Microiontophoretic studies of the effects of L-proline on neurons in the mammalian central nervous system

Central mechanism of the cardiovascular responses caused by L-proline microinjected into the paraventricular nucleus of the hypothalamus in unanesthetized rats

Previously, we reported that microinjection of L-proline (L-Pro) into the paraventricular nucleus of the hypothalamus (PVN) caused vasopressin-mediated pressor responses in unanesthetized rats. In the present study, we report on the central mechanisms involved in the mediation of the cardiovascular effects caused by the microinjection of L-Pro into the PVN. Microinjection of increasing doses of L-Pro (3-100nmol/100nL) into the PVN caused dose-related pressor and bradycardic responses. No cardiovascular responses were observed after the microinjection of equimolar doses (33nmol/100nL) of its isomer D-Proline (D-Pro) or Mannitol. The PVN pretreatment with either a selective non-NMDA (NBQX) or selective NMDA (LY235959 or DL-AP7) glutamate receptor antagonists blocked the cardiovascular response to L-Pro (33nmol/100nL). The dose-effect curve for the pretreatment with increasing doses of LY235959 was located at the left in relation to the curves for NBQX and DL-AP7, showing that LY235959 is more potent than NBQX, which is more potent than DL-AP7 in inhibiting the cardiovascular response to L-Pro. The cardiovascular response to the microinjection of L-Pro into the PVN was not affected by local pretreatment with N^ω-Propyl-l-arginine (N-Propyl), a selective inhibitor of the neuronal nitric oxide synthase (nNOS), suggesting that NO does not mediate the responses to L-Pro in the PVN. In conclusion, the results suggest that ionotropic receptors in the PVN, blocked by both NMDA and non-NMDA receptor antagonists, mediate the pressor response to L-Pro that results from activation of PVN vasopressinergic magnocellular neurons and vasopressin release into the systemic circulation.

Activity of prolidase isoenzymes in the rat brain: subcellular and regional distribution during development

Prolidase deficiency is characterized by chronic ulcerative dermatitis, mental retardation, and frequent infections. In the present study we examined the characteristics of rat brain prolidase isoenzymes. Prolidase isoenzymes (PD I and PD II) were isolated from the rat brain using DEAE cellulose column chromatography. PD I showed higher activity against seryl-proline and alanyl-proline, while PD II was particularly active against methionyl-proline. Prolidase activity in the whole brain and in the different brain regions showed higher activity against methionyl-proline and seryl-proline. PD II activity was highest in the hippocampus, followed by the cerebellum, cerebral cortex, caudatum, and the midbrain. The most rapid changes in the activities of PD I and PD II occurred perinatally, with a peak at three days before birth and a nadir at two days after birth, which then gradually increased until 21 days. N-benzyloxycarbonyl-l-proline inhibited PD I activity against various substrates in a dose-dependent manner. In contrast, there was no inhibition of PD II activity against methionyl-proline at low concentrations. In summary, these data suggest that maintenance of levels of proline, other amino acids and peptides containing proline in the rat brain is regulated by prolidase isoenzymes. The age-related alterations in PD I and PD II also may help to elucidate the fundation of prolidase isoenzymes in brain nervous system.

L-proline microinjected into the rat ventrolateral medulla induces a depressor response distinct from L-glutamate

The neurotransmitter candidate L-proline elicits changes in the cardiovascular system via actions in the brainstem. However, its action have not yet been determined in the ventrolateral medulla (VLM), a brain region critical in mediating vasomotor sympathetic nervous system responses. Microinjections of L-glutamate produce depressor responses in the caudal (C) VLM, but pressor responses in the rostral (R) VLM and the caudal pressor area (CPA) in the far caudal CVLM. The present study tested whether microinjections of l-proline in the VLM produce a pattern of hemodynamic responses distinct from that of l- glutamate. Urethane-anesthetized rats received arterial catheters and were implanted with flow probes around the abdominal aorta (supplies hindquarters). The surface of each rat's VLM was then exposed. L-Proline induced dose- dependent depressor responses in the CVLM (0.003-1.0 M, 34 nl), but did not induce hemodynamic responses in sites of the RVLM (0.01-1.0 M, 34 nl) that responded to L-glutamate (0.01 M, 34 nl). L-Proline injections (0.1 M, 34 nl) induced rapid and consistent depressor responses correlated with coincident decreases in hindquarter resistance (arterial blood pressure/flow) in the CVLM and CPA, but only inconsistent responses in a few sites in the RVLM. In summary, L-proline induced a distinct pattern of depressor responses preferentially in caudal regions of the VLM, and these depressor effects were associated with decreases in hindquarter resistance. These findings indicate that L-proline may have unique roles including cardiovascular regulation independently from L-glutamate, especially in caudal region of the VLM, via a mechanism that involves altering hindquarter resistance.

Depressor and bradycardic actions of L-proline injected into the nucleus tractus solitarii of anesthetized rats

L-Glutamate has been considered to be a neurotransmitter in the nucleus tractus solitarius (NTS) of the afferent baroreflex pathway, though this has not yet been decisively shown. A bolus injection of a neurotransmitter candidate amino acid L-proline into the cisterna magna and that of L-glutamate shows the same pressor action in the freely moving rat, but the actual nuclei responding L-proline remain undetermined. Besides L-glutamate, L-proline might be another candidate amino acid in the NTS. The present study was therefore performed to characterize the circulatory action of L-proline injected into the NTS where responses to glutamate in the anesthetized rat had already been shown. The NTS was first determined as a site on the dorsal surface of the medulla where a microinjection of L-glutamate decreased arterial pressure and heart rate. Microinjected L-proline (1.65 to 13.2 nmol, 33 nl) into the NTS decreased arterial pressure and heart rate in a dose-dependent manner. The injection of a mixed solution (66 nl) of kynurenate, an ionotropic excitatory amino acid receptors antagonist (1.32 nmol), and L-proline (6.6 nmol) into the NTS abolished the depressor and bradycardic actions with L-proline alone (6.6 nmol, 66 nl). However, a mixture of an increased concentration of kynurenate (6.6 nmol) with glutamate augmented the actions seen with glutamate alone (0.66 nmol, 66 nl). D-Proline (13.2 nmol, 66 nl), the optic isomer of L-proline, produced no change in arterial pressure or heart rate, suggesting that the actions of L-proline in the NTS were optically specific. The results indicate that L-proline but not D-proline induces its depressor and bradycardic actions through ionotropic excitatory amino acid receptors in the NTS of the anesthetized rat. L-Proline may become a candidate transmitter of baroreceptor information in the NTS.

Kynurenic acid inhibits circulatory responses to intracisternally injected L-proline in conscious rats

To investigate the modes of action of potential neurotransmitters for cardiovascular control, amino acids and an antagonist were injected intracisternally into conscious rats. Blood pressure and superior mesenteric flow were measured with cannulae and electromagnetic flow probes that had been implanted in a previous operation under pentobarbitone anaesthesia. L-Proline, L-glutamate and L-arginine (10, 2 and 10 micromol, respectively) caused similar increases in blood pressure and mesenteric vascular resistance. Prior injection of kynurenic acid (0.1 micromol), a broad spectrum antagonist of ionotropic excitatory amino acid receptors, completely blocked the circulatory effects of L-proline, significantly reduced those of L-glutamate but had little effect on responses to L-arginine. These results suggest that the central pressor pathways activated by L-proline, a potential endogenous neurotransmitter, are mediated by ionotropic excitatory amino acid receptors.

Characteristics and regulation of proline transport in cultured glioblastoma cells

L-Proline transport in C6 glioblastoma cells takes place mainly via a saturable Na(+)-dependent mechanism. The uptake process can be discriminated into two components, system A and system ASC. A minor proportion of L-proline transport is carried out by the ASC system, which appears to be constitutively expressed by the cell, but most is by system A which shows adaptive responses to amino acid deprivation and sensitivity to N-methyl-alpha-aminoisobutyric acid. The transport system is inhibited by proline derivatives, such as methyl and benzyl esters, and also hydroxyproline, and is stereospecific. Incubation of glioblastoma cells with phorbol 12-myristate 13-acetate led to concentration- and time-dependent decreases in L-proline transport. This effect could be mimicked by exogenous phospholipase C. Proline transport is significantly stimulated in the presence of Ca(2+)-mobilization agents and strongly inhibited in the absence of Ca2+. The present data suggest a complex regulation of L-proline transport by different kinases in glioblastoma cells.

NMDA receptor-mediated depolarizing action of proline on CA1 pyramidal cells

This study investigated the actions of proline on CA1 hippocampal pyramidal cells with use of slice preparations. Bath-applied L-proline first induced these cells to fire multiple orthodromic population spikes in response to a single stimulus and then blocked their response to both orthodromic and antidromic stimulation. These effects could be explained by postsynaptic depolarization followed by depolarization block. Grease-gap studies confirmed that L-proline depolarizes CA1 pyramidal cells. D-Proline was inactive in these tests. Excitatory amino acid antagonists reduced depolarizing responses to proline and N-methyl-D-aspartate (NMDA) in parallel. Mn2+ failed to attenuate proline-evoked depolarizations at concentrations that substantially inhibited synaptic transmission, but at a higher concentration it reduced responses to both proline and NMDA. These results suggest that proline depolarized CA1 pyramidal cells mainly by activating postsynaptic NMDA receptors. The neuroexcitatory and neurotoxic actions of proline in the hippocampus may contribute to the seizures and mental retardation associated with hyperprolinemia.

Autoradiographic localization of proline uptake in excitatory hippocampal pathways

An autoradiographic method was developed to localize sites of high-affinity, Na(+)-dependent proline uptake in the rat hippocampal formation. Hippocampal slices were incubated with [3H]proline, fixed with a glutaraldehyde/carbodiimide mixture, and cut into frozen sections. The sections were coated with photographic emulsion and autoradiograms were prepared. Autoradiographic grain densities were highest over the inner and outer thirds of the dentate molecular layer, followed by stratum lacunosum-moleculare of area CA3. Stratum oriens and stratum radiatum of area CA1 and CA3 were fairly intensely labeled. The pyramidal and granule cell body layers, stratum lucidum of area CA3, and middle third of the dentate molecular layer were lightly labeled. Effects of surgical and kainic acid lesions suggested that the lateral perforant path, associational-commissural fibers in the fascia dentata, and Schaffer collateral-commissural-ipsilateral stratum oriens fibers have considerable proline uptake capacity. In contrast, the medial perforant path and the mossy fibers appear to accumulate little or no proline. These results suggest that high-affinity, Na(+)-dependent uptake of proline is expressed by a subset of hippocampal glutamate pathways. The relative capacities of glutamate terminal populations to transport glutamate and proline varies widely. Proline was previously shown to possess neuroexcitatory and excitotoxic properties in the rat hippocampal formation. Taken together, these findings argue that proline plays a role in excitatory transmission. In elucidating this role, comparisons between medial and lateral divisions of the perforant path may prove especially advantageous.

Molecular cloning and expression of a high affinity L-proline transporter expressed in putative glutamatergic pathways of rat brain

We have used the polymerase chain reaction (PCR) with degenerate oligonucleotides derived from two conserved regions of the norepinephrine and gamma-aminobutyric acid transporters to identify novel Na(+)-dependent transporters in rat brain. One PCR product hybridized to a 4.0 kb RNA concentrated in subpopulations of putative glutamatergic neurons including mitral cells of the olfactory bulb, pyramidal cells of layer V of the cerebral cortex, pyramidal cells of the piriform cortex, and pyramidal cells of field CA3 of the hippocampus. Transient expression of the cognate cDNA conferred Na(+)-dependent L-proline uptake in HeLa cells that was saturable (Km = 9.7 microM) and exhibited a pharmacological profile similar to that for high affinity L-proline transport in rat brain slices. The cloned transporter cDNA predicts a 637 aa protein with 12 putative transmembrane domains and exhibits 44%-45% amino acid sequence identity with other members of the emerging family of neurotransmitter transporters. These findings support a synaptic role for L-proline in specific excitatory pathways in the CNS.

Proline and proline derivatives as anticonvulsants

1. The anticonvulsant properties of L-proline, of proline derivatives (trans-4-hydroxy-L-proline, cis-4-hydroxy-D-proline, 3,4-dehydro-D,L-proline) and of D- and L-pipecolic acid were studied alone and in combination with vigabatrin (R/S-4-aminohex-5-enoic acid). 3-Mercaptopropionic acid and pentylenetetrazol-induced convulsions in mice were used as animal models of epilepsy. 2. Proline and proline derivatives are weak anticonvulsants if given alone in doses up to 10 mmol/kg, however, they are capable of potentiating the anticonvulsant effects of vigabatrin, in a manner similar to that reported previously for glycine, and some glycine derivatives. Among the compounds tested, trans-4-hydroxy-L-proline was the most potent anticonvulsant in combination with the indirect GABA agonist vigabatrin. 3. A potential explanation for the synergistic anticonvulsant effect of the combination of the GABA agonist and proline is the presumed role of proline as inhibitory neurotransmitter, and/or its glutamate antagonistic effects. 4. The current study points out the lack of basic knowledge on the neurochemistry and pharmacology of proline and hydroxyproline.

L-proline depolarizes rat spinal motoneurones by an excitatory amino acid antagonist-sensitive mechanism

1 Isolated spinal cords prepared from neonatal rats were used to examine the effects of L-proline (L-Pro). 2 L-Pro (1-8 mM) depolarized ventral and dorsal roots in a dose-dependent manner with one sixth of the potency of L-glutamate (L-Glu). L-Pro was four times more potent than D-Pro. Prolonged application of L-Pro produced a plateau depolarization of motoneurones with no apparent fade. 3 Omission of calcium ions from the medium potentiated the depolarizing actions of L-Pro, L-Glu and quisqualate. 4 L-Pro was antagonized by concentrations of 2-amino-5-phosphonovalerate (25 microM), gamma-D-glutamylglycine (100 microM) and Mg2+ ions (1 mM) that depressed responses to N-methyl-D-aspartate (NMDA). The NMDA receptor-mediated component of the response to L-Pro was estimated to be 60-70%. 5 These data suggest that L-Pro should be considered as a possible excitatory neurotransmitter and that, because L-Pro is a neutral compound, excitatory amino receptors may not require an agonist to possess two anionic groups and one cationic group.

Sodium-dependent proline uptake in the rat hippocampal formation: association with ipsilateral-commissural projections of CA3 pyramidal cells

Na+-dependent uptake of L-[3H]proline was measured in a crude synaptosomal preparation from the entire rat hippocampal formation or from isolated hippocampal regions. Among hippocampal regions, Na+-dependent proline uptake was significantly greater in areas CA1 and CA2-CA3-CA4 than in the fascia dentata, whereas there was no marked regional difference in the distribution of Na+-dependent gamma-[14C]aminobutyric acid ([14C]GABA) uptake. A bilateral kainic acid lesion, which destroyed most of the CA3 hippocampal pyramidal cells, reduced Na+-dependent proline uptake by an average of 41% in area CA1 and 52% in area CA2-CA3-CA4, without affecting the Na+-dependent uptake of GABA. In the fascia dentata, neither proline nor GABA uptake was significantly altered. Kinetic studies suggested that hippocampal synaptosomes take up proline by both a high-affinity (KT = 6.7 microM) and a low-affinity (KT = 290 microM) Na+-dependent process, whereas L-[14C]glutamate is taken up predominantly by a high-affinity (KT = 6.1 microM) process. A bilateral kainic acid lesion reduced the Vmax of high-affinity proline uptake by an average of 72%, the Vmax of low-affinity proline uptake by 44%, and the Vmax of high affinity glutamate uptake by 43%, without significantly changing the affinity of the transport carriers for substrate. Ipsilateral-commissural projections of CA3 hippocampal pyramidal cells appear to possess nearly as great a capacity for taking up proline as for taking up glutamate, a probable transmitter of these pathways. Therefore proline may play an important role in transmission at synapses made by the CA3-derived Schaffer collateral, commissural, and ipsilateral associational fibers.

A glial-neuronal-glial communication system in the mammalian central nervous system

Previous studies have demonstrated that when tritiated proline [( 3H]Pro) is injected into the dorsal column nuclei (DCN) of cats, it labels macroglial cells, but fails to label neurons at the injection site. (Tritiated leucine [( 3H]Leu) in contrast, labels both neurons and some glial cells.) Despite the failure of [3H]Pro to label DCN neurons, labeling is still observed in DCN terminal targets. This result suggests that glial cells are involved in the translocation of [3H]Pro-labeled molecules from one part of the brain to another. The purpose of the present experiment was to use electron microscopic autoradiographic techniques to characterize the labeling produced in internal arcuate fiber tract axons arising from DCN neurons 24 h after injections of [3H]Pro (or [3H]Leu, for comparison) into DCN. It was reasoned that, if the translocation of [3H]Pro-labeled molecules from DCN to its targets is indeed carried out by glial cells, then only glial elements associated with the fibers should be labeled following [3H]Pro injections of DCN. If, on the other hand, the translocation involves an initial transfer of [3H]Pro-labeled molecules into neuronal perikarya followed by axonal transport, then only axoplasmic elements along the fiber pathway should be labeled. Injections of [3H]Pro into DCN labeled axoplasmic elements in samples of axons from the internal arcuate tract both 'near' (0.5-0.8 mm) and 'far' (2-4 mm) from the injection site at about an equal absolute density. However, glial elements associated with the axons were also labeled in both samples, but much more densely in the 'near' than in the 'far' axons. Injections of [3H]Leu labeled axoplasm more densely than did [3H]Pro (by a factor of 4 in the 'far' samples). Glial labeling by [3H]Leu near the injection site was much less than that of [3H]Pro, but, 'far' from the injection, the levels of [3H]Leu and [3H]Pro glial labeling were comparable. Taken together with the results of other studies, these data support the existence of a previously unrecognized system of communication between glial cells and neurons. In this putative system (Fig. 9), molecules containing both [3H]Leu and [3H]Pro are transferred from glial cells into adjacent neuronal soma and transported down the length of the axon where, all along the way, some of them are transferred from the axon into adjacent glial processes. The system is more readily apparent when [3H]Pro is used because of its avid and preferential uptake by glial cells. Potential functions of such a system are unknown, but could be trophic, protective and/or informative.

Proline in the cerebrospinal fluid of normal subjects and Alzheimer's-disease patients, as determined with a new double-labeling assay technique

Past studies have implicated proline involvement in the function of memory and learning. A new micromethod has been developed that is suitable for measuring proline accurately in as little as 0.1 ml of CSF. In normal human CSF, the average proline level was found to be consistently about 1.3 microM. In the CSF of patients with Alzheimer's disease and mixed dementias, the levels of proline showed no statistically significant difference from proline levels in the CSF of normal controls. Furthermore, the proline levels in the CSF of the Alzheimer's disease patients did not reflect, consistently, the cognitive deficits or the symptomatic severity of the disease. Proline levels in CSF showed no statistically significant change with the age of individuals tested.

The nature of the chick's magnesium-sensitive retinal spreading depression

Spreading depression (SD) in the chick retina is completely suppressed by 10 mM MgCl2 in the bathing solution (Mg-sensitive SD). However, after increasing the KCl concentration in the Mg solution to values between 10 and 20 mM the retina can again exhibit SDs (Mg-insensitive SD). It has been postulated that the Mg-sensitive SD is a glutamatergic phenomenon. This is supported by the effect of four gl(utamate)-antagonists--L-proline, glutamic acid diethyl ester (GDEE), D-alpha-aminoadipate (D-AA), and 2-amino-4-phosphonobutyrate (APB)--which all suppressed this type of SD. It was suggested that this effect is due to competitive binding of glutamate involved in the Mg-sensitive SD and the gl-antagonist to glutamate receptors. The suppression of SD could be reversed by washing the preparation in a physiologic salt solution. The gl-antagonists in relatively high concentrations had a cytotoxic effect which, when severe, suppressed SD and prevented the recovery of this phenomenon by washing the compound out of the tissue. The compounds examined had, in addition to their gl-antagonistic properties, a gl-agonistic effect, which was postulated to enhance the Na+ permeability of neural membranes resulting in a release of K+ into the extracellular space. In preparations bathed in 10 mM MgCl2 (which suppresses Mg-sensitive SDs) the four compounds investigated promoted Mg-insensitive SDs supposedly when the extracellular K+ concentration reached values between 10 and 20 mequiv.

Iminoglycine transport system in synaptosomes and its interaction with enkephalins

Evidence is presented which suggests that proline, pipecolic acid, and glycine are accumulated by a common transport system in rat brain cortical synaptosomes and synaptosomal plasma membrane vesicles. This system is Na+ dependent and appears to be similar to the iminoglycine transport system present in renal tubules and in renal brush border membranes. The opioid pentapeptides Leu- and Met-enkephalin specifically inhibit the uptake of these three imino/amino acids, presumably by interaction with a nonopioid receptor, since the inhibition is not affected by the opiate antagonist naloxone and occurs with des-tyrosyl enkephalins as well as with the intact pentapeptides.

Selective neuron loss after cerebral ischemia in the rat: possible role of transmitter glutamate

Male Wistar rats were subjected to 20 min of cerebral ischemia by means of 4-vessel occlusion. The topography of regional, selective neuron loss in this model corresponded to areas with pronounced glutamate high affinity uptake (presynaptic receptors), suggesting that transmitter glutamate is involved in the mechanism of neuron damage. One group of animals was injected with the glutamate antagonist, glutamic acid diethyl ester (GDEE) before ischemia. The regional neuron loss was rated using a semiquantitative scale. No statistically significant difference was found between the groups. The results do not exclude a possible role of transmitter glutamate in the pathogenesis of ischemic brain damage. More specific and potent glutamate antagonists are needed in order to clarify such a mechanism.

Primary afferent depolarization and changes in extracellular potassium concentration induced by L-glutamate and L-proline in the isolated spinal cord of the frog

To test the hypothesis that L-proline acts as an antagonist on glutamate receptors [17, 18], the interaction between L-glutamate and L-proline was studied in the isolated spinal cord of the frog. Glutamate at concentrations of 10(-6) -5 x 10(-3) mol/l depolarized the primary afferent fibres and increased extracellular potassium concentration, [K+]e, by 0.3-4 mmol/l. Repeated applications lead to inactivation of the response. L-Proline at 5 x 10(-3) -10(-2) mol/l, also depolarized the primary afferents and increased [K+]e by 0.5-2 mmol/l, but there was only a slight decrease of the effects after repeated application. The effects were additive when the amino acids were applied simultaneously. The effect of L-proline was still present when it was applied during inactivation of the glutamate receptors. This suggests that L-glutamate and L-proline act on different receptors.

L-Proline inhibition of glutamate release

The influence of 1 mM-L-proline on electrically stimulated release of endogenous glutamate from slices of rat frontal cortex was measured. One mM-L-proline approximates the in vitro concentration that inhibits glutamate-induced spreading depression. Also, this L-proline level has been reported to be present in brain after an intraperitoneal injection that induced amnesia in chicks. L-Proline but not D-proline exhibits an inhibitory effect on glutamate release, thus supporting the suggestion that this mechanism may play a role in memory retention and/or formation.

The dual effect of L-proline on spreading depression in the chicken retina

Evidence was presented for a glutamate agonistic effect of L-proline which promotes K+-based spreading depressions (SD) in chick retinas at relatively high concentrations (5 mM), in addition to an antagonistic effect which inhibits glutamate-based SDs at lower (2 mM) concentrations. Together these effects explain the observed biphasic effect of L-proline on the incidence of SD in the retina.

Impairment of instrumental learning in rats by glutamic acid diethyl ester

Glutamic acid diethyl ester (GDEE), a putative antagonist of glutamate-induced neuronal excitations, was administered prior to an instrumental conditioning task motivated by food reinforcement. A profound impairment of learning was produced in animals receiving 240 or 480 mg/kg of GDEE. Performance was not impaired by GDEE in rats that had previously learned the task. These findings support suggestions that central excitatory processes play an important role in learning phenomena, in particular when these learning phenomena involve acquisition of new behavioral patterns.

L-proline as a glutamate antagonist at a crustacean neuromuscular junction

The fast as well as the slow contractions of the adductor muscle in the claw of Procambarus clarkii are inhibited by L-proline. This inhibition is dose dependent and decreases with increasing frequency of stimulation of the "slow" fiber. Contractions caused by perfusing the adductor muscle with L-glutamate solutions are also inhibited by L-proline. The inhibiting potency of L-proline is small; the effective concentration of this amino acid is 50--100 times that of the L-glutamate applied. It was postulated that the inhibitory effect of L-proline is based on competition for excitatory receptor sites of L-glutamate, which causes depolarization and contraction, and L-proline, which lacks these actions. Theoretical considerations suggested a linear relationship between the stimulating L-glutamate and the just-inhibiting L-proline concentrations. Experimental evidence supported this model.

Effects of L-proline and some of its analogs on retinal spreading of depression

L-Proline inhibits glutamate-based spreading depressions (SDs) at low concentrations (2--2.5 mM) and promotes K+-based SDs at higher concentrations (5 mM). The inhibition of glutamate-based SDs was postulated to be due to competition of L-glutamate and L-proline for glutamate receptors on somatic and dendritic plasma membranes. The binding of proline to glutamate receptors was furthermore postulated to result in a release of K+ from the intracellular compartment, enhancing the extracellular K+ concentration sufficiently to promote K+-based SDs. A proline analog, L-baikiain, containing a double bond and one more C atom in the ring structure than proline had similar effects as the latter amino acid, but an analog, L-azetidine-2-carboxylic acid, with one less C atom in the ring had little effect on SD in the retina.

L-Proline and related compounds: correlation of structure, amnesic potency and anti-spreading depression potency

The effects of L-proline, D-proline, and L-azetidine-2-carboxylic acid (L-A.2.C., the lower homolog of L-proline) have been compared in two systems. L-Proline is more potent than either analog in causing amnesia of one-trial avoidance conditioning of the 2-day-old chick and in preventing mechanically induced spreading depression in the retina isolated from 2-3-week-old chicks. The results suggest that the L-configuration and the proper molecular size are essential for the effects of L-proline upon memory and upon spreading depression. This level of specificity is greater than that involved in protein synthesis because L-A.2.C. is incorporated into protein in place of L-proline, in several protein-synthesizing systems.

The interaction of four putative glutamate antagonists with glutamate and their effects on the toad spinal cord

1. Four putative glutamate antagonists (L-glutamate diethyl ester, L-glutamate dimethyl ester, L-proline and 1-hydroxy-3-amino-pyrrolidone-2) were tested on the isolated hemisected toad spinal cord. 2. 1-hydroxy-3-amino-pyrrolidone-2 (10(-3)-10(-2) M) selectively antagonized the depolarizations evoked in both dorsal and ventral roots by applications of L-glutamate (5 X 10(-4) M). 3 1-hydroxy-3-amino-pyrrolidone-2 also antagonized the depolarizations evoked in both dorsal and ventral roots by stimulation of the adjacent dorsal root. 4. The dimethyl and diethyl esters of L-glutamate and L-proline had their own depolarizing actions on the dorsal and ventral roots, and neither potentiated nor antagonized the effects of L-glutamate. 5. The results with 1-hydroxy-3-amino-pyrrolidone-2 offer further evidence for the involvement of L-glutamate and L-aspartate in synaptic transmission in the amphibian spinal cord.