Highlights Daily injection of serotonin before a training session accelerated defensive reflex conditioning in snails.Daily injection of 5-hydroxytryptophan before a training session in snails with a deficiency of serotonin induced by the “neurotoxic” analog of serotonin 5,7-dihydroxytryptamine, restored the ability of snails to learn.After injection of the “neurotoxic” analogs of serotonin 5,6- and 5,7-dihydroxytryptamine as well as serotonin, depolarization of the membrane and decrease of the threshold potential of premotor interneurons was observed.We studied the role of serotonin in the mechanisms of learning in terrestrial snails. To produce a serotonin deficit, the “neurotoxic” analogs of serotonin, 5,6- or 5,7-dihydroxytryptamine (5,6/5,7-DHT) were used. Injection of 5,6/5,7-DHT was found to disrupt defensive reflex conditioning. Within 2 weeks of neurotoxin application, the ability to learn had recovered. Daily injection of serotonin before a training session accelerated defensive reflex conditioning and daily injections of 5-HTP in snails with a deficiency of serotonin induced by 5,7-DHT restored the snail's ability to learn. We discovered that injections of the neurotoxins 5,6/5,7-DHT as well as serotonin, caused a decrease in the resting and threshold potentials of the premotor interneurons LPa3 and RPa3.

We consider the evolution of an isentropic thermal instability in the atomic zone of a photodissociation region (PDR). In this zone, gas heating and cooling are associated mainly with photoelectric emission from dust grains and fine-structure lines ([C ii] 158, [O i] 63, and [O i] 146 µm), respectively. The instability criterion has a multiparametric dependence on the conditions of the interstellar medium. We found that instability occurs when the intensity of the incident far-ultraviolet field G 0 and gas density n are high. For example, we have 3 × 10 3 < G 0 < 10 6 and 4.5 × 10 4 < n < 10 6 cm −3 at temperatures 360 < T < 10 4 K for typical carbon and oxygen abundances ξ C = 1.4 × 10 −4 and ξ O = 3.2 × 10 −4 . The instability criterion depends on the relation between ξ C and ξ O abundances and line opacities. We also give examples of observed PDRs where instability could occur. For these PDRs, the characteristic perturbation growth time is t inst ∼ 10 3 -10 4 yr and the distance characterizing the formation of secondary waves is L ∼ 10 −3 -5 × 10 −2 pc. For objects that are older than t inst and have sizes of the atomic zone larger than L, we expect that instability influences the PDR structure significantly. The presence of multiple shock waves, turbulent velocities of several kilometers per second and inhomogeneities with higher density and temperature than the surrounding medium can characterize isentropic thermal instability in PDRs.

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We report here a study of the effects of blockade of protein synthesis with anisomycin during reactivation of a contextual conditioned reflex in the common snail. The amplitudes of the defensive reactions of snails to standard tactile stimulation before training were identical in two conditions: 1) testing of responses of snails fixed by the shell to a plastic ball floating in water and 2) on the surface of the terrarium glass. After applying electric shocks to the snails' skin for 5 days, a significant difference in responses reflecting the formation of a contextual conditioned reflex was seen in only one of the contexts. Placing trained snails in the same context (reminding) two days after training with simultaneous injection of anisomycin led to significant weakening of training, while control injections of physiological saline produced no such changes. These data suggest that the mechanisms of memory consolidation after training and reminding are not identical.