Infralimbic prefrontal cortex structural and functional connectivity with the limbic forebrain: a combined viral genetic and optogenetic analysis

Abstract

The medial prefrontal cortex is critical for contextual appraisal, executive function, and goal-directed behavior. Additionally, the infralimbic (IL) subregion of the prefrontal cortex has been implicated in stress responding, mood, and fear memory. However, the specific circuit mechanisms that mediate these effects are largely unknown. To date, IL output to the limbic forebrain has been examined largely qualitatively or within a restricted number of sites. To quantify IL presynaptic input to structures throughout the forebrain, we utilized a lentiviral construct expressing synaptophysin-mCherry. Thus, allowing quantification of IL efferents that are specifically synaptic, as opposed to fibers of passage. Additionally, this approach permitted the determination of IL innervation on a sub-structural level within the multiple heterogeneous limbic nuclei. To examine the functional output of the IL, optogenetic activation of IL projections was followed by quantification of neuronal activation throughout the limbic forebrain via fos-related antigen (Fra). Quantification of synaptophysin-mCherry indicated that the IL provides robust synaptic input to a number of regions within the thalamus, hypothalamus, amygdala, and bed nucleus of the stria terminalis, with limited input to the hippocampus and nucleus accumbens. Furthermore, there was high concordance between structural connectivity and functional activation. Interestingly, some regions receiving substantial synaptic input did not exhibit significant increases in Fra immunoreactivity. Collectively, these studies represent a step toward a comprehensive and quantitative analysis of output circuits. This large scale efferent quantification or ‘projectome’ also opens the door for data-driven analyses of the downstream synaptic mechanisms that mediate the integrative aspects of cortico-limbic interactions.

Bed nucleus of the stria terminalis regulates fear to unpredictable threat signals

“…The amygdala, PFC, and hippocampus have strong connections with the BNST (Canteras 410 and Swanson, 1992;deCampo and Fudge, 2013;Dong et al, 2001;Glangetas et al, 2017;Johnson 411 et al, 2018;McDonald et al, 1999;Reichard et al, 2017;Reynolds and Zahm, 2005;Torrisi et 412 al., 2015;Vertes, 2004;Weller and Smith, 1982;Wood et al, 2018). In line with previous work, 413 the expression of freezing (whether elicited by a forward or, in our case, backward CS) increased 414 the number of Fos-positive neurons in these regions (Herry et al, 2008;Jin and Maren, 2015;415 Knapska and Maren, 2009;Lemos et al, 2010;Wang et al, 2016).…”

Top‐down and bottom‐up control of stress‐coping

“…This analysis of structural connectivity was combined with optogenetic activation of infralimbic glutamatergic neuronal efferents and quantification of target neurone activation by c-fos related antigen immunostaining. Such an approach revealed functional connectivity of the infralimbic PFC in particular with neurones of the medial dorsal thalamus, posterior hypothalamus and parts of the central/basolateral amygdala 124. Interestingly, some regions with extensive structural connectivity, such as BNST, showed only limited activation, but this discrepancy may be clarified when the database is extended with additional infralimbic ensembles and variation in environmental conditions.…”

“…121 Projections from infralimbic origin innervate the locus coeruleus and NTS, and also intercalate amygdala neurones that exert GABA-ergic control over the central amygdala. 122 In very recent studies, 124 concluded causality between this "loss of a GR-mediated brake on interneurone activity" and consequent increased synaptic inhibition of the PFC excitatory outflow. 122 In very recent studies, 124 concluded causality between this "loss of a GR-mediated brake on interneurone activity" and consequent increased synaptic inhibition of the PFC excitatory outflow.…”

“…122,123 Prelimbic-and infralimbic PFC regions are important for fear expression and extinction. 122 In very recent studies, 124 concluded causality between this "loss of a GR-mediated brake on interneurone activity" and consequent increased synaptic inhibition of the PFC excitatory outflow. During chronic stress, the GR-dependent hypo-activity and atrophy of the mPFC eventually may lead to a disinhibited HPA axis and increased emotional behaviour, a condition that had been achieved previously by infralimbic GR knockdown.…”

Prefrontal Cortex Regulates Chronic Stress-Induced Cardiovascular Susceptibility

“…The rodent homolog of BA25 is infralimbic cortex (IL) (Öngür et al, 2003;Uylings et al, 2003;Vertes, 2004). This subregion of ventral mPFC provides inputs to stress-integrative nuclei, including the posterior hypothalamus and brainstem autonomic centers (Gabbott et al, 2005;Myers et al, 2014aMyers et al, , 2016Wood et al, 2019). Our previous studies reduced glutamate outflow from the IL in rats undergoing chronic variable stress (CVS), promoting hyperactivation of the hypothalamic-pituitary-adrenal axis (HPA) axis .…”