Location and function of the slow afterhyperpolarization channels in the basolateral amygdala

The basolateral amygdala (BLA) assigns emotional significance to sensory stimuli. This association results in a change in the output (action potentials) of BLA projection neurons in response to the stimulus. Neuronal output is controlled by the intrinsic excitability of the neuron. A major determinant of intrinsic excitability in these neurons is the slow after hyperpolarization (sAHP) that follows action potential (AP) trains and produces spike-frequency adaptation. The sAHP is mediated by a slow calcium-activated potassium current (sI(AHP)), but little is known about the channels that underlie this current. Here, using whole-cell patch-clamp recordings and high-speed calcium imaging from rat BLA projection neurons, we examined the location and function of these channels. We determined the location of the sI(AHP) by applying a hyperpolarizing voltage step during the sI(AHP) and measuring the time needed for the current to adapt to the new command potential, a function of its electrotonic distance from the somatic recording electrode. Channel location was also probed by focally uncaging calcium using a UV laser. Both methodologies indicated that, in BLA neurons, the sI(AHP) is primarily located in the dendritic tree. EPSPs recorded at the soma were smaller, decayed faster, and showed less summation during the sAHP. Adrenergic stimulation and buffering calcium reduced the sAHP and the attenuation of the EPSP during the sAHP. The sAHP also modulated the AP in the dendrite, reducing the calcium response evoked by a single AP. Thus, in addition to mediating spike-frequency adaptation, the sI(AHP) modulates communication between the soma and the dendrite

Drug-Driven AMPA Receptor Redistribution Mimicked by Selective Dopamine Neuron Stimulation

Addictive drugs have in common that they cause surges in dopamine (DA) concentration in the mesolimbic reward system and elicit synaptic plasticity in DA neurons of the ventral tegmental area (VTA). Cocaine for example drives insertion of GluA2-lacking AMPA receptors (AMPARs) at glutamatergic synapes in DA neurons. However it remains elusive which molecular target of cocaine drives such AMPAR redistribution and whether other addictive drugs (morphine and nicotine) cause similar changes through their effects on the mesolimbic DA system

Inhibitory circuits of mesolimbic system involved in drug addiction

All addictive drugs target the mesolimbic dopamine system that originates in the ventral tegmental area (VTA). VTA dopamine neurons project mainly to the nucleus accumbens (NAc), from where inhibitory medium spiny neurons (MSNs) project back to the VTA. Cocaine-evoked synaptic plasticity has been observed in excitatory transmission of the NAc and in the VTA but it remains unknown whether the GABAergic projections from the NAc to the VTA also undergo synaptic plasticity in response to cocaine treatment. Here, we used optogenetic projection targeting to selectively stimulate axons of NAc MSNs to record inhibitory postsynaptic currents (IPSCs) in acute VTA slices. We observed a preferential connectivity of MSNs onto VTA GABA neurons, which was confirmed in vivo. Extracellular single unit recordings in the VTA of anaesthetised mice revealed a strong inhibition of GABA neurons in response to the optogenetic stimulation of MSN terminals. In contrast, dopamine neurons showed increased firing rates in response to light stimulation, confirming a disinhibitory action of MSNs onto dopamine cells. Combining retrograde labeling with attenuated cholera toxine and immunohistochemistry we further show that the MSNs projecting directly to the VTA express D1Rs. We tested whether the synapses between MSNs and VTA GABA neurons undergo synaptic plasticity. A high frequency light stimulation protocol successfully potentiatiated IPSCs in VTA GABA cells by 81 ± 20%. Synaptic potentiation was insensitive to the Ca2+ buffer BAPTA in the postsynaptic cell but was prevented by blocking L-type voltage gated Ca2+ channels, indicating that the potentiation is induced presynaptically. The potentiation was mimicked by the adenylat cyclase activator forskolin, indicating that the cAMP-PKA cascade is involved in the mechanism. Further, using 2-photon imaging of activity induced uptake of the styryl dye FM4-64 at MSN terminals, we show that HFS induced potentiation of MSN-VTA GABA neuron synapses is expressed by an increased number of active release sites. We then treated mice with five daily cocaine injections and observed that HFS and forskolin were no longer able to induce inhibitory potentiation ex vivo 24 hours after the last injection. In accordance with an occlusion scenario, we observed paired pulse depression of light evoked IPSCs after cocaine treatment, indicative of increased GABA release at MSN-VTA GABA neuron synapses. Our data suggest that cocaine potentiates inhibitory transmission of D1-MSNs onto VTA GABA neurons via a PKA dependent mechanism. Taken together, these results suggest that cocaine may disinhibit VTA dopamine neurons by increasing GABA release onto VTA GABA neurons

Cocaine disinhibits dopamine neurons by potentiation of GABA transmission in the ventral tegmental area

Drug-evoked synaptic plasticity in the mesolimbic system reshapes circuit function and drives drug-adaptive behavior. Much research has focused on excitatory transmission in the ventral tegmental area (VTA) and the nucleus accumbens (NAc). How drug-evoked synaptic plasticity of inhibitory transmission affects circuit adaptations remains unknown. We found that medium spiny neurons expressing dopamine (DA) receptor type 1 (D1R-MSNs) of the NAc project to the VTA, strongly preferring the GABA neurons of the VTA. Repeated in vivo exposure to cocaine evoked synaptic potentiation at this synapse, occluding homosynaptic inhibitory long-term potentiation. The activity of the VTA GABA neurons was thus reduced and DA neurons were disinhibited. Cocaine-evoked potentiation of GABA release from D1R-MSNs affected drug-adaptive behavior, which identifies these neurons as a promising target for novel addiction treatments

Cortical dendritic activity correlates with spindle-rich oscillations during sleep in rodents

Different stages of sleep, marked by particular electroencephalographic (EEG) signatures, have been linked to memory consolidation, but underlying mechanisms are poorly understood. Here, the authors show that dendritic calcium synchronisation correlates with spindle-rich sleep phases

Addictive drugs cause rectification via AMPAR redistribution.

<p>(<b>A</b>) Representative traces of AMPAR excitatory postsynaptic currents recorded at −70, 0 and +40 mV. Examples are shown from recordings 24 h post injection. (<b>B</b>) Individual and averaged normalized rectification indices (RIs) (mean ± s.e.m) of saline and each drug treatment. RIs of morphine (2.12±0.27), nicotine (2.06±0.23) and cocaine (1.72±0.14) groups were significantly different from the saline (1.12±0.08) control group (F_(3,35) = 4.93, p<0.01, ANOVA. n = 7–15). (<b>C</b>) Representative electron micrographs of VTA sections from saline- or drug-treated animals. Large profiles (arrows) represent tyrosine hydroxylase (TH) immunoreactivity in dendrites (Den) forming asymmetrical synapses with boutons (b), and small profiles (arrowheads) represent GluA2 immunoreactivity. (<b>D</b>) Number of small profiles plotted against the distance from the postsynaptic density. (<b>E</b>) Same as in (C) but staining against PSD 95. (<b>F</b>) Same quantification as in (D) but for PSD 95.</p

Cocaine drives the insertion of GluA2-lacking AMPARs via its effect on the DAT.

<p>(<b>A</b>) Single unit extracellular <i>in vivo</i> recordings (above) and corresponding firing rate plots (below) of VTA neurons during a single i.p. injection of 15 mg/kg cocaine in either WT (left) or DAT_KI (right) mice. Black bar denotes injection time, (a) and (b) denote points from which example traces were taken. (<b>B</b>) The resulting inhibition of neuron firing rate observed in WT mice (38±3.3%) was not present in DAT_KI mice (94.9±1.4%). n = 4–5, t₍₇₎ = 16.5, p<0.0001. (<b>C</b>) Representative AMPAR excitatory postsynaptic currents recorded at −60, 0 and +30 mV (normalized to +40 mV AMPAR component) and RIs (<b>D</b>) of WT and DAT_KI mice 24 h post cocaine injection. Linearity corresponds to and RI of 1. Mean RI = 1.95±0.17 in WT, and 1.12±0.08 in DAT_KI; F_(2–22) = 9.8, p<0.001, n = 5–9). All data are expressed as mean ± sem.</p