Joel L. Davis

D-proline: Stereospecificity and sodium chloride dependence of lethal convulsant activity in the chick

Two- and five-day old chicks were injected intraventricularly with D-proline and structurally related compounds. D-proline produced convulsions and lethality, but was non-amnestic, whereas the naturally-occurring isomer, L-proline, was non-convulsant and non-toxic but amnestic. D-proline convulsions were accompanied by decreased high frequency in the EEG and increased slow wave activity. High amplitude spiking was not observed. The lethality of D-proline was saline-dependent. Control experiments ruled out possible toxic factors such as hypertonicity, pH pyrogens, injection volume, or needle misplacement. The results demonstrate that saline and distilled water are not equivalent injection vehicles. A sodium-free vehicle may lead to artifacts but is advantageous in experiments in which amino acid transport must be minimized.

Brain glutamate inhibition and amnesia: evidence provided by proline analog action

The action of proline and its analog as glutamate antagonists was investigated in CNS tissue of the neonatal chick. Avian brain slices were incubated in low concentrations of L-proline, D-proline, DL-3,4-dehydro-proline, L-prolyl-L-proline, or in avian physiological salt solution before depolarization was induced by application of 45 mM K+. Glutamate was determined in the efflux material collected both before and after tissue stimulation. The release of endogenous glutamate was inhibited significantly by exposure to L-proline, DL-3,4-dehydroproline and L-prolyl-L-proline. The degree of glutamate inhibition correlated with the amnestic potency of these substances. The manner in which these results strengthen the hypothesis of glutamate involvement in memory processes is discussed.

Memory enhancement induced in chicks by L-prolyl-L-leucyl-glycineamide ☆

Two-day-old chicks were injected either intraventricularly or intraperitoneally with saline or a L-prolyl-L-leucyl-glycinamide solution. This C-terminal tripeptide of oxytocin produced retrograde enhancement when injected centrally but not peripherally. Possible memory mechanisms are discussed in light of this peptides relationship to oxytocin, MSH, and dopaminergic systems.

Arginine vasopressin enhances memory retroactively in chicks.

White Leghorn cockerels were trained in a one-trial passive avoidance task to suppress their spontaneous pecking behavior at a visually attractive target by precoating the target with an aversive liquid. In a series of experiments, chicks were injected intraventricularly (5 μl/hemisphere) or subcutaneously (10 μl) with either 2.5 or 0.25 μg of arginine vasopressin or with saline. These injections took place 1 min after the 10-sec training period. In one experiment, two groups were injected at 9 or 59 min posttraining. Twenty-four hours after injection, chicks were tested for enhancement of memory for the training task, by presenting them with an uncoated peck target. Chicks that pecked less than did the saline controls were considered to demonstrate enhancement of memory. In all experiments performed with chicks receiving arginine vasopressin between 1 and 9 min posttraining, retroactive enhancement of memory was observed. Chicks receiving injection at 59 min did not exhibit enhancement. These results were interpreted as showing that arginine vasopressin can improve memory retention in a retroactive manner and that there appears to be a temporal limitation on its effect. The response of both avian and mammalian cardiovascular systems to arginine vasopressin is discussed.

Amnestic potency of proline analogs correlates with anti-spreading depression potency

L-Proline and some of its analogs have been shown to prevent spreading depression (SD) in the chick retina at relatively low concentrations and to impair memory processing without provoking toxic or electrophysiological disturbances. Both effects are hypothesized to be caused by inhibition of the effects of glutamate released into the extracellular space. L-Proline, its D-enantiomer, six proline analogs including two homologs (L-azetidine-2-carboxylic acid and DL-pipecolic acid), and five other compounds were examined for their effects on spreading depression and their amnestic and electrophysiological effects. L-Proline, L-baikiain, DL-3,4-dehydroproline, and L-4-hydroxyproline all reduced the incidence of SD in the chick retina and proved to be amnestic. D-Proline, L-pyroglutamic acid, L-azetidine-2-carboxylic acid, DL-pipecolic acid, L-glutamic acid diethylester, L-isoleucine and L-norleucine neither depressed SD nor caused retrograde amnesia. L-Prolyl-L-proline and L-glutamine did not depress SD at low concentrations but had significant amnestic effects. None of the listed compounds induced EEG disturbances. Implications for memory mechanisms are discussed in the light of these results.

Dose-dependent and time-dependent action of oxytocin on chick memory

Experiments were conducted to investigate the dose-related and time-dependent effects of oxytocin on memory for a one-trial conditioned taste aversion task using two-day old chicks. Oxytocin was administered intracerebrally 1 min posttraining to 5 groups of chicks in dose levels differing by a factor of 10 and ranging from 5.0 pg to 50 ng. A second experiment tested the time-dependent nature of the neuropeptides action. In this experiment the oxytocin (5.0 ng) was administered at either 1 min, 9 min or 59 min posttraining. In both experiments saline-injected control groups were included. The taste aversion training for all experiments consisted of presenting an attractive lure, coated with an aversive liquid (EtOH), to each chick for a 10-s training trial. Most chicks pecked 1 or 2 times at the lure before inhibiting any further response. The retention testing took place 24 h after the training and consisted of presenting the dry, uncoated lure to each chick for an additional 10 s. Chicks that avoided pecking at the lure were considered to have exhibited enhanced retention. The groups of chicks receiving 50 pg to 50 ng of oxytocin exhibited enhancement of retention, as did the 1 min group of the time-dependent experiment. These results are compared to the effects on memory consolidation in chicks induced by vasopressin and L-prolyl-L-leucyl-glycineamide. The apparent conflict between these results and those obtained in mammalian studies with oxytocin are discussed.

Differences in learning between hyperprolinemic mice and their congenic controls

These experiments expanded earlier work on hyperprolinemic mice which showed learning deficits. The following behavioral tasks were used: step-through, passive avoidance; T-maze acquisition; shuttlebox acquisition, and radial-arm maze. Mouse species included PRO/Re-bb (genetically hyperprolinemic mice) and PRO/Re-aa (congenic nonhyperprolinemic controls) obtained from the Jackson Breeding Laboratories. Hyperprolinemic mice were impaired in acquiring T-maze and shuttlebox footshock avoidance behavior. One-trial passive avoidance behavior did not clearly differentiate between the groups. Radial maze performance was poor in both groups due possibly to observed acrophobia and lack of exploratory behavior. The results of this study combined with previously published work suggest that high-brain proline in conjunction with other amino acid changes account for the learning deficits.

L-prolyl-L-arginyl-glycineamide induces memory enhancement in chicks

Two-day-old chicks were injected either intraventricularly or intraperitoneally with saline or a L-prolyl-L-arginyl-glycineamide solution. This C-terminal tripeptide of arginine vasopressin produced dose dependent enhancement effects when injected centrally but not peripherally. Physical debilitation and/or aversive effects of the peptide were eliminated as the cause of the decreased responding noted in memory enhancement studies using this avoidance paradigm. Possible memory mechanisms are discussed in light of this peptides relationship to vasopressin, vasotocin, and L-propyl-L-leucyl-glycineamide.

Intraventricular N-acetyl-L-glutamate induces retrograde amnesia in chicks

Abstract Two-day-old chicks were injected intraventricularly with saline or an N-acetyl-L-glutamate solution. The amino acid analog produced retrograde amnesia without concomitant electrophysiological seizure activity. A mechanism involving the disruption of activity at memory-related glutamate synapses was suggested for the observed amnestic effect.

Memory enhancement induced in chicks by anesthetic doses of chlorpromazine

Three experiments describe retroactive memory enhancement in chicks by chlorpromazine (CPZ), a potent membrane stabilizer at anesthetic doses. First, chicks trained on a one-trial peck suppression task under moderate aversant conditions and given 60–144 mg CPZ/kg, intraperitoneally, displayed increased memory retention 24 h later when compared with chicks given saline or 15–36 CPZ/kg. The second experiment confirmed the memory enhancement using a different moderate aversant on a different peck target and extended the effect to 48 h. The third experiment demonstrated the CPZ effect to be retrograde because the enhancement observed when CPZ was injected 1 min or 3 min after training did not occur when injection was delayed 4 h. One speculative interpretation of retrograde enhancement with CPZ includes the stabilization of swollen dendritic spines, which may facilitate neuronal pathways.