Brief Bursts Self-Inhibit and Correlate the Pyramidal Network

Inhibitory pathways are an essential component in the function of the neocortical microcircuitry. Despite the relatively small fraction of inhibitory neurons in the neocortex, these neurons are strongly activated due to their high connectivity rate and the intricate manner in which they interconnect with pyramidal cells (PCs). One prominent pathway is the frequency-dependent disynaptic inhibition (FDDI) formed between layer 5 PCs and mediated by Martinotti cells (MCs). Here, we show that simultaneous short bursts in four PCs are sufficient to exert FDDI in all neighboring PCs within the dimensions of a cortical column. This powerful inhibition is mediated by few interneurons, leading to strongly correlated membrane fluctuations and synchronous spiking between PCs simultaneously receiving FDDI. Somatic integration of such inhibition is independent and electrically isolated from monosynaptic excitation formed between the same PCs. FDDI is strongly shaped by I(h) in PC dendrites, which determines the effective integration time window for inhibitory and excitatory inputs. We propose a key disynaptic mechanism by which brief bursts generated by a few PCs can synchronize the activity in the pyramidal network.     

ATP inhibits smooth muscle Ca2(+)-activated K+ channels

There has been much recent interest in the roles played by smooth-muscle K+ channels in protecting cells against ischemic and anoxic insults and in therapeutic vaso- and bronchodilation (Buckingham 1990; Longmore & Weston 1990). A K+ channel, which is uniquely sensitive to cytoplasmic ATP (KATP), has been identified as a likely candidate for mediating these important functions (Standen et al. 1989). We now show, by using electrophysiological techniques in three different types of smooth muscle, that a large-conductance voltage and Ca2(+)-sensitive channel, otherwise indistinguishable from the the large-conductance Ca2(+)-activated K+ channel (BK channel), is also sensitive to cytoplasmic ATP and cromakalim. ATP, in a dose-dependent manner, decreased the probability of channel opening (Po) of rabbit aortic, rabbit tracheal and pig coronary artery BK channels with a Ki of 0.2-0.6 mM. Cromakalim, 10 microM, partially reversed the ATP induced inhibition and increased Po. Our observations raise the possibility that the ubiquitous BK channel may play a role during pathophysiological events.     

Calcium-Dependent Transglutaminase of Rat Sympathetic Ganglion in Development and After Nerve Injury

The activity of transglutaminase (TG) was examined in the rat superior cervical ganglion (SCG) during development and after postganglionic nerve crush. During postnatal development the enzyme activity is increased by sevenfold in parallel to protein content of the ganglion and reaches adult levels by day 35 after birth. The endogenous activity (enzyme activity assayed in the absence of the exogenous substrate) during development is transiently elevated with a peak at day 21 postnatal. In the adult ganglion the enzyme specific activity is evenly distributed in all subcellular compartments, but most of it is contained in the cytosol. Within the first hour after axotomy TG activity is rapidly and transiently elevated. The peak value, 80% above control levels, is attained by 30 min postoperative. At this time the activity is increased in all subcellular fractions, but the endogenous activity is selectively increased in the fraction containing nuclei. The enhanced TG activity after axotomy can be prevented by topical treatments with verapamil, an inhibitor of voltage-dependent calcium fluxes across excitable membranes, or with the calcium chelator EGTA. The results show that intracellular TG activity is present in the SCG and that it increases with postnatal growth of the ganglion. After axotomy the enzyme activity is rapidly and transiently increased in the ganglion and this elevation critically depends on calcium fluxes.     

Pyrimidine synthesis in tissue culture

Abstract— Myelinated cerebellar tissue culture (organ culture) was used to assess the salvage and de novo pathways of pyrimidine synthesis in mammalian brain. Radioactive orotic acid and carbamyl aspartic acid were readily incorporated into UMP and into perchloric acid-insoluble RNA. The incorporation was effectively blocked by azauridine. Neither radioactive sodium bicarbonate or citrulline was incorporated into UMP or blocked by azauridine. [³H]Uridine, on the other hand, rapidly entered the cultures, was incorporated into UMP and perchloric acid-insoluble material, and was partially inhibited by azauridine. The failure to demonstrate activity of carbamyl phosphate synthetase suggests the potential importance of the salvage pathway and the likely dependence of the brain upon exogenous and endogenous pyrimidine precursors.     

Strain differences in rat adrenal biosynthetic enzymes and stress-induced increases in plasma catecholamines.

We have examined in two inbred rat strains basal and stress-induced increases in plasma levels of epinephrine (EPI) and norepinephrine (NE) and compared these with activities of the adrenal enzymes involved in the synthesis of catecholamines. There were no differences in basal levels of NE and EPI in plasma of adult male rats of the Wistar-Kyoto (WKY) and Brown-Norway (B-N) strains. However, following 5 min. of intermittent footshock, plasma levels of both catecholamines were twice as high in WKY rats as in B-N rats. In the adrenals of unstressed rats, activities of tyrosine hydroxylase and dopamine-beta-hydroxylase were significantly higher in B-N rats. In addition, the adrenal weights and the contents of NE but not EPI were greater in B-N rats. Thus, in these two rat strains, the capacity of the adrenal gland to synthesize and store catecholamines appeared to be inversely related to plasma levels of NE and EPI after stress. The differences between the strains appeared to be due to differences in the rates of removal of catecholamines from the peripheral circulation as well as to differences in the rate of release of catecholamines from the sympatho-adrenal medullary system. Thus biosynthetic enzyme activities need not be related directly to the capacity to release and elevate plasma levels of catecholamines following stressful stimulation.     

Dermal and intravascular Langerhans cells at sites of passively induced allergic contact sensitivity.

In order to further define the role of Langerhans cells in contact allergic reactions, passive transfer studies were done in guinea pigs using 2,4-dinitro-1-chlorobenzene (DNCB)-sensitive donor cells. Langerhans cells were found in the lumen of dermal vessels resembling lymphatics at 2, 3, 15, and 48 hr after DNCB challenge. In contrast to the previously reported findings in actively sensitized guinea pigs, the changes involving Langerhans cells in passively sensitized guinea pigs were mainly noted in the dermis. These consisted of increased numbers of Langerhans cells and of mononuclear cells apposed to Langerhans cells 3 or more hours after challenge with DNCB. The increased Langerhans cell population in the dermis and the presence of Langerhans cells in dermal vessels in specifically challenged sites in adoptive immune reactions furnishes further support for a significant role of Langerhans cells in the interaction between antigen and sensitized cells.     

CERAMIDE GLYCOSYLTRANSFERASES IN CULTURED RAT CEREBELLUM: CHANGES WITH AGE, WITH DEMYELINATION, AND WITH INHIBITION OF MYELINATION BY 5-BROMO-2''-DEOXYURIDINE OR EXPERIMENTAL ALLERGIC ENCEPHALOMYELITIS SERUM

—UDP-galactose:ceramide galactosyltransferase (CGalT) (E.C. 2.4.1.62) and UDP-glucose:ceramide glucosyltransferase (CGlcT) activities were measured in myelinating cultures of newborn rat cerebellum. Specific activities were measured at various days in vitro and the pattern of activities compared with that reported for in vivo tissue. Cultures demyelinated by incubation with media containing 22% serum from rabbits in which experimental allergic encephalomyelitis (EAE) was induced by injection with whole guinea-pig spinal cord, had 28% of CGalT specific activity and 86% of CGlcT specific activity measured in control cultures. Cultures in which myelination was inhibited by maintenance on media containing 0.15 mm -5-bromo-2′-deoxyuridine (BUdR) had 10% of CGalT specific activity and 118% of CGlcT specific activity of control cultures. Cultures in which myelination was inhibited by maintenance on media containing 2% EAE serum had 12% of the CGalT specific activity of control cultures. The data suggest that in vitro CGalT is predominantly a glial enzyme while CGlcT occurs primarily in neurons, and that the reduced CGalT activity may be involved in the mechanism of myelination inhibition by BUdR and by EAE serum.