Marie E. Gibbs

Psychobiology of memory: Towards a model of memory formation

Abstract Evidence from the use of inhibitory drugs and antagonists to these drugs suggest a three phase model of memory formation, with phases sequentially dependent. Hyperpolarization due to potassium conductance changes following learning are postulated to underlie the formation of a short-term memory phase. Hyperpolarization associated with sodium pump activity appears to be involved in the formation of the succeeding labile phase. Long-term memory formation appears to involve sodium pump associated amino acid uptake occuring during labile phase formation. Protein synthesis is accepted as underlying the formation of long-term memory. Although reference is made to available evidence in the literature, this review deals in detail with evidence from our laboratories.

Inhibition of glycogenolysis in astrocytes interrupts memory consolidation in young chickens

Glycolysis and glycogenolysis are involved in memory processing in day‐old chickens and, aside from the provision of energy for neuronal and astrocytic energy metabolism these pathways enable astrocytes to supply neurones with precursor for transmitter glutamate by glucose‐based de novo synthesis. We have previously shown that memory processing for bead discrimination learning is dependent on glycolysis; however, the metabolic inhibitor used, iodoacetate, inhibits pyruvate formation from both glucose and glycogen. At specific time points after training transient reductions in brain glycogen content occur, mirrored by increases in glutamate/glutamine content. In the present study, we used intracerebral injection of a glycogen phosphorylase inhibitor, 1,4‐dideoxy‐1,4‐imino‐D‐arabinitol (DAB), which does not affect glucose breakdown, to evaluate the role of glycogen metabolism in memory consolidation. Dose‐dependent inhibition of learning occurred when DAB was administered at specific time periods in relation to training: (i) 5 min before training, (ii) around 30 min posttraining, and (iii) 55 min posttraining. After injection at either of the two earlier periods, memory disappeared after consolidation 30 min postlearning, and after injection 55 min after learning memory was absent at 70 min. The memory loss caused by early administration could be prevented after training by central injection of the glutamate precursor glutamine or the astrocyte‐specific substrate acetate together with aspartate, substituting for pyruvate carboxylation. Thus, glycogenolysis is essential for learning in this paradigm and, aside from energy supply considerations, we suggest that an important role for glycogenolysis is to provide neurones with glutamine as the precursor for neuronal glutamate and GABA.

Role of adrenoceptor subtypes in memory consolidation

Noradrenaline release in areas within the forebrain occurs following activation of noradrenergic cells in the locus coeruleus (LoC). Release of noradrenaline by attentional/arousal/vigilance factors appears to be essential for learning and is responsible for the consolidation of memory. Noradrenaline can activate any of nine different adrenoceptor (AR) subtypes in the brain and selectivity of action may be achieved by the spatial location and relative density of the AR subtypes, by different affinities of the different subtypes and by temporal selectivity in terms of when the different ARs are activated in the memory formation process. This review examines the use of selective agonists and antagonists to determine the roles of the AR subtypes in the one-trial discriminated avoidance learning paradigm in the chick. A model is developed that integrates noradrenergic activity in basal ganglia (lobus parolfactorius (LPO)) and association cortex (intermediate medial hyperstriatum ventrale (IMHV)) leading to the consolidation of memory 30 min after training. There is evidence that beta(2)- and beta(3)-ARs are important in the association area but require input from alpha(2)-AR stimulated activity in the basal ganglia for consolidation. On the other hand, alpha(1)-AR activation in the IMHV is inhibitory and prevents consolidation. While there is no role for beta(1)-ARs in memory consolidation, they play a role in short-term memory (STM). The use of the precocial chick has clear advantages in having a temporally discrete learning task which allows for discrimination memory and whose development can be followed at discrete intervals after learning. These studies reveal clear roles for AR subtypes in the formation and consolidation of memory in the chick, which have allowed the development of a model that can now be tested in mammalian systems.

Astrocytic involvement in learning and memory consolidation

Astrocytes play fundamental roles in brain function, interacting with neurons and other astrocytes, yet their role in learning is not widely recognized. This review focuses on astrocytic involvement in memory consolidation following bead discrimination learning in day-old chick and draws parallels to mammalian learning, providing strong empirical support for the conclusion that the described neuronal-astrocytic interactions are universally valid. It identifies specific mechanisms whereby astrocytes support memory consolidation. Uptake of glucose, stimulated in astrocytes by beta(3)-noradrenergic receptor activation, provides energy by glycolytic/oxidative metabolism. Unlike neurons, astrocytes carry out net synthesis of tricarboxylic acid cycle intermediates needed for synthesis of transmitter glutamate formed by rapid degradation of glucose-derived glycogen and stimulated by beta(2)-noradrenergic receptor activation. This makes learning dependent on glycogenolysis and its stimulation by noradrenaline. Astrocytes take up most synaptically released glutamate, terminating transmitter activity and returning glutamate to neurons in a glutamate-glutamine cycle, interference with which abolishes learning. The various astrocytic activities follow a rigidly controlled time schedule, easily determined after bead discrimination learning but also detectable in other paradigms.

Glycogen is a preferred glutamate precursor during learning in 1-day-old chick: Biochemical and behavioral evidence

Bead discrimination training in chicks sets in motion a tightly timed series of biochemical events, including glutamate release, increase in forebrain level of glutamate and utilization of glycogen and glucose. Inhibition of glycogen breakdown by the glycogen phosphorylase inhibitor 1,4‐dideoxy‐1,4‐imino‐D‐arabinitol (DAB) around the time of training abolishes the increase in glutamate 5 min posttraining in the left hemisphere, in spite of uninhibited glucose metabolism. It also reduces the contents of glutamate, glutamine, and aspartate in the right hemisphere. Behavioral evidence supports the conclusion that glucose breakdown serves to provide energy, whereas glycogen acts as a substrate for glutamate, glutamine, and aspartate formation, requiring both pyruvate dehydrogenation to acetyl coenzyme A and pyruvate carboxylation in astrocytes. Inhibition of memory consolidation caused by DAB or 2‐deoxyglucose (2‐DG), an inhibitor of glucose phosphorylation without effect on glycogen metabolism, was challenged by intracerebral administration of acetate, aspartate, glutamine, lactate or glucose. DAB‐mediated memory inhibition was successfully challenged by administration at 0 or 20 min posttraining of acetate (an astrocyte‐specific acetyl CoA precursor) together with aspartate, substituting for pyruvate carboxylation, or of glutamine at 0–2.5 or 30 min posttraining. 2‐DG‐mediated memory impairment was not challenged by acetate with or without aspartate at 0 time but was challenged by acetate without aspartate at 20 min. Lactate, a substrate for both dehydrogenation and pyruvate carboxylation challenged both DAB and 2‐DG. Doses of DAB and 2‐DG which, on their own were subeffective, were not additive, further supporting the existence of one pathway using glucose and another using glycogen.

Behavioural stages in memory formation

Day-old chickens trained in pairs on an aversive discrimination task yielded a retention function with two points of reduced retention, at 15 and 55 min after learning. These points of temporary reduction in retention were interpreted as reflecting change-over of recall from three successive phases in memory formation. Chicks trained in isolation showed the same retention function as paired chicks except that the second point of reduced retention occurred at 70 min after learning. It was suggested that isolation prolonged the availability for recall of the second phase of memory formation. The findings are consistent with and support a three phase, behaviourally sequentially dependent, model of memory formation previously postulated on the basis of pharmacological studies.

Both non-NMDA and NMDA glutamate receptors are necessary for memory consolidation in the day-old chick

Day-old chicks (black Australorp-white Leghorn) trained to avoid an aversive stimulus will usually retain memory for this event indefinitely. The passive avoidance task used involves a period of pretraining where chicks peck freely at two differently colored glass beads, a single training trial where one of the beads is coated in a chemical aversant eliciting typical disgust reactions from the chicks, and a test trial where both beads are presented dry, and discrimination memory is demonstrated by avoidance of the previously aversive bead with continued pecking of the nonaversive bead. Intracranial administration of a N-methyl-D-aspartate glutamate receptor antagonist (50 microM 2-amino-5-phosphopentanoate) immediately after or prior to learning, or a non-N-methyl-D-aspartate glutamate receptor antagonist (100 microM 6,7-dinitro-quinoxaline-2,3-dione) between 10 and 25 min after learning, resulted in amnesia for this task at 80 to 90 min post-training. These data indicate that processes dependent on N-methyl-D-aspartate and non-N-methyl-D-aspartate glutamate receptor activation are necessary for memory consolidation of a passive avoidance task in the day-old chick. Since these agents must be administered during the earlier stages of memory formation to cause amnesia, the receptors are probably activated close to the time of learning. The delayed effect of these antagonists, however, suggests that memory is independent of these receptors until quite late in the memory consolidation process.

Transient increase in forebrain muscarinic cholinergic receptor binding following passive avoidance learning in the young chick

Abstract The concentration of the muscarinic cholinergic receptor in two regions of the chick brain was measured by atropine-sensitive binding of the ligand [ 3 H]quinuclidinyl benzilate at varying times after training 1–2 day old chicks on a one-trial passive avoidance task. Thirty min after training, there was a 21% increase ( P 3 H]quinuclidinyl benzilate binding/mg protein in the chick fore brain, though not the optic lobes. The elevation was not significant at 10 min after training and had disappeared in 3 h. The elevation could not be induced merely by making the birds taste the aversive stimulus, methylanthranilate, whilst agents which disrupted acquisition and memory, ouabain and cyclo heximide, also abolished the training-induced rise in [ 3 H]quinuclidinyl benzilate binding activity. We suggest that increased [ 3 H]quinuclidinyl benzilate binding in the chick forebrain is a correlate of the early phases of memory fixation.

Complex Roles of Glutamate in the Gibbs—Ng Model of One-trial Aversive Learning in the New-born Chick

Glutamate is the most widespread excitatory transmitter in the CNS and is probably involved in LTP, a neural phenomenon which may be associated with learning and memory formation. Intracerebral injection of large amounts of glutamate between 5 min and 2.5 min after passive avoidance learning in young chicks inhibits short-term memory, which occurs between 0 and 10 min post-learning in a three-stage model of memory formation first established by Gibbs and Ng(25) [Physiol. Behav. 23:369-375; 1979]. This effect may be attributed to non-specific excitation. Blockade of glutamate uptake by L-aspartic and beta-hydroxamate also abolishes this stage of memory, provided the drug is administered within 2.5 min of learning. Interference with either production of percursors for transmitter glutamate in astrocytes or with glutamate receptors is also detrimental to memory formation, but the effects appear much later. After its release from glutamatergic neurons, glutamate is, to a large extent, accumulated into astrocytes where it is converted to glutamine, which can be returned to glutamatergic neurons and reutilized for synthesis of transmitter glutamate, and partly oxidized as a metabolic substrate. The latter process leads to a net loss of transmitter glutamate which can be compensated for by de novo synthesis of a glutamate precursor alpha-ketoglutarate (alpha KG) in astrocytes, a process which is inhibited by the astrocyte-specific toxin fluoroacetate (R. A. Swanson, personal communication). Intracerebral injection of this toxin abolishes memory during an intermediate stage of memory processing occurring between 20 and 30 min post-training (50) [Cog. Brain Res, 2:93-102; 1994]. Injection of methionine sulfoximine (MSO), a specific inhibitor of glutamine synthetase, which interferes with the re-supply of transmitter glutamate to neurons by inhibition of glutamine synthesis in astrocytes, has a similar effect. This effect of MSO is prevented by intracerebral injection of glutamate, glutamine, or a combination and alpha KG and alanine. MSO must be administered before learning, but does not interfere with acquisition since short-term memory remains intact. Administration of either the NMDA antagonist AP5, the AMPA antagonist DNQX, or the metabotropic receptor antagonist MCPF, also induces amnesia. Memory loss in each case does not occur until after 70 min post-training, during a protein synthesis-dependent long-term memory stage which begins at 60 min following learning. However, to be effective, AP5 must be administered within 60 s following learning, MCPG before 15 min post-learning, and DNQX between 15 and 25 min after learning. Together, these findings suggest that learning results in an immediate release of glutamate, followed by a secondary release of this transmitter at later stages of processing of the memory trace, and that one or both of these increases in extracellular glutamate concentration are essential for the consolidation of long-term memory. Since both fluoroacetate and MSO act exclusively on glial cells, the findings also show that neuronal-glial interactions are necessary during the establishment of memory.

Memory formation processes in weakly reinforced learning.

Day-old chicks trained on a single-trail passive avoidance learning task, with varying concentrations of the aversive stimulus (methyl anthranilate), truncated retention functions for low concentrations. The retention function for a 20% v/v dilution of methyl anthranilate in absolute ethanol yielded high retention levels until approximately 40 to 45 minutes following learning. This retention function appears to consist of only the short-term and intermediate (phase A) memory stages of Gibbs and Ngs three-stage model of memory formation, with the short-term stage susceptible to inhibition by monosodium glutamate, and the intermediate stage by ouabain and dinitrophenol. The results suggest that processing of memory into the relatively permanent long-term stage may depend on the strength of the reinforcer in aversive learning.