Coordinate synaptic mechanisms contributing to olfactory cortical adaptation

Abstract

Anterior piriform cortex (aPCX) neurons rapidly filter repetitive odor stimuli despite relatively maintained input from mitral cells. This cortical adaptation is correlated with short-term depression of afferent synapses, in vivo. The purpose of this study was to elucidate mechanisms underlying this nonassociative neural plasticity using in vivo and in vitro preparations and to determine its role in cortical odor adaptation. Lateral olfactory tract (LOT)-evoked responses were recorded in rat aPCX coronal slices. Extracellular and intracellular potentials were recorded before and after simulated odor stimulation of the LOT. Results were compared with in vivo intracellular recordings from aPCX layer II/III neurons and field recordings in urethane-anesthetized rats stimulated with odorants. The onset, time course, and extent of LOT synaptic depression during both in vitro electrical and in vivo odorant stimulation methods were similar. Similar to the odor specificity of cortical odor adaptation in vivo, there was no evidence of heterosynaptic depression between independent inputs in vitro. In vitro evidence suggests at least two mechanisms contribute to this activity-dependent synaptic depression: a rapidly recovering presynaptic depression during the initial 10-20 sec of the post-train recovery period and a longer lasting (approximately 120 sec) depression that can be blocked by the metabotropic glutamate receptor (mGluR) II/III antagonist (RS)-alpha-cyclopropyl-4-phosphonophenylglycine (CPPG) and by the beta-adrenergic receptor agonist isoproterenol. Importantly, in line with the in vitro findings, both adaptation of odor responses in the beta (15-35 Hz) spectral range and the associated synaptic depression can also be blocked by intracortical infusion of CPPG in vivo.

Figure 1.

Piriform cortex adaptation to 50 sec odor exposure, in vivo. A, Example of an in vivo, intracellular recording from a layer II/III aPCX neuron during a 50 sec exposure to the odorant eugenol. The first 20 sec of the odorant stimulus are shown on the left. In this cell, eugenol evoked a single spike followed by several seconds of respiration entrained subthreshold oscillations. On the right, membrane potential averages triggered off the respiratory cycle are shown (inhalation is up in the respiration trace). Note the large respiration-linked odor-evoked depolarization during the first 5 sec of odor stimulation, which is mostly absent by the end of the 50 sec stimulus. The horizontal line in these traces corresponds to the -76 mV resting membrane potential. B, Mean respiration-entrained PSP peak-to-peak amplitude before and during 50 sec odorant exposure (n = 15 cells). Note the rapid decrease in response magnitude over the first 20 sec. C, LOT-evoked, intracellularly recorded EPSPs were monitored before and after 50 sec of odorant stimulation (A, B). EPSP initial slope was significantly depressed immediately after odor offset (asterisk signifies significantly different from baseline, p < 0.05) and recovered within 100 sec (n = 15 cells). Inset shows a representative example of in vivo, intracellularly recorded, LOT-evoked EPSPs before and after 50 sec of odor stimulation. Bars indicate duration of odor exposure.

Figure 2.

In vitro, 50 sec of simulated odor experience (train stimulation as described in Materials and Methods) results in aPCX synaptic depression. A, Representative example of an individual in vitro test run with extracellular fEPSPs before and after train stimulation. B, Average fEPSP data for 27 slices showing aPCX cortical adaptation. Asterisk signifies significant difference from baseline (p < 0.05). C, Within-train development of cortical adaptation recorded intracellularly in layer II/III aPCX neurons in vitro (n = 7 cells). Waveforms are examples of intracellular responses to the first LOT train stimulus and after 10 and 50 sec of trains repeated at 0.5 Hz. Bars indicate duration of trains stimulation.

Figure 3.

Overlay of in vivo odor exposure-induced LOT synaptic depression (connected filled squares; data from Fig. 1C), in vitro train-induced LOT synaptic depression (connected open squares; data from Fig. 2B), and in vivo odor exposure-induced odor-evoked PSP depression (filled circles and trend line; adapted from Wilson, 1998b) showing similarity of recovery time course for each paradigm. Both the time course and magnitude of in vitro train-induced depression and in vivo odor-evoked depression were similar (see text). The in vivo LOT-evoked EPSPs did not depress to the same extent as in the other two paradigms, presumably because of the fact that only a small subset of electrically activated LOT synapses would have been activated by the habituating odor exposure. Bar indicates duration of odor exposure or trains stimulation.

Figure 4.

In vitro synaptic depression is not dependent on stimulus intensity but is dependent on stimulus duration. A, Magnitude and time course of recovery of in vitro cortical adaptation are independent of afferent stimulus intensity. Synaptic depression during the first minutes post-train does not differ between groups after 50 sec of train stimulation at varying stimulus intensities (n = 6 slices per intensity). B, Varying the duration of train stimulation significantly altered the magnitude of synaptic depression during the first minutes post-train between groups, with 10 sec insufficient to induce depression (n = 6 slices per duration). Asterisks signify significant differences between the 10 sec duration group and all other groups (p < 0.05). Bars indicate duration of trains stimulation.

Figure 5.

In vitro synaptic depression is homosynaptic. LOT-evoked, intracellularly recorded EPSPs were monitored before and after a 50 sec train stimulation to the trained pathway, whereas the untrained pathway was left unstimulated. A, Intracellular EPSPs recorded after test stimuli given to the trained and untrained pathways in a single cell before and within the first 30 sec after the train protocol. B, Train-induced depression was selective to the stimulated pathway (n = 8 cells). An asterisk signifies significant difference from baseline (p < 0.05). Bar indicates duration of trains stimulation.

Figure 6.

In vitro PPF was significantly decreased immediately post-train but returned to baseline before recovery of cortical adaptation (n = 9 slices). Asterisks signify significant differences in PPF from baseline (p < 0.05). Horizontal lines represent baseline levels for PPF (top line) and fEPSP (bottom line). Bar indicates duration of trains stimulation.

Figure 7.

Blockade of mGluR II/III receptors with CPPG or activation of noradrenergic β-receptors reduces train-induced depression. A, CPPG application (n = 9 slices) had no effect on baseline response slope but resulted in a significant reduction in post-train depression, with fEPSPs returning to baseline within 30 sec post-train. Asterisks signify significant difference between drug conditions (p < 0.05). B, CPPG had no effect on within-train response magnitude. C, Activation of noradrenergic β-receptors with isoproterenol in vitro had no effect on baseline fEPSP response slope but significantly reduced post-train synaptic depression (n = 9 slices). Asterisks signify significant difference between drug conditions (p < 0.05). D, Isoproterenol had no effect on within-train response magnitude. These data suggest that the onset and early phase of cortical adaptation rely on a different mechanism than the later phases, which are blocked by both CPPG and isoproterenol. Bars indicate duration of trains stimulation.

Figure 8.

Blockade of mGluR II/III receptors with CPPG in vivo decreases the extent of post-odor synaptic depression and adaptation of odor responses. A, Example of an odor response recorded in aPCX layer during the initial portion of the 50 sec conditioning odor. B, Response to the 2 sec test odor given 50 sec after the cessation of the conditioning odor in the control animal shown in A. C, LOT shock-evoked response slopes in aPCX were depressed after 50 sec of odor exposure. CPPG infusion significantly decreased the extent of synaptic depression. Asterisks signify significant difference between drug conditions (p < 0.05). D, Fifty seconds of odor presentation resulted in adaptation to a 2 sec odor given 50 sec after the conditioning odor. CPPG infusion significantly decreased the extent of odor adaptation. Asterisks signify significant difference between drug conditions (p < 0.05). Bars indicate duration of odor exposure.