Alleviating Hypertension by Selectively Targeting Angiotensin Receptor-Expressing Vagal Sensory Neurons

Cardiovascular homeostasis is maintained, in part, by neural signals arising from arterial baroreceptors that apprise the brain of blood volume and pressure. Here, we test whether neurons within the nodose ganglia that express angiotensin type-1a receptors (referred to as NGAT1aR) serve as baroreceptors that differentially influence blood pressure (BP) in male and female mice. Using Agtr1a-Cre mice and Cre-dependent AAVs to direct tdTomato to NGAT1aR, neuroanatomical studies revealed that NGAT1aR receive input from the aortic arch, project to the caudal nucleus of the solitary tract (NTS), and synthesize mechanosensitive ion channels, Piezo1/2 To evaluate the functionality of NGAT1aR, we directed the fluorescent calcium indicator (GCaMP6s) or the light-sensitive channelrhodopsin-2 (ChR2) to Agtr1a-containing neurons. Two-photon intravital imaging in Agtr1a-GCaMP6s mice revealed that NGAT1aR couple their firing to elevated BP, induced by phenylephrine (i.v.). Furthermore, optical excitation of NGAT1aR at their soma or axon terminals within the caudal NTS of Agtr1a-ChR2 mice elicited robust frequency-dependent decreases in BP and heart rate, indicating that NGAT1aR are sufficient to elicit appropriate compensatory responses to vascular mechanosensation. Optical excitation also elicited hypotensive and bradycardic responses in ChR2-expressing mice that were subjected to deoxycorticosterone acetate (DOCA)-salt hypertension; however, the duration of these effects was altered, suggestive of hypertension-induced impairment of the baroreflex. Similarly, increased GCaMP6s fluorescence observed after administration of phenylephrine was delayed in mice subjected to DOCA-salt or chronic delivery of angiotensin II. Collectively, these results reveal the structure and function of NGAT1aR and suggest that such neurons may be exploited to discern and relieve hypertension.

Circuit and Cell-Specific Contributions to Decision Making Involving Risk of Explicit Punishment in Male and Female Rats

Decision making is a complex cognitive process that recruits a distributed network of brain regions, including the basolateral amygdala (BLA) and nucleus accumbens shell (NAcSh). Recent work suggests that communication between these structures, as well as activity of cells expressing dopamine (DA) D2 receptors (D2R) in the NAcSh, are necessary for some forms of decision making; however, the contributions of this circuit and cell population during decision making under risk of punishment are unknown. The current experiments addressed this question using circuit-specific and cell type-specific optogenetic approaches in rats during a decision making task involving risk of punishment. In experiment 1, Long-Evans rats received intra-BLA injections of halorhodopsin or mCherry (control) and in experiment 2, D2-Cre transgenic rats received intra-NAcSh injections of Cre-dependent halorhodopsin or mCherry. Optic fibers were implanted in the NAcSh in both experiments. Following training in the decision making task, BLA→NAcSh or D2R-expressing neurons were optogenetically inhibited during different phases of the decision process. Inhibition of the BLA→NAcSh during deliberation (the time between trial initiation and choice) increased preference for the large, risky reward (increased risk taking). Similarly, inhibition during delivery of the large, punished reward increased risk taking, but only in males. Inhibition of D2R-expressing neurons in the NAcSh during deliberation increased risk taking. In contrast, inhibition of these neurons during delivery of the small, safe reward decreased risk taking. These findings extend our knowledge of the neural dynamics of risk taking, revealing sex-dependent circuit recruitment and dissociable activity of selective cell populations during decision making.SIGNIFICANCE STATEMENT Until recently, the ability to dissect the neural substrates of decision making involving risk of punishment (risk taking) in a circuit-specific and cell-specific manner has been limited by the tools available for use in rats. Here, we leveraged the temporal precision of optogenetics, together with transgenic rats, to probe contributions of a specific circuit and cell population to different phases of risk-based decision making. Our findings reveal basolateral amygdala (BLA)→nucleus accumbens shell (NAcSh) is involved in evaluation of punished rewards in a sex-dependent manner. Further, NAcSh D2 receptor (D2R)-expressing neurons make unique contributions to risk taking that vary across the decision making process. These findings advance our understanding of the neural principles of decision making and provide insight into how risk taking may become compromised in neuropsychiatric diseases.

Circuit and cell-specific contributions to decision making involving risk of explicit punishment in male and female rats

Decision making is a complex cognitive process that recruits a distributed network of brain regions, including the basolateral amygdala (BLA) and nucleus accumbens shell (NAcSh). Recent work suggests that communication between these structures, as well as activity of cells expressing dopamine D2 receptors (D2R) in the NAcSh, are necessary for some forms of decision making; however, the contributions of this circuit and cell population during decision making under risk of punishment are unknown. The current experiments addressed this question using circuit- and cell type-specific optogenetic approaches in rats during a decision-making task involving risk of punishment. In Experiment 1, Long-Evans rats received intra-BLA injections of halorhodopsin or mCherry (control) and in Experiment 2, D2-Cre transgenic rats received intra-NAcSh injections of Cre-dependent halorhodopsin or mCherry. Optic fibers were implanted in the NAcSh in both experiments. Following training in the decision-making task, BLA→NAcSh or D2R-expressing neurons were optogenetically inhibited during different phases of the decision process. Inhibition of the BLA→NAcSh during deliberation (the time between trial initiation and choice) increased choice of the large, risky reward (increased risk taking). Similarly, inhibition during delivery of the large, punished reward increased risk taking, but only in males. Inhibition of D2R-expressing neurons in the NAcSh during deliberation increased risk taking. In contrast, inhibition of these neurons during delivery of the small, safe reward decreased risk taking. These findings extend our knowledge of the neural dynamics of risk taking, revealing sex-dependent circuit recruitment and dissociable activity of selective cell populations during decision making.

A rapid in vitro assay for evaluating the effects of acetylcholinesterase inhibitors and reactivators in the rat basolateral amygdala

We established a novel brain slice assay to test the ability of acetylcholinesterase (AChE) reactivators to prevent ACh-induced M1 muscarinic acetylcholine receptor (mAChR) dependent hyperexcitability observed after exposure to the organophosphate (OP)-based AChE inhibitor and sarin surrogate 4-nitrophenyl isopropyl methylphosphonate (NIMP). Whole-cell patch clamp recordings were used to evaluate the response of pyramidal neurons in the rat basolateral amygdala (BLA) to brief (1 min) bath application of ACh (100 μM), either in control conditions, or after exposure to NIMP ± an AChE reactivator. Bath application of ACh produced atropine- and pirenzepine-sensitive inward currents in voltage clamped BLA pyramidal neurons, and increased the frequency of spontaneous EPSCs, suggesting robust activation of M1 mAChRs. Responses to ACh were increased ~3–5 fold in slices that had been preincubated in NIMP, and these effects were reversed in a concentration dependent manner by exposure to a commercially available AChE reactivator. The current work outlines a simple assay that can be used to evaluate the efficacy of both known and novel AChE reactivators in an area of the limbic system that likely contributes to seizures after acute exposure to OP-based AChE inhibitors.

Intrahypothalamic effects of oxytocin on PVN CRH neurons in response to acute stress

Much of the centrally available oxytocin (OT) is synthesized in magnocellular neurons located in the paraventricular nucleus of the hypothalamus. This same area is home to parvocellular corticotropin-releasing hormone (CRH) synthesizing neurons that regulate activation of the hypothalamic-pituitary-adrenal (HPA) axis. A large body of data indicates that complex interactions between these systems inextricably link central OT signaling with the neuroendocrine response to stress. This review focuses on a small but diverse set of cellular and synaptic mechanisms that have been proposed to underlie intrahypothalamic OT/CRF interactions during the response to acute stress.

Abstract 060: Angiotensin Type-2 Receptors In The Median Preoptic Nucleus Demarcate A Novel Depressor Neural Circuit In Mice

Blood pressure (BP) is under constant control and regulation by an extensive neural network that spans a number of brain regions. This includes the median preoptic nucleus (MnPO) which directly projects to the pre-autonomic neurons of the hypothalamus to increase BP. While angiotensin type-1 receptors (AT 1 R) are expressed in the MnPO and project to the hypothalamus to increase BP, the role of angiotensin type-2 receptors (AT 2 R) is not clear. Here, we combine the use of AT 2 R-Cre mice, neuroanatomical techniques, in vivo optogenetics, pharmacology, and cardiovascular physiology to test the hypothesis that AT 2 R neurons of MnPO (MnPO AT2R ) elicit cardioprotective effects. We found an abundance of AT 2 R-containing neurons in the MnPO that expressed the mRNA(s) for AT 2 R with high fidelity (~77%; 241 of 313 neurons; n=7). Using RNAscope in situ hybridization, we found that ~ 42% (153 of 364 neurons; n=8) of MnPO AT2R expressed genetic markers for GABA, an inhibitory neurotransmitter. Next, we delivered a Cre-inducible AAV to direct expression of channelrhodopsin-2 (ChR2) and/or enhanced yellow fluorescent protein (eYFP) within MnPO AT2R of AT 2 R-Cre mice. Using in vitro electrophysiology, we confirmed functional expression of ChR2 within MnPO AT2R , with pulses of light reliably producing action potentials (n= 12 neurons/3 animals). Subsequently, we found that activation of MnPO AT2R elicited inhibitory post-synaptic currents in neighboring AT 2 R-negative neurons. The in vivo optogenetic excitation of MnPO AT2R elicited depressor responses in both adult male (ΔSBP: +2.8 ± 1.3 vs. -4.1 ± 1.9 mmHg, p<0.05 eYFP vs. ChR2, n=15) and female mice (ΔSBP: +0.8 ± 2.2 vs.-5.3 ± 1.9 mmHg, p<0.05 eYFP vs. ChR2, n=5). These responses were abolished in the presence of the autonomic ganglion blocker, hexamethonium. Finally, to assess whether the depressor effects were AT 2 R-mediated, we administered the selective AT 2 R-agonist, compound 21, into the MnPO and observed depressor responses (ΔSBP: +0.5 ± 0.3 vs.-4.9 ± 1.0 mmHg, p<0.05 Saline vs. C21, n=7). In conclusion, our results demonstrate that MnPO AT2R are depressor inhibitory neurons that can be targeted with compound 21 to lower blood pressure.

A Novel Organ-Specific Approach to Selectively Target Sensory Afferents Innervating the Aortic Arch

The brain maintains cardiovascular homeostasis, in part, via the arterial baroreflex which senses changes in blood pressure (BP) at the level of the aortic arch. Sensory afferents innervating the aortic arch employ baroreceptors to convert stretch exerted on the arterial wall into action potentials carried by the vagus nerve to second order neurons residing within the nucleus of the solitary tract (NTS). Although the baroreflex was described more than 80 years ago, the specific molecular, structural, and functional phenotype of the baroreceptors remain uncharacterized. This is due to the lack of tools that provide the genetic and target organ specificity that is required to selectively characterize baroreceptor afferents. Here, we use a novel approach to selectively target baroreceptors. Male mice on a C57BL/6J background were anesthetized with isoflurane, intubated, and artificially ventilated. Following sternotomy, the aortic arch was exposed, and a retrograde adeno-associated virus was applied to the aortic arch to direct the expression of channelrhoropsin-2 (ChR2) and/or tdTomato (tdTom) to sensory afferents presumably functioning as baroreceptors. Consistent with the structural characteristics of arterial baroreceptors, robust tdTom expression was observed in nerve endings surrounding the aortic arch, within the fibers of the aortic depressor and vagus nerves, cell bodies of the nodose ganglia (NDG), and neural projections to the caudal NTS (cNTS). Additionally, the tdTom labeled cell bodies within the NDG also expressed mRNAs coding for the mechanically gated ion channels, PIEZO-1 and PIEZO-2. In vitro electrophysiology revealed that pulses of blue light evoked excitatory post-synaptic currents in a subset of neurons within the cNTS, suggesting a functional connection between the labeled aortic arch sensory afferents and second order neurons. Finally, the in vivo optogenetic stimulation of the cell bodies of the baroreceptor expressing afferents in the NDG produced robust depressor responses. Together, these results establish a novel approach for selectively targeting sensory neurons innervating the aortic arch. This approach may be used to investigate arterial baroreceptors structurally and functionally, and to assess their role in the etiology or reversal of cardiovascular disease.

Attenuated NMDAR signaling on fast-spiking interneurons in prefrontal cortex contributes to age-related decline of cognitive flexibility

Ionotropic glutamate receptors of the NMDA and AMPA subtypes transduce excitatory signaling on neurons in the prefrontal cortex (PFC) in support of cognitive flexibility. Cognitive flexibility is reliably observed to decline at advanced ages, coinciding with changes in PFC glutamate receptor expression and neuronal physiology. However, the relationship between age-related impairment of cognitive flexibility and changes to excitatory signaling on distinct classes of PFC neurons is not known. In this study, one cohort of young adult (4 months) and aged (20 months) male F344 rats were characterized for cognitive flexibility on an operant set-shifting task. Expression of the essential NMDAR subunit, NR1, was correlated with individual differences in set-shifting abilities such that lower NR1 in the aged PFC was associated with worse set-shifting. In contrast, lower expression of two AMPAR subunits, GluR1 and GluR2, was not associated with set-shift abilities in aging. As NMDARs are expressed by both pyramidal cells and fast-spiking interneurons (FSI) in PFC, whole-cell patch clamp recordings were performed in a second cohort of age-matched rats to compare age-associated changes on these neuronal subtypes. Evoked excitatory postsynaptic currents were generated using a bipolar stimulator while AMPAR vs. NMDAR-mediated components were isolated using pharmacological tools. The results revealed a clear increase in AMPA/NMDA ratio in FSIs that was not present in pyramidal neurons. Together, these data indicate that loss of NMDARs on interneurons in PFC contributes to age-related impairment of cognitive flexibility.

Dendritic osmosensors modulate activity-induced calcium influx in oxytocinergic magnocellular neurons of the mouse PVN

Hypothalamic oxytocinergic magnocellular neurons have a fascinating ability to release peptide from both their axon terminals and from their dendrites. Existing data indicates that the relationship between somatic activity and dendritic release is not constant, but the mechanisms through which this relationship can be modulated are not completely understood. Here, we use a combination of electrical and optical recording techniques to quantify activity-induced calcium influx in proximal vs. distal dendrites of oxytocinergic magnocellular neurons located in the paraventricular nucleus of the hypothalamus (OT-MCNs). Results reveal that the dendrites of OT-MCNs are weak conductors of somatic voltage changes; however, activity-induced dendritic calcium influx can be robustly regulated by both osmosensitive and non-osmosensitive ion channels located along the dendritic membrane. Overall, this study reveals that dendritic conductivity is a dynamic and endogenously regulated feature of OT-MCNs that is likely to have substantial functional impact on central oxytocin release.

Targeting angiotensin type 2 receptors located on pressor neurons in the nucleus of the solitary tract to relieve hypertension in mice

AIMS

These studies evaluate whether angiotensin type-2 receptors (AT2Rs) that are expressed on γ-aminobutyric acid (GABA) neurons in the nucleus of the solitary tract (NTS) represent a novel endogenous blood pressure lowering mechanism.

METHODS AND RESULTS

Experiments combined advanced genetic and neuroanatomical techniques, pharmacology, electrophysiology and optogenetics in mice to define the structure and cardiovascular-related function of NTS neurons that contain AT2R. Using mice with Cre-recombinase directed to the AT2R gene, we discovered that optogenetic stimulation of AT2R-expressing neurons in the NTS increases GABA release and blood pressure. To evaluate the role of the receptor, per se, in cardiovascular regulation, we chronically delivered C21, a selective AT2R agonist, into the brains of normotensive mice and found that central AT2R activation reduces GABA-related gene expression and blunts the pressor responses induced by optogenetic excitation of NTS AT2R neurons. Next, using in situ hybridization, we found that the levels of Agtr2 mRNAs in GABAergic NTS neurons rise during experimentally-induced hypertension, and we hypothesized that this increased expression may be exploited to ameliorate the disease. Consistent with this, final experiments revealed that central administration of C21 attenuates hypertension, an effect that is abolished in mice lacking AT2R in GABAergic NTS neurons.

CONCLUSIONS

These studies unveil novel hindbrain circuits that maintain arterial blood pressure, and reveal a specific population of AT2R that can be engaged to alleviate hypertension. The implication is that these discrete receptors may serve as an access point for activating an endogenous depressor circuit.

TRANSLATIONAL PERSPECTIVE

Hypertension is a widespread health problem and risk factor for cardiovascular disease and stroke. Although treatment options exist, many patients suffer from resistant hypertension, which is associated with enhanced sympathetic drive. Thus, many available therapeutics focus on dampening pressor mechanisms. The present studies take the alternative approach of treating hypertension by exploiting an endogenous depressor mechanism.

Endogenous oxytocin inhibits hypothalamic corticotrophin-releasing hormone neurones following acute hypernatraemia

Significant prior evidence indicates that centrally acting oxytocin robustly modulates stress responsiveness and anxiety-like behaviour, although the neural mechanisms behind these effects are not entirely understood. A plausible neural basis for oxytocin-mediated stress reduction is via inhibition of corticotrophin-releasing hormone (CRH) neurones in the paraventricular nucleus of the hypothalamus (PVN) that regulate activation of the hypothalamic-pituitary-adrenal axis. Previously, we have shown that, following s.c. injection of 2.0 mol L^-1 NaCl, oxytocin synthesising neurones are activated in the rat PVN, an oxytocin receptor (Oxtr)-dependent inhibitory tone develops on a subset of parvocellular neurones and stress-mediated increases in plasma corticosterone levels are blunted. In the present study, we utilised transgenic male CRH-reporter mice to selectively target PVN CRH neurones for whole-cell recordings. These experiments reveal that acute salt loading produces tonic inhibition of PVN CRH neurones through a mechanism that is largely independent of synaptic activity. Further studies reveal that a subset of CRH neurones within the PVN synthesise mRNA for Oxtr(s). Salt induced Oxtr-dependent inhibitory tone was eliminated in individual PVN CRH neurones filled with GDP-β-S. Additional electrophysiological studies suggest that reduced excitability of PVN CRH neurones in salt-loaded animals is associated with increased activation of inwardly rectifying potassium channels. Nevertheless, substantial effort to recapitulate the core effects of salt loading by activating Oxtr(s) with an exogenous agonist produced mixed results. Collectively, these results enhance our understanding of how oxytocin receptor-mediated signalling modulates the function of CRH neurones in the PVN.

Region-specific effects of HIV-1 Tat on intrinsic electrophysiological properties of pyramidal neurons in mouse prefrontal cortex and hippocampus

Human immunodeficiency virus (HIV)-1 transactivator of transcription protein (Tat) is a viral protein that promotes transcription of the HIV genome and possesses cell-signaling properties. Long-term exposure of central nervous system (CNS) tissue to HIV-1 Tat is theorized to contribute to HIV-associated neurodegenerative disorder (HAND). In the current study, we sought to directly evaluate the effect of HIV-1 Tat expression on the intrinsic electrophysiological properties of pyramidal neurons located in layer 2/3 of the medial prefrontal cortex and in area CA1 of the hippocampus. Toward that end, we drove Tat expression with doxycycline (100 mg·kg^-1·day^-1 ip) in inducible Tat (iTat) transgenic mice for 7 days and then performed single-cell electrophysiological studies in acute tissue slices made through the prefrontal cortex and hippocampus. Control experiments were performed in doxycycline-treated G-tg mice, which retain the tetracycline-sensitive promoter but do not express Tat. Our results indicated that the predominant effects of HIV-1 Tat expression are excitatory in medial prefrontal cortical pyramidal neurons yet inhibitory in hippocampal pyramidal neurons. Notably, in these two populations, HIV-1 Tat expression produced differential effects on neuronal gain, membrane time constant, resting membrane potential, and rheobase. Similarly, we also observed distinct effects on action potential kinetics and afterhyperpolarization, as well as on the current-voltage relationship in subthreshold voltage ranges. Collectively, these data provide mechanistic evidence of complex and region-specific changes in neuronal physiology by which HIV-1 Tat protein may promote cognitive deficits associated with HAND.NEW & NOTEWORTHY We drove expression of human immunodeficiency virus (HIV)-1 transactivator of transcription protein (Tat) protein in inducible Tat (iTat) transgenic mice for 7 days and then examined the effects on the intrinsic electrophysiological properties of pyramidal neurons located in the medial prefrontal cortex (mPFC) and in the hippocampus. Our results reveal a variety of specific changes that promote increased intrinsic excitability of layer II/III mPFC pyramidal neurons and decreased intrinsic excitability of hippocampal CA1 pyramidal neurons, highlighting both cell type and region-specific effects.

Optogenetic dissection of basolateral amygdala contributions to intertemporal choice in young and aged rats

Across species, aging is associated with an increased ability to choose delayed over immediate gratification. These experiments used young and aged rats to test the role of the basolateral amygdala (BLA) in intertemporal decision making. An optogenetic approach was used to inactivate the BLA in young and aged rats at discrete time points during choices between levers that yielded a small, immediate vs. a large, delayed food reward. BLA inactivation just prior to decisions attenuated impulsive choice in both young and aged rats. In contrast, inactivation during receipt of the small, immediate reward increased impulsive choice in young rats but had no effect in aged rats. BLA inactivation during the delay or intertrial interval had no effect at either age. These data demonstrate that the BLA plays multiple, temporally distinct roles during intertemporal choice, and show that the contribution of BLA to choice behavior changes across the lifespan.

Abstract 084: Neurons in the Nodose Ganglion that Express Angiotensin Type 1a Receptors Function as Primary Baroreceptor Afferents: An in vitro and in vivo Optogenetic Study

The baroreflex is an essential regulator of blood pressure (BP); whereby, baroreceptors sense acute changes in BP and convey this information, via the nodose ganglia (NG), to the nucleus of the solitary tract (NTS). Manipulation of the baroreflex may provide a novel strategy in the treatment of hypertension. To that end, our neuroanatomical studies revealed a dense localization of angiotensin type 1a receptor (AT1a)-containing neuronal cell bodies in the NG and terminals in the NTS, thereby positioning them to play a role in the baroreflex. We, therefore, hypothesized that AT1a neurons residing in the NG potently influence cardiovascular function. Male and female mice expressing channelrhodopsin-2 (ChR2) and yellow fluorescent protein (eYFP) specifically in AT1a-expressing neurons (AT1aR-ChR2-eYFP) were used to determine the function of these neurons. In vitro patch clamp electrophysiological recordings from neurons in the NTS receiving axons expressing ChR2-eYFP revealed that optogenetic stimulation (473nm) reliably evoked excitatory postsynaptic currents (EPSCs). In concurrence with studies demonstrating that baroreceptor afferents utilize glutamate in the NTS, these EPSCs were eliminated by the presence of glutamate receptor antagonists. Next, the NG of anesthetized mice were subjected to optogenetic stimulation of varying frequencies (1, 15, 30Hz) for 1 min, and BP and HR responses were assessed. Optogenetic stimulation of these AT1a-expressing neurons led to significant decreases in mean arterial pressure (ΔMAP= -16±3*, -36±4*, -44±7* mmHg) and HR (ΔHR= -13±7, -104±44*, -163±59* bpm) in AT1a-ChR2-eYFP mice (n=6), but there was no effect on control mice harboring only the stop-FLOX-ChR2-eYFP gene (n=5; ΔMAP= 0±2, -2±1, 5±5 mmHg; ΔHR= 4±4, 3±4, -13±21 bpm). Additionally, AT1a-ChR2-eYFP mice rendered hypertensive via DOCA-salt (n=7) exhibited a dampened response to optogenetic stimulation (ΔMAP= -7±1#, -14±3*#, -30±6*# mmHg; ΔHR= -29±34, -71±37*, -178±50* bpm). Collectively, these data suggest that AT1a neurons in the NG are key regulators of the baroreflex, and may serve as a target for antihypertensive therapeutics. (p<0.05, *significantly different from control, #significantly different from normotensive AT1a-ChR2-eYFP)

Chronic salt-loading reduces basal excitatory input to CRH neurons in the paraventricular nucleus and accelerates recovery from restraint stress in male mice

Neurons synthesizing corticotrophin-releasing hormone (CRH) in the paraventricular nucleus of the hypothalamus (PVN) are activated during acute stress and act via the hypothalamic-pituitary-adrenal (HPA) axis to increase systemic levels of corticosterone (CORT). Recent data indicates that CRH neurons in the PVN are inhibited by acute salt-loading, and that this inhibition blunts the response to restraint stress as measured by increases in plasma CORT. The current study evaluates the effects of chronic rather than acute salt-loading on stress-induced activation of the HPA axis. Relative to euhydrated controls, chronic salt-loading over a 5-day period elevated plasma sodium and fluid intake without eliciting hypovolemia or substantial alterations in food intake or body weight. Chronic salt-loading also decreased expression of CRH mRNA in the anterior but not posterior portion of the PVN. Similarly, whole cell patch clamp recordings revealed that salt-loading effectively decreases spontaneous excitatory input to CRH neurons in the PVN without altering spontaneous inhibitory input. Generally consistent with these observations, chronic salt attenuated HPA axis activation as indicated by a significant reduction of plasma CORT during recovery from restraint stress.

Alcohol consumption increases basal extracellular glutamate in the nucleus accumbens core of Sprague-Dawley rats without increasing spontaneous glutamate release

Glutamate neurotransmission in the nucleus accumbens core (NAc) mediates ethanol consumption. Previous studies using non-contingent and voluntary alcohol administration in inbred rodents have reported increased basal extracellular glutamate levels in the NAc. Here, we assessed basal glutamate levels in the NAc following intermittent alcohol consumption in male Sprague-Dawley rats that had access to ethanol for 7 weeks on alternating days. We found increased basal NAc glutamate at 24 h withdrawal from ethanol and thus sought to identify the source of this glutamate. To do so, we employed a combination of microdialysis, slice electrophysiology and western blotting. Reverse dialysis of the voltage-gated sodium channel blocker tetrodotoxin did not affect glutamate levels in either group. Electrophysiological recordings in slices made after 24 h withdrawal revealed a decrease in spontaneous excitatory postsynaptic current (sEPSC) frequency relative to controls, with no change in sEPSC amplitude. No change in metabotropic glutamate receptor 2/3 (mGlu2/3) function was detected as bath application of the mGlu2/3 agonist LY379268 decreased spontaneous and miniature EPSC frequency in slices from both control and ethanol-consuming rats. The increase in basal glutamate was not associated with changes in the surface expression of GLT-1, however, a decrease in slope of the no-net-flux dialysis function was observed following ethanol consumption, indicating a potential decrease in glutamate reuptake. Taken together, these findings indicate that the increase in basal extracellular glutamate occurring after chronic ethanol consumption is not mediated by an increase in action potential-dependent glutamate release or a failure of mGlu2/3 autoreceptors to regulate such release.

Age-related changes in tonic activation of presynaptic versus extrasynaptic γ-amniobutyric acid type B receptors in rat medial prefrontal cortex

The present study examined the effect of age on both glutamatergic and γ-aminobutyric acid mediated (GABAergic) signaling in the rodent medial prefrontal cortex (mPFC), with an emphasis on revealing novel changes contributing to increased inhibition in age. Whole-cell patch clamp recordings were obtained from layer 2/3 mPFC pyramidal neurons in acute cortical slices prepared from either young (4 months) or aged (20-24 months) male F344 rats. Results indicated that GABAB receptors on GABAergic, but not on glutamatergic, inputs to layer 2/3 pyramidal cells are tonically activated by ambient GABA in young animals and further demonstrated that this form of tonic inhibition is significantly attenuated in aged mPFC. Moreover, concurrent with loss of tonic presynaptic GABAB autoreceptor activation, layer 2/3 pyramidal cells in aged mPFC are subjected to increased tonic activation of extrasynaptic GABAA and GABAB receptors. These data demonstrate a shift in the site of GABAB receptor-mediated inhibitory tone in the aged mPFC that clearly promotes increased inhibition of pyramidal cells in aged animals, and that may plausibly contribute to impaired executive function.

Oxytocin depolarizes fast-spiking hilar interneurons and induces GABA release onto mossy cells of the rat dentate gyrus

Delivery of exogenous oxytocin (OXT) to central oxytocin receptors (OXT-Rs) is currently being investigated as a potential treatment for conditions such as post-traumatic stress disorder (PTSD), depression, social anxiety, and autism spectrum disorder (ASD). Despite significant research implicating central OXT signaling in modulation of mood, affect, social behavior, and stress response, relatively little is known about the cellular and synaptic mechanisms underlying these complex actions, particularly in brain regions which express the OXT-R but lie outside of the hypothalamus (where OXT-synthesizing neurons reside). We report that bath application of low concentrations of the selective OXT-R agonist Thr4,Gly7-OXT (TGOT) reliably and robustly drives GABA release in the dentate gyrus in an action potential dependent manner. Additional experiments led to identification of a small subset of small hilar interneurons that are directly depolarized by acute application of TGOT. From a physiological perspective, TGOT-responsive hilar interneurons have high input resistance, rapid repolarization velocity during an action potential, and a robust afterhyperpolarization. Further, they fire irregularly (or stutter) in response to moderate depolarization, and fire quickly with minimal spike frequency accommodation in response to large current injections. From an anatomical perspective, TGOT responsive hilar interneurons have dense axonal arborizations in the hilus that were found in close proximity with mossy cell somata and/or proximal dendrites, and also invade the granule cell layer. Further, they have primary dendrites that always extend into the granule cell layer, and sometimes have clear arborizations in the molecular layer. Overall, these data reveal a novel site of action for OXT in an important limbic circuit, and represent a significant step towards better understanding how endogenous OXT may modulate flow of information in hippocampal networks. © 2016 Wiley Periodicals, Inc.

Increasing brain angiotensin converting enzyme 2 activity decreases anxiety-like behavior in male mice by activating central Mas receptors

Over-activation of the brain renin-angiotensin system (RAS) has been implicated in the etiology of anxiety disorders. Angiotensin converting enzyme 2 (ACE2) inhibits RAS activity by converting angiotensin-II, the effector peptide of RAS, to angiotensin-(1-7), which activates the Mas receptor (MasR). Whether increasing brain ACE2 activity reduces anxiety by stimulating central MasR is unknown. To test the hypothesis that increasing brain ACE2 activity reduces anxiety-like behavior via central MasR stimulation, we generated male mice overexpressing ACE2 (ACE2 KI mice) and wild type littermate controls (WT). ACE2 KI mice explored the open arms of the elevated plus maze (EPM) significantly more than WT, suggesting increasing ACE2 activity is anxiolytic. Central delivery of diminazene aceturate, an ACE2 activator, to C57BL/6 mice also reduced anxiety-like behavior in the EPM, but centrally administering ACE2 KI mice A-779, a MasR antagonist, abolished their anxiolytic phenotype, suggesting that ACE2 reduces anxiety-like behavior by activating central MasR. To identify the brain circuits mediating these effects, we measured Fos, a marker of neuronal activation, subsequent to EPM exposure and found that ACE2 KI mice had decreased Fos in the bed nucleus of stria terminalis but had increased Fos in the basolateral amygdala (BLA). Within the BLA, we determined that ∼62% of GABAergic neurons contained MasR mRNA and expression of MasR mRNA was upregulated by ACE2 overexpression, suggesting that ACE2 may influence GABA neurotransmission within the BLA via MasR activation. Indeed, ACE2 overexpression was associated with increased frequency of spontaneous inhibitory postsynaptic currents (indicative of presynaptic release of GABA) onto BLA pyramidal neurons and central infusion of A-779 eliminated this effect. Collectively, these results suggest that ACE2 may reduce anxiety-like behavior by activating central MasR that facilitate GABA release onto pyramidal neurons within the BLA.

Molecular aspects of age-related cognitive decline: the role of GABA signaling

Alterations in inhibitory interneurons contribute to cognitive deficits associated with several psychiatric and neurological diseases. Phasic and tonic inhibition imparted by γ-aminobutyric acid (GABA) receptors regulates neural activity and helps to establish the appropriate network dynamics in cortical circuits that support normal cognition. This review highlights basic science demonstrating that inhibitory signaling is altered in aging, and discusses the impact of age-related shifts in inhibition on different forms of memory function, including hippocampus-dependent spatial reference memory and prefrontal cortex (PFC)-dependent working memory. The clinical appropriateness and tractability of select therapeutic candidates for cognitive aging that target receptors mediating inhibition are also discussed.

Hydration and beyond: neuropeptides as mediators of hydromineral balance, anxiety and stress-responsiveness

Challenges to body fluid homeostasis can have a profound impact on hypothalamic regulation of stress responsiveness. Deficiencies in blood volume or sodium concentration leads to the generation of neural and humoral signals relayed through the hindbrain and circumventricular organs that apprise the paraventricular nucleus of the hypothalamus (PVH) of hydromineral imbalance. Collectively, these neural and humoral signals converge onto PVH neurons, including those that express corticotrophin-releasing factor (CRF), oxytocin (OT), and vasopressin, to influence their activity and initiate compensatory responses that alleviate hydromineral imbalance. Interestingly, following exposure to perceived threats to homeostasis, select limbic brain regions mediate behavioral and physiological responses to psychogenic stressors, in part, by influencing activation of the same PVH neurons that are known to maintain body fluid homeostasis. Here, we review past and present research examining interactions between hypothalamic circuits regulating body fluid homeostasis and those mediating behavioral and physiological responses to psychogenic stress.

A method for high purity intestinal epithelial cell culture from adult human and murine tissues for the investigation of innate immune function

Intestinal epithelial cells (IECs) serve as an important physiologic barrier between environmental antigens and the host intestinal immune system. Thus, IECs serve as a first line of defense and may act as sentinel cells during inflammatory insults. Despite recent renewed interest in IEC contributions to host immune function, the study of primary IEC has been hindered by lack of a robust culture technique, particularly for small intestinal and adult tissues. Here, a novel adaptation for culture of primary IEC is described for human duodenal organ donor tissue as well as duodenum and colon of adult mice. These epithelial cell cultures display characteristic phenotypes and are of high purity. In addition, the innate immune function of human primary IEC, specifically with regard to Toll-like receptor (TLR) expression and microbial ligand responsiveness, is contrasted with a commonly used intestinal epithelial cell line (HT-29). Specifically, TLR expression at the mRNA level and production of cytokine (IFNγ and TNFα) in response to TLR agonist stimulation is assessed. Differential expression of TLRs as well as innate immune responses to ligand stimulation is observed in human-derived cultures compared to that of HT-29. Thus, use of this adapted method to culture primary epithelial cells from adult human donors and from adult mice will allow for more appropriate studies of IECs as innate immune effectors.

Prefrontal cortical GABAergic dysfunction contributes to age-related working memory impairment

Working memory functions supported by the prefrontal cortex decline in normal aging. Disruption of corticolimbic GABAergic inhibitory circuits can impair working memory in young subjects; however, relatively little is known regarding how aging impacts prefrontal cortical GABAergic signaling and whether such changes contribute to cognitive deficits. The current study used a rat model to evaluate the effects of aging on expression of prefrontal GABAergic synaptic proteins in relation to working memory decline, and to test whether pharmacological manipulations of prefrontal GABAergic signaling can improve working memory abilities in aged subjects. Results indicate that in aged medial prefrontal cortex (mPFC), expression of the vesicular GABA transporter VGAT was unchanged; however, there was a significant increase in expression of the GABA synthesizing enzyme GAD67, and a significant decrease in the primary neuronal GABA transporter GAT-1 and in both subunits of the GABA(B) receptor (GABA(B)R). Expression of VGAT, GAD67, and GAT-1 was not associated with working memory ability. In contrast, among aged rats, GABA(B)R expression was significantly and negatively associated with working memory performance, such that lower GABA(B)R expression predicted better working memory. Subsequent experiments showed that systemic administration of a GABA(B)R antagonist, CGP55845, dose-dependently enhanced working memory in aged rats. This enhancing effect of systemic CGP55845 was reproduced by direct intra-mPFC administration. Together, these data suggest that age-related dysregulation of GABAergic signaling in prefrontal cortex may play a causal role in impaired working memory and that targeting GABA(B)Rs may provide therapeutic benefit for age-related impairments in executive functions.

Acute hypernatremia exerts an inhibitory oxytocinergic tone that is associated with anxiolytic mood in male rats

Anxiety disorders are the most common psychiatric illnesses and are associated with heightened stress responsiveness. The neuropeptide oxytocin (OT) has garnered significant attention for its potential as a treatment for anxiety disorders; however, the mechanism mediating its effects on stress responses and anxiety is not well understood. Here we used acute hypernatremia, a stimulus that elevates brain levels of OT, to discern the central oxytocinergic pathways mediating stress responsiveness and anxiety-like behavior. Rats were rendered hypernatremic by acute administration of 2.0 M NaCl and had increased plasma sodium concentration, plasma osmolality, and Fos induction in OT-containing neurons relative to 0.15 M NaCl-treated controls. Acute hypernatremia decreased restraint-induced elevations in corticosterone and created an inhibitory oxytocinergic tone on parvocellular neurosecretory neurons within the paraventricular nucleus of the hypothalamus. In contrast, evaluation of Fos immunohistochemistry determined that acute hypernatremia followed by restraint increased neuronal activation in brain regions receiving OT afferents that are also implicated in the expression of anxiety-like behavior. To determine whether these effects were predictive of altered anxiety-like behavior, rats were subjected to acute hypernatremia and then tested in the elevated plus maze. Relative to controls given 0.15 M NaCl, rats given 2.0 M NaCl spent more time in the open arms of the elevated plus maze, suggesting that acute hypernatremia is anxiolytic. Collectively the results suggest that acute elevations in plasma sodium concentration increase central levels of OT, which decreases anxiety by altering neuronal activity in hypothalamic and limbic nuclei.

Marijuana, endocannabinoids, and epilepsy: potential and challenges for improved therapeutic intervention

Phytocannabinoids isolated from the cannabis plant have broad potential in medicine that has been well recognized for many centuries. It is presumed that these lipid soluble signaling molecules exert their effects in both the central and peripheral nervous system in large part through direct interaction with metabotropic cannabinoid receptors. These same receptors are also targeted by a variety of endogenous cannabinoids including 2-arachidonoyl glycerol and anandamide. Significant effort over the last decade has produced an enormous advance in our understanding of both the cellular and the synaptic physiology of endogenous lipid signaling systems. This increase in knowledge has left us better prepared to carefully evaluate the potential for both natural and synthetic cannabinoids in the treatment of a variety of neurological disorders. In the case of epilepsy, long standing interest in therapeutic approaches that target endogenous cannabinoid signaling systems are, for the most part, not well justified by available clinical data from human epileptics. Nevertheless, basic science experiments have clearly indicated a key role for endogenous cannabinoid signaling systems in moment to moment regulation of neuronal excitability. Further it has become clear that these systems can both alter and be altered by epileptiform activity in a wide range of in vitro and in vivo models of epilepsy. Collectively these observations suggest clear potential for effective therapeutic modulation of endogenous cannabinoid signaling systems in the treatment of human epilepsy, and in fact, further highlight key obstacles that would need to be addressed to reach that goal.

Cannabinoid receptor agonists potentiate action potential-independent release of GABA in the dentate gyrus through a CB1 receptor-independent mechanism

We report a novel excitatory effect of cannabinoid agonists on action potential-independent GABAergic transmission in the rat dentate gyrus. Specifically, we find that both WIN55,212-2 and anandamide increase the frequency of miniature IPSCs (mIPSCs)recorded from hilar mossy cells without altering event amplitude, area, rise time, or decay. The effect of WIN55,212-2 on mIPSCs is insensitive to AM251 and preserved in CB1 −/− animals,indicating that it does not depend on activation of CB1 receptors. It is also insensitive to AM630 and unaffected by capsazepine suggesting that neither CB2 nor TRPV1 receptors are involved. Further, it is blocked by pre-incubation in suramin and by a selective protein kinase A inhibitor (H-89), and is mimicked (and occluded) by bath application of forskolin. Similar CB1 receptor-independent facilitation of exocytosis is not apparent when recording evoked IPSCs in the presence of AM251, suggesting that the exocytotic mechanism that produces WIN55,212-2 sensitive mIPSCs is distinct from that which produces CB1 sensitive and action potential-dependent release. Despite clear independence from action potentials, WIN55,212-2 mediated facilitation of mIPSCs requires calcium, and yet is insensitive to chelation of calcium in the postsynaptic cell. Finally, we demonstrate that both bath application of 2-arachidonoylglycerol(2-AG) and depolarization-induced release of endogenous cannabinoids have minimal effect on mIPSC frequency. Cumulatively, our results indicate that cannabinoid ligands can selectively facilitate action potential-independent exocytosis of GABA in the rat dentate gyrus, and further emphasize that this new cannabinoid sensitive signalling system is distinct from previously described CB1 receptor-dependent systems in numerous respects.

Two distinct and activity-dependent mechanisms contribute to autoreceptor-mediated inhibition of GABAergic afferents to hilar mossy cells

We report that bath application of 3 mum carbachol (CCh), a muscarinic acetylcholine receptor agonist, reduces evoked IPSC amplitude recorded from hilar mossy cells in the rat dentate gyrus through a presynaptic mechanism. While CCh has been shown to inhibit evoked IPSCs in other systems, this effect is intriguing in that it does not require inhibitory action of either presynaptic muscarinic receptors or presynaptic cannabinoid receptors. Previous work from our lab has shown that identical application of CCh produces an action potential-dependent increase in ambient GABA in this system; however, inhibition of evoked IPSCs produced by both 3 and 10 mum CCh is insensitive to the GABA(B) antagonist CGP52432. Therefore we hypothesized that CCh-mediated inhibition of evoked IPSCs might be produced by activity-dependent increases in ambient GABA and subsequent activation of presynaptic GABA(A) receptors. Consistent with that hypothesis, we report that CCh-mediated inhibition of evoked IPSCs appears to be well correlated with CCh-mediated facilitation of spontaneous IPSCs and that CCh does not affect GABA(B)-mediated IPSCs recorded in the presence of the GABA(A) receptor antagonist picrotoxin. Intriguingly, however, we found that bath application of the GAT-1 transport blocker NO-711 (1 mum) produces inhibition of evoked IPSCs that is reversed by CGP52432, and that lower doses of CCh produce inhibition with greater CGP52432 sensitivity. These observations, combined with subsequent work on multiple pulse depression, reveal that feedback inhibition of GABAergic afferents to hilar mossy cells is governed by a complex relationship between two distinct and activity-dependent mechanisms.

Muscarinic receptor activation modulates the excitability of hilar mossy cells through the induction of an afterdepolarization

In the present study we used electrophysiological techniques in an in vitro preparation of the rat dentate gyrus to examine the effect of muscarinic acetylcholine receptor activation on the intrinsic excitability of hilar neurons. We found that bath application of muscarine caused a direct depolarization in approximately 80% of mossy cells tested, and also produced a clear afterdepolarization (ADP) in nearly 100% of trials. The ADP observed in hilar mossy cells is produced by the opening of a Na(+) permeant and yet largely TTX insensitive ion channel. It requires an increase in postsynaptic calcium for activation, and is blocked by flufenamic acid, an antagonist of a previously identified calcium activated non-selective cation channel (I(CAN)). Further, we demonstrate that induction of an ADP in current clamp causes release of cannabinoids, and subsequent depression of GABAergic transmission that is comparable to that produced in the same cells by a more conventional 5s depolarization in voltage clamp. By contrast, other types of hilar neurons were less strongly depolarized by bath application of muscarinic agonists, and uniformly lacked a similar muscarinic ADP. Overall, the data presented here extend our understanding of the specific mechanisms through which muscarinic agonists are likely to modulate neuronal excitability in the hilar network, and further reveal a mechanism that could plausibly promote endocannabinoid mediated signaling in vivo.

mGluR-mediated and endocannabinoid-dependent long-term depression in the hilar region of the rat dentate gyrus

We report that bath application of the group I mGluR agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) causes acute inhibition of evoked IPSCs recorded from hilar mossy cells, and that significant long-term depression (LTD) of synaptic transmission remains following washout of DHPG. Subsequent experiments using minimal stimulation techniques revealed that expression of both acute and long-term effects of DHPG are restricted to a subset of GABAergic afferents that are also sensitive to depolarization-induced suppression of inhibition (DSI). Experiments with a selective CB1 antagonist and with transgenic animals lacking CB1 receptors indicate that all effects of DHPG, like DSI, depend on activation of CB1 receptors. Further work with selective mGluR antagonists suggests a direct involvement of mGluR1 receptors. Interestingly, we also report that induction of LTD under our experimental conditions does not require prior direct somatic depolarization via the patch pipette and does not appear to depend critically on the level of activity in incoming GABAergic afferents. Collectively, these results represent the first characterization of mGluR-mediated and endocannabinoid-dependent LTD in the hilar region of the dentate gyrus. The dentate gyrus is thus one of relatively few areas where this mechanism has clearly been demonstrated to induce long-term modulation of inhibitory synaptic transmission.

Excitatory afferents to CA3 pyramidal cells display differential sensitivity to CB1 dependent inhibition of synaptic transmission

Recent advances in immunohistochemical techniques have, contrary to earlier reports, positively identified CB1 receptors on glutamatergic terminals in the hippocampus. Further work has implicated these receptors in modulation of susceptibility to kainic acid induced seizures. Based on these results, the current study was designed to test the hypothesis that both exogenous and endogenous cannabinoids can selectively modulate glutamatergic afferents to CA3 pyramidal cells, and that such modulation is mediated by cannabinoid type 1 (CB1) receptors. Towards that end we employed either conventional or two-photon guided minimal stimulation techniques to isolate mossy fiber and/or associational/commissural (A/C) inputs to CA3 pyramidal cells. We report that bath application of WIN55,212-2 selectively inhibits minimally evoked A/C inputs to CA3 pyramidal cells, without significantly altering simultaneously recorded mossy fiber inputs. Further, we find that WIN55,212-2 mediated inhibition of A/C inputs is completely blocked by the CB1 selective antagonist AM-251 and absent in CB1(-/-) animals, suggesting a dependence on CB1 receptors. Finally, we demonstrate that depolarization of CA3 pyramidal cells leads to calcium dependent release of endogenous cannabinoids that transiently inhibit A/C mediated responses, and that this effect is also sensitive to both AM-251 and the muscarinic acetylcholine receptor antagonist atropine. To our knowledge this represents the first demonstration of depolarization induced suppression of excitation in area CA3 of the hippocampus. Collectively, these results provide new information relevant to developing a thorough understanding of how ECs modulate excitatory transmission in an area that is both essential for the acquisition of new memories and intimately involved in epileptogenesis.

Presynaptic inhibition of excitatory afferents to hilar mossy cells

The hippocampus contains one very strong recurrent excitatory network formed by associational connections between CA3 pyramidal cells and another that depends largely on a disynaptic excitatory pathway between dentate granule cells. The recurrent excitatory network in CA3 has long been considered a possible location of autoassociative memory storage, whereas changes in the level and arrangement of recurrent excitation between granule cells are strongly implicated in epileptogenesis. Hilar mossy cells are likely to receive collateral input from CA3 pyramidal cells and they are key intermediaries (by mossy fiber inputs) in the recurrent excitatory network between granule cells. The current study uses minimal stimulation techniques in an in vitro preparation of the rat dentate gyrus to examine presynaptic modulation of both mossy fiber and non-mossy fiber inputs to hilar mossy cells. We report that both mossy fiber and non-mossy fiber inputs to hilar mossy cells express presynaptic gamma-aminobutyric acid type B (GABA(B)) receptors that are subject to tonic inhibition by ambient GABA. We further find that only non-mossy fiber inputs express presynaptic muscarinic acetylcholine receptors, but that bath application of cholinergic agonists produces action potential-dependent increases in ambient GABA that can indirectly inhibit mossy fiber inputs. Finally, we demonstrate that mossy cells express high-affinity postsynaptic GABA(A) receptors that are also capable of detecting changes in ambient GABA produced by cholinergic agonists. Our results are among the first to directly characterize these important collateral inputs to hilar mossy cells and may help facilitate informed comparison between primary and collateral projections in two major excitatory pathways.

Endocannabinoids in the dentate gyrus

Recent years have produced rapid and enormous growth in our understanding of endocannabinoid-mediated signaling in the CNS. While much of the recent progress has focused on other areas of the brain, a significant body of evidence has developed that indicates the presence of a robust system for endocannabinoid-mediated signaling in the dentate gyrus. This chapter will provide an overview of our current understanding of that system based on available anatomical and physiological data.

Endocannabinoid-Mediated Depolarization-Induced Suppression of Inhibition in Hilar Mossy Cells of the Rat Dentate Gyrus

Hilar mossy cells represent a unique population of local circuit neurons in the hippocampus and dentate gyrus. Here we use electrophysiological techniques in acute preparations of hippocampal slices to demonstrate that depolarization of a single hilar mossy cell can produce robust inhibition of local GABAergic afferents. This depolarization-induced suppression of inhibition (DSI) can be observed as a transient reduction in frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) or as a transient reduction in amplitude of evoked IPSCs (eIPSCs). We find that DSI of eIPSCs as observed in hilar mossy cells is enhanced by activation of muscarinic acetylcholine receptors, blocked by chelation of postsynaptic calcium, and critically dependent on retrograde activation of presynaptic cannabinoid type 1 (CB1) receptors. We further report that activation of CB1 receptors on GABAergic afferents to hilar mossy cells (by either endogenous or exogenous agonists) preferentially inhibits calcium-dependent exocytosis and that endocannabinoid-dependent retrograde signaling in this system is subject to tight spatial constraints.

Medial septal/diagonal band cells express multiple functional nicotinic receptor subtypes that are correlated with firing frequency

The medial septum-diagonal band (MS/DB) contains primarily cholinergic and GABAergic neurons that project to the hippocampus, and are important for learning and memory. Whole-cell patch clamp methods with brain slices from p11--p20 rats were used to measure MS/DB cell responses to focal somatic application of 1mM acetylcholine (ACh) and a series of current pulses was applied in order to assess firing frequencies and the presence of hyperpolarization-activated currents (Ih). We identified three types of cells: (1) cells with fast inward currents blocked by methyllycaconitine (MLA) with slow firing rates (3--12 Hz), accommodating action potentials, and no Ih (n=20); (2) cells with currents that had both fast (MLA-sensitive) and slow components that were blocked with mecamylamine (MEC) that showed fast firing (up to 60 Hz) and slow firing (up to 3 Hz), with accommodating and non-accommodating action potentials (n=46), 33% of which had Ih; and (3) cells not responsive to ACh with moderate firing rates (10--42 Hz), some with accommodating action potentials and some without (n=19), of which 92% had Ih. These results are among the first to demonstrate functional nicotinic receptors in the MS/DB. The data suggest that these receptors include alpha 7 and non-alpha 7 subtypes and that the expression of each is correlated with firing frequency and the presence of Ih. Responses to ACh were not affected by tetrodotoxin (TTX) and CdC l(2) but were blocked by MLA or MLA and MEC, suggesting that these currents involve direct activation of nicotinic receptors.

Septal innervation regulates the function of α7 nicotinic receptors in CA1 hippocampal interneurons

The hippocampus receives substantial input from the medial septum/diagonal band of broca (MS/DB) via the fibria-fornix (FF). Projections from the MS/DB innervate hippocampal interneurons that express alpha7 nicotinic receptors and regulate excitation in principal cell populations. In the present report we used stereotaxic surgery, whole-cell patch clamping, and immunohistochemical techniques to evaluate the effects of FF and MS/DB lesions on alpha7 nicotinic receptors in stratum radiatum interneurons. Focal somatic application of ACh (1 mM) evoked methyllycaconitine (MLA)-sensitive currents that were markedly reduced following aspirative lesions of the FF. Reductions in current amplitudes were prevented or restored to levels not significantly different from controls following in vivo treatment with the alpha7-selective agonist GTS-21, and GTS-21 treatment did not change current amplitudes measured in tissue from unlesioned animals. MS/DB injections of the selective cholinergic neurotoxin 192 IgG-saporin did not affect alpha7 receptor currents, although MS/DB ChAT and hippocampal AChE immunolabeling were significantly reduced. In contrast, kainic acid lesions of the MS/DB, potentially more selective for GABAergic projection neurons, produced significant reductions in current amplitudes. These findings are the first to show functional changes in alpha7 receptors following hippocampal denervation and suggest that MS/DB hippocampal innervation regulates functional aspects of hippocampal alpha7 receptors. The results confirm hippocampal alpha7 nicotinic receptors as viable therapeutic targets in diseases that involve degradation of the septohippocampal pathway and may indicate that GABAergic MS/DB hippocampal input plays a more substantial role in the regulation of alpha7 nicotinic receptor function than MS/DB hippocampal cholinergic input.

Nicotinic receptors on local circuit neurons in dentate gyrus: a potential role in regulation of granule cell excitability

Although the dentate gyrus is one of the primary targets of septo-hippocampal cholinergic afferents, relatively little is known about the cholinergic physiology of neurons in the area. By combining whole cell patch-clamp recording with brief local application of exogenous agonists in horizontal slices, we found that there is robust expression of functional somatic alpha 7-containing nicotinic acetylcholine receptors (nAChRs) on molecular layer interneurons, hilar interneurons, and the glutamatergic mossy cells of the dentate hilus. In contrast, the principal neurons of the dentate gyrus, the granule cells, are generally unresponsive to focal somatic or dendritic application of ACh in the presence of atropine. We also demonstrate that cholinergic activation of alpha 7-containing nAChRs on the subgranular interneurons of the hilus can produce methyllycaconitine-sensitive GABAergic inhibitory postsynaptic currents (IPSCs) in nearby granule cells and enhance the amplitude of an electrically evoked monosynaptic IPSC. Further, activation of alpha 7-containing nAChRs on subgranular interneurons that is timed to coincide with synaptic release of glutamate onto these cells will enhance the functional inhibition of granule cells. These findings suggest that a complex interplay between glutamatergic afferents from the entorhinal cortex and cholinergic afferents from the medial septum could be involved in the normal regulation of granule cell function. Such a relationship between these two afferent pathways could be highly relevant to the study of both age-related memory dysfunction and disorders involving regulation of excitability, such as temporal lobe epilepsy.

Gating kinetics of the alpha1I T-type calcium channel

The alpha1I T-type calcium channel inactivates almost 10-fold more slowly than the other family members (alpha1G and alpha1H) or most native T-channels. We have examined the underlying mechanisms using whole-cell recordings from rat alpha1I stably expressed in HEK293 cells. We found several kinetic differences between alpha1G and alpha1I, including some properties that at first appear qualitatively different. Notably, alpha1I tail currents require two or even three exponentials, whereas alpha1G tails were well described by a single exponential over a wide voltage range. Also, closed-state inactivation is more significant for alpha1I, even for relatively strong depolarizations. Despite these differences, gating of alpha1I can be described by the same kinetic scheme used for alpha1G, where voltage sensor movement is allosterically coupled to inactivation. Nearly all of the rate constants in the model are 5-12-fold slower for alpha1I, but the microscopic rate for channel closing is fourfold faster. This suggests that T-channels share a common gating mechanism, but with considerable quantitative variability.

Apparent change in ion selectivity caused by changes in intracellular K(+) during whole-cell recording

In whole-cell recordings from HEK293 cells stably transfected with the delayed rectifier K(+) channel Kv2.1, long depolarizations produce current-dependent changes in [K(+)](i) that mimic inactivation and changes in ion selectivity. With 10 mM K(o)(+) or K(i)(+), and 140-160 mM Na(i,o)(+), long depolarizations shifted the reversal potential (V(R)) toward E(Na). However, similar shifts in V(R) were observed when Na(i,o)(+) was replaced with N-methyl-D-glucamine (NMG(+))(i, o). In that condition, [K(+)](o) did not change significantly, but the results could be quantitatively explained by changes in [K(+)](i). For example, a mean outward K(+) current of 1 nA for 2 s could decrease [K(+)](i) from 10 mM to 3 mM in a 10 pF cell. Dialysis by the recording pipette reduced but did not fully prevent changes in [K(+)](i). With 10 mM K(i,o)(+), 150 mM Na(i)(+), and 140 mM NMG(o)(+), steps to +20 mV produced a positive shift in V(R), as expected from depletion of K(i)(+), but opposite to the shift expected from a decreased K(+)/Na(+) selectivity. Long steps to V(R) caused inactivation, but no change in V(R). We conclude that current-dependent changes in [K(+)](i) need to be carefully evaluated when studying large K(+) currents in small cells.

Synaptic potentials mediated via alpha-bungarotoxin-sensitive nicotinic acetylcholine receptors in rat hippocampal interneurons

Exogenous application of acetylcholine elicits inward currents in hippocampal interneurons that are mediated via alpha-bungarotoxin-sensitive nicotinic acetylcholine receptors, but synaptic responses mediated via such receptors have never been reported in mammalian brain. In the present study, EPSCs were evoked in hippocampal interneurons in rat brain slices by electrical stimulation and were recorded by using whole-cell voltage-clamp techniques. Nicotinic EPSCs were isolated pharmacologically, using antagonists to block other known types of ligand-gated ion channels, and then were tested with either alpha-bungarotoxin or methyllycaconitine, which are selective antagonists for nicotinic acetylcholine receptors that contain the alpha7 receptor subunit. Each antagonist proved highly effective at reducing the remaining synaptic current. Evoked alpha7-mediated nicotinic EPSCs also were desensitized by superfusion with 1 microM nicotine, had extrapolated reversal potentials near 0 mV, and showed strong inward rectification at positive potentials. In several interneurons, methyllycaconitine-sensitive spontaneous EPSCs also were observed that exhibited a biphasic decay rate very similar to that of the alpha7-mediated evoked response. These studies provide the first demonstration of a functional cholinergic synapse in the mammalian brain, in which the primary postsynaptic receptors are alpha-bungarotoxin-sensitive nicotinic acetylcholine receptors.

Acetylcholine activates an alpha-bungarotoxin-sensitive nicotinic current in rat hippocampal interneurons, but not pyramidal cells

The effects of acetylcholine on both pyramidal neurons and interneurons in the area CA1 of the rat hippocampus were examined, using intracellular recording techniques in an in vitro slice preparation. In current-clamp mode, fast local application of acetylcholine (ACh) to the soma of inhibitory interneurons in stratum radiatum resulted in depolarization and rapid firing of action potentials. Under voltage-clamp, ACh produced fast, rapidly desensitizing inward currents that were insensitive to atropine but that were blocked by nanomolar concentrations of the nicotinic alpha7 receptor-selective antagonists alpha-bungarotoxin (alphaBgTx) and methyllycaconitine. Nicotinic receptor antagonists that are not selective for alpha7-containing receptors had little (mecamylamine) or no effect (dihydro-beta-erythroidine) on the ACh-induced currents. Glutamate receptor antagonists had no effect on the ACh-evoked response, indicating that the current was not mediated by presynaptic facilitation of glutamate release. However, the current could be desensitized almost completely by bath superfusion with 100 nM nicotine. In contrast to those actions on interneurons, application of ACh to the soma of CA1 pyramidal cells did not produce a detectable current. Radioligand-binding experiments with [125I]-alphaBgTx demonstrated that stratum radiatum interneurons express alpha7-containing nAChRs, and in situ hybridization revealed significant amounts of alpha7 mRNA. CA1 pyramidal cells did not show specific binding of [125I]-alphaBgTx and only low levels of alpha7 mRNA. These results suggest that, in addition to their proposed presynaptic role in modulating transmitter release, alpha7-containing nAChRs also may play a postsynaptic role in the excitation of hippocampal interneurons. By desensitizing these receptors, nicotine may disrupt this action and indirectly excite pyramidal neurons by reducing GABAergic inhibition.

Elevation of basal protein kinase C activity increases ethanol sensitivity of GABA(A) receptors in rat hippocampal CA1 pyramidal neurons

The ability of ethanol to enhance GABA(A) receptor function remains controversial; conflicting observations have been made even in the same brain region, and when using apparently similar methodologies. In this study we characterized a single protocol variable, the initial incubation temperature of brain slices, that had dramatic effects on the ethanol sensitivity of GABA(A) inhibitory postsynaptic currents (IPSCs) recorded from rat hippocampal CA1 pyramidal neurons. Incubation of hippocampal slices at relatively low temperatures (11-15 degrees C) immediately after slice preparation significantly affected a number of physiological and biochemical parameters. Such slices showed a decrease in extracellular inhibitory postsynaptic potential amplitude, a significant increase in the ethanol sensitivity of GABA(A) IPSCs in CA1 pyramidal neurons, no change in pentobarbital or flunitrazepam potentiation of IPSCs, and an increase in basal protein kinase C (PKC) activity relative to slices incubated at 31-33 degrees C. In addition, the increase in ethanol sensitivity of GABA(A) IPSCs was blocked by chelerythrine, a selective inhibitor of PKC. These results suggest that differences in hippocampal slice incubation protocols may have contributed to the disparate results of previous investigations of ethanol modulation of GABA(A) receptor-mediated synaptic transmission in the rat hippocampus. In addition, these findings provide further evidence that PKC activity positively modulates the interaction between ethanol and GABA(A) receptors in the mammalian brain.

Cholinergic deafferentation enhances rat hippocampal pyramidal neuron responsiveness to locally applied nicotine

We tested whether cholinergic denervation of the hippocampus of young rats would result in an enhancement of CA1 pyramidal cell responsiveness to nicotine. Electrolytic lesions of the medial septal area were performed in young male Fisher 344 rats. One month later the rats were anesthetized with pentobarbital and nicotine was locally applied to CA1 pyramidal neurons using pressure microejection. The dose of nicotine required to excite the pyramidal neurons was significantly lower for cells recorded from rats with septal lesions. However, no changes in hippocampal cytisine or alpha-bungarotoxin binding were found.

Platelet-derived growth factor induces a long-term inhibition of N-methyl-D-aspartate receptor function

Platelet-derived growth factor (PDGF) is a multifunctional protein that plays important roles in many tissues, including the mammalian central nervous system. PDGF and PDGF receptors (PDGFRs) are expressed in virtually every region of the central nervous system where they are involved in the development, survival, growth, and differentiation of both neuronal and glial cells. We now report that a brief activation of PDGFRs produced a long-lasting inhibition of N-methyl-D-aspartate (NMDA)-dependent excitatory postsynaptic currents in CA1 pyramidal neurons in rat hippocampal slices. PDGF also inhibited NMDA receptors (NMDA-Rs) in cultured hippocampal neurons by a mechanism that involves a decrease in single channel open probability. Non-NMDA receptor function was not affected by PDGF in hippocampal neurons. Experiments with mutant PDGFRs and chelation of intracellular Ca2+ in Xenopus oocytes indicate that this inhibition depends on a phospholipase C-gamma-induced elevation of intracellular Ca2+ levels. The PDGF-induced inhibition of NMDA-Rs is produced by a mechanism different than the well characterized phenomenon of Ca2+-dependent NMDA-R run down because the effect of PDGF was blocked by the phosphatase inhibitor, calyculin A, and was not affected by the microtubule polymerizing agent, phalloidin. Because elevations of PDGF levels are associated with neurological trauma or disease, we propose that PDGF can exert neuroprotective effects by inhibiting NMDA-R-dependent excitotoxicity.