A variety of experimental data support this hypothesis. Reductions in dopaminergic function associated with alpha-methyl-para-tyrosine administration can induce depressive symptoms in susceptible individuals(Bremner et al. 2003; Hasler et al. 2008). Conversely, dopamine receptor agonists (e.g., pramipexole) exert antidepressant effects in placebo-controlled studies(Willner 2000; Zarate et al. 2004). In MDD-subjects DA turnover appears abnormally decreased, as concentrations of the DA metabolite, homovanillic acid(HVA), consistently are reduced in the cerebrospinal fluid(CSF) and jugular vein plasma in vivo(Lambert et al. 2000; Willner 2000)--particularly in depressives who manifest psychomotor retardation or melancholic features(Asberg et al. 1984) and in the caudate and accumbens post mortem in suicide victims(Bowden et al. 1997). Neuroimaging studies of MDD showed reduced [¹¹C]L-DOPA uptake across the blood-brain-barrier(Agren and Reibring 1994) and increased striatal binding to D₂/D₃-receptor radioligands that were sensitive to endogenous DA concentrations, although these latter findings were limited to cases who showed psychomotor slowing(Ebert et al. 1996; Drevets et al. 2005). In such cases the elevated D₂/D₃-receptor binding may have reflected either reduced intrasynaptic DA concentrations, or compensatory up-regulation of D₂/D₃-receptor density or affinity(Todd et al. 1996; Laruelle and Huang 2001). Nevertheless, studies whose samples were not predominantly composed of psychomotor slowed cases found no difference in D_2/3-receptor levels during depression(Klimke et al. 1999; Parsey et al. 2001; Montgomery et al. 2007; Hirvonen et al. 2008). Similarly, some(Meyer et al. 2001) but not other(Brunswick et al. 2003; Argyelan et al. 2005; Yang et al. 2008) studies of striatal DA transporter(DAT) binding reported reduced availability in MDD-subjects versus controls.

The specific DA receptor subtypes that mediate dopaminergic function in reward processing, emotional behavior and depression remain incompletely understood, partly due to the paucity of highly selective agonists and antagonists. In mice phenotypic analysis of DA receptor knockouts identified roles for the D₁, D₂ and D₃ receptor subtypes in mediating dopamine’s effects on reward processing and/or emotional behavior. A complex role for D₁-receptors in particular was supported by both preclinical and clinical evidence. In genetically-engineered mice deletion of the D₁-receptor attenuated the reinforcing properties of rewarding stimuli [reviewed in(Holmes et al. 2004)]. Nevertheless, the euphoric effects of cocaine appeared blunted by D₁-receptor-like antagonist administration in cocaine addicts(Romach et al. 1999; Waddington et al. 2001; Holmes et al. 2004). Moreover, in rats the reduction in sucrose consumption resulting from chronic mild stress, a purported model of anhedonia, was associated with increased D₁-receptor density in the caudate-putamen(but not the accumbens or amygdala)(Papp et al. 1994), and in humans with schizophrenia, D₁-receptor antagonists alleviated “negative” symptoms such as anhedonia and amotivation(Den Boer et al. 1995; Karle et al. 1995).

With respect to other emotional states, in rats intra-amygdaloid injection of D₁-receptor antagonists exerted anxiolytic effects(de la Mora et al. 2005) and impaired retention of inhibitory avoidance learning(fear-based memory)(Lalumiere et al. 2004). Moreover, D₁-receptor knockout mice showed deficits in fear extinction and reversal learning (putative correlates of resilience to stress or adaptation to behavioral reinforcement, respectively), and abnormal long-term potentiation of synapses on PFC neurons(El-Ghundi et al. 1999; El-Ghundi et al. 2001; Huang et al. 2004). These data appeared consistent with evidence that an optimal range of D₁-receptor stimulation is required to facilitate working memory, as intracortical administration of either agonists or antagonists for D₁-receptors impairs working memory performance in rodents and primates(Arnsten et al. 1994; Williams and Goldman-Rakic 1995; Granon et al. 2000; Robbins 2000).

In MDD the D₁-receptor has been assessed by genetic, post-mortem and neuroimaging studies. Although polymorphisms in the human D₁-receptor gene have not been associated specifically with the vulnerability for developing MDD(Koks et al. 2006), one study(Severino et al. 2005)(but not another(Kato 2007)) found an association with bipolar disorder (BD). Post-mortem studies of D₁-receptor binding found no difference in the striatum(Bowden et al. 1997) or right amygdala(Klimek et al. 2002) in MDD-subjects who were unmedicated at the time of death (n=28 and 11, respectively) versus controls, although one of these found increased D₁-receptor density and reduced affinity in the accumbens in medicated suicide victims(Bowden et al., 1997). Finally, a PET-[¹¹C]SCH-23390 study reported reduced D₁-receptor binding in the striatum bilaterally in 10 depressed MDD-subjects with anger attacks(Dougherty et al. 2006). It remained unclear whether this finding would generalize to other MDD subtypes. In subjects with BD(n=10), a PET-[¹¹C]SCH-23390 study reported no difference in striatal D₁-receptor binding relative to controls, although only three of the subjects were depressed(Suhara et al. 1992). The latter study also reported reduced binding in the frontal cortex of BD-subjects, but because ~one-quarter of [¹¹C]SCH-23390 binding in the frontal cortex is attributable to 5-HT_2A receptor binding, this difference remained difficult to interpret(Ekelund J et al. 2006).

The rank order of the [¹¹C]NNC BP_ND values was caudate, putamen>AVS>amygdala>insula > ACC >cerebellum(Table 2), consistent with the relative D₁-receptor densities measured post-mortem in humans(Cortes et al. 1989).

The mean D₁-receptor BP_ND was 14% lower in the left MC of the MDD-subjects versus controls(T=2.21, p=0.03, Figure 1, Figure 2; Table 2). The magnitude of this difference was similar after removing the 3 subjects with a remote history of substance abuse (12.6%), the 7 previously-medicated MDD-subjects(13.5%) or the 5 subjects with co-morbid phobias(11%). In the remaining ROIs mean D₁-receptor BP_ND did not differ between the control and MDD groups(Table 2).

In post-hoc assessments, a main effect of hemisphere was evident in several regions (Figure 1, Figure 3), including AVS(F=87.8, p=8.01e⁻¹¹), VP(F=8.84, p=3.10e⁻²), DP(F=25.5, p=1.56e⁻⁵), MC(F=308, p=1.09e⁻⁵) and DC(F=98.5, p=1.77e⁻⁴). A group-by-gender-by-hemisphere interaction(F=7.33, p=0.011) was detected in the DP, which was accounted for by a greater interhemispheric difference in BP_ND in female controls versus female MDD-subjects(Figure 6). This interaction remained significant after including age as a covariate, or after removing from analysis the subjects who had a remote history of substance abuse(F=4.44, p=0.044), previous psychotropic medication exposure(F=6.17, p=0.020) or co-morbid phobia(F=5.83, p=0.023).

Age correlated with BP_ND in the left VP(ρ=−0.34, p=0.04), left DP(ρ=−0.33, p=0.04), right MC(ρ=−0.38, p=0.02), bilateral DC(left: ρ =−0.39, p=0.02; right: ρ =−0.38 p=0.02) and left insula(ρ =−0.42, p=0.01). The relationship between BP_ND and age showed a trend toward significance in right VP(ρ=−0.32, p=0.06), left MC(ρ=−0.29, p=0.08) and left insula(ρ=−0.32, p=0.052) but did not approach significance(p>.10) in the right DP, AVS, ACC, or amygdala. Analyses were covaried for age where relevant.

A main effect of gender on BP_ND was significant in the AVS(F=4.21, p=0.048), VP(F=7.28, p=0.043), MC(F=9.72, p=0.026), and DC(F=9.77, p=0.026) but not in the DP(F=1.38, p=0.249). There were no significant group-by-gender interaction in any region(p>0.1).

Exploratory, post hoc clinical correlations showed that BP_ND values in the left MC correlated inversely with illness duration(r= −0.53, p=0.02; Figure 4), but did not correlate significantly with depression(MADRS), anxiety(HAM-A) or anhedonia(IDS-C anhedonia subscale) severity(i.e., p>0.05). Reaction times on the RVIP correlated negatively with D₁-receptor binding in the left MC in controls(r=−0.453, p=0.007) but not MDD-subjects(r= −0.096, p=0.358). The ratio of left-to-right BP_ND values in the MC correlated inversely with IDS-C anhedonia subscale ratings(r=−0.65, p=0.0040) but not with depression or anxiety ratings(p>0.4). MDD-subjects with comorbid phobias did not differ in D₁-receptor BP_ND in the left MC than non-phobic depressives.

Depression severity correlated negatively with BP_ND in right DP(MADRS ρ=−0.61, p=0.007, Figure 6). Anxiety severity(HAM-A) correlated negatively with BP_ND in right DP(ρ=−0.48, p=0.045). Anhedonia ratings, age-at-onset, and illness-duration did not correlate significantly with BP_ND in the DP(p>.05). In right DP, MDD-subjects with comorbid phobias showed lower D₁-receptor BP_ND than non-phobic depressives(Z=−2.3, p=0.019).

The abnormalities in striatal BP_ND in MDD likely reflect alterations in D₁-receptor density and/or affinity for [¹¹C]NNC-112. Although [₁₁C]NNC-112 binding is reversible in vivo(Abi-Dargham et al. 2000), it is insensitive to displacement by endogenous dopamine(Chou et al. 1999). Among DA receptors NNC-112 is highly selective for D₁-receptors(e.g., the D₁:D₂ receptor binding ratio=4500)(Ekelund et al. 2007). The major limitation in specificity is that [¹¹C]NNC-112 is only 2–3-fold more selective for D₁-receptors than 5-HT_2A receptors(Ekelund et al. 2007). Thus in the frontal cortex, where the 5-HT_2A receptor and D₁-receptor concentrations are comparable, up to 25% of [₁₁C]NNC-112 binding is attributable to 5-HT_2A-receptor binding(Ekelund J et al. 2006; Slifstein et al. 2007). In the striatum, however, where the ratio of D₁:5-HT_2A receptor density is relatively higher, the 5-HT_2A contribution to the measured BP_ND is negligible(Slifstein et al. 2007). Thus, the abnormalities observed in the MC and DP in MDD most likely reflect differences in D₁-receptor binding.

The mean 14% decrement in D₁-receptor BP_ND in the left MC in our MDD-subjects appeared consistent with the 13% reduction in binding in the left striatum reported by Dougherty et al (2002) in MDD-subjects with anger attacks. Our data extended this previous finding by showing that the left striatal D₁-receptor binding also was decreased in a depressed sample selected more generally according to MDD criteria. We did not observe any abnormality in the right striatum, however, raising the possibility that a deficit in right striatal D₁-receptor binding may be specific to MDD-subjects with anger attacks.

Our data more specifically localized the abnormality in left striatal D₁-receptor binding in MDD to the left MC, which showed the greatest magnitude-of-difference and effect-size across the striatal ROI(figure 1, figure 3). Because of the proximity of the MC to other striatal ROI, measured signals from the MC were weakly influenced by those from the AVS and DC which were partly continuous with the MC(Drevets et al.1999, 2001). Since parts of these ROI were separated by less than the 6 mm resolution of our PET measures, demonstrating differential regional abnormalities in MDD depended upon showing that the mean difference in BP_ND was greater in the MC than in the DCA or AVS. This approach was facilitated by the low [₁₁C]NNC-112 specific binding in extrastriatal areas, which reduced the number of comparisons required across adjacent regions. The ability to assess relative differences in radiotracer concentration across conditions in ROI separated by less than the FWHM resolution is central to PET’s utility in localizing voxels of maximal difference in brain mapping studies(Fox et al. 1986; Friston et al. 1996). Because of these spatial resolution limitations, however, the non-significant trends seen in the left AVS and left DC(figure 1) are difficult to interpret since they may reflect spilling in of radioactivity from the MC(Links et al. 1996).

The MC region is the major striatal target of predominantly ipsilateral, afferent projections from the ACC and orbitofrontal cortex(OFC)(Ferry et al. 2000; Haber et al. 2006). Since the grey matter volume and/or neuronal counts in these cortical areas is reduced in MDD(Drevets et al. 2008), the regional specificity of the reduction in D₁-receptor binding to the MC raises the possibility that this abnormality reflects a reduction in afferent neuronal terminals from the cortex, and thus in the density of synapses where D₁-receptors are expressed post-synaptically. This hypothesis appears compatible with findings that in MDD both the reduction in D₁-receptor binding in the MC(figure 1) and the reduction in grey matter volume in the ACC and OFC are predominantly left-lateralized(Drevets et al. 2008). This hypothesis may also be compatible with the finding that the D₁-receptor binding in the left MC correlated inversely with illness duration(figure 4), since the reductions in ACC volume reportedly worsen across time in mood disorders(Koo et al. 2008). In contrast, perturbations in D₁-receptor binding in MDD may be less likely to reflect local changes in receptor expression associated with changes in DA release, as the severe DA deficiency state accompanying Parkinson’s Disease is not associated with changes in D₁-receptor binding or density(Pimoule et al. 1985; Shinotoh et al. 1993). Moreover, while changes in D₁-receptor regulation at the level of G-protein coupling may exist in the DA depleted striatum(Gerfen et al. 2002), such a change would not likely be evinced by corresponding changes in D₁-receptor antagonist binding to PET radioligands(Pimoule et al. 1985; Shinotoh et al. 1993) which typically bind to receptors in high versus low affinity states with comparable potency.

Post-hoc analyses revealed an altered interhemispheric ratio of D₁-receptor BP_ND that differed significantly between depressed and control females in the DP. Specifically, the depressed females showed a loss of the asymmetry in D₁-receptor binding that was evident in the controls(Figure 6). This difference was explained by D₁-receptor BP_ND values that were non-significantly lower in the left DP and non-significantly higher in the right DP in depressed females versus control females(figure 6).

In other striatal regions the D₁-receptor binding also showed highly significant laterality effects in healthy controls, which did not differ significantly between groups. In the caudate and AVS(accumbens area), the BP_ND was lower in the left hemisphere than the right. In contrast, in the putamen the direction of the laterality effect was reversed(left >right). This difference between caudate and putamen may explain the absence of laterality effects in striatal [₁₁C]SCH-23390 binding in(Dougherty et al. 2006), who combined caudate and putamen into a single ROI. Other studies employing neuroimaging or autoradiographic techniques to assess D₁-receptor binding in depressed or healthy humans(De Keyser et al. 1988; Suhara et al. 1991; Suhara et al. 1992; Bowden et al. 1997) or experimental animals(Cortes et al. 1989; Camps et al. 1990) did not examine laterality effects.

Asymmetrical D₁-receptor binding across hemispheres is consistent with several reports of lateralized function within the dopaminergic system. The amount of DA release in human striatal subregions in response to unpredicted reward(Martin-Soelch et al. 2007), DA release in the rodent caudate during voluntary behavior(Yamamoto et al. 1982), and preference for self-stimulation induced by localized amphetamine administration in rats all show prominent laterality effects(Glick et al. 1980; Glick et al. 1981). In healthy humans greater D₂-receptor availability in the left relative to the right striatum was associated with greater positive incentive motivation (Tomer et al. 2008). In right-handed humans DA concentrations were higher in the left putamen than the right(Glick et al. 1982; de la Fuente-Fernandez et al. 2000), and in marmosets levels of DA and HVA were higher in the right caudate and putamen than the left(Silva et al. 2007). Consistent with these data humans also showed higher dopamine transporter density in the left putamen and caudate versus the right, irrespective of handedness(van Dyck et al. 2002). [Although the lateralization of motor dominance(“handedness”) has been associated with variation in elements of the dopaminergic system including COMT(Savitz et al. 2007), it remains unclear whether handedness also would influence D₁-receptor BP_ND.] Finally, our finding that the loss of normal asymmetry in D₁-receptor BP_ND in MDD appeared specific to females was notable given evidence that the direction of asymmetry preference for self-stimulation in rats showed prominent sex-effects(Glick and Badalamenti 1986).

Since the mesocorticolimbic dopaminergic system plays major roles in the neural processing of reward and motivation, it is conceivable that either a deficit in dopaminergic function in the left MC, or alterations in the interhemispheric ratio of D₁-receptors that may result from such a deficit, may disrupt the normally lateralized function of this system and thereby contribute to the depressed mood, anhedonia and amotivation associated with MDD(Nestler and Carlezon 2006; Liu et al. 2008). Compatible with this hypothesis, the ratio of left-to-right D₁-receptor binding in the MC correlated inversely with the IDS-C anhedonia subscale(r=−0.65, p=0.0040). In the left MC functional abnormalities have been identified during reward processing in MDD. In independent samples of MDD-subjects the magnitude of DA release during unpredicted monetary reward and the hemodynamic response to anticipated monetary reward were abnormally blunted specifically in the left MC(Drevets 2008).

The relationship between the response latencies on a sustained attention task (RVIP) and D₁-receptor binding in the left MC in healthy controls suggests that the abnormal D₁-binding group in MDD also may play a role in the psychomotor or cognitive impairment associated with MDD. The mean response latency was 12% slower in depressives than controls, as expected(Tavares et al. 2003), although we were underpowered to establish the statistical significance of this difference. Although reaction times measured on the RVIP reflect both psychomotor speed and sustained attention, these variables generally are confounded on neuropsychological assessments of psychomotor speed. In controls the response latencies correlated inversely with BP_ND in the left MC (explaining 20.5% of the variance) implying the reduction in D₁-receptor binding in this region in MDD may contribute to the psychomotor slowing associated with depression. The absence of a normal correlation between reaction time and MC-BP_ND in the MDD group also may be compatible with this hypothesis. Notably, psychomotor slowing previously was associated with reduced dopaminergic function in the left caudate in MDD, as (Martinot et al. 2001) reported that [¹⁸F]DOPA was reduced specifically in the left caudate in MDD patients with psychomotor retardation compared to both healthy controls and MDD patients without psychomotor slowing.

The inverse correlation between D₁-receptor binding and age is consistent with the results of previous PET(Suhara et al. 1991; Iyo and Yamasaki 1993; Wang et al. 1998) or autoradiography studies(Klimek et al. 2002) conducted in humans or experimental animals(Suzuki et al. 2001). In humans Wang et al. (1998) reported a 6–7% decline in [¹¹C]SCH-23390 binding to D₁-receptors per decade in the caudate and putamen, comparable to the 3–6% decline in the BP_ND for [₁₁C]NNC-112 per decade detected herein (assessed relative to the y-intercept for predicted value at birth; figure 5). In contrast, the relationship between BP_ND and age was not significant in the amygdala, consistent with the data of Klimek et al.(2002) who observed no significant age effect on D₁-receptor density measured post mortem in the human amygdala.

A limitation of our study was that we modeled the BP_ND for [¹¹C]NNC-112 using a simplified reference tissue model to avoid arterial cannulation. This approach used the time-tissue radioactivity concentration in a reference region, the cerebellum, to estimate the concentration of free plus non-specifically bound radiotracer in all regions. However, without arterial plasma we were unable to measure the distribution volume of [¹¹C]NNC-112 in the cerebellum. Consequently, a difference in BP_ND between groups could have been accounted for by abnormal [¹¹C]NNC-112 binding either in the region-of-interest or the reference tissue. However, an abnormality of tracer uptake in the reference tissue could not account for the regionally specific group difference or the group-by-gender-by-hemisphere interaction found in the left MC and the DP, respectively.