Putative transmitters in denervated olfactory complex

Laminar distribution of putative neurotransmitter amino acids and ligand binding sites in the dog olfactory bulb

Coronal sections of frozen dog olfactory bulb have been dissected into four anatomically distinct layers. The laminar distribution of ten amino acids, the dipeptide carnosine, and nine [3H]ligand binding sites in these layers was determined. GABA and tyrosine levels were highest in the mitral cell-granule cell layer, and glutamate levels were slightly elevated in the glomerular layer. The distributions of all other amino acids did not show significant differences across the layers. Carnosine was predominantly localized in the fiber and glomerular layers. With the exception of quinuclidinyl benzilate, the [3H]ligand binding sites showed more discrete distributions. Muscimol, diazepam, kainic acid, and spiroperidol binding were predominantly localized in the mitral cell-granule cell layer, where clonidine binding was at a minimum. Dihydromorphine binding was high in both the fiber and the mitral cell-granule cell layers. Carnosine binding was maximal in the glomerular layer. The implications of these observations with regard to biochemical and neurophysiological data are discussed.

Net uptake of aspartate by a high-affinity rat cortical synaptosomal transport system

Reuptake of neuroactive amino acids by high affinity transport systems (Km approximately 10 micrometer) is thought to terminate the action of these substances. Since homoexchange can complicate the interpretation of uptake experiments, it is necessary to demonstrate net inward transport of neuroactive amino acids before uptake can be considered as a likely mechanism for transmitter inactivation. When rat cortical synaptosomes are incubated with 10 micrometer [14C]L-aspartate, net (chemical) and apparent (radioactive) uptake into the synaptosomal fractions are equivalent. Although there is net aspartate uptake into the synaptosome fraction, aspartate exchange can be demonstrated in a variety of conditions. Net uptake exhibits the characteristics of high-affinity transport systems including Na+-and temperature-dependence. Furthermore, KCl (or RbCl)--1 micrometer--are required for net uptake but not radioactive or apparent uptake. LiCl, NH4Cl, CsCl, and choline chloride fail to support net uptake. Ouabain (0.1 micrometer) inhibits net uptake to a greater extent than apparent uptake. Although glutamate inhibits aspartate uptake (and vice versa), the net uptake of the combination is greater than that of each alone. The demonstration of net uptake of aspartate by a high-affinity system is consonant with the idea that this system may play a role in its inactivation in the synaptic region.

Depression of evoked potentials in brain slices by adenosine compounds

1 A study has been made of the action of adenosine on surface slices of guinea-pig olfactory cortex in vitro. 2 With extracellular recordings from the pial surface and stimulation of the presynaptic input, the lateral olfactory tract (LOT) generated a monosynaptic negative wave representing dendritic excitatory potentials. This negative wave was depressed by bath application of 1 micron adenosine with increasing effect up to 1 mM. Adenosine 5'--triphosphate (ATP), adenosine 5'-monophosphate (AMP) and cyclic adenosine 3',5'-monophosphate (cyclic AMP) had similar depressant actions. Adenine and guanosine were very weak depressants. 3 Theophylline concentrations in the range 10 micron to 3 mM progressively antagonized the action of adenosine. 4 Dibuyryl cyclic AMP (100 micron) and agents which increase intracellular cyclic AMP were not depressants, suggesting that the action of adenosine was not cyclic AMP-mediated. 5 Intracellular recordings confirmed the depressant effect of adenosine on excitatory potentials generated by LOT stimulation and also showed that postsynatpic action potentials and the membrane of the soma were unaffected by adenosine. 6 Since presynaptic action potentials were also unaffected by adenosine, these experiments suggest that adenosine reduces excitatory transmission at LOT synapses and fortifies the idea that adenosine has a 'neurohumoral' action.

Aspartic acid and glutamic acid levels in the cochlear nucleus after auditory nerve lesion

Aspartic acid, glutamic acid and alanine were measured in the cochlear nucleus after lesioning the auditory nerve by cochlear ablation. Ultrastructural analysis of the cochlear nucleus showed that most primary auditory terminals were degenerating one day after cochlear ablation; the terminals were enlarged and the number of synaptic vesicles was reduced. Primary auditory terminals were virtually gone three days after cochlear ablation. Aspartic acid decreased after cochlear ablation in parallel with the morphological degeneration of the primary auditory terminals. The level of total aspartic acid in the cochlear nucleus had decreased more than 8% one day after cochlear ablation and more than 30% after two days, and remained at this level up to 28 days. Glutamic acid also decreased in the cochlear nucleus after cochlear ablation but not in parallel with the morphological degeneration of the primary auditory terminals. Following a slight increase one day after cochlear ablation, total glutamic acid decreased about 10% after two days and continued to decrease slowly through to day 28. Alanine dropped slowly after cochlear ablation and not in parallel with the degeneration of the primary terminals. Levels of other amino acids measured were unchanged or had increased two days after cochlear ablation. Aspartic acid and glutamic acid did not decrease in the superficial layers of the dorsal cochlear nucleus, an area receiving little or no primary innervation.

Cellular events induced in the molecular layer of the piriform cortex by ablation of the olfactory bulb in the mouse

Cellular events associated with degeneration of the projection of the olfactory bulb to the molecular layer of the piriform cortex of the mouse have been studied with rapid-Golgi and Fink-Heimer impregnations and with the electron microscope. Four classes of axon terminals: s-1, s-d, f-1, and f-d, are differentiated on the basis of whether the synaptic vesicles are spherical or flattened and whether the axoplasm is lightly or darkly stained. The majority of s-1 terminals, the predominant class in sublamina Ia of the molecular layer, degenerate after bulb ablation. Degeneration of axon terminals is associated with dilation and, eventually, degeneration of segments of dendrites in Ia. Both s-1 and s-d terminals contribute to a partial reconstitution of the neuropil of Ia during the weeks after bulb ablation.