The physiological role of 5-HT2A receptors in working memory

Abstract

Dorsolateral prefrontal cortex has an essential role in the cognitive process of working memory, dysfunction of which is considered to be a core deficit in schizophrenia. Although this cortical region is densely innervated with 5-HT2A receptors to which atypical antipsychotic drugs bind with high affinity, little is known of the influence of this serotonin receptor subtype on prefrontal function. We addressed this issue by examining the effects of iontophoresis of selective receptor ligands on prefrontal neurons possessing spatially tuned delay activity, or "memory fields," in monkeys performing a delayed-response task. Memory fields of putative pyramidal cells were attenuated by iontophoresis of 5-HT2A antagonists, which primarily produced a reduction in delay activity for preferred target locations. Conversely, 5-HT2A stimulation by alpha-methyl-5-HT or 5-HT itself, accentuated the spatial tuning of these neurons by producing a modest increase in activity for preferred target locations and/or a reduction in activity for nonpreferred locations. The agonist effects could be reversed by the selective antagonist MDL100,907, and were dose-dependent, such that high levels attenuated spatial tuning by profoundly reducing delay activity. A role for feedforward inhibitory circuitry in these effects was supported by the finding that 5-HT2A blockade also attenuated the memory fields of putative interneurons. We conclude that prefrontal 5-HT2A receptors have a hitherto unrecognized role in the cognitive function of working memory, which involves actions at both excitatory and inhibitory elements within local circuitry.

Fig. 1.

Region of recording and experimental paradigm.A, Left hemisphere view of the macaque brain showing the calculated position of the recording chamber (gray circle) over caudal areas 46 and 8a (as, arcuate sulcus; ps, principal sulcus). B,Anatomical MRI at the same rostrocaudal level as the center of the circle shown in A (∼ 27 mm anterior to ear, bar zero). The estimated lateral position of the chamber is shown inwhite with the center marked by a dashed line. C, Schematic view of the ODR task.Top left panel shows the position of the central fixation point and the possible position of the eight peripheral cues. One trial is depicted below and to the right, where the target at 135° is displayed during the cue period and the correct response is portrayed by the arrow in the response (Resp.) epoch (Pre, presaccadic epoch;Post, postsaccadic epoch).

Fig. 2.

Effects of 5-HT_2A blockade on RS neurons. A, Rastergrams and average histograms of the activity of one RS neuron (C, cue; D, delay; R, response period; bin = 50 msec). Thetop panel shows activity during the control condition in which an elevation of activity can be seen during the delay and early response period for the preferred direction at 315° (left), and only a postsaccadic response for the nonpreferred location at 135° (right). Iontophoresis of MDL100,907 (middle panel) attenuated the delay and presaccadic activity at 315° and the postsaccadic activity for the opposite target location. After drug application was ceased (bottom panel), the delay activity partially returns toward that in control. B, Mean and SE polar plots of the firing rate of the same cell during the delay period for each target location (arrow: vector angle of tuning;inner circle indicates background activity). A memory field can be seen in the control condition (top) that is diminished by the 5-HT_2A antagonist (middle). In recovery (bottom), the memory field returns in a similar shape, although smaller in size.C, Population analysis of the delay activity of 28 tuned RS cells tested with 5-HT_2A antagonists in the control (left) and drug (right) conditions. As described in Materials and Methods, the cells are first normalized for their preferred target location which is set to 0° and then the activity for each target location, relative to the preferred location is taken as a ratio to the mean for the delay activity of the cell across all target directions (shown byline). The histogram therefore depicts the dispersion of delay activity between preferred and nonpreferred target locations. A clear spatial profile can be seen in the control condition, which is highly diminished under 5-HT_2A blockade (asterisks denote significant differences from the preferred direction). D, Histogram showing the percentage change in delay activity produced by 5-HT_2Ablockade (for the same neuronal population), relative to the activity in the control condition for each target location (preferred direction again normalized to 0°). A reduction in activity can be seen for the preferred direction, greater than that for the two adjacentlocations, and a small increase in activity is evident for opposite locations in space.

Fig. 3.

Effects of 5-HT_2A stimulation on RS neurons. A, Neuronal activity of an RS neuron showing a small response during the delay period for targets at 135° but not at 315° in the control condition. Iontophoresis of α-methyl-5-HT boosts the delay activity for the preferred direction while, at the same time, it depresses activity for the opposite location. The same cell tested with MDL100,907 (bottom panel) shows a complete abolition of its previous response. B, The memory field of this cell exhibits modest tuning during the control condition (top), which is sharpened by application of the agonist (middle), but delay activity loses spatial specificity altogether after application of MDL100,907.C, Population analysis for 11 RS cells tested with the agonist reveals signs of a spatial profile in response in control (left) that is dramatically augmented by the agonist (right). Note that one cell was excluded from this analysis because it showed changes in activity for opposite directions in space between the first and second half of the delay period.D, Overall, the agonist produces a larger reduction in the delay activity for nonpreferred target locations than that for the preferred location in this population of cells. Conventions as in previous figure.

Fig. 4.

Effect of serotonin on delay activity in an RS cell. A, In the control condition the cell shows barely any distinction in its response for the 0° and 180° targets. However, on application of 5-HT at just 10 nA there is a marked enhancement of the firing of the cell that particularly accentuates the delay activity on 0° trials. Subsequent coapplication of MDL100,907 (bottom panel) dramatically reduces the firing rate and attenuates the previous selective response in the delay period. B, The delay activity of this RS cell does not show any spatial specificity in control but it develops into a significant memory field (TF = 1) when 5-HT is applied (middle panel). Coapplication of MDL100,907 produces a substantial reduction in the delay activity but sharply limits firing to a small region of space and, as a consequence, improves spatial tuning (TF = 2). Conventions as in previous figures.

Fig. 5.

Dose-dependent effects of α-methyl-5-HT. This RS cell possessed a rather broad memory field in the control condition (top), which is clearly sharpened by iontophoresis of α-methyl-5-HT at 50 nA, despite some reduction in the overall level of delay activity. When agonist application was increase to 75 nA (bottom), the delay was further reduced to the extent that the memory field was dramatically attenuated.

Fig. 6.

Effects of 5-HT_2A blockade on FS neurons. A, An FS neuron which, in the control condition (top panel), fired during the delay period on trials in which the target was at 45°, and in the postsaccadic epoch for the opponent target location (225°). Application of ritanserin gradually abolished the delay activity for the preferred target location but cue and presaccadic activity for this location, as well as the postsaccadic activity for the opposite target location, persisted longer during drug application. B, The memory field of this FS cell included responses to two adjacent targets at 45° and 90° that were both diminished after the application of ritanserin.C, A population of 13 FS cells that were tuned in the control condition showed a clear spatial profile in their delay activity that was practically abolished by 5-HT_2A blockade.D, Rather than generally reducing the activity of this cell type, the 5-HT_2A antagonists produced an overall increase in delay activity for all targets other than the preferred location in this cell population. Conventions as in previous figures.