Amino acids and memory consolidation in the cricket. II: Effect of injected amino acids and opioids on memory

Involvement of amino acids, opioids, nitric oxide, and NMDA receptors in learning and memory consolidation in crickets

The effect of injections of selected amino acids and of N-methyl-D-aspartate (NMDA); morphine; and NMDA, nitric oxide (NO), and opioid inhibitors given before a maze-learning was investigated. Thirsty crickets (Pteronemobius sp) were trained to turn only to one side of a symmetrical Y-shaped maze using reinforcements with water. The insects retained the learned task 24 h later. N2 anoxia applied immediately after training produced retrograde amnesia. Injections of alanine (Ala), arginine (Arg), glutamine (Gln), morphine, or NMDA prior to training blocked the amnesic action of anoxia. Naloxone, an opioid antagonist, blocked long-term memory formation, but not learning, whereas hemoglobin or 2-amino-5-phosphonovaleric acid (APV), NO and NMDA antagonists respectively, blocked both. The antiamnesic effect of Morphine and Arg, but not that of Ala or NMDA was blocked by naloxone. The results suggest involvement of NMDA receptors and NO and thus of long-term potentiation phenomena in learning and in memory consolidation, whereas other neuromodulatory systems related to Arg, and opiate receptors, are only involved in memory consolidation.

The opiate system in invertebrates

The presence in diverse species of a similar mode of communication, that of a soluble messenger binding to a receptor, raises the question as to whether the specific components of this system are equally widespread. Do invertebrates use the same hormones and receptors as vertebrates do? Invertebrates ranging from unicellular organisms to insects have been shown to contain opiate-like peptides and binding sites, and they exhibit biological responses to opiates. However, critical genetic data are lacking. It is not known how signal systems arise phylogenetically, but it is conceivable that signal molecules that are already present cause the formation of their own receptors from membrane proteins.

Amino acid levels during learning and memory consolidation of an aversive conditioning task in crickets

We followed the titer of free amino acids in nervous ganglia and hemolymph of the cricket Pteronemobius sp. at different times during and after a shock avoidance training that included one experimental group and three controls. The results showed that Tau, urea, Thr, His, GABA, and an unidentified compound (Q) increased their titer in ganglia and hemolymph during training, whereas Ala, Arg, Val, Glu, Ser, and one or all of the group formed by Cys, Phe, Ile, Leu, and Trp decreased theirs concomitantly to memory consolidation. The difference in the rate of experimental insects and their yoked slaves to consolidate the learned task was reflected in the changes of the titers of the amino acids mentioned above. The data add to the evidence for a direct involvement of these amino acids in modulating the memory consolidation process.

Morphine analgesia, tolerance and addiction in the cricket Pteronemobius sp. (Orthoptera, Insecta)

The escape reaction time (ERT) of the cricket Pteronemobius sp. from the heated box begins at 48 degrees and increases with temperature until 56 degrees C, beyond which there is no further increase. The ERT (2.2 +/- 1.39 s) from the hot box at 54 degrees C is used as a model for studying the analgesic effects of opiates. Results of the present paper show that the ERT did not change after injecting the insect in the abdominal haemocoel with 0.9% saline solution, but ERT increased when 0.32, 0.52 or 0.69 mg/g of morphine is injected in the same place. The maximum ERT increase is reached at 90 min after drug injection, and the drug effect disappears 3 h after the injection. At 90 min after drug injections, the dose of 0.50 mg/g of morphine produces 50% of ERT increase, and it is referred to as the median analgesic dose (D50). 1.05 mg/g of morphine produces an ERT longer than 30 s that results in an irreversible damage to the insect. Sixty-four micrograms/g of naloxone given in addition to D50 of morphine fully blocked the effect of morphine during its 3-h action. However, more than 64 micrograms/g of naloxone alone also increase the ERT in the cricket, similar to what has been described for vertebrates. Four daily morphine injections of D50 decreased ERT in such a way that, at the fourth day, the ERT is similar to the ERT produced by saline solution; i.e., tolerance is shown. The suppression of daily morphine injections of D50 during the fifth day produced a hyperresponse to vibration (big jumps) not shown in the case of the injections of saline solution; i.e., addiction is shown.

Endogenous opiates: 1990

This paper, an examination of works published during 1990, is thirteenth in a series of our annual reviews of the research involving the behavioral, nonanalgesic, effects of the endogenous opiate peptides. The specific topics this year include stress; tolerance and dependence, eating; drinking; gastrointestinal, renal, and hepatic functions; mental illness; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; locomotor activity; sex, pregnancy, development, and aging; immunological responses; and other behavior.

Amino acids and memory consolidation in the cricket. I: Changes in the titer of free amino acids in nervous tissue after learning

The involvement of certain amino acids in the memory consolidation process was investigated in the cricket Pteronemobius sp. Thirsty crickets were trained to constantly turn to one side of a symmetrical Y-shaped maze using reinforcement with water. Controls were trained to turn to both sides of the maze according to a random program. Animals were sacrificed immediately after training and free amino acid fractions were isolated from whole brain, subesophagic, prothoracic, mesothoracic and metathoracic ganglia homogenates and analyzed by high pressure liquid chromatography. A complex pattern of variation in the titer of amino acids emerged after learning, where the changes differed among the various ganglia. The most conspicuous change was an increase in the levels of urea and an amino acid-like compound related to the urea cycle, in all ganglia except the subesophagic one, if compared to controls. Arginine increased in the subesophagic ganglion, but decreased significantly in the metathoracic ganglion. The variation of ganglionic amino acid levels and its possible relation to mnemonic processes is discussed.