A Golgi study on the neuronal organization of the interhemispheric cortex in the mouse. I. Projection neurons

Long-term potentiation and depression of synaptic transmission in rat posterior cingulate cortex

We used stimulation of corpus callosum (CAL) and the subiculo-cingulate tract (SCT), in an in vitro brain slice preparation, to study activity-dependent changes in synaptic efficacy in posterior cingulate cortex (PCC). SCT stimulation monosynaptically excites the apical dendrites of deep laminae (V-VI) pyramidal neurons, while CAL afferents drive these same cells via synapses on their basal dendrites. In contrast, most superficial laminae (II/III-IV) pyramids appear to be driven polysynaptically via ascending axonal collaterals of deep pyramids. In slices retaining these connectivities, we contrasted characteristics of synaptic plasticity in superficial vs deep laminae field and intracellular potentials evoked by conditioning stimuli given at frequencies of 100, 20, 8, 5 and 1 Hz. Tetanic stimulation (100 Hz) of SCT or CAL yielded homosynaptic long-term potentiation (LTP) of each pathway, while stimulus trains of 8-20 Hz did not. 1-5 Hz stimulation of SCT and CAL elicited homosynaptic long-term depression (LTD) of synaptic strength in each pathway. Associative LTD was induced by interleaving 5 Hz pulses to the SCT pathway with 100 Hz theta-burst stimulation of CAL, but was not induced when these stimulus loci were switched. Heterosynaptic non-associative LTD was also observed in the alternate pathway following tetanization of either SCT or CAL. In all cases, LTP and LTD were observed only in deep laminae recordings. In contrast, superficial records showed only paired-pulse facilitation and short-term post-tetanic potentiation. In in vivo experiments in anaesthetized rats, PCC responses to SCT stimulation were contrasted with responses to stimulation of anteroventral and anterodorsal thalamic nuclei (AV/AD). SCT-elicited field potentials closely resembled those evoked in the slice, with maximal amplitude tuned to the 4-8 Hz frequency band. AV/AD stimulation elicited field potentials which were not frequency tuned. Overall, these data suggest that the acute circuit properties of PCC superficial laminae, modulated by thalamic input and synaptic plasticity in deep laminae, can transform hippocampal synaptic inflow before relaying it to extracingulate targets.

Microcircuitry of posterior cingulate cortex in vitro: electrophysiology and laminar analysis using the current source density method

We used current source density (CSD) analysis of a laminar profile of subicular stimulus-evoked field potentials recorded in cortical slices in vitro to characterize the interlaminar microcircuitry of posterior cingulate cortex. Neuroanatomic and electrophysiologic data indicate that subiculocingulate tract (SCT) afferents monosynaptically excite apical dendrites of deep laminae (V-VI) neurons, evoking pure EPSPs, while superficial laminae (II/III-IV) neurons are driven polysynaptically, evoking a mixture of longer latency EPSPs and IPSPs. Consistent with this model, CSD analysis of field potential laminar profiles supports the conclusion that activation of excitatory subicular afferent terminal fields in superficial laminae of cingulate cortex elicits primary monosynaptic activation of apical dendrites of deep lamina (V-VI) pyramids. Subsequent EPSP propagation to the somata of these pyramids generated synchronous action potential discharges which appeared to elicit delayed polysynaptic activation of superficial laminae pyramids and interneurons. Latency differences between SCT-stimulus-evoked EPSPs and action potentials in superficial and deep laminae were minimized by stimulus train frequencies of 5-8 Hz, indicating that the proposed microcircuitry can show functional tuning at frequencies characteristic of hippocampal neuronal activity (theta). Such tuning suggests that hippocampal output activity frequency and phase locked to theta rhythm will be preferentially gated through cingulate cortex.

Dendritic bundling in layer I of granular retrosplenial cortex: intracellular labeling and selectivity of innervation

The extrinsic projections to and from the retrosplenial cortex have been studied in detail, but the intrinsic circuitry within this region has been characterized less completely. To further define the internal connections, small injections of the retrograde, fluorescent tracer Fluorogold were made into the retrosplenial cortex of the rat. These injections label neurons in layers II-V of the contralateral homotopic cortex. In layers III-V, the labeled neurons are present over an area much larger than the injection site, but in layer II neurons are labeled in a very precise homotopic pattern. Following these injections, only the neurons in layer II display heavily labeled apical dendrites, and these labeled dendrites form tight bundles in layer Ic and Ib of the cortex and spread out in layer Ia. An examination of Golgi-stained material demonstrates that most of the neurons in layer II are small pyramidal cells with 2-3 small basal dendrites and a single, large apical dendrite that arborizes extensively in layer Ia. To verify the structure of the layer II neurons, they were intracellularly filled with Lucifer yellow. Examination of these labeled cells confirms the observations from the Golgi-stained material and demonstrates that many apical dendrites of the layer II cells angle acutely, apparently to join a bundle and/or avoid an interbundle space. Tract tracing experiments demonstrate that the anteroventral nucleus of the thalamus appears to project selectively to the region containing the dendritic bundles, whereas intracortical projections appear to terminate in layers Ib and Ic in the 30-200 microns spaces between the bundles. Furthermore, the areas containing the bundles display dense AChE staining, but the interbundle spaces are almost free of AChE staining. These findings demonstrate a form of dendritic bundling that is input and output specific and may play an important role in the regulation of thalamic inputs to the cingulate cortex.

Structure of layer II in cat primary auditory cortex (AI)

The cytoarchitecture, myeloarchitecture, neuronal architecture, and intrinsic and laminar organization of layer II were studied in the primary auditory cortex (AI) of adult cats. The chief goal was to describe the different types of cells and axons to provide a framework for experimental studies of corticocortical connections or of neurons accumulating putative neurotransmitters. A further goal was to differentiate layer II from layer III. Layer II extends from 150-200 micron to about 400 micron beneath the pia and has two subparts. The superficial stratum, layer IIa, has many small, chiefly non-pyramidal neurons, primarily with round or oval perikarya, and a sparse, fine, and irregularly arranged axonal plexus. Layer IIb somata are larger and more densely packed and there is a more developed vertical and lateral axonal plexus. The border with layer III was marked by numerous large pyramidal cells with a thicker apical dendrite with more developed basal dendritic arbors than those of layer II pyramidal cells. Eight varieties of neurons were recognized in Golgi-impregnated material. These included small and medium-sized pyramidal cells, whose apical dendrites often ramified in layer I; bipolar and bitufted cells with polarized, sparse dendritic arbors; small smooth or sparsely spinous multipolar cells with radiating dendrites and small dendritic fields; spinous multipolar cells, whose large dendritic fields had more extensive apical than basal arbors; large sparsely spinous multipolar cells with smooth, robust apical dendrites; tufted multipolar cells with highly developed apical dendrites and some dendritic appendages; and extraverted multipolar cells with a broad, candelabra-shaped dendritic configuration, and with most dendrites oriented at right angles to the pia. The axons of the different cell types had the following general dispositions: those arising from the pyramidal cells could often be traced into the white matter but had many local branches as well; those of the other neurons had more or less extensive local axonal collateral systems and fewer branches which appeared to be corticofugal. However, the complete trajectory of the axons was not always impregnated in the adult material.(ABSTRACT TRUNCATED AT 400 WORDS)

A Golgi study on the neuronal organization of the interhemispheric cortex in the mouse

The intrinsic neurons in the interhemispheric cortex (IHC) were studied by the rapid Golgi method in the young mouse. In each of the five layers of the IHC, a wide variety of intrinsic neurons were observed. They were classified into several groups according mainly to the patterns of axonal and dendritic distribution. The Cajal-Retzius cells were most frequently seen in layer I. The dendrites and axons of these neurons ran irregularly in the plane parallel to the pial surface of the IHC. Many neurons in layers II-V were observed to send their axons to layer I. Some of these neurons took the form of the inverted pyramidal neurons. The axons of some neurons in layers II, III and IV formed dense axonal plexuses usually in layer III and rarely in layer II. The dendrites of many neurons in layers IV and V extended into the cingulum. The stellate neurons embedded in the cingulum might be the dislocated neurons of layer V.