Accumulation of 3 H-glycine in interneurons of the cat spinal cord

Looking at neurotransmitters in the microscope

This review article covers the early period of my career. I first summarize research initiated by the late Nils-Ake Hillarp, after his appointment in 1962 as professor in the Department of Histology at Karolinska Institutet. He only lived for three more years, but during this short period he started up a group of ten students who explored various aspects of the three monoamine transmitters, dopamine, noradrenaline and 5-hydroxytryptamine, using the new formaldehyde fluorescence method developed by Bengt Falck and Hillarp in Lund. This method allowed visualization of the cellular localization in the microscope of these monoamines, which introduced a new discipline in neurobiology-chemical neuroanatomy. I then deal with work aiming at localizing the monoamines at the ultrastructural level, as well as attempts to use radioactively labeled aminoacids, especially gamma-aminobutyric acid (GABA), and autoradiography, to identify, in the microscope, neurons using such transmitters. Finally, our immunohistochemical work together with Kjell Fuxe and the late Menek Goldstein, using antibodies to four monoamine-synthesizing enzymes is summarized, including some aspects on the adrenaline neurons, which had escaped detection with the Falck-Hillarp technique.

The influence of glycine and related compounds on spinal cord injury-induced spasticity

Spasticity is a frequent and complex sequel to spinal cord injury. The neurochemical basis for the origin of spasticity is largely unknown. Glycine is among the most abundant neurotransmitters in the spinal cord. However, the role of glycine and related compounds in spasticity have received little attention. An ischemic spinal cord injury was created in rabbits, by an intraaortic balloon occlusion technique, which produced lower limb spasticity. A catheter was inserted into the cisterna magna and the spinal cord was bathed with 100 microM solutions of glycine, strychnine, D-serine, beta-alanine, MK-801, or artificial CSF for 4 hours at a rate of 10 microliters/min. H-reflexes were monitored before and during infusion by stimulating the posterior tibial nerve and recording from the plantar surface of the foot. Glycine, D-serine, and MK-801 depressed the H wave, strychnine produced a heightened H wave, and beta-alanine caused no significant changes. These results indicate that glycine and related compounds may influence spasticity.

Co-localization of choline acetyltransferase and postsynaptic glycine receptors in motoneurons of rat spinal cord demonstrated by immunocytochemistry

Male rats were perfused with paraformaldehyde and picric acid. The cervical spinal cord was cryosectioned and immunostained with a monoclonal antibody against the postsynaptic receptor for the neurotransmitter glycine. The anterior horn contained glycinoceptive neurons of varying morphology. Cholinergic cells were identified in the same tissue sections when subsequently immunostained with a monoclonal antibody against choline acetyltransferase, the biosynthetic enzyme of acetylcholine. Immunoreactivity for the glycine receptor was detected in the plasma membrane and for choline acetyltransferase in the perikaryal cytoplasm of identical anterior horn cells, classified as small, medium and large motoneurons. This suggests that motoneurons have receptors for glycine on their cell surface.

Localization of axonally transported [3H]glycine in vesicles of identified neurons

A specific association of axonally transported, free [3H]glycine with vesicles in the identified neurons R3-R15 of Aplysia is demonstrated by high resolution autoradiography. The association of glycine with vesicles, the first such finding in any animal for a neuroactive amino acid, adds to evidence that glycine may be utilized as a neuro-chemical messenger by R3-R14.

Identification of GABA neurons in rat cortical cultures by GABA uptake autoradiography

Autoradiographic studies of rat cortical cultures were conducted with tritiated transmitters and related drugs. Autoradiographs prepared from cultures incubated in [3H]GABA showed selective labeling: dense accumulations of silver grains over the somas and all processes of approximately 30-50% of the neuronal population, few grains over the non-neuronal cells. This labeling was blocked by diaminobutyric acid (DABA) and sodium-free media but not by beta-alanine and thus has the characteristics of GABA uptake in other neuronal systems. There were no obvious differences in the size, shape, number of processes or distribution in the culture between neurons which accumulated GABA and those which did not. Similar cultures incubated in either [3H]glycine or [3H]glutamate and processed by autoradiography resulted in a much different distribution of silver grains than that seen for [3H]GABA. Following incubation in [3H]glycine, silver grains were distributed uniformly over all cells in the culture, both neuronal and non-neuronal. This distribution suggests a metabolic and not a neurotransmitter role for glycine in the cultures, as would be expected of neuronal cells derived from cerebral cortex. Glutamate incubations resulted in the appearance of silver grains over only the non-neuronal cells with very few over the neuronal population. Autoradiograms were also prepared following incubation in the potent GABA receptor agonist [3H]muscimol. These autoradiograms were indistinguishable from those obtained following [3H]GABA incubation. Thus, a finite population of neurons was densely labeled, the labeling was blocked by the GABA uptake inhibitors DABA, nipecotic acid, guvacine and Na+-free media, while substances which interact with the GABA receptor, bicuculline methiodide, THIP, isoguvacine and the noncompetitive antagonist, picrotoxin, were without effect. These results demonstrate that the affinity of muscimol for the GABA uptake site far outweighs its affinity for the GABA receptor site in autoradiographic experiments where intact cells are employed, presumably because its binding to receptors is fleeting. Therefore, muscimol autoradiography may not be informative about GABA receptor localization. These autoradiographic studies suggest that nearly half the neurons in our culture system are GABA neurons but disclosed no morphological handle for GABA neurons.

Specific glycine uptake by identified neurons of Aplysia californica. I. Autoradiography

The identified giant neurons R3-R14 in the Aplysia parietovisceral ganglion (PVG) have a rapid, Na+-dependent and Hg2+-sensitive uptake system for glycine not found in neighboring neurons. In autoradiographs of PVG incubated in [3H]glycine (glutaraldehyde fix), the cytoplasms of R3-R14 have 3--4 times more silver grains (No./100 sq.micrometer) than other neurons. The glycine uptake system in R3-R14 is selective (alanine, serine, leucine, and proline are taken up equally by all neurons) and is unaffected by reserpine and anisomycin. Neurons R3-R14 contain 2 times less label when ganglia are fixed in formaldehyde than when glutaraldehyde is used as a fixative. Because formaldehyde fixes free amino acids poorly, much of the glycine taken up by R3-R14 is, therefore, not incorporated into protein. In autoradiographs of PVG incubated in [3H]glycine, silver grains are distributed randomly throughout the cytoplasm and nucleus of R3-R14; no association of the grains with the dense core granules characteristic of these neurons7 or other cellular components was found. In contrast, grains in the neurosecretory "bag cells" of the PVG were clustered in numerous discrete areas of the cytoplasm (Golgi complex areas) and the nucleus was only sparsely labeled. The existence of a rapid and selective glycine uptake system in R3-R14, together with their high endogenous glycine concentrations17, suggests that glycine may be a neurotransmitter in these neurons.

Uptake of L-alanine, glycine and L-serine in the pigeon central nervous system

The uptake characteristics of L-alpha-alanine, glycine and L-serine into crude mitochondrial fractions from pigeon telencephalon, tectum and spinal cord were determined. Sodium dependent high affinity uptake systems were found for all 3 amino acids in all 3 brain regions, except for glycine in the telencephalon. The mutual inhibition of the high affinity uptake by the 3 amino acids was measured in the tectum. Alanine uptake was inhibited competitively by serine and glycine. The inhibition of serine uptake by alanine and glycine was also competitive. In contrast, glycine uptake inhibition by alanine and serine was incomplete and neither competitive, non-competitive nor uncompetitive. The effect of various chemicals on the uptake of the 3 amino acids was measured in the tectum at a substrate concentration of 10(-5) M. Four groups could be distinguished: (1) Substances with no effect on the uptake of all 3 amino acids, (2) substances which inhibited the uptake of all 3 amino acids to a similar degree, (3) substances which inhibited the uptake of alanine and serine more than glycine and (4) substances which inhibited glycine uptake more than alanine and serine uptake. From these results we conclude: Alanine and serine are probably taken up by the same transport system. This system can possibly also use glycine as substrate. Most of the glycine high affinity uptake, however, is due to a specific glycine transport system. This system was found only in tectum and spinal cord and is probably the same uptake system as known in the spinal cord of other vertebrates.

Inhibition of dopamine release in the cat caudate nucleus by nigral application of glycine

"Encéphale isolé" cats were implanted with two push-pull cannulae, one in the left caudate nucleus and the other in the ipsilateral substantia nigra. L-3,5(-3)H-Tyrosine was introduced continuously into the caudate nucleus to study the release of 3H-DA. Glycine (10(-5)M) added to the superfusing medium of the push-pull cannula inserted into the substantia nigra reduced (25%) the spontaneous release of 3H-DA. Conversely, strychnine (10(-5)M) slightly stimulated the 3H-transmitter release. The inhibiting effect of glycine (10(-5)M) on 3H-DA release was no longer seen in the presence of strychnine (10(-5) M). The results support the hypothesis of a tonic glycinergic inhibitory control of the activity of the nigrostriatal dopaminergic neurons.