Effect of an inhibitor of dopamin-beta-hydroxylase on the acquistion and retention of four different avoidance tasks in mice

Locus ceruleus activation initiates delayed synaptic potentiation of perforant path input to the dentate gyrus in awake rats: a novel beta-adrenergic- and protein synthesis-dependent mammalian plasticity mechanism

Norepinephrine, acting through beta-adrenergic receptors, is implicated in mammalian memory. In in vitro and in vivo studies, norepinephrine produces potentiation of the perforant path-dentate gyrus evoked potential; however, the duration and dynamics of norepinephrine-induced potentiation have not been explored over extended time periods. To characterize the long-term effects of norepinephrine on granule cell plasticity, the present study uses glutamatergic activation of the locus ceruleus (LC) to induce release of norepinephrine in the hippocampus of the awake rat and examines the subsequent modulation of the dentate gyrus evoked potential for 3 hr (short term) and 24 hr (long term) after LC activation. LC activation initiates a potentiation of the field EPSP slope observed 24 hr later. This late-phase potentiation of the synaptic potential is not preceded by early phase potentiation, although spike potentiation can be seen both immediately after, and 24 hr after, LC activation. Intracerebroventricular infusion of the beta-adrenergic antagonist, propranolol, or the protein synthesis inhibitor, anisomycin, before LC activation blocks the potentiation of perforant path input observed at 24 hr. The initiation of late-phase synaptic potentiation observed at 24 hr but not at the 3 hr after LC activation parallels the observation of a cAMP- and protein synthesis-dependent long-lasting synaptic facilitation in Aplysia that is not preceded by short-term synaptic facilitation. Locus ceruleus-initiated synaptic potentiation may selectively support long-term, rather than short-term, memory. The observation of selective initiation of long-term synaptic facilitation in a mammalian brain, as in invertebrates, is additional evidence that these two forms of memory depend on separable biological mechanisms.

[Memory consolidation and posttraumatic stress disorder]

Extensive evidence from animal and human studies has shown that memory formation is enhanced by an endogenous modulatory system mediated by stress hormones and activation of the amygdala. This system is an evolutionarily adaptive method of enhancing important memories. Under emotional stress, this system is activated promoting the formation of vivid, long lasting traumatic memories, which are the hallmark of PTSD. The understanding of the mechanisms underlying memory modulation might lead to an improved ability to assess and treat PTSD.

Behavioural pharmacology and its contribution to the molecular basis of memory consolidation

Recent findings have significantly advanced our understanding the mechanisms of memory formation. Most of these advances stemmed from behavioural pharmacology research involving, particularly, the localized infusion of drugs with specific molecular actions into specific brain regions. This approach has revealed brain structures involved in different memory types and the main neurotransmitter systems and sequence of metabolic cascades that participate in memory consolidation. Biochemical studies and, in several cases, studies of genetically manipulated animals, in which receptors or enzymes affected by the various drugs were absent or overexpressed, have complemented the pharmacological research. Although most studies have concentrated on the involvement of the hippocampus, many have also investigated the entorhinal cortex, other regions of the cortex, and the amygdala. Behavioural pharmacology has been of crucial importance in establishing the major neurohumoral and hormonal systems involved in the modulation of memory formation. These systems act on specific steps of memory formation in the hippocampus and in the entorhinal, parietal, and cingulate cortex. A specialized system mediated by the basolateral amygdaloid nucleus, and involving several neuromodulatory systems, is activated by emotional arousal and serves to regulate memory formation in other brain regions. The core mechanisms involved in the formation of explicit (declarative) memory are in many respects similar to those of long-term potentiation (LTP), particularly in the hippocampus. However, there are also important differences between memory formation and LTP. Memory formation involves numerous modulatory influences, the co-participation of various brain regions other than the hippocampus, and some properties that are specific to memory and absent in LTP (i.e. flexibility of response). We discuss the implications of these similarities and differences for understanding the neural bases of memory.

Additive effect of three noradrenergic genes (ADRA2a, ADRA2C, DBH) on attention-deficit hyperactivity disorder and learning disabilities in Tourette syndrome subjects

Halperin et al. (Halperin JM. Newcorn JH, Koda VH, Pick L, McKay KE, Knott P. Noradrenergic mechanisms in ADHD children with and without reading disabilities: a replication and extension. J Am Acad Child Adolesc Psychiatry 1997: 36: 1688 1696) reported a significant increase in plasma norepinephrine (NE) in attention-deficit hyperactivity disorder (ADHD) children with reading and other cognitive disabilities compared to ADHD children without learning disabilities (LD). We examined the hypothesis that ADHD + LD was associated with NE dysfunction at a molecular genetic level by testing for associations and additive effects between polymorphisms at three noradrenergic genes the adrenergic alpha2A receptor (ADRA2A), adrenergic alpha2C receptor (ADRA2C), and dopamine beta-hydroxylase (DBH) genes. A total of 336 subjects consisting of 274 individuals with Tourette syndrome (TS) and 62 normal controls were genotyped. Regression analysis showed a significant correlation between scores for ADHD, a history of LD, and poor grade-school academic performance that was greatest for the additive effect of all three genes. Combined, these three genes accounted for 3.5% of the variance of the ADHD score (p = 0.0005). There was a significant increase in the number of variant NE genes progressing from subjects without ADHD (A-) or learning disorders (LD-) to A + LD - to A - LD + to A + LD + (p = 0.0017), but no comparable effect for dopamine genes. These data support an association between NE genes and ADHD, especially in ADHD + LD subjects.

Polygenic inheritance of Tourette syndrome, stuttering, attention deficit hyperactivity, conduct, and oppositional defiant disorder: the additive and subtractive effect of the three dopaminergic genes--DRD2, D beta H, and DAT1

Polymorphisms of three different dopaminergic genes, dopamine D2 receptor (DRD2), dopamine beta-hydroxylase (D beta H), and dopamine transporter (DAT1), were examined in Tourette syndrome (TS) probands, their relatives, and controls. Each gene individually showed a significant correlation with various behavioral variables in these subjects. The additive and substractive effects of the three genes were examined by genotyping all three genes in the same set of subjects. For 9 of 20 TS associated comorbid behaviors there was a significant linear association between the degree of loading for markers of three genes and the mean behavior scores. The behavior variables showing the significant associations were, in order attention deficit hyperactivity disorder (ADHD), stuttering oppositional defiant, tics, conduct, obsessive-compulsive, mania, alcohol abuse and general anxiety-behaviors that constitute the most overt clinical aspects of TS. For 16 of the 20 behavior scores there was a linear progressive decrease in the mean score with progressively lesser loading for the three gene markers. These results suggest that TS, ADHD, stuttering oppositional defiant and conduct disorder, and other behaviors associated with TS, are polygenic, due in part to these three dopaminergic genes, and that the genetics of other polygenic psychiatric disorders may be deciphered using this technique.

Analysis of the avoidance learning deficit induced by the serotonin releasing compound p-chloroamphetamine

The effects of the serotonin-releasing compound p-chloroamphetamine (PCA, 2.5 mg/kg) on avoidance acquisition, retention and memory retrieval were examined in male Sprague-Dawley rats using a one-way active avoidance and a one-trial passive avoidance task. The drug was injected IP prior to training, following acquisition and prior to the retention test 24 hr after training using a state-dependent design. In the normal context situation pretraining administration of PCA markedly impaired active avoidance acquisition, but PCA-treated rats did not differ from controls in their retention performance when tested 24 hr after training. In the dark/light box test pretraining administration of PCA caused a dose-dependent impairment of both active and passive avoidance retention which could not be explained in terms of changes in locomotor activity or behavioural disinhibition at the time of testing or state-dependent retention. Post-training administration of PCA failed to affect avoidance retention in both tasks. The drug was found to impair memory retrieval in a dose- and time-dependent fashion in the one-way active but not in the passive avoidance test. Pretraining administration of PCA produced a progressive loss of passive and active avoidance performance at increasingly longer retention intervals. The present results suggest that serotonin has dual effects on processes underlying learning and memory involving effects on both associative and non-associative learning processes in the rat. The time-dependent loss of memory retention following 5-HT release indicates that serotonin has a role in the way information is processed in the brain.

Selective brain noradrenaline depletion induced by the neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP 4) does not prevent the memory facilitation induced by a muscarinic agonist in mice

These experiments investigated the effects of central noradrenaline (NA) depletion and its interaction with cholinergic and dopaminergic mechanisms upon retention of a passive-avoidance response in mice. The NA selective neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP 4) (50 mg/kg IP, 7 days) was injected into mice to produce depletion of NA in frontal cortex, hypothalamus, cerebellum, midbrain and brain stem without any significant change in dopamine (DA) levels in frontal cortex, striatum, hypothalamus and midbrain. Depletion of brain NA produced by DSP 4 was significantly but not completely prevented by the NA uptake inhibitor desmethylimipramine (DMI) (10 mg/kg IP, 30 min before DSP 4 injection). Despite the marked NA depletion, DSP 4 neither impaired the retention of a passive-avoidance response in mice nor prevented the enhancement of retention of this response induced by the central muscarinic agonist oxotremorine (OTM) (0.05 mg/kg IP, immediately after training. This lack of effect of DSP 4 on retention was prevented neither by DMI nor by the serotonin uptake inhibitor fluoxetine (5 mg/kg IP, 30 min before DSP 4 injection). The enhancement of retention induced by OTM in the groups of mice injected with either water or DSP 4 was prevented by atropine (0.5 mg/kg IP, 20 min before training) but not by methylatropine in the same experimental conditions. This suggests that both in controls and DSP 4-pretreated mice, the primary effect of OTM is due to an interaction with muscarinic brain receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacological investigations of neurotransmitter involvement in passive avoidance responding: a review and some new results

The roles of acetylcholine (ACh), noradrenaline (NA), dopamine (DA) and serotonin (5-HT) in passive avoidance responding are examined by reviewing previous studies of the effects on this task of drugs which alter the functioning of these neurotransmitter systems and also by presenting the results of a new study. This new study includes a number of drugs which do not seem to have been examined before, namely pilocarpine, pempidine, pentolinium, tetrabenazine, desipramine, clonidine, isoprenaline, pimozide, fluoxetine, L-tryptophan, methysergide and cyproheptadine. Because there is large variability in the effects of any one drug or class of drugs on passive avoidance responding, it is difficult to determine the exact involvement of the various neurotransmitter systems. There is also little good evidence that drug effects on performance of the passive avoidance response are caused by drug-induced changes in learning and memory processes or by state-dependent effects. Three other factors which may influence performance of the passive avoidance response-shock sensitivity, the biochemical response to stress and locomotor activity-are discussed and may be responsible for many of the drug-induced changes in passive avoidance responding.

Reversal of guanethidine- and diethyldithiocarbamate-induced amnesia by peripherally-administered catecholamines

The effect of peripherally-administered catecholamines on guanethidine- and diethyldithiocarbamate-induced amnesia of a PA training in mice was investigated. The amnesic effect of guanethidine could be blocked with 50 mg/kg DA, or 0.75 mg/kg NE when given either before, immediately, or 10 min after but not 90 min following PA. Epinephrine or a lower dose of DA could be attenuate the guanethidine-induced amnesia. The amnesic effect of diethyldithiocarbamate could be blocked with 50 mg/kg DA, 0.75 mg/kg NE or 0.5 mg/kg E when given either before, immediately or 10 ming after but not 90 min following PA. The amnesic effects of these compounds were interpreted in terms of their peripheral antiadrenergic actions.

The role of opioid peptides in memory and learning

Evidence is discussed which points to the existence of a physiologic amnesic mechanism mediated by beta-endorphin and perhaps by other opioid peptides as well. This mechanism is triggered by various forms of training and by either painful or painless stimulation. It may operate through the inhibition of central dopaminergic and beta-adrenergic systems that modulate the memory consolidation process. This amnesic mechanism in unrelated to the regulation of pain perception, and operates at opioid peptide levels several orders of magnitude below those that are needed to cause analgesia or other effects. In addition, shuttle avoidance and habituation learning seem to be dependent on a state induced by the release of beta-endorphin. It is possible that this may be related to the amnesic properties of this substance. Therefore, it appears that the endogenous opioid peptides may exert their primary function in the modulation of memory processes.

Memory facilitation by naloxone is due to release of dopaminergic and beta-adrenergic systems from tonic inhibition

The post-training IP administration of naloxone (0.8 mg/kg) facilitates memory consolidation of the habituation of a rearing response to a tone in rats. Amphetamine (1.0 - 2.5 mg/kg or nicotine (0.2 - 0.5 mg/kg), and amphetamine (2.5 mg/kg) plus nicotine (0.5 mg/kg) have no effect. The higher doses of amphetamine or nicotine, however, when given together with a dose of naloxone which is ineffective alone (0.2 mg/kg), markedly enhance consolidation. Haloperidol (0.5 mg/kg), propranolol (0.5 mg/kg), and phenoxybenzamine (2.0 mg/kg) have no effect on their own; whereas tolazoline (2.0 mg/kg) impairs consolidation. The effect of naloxone (0.8 mg/kg) is antagonized by haloperidol and by propranolol, but not by phenoxybenzamine or tolazoline. The results suggest that naloxone causes memory facilitation through the release of central dopaminergic and beta-adrenergic mechanisms from a tonic inhibitory influence of endogenous opiate peptide systems.

Effect of beta-endorphin and naloxone on acquisition, memory, and retrieval of shuttle avoidance and habituation learning in rats

Naloxone impairs acquisition of shuttle avoidance behavior (0.8 mg/kg IP) and habituation to a rearing response to a tone (1.6 mg/kg IP) in rats. beta-Endorphin (2 microgram/kg IP) has no effect on acquisition, but, when given prior to test sessions, facilitates retrieval of the two tasks. Naloxone has no effect of its own upon retrieval. In addition to these effects, the pretraining administration of beta-endorphin disrupts, and that of naloxone facilitates retention of the two tasks. The results are consistent with the hypothesis that these two forms of learning are state-dependent on the release of beta-endorphin (and, perhaps, of other opiate peptides as well), that this substance is released during training in a sufficient amount for this purpose, and that, in addition, there is a physiological amnesic mechanism mediated by opiate peptides. Furthermore, the results are also consistent with previous observations that beta-endorphin is released from the rat brain during training, but not during test sessions of the two tasks (Izquierdo et al., 1980b). The possibility is discussed that state-dependency and the amnesic effect comprise one single, rather than two separate mechanisms.