A Focus on the Functions of Area 25

Abstract

Subcallosal area 25 is one of the least understood regions of the anterior cingulate cortex, but activity in this area is emerging as a crucial correlate of mood and affective disorder symptomatology. The cortical and subcortical connectivity of area 25 suggests it may act as an interface between the bioregulatory and emotional states that are aberrant in disorders such as depression. However, evidence for such a role is limited because of uncertainty over the functional homologue of area 25 in rodents, which hinders cross-species translation. This emphasizes the need for causal manipulations in monkeys in which area 25, and the prefrontal and cingulate regions in which it is embedded, resemble those of humans more than rodents. In this review, we consider physiological and behavioral evidence from non-pathological and pathological studies in humans and from manipulations of area 25 in monkeys and its putative homologue, the infralimbic cortex (IL), in rodents. We highlight the similarities between area 25 function in monkeys and IL function in rodents with respect to the regulation of reward-driven responses, but also the apparent inconsistencies in the regulation of threat responses, not only between the rodent and monkey literatures, but also within the rodent literature. Overall, we provide evidence for a causal role of area 25 in both the enhanced negative affect and decreased positive affect that is characteristic of affective disorders, and the cardiovascular and endocrine perturbations that accompany these mood changes. We end with a brief consideration of how future studies should be tailored to best translate these findings into the clinic.

Keywords: anhedonia; anticipatory arousal; area 25; autonomic; emotion; infralimbic; negative affect.

Figure 1

The connectivity of area 25 in macaques and the infralimbic cortex (IL) in rodents. In the macaque (A) area 25 has widespread efferent and afferent connections with many cortical and subcortical regions that are comparable to the efferent and afferent connections of IL in the rat (B). Afferents and efferents are depicted superimposed on orbital, medial and lateral views of the macaque and rat brains based on anterograde and retrograde tracing studies. Macaque: [1,4,5,6,7,8,9,10,11,12,13,14,15]. Rat: [4,6,9]. Abbreviations: AC, Anterior Cingulate; Acb, Accumbens; AIa/p, Agranular Insula anterior/posterior; Amyg, Amygdala (AA, anterior amygdala; AB, Accessory Basal; BMA, basomedial; BLA, basolateral; Bpc/mc, Basal parvicellular/magnocellular; CE, CM, centromedial; central; COA, cortical; LA, lateral; MEA, medial; PA, posterior; TR, Amygdalo-piriform transition zone); AON, Anterior Olfactory Nucleus; BF, Basal Forebrain (BNST, Bed Nucleus of the Stria Terminalis; DB, Diagonal Band of Broca; EN, endopiriform nucleus; LS, Lateral Septum; MPO, Medial Preoptic Area MS, Medial Septum; SI, Substantia Innominata; TTv/d, ventral/dorsal Taenia Tecta); cc, Corpus Callosum; Cdm, medial Caudate; CLA, Claustrum; CLi, Central linear nucleus; CPu, CaudatePutamen; EC, Entorhinal Cortex; ECT, Ectorhinal Cortex; FPm, Frontal Polar Cortex, medial; HF, Hippocampal Formation, Hyp, Hypothalamus (AH, anterior nucleus; AHAvm, Anterior Hypothalamic area, ventromedial; DHA, Dorsal Hypothalamic area; DMH, Dorsomedial, LHA, Lateral Hypothalamic area; LPOA, lateral Preoptic area; MB, Mammillary bodies; MPOA, medial Preoptic area; Pef, Perifornical; PH, Posterior nucleus; PHA, Posterior Hypothalamic area; PeriMam, Peri-mammillary; SupraMam, Supra-mammillary; TM, Tubero-mammillary; ParaMam, Paramammillary); IL, Infralimbic;; IP, interpeduncular nucleus; OFC, Orbitofrontal cortex (LO, lateral orbital; VLO, ventrolateral orbital; VO, ventral orbital; MO, medial orbital); Opro, Orbital proisocortex; OPAI, Orbital periallocortex; Pons/Medulla (Barrington’s N, Barrington’s Nucleus; CN, Cuneiform nucleus; dlPAG, dorsolateral Periaqueductal Grey; DMNvagus, Dorsal Motor Nucleus of Vagus; DR, Dorsal Raphe; LC, Locus Coeruleus; LDT, Laterodorsal tegmental nucleus; MR, Median Raphe; NI, Nucleus Incertus; NSTract, Nucleus of the Solitary Tract; PBl/m, Parabrachial lateral/medial; RF, Reticular Formation); pg, perigenual; PL, Prelimbic; PRC, Perirhinal Cortex; RPO, Nucleus pontis oralis; RSC, Retrosplenial Cortex; SNc, Substantia nigra pars compacta; Thalamus: (Am, medial Anterior; CEs/m/dc, Central superior/medial/ densocellular; IAM, Interanteriomedial; IMD, Intermediodorsal; LD, Lateral dorsal; LH, Lateral Habenula; Lim, Limitans; LP, Lateral posterior; MDpc/mc, Mediodorsal parvocellular.magnocellular; PF, parafascicular; PM, medial Pulvinar; PT, Parataenial; PV, Paraventricular, R, Reuniens; RH, Rhomboid; sc, subcallosal; SM, Submedial; Vamc, Ventral Anterior magnocellular); STG, Superior Temporal Gyrus, STS, Superior Temporal Sulcus; TEI, TF, TH, TP, Temporal pole; VTA, Ventral Tegmental Area; Brodmann’s Areas, 9, 10, 11, 12, 13, 14, 23, 24, 25, 28, 30, 32, 35, 36, 46, TE1, TF, TH. Nomenclature of prefrontal cortex parcellation in macaque based on [15].

Figure 2

Examples of the relationship between cardiovascular and endocrine responsivity and area 25/infralimbic (IL) activity in emotional and non-emotional conditions in human, macaque, marmoset and rat. (A) In humans, (i) stimulation of area 25 via deep brain stimulation (red circles indicate stimulation loci) causes a brief hypertension followed by pronounced hypotension in anesthetized humans [34] while (ii) the administration of peripheral cortisol decreases area 25 activation to sad stimuli (brain region shaded blue) in the absence of any stressors [36]. (B) In macaques (i) stimulation of area 25 in anesthetized animals caused a brief hypertension followed by pronounced hypotension [22], similar to humans, and (ii) cortisol release correlates with ventromedial prefrontal cortex (vmPFC) activity during neutral conditions (large demarcated area), and specifically with area 25 during both stressful and neutral conditions (small demarcated area; [37]). (C) In marmosets, pharmacological inactivation of area 25 with GABA A and B agonists (muscimol and baclofen; ‘musbac’) in a neutral condition caused a reduction in heart rate (HR) and mean arterial blood pressure (MAP) that was accompanied by an increase in heart rate variability (HRV). Subdivision of the cardiac vagal and cardiac sympathetic indices (CVI and CSI) revealed that this HRV change was caused by a selective increase in the parasympathetic CVI [38]. (D) In rats (i) inactivation of IL had no effect on HR and MAP [39], unlike humans and non-human primates, but (ii) selective glucocorticoid receptor (GCR) silencing within the IL reduced activity within the open field test indicating that cortisol can modulate IL’s impact on emotional behavior [40]. Thus, whereas manipulation of area 25 in a neutral setting consistently modulates cardiovascular function in humans and non-human primates, such changes are not apparent after IL manipulation in the rat. However, glucocorticoids appear to modulate negative emotion in both area 25 and IL indicating similarities in some functional domains, but not others.

Figure 3

Over-activation and inactivation of area 25 in marmoset has largely opposing effects on tests measuring the regulation of responsivity to rewarding and punishing stimuli. Top from left. Over-activation via presynaptic glutamatergic disinhibition increased the bias away from punishment in an approach-avoidance decision making paradigm [140], and over-activation via inhibition of the excitatory glutamate amino acid transporter (EAAT2; with dihydrokainic acid [DHK]) increased anxiety-like behavior in the human intruder paradigm, increased the break point in a progressive ratio task, left sucrose consumption unaltered, and blunted the anticipation of food reward during appetitive conditioning [141]. Bottom from left. In contrast, inactivation with GABA A and B agonists (muscimol and baclofen) decreased the avoidance of punishment during approach-avoidance decision making [140] and reversed the blunted cardiovascular responsivity to threat in high trait anxious animals [46]. Inactivation also blunted the threat-induced anticipatory increases in heart rate and vigilant scanning (not shown) during Pavlovian threat conditioning and enhanced the rate of threat extinction [38], but had no effect on reward anticipation during appetitive conditioning [141]. *, p < 0.05, ^†, p < 0.05, main effect of manipulation; error bars indicate SEM. Thus, in general, over-activation blunted appetitive responses whilst enhancing threat-induced responses whilst inactivation primarily dampened threat-induced responses.

Figure 4

Re-thinking the role of area 25 in threat extinction in humans, marmosets and rats. (A) Human neuroimaging of extinction recall has identified regions of the subcallosal zone (scACC) in which the deactivation induced by the conditioned stimulus, CS+ is blocked following successful extinction recall [160,161,162]. However, these regions of activity are generally more rostral than area 25. (B) In marmosets, inactivation of area 25 with GABA A and B agonists (muscimol and baclofen; closed circles) hastened the behavioral extinction of an aversive (rubber snake) Pavlovian conditioned association [38]. In contrast (C) IL inactivation in rats (muscimol; closed circles) impeded the behavioral extinction and extinction recall of conditioned footshock. Redrawn from Sierra-Mercado et al., [157]. *, p < 0.05; #, p < 0.05, manipulation × CS interaction; error bars indicate SEM. Arrow indicates point of inactivation.

Figure 5

A summary of the physiological and behavioral functions associated with human and monkey area 25 and rodent infralimbic (IL). These representative studies illustrate, (i) the similarities and differences in the functional effects of manipulations of area 25 in monkeys and IL in rodents; (ii) how these map onto effects in humans; and (iii) where there are gaps in our knowledge. Blue bars denote reductions in activity, red bars denote increases in activity whilst green bars denote either that the direction of effects in area 25 are unclear, or that the effects may not be specific to area 25. Thus, if red and blue bars are going in the same direction (as in rodent despair), or if the same colored bars are going in opposite directions (as in rodent anxiety), the results are inconsistent. Headings above the bars indicate physiological measure if in italics, and the behavioral paradigm if non-italic. Hatching indicates correlations rather than manipulations. In terms of physiology, there is reasonable correspondence between monkey and rodent with respect to the reductions/increases in cardiovascular activity following reductions/increases in area 25/IL activity, especially during stress; although there are exceptions (see [39]). Any correspondence with the effects of stimulation in humans is unclear, however, because the excitatory versus inhibitory effects of stimulation on area 25 activity are unknown; if indeed the effects are due to changes in area 25 at all, since effects on fibers of passage cannot be ruled out. In relation to cortisol, there is agreement between correlatory findings in humans and monkeys that indicate positive correlations between cortisol levels and area 25 activity in monkeys and area 25 functional connectivity in humans. However, this similarity does not extend to rodents [57], as radiofrequency ablation of the IL increases corticosteroids; although whether the ablation effects are specific to the IL cannot be determined. Whilst immune function is related to area 25 activity in humans, this hasn’t yet been addressed in monkeys or rodents. With respect to behavior, changes in activity in area 25 in relation to depression can be variable, but successful treatment, especially in treatment resistant patients following DBS, is very often associated with reductions in area 25 activity. In line with this, the most consistent effects in both monkey area 25 and rodent IL are the overactivation-induced anhedonia-like effects. In contrast, the effects in monkeys and rodents of area 25/IL manipulations on conditioned threat responses and their extinction appear opposite, while in rodent studies the effects on despair-like and anxiety-like behaviors are inconsistent. Abl, ablation manipulation; AP5, AP-5 (NMDA antagonist); ACTH, adrenocorticotrophic hormone; Ant, Anticipatory arousal; Bic, Bicuculline (GABA_A antagonist); CoCl, Cobalt chloride (silences activity); CoT, conditioned threat; Cort, Corticosterone; CVI, Cardiac vagal index of heart rate variability; CVS, Cardiovascular system; DBS, Deep brain stimulation; dCS, d-Cycloserine (NMDA co-agonist); DHK, Dihydrokainic acid (EAAT2 inhibitor); DM, Decision making (approach-avoidance); EPM, Elevated plus maze; Ext, Extinction; ExR, Extinction recall; Fdg, Feeding behaviour; FST, Forced Swim Test; Gen, genetic manipulation; HInt, Human intruder test; HR, Heart rate; HRV, heart rate variability; IL1β, Interleukin 1β; Les, lesion manipulation; LY/C, LY341495 (mGluR2/3 antagonist) and CGP52432 (GABA_B antagonist); M, Muscimol (GABA_A agonist); MAP, mean arterial pressure; MB, Muscimol (GABA_A agonist) and baclofen (GABA_B agonist); Md, Mood; Mot, Motivational arousal; NAff, Negative affect; NSF, Novelty suppressed feeding, OF, Open Field test; Opto, Optogenetic manipulation; PAff, Positive affect; ReExt, Re-extinction; Sad, Response to sad faces; SPT, Sucrose preference test; SSRI, Selective serotonin reuptake inhibitor; Stim, Stimulation manipulation; TST, Tail suspension test; V, Veratrine (Sodium channel activator). Numbers indicate the relevant reference.