Light- and electron-microscopic study of electrophysiologically characterized neurons in the mediolateral part of the lateral spetum of the guinea-pig.

In slices of guinea-pig brains, 36 neurons located in the mediolateral part of the lateral septum were stained intracellularly with horseradish peroxidase (n = 28) or biocytin (n = 8) after electrophysiological characterization. These neurons belonged to class A neurons (n = 23), which generated pronounced Ca(++)-dependent high-threshold spikes in control medium, or to class C neurons (n = 9), which were recognized by the occurrence of small-amplitude sodic spikes followed by slower larger calcic spikes. The present results demonstrate that, despite the variety of individual cell types, the major morphological population (30/36 cells) was composed of a homogeneous class of large-sized neurons that displayed thick primary dendrites and abundant dendritic appendages. The remaining 6 cells were small-sized, poorly-spiny neurons. Somatic spines were observed on 5 out of the 30 large cells and on one out of the six smaller cells. Labeled axons were mainly oriented to the anterior commissure. The axons of nine cells richly collateralized near the perikaryon. Ultrastructural examination of 3 horseradish peroxidase-injected cells showed indented nuclei, classic organelles and somatic spines. Terminal boutons established symmetric synapses with the injected cells. These results describe the morphological features of electrophysiologically identified neurons and indicate that class A and class C neurons are distributed among morphological populations differing in perikaryal size. This suggests that the different electrical properties of class A and class C neurons reflect recordings from different parts of the neuron rather than from neurons of different types. Furthermore, the present findings demonstrate that, in the guinea-pig, electrical and morphological characteristics of somatospiny neurons are comparable with those of non-somatospiny neurons. Somatospiny neurons have a recognized integrative role in the hippocampo-septo-hypothalamic complex.

Comparative distribution of calbindin and Met-enkephalin immunoreactivities in the guinea-pig lateral septum, with reference to electrophysiologically characterized neurons in the mediolateral part.

The distribution both of Met-enkephalinergic nerve terminals and of calbindin-containing neurons was investigated in the guinea-pig lateral septum, using a double-immunostaining technique. The findings show that the two immunoreactivities overlapped for neurons located in areas of the dorsal part and of the mediolateral part of the lateral septum. Nine cells of the mediolateral part which were electrophysiologically characterized and intracellularly labelled were subjected to the double-immunostaining protocol. All these cells displayed characteristic discharges due to the activation of high-threshold Ca2+ conductances. Two of them contained calbindin and were the target of enkephalinergic inputs; they possessed somatic spines. These data demonstrate: (1) that the guinea-pig lateral septum contains subpopulations of calbindin neurons which are postsynaptic to enkephalinergic inputs; (2) that Ca2+ conductances are not related to the presence of calbindin; (3) that somatospiny neurons, which are involved in the regulation of the hippocampo-septo-hypothalamic complex, contain calbindin and are the target of enkephalinergic endings.

Morphological analysis of the neurons in the area of the hypothalamic magnocellular dorsal nucleus of the guinea pig.

In the guinea-pig hypothalamus, a group of enkephalinergic cells forms a well-circumscribed nuclear area called the magnocellular dorsal nucleus (MDN). This nucleus gives rise to a prominent projection to the lateral septum: the hypothalamo-septal enkephalinergic pathway. In the present study, MDN neurons visualized by Golgi impregnation were subjected to morphological analysis in order to define the potential segregation of cellular types within the MDN. This study was complemented by additional observations of MDN neurons intracellularly injected by Lucifer yellow (LY) or horseradish peroxidase (HRP) during the in vitro incubation of hypothalamic slices. The following results were obtained from the analysis of 200 neurons: 163 Golgi-impregnated cells plus 37 injected cells (LY = 14; HRP = 23). Thirteen HRP-injected cells were precisely located in the MDN and 10 were located in the perifornical area surrounding the MDN. Four different cellular types were identified. Type-I neurons (41%) displayed a globular perikaryon, a variable number of primary dendrites that were poorly ramified, no preferential orientation, and an axon emerging from the perikaryon. Type-II neurons (30.5%) had a triangular perikaryon, three well-ramified primary dendrites, an orientation perpendicular to the third ventricle, and an axon emerging from the perikaryon. Type-III neurons (22%) exhibited a spindle-shaped perikaryon, two opposed well-ramified primary dendrites, an orientation perpendicular to the third ventricle, and an axon emerging from a primary dendrite. Type-IV neurons (6.5%), showed a globular perikaryon, a variable number of primary dendrites, poorly ramified dendrites, an orientation parallel to the third ventricle, and an axon whose orientation could not be identified. Neurons labeled after intracellular injection belonged to the first three cellular types.

Electrical properties of neurons in the mediolateral part of the lateral septum: intracellular recordings from guinea-pig brain slices.

Membrane properties of 174 neurons were studied in the mediolateral part of the lateral septum (LSml) using an in vitro slice preparation of guinea-pig brain. Intracellular recordings were correlated with morphological data obtained from 34 neurons intracellularly stained with horseradish peroxidase (HRP). Neurons were divided into three classes according to their electrical responses. Class A and B neurons displayed the common property of an overshooting of spikes in response to the direct application of weak depolarizing current pulses. Class A neurons (59.2% of the total) generated tetrodotoxin-insensitive, high-threshold Ca2+ spikes in a control medium. Class B neurons (20.7% of the total) generated high-threshold Ca2+ spikes only if tetraethylammonium was used to block delayed rectifying K+ current. Features common to class A and B neurons included the inactivation of Na+ conductance, the participation of high-threshold Ca2+ conductance in the generation of spikes--when repetitive discharges were elicited by strong depolarizing current pulses--and Cs(+)-sensitive, Ba(2+)-insensitive anomalous rectification. Class C neurons (20.1% of the total) displayed discharges comprising small-amplitude Na+ spikes followed by slow and large Ca2+ spikes, suggesting a locus of impalement which was not the soma. HRP-filled class B neurons (n = 5) were characterized by small to medium perikarya with spindly dendrites. The majority of HRP-filled class A (15/21) and all class C (n = 8) neurons showed large perikarya with thick primary dendrites and spiny dendritic branches. Thus, class A and C neurons typify the guinea-pig LSml in their morphological characteristics and in their ability to generate high-threshold Ca2+ spikes in a control medium.

[GABA acts through GABA A receptors on neurons of the hypothalamic magnocellular dorsal nucleus in the guinea-pig: in vitro intracellular study]. / Le GABA agit par des récepteurs GABAA sur les neurones du noyau magnocellulaire dorsal hypothalamique du cobaye: étude intracellulaire in vitro.

In guinea-pig hypothalamic slices, GABA, applied through perfusion medium or by iontophoresis, evoked a hyperpolarization or a depolarization in enkephalinergic neurons of the magnocellular dorsal nucleus (MDN). This effect was accompanied by an inhibition of action potentials. It persisted in the presence of tetrodotoxin and involved GABAA receptors, coupled to a chloride conductance. Our results are discussed in relation to immunocytochemical data on the enkephalinergic neurons of MDN, gathered after an increase of intracerebral GABA concentration.