Dopamine attenuates evoked inhibitory synaptic currents in central amygdala neurons.

The central nucleus of the amygdala (CeA) plays a critical role in regulating the behavioral, autonomic and endocrine response to stress. Dopamine (DA) participates in mediating the stress response and DA release is enhanced in the CeA during stressful events. However, the electrophysiological effects of DA on CeA neurons have not yet been characterized. Therefore, the purpose of this study was to identify and characterize the effect of DA application on electrophysiological responses of CeA neurons in coronal brain sections of male Sprague-Dawley rats. We used whole-cell patch-clamp electrophysiological techniques to record evoked synaptic responses and to determine basic membrane properties of CeA neurons both before and after DA superfusion. DA (20-250 µM) did not significantly alter membrane conductance over the voltage range tested. However, DA significantly reduced the peak amplitude of evoked inhibitory synaptic currents in CeA neurons. Pretreatment with the D(2) receptor antagonist eticlopride failed to significantly block the inhibitory effects of DA. In contrast, pretreatment with the D(1) receptor antagonist SCH-23390 significantly reduced the effects of DA on evoked inhibitory neurotransmission in these neurons. Moreover, bath superfusion of the specific D(1) receptor agonist SKF-39393, but not the D(2) receptor agonist quinpirole, significantly reduced peak amplitude of evoked inhibitory synaptic events. DA reduced the frequency of miniature IPSCs without altering the amplitude, while having no effect on the amplitude of IPSCs elicited by pressure application of GABA. These results suggest that DA may modulate inhibitory synaptic transmission in CeA through D(1) receptor activation primarily by a presynaptic mechanism.

Mu-opioid receptors selectively regulate basal inhibitory transmission in the central amygdala: lack of ethanol interactions.

Endogenous opioid systems are implicated in the actions of ethanol. For example, mu-opioid receptor (MOR) knockout (KO) mice self-administer less alcohol than the genetically intact counterpart wild-type (WT) mice (Roberts et al., 2000). MOR KO mice also exhibit less anxiety-like behavior than WT mice (Filliol et al., 2000). To investigate the neurobiological mechanisms underlying these behaviors, we examined the effect of ethanol in brain slices from MOR KO and WT mice using sharp-electrode and whole-cell patch recording techniques. We focused our study in the central nucleus of the amygdala (CeA) because it is implicated in alcohol drinking behavior and stress behavior. We found that the amplitudes of evoked inhibitory postsynaptic currents (IPSCs) or inhibitory postsynaptic potentials (IPSPs) were significantly greater in MOR KO mice than WT mice. In addition, the baseline frequencies of spontaneous and miniature GABA(A) receptor-mediated inhibitory postsynaptic currents were significantly greater in CeA neurons from MOR KO than WT mice. However, ethanol enhancements of evoked IPSP and IPSC amplitudes and the frequency of miniature IPSCs were comparable between WT and MOR KO mice. Baseline spontaneous and miniature excitatory postsynaptic currents (EPSCs) and ethanol effects on EPSCs were not significantly different between MOR KO and WT mice. Based on knowledge of CeA circuitry and projections, we hypothesize that the role of MOR- and GABA receptor-mediated mechanisms in CeA underlying reinforcing effects of ethanol operate independently, possibly through pathway-specific responses within CeA.

Differential sensitivity of GABA A receptor-mediated IPSCs to cannabinoids in hippocampal slices from adolescent and adult rats.

The impairment of learning and memory is one of the most powerful and least understood effects of marijuana although the hippocampal formation appears to be one CNS region mediating these effects. We have shown that systemic injection of Delta9-tetrahydrocannabinol (THC), an active component of marijuana, impairs spatial learning more efficaciously in adolescent rats, compared with adult rats, but there have been no studies of the cellular mechanisms underlying this developmental sensitivity. In this study, we examined cannabinoid-mediated activity in hippocampal area CA1 neurons in brain slices from adolescent and adult rats. The magnitude of endocannabinoid-mediated synaptic functions such as long-term depression of inhibition was greater in the hippocampal slices from adolescent rats than in those from adults. The effect of R-(+)-(2,3-dihydro-5-methyl-3-[(4-morpholinyl)methyl]pyrrolo[1,2,3-de]-1,4-benzoxazine-6-yl)(1-naphtalenyl) methanone mesylate (WIN55,212-2), an exogenous cannabinoid agonist, to suppress GABA(A) receptor-mediated synaptic responses was also greater in the hippocampal slices from adolescent rats than in those from adults. However, tonic endocannabinoid effects, shown as an increase of the spontaneous IPSC frequency by N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (AM251), a specific CB1 receptor antagonist, were greater in CA1 neurons from adult rats than in those from adolescent rats. On the other hand, WIN55,212-2 suppressed glutamate-mediated excitatory neurotransmission in CA1 pyramidal cells from adolescent and adult rats with similar efficacy. These results indicate that inhibitory synaptic function in the adolescent hippocampus is more sensitive to cannabinoid effects and may account, in part, for the greater sensitivity of adolescent animals to THC-induced memory impairment.

Presynaptic delta opioid receptors regulate ethanol actions in central amygdala.

Endogenous opioid systems are implicated in the reinforcing effects of ethanol consumption. For example, delta opioid receptor (DOR) knockout (KO) mice show greater ethanol consumption than wild-type (WT) mice (Roberts et al., 2001). To explore the neurobiological correlates underlying these behaviors, we examined effects of acute ethanol application in brain slices from DOR KO mice using whole-cell patch recording techniques. We examined the central nucleus of amygdala (CeA) because the CeA is implicated in alcohol reinforcement (Koob et al., 1998). We found that the acute ethanol effects on GABA(A) receptor-mediated inhibitory postsynaptic currents (IPSCs) were greater in DOR KO mice than in WT mice. Ethanol increased the frequency of miniature IPSCs (mIPSCs) significantly more in DOR KO mice than in WT mice. In CeA of WT mice, application of ICI 174864 [[allyl]2-Tyr-alpha-amino-isobutyric acid (Aib)-Aib-Phe-Leu-OH], a DOR inverse agonist, augmented ethanol actions on mIPSC frequency comparable with ethanol effects seen in DOR KO mice. Superfusion of the selective DOR agonist D-Pen(2),D-Pen(5)-enkephalin decreased the mean frequency of mIPSCs; this effect was reversed by the DOR antagonist naltrindole. These findings suggest that endogenous opioids may reduce ethanol actions on IPSCs of CeA neurons in WT mice through DOR-mediated inhibition of GABA release and that the increased ethanol effect on IPSCs in CeA of DOR KO mice could be, at least in part, due to absence of DOR-mediated inhibition of GABA release. This result supports the hypothesis that endogenous opioid peptides modulate the ethanol-induced augmentation of GABA(A) receptor-dependent circuitry in CeA (Roberto et al., 2003).

Protein phosphatases mediate depotentiation induced by high-intensity theta-burst stimulation.

We have previously reported that varying stimulus intensity produces qualitatively different types of synaptic plasticity in area CA1 of hippocampal slices: brief low-intensity (LI) theta-burst (TB) stimuli induce long-term potentiation (LTP), but if the stimulus intensity is increased (to mimic conditions that may exist during seizures), LTP is not induced; instead, high-intensity (HI) TB stimuli erase previously induced LTP ("TB depotentiation"). We now have explored the mechanisms underlying TB depotentiation using extracellular field recordings with pharmacological manipulations. We found that TB depotentiation was blocked by okadaic acid and calyculin A (inhibitors of serine/threonine protein phosphatases PP1 and PP2A), FK506 (a specific blocker of calcineurin, a Ca(2+)/calmodulin (CaM) protein phosphatase), and 8-Br-cAMP (an activator of protein kinase A) with 3-isobutyl-1-methylxanthine (IBMX, a phosphodiesterase inhibitor). These results suggest that protein phosphatase pathways are involved in the TB depotentiation similar to other type of down-regulating synaptic plasticity such as low-frequency stimulation (LFS)-induced long-term depression (LTD) and depotentiation in the rat hippocampus. However, TB depotentiation and LFS depotentiation could have differential functional significance.

Properties of the pathways from the lateral amygdal nucleus to basolateral nucleus and amygdalostriatal transition area.

Studies have revealed that the amygdala formation is involved in emotional learning, attention, and autonomic functions. Although intra-amygdala connections have been described anatomically, the functional characteristics of these connections are not well understood. We used a rat brain slice preparation with a voltage-sensitive imaging system to compare the electrophysiological characteristics of intra-amygdala pathways. Electrical stimuli delivered to the lateral nucleus (La) caused the optical signal to propagate to basolateral nucleus (BL) and amygdalostriatal transition area (AStr), but not the central nucleus (Ce), consistent with previous anatomical studies, including the recently characterized projections from La to AStr. The velocity of propagation of the evoked potential along the La-AStr pathway was significantly faster than that along the La-BL pathway. In addition, the efficiency of the signal transmission (determined by the rate of decay) along the La-AStr pathway was higher than that along the La-BL pathway. Also, AStr possessed a distinct property of temporal summation of La signals. On the other hand, the La-BL pathway possessed a significantly higher sensitivity to bicuculline/picrotoxin and a stronger paired-pulse inhibition than the La-AStr pathway. Furthermore, the La-BL pathway expressed a higher D-2-amino-5-phosphonovaleric acid (a NMDA blocker) sensitivity than the La-AStr pathway. These results suggest that the La-AStr pathway, which conducts signals with high velocity and less attenuation, may be involved in rapid reflexive responses during fear-induced behavior, whereas the La-BL pathway facilitates signal integration and learning.