Showing papers by "Pavel M. Balaban published in 2000"

Retrograde signalling with nitric oxide at neocortical synapses.

Abstract: Long-term changes of synaptic transmission in slices of rat visual cortex were induced by intracellular tetanization: bursts of short depolarizing pulses applied through the intracellular electrode without concomitant presynaptic stimulation. Long-term synaptic changes after this purely postsynaptic induction were associated with alterations of release indices, thus providing a case for retrograde signalling at neocortical synapses. Both long-term potentiation and long-term depression were accompanied by presynaptic changes, indicating that retrograde signalling can achieve both up- and down-regulation of transmitter release. The direction and the magnitude of the amplitude changes induced by a prolonged intracellular tetanization depended on the initial properties of the input. The inputs with initially high paired-pulse facilitation (PPF) ratio, indicative of low release probability, were most often potentiated. The inputs with initially low PPF ratio, indicative of high release probability, were usually depressed or did not change. Thus, prolonged postsynaptic activity can lead to normalization of the weights of nonactivated synapses. The dependence of polarity of synaptic modifications on initial PPF disappeared when plastic changes were induced with a shorter intracellular tetanization, or when the NO signalling pathway was interrupted by inhibition of NO synthase activity or by application of NO scavengers. This indicates that the NO-dependent retrograde signalling system has a relatively high activation threshold. Long-term synaptic modifications, induced by a weak postsynaptic challenge or under blockade of NO signalling, were nevertheless associated with presynaptic changes. This suggests the existence of another retrograde signalling system, additional to the high threshold, NO-dependent system. Therefore, our data provide a clear case for retrograde signalling at neocortical synapses and indicate that multiple retrograde signalling systems, part of which are NO-dependent, are involved.

Optical Recording of Odor-Evoked Responses in the Olfactory Brain of the Naïve and Aversively Trained Terrestrial Snails

Abstract: Olfaction in terrestrial gastropod mollusks (snails and slugs) is an important sensory modality for locating and differentiating odors during feeding and orientation. Mollusks learn a variety of new odors. The acquisition of olfactory memory in Helix require the olfactory stimulation of the sensory epithelia on the anterior tentacles, whereas the recall of memory needs posterior tentacles to be intact (Friedrich and Teyke 1998). Pairing electric shock with food results in a strong aversion to the taste and smell of the conditioned kind of food in snail Helix (Balaban 1993). Likewise, Limax tends to avoid an area with a strong concentration of the odor that previously had been paired with noxious quinidine sulfate (Sahley et al. 1981). The procerebral lobe (PC) of cerebral ganglia in terrestrial snails is directly involved in olfactory information processing (Gelperin and Tank 1990; Chase and Tolloczko 1993). The PC constitutes the most dorsal part of the cerebral (supraesophageal) ganglion of the snail's nerve ring. Two major parts can be distinguished in the PC: a neuropile and the cell body layer. The neuropile occupies the medial and core position, while the cell body layer joins the neuropile at the lateral side (Zaitseva 1991; Chase and Tolloczko 1993). The borders of the cell body layer partially surround the neuropile on the ventral and dorsal sides. A transverse section of the procerebral cell body layer shows a half-moon shape, whereas the neuropile is nested (Ermentrout et al. 1998). The PC of the terrestrial mollusk contains up to 80% of all the neurons of the central nervous system (CNS). The procerebral neurons are the smallest in the entire nervous system (5–7 μm). They are mostly monopolar, sending the neurites into the neuropile of PC (Zaitseva 1991; Chase and Tolloczko 1993). The spontaneous or stimulus-induced oscillations are common in olfactory systems of both vertebrates and invertebrates. Odor-evoked 20–40-Hz oscillations have been found in mushroom bodies and antennal lobes of insects (Laurent and Davidowitz 1994). Moreover, Wehr and Laurent (1996) demonstrated that odor attributes may be coded in the temporal sequences of firing in oscillating neuronal assemblies of the antennal lobe. Oscillatory olfactory neurons were described in the protocerebrum of the crayfish (Mellon et al. 1992). Oscillations were recorded optically in the olfactory lobe of turtles. Each inspiration of a mammal is followed by several cycles of high-frequency (60–90 Hz) oscillations of a local field potential in the granular cell layer of the olfactory bulb (Gray and Skinner 1988). Low-frequency (∼1 Hz) spontaneous oscillations of the local field potential have been described in the PC of the slug Limax maximus (Gelperin and Tank 1990). The waves of activity propagate across the PC from the apical end toward the base. The front of the wave is presented by a linear zone of depolarization followed by a larger and higher-amplitude zone of hyperpolarization. The propagation velocity is ∼1.1 mm/sec (Delaney et al. 1994). It was shown that odor application causes the electrical activity in the procerebral lobe of the slug Limax to switch transiently from the state of propagating waves to a state with no or only a small phase difference along the lobe (Delaney et al. 1994). Application of different odors in different concentrations induces transient changes in PC oscillations that differ remarkably from each other (Gervais et al. 1996). In this work, we analyzed responses to odor application in the PC of the terrestrial snail Helix, using absorption voltage-sensitive dyes and a 124-photodiode array for optical recording of electrical events. To estimate the role of spontaneous oscillations in odor encoding, we trained the snails to avoid the odor cineole using paired presentations of cineole and electric shock. Differences in parameters of the PC activity in two groups of snails subjected to explicitly unpaired and paired procedures reflected changes in neural firing in certain areas of the olfactory lobe.