Influence of microelectrophoretically applied acetylcholine on the responsiveness of hippocampal and lateral geniculate neurones

Medial septal modulation of hippocampal theta cell discharges

The effect of small electrolytic lesions in various areas of the septum on the behavioral correlates and firing repertoires of hippocampal theta cells, was investigated in the freely moving rabbit. Lesions localized to the medial septum were found to abolish both slow wave theta and the rhythmic firing of CA1 and dentate layer theta cells, in both the type 1 theta (movement) and type 2 theta (sensory processing) behavior conditions. Small lesions of the diagonal band, lateral septum and fimbria/fornix regions only affected rhythmicity to the extent that they also involved the medial septal region. The same medial septal lesions that abolished rhythmicity were also shown to reduce the mean discharge rate of theta cells occurring during the type 1 movement condition by approximately 50%, while the discharge rate occurring during the type 2 sensory processing condition did not change significantly. Behavioral changes were also only observed for lesions involving the medial septum. The importance of afferent input from the medial septum in the generation of hippocampal theta cell rhythmicity was discussed.

An analysis of cholinoceptive neurons in the hippocampal formation by direct microinfusion

Microinfusions of cholinergic agents were made in various sites in the dorsal hippocampal formation of urethane anaesthetized rats. Infusions of eserine or carbachol elicited hippocampal theta activity when made in areas containing high levels of cholinergic markers: the stratum oriens and radiatum of the CA1 and CA3, the stratum moleculare and stratum granulosum of the dentate gyrus and the infragranular region of the hilus. Subsequent infusions of atropine sulfate antagonized the theta activity. Control infusions of equal volumes of saline in active sites were without effect. Infusions of eserine or carbachol in the vicinity of the hippocampal fissure, the stratum lacunosum/moleculare of the CA1 or CA3, in the deep regions of the hilus, and in the lateral ventricle and overlying neocortex, were also without effect. Furthermore, in active sites, the latency to onset of theta and subsequent theta frequency, were both directly related to the total amount of carbachol infused. Thus, areas in which theta could be elicited with a cholinergic agonist (carbachol), or an anticholinesterase (eserine) and antagonized with atropine, were found to correspond well to areas previously found to contain a high density of cholinoceptive neurons, using autoradiographic and immunohistochemical techniques. These results provide further support for the involvement of acetylcholine as a neurotransmitter in the generation of type 2 theta in the hippocampal formation.

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.

The pharmacology of hippocampal theta cells: evidence that the sensory processing correlate is cholinergic

The firing repertoires of theta cells in the CA1 and dentate layers of the hippocampal formation of the freely moving rabbit were analyzed during 3 behavioral conditions: (1) voluntary motor patterns, termed type 1 theta behaviors; (2) automatic motor patterns, termed type 2LIA behaviors; (3) alert immobility with presentation of sensory stimuli, termed type 2 theta behavior. Cholinergic manipulations were shown to effect the firing repertoires of theta cells during the type 2 theta behavior condition (sensory processing) and not the other two behavioral conditions. A hypothesis of a sensorimotor processing function of the hippocampal formation is presented and discussed.

Voltage-clamp analysis of muscarinic excitation in hippocampal neurons

Pyramidal cells in the CA1 field of guinea pig hippocampal slices were voltage-clamped using a single microelectrode, at 23-30 degrees C. Small inwardly relaxing currents triggered by step hyperpolarizations from holding potentials of -80 to -40 mV were investigated. Inward relaxations occurring for negative steps between -40 mV and -70 mV resembled M-currents of sympathetic ganglion cells: they were abolished by addition of carbachol, muscarine or bethanechol, as well as by 1 mM barium; the relaxations appeared to invert at around -80 mV; they became faster at more negative potentials; and the inversion potential was shifted positively by raising external K+ concentration. Inward relaxations triggered by steps negative to -80 mV, in contrast, appeared to reflect passage of another current species, which has been labelled IQ. Thus IQ did not invert negative to -80 mV, it was insensitive to muscarinic agonists or to barium, and it was blocked by 0.5-3 mM cesium (which does not block IM). Turn-on of IQ causes the well known droop in the hyperpolarizing electrotonic potential in these cells. The combined effects of IQ and IM make the steady-state current-voltage relation of CA1 cells slightly sigmoidal around rest potential. It is suggested that activation of cholinergic septal inputs to the hippocampus facilitates repetitive firing of pyramidal cells by turning off the M-conductance, without much change in the resting potential of the cell.

Cholinergic excitation of mammalian hippocampal pyramidal cells

Responses of CA1 pyramidal neurons to ACh were recorded with intracellular microelectrodes utilizing the in vitro guinea pig hippocampal slice preparation. ACh was delivered by drop or iontophoretic application to stratum oriens or stratum radiatum. Threshold dose for drop application was 1 mM. An initial hyperpolarization of 3.1 +/- 1.8 (S.D.) mV associated with a decrease in membrane input resistance (RN) of 21 +/- 9% (S.D.) occurred in about half the cells. This result is consistent with a presynaptic action of ACh mediated through excitation of inhibitory interneurons. This interpretation was supported by recordings of cholinergic excitatory responses from presumed interneurons, and repetitive spontaneous IPSPs from pyramidal neurons during the hyperpolarization. ACh evoked a slow depolarization (14.3 +/- 10.8 (S.D.) mV) accompanied by a peak increase in apparent input resistance (Ra) of about 60% in the majority of cells. Large increases in spike frequency were associated with these events but action potential shape was unchanged. Plots of Ra versus membrane potential following ACh application revealed that Ra increases were proportionately higher at depolarized membrane potential levels (less than or equal to -70 mV) in some neurons. In these cells Ra was increased significantly at -60 mV (28%), but only 6% at -75 mV. These results are consistent with the conclusion that ACh reduces a voltage-dependent gK, distinct from delayed rectification. ACh also induced a non-voltage-dependent increase in Ra in some cells. ACh-evoked changes in Ra were long-lasting and gave rise to alterations in firing mode, with development of burst generation. ACh also transiently blocked after hyperpolarizations which followed spike trains in pyramidal neurons and presumed interneurons, an action which may be related to effects on a Ca2+-activated gK.

Multiple actions of acetylcholine on hippocampal pyramidal cells in organotypic explant cultures

Hippocampal cultures were prepared from 7- to 10-day-old rats by means of the roller-type technique. The preservation of the characteristic hippocampal cytoarchitecture allowed, after many weeks in vitro, impalement of pyramidal cells by microelectrodes under visual control. Application of 10(-7) to 10(-5) M acetylcholine to the bath depolarized hippocampal pyramidal cells, strongly increased their rate of firing and induced paroxysmal depolarization shifts. This depolarizing action was accompanied by a reduction in the amplitude of evoked postsynaptic potentials. Whereas it was not clear whether the decrease in the amplitude of the excitatory postsynaptic potentials was only a result of membrane depolarization, acetylcholine clearly and reversibly reduced the potency of evoked inhibitory postsynaptic potentials. Iontophoresis of acetylcholine to the perisomatic region of pyramidal neurons, like acetylcholine applied to the bath, increased their firing rate and powerfully decreased the amplitude and duration of spontaneous and evoked inhibitory postsynaptic potentials. In contrast, iontophoresis of acetylcholine in the pyramidal cell layer at a distance from the recorded neuron generated a hyperpolarizing response associated with a reduction in firing rate. At high current strength, the initial hyperpolarization was (often) followed by a paroxysmal depolarization shift. High frequency electrical stimulation with electrodes located close to the acetylcholine pipette in the pyramidal cell layer (i.e. about 1 mm away from the recorded neuron) mimicked the acetylcholine effect. Resistance measurements indicated that membrane input resistance was decreased in the majority of cells during application of acetylcholine. This decrease in membrane resistance may result from a direct action of acetylcholine or from an increased synaptic activity. Synaptic alterations induced by acetylcholine were quick in onset and in recovery, while the increase in the rate of firing occurred somewhat later. Atropine (10(-5) M), which had no significant action by itself, completely abolished the action of acetylcholine applied to the bath or by iontophoresis. In contradistinction, naloxone did not influence the acetylcholine effects, although opiates and opioid peptides produce paroxysmal depolarization shifts in pyramidal cells which resemble those induced by acetylcholine. Addition of 8-16 mM magnesium to the bathing solution or exposure of the cultures to a calcium-free solution containing 1 mM cobalt abolished the effects of acetylcholine. In the presence of 10(-6) g/ml tetrodotoxin, 10(-5) M acetylcholine decreased the membrane input resistance of pyramidal cells, reduced their threshold for the generation of tetrodotoxin-resistant spikes and generated paroxysmal depolarization shifts in a proportion of pyramidal cells...

Distortions of vision and pain: two functional facets of D-lysergic acid diethylamide

D-lysergic acid diethylamide (LSD) produces distortions of visual perception and analgesia. Evidence is advanced from a functional standpoint that the observed visual effects result from an attenuation of light-evoked input to the dorsal lateral geniculate nucleus (LGN) from the purely centripetal pathways of the retina. More slowly responding visual afferents or those with more complex receptive fields seem to be affected most. LSD analgesia, accompanied by severe psychotic symptoms, appears to result from drug actions on a centrifugally controlled pain system involving neurons of the midbrain raphe.

Localization of cholinergic muscarinic receptors in rat brain by light microscopic radioautography

Injection of a potent, cholinergic muscarinic antagonist, [3H]3-quinuclidinyl benzilate ([3H]QNB), results in a localization of the drug to muscarinic receptors in rat brain. The distribution of these drug receptors was examined in various brain regions previously thought to contain cholinergic neurons. They were localized to dendritic regions in the hippocampus, corpus striatum, nucleus accumbens and cerebral cortex. Particularly in the hippocampus, the receptor distribution may correspond to that for cholinergic nerve terminals.

GDEE antagonism of iontophoretic amino acid excitations in the intact hippocampus and in the hippocampal slice preparation

Glutamic acid diethylester (GDEE) reversibly antagonized excitations produced by glutamate and aspartate but not those produced by acetylcholine when applied iontophoretically to rat CA1 hippocampal neurons in penthrane (methoxyfluorane) anesthetized rats and to CA1 neurons in in vitro slice preparations. GDEE did not appear to differentiate between the excitations produced by glutamate aspartate and appeared to be a more potent antagonist than has previously been reported. CA1 cells were remarkably sensitive to acetylcholine; 5-50 nA being sufficient to produce marked amino acid-like excitations, which were unrelated to the pH of the acetylcholine. The nature of the responses to applied substances was virtually identical between the intact animal and the in vitro slice preparation. A description of the in vitro technique is given as an Appendix.

Regeneration of central cholinergic neurones in the adult rat brain

The regrowth of lesioned central acetylcholinesterase (AChE)-positive axons in the adult rat was studied in irides implanted to two different brain sites: in the caudal diencephalon and hippocampus, and in the hippocampal fimbria. At both implantation sites the cholinergic septo-hippocampal pathways were transected. At 2-4 weeks after lesion, newly formed, probably sprouting fibres could be followed in abundance from the lesioned proximal axon stumps into the iris transplant. Growth of newly formed AChE-positive fibres into the transplant was also observed from lesioned axons in the anterior thalamus, and to a minor extent also from the dorsal and ventral tegmental AChE-positive pathways and the habenulo-interpeduncular tract. The regrowth process of the sprouting AChE-positive, presumed cholinergic fibres into the iris target was studied in further detail in whole-mount preparations of the transplants. For this purpose the irides were removed from the brain, unfolded, spread out on microscope slides, and then stained for AChE. During the first 2-4 weeks after transplantation the sprouting central fibres grew out over large areas of the iris. The new fibres branched profusely into a terminal plexus that covered maximally about half of the iris surface, and in some areas the patterning of the regenerated central fibres mimicked closely that of the normal autonomic cholinergic innervation of the iris. In one series of experiments the AChE-staining was combined with fluorescence histochemical visualization of regenerated adrenergic fibres in the same specimens. In many areas there was a striking congruence in the distributional patterns of the regenerated central cholinergic and adrenergic fibres in the transplant. This indicates that - as in the normal iris - the sprouting cholinergic axons (primarily originating in the lesioned septo-hippocampal pathways) and adrenergic axons (primarily originating in the lesioned axons of the locus neurones) regenerate together along the deneravated Schwann cell sheaths. From a comparison between the central reinnervation process and the process of reinnervation of the iris by peripheral cholinergic axons after transplantation to the anterior eye chamber, it is concluded that the regenerative capacity of central cholinergic neurones (above all the septo-hippocampal system) is not much inferior to that of their peripheral counterparts when given similar growth conditions. Moreover, central cholinergic neurones seem partly able to replace the peripheral ones in the reinnervation of a denervated peripheral target.

Peptide and acetylcholine action on neurones of the cat subfornical organ

Angiotensin II, related oligopeptides and acetylcholine were tested on neurones of the cat subfornical organ (SFO). Angiotensin II activates SFO-neurones by local administration onto the surface, by i.v. injection or with the aid of microiontophoretic techniques. Application of related oligopeptides, bradykinin, physalaemin and eledoisin showed no comparable results. Activation of neurones similar to those observed after angiotensin II was obtained with acetylcholine. About 30% of the cells tested were excited by both substances, 56% of tested SFO-cells responded only to angiotensin II, but not to acetylcholine. Atropine sulphate prevents specifically the acetylcholine excitation. Since angiotensin II is involved in regulatory mechanism of thirst, these results suggest the possibility that the SFO is one of the sites, where dipsogenic receptors for this circulating peptide are located.