Evidence for the connections between a clutch cell and a corticotectal neuron in area 17 of the cat visual cortex

Evidence is presented for the synaptic connectivity between a physiologically characterized and intracellularly filled GABAergic interneuron and a corticotectal pyramidal neuron in area 17 of the cat visual cortex. The interneuron was located in layer 4 and had the morphological characteristics of a clutch cell. The physiological data demonstrated that the clutch cell received direct X-type innervation from the dorsal lateral geniculate nucleus. These results indicate that a GABAergic neuron is directly involved during the first cortical stages of geniculocorticotectal interactions. Furthermore, the proximal location of the clutch-cell inputs to the labelled dendrite suggests a strategic siting of intracortical feedforward inhibition.     

A quantitative analysis of the local connectivity between pyramidal neurons in layers 2/3 of the rat visual cortex

This study provides a detailed quantitative estimate for local synaptic connectivity between neocortical pyramidal neurons. A new way of obtaining such an estimate is presented. In acute slices of the rat visual cortex, four layer 2 and four layer 3 pyramidal neurons were intracellularly injected with biocytin. Axonal and dendritic arborizations were three-dimensionally reconstructed with the aid of a computer-based camera lucida system. In a computer experiment, pairs of pre- and postsynaptic neurons were formed and potential synaptic contacts were calculated. For each pair, the calculations were carried out for a whole range of distances (0 to 500 μm) between the presynaptic and the postsynaptic neuron, in order to estimate cortical connectivity as a function of the spatial separation of neurons. It was also differentiated whether neurons were situated in the same or in different cortical layers. The data thus obtained was used to compute connection probabilities, the average number of contacts between neurons, the frequency of specific numbers of contacts and the total number of contacts a dendritic tree receives from the surrounding cortical volume. Connection probabilities ranged from 50% to 80% for directly adjacent neurons and from 0% to 15% for neurons 500 μm apart. In many cases, connections were mediated by one contact only. However, close neighbors made on average up to 3 contacts with each other. The question as to whether the method employed in this study yields a realistic estimate of synaptic connectivity is discussed. It is argued that the results can be used as a detailed blueprint for building artificial neural networks with a cortex-like architecture. Received: 30 March 1999 / Accepted in revised form: 5 August 1999     

Active endocytosis and microtubule remodeling restore compressed pyramidal neuron morphology in rat cerebral cortex

Previous studies have shown that compression alone reduced the thickness of rat cerebral cortex and apical dendritic lengths of pyramidal neurons without apparent cell death. Besides, decompression restored dendritic lengths at different degrees depending on duration of compression. To understand the mechanisms regulating dendritic shortening and lengthening upon compression and decompression, we applied transmission electron microscopy to examine microtubule and membrane structure of pyramidal neurons in rat sensorimotor cortex subjected to compression and decompression. Microtubule densities within apical dendritic trunks decreased significantly and arranged irregularly following compression for a period from 30?min to 24?h. In addition, apical dendritic trunks showed twisted contour. Two reasons are accounted for the decrease of microtubule density within this period. First, microtubule depolymerized and resulted in lower number of microtubules. Second, the twisted membrane widened the diameters of apical dendritic trunks, which also caused a decrease in microtubule density. Interestingly, these compression-induced changes were quickly reversed to control level following decompression, suggesting that these changes were accomplished passively. Furthermore, microtubule densities were restored to control level and the number of endocytotic vesicles significantly increased along the apical dendritic membrane in neurons subjected to 36?h or longer period of compression. However, decompression did not make significant changes on dendrites compressed for 36?h, for they had already shown straight appearance before decompression. These results suggest that active membrane endocytosis and microtubule remodeling occur in this adaptive stage to make the apical dendritic trunks regain their smooth contour and regular microtubule arrangement, similar to that of the normal control neurons.     

Synaptic and non-synaptic plasticity between individual pyramidal cells in the rat hippocampus in vitro

We report here two forms of activity-dependent plasticity of the transfer of neuronal information between pairs of monosynaptically coupled pyramidal cells. In a first part, we discuss the induction of long-term bidirectional changes in excitatory synaptic transmission following defined regimes of neuronal activity. In a second part, we provide evidence that the conditions in which the presynaptic action potential is elicited determine whether it will successfully propagate along the presynaptic axon.     

Columnar organization of pyramidal cells of the visual cortex in the albino rat]

1. The arrangement of the pyramidal cells of the visual cortex has been investigated by light and electron microscopy in 36 albino rats. 2. It was shown, that the apical dendrites form bundles. The number of dendrites per bundle is about 5 in the lower lamina Vb and 7.5 in the upper lamina Va, the diameter of bundles is about 25.7 mum. The average distance between the centers of bundles is 76,4 mum, and between the peripheries 53.1 mum. 3. The results are compared with physiological and morphological findings. It was shown, that there is an agreement of diameter of bundles und their lateral branching dendrites with diameters of columns of cells and of terminal branchings of specific afferents. 4. The morphological results are graphically reconstructed.     

Synaptic relationships between proopiomelanocortin- and ghrelin-containing neurons in the rat arcuate nucleus

Both proopiomelanocortin (POMC) and ghrelin peptides are implicated in the feeding regulation. The synaptic relationships between POMC- and ghrelin-containing neurons in the hypothalamic arcuate nucleus were studied using double-immunostaining methods at the light and electron microscope levels. Many POMC-like immunoreactive axon terminals were found to be apposed to ghrelin-like immunoreactive neurons and also to make synapses with ghrelin-like immunoreactive neuronal perikarya and dendritic processes. Most of the synapses were symmetrical in shape. A small number of synapses made by ghrelin-like immunoreactive axon terminals on POMC-like immunoreactive neurons were also identified. Both the POMC- and ghrelin-like immunoreactive neurons were found to contain large dense granular vesicles. These data suggest that the POMC-producing neurons are modulated via synaptic communication with ghrelin-containing neurons. Moreover, ghrelin-containing neurons may also have a feedback effect on POMC-containing neurons through direct synaptic contacts.     

Synaptic relationships between GABA-immunoreactive neurons and an identified uniglomerular projection neuron in the antennal lobe of Periplaneta americana: a double-labeling electron microscopic study

Two types of central neurons in the antennal lobe of the American cockroach Periplaneta americana were labeled with a combination of two specific markers. Their synaptic contacts were characterized and their distribution on the neurons examined. A uniglomerular pheromone-sensitive projection neuron with dendritic arbor in the male-specific macroglomerulus (attractant neuron) was characterized physiologically by intracellular recording and then filled with biocytin, which was converted to a marker for this individual neuron by a preembedding procedure. In a postembedding procedure local, multiglomerular interneurons were marked by immunogold labeling of GABA. Two kinds of synaptic contacts were found on the attractant neuron. (i) Input synapses from GABA-immunoreactive profiles. There were many of these, which (together with results of previous studies) suggests that local interneurons mediate polysynaptic transmission from antennal receptor fibers to the projection neuron. (ii) Output synapses onto GABA-immunoreactive profiles and onto non-identified neurons. These contacts indicate that signals generated by the projection neurons in a given glomerulus are passed back to multiglomerular interneurons and hence are also transmitted to other glomeruli.     

Synaptic relationships between GABA-immunoreactive neurons and an identified uniglomerular projection neuron in the antennal lobe of Periplaneta americana: a double-labeling electron microscopic study

Summary Two types of central neurons in the antennal lobe of the American cockroach Periplaneta americana were labeled with a combination of two specific markers. Their synaptic contacts were characterized and their distribution on the neurons examined. A uniglomerular pheromone-sensitive projection neuron with dendritic arbor in the male-specific macroglomerulus (attractant neuron) was characterized physiologically by intracellular recording and then filled with biocytin, which was converted to a marker for this individual neuron by a preembedding procedure. In a postembedding procedure local, multiglomerular interneurons were marked by immunogold labeling of GABA. Two kinds of synaptic contacts were found on the attractant neuron. (i) Input synapses from GABA-immunoreactive profiles. There were many of these, which (together with results of previous studies) suggests that local interneurons mediate polysynaptic transmission from antennal receptor fibers to the projection neuron. (ii) Output synapses onto GABA-immunoreactive profiles and onto non-identified neurons. These contacts indicate that signals generated by the projection neurons in a given glomerulus are passed back to multiglomerular interneurons and hence are also transmitted to other glomeruli.     

A theory for the acquisition and loss of neuron specificity in visual cortex

We assume that between lateral geniculate and visual cortical cells there exist labile synapses that modify themselves in a new fashion called threshold passive modification and in addition, non-labile synapses that contain permanent information. In the theory which results there is an increase in the specificity of response of a cortical cell when it is exposed to stimuli due to normal patterned visual experience. Non-patterned input, such as might be expected when an animal is dark-reared or raised with eyelids sutured, results in a loss of specificity, with details depending on whether noise to labile and non-labile junctions is correlated. Specificity can sometimes be regained, however, with a return of input due to patterned vision. We propose that this provides a possible explanation of experimental results obtained by Imbert and Buisseret (1975); Blakemore and Van Sluyters (1975); Buisseret and Imbert (1976); and Frégnac and Imbert (1977, 1978).This work was supported in part by a grant from the Ittleson Foundation, Inc.     

Modulation of pyramidal response in the rat sensorimotor cortex after combined stimulation of the lateral hypothalamus and the sensorimotor cortex

It was shown during experiments on unrestrained rats that rhythmic stimulation of the pyramidal tract produced a statistically significant increase in the functional activity of neuronal populations of the sensorimotor cortex, manifesting as potentiation of the primary, positive phase of pyramidal cortical response. Combined rhythmically matched stimulation of the pyramidal tract and of the lateral hypothalamus leads to statistically significant enhancement in potentiation of the positive phase of pyramidal cortical response compared with effects produced independently of hypothalamic involvement. When stimulation of the pyramidal tract and the lateral hypothalamus are combined with stimulation applied at the same periodicity to the sensorimotor cortex, a further statistically significant enhancement in potentiation of the positive phase of pyramidal cortical response is seen in addition to the potentiating effect produced by hypothalamic stimulation.Institute for Brain Research of the All-Union Scientific Center of Mental Health, Academy of Medical Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 18, No. 3, pp. 367–373, May–June, 1986.     

Ovarian hormones after postnatal day 20 reduce neuron number in the rat primary visual cortex

Previous work from our lab has documented a sex difference in neuron number in the binocular region of the adult rat primary visual cortex (Oc1B), with males having 19% more neurons than females. In the present study, the role of developmental steroid hormones in the formation of this difference was explored. Male and female rats underwent neonatal hormone manipulation (female + testosterone or dihydrotestosterone; male + flutamide) followed by gonadectomy on postnatal day 20. Animals that did not undergo hormone manipulation were either gonadectomized or sham operated at day 20. Neuron number was quantified in the monocular (Oc1M) and binocular (Oc1B) subfields of the adult rat primary visual cortex using the optical disector technique. As adults, day 20 gonadectomized females, as well as females + testosterone and females + dihydrotestosterone, had significantly more neurons than intact females. There was no difference in neuron number between postnatal day 20 gonadectomized males, males + flutamide, and intact males. Also, intact males had significantly more neurons than intact females in both in Oc1M and Oc1B. It appears that ovarian steroids after day 20 are the primary cause of the lower number of neurons in the primary visual cortex of the female rat.