“Cubos y prismas rectos”

First, the application of the conditioning stimuli (100 pulses at 50 Hz, x 4 at 0.1 Hz), in control medium, does not induce lasting synaptic plasticity (Otani et al., 1998, 1999; Fig. 1B). The same stimuli, however, induce LTD when delivered at the end of a 10-15 min bath-application of dopamine

in ascorbic acid) (Otani et al., 1998, 1999; Fig. 1C). A series of our in-depth studies revealed the underlying mechanisms of this dopamine- facilitated LTD as follows (see also Otani et al., 2003 for review).

train (100 pulses at 50 Hz). In short-burst protocol, stimuli consisted of five or six episodes (8 – 10 sec interval) of 80 or 180 ms duration of stimuli (5 or 10 pulses at 50 Hz). An increase or decrease of the synaptic responses was expressed as a percent change of the initial rising slope period from response onset) from baseline level. Synaptic responses evoked during the conditioning stimuli were recorded in magnetic tape for off-line analyses.

2.2 LTD: Previous Findings

Dopamine acts on both D1-like and D2-like receptors. Stimulation of either class of receptors appears sufficient to facilitate LTD induction. The dopamine-facilitated LTD is NMDA (N-methyl-D-aspartate) receptor-independent.

Induction of the dopamine-facilitated LTD requires postsynaptic depolarization during conditioning. Indeed, dopamine enhances postsynaptic responses during high-frequency conditioning stimuli. Induction of the dopamine-facilitated LTD requires synaptic activation of both group I and group II metabotropic glutamate receptors (mGluRs) during conditioning.

A mechanism of the cooperation between dopamine receptors and the mGluRs is convergent postsynaptic activation of mitogen-activated protein kinases (MAP kinases).

Mechanisms of group II mGluR involvement include postsynaptic activation of phospholipase C and consequential protein kinase C activation and internal release (Otani et al., 2002).

2.3 LTP: Previous Findings

In our slice condition, the conditioning stimuli coupled to a dopamine bath-application always induced LTD and never induced LTP. By contrast, in the anesthetized rats, release of dopamine in the PFC by ventral tegmental stimulation facilitates LTP (Gurden et al., 1999, 2000). We reasoned that the lack of baseline dopamine receptor stimulation in our slice condition may be a source for the discrepancy. For example, in freely moving rats, ventral

tegmental neurons discharge spontaneously and raise their firing rate phasically at various behavioral occasions (Kosobud et al., 1994; Kiyatkin and Rebec, 1998). In fact, dopamine concentration in rat PFC is tonically maintained (Takahata and Moghaddam, 2000) and increases phasically during conditioned and unconditioned appetitive tasks (Bassareo and Di Chiara, 1997) as well as during conditioned and unconditioned aversive tasks (Feenstra et al., 2001). Under anesthesia also, there is always a basal level of dopamine in the PFC (Gurden et al., 2000). In contrast, we usually allow brain slices to recover from the dissection insult for 3 hours or longer. Because the axons of dopaminergic afferents are severed in our condition, the dopamine receptors are largely left unstimulated during this period. Routinely, we detect no effects of dopamine receptor antagonists on baseline synaptic responses, which should otherwise occur if dopamine receptors are stimulated, since the presence of dopamine reduces synaptic responses (Otani et al., 1998).

We tested therefore in our slice preparation the effect of prior application of dopamine on later plasticity induction (Blond et al., 2002). First, we bath- applied dopamine identically as in our previous studies. When the responses recovered from the acute transient depression by the dopamine, dopamine was applied for the second time, and this second dopamine was coupled to 50 Hz stimuli. This procedure induced LTP (Fig. 1D). The inhibitory effect of the second dopamine on the synaptic responses is always smaller than the first dopamine (–4.3 ± 4.6% vs –33 ± 5.5%, p<0.0025), suggesting that the first dopamine triggers some lasing "priming" intracellular effects (see Otani et al., 2003).

2.4 Present Focus

In this chapter, based on the following reasons, we will focus on the point 3 in 2.2, i.e. the dopamine effect on postsynaptic responses during conditioning stimuli. We included some newly conducted experiments.

1.

2.

This dopamine effect has not been thoroughly analyzed in our past studies (Otani et al., 1998, 1999).

This effect suggests that signal-to-noise ratio might be exaggerated by dopamine. Thus, dopamine severely reduces (by about 40%; Otani et al., 1998, 1999) synaptic responses to single 0.033 Hz stimuli (Fig. 1B). A reduction of the low-frequency synaptic responses and an enhancement of high-frequency synaptic responses might mean that in the presence of dopamine, "background noise" is filtered while "significant" events are amplified.

By contrast, dopamine prolonged postsynaptic depolarization during 50 Hz stimulus train. We computed the amount of depolarization every 100 ms following a train onset (calculated in 10 ms windows). This generates twenty voltage values from one neuron for one train episode. We then drew mean decay curve of the voltage values in each experimental group for each of the four train episodes. Figure 2B shows that dopamine prolonged postsynaptic depolarization in the first train episode compared to control (two-way ANOVA, F(19,342)=1,743, p<0.03). The effect is relatively small but consistent. Significant interactions were not seen in later train episodes, but two-tailed t-test revealed that the depolarization 400 ms after train onset in

3. Other studies report dopaminergic enhancement of postsynaptic depolarization through the modulation of tetrodotoxin-sensitive, slowly- inactivating persistent current (Yang and Seamans, 1996; Gorelova and Yang, 2000; but see Geijo-Barrientos and Pastore, 1995; Gulledge and Jaffe, 1998). However, in these studies, the effect was tested by direct postsynaptic current injection. Detailed analyses of the dopamine effect with synaptically-applied input will add important information to the literature.

3.

EFFECTS OF DOPAMINE ON POSTSYNAPTIC