Morphological and electrophysiological evidence for electrotonic coupling of rat dorsolateral septal nucleus neurons in vitro

Dopamine-mediated regulation of striatal neuronal and network interactions

The dopaminergic system exerts dynamic modulation of glutamatergic afferent drive that is dependent on the temporal pattern of the dopaminergic input and the subtypes of striatal neurons affected. The differences in feedforward inhibition between striatal neurons comprising the direct and indirect output pathway confer distinct response-pattern differences in their respective targets,supporting brief bursts of activity in Type-I neurons but attenuating repetitive activity in Type-II cells. This temporal patterning is further modulated by NO-mediated signaling, and by tonic and phasic dopamine-mediated stimulation, which exerts preferential actions on indirect and direct output neurons, respectively. As a result,the striatal network is forced into state-dependent patterns of activity that differentially regulate muscle tone and voluntary motor activity via distinct output projections from the striatum.

N-ethylmaleimide selectively blocks presynaptic GABA-B autoreceptor but not heteroreceptor-mediated inhibition in adult rat striatal slices

Intracellular recording techniques were used in slices of adult rat striatum to compare the sensitivity of presynaptic gamma-aminobutyric acid type B (GABA-B) autoreceptor and heteroreceptor-mediated inhibition to the sulfhydryl alkylating agent, N-ethylmaleimide (NEM). NEM (100 microM) alone had no significant effect on resting potential or input resistance and did not consistently effect stimulus-evoked responses. Treatment of slices with NEM for up to 1 h, either in the presence or the absence of the GABA-B receptor-specific agonist, baclofen (BAC; 100 microM), completely blocked the BAC-induced depression of inhibitory, but not excitatory, postsynaptic potentials. This differential sensitivity to NEM alkylation suggests that in the adult rat striatum, the presynaptic GABA-B autoreceptors and heteroreceptors may exhibit distinct receptor-effector coupling mechanisms.

Intrinsic controls of intracellular calcium and intercellular communication in the regulation of neuroendocrine cell activity

1. The magnocellular hypothalamoneurohypophysial system, consisting chiefly of the supraoptic and paraventricular nuclei and their axonal projections to the pituitary neural lobe, has become a model for the study of neuroendocrine cell morphology, function, and plasticity. 2. Decades of research have produced a wealth of knowledge about the physiological conditions that activate this system, the peripheral target tissues affected by its outputs, and its capacity to undergo use-dependent, reversible reorganization. 3. Earlier research on the neural control of this system concentrated largely on the synaptic inputs that influence the activity of these magnocellular neurons and, while that task is still far from completed, methods have now been developed that permit insights to be gained into the control exercised by intrinsic cellular and molecular mechanisms. 4. This article reviews the current state of knowledge of roles played by these intrinsic mechanisms, including influences of intracellular calcium buffering, calcium release from internal stores and intercellular communication through gap junctions, in the control of neuroendocrine cell activity.

Acute exposure to 25-hydroxy-cholesterol selectively reduces GABAb and not GABAa receptor-mediated synaptic inhibition

Intracellular recording techniques were used to study the effects of the cholesterol oxide, 25-hydroxycholesterol (25-OH-Chol), on gamma-aminobutyric acid (GABA) receptor-mediated inhibitory postsynaptic potentials (IPSPs) in brain slices of the rat lateral septum. Superfusion of 25-OH-Chol increased the peak amplitude of the GABAa IPSP in more than half of the neurons tested, many of which exhibited a similar increase in the GABAb IPSP. However, some neurons exhibited a gradual decrease in input resistance and a selective reduction or blockade of the GABAb IPSP during prolonged exposure. Cholesterol partly mimicked the effects of 25-OH-Chol. These findings indicate that 25-OH-Chol can selectively reduce or block metabotropic GABAb while sparing ionotropic GABAa receptor-mediated synaptic inhibition. Our results indicate that brain slices can be used to study the effects of short term alterations in cholesterol on the excitability and synaptic integration properties of neurons.

Retrograde labeling of rat dorsolateral septal nucleus neurons following intraseptal injections of WGA-HRP

Projection neurons in the rat dorsolateral septal nucleus (DLSN) were retrogradely labeled following intraseptal injection of wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP). Injections of WGA-HRP centered in the medial septum (MS) and parts of the intermediate and ventrolateral subdivisions of the lateral septum retrogradely labeled only a few centrally scattered multipolar-shaped neurons. In contrast, injections placed in the nucleus of the diagonal band of Broca (DBB) consistently resulted in labeling of DLSN neurons of all sizes and shapes. Large injections in rostral DBB appeared to retrogradely label every DLSN neuron, while similar injections in caudal DBB only labeled neurons in restricted regions of the nucleus. A collection of small cells forming the ventricular border of caudal DLSN and a group of larger cells situated in the dorsolateral tip of rostral DLSN were consistently labeled following each DBB injection. The pattern of retrogradely labeled neurons in the DLSN appeared in a complementary fashion to that seen in the other lateral septal nuclei. Our findings support the conclusion that the DLSN is a morphologically heterogeneous nucleus consisting almost entirely of projection neurons. The pattern of retrograde labeling in the lateral septum suggests that these projection neurons may be topographically organized since distinct subpopulations of cells were labeled following different injections in the MS/DBB complex.

Ultrastructural characterization of neurons recorded intracellularly in vivo and injected with lucifer yellow: advantages of immunogold-silver vs. immunoperoxidase labeling

Immunoperoxidase labeling of lucifer yellow provides a sensitive method for morphological characterization of neurons recorded intracellularly in vitro or in vivo. However, the reaction product is often so dense that it obscures ultrastructural details necessary for the analysis of synaptic contacts onto individually filled neurons. In the present study, we describe a silver intensification procedure using 1 nm gold labeling of lucifer yellow as an optimal means for immunocytochemically identifying single physiologically characterized neurons at the ultrastructural level. Single neurons in the frontal cortex of anesthetized rats were impaled in vivo and filled with lucifer yellow. The brains were then perfused with an acrolein fixative. Single vibratome sections through the recording site were reacted with a rabbit antibody directed against lucifer yellow followed by goat anti-rabbit 1 nm gold-labeled IgG and silver intensified. For comparison, additional sections were processed for immunoperoxidase detection of lucifer yellow. Labeled sections were processed for light microscopy or embedded in plastic for electron microscopy. The immunogold-silver label as well as peroxidase reaction product of lucifer yellow was readily detected in cell bodies, proximal and distal dendrites, and spines. However, in contrast to immunoperoxidase, the immunogold-silver reaction did not obscure subcellular organelles. Most importantly, the synaptic junctions formed by afferents to the filled neuron were more easily identifiable following the immunogold-silver procedure. This clear visualization of postsynaptic densities is essential for examining synaptic circuitry between afferents and physiologically characterized neurons.

Functional aspects of central electrical coupling in mechanoreceptor afferents of crayfish

Cuticular stress detector afferents (CSD1 and CSD2) in the walking legs of crayfish have been studied in an in vitro preparation allowing intracellular recordings of single neurones during mechanical stimulation of the organs. Lucifer yellow staining of single fibres showed them to be dye-coupled through junctions presumably located near their point of entry into the ganglion. In intracellular recordings from identified sensory fibres with 30-80-mV impulses, coupled afferent spikes were seen as depolarizing potentials of short duration but smaller amplitude. Coupling sometimes involved fibres with quite different response characteristics to mechanical stimuli. Coupled spikes summated during strong mechanical stimuli. In silent CSD fibres, antidromic spikes could occur which were triggered by the depolarization due to coupling. Antidromic spikes were more frequent during strong stimuli and were in some cases shown to be triggered in a cooperative way when two afferent spikes synchronously reached the ganglion. Our results suggest that central coupling of primary afferents may be a widespread phenomenon in Crustacea and that it provides a threshold mechanism for selective amplification of strong stimuli.

Dye coupling between dorsal raphe neurones

Neurones in the dorsal raphe nucleus (DRN) were impaled and filled with biocytin in coronal slices of midbrain taken from young adult rats. The electrophysiological properties and gross morphology of the cells were similar to those reported previously for serotonergic neurones in the DRN. Of 27 cases in which filled neurones were recovered in histological material, almost half (48%) showed labelling of two or three cells, although only one cell had been recorded from. Coupled cells were identified as close or distantly coupled, depending on the distance from the soma of the presumed impaled cell (23.5 +/- 15 microns, n = 7 and 150 +/- 26.5 microns, n = 10 respectively). Whereas close-coupled cells may have been artefactually "coupled" by the penetrating electrode, coupling between distant cells is most likely to be a result of transfer of biocytin through gap junctions. Camera lucida reconstructions of pairs of labelled cells revealed extensive overlap of dendritic fields and numerous crossings between dendrites. When examined at high magnification under a light microscope, many of the crossing dendrites were found to travel in different focal planes. Nevertheless, for each pair of cells, at least one point of close apposition was observed between dendrites or between the axon and a dendrite of the presumed impaled and coupled cell. The incidence of dye coupling between neurones in the DRN may reflect a relatively high level of electronic coupling between the neurones. This form of coupling may be important in determining the synchronous nature of firing of neurones in the DRN.