-receptor-mediated responses whilst reducing excitatory synaptic transmission and inhibiting long-term potentiation. However, by using zinc-chelating agents, endogenously released zinc following high-frequency stimulation in the stratum lucidum does not appear to have any modulatory role in excitatory synaptic transmission and long-term potentiation. These results indicate that an increase in the level of extracellular zinc can limit excitatory synaptic transmission in the CA1 or CA3 region and further suggests that pathologies that can be related to excessive levels of endogenous zinc may have implications for synaptic plasticity in CA3 neurons.     

Modulation of excitability in Aplysia tail sensory neurons by tyrosine kinases

Tyrosine kinases have recently been shown to modulate synaptic plasticity and ion channel function. We show here that tyrosine kinases can also modulate both the baseline excitability state of Aplysia tail sensory neurons (SNs) as well as the excitability induced by the neuromodulator serotonin (5HT). First, we examined the effects of increasing and decreasing tyrosine kinase activity in the SNs. We found that tyrosine kinase inhibitors decrease baseline SN excitability in addition to attenuating the increase in excitability induced by 5HT. Conversely, functionally increasing cellular tyrosine kinase activity in the SNs by either inhibiting opposing tyrosine phosphatase activity or by direct injection of an active tyrosine kinase (Src) induces increases in SN excitability in the absence of 5HT. Second, we examined the interaction between protein kinase A (PKA), which is known to mediate 5HT-induced excitability changes in the SNs, and tyrosine kinases, in the enhancement of SN excitability. We found that the tyrosine kinases function downstream of PKA activation since tyrosine kinase inhibitors reduce excitability induced by activators of PKA. Finally, we examined the role of tyrosine kinases in other forms of 5HT-induced plasticity in the SNs. We found that while tyrosine kinase inhibitors attenuate excitability produced by 5HT, they have no effect on short-term facilitation (STF) of the SN-motor neuron (MN) synapse induced by 5HT. Thus tyrosine kinases modulate different forms of SN plasticity independently. Such differential modulation would have important consequences for activity-dependent plasticity in a variety of neural circuits.     

Rat hippocampal neurons in culture: responses to electrical and chemical stimuli

Intracellular activity was recorded from dissociated rat hippocampal neurons maintained in tissue culture conditions for 4-6 wk. The cells developed dense interconnections and had typical morphological characteristics similar to hippocampal neurons in situ. The recorded neurons possessed similar electrophysiological properties to those observed in situ or in a slice preparation. Their input resistance (42 M omega), resting membrane potential (-60 mV), membrane time constant (16.2 ms), total electrotonic length (0.92), and spike size (68.3 mV) were similar to values obtained in hippocampal cells in a slice. The connections among adjacent neurons were largely inhibitory. The inhibitory postsynaptic potentials (IPSPs) had longer durations than excitatory postsynaptic potentials (EPSPs) when these were detected. Synaptic delay varied between 0.3 and 3.0 ms. There were no electrotonic connections among neurons. Reciprocal connections were common. Most neurons reacted to acetylcholine (ACh) by an increase in frequency of spontaneous EPSPs, action-potential discharges, and IPSPs. Concurrently, there was a marked reduction in the magnitude of the evoked PSPs tested in pairs of cells. This effect is probably presynaptic to the recorded neurons. A statistical analysis of quantal properties of the synaptic interactions among neurons revealed that ACh causes a reduction of magnitude of PSPs by reducing the number of releasing elements (m). This effect is different from the reduction of evoked PSPs caused by postsynaptic depolarization.     

Effects of septal and hippocampal stimuli on paraventricular nucleus neurons

The electrical activity of 125 neurons within the hypothalamic paraventricular nucleus was recorded in urethan-anaesthetized male rats. Spontaneous activity of the cells and their responses following electrical stimuli delivered to the ipsilateral lateral septum and dorsal hippocampus were recorded. The mean firing rate of all the cells recorded was3.5 ± 0.4 Hz and the majority were located within the dorsal and medial components of the paraventricular nucleus. Forty-six percent of the cells were inhibited following stimulation of the lateral septum (onset,22.8 ± 6.7 ms; offset,195.1 ± 28.5 ms). Inhibitory responses to dorsal hippocampus stimulation were recorded from 44% of all cells (onset,28.1 ± 4.7ms; offset,180.7 ± 28.7ms). Stimulation of both sites caused excitation of equal proportions (26%) of the cells tested (lateral septum onset, 47.7 ± 4.5 ms; offset, 64.8 ± 6.6 ms; dorsal hippocampus onset,48.7 ± 5.6 ms; offset, 72.3 ± 8.8 ms). Of the sub-population of cells identified as projecting to the median eminence, inhibition was recorded from 50% following lateral septum stimulation and 43% following dorsal hippocampus stimulation, excitatory responses being recorded from only 9% of cells tested. The excitatory responses were only recorded from phasically firing, vasopressin-secreting cells identified as projecting to the median eminence, and also to the neurohypophysis. Following stimulation of either site, more phasic cells were excited whilst only few were inhibited. Continuously active cells, identified as projecting to the neurohypophysis, showed more mixed responses following stimulation.

The specificity of the response categories, according to cell-type, and the high degree of convergence of responses (79%;P < 0.05) appear to correlate with a variety of neuroendocrine studies concerning the regulation of anterior and posterior pituitary hormone secretion and the results are discussed in relation to such studies.     

Modulation of large conductance calcium-activated potassium channels from rat hippocampal neurons by glutathione

We have investigated the modulation of neuronal large conductance Ca2+-activated K+ channels by glutathione. Single channel recordings were made from cultured neonatal rat hippocampal neurons by using excised inside-out patch clamp method. Glutathione, a physiological sulfhydryl specific reducing reagent, increased channel activities in concentration dependent manner with half activation concentration of 710 microM. Conversely, oxidized form of glutathione inhibited channel activities with half inhibition concentration of 520 microM. Our results provide direct evidence that when neuronal large conductance Ca2+-activated K+ channels are exposed to reducing or oxidizing environments, channel activities are increased or decreased in opposite directions due to the redox modification. This may constitute an important regulatory mechanism of neuronal Ca2+-activated K+ channel activities.     

Slow cholinergic excitation of guinea pig hippocampal neurons is mediated by two muscarinic receptor subtypes

Stimulation of cholinergic fibers or bath application of carbachol (0.1-10 microM) induced a slow excitability increase in CA3 neurons and dentate granule cells of hippocampal slices. This effect which was antagonized by atropine (1 microM) was mediated by two receptor subtypes: a pirenzepine (10 microM)-insensitive receptor, 'M2', and a pirenzepine (1 microM)-sensitive receptor, 'M1'. The M2-receptor activation led to a blockade of slow afterhyperpolarizations following trains of action potentials and to the occurrence of threshold-activated plateau-depolarizations associated with a conductance increase. The M1-receptor mediated a membrane depolarization sometimes associated with a conductance decrease which reversed its polarity at membrane potentials negative to -80 mV. The 'slow excitatory postsynaptic potential' which results from activation of cholinergic fibers is thus caused by the activation of two receptor subtypes.     

Catecholamine luminescence reaction in neurons of the spinal sensory ganglion

The spinal ganglia of 45 cats were studied by the method of Falck and Hillarp in Krokhina's modification, and parallel investigations were made by Nissl and Unna's methods, staining for lipofuscin, and by the Masson-Fontana method for serotonin. Two types of neurons were discovered: neurons of one type had yellowish-white granules in the region of the axon hillock and speckled yellowish granules and diffuse greenish luminescence in the perikaryon, while those of the second type had orange granules, gathered into small clumps all over the region of the perikaryon. Neurons of both types were surrounded by single adrenergic nerve fibers.     

Modulation of N-type Ca2+ channels by intracellular pH in chick sensory neurons

Both physiological and pathological neuronal events, many of which elevate intracellular [Ca2+], can produce changes in intracellular pH of between 0.15 and 0.5 U, between pH 7.4 and 6.8. N-type Ca2+ channels, which are intimately involved in exocytosis and other excitable cell processes, are sensitive to intracellular pH changes. However, the pH range over which N-type Ca2+ channels are sensitive, and the sensitivity of N-type Ca2+ channels to small changes in intracellular pH, are unknown. We studied the influence of intracellular pH changes on N-type calcium channel currents in dorsal root ganglion neurons, acutely isolated from 14-day-old chick embryos. Intracellular pH was monitored in patch-clamp recordings with the fluorescent dye, BCECF, and manipulated in both the acidic and basic direction by extracellular application of NH4+ in the presence and absence of intracellular NH4+. Changes in intracellular pH between 6.6 and 7.5 produced a graded change in Ca2+ current magnitude with no apparent shift in activation potential. Intracellular acidification from pH 7.3 to 7.0 reversibly inhibited Ca2+ currents by 40%. Acidification from pH 7.3 to pH 6.6 reversibly inhibited Ca2+ currents by 65%. Alkalinization from pH 7.3 to 7.5 potentiated Ca2+ currents by approximately 40%. Channels were sensitive to pHi changes with high intracellular concentrations of the Ca2+ chelator, bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid, which indicates that the effects of pHi did not involve a Ca2+-dependent mechanism. These data indicate that N-type Ca2+ channel currents are extremely sensitive to small changes in pHi in the range produced by both physiological and pathological events. Furthermore, these data suggest that modulation of N-type Ca2+ channels by pHi may play an important role in physiological processes that produce small changes in pHi and a protective role in pathological mechanisms that produce larger changes in pHi.     

Recovery of hippocampal cholinergic activity by transplantation of septal neurons in AF64A treated rats

Embryonic septal neurons were transplanted into the hippocampus of adult rats which had received lateral-ventricular administration of AF64A, a cholinergic neurotoxin, and the effects on hippocampal cholinergic activity were studied. One week after AF64A administration, we injected dissociated septal cell suspension into the dorsal hippocampus, unilaterally. About 3 months after the transplantation, acetylcholine (ACh)-rich septal grafts formed extensive acetylcholinesterase (AChE)-positive fibers into the host hippocampus, recovering choline acetyltransferase (ChAT) level only in the grafted side. These results indicate that septal implants can produce a partial recovery of the cholinergic activity in the chemically damaged hippocampus.     

Septal cholinergic neurons suppress seizure development in hippocampal kindling in rats: comparison with noradrenergic neurons

Widespread lesions of forebrain cholinergic or noradrenergic projections by intraventricular administration of 192 IgG-saporin or 6-hydroxydopamine, respectively, accelerate kindling epileptogenesis. Here we demonstrate both quantitative and qualitative differences between the two lesions in their effects on hippocampal kindling in rats. Epileptogenesis was significantly faster after noradrenergic as compared to cholinergic denervation, and when both lesions were combined, kindling development resembled that in animals with 6-hydroxydopamine lesion alone. Furthermore, whereas the 192 IgG-saporin lesion promoted the development only of the early stages of kindling, administration of 6-hydroxydopamine or both neurotoxins accelerated the late stages also. To investigate the contribution of different subparts of the basal forebrain cholinergic system to its seizure-suppressant action in hippocampal kindling, 192 IgG-saporin was injected into medial septum/vertical limb of the diagonal band of Broca or nucleus basalis magnocellularis, leading to selective hippocampal or cortical cholinergic deafferentation, respectively. The denervation of the hippocampus facilitated kindling similar to the extensive lesion caused by intraventricular 192 IgG-saporin, whereas the cortical lesion had no effect. These results indicate that although both noradrenergic and cholinergic projections to the forebrain exert powerful inhibitory effects on hippocampal kindling epileptogenesis, the action of the cholinergic system is less pronounced and occurs specifically prior to seizure generalization. In contrast, noradrenergic neurons inhibit the development of both focal and generalized seizures. The septo-hippocampal neurons are responsible for the antiepileptogenic effect of the cholinergic system in hippocampal kindling, whereas the cortical projection is not significantly involved. Conversely, we have previously shown [Ferencz I. et al. (2000) Eur. J. Neurosci., 12, 2107-2116] that seizure-suppression in amygdala kindling is exerted through the cortical and not the hippocampal cholinergic projection. This shows that, depending on the location of the primary epileptic focus, i.e. the site of stimulation, basal forebrain cholinergic neurons operate through different subsystems to counteract seizure development in kindling.     

Neurotrophic effects of hippocampal target cells on developing septal cholinergic neurons in culture

The influence of hippocampal target cells on the development of cholinergic septal neurons was studied in rotation-mediated reaggregating cell cultures. Brain cells from 15-day-old mouse embryos were obtained from: septum, containing cholinergic cells which project to the hippocampus; hippocampus which contains target cells for the septal cholinergic neurons; and cerebellum, containing cells which are not targets for the septal cholinergic cells. The cells were then cultured for 3 weeks in a rotary incubator in the following combinations: septal cells alone; hippocampal cells alone; cerebellar cells alone; septal-hippocampal cells together; and septal-cerebellar cells together. After harvesting, fixation, and embedding, 50 micron sections were cut and processed for visualization of acetylcholinesterase activity. Sections from reaggregates containing either hippocampal or cerebellar cells alone contained only a few acetylcholinesterase-positive cells, but no positive fibers. Sections from septal-hippocampal coaggregates revealed a pattern of well-defined, fine-caliber acetylcholinesterase-positive fibers with extensive arborizations and varicosities suggesting axonal proliferation. In septal-cerebellar coaggregates, acetylcholinesterase-positive fibers appeared to be degenerating and distinct areas were observed which were essentially devoid of acetylcholinesterase fibers. In some experiments, either cerebellar or hippocampal cells were labeled with wheatgerm agglutinin-rhodamine prior to culture in order to identify these cells in the resulting reaggregates. Analysis of sections from these studies showed that acetylcholinesterase fibers were excluded from regions of coaggregates containing cerebellar cells, but were present in regions of coaggregates containing hippocampal cells. Finally, cell counts of acetylcholinesterase-positive cells in the various combinations revealed that these putative cholinergic neurons were significantly more numerous in septal-hippocampal coaggregates (271 +/- 19 per 10(6) septal cells added) than in septal reaggregates (38 +/- 6 per 10(6) septal cells added) or septal-cerebellar coaggregates (85 +/- 29 per 10(6) septal cells added). These results, taken together, suggest that hippocampal target cells influence the development and survival of cholinergic neurons.     

Modulation of corticomuscular coherence by peripheral stimuli

The purpose of this study was to investigate the effects of peripheral afferent stimuli on the synchrony between brain and muscle activity as estimated by corticomuscular coherence (CMC). Electroencephalogram (EEG) from sensorimotor cortex and electromyogram (EMG) from two intrinsic hand muscles were recorded during a key grip motor task, and the modulation of CMC caused by afferent electrical and mechanical stimulation was measured. The particular stimuli used were graded single-pulse electrical stimuli, above threshold for perception and activating cutaneous afferents, applied to the dominant or non-dominant index finger, and a pulsed mechanical displacement of the gripped object causing the subject to feel as if the object may be dropped. Following electrical stimulation of the dominant index finger, the level of β-range (14–36 Hz) CMC was reduced in a stimulus intensity-dependent fashion for up to 400 ms post-stimulus, then returned with greater magnitude before falling to baseline levels over 2.5 s, outlasting the reflex and evoked changes in EMG and EEG. Subjects showing no baseline β-range CMC nevertheless showed post-stimulus increases in β-range CMC with the same time course as those with baseline β-range CMC. The mechanical stimuli produced similar modulation of β-range CMC. Electrical stimuli to the non-dominant index finger produced no significant increase in β-range CMC. The results suggest that both cutaneous and proprioceptive afferents have access to circuits generating CMC, but that only a functionally relevant stimulus produces significant modulation of the background β-range CMC, providing further evidence that β-range CMC has an important role in sensorimotor integration.     

Modulation of Ca-channel current by an adenosine analog mediated by a GTP-binding protein in chick sensory neurons

Inhibitory modulation of the high-voltage-activated (HVA) Ca-channel current by 2-chloroadenosine (2CA) was studied in chick sensory neurons using the whole-cell clamp method. 2CA reduced the CTX-sensitive HVA-current (Aosaki and Kasai 1989) in a dose-dependent manner with aK _d of 0.8 M. The inhibition by 2CA was also voltage-dependent, being maximal at hyperpolarized potentials, and completely removed at potentials more positive than 30 mV. This voltage-dependence of 2CA action was also evident as a progressive increase in Ca-channel current magnitude during a depolarization which could be described by a single exponential function and which became faster at larger depolarizations. The concentration of 2CA affected the steady-state reduction in Ca-channel current, but did not alter the time-course of current increase during depolarization. The voltage-dependent effect of 2CA was mimicked by intracellular application of GTP-S, but not by phorbol ester, arachidonic acid or nitroprusside. These results are consistent with model in which 2CA activates a G-protein, which then unmasks an additional activation gate on the Ca-channel.     

Differential modulation of single voltage-gated calcium channels by cholinergic and adrenergic agonists in adult hippocampal neurons.

1. Pharmacologic agents known to modulate long-term potentiation (LTP) at the mossy fiber-to-CA3 pyramidal neuron synapse were tested for their effects on the activity of single voltage-gated calcium channels in adult CA3 pyramidal neurons. 2. Single-channel current recordings of three types of voltage-gated calcium channels were made from acutely exposed CA3 pyramidal neurons of the adult guinea pig hippocampus. 3. The beta-adrenergic agonist isoproterenol (10 microM), which is known to enhance LTP, increased the activity of the two high-threshold calcium channels (N and L) with no striking effect on the low-threshold (T) channel. 4. The muscarinic agonists carbachol and muscarine (1-10 microM), the latter of which has been shown to inhibit LTP, decreased the probability of opening of L channels, increased the probability of opening of T channels, and had no effect on N channels. The effects were blocked by 0.1 microM atropine. 5. These results are consistent with the hypothesis that neuromodulation of mossy fiber LTP occurs, at least in part, through the modulation of postsynaptic, voltage-gated calcium channels.