Serotonin receptor expression in human prefrontal cortex: balancing excitation and inhibition across postnatal development

Abstract

Serotonin and its receptors (HTRs) play critical roles in brain development and in the regulation of cognition, mood, and anxiety. HTRs are highly expressed in human prefrontal cortex and exert control over prefrontal excitability. The serotonin system is a key treatment target for several psychiatric disorders; however, the effectiveness of these drugs varies according to age. Despite strong evidence for developmental changes in prefrontal Htrs of rodents, the developmental regulation of HTR expression in human prefrontal cortex has not been examined. Using postmortem human prefrontal brain tissue from across postnatal life, we investigated the expression of key serotonin receptors with distinct inhibitory (HTR1A, HTR5A) and excitatory (HTR2A, HTR2C, HTR4, HTR6) effects on cortical neurons, including two receptors which appear to be expressed to a greater degree in inhibitory interneurons of cerebral cortex (HTR2C, HTR6). We found distinct developmental patterns of expression for each of these six HTRs, with profound changes in expression occurring early in postnatal development and also into adulthood. However, a collective look at these HTRs in terms of their likely neurophysiological effects and major cellular localization leads to a model that suggests developmental changes in expression of these individual HTRs may not perturb an overall balance between inhibitory and excitatory effects. Examining and understanding the healthy balance is critical to appreciate how abnormal expression of an individual HTR may create a window of vulnerability for the emergence of psychiatric illness.

Figure 1. Developmental pattern of expression of Gα_i-coupled serotonin receptors: HTR1A and HTR5A.

(a) Normalized qPCR expression for HTR1A in human prefrontal cortex across postnatal development. The expression of HTR1A does not significantly change across the developmental groups (F(6,53)  = 0.6, p = 0.8). (b) Normalized qPCR expression for HTR5A in human prefrontal cortex across postnatal development. There is a significant developmental change in HTR5A expression (F(6,43) = 2.9, p<0.02), Asterisks show the significance levels of an ANOVA post-hoc Fisher LSD test (* p<0.05).

Figure 2. Developmental pattern of expression of Gα_q-coupled serotonin receptors: HTR2A and HTR2C.

(a) Normalized qPCR expression for HTR2A in human prefrontal cortex across postnatal development. The expression of HTR2A changes significantly across the developmental groups (F(6,53)  = 5.3, p<0.001) (b) Normalized qPCR expression for HTR2C in human prefrontal cortex across postnatal development. There is a significant developmental change in HTR2C expression (F(6,52)  = 4.7, p = 0.001). Asterisks show the significance levels of an ANOVA post-hoc Fisher LSD test (*p<0.05,**p<0.01).

Figure 3. Developmental pattern of expression of Gα_s-coupled serotonin receptors: HTR4 and HTR6.

(a) Normalized qPCR expression for HTR4 in human prefrontal cortex across postnatal development. The expression of HTR4 changes significantly across the developmental groups (F(6,50) = 4.6, p = 0.001). (b) Normalized qPCR expression for HTR6 in human prefrontal cortex across postnatal development. There is also a significant developmental change in HTR6 expression (F(6,50) = 4.9, p = 0.001). Asterisks show the significance levels of an ANOVA post-hoc Fisher LSD test (*p<0.05, **p<0.01).

Figure 4. Working model to illustrate the balanced developmental expression of HTRs with excitatory effects on prefrontal cortex and those with inhibitory effects.

The complex cellular localization of the HTRs and their different excitatory or inhibitory effects makes it difficult to conceptualize how the developmental changes in their expression would affect the prefrontal cortex. Several assumptions were required in order to formulate a working model that attempted to synthesize the potential consequences of these developmental changes. The mRNA expression of each serotonin receptor was normalized to its individual peak expression which was given the value of one. Receptors on which serotonin has an excitatory effect were assigned a positive contribution value while those receptors on which serotonin has an inhibitory effect were assigned a negative value. The net HTR contribution to interneuron activity predicted by this model is plotted in black and was calculated as follows: (HTR2A + HTR2C + HTR6 – HTR1A). The net HTR contribution to pyramidal neuron activity predicted by this model is plotted in gray and was calculated as follows: (HTR2A + HTR4 - HTR1A - HTR5A). The overall effect of serotonin receptors on the output from the prefrontal cortex was based on the predominant localization of each receptor. This net prefrontal effect is illustrated by the dashed line which takes into account that excitation on inhibitory interneurons would likely result in inhibition of the firing of the pyramidal cells responsible for output. Note this working model suggests a lack of a large developmental change in the predicted overall net effect of HTRs on the prefrontal cortex, despite an overall trend towards increased expression of the excitatory serotonin receptors within the first decade of life. However, the 5HTR subtypes responsible for maintaining this balance appear to change.