Dynamic Network Connectivity (DNC)

Our recent iontophoretic studies have shown that the persistent firing of dlPFC Delay neurons is highly dependent on NMDA receptors, including those with NR2B subunits. These NMDA receptos are found exclusively within the synapse, and are not extrasynaptic as they are in hippocampus. Iontophoresis of an NMDA-NR2B receptor blocker markedly reduced persistent firing (shown in red), while iontophoresis of an AMPA antagonist had much more subtle effects (green). In contrast, Response cells showed loss of firing to AMPA blockade (not shown). These physiological data are consistent with computational models by Xiao-Jing Wang predicting that persistent neuronal firing requires the slower kinetics of the NR2B receptor, while the faster kinetics of the AMPA receptor cause network collapse.

If you are interested: Wang MJ, Yang Y, Wang CJ, Gamo NJ, Jin LE, Mazer JA, Morrison JH, Wang X-J, Arnsten AF (2013) NMDA receptors subserve working memory persistent neuronal firing In dorsolateral prefrontal cortex. Neuron 77: 736-749

Systemic administration of the NMDA antagonist, ketamine, impaired working memory performance and reduced Delay cell firing. However, ketamine increased the firing of post-saccadic Response cells, similar to that seen in rodents. Thus, dlPFC neurons do not respond in a unitary way to systemic ketamine administration. This information should be kept in mind when viewing fMRI data in humans.

NMDA receptors need to sit in a depolarized membrane in order to relieve their Mg+ block and be able to transmit Na+ and Ca++. Thus, for layer III dlPFC NMDA circuits to communicate, the synapse must be depolarized. In many synapses, this function is done by AMPA receptor stimulation, but for dlPFC Delay cells, nicotinic a7 receptors (nic-a7R) seem to play that key role. Nic-a7R are localized within or next to the post-synaptic density indlPFC layer III spines, and nic-a7R stimulation enhances, while blockade reduces, Delay cell firing. Importantly, NMDA can excite Delay cells under normal conditions, but is unable to excite the cell if nic-a7R are blocked.

Relevance to State of Arousal

Acetylcholine is released when we are awake, but not in deep sleep. Thus, the lack of cholinergic stimulation of nic-a7R in dlPFC when we are asleep may explain why dlPFC circuits are unable to communicate, and why we are unconscious when in deep sleep.

Relevance to Schizophrenia

It has long been appreciated that nic-a7R are genetically altered in some families with schizophrenia, and that most patients with schizophrenia smoke as a form of self-medication. These data help to explain why genetic alterations in nic-a7R would be so devastating to dlPFC cognitive function, and why their stimulation by nicotine would be therapeutic.

If you are interested: Yang et al (2013) Nicotinic a7 receptors enhance NMDA cognitive circuits in dorsolateral prefrontal cortex. Proc Natl Acad Sci U S A. 110:12078-83.

High levels of NMDA receptor activation can generate high levels of internal calcium, and a variety of mechanisms (e.g. mGluR1/5 stimulation) can also initiate IP3 receptor mediated Ca²⁺ release from the spine apparatus (white asterisk), which is a specialization of the endoplasmic reticulum in the spine. Ca²⁺ increases cAMP generation, and cAMP-PKA in turn increases IP3 receptor calcium release, thus creating feedforward Ca²⁺-cAMP signaling.

A variety of Ca²⁺- and cAMP-PKA sensitive K⁺ channels can be found near the synapse or in the spine neck, e.g. SK, KCNQ and HCN channels. Opening of these K⁺ channels weakens the network input; e.g. low dose blockade of HCN channels strengthens delay-related firing. These mechanisms can serve as negative feedback to hold recurrent excitatory inputs in check, and can also be purposefully activated to gate network inputs or rapidly take prefrontal cortex “off-line” during stress exposure.

If you are interested:

• Wang M, Ramos B, Paspalas C, Shu Y, Simen A, Duque A, Vijayraghavan S, Brennan A, Dudley AG, Nou E, Mazer JA, McCormick DA, Arnsten AFT (2007) Alpha2A-adrenoceptor stimulation strengthens working memory networks by inhibiting cAMP-HCN channel signaling in prefrontal cortex. Cell 129: 397-410.
• Paspalas CD, Min Wang M, Arnsten AFT (2012) Constellation of HCN Channels and cAMP regulating proteins in dendritic spines of the primate prefrontal cortex - Potential substrate for working memory deficits in schizophrenia. Cereb Cortex 23: 1643-1654.
• Arnsten AFT, Wang MJ, Paspalas CD (2012) Neuromodulation of thought: Flexibilities and vulnerabilities in prefrontal cortical network synapses. Neuron 76: 223-239.

Dopamine D1R are concentrated on spines, often next to HCN channels near the synapse in layer III dlPFC circuits. Physiological data indicate an inverted U dose response (see following topic), whereby low dose D1R stimulation gates out “noise” by reducing firing to a neuron’s nonpreferred inputs, while high doses of D1R stimulation suppress all firing, e.g. as likely occurs during stress exposure. Some of these actions likely involve D1R-cAMP opening of HCN and KCNQ channels, as shown in these images.

However, in vitro studies suggest that D1R also may reduce firing by decreasing glutamate release from terminals, and by activating GABA interneurons. In our studies, fast-spiking neurons (likely GABAergic initerneurons) were decreased rather than increased by D1R stimulation, but this may have been due to the geometry of drug distribution in the tissue. The marked suppression of firing with high dose D1R stimulation likely contributes to stress-induced cognitive deficits.

If you are interested:

• Vijayraghavan S, Wang M, Birnbaum SG, Bruce CJ, Williams GV, Arnsten AFT (2007) Inverted-U dopamine D1 receptor actions on prefrontal neurons engaged in working memory. Nature Neuroscience 10: 376-384.
• Arnsten AFT, Wang MJ, Paspalas CD (2012) Neuromodulation of thought: Flexibilities and vulnerabilities in prefrontal cortical network synapses. Neuron 76: 223-239.

Feedforward Ca2+-cAMP signaling is held in check by the phosphodiesterase, PDE4A, which is anchored to the spine apparatus by DISC1 (Disrupted In SChizophrenia1). PDE4A catabolizes cAMP, and thus may be particularly important to restore dlPFC cognitive function following stress exposure when high levels of cAMP are generated, eg by dopamine D1R stimulation.

This figure shows that inhibition of PDE4 activity by etazolate markedly reduces delay-related neuronal firing in monkey dlPFC, but is rescued by low dose HCN channel blockade. As PDE4A and DISC1 can both be genetically altered in schizophrenia, loss of this key regulation may lead to weaker dlPFC function in patients as well.

If you are interested:

• Wang M, Ramos B, Paspalas C, Shu Y, Simen A, Duque A, Vijayraghavan S, Brennan A, Dudley AG, Nou E, Mazer JA, McCormick DA, Arnsten AFT (2007) Alpha2A-adrenoceptor stimulation strengthens working memory networks by inhibiting cAMP-HCN channel signaling in prefrontal cortex. Cell 129: 397-410.
• Paspalas CD, Min Wang M, Arnsten AFT (2013) Constellation of HCN Channels and cAMP regulating proteins in dendritic spines of the primate prefrontal cortex - Potential substrate for working memory deficits in schizophrenia. Cereb Cortex 23: 1643-1654.
• Arnsten AFT, Wang MJ, Paspalas CD (2012) Neuromodulation of thought: Flexibilities and vulnerabilities in prefrontal cortical network synapses. Neuron 76: 223-239.

Feedforward Ca²⁺-cAMP signaling is also inhibited by a2A-adrenoceptors (a2A-AR), which are localized next to HCN channels on spines in layer III of the primate dlPFC. They can be found near the synapse and in the thin spine neck. Blockade of a2A-AR with yohimbine reduces delay-related firing; firing is rescued by HCN channel blockade.

Conversely, stimulation of a2A-AR with guanfacine enhances delay-related firing, consistent with strengthening network connectivity. Guanfacine is now in clinical use for the treatment of a number of prefrontal disorders.

If you are interested:

• Wang M, Ramos B, Paspalas C, Shu Y, Simen A, Duque A, Vijayraghavan S, Brennan A, Dudley AG, Nou E, Mazer JA, McCormick DA, Arnsten AFT (2007) Alpha2A-adrenoceptor stimulation strengthens working memory networks by inhibiting cAMP-HCN channel signaling in prefrontal cortex. Cell 129: 397-410.
• Arnsten AFT, Wang MJ, Paspalas CD (2012) Neuromodulation of thought: Flexibilities and vulnerabilities in prefrontal cortical network synapses. Neuron 76: 223-239.