Biology subjects

Kramer, P. F.

Publications and source records attributed to Kramer, P. F..

4 recordsLinked to original sources

bioRxiv2025

Excitatory nicotinic signaling drives action potential bursting in dopaminergic axons

Nicotinic acetylcholine receptors (nAChRs) facilitate striatal dopamine transmission but also suppress dopamine release during high-frequency stimulation, suggesting they act as as low pass filters of dopamine release. Because axonal excitability is key a determinant of transmission, we combined axonal recordings and calcium imaging to define the physiological conditions under which nAChRs bidirectionally control dopaminergic axons. Activation of cholinergic interneuron (CINs) recruited nAChRs to enhance dopaminergic axon signals under moderate activation but suppressed signals after strong high-frequency stimulation. Axonal recordings revealed that single-pulse striatal stimulation triggered a rapid ([~]125 Hz) burst of 2-3 nAChR-driven spikes in dopaminergic axons followed by a brief refractory period that inhibited further axon spiking. In sum, we show that nAChRs mainly enhance local excitability of striatal dopaminergic axons but also trigger axonal bursting that suppresses axonal excitability. This mechanism expands the computational power of dopaminergic axons and explains the apparent nAChR-mediated low-pass filtering of dopamine release.

neuroscience↗

bioRxiv2024

Presynaptic GABAA receptors control integration of nicotinic input onto dopaminergic axons in the striatum

Axons of dopaminergic neurons express gamma-aminobutyric acid type-A receptors (GABAARs) and nicotinic acetylcholine receptors (nAChRs) which are both independently positioned to shape striatal dopamine release. Using electrophysiology and calcium imaging, we investigated how interactions between GABAARs and nAChRs influence dopaminergic axon excitability. Direct axonal recordings showed that benzodiazepine application suppresses subthreshold axonal input from cholinergic interneurons (CINs). In imaging experiments, we used the first temporal derivative of presynaptic calcium signals to distinguish between direct- and nAChR-evoked activity in dopaminergic axons. We found that GABAAR antagonism with gabazine selectively enhanced nAChR-evoked axonal signals. Acetylcholine release was unchanged in gabazine suggesting that GABAARs located on dopaminergic axons, but not CINs, mediated this enhancement. Unexpectedly, we found that a widely used GABAAR antagonist, picrotoxin, inhibits axonal nAChRs and should be used cautiously for striatal circuit analysis. Overall, we demonstrate that GABAARs on dopaminergic axons regulate integration of nicotinic input to shape presynaptic excitability.

neuroscience↗

bioRxiv2022

Fast-synaptic axo-axonal transmission from striatal cholinergic interneurons onto dopamine fibers.

Transmission from striatal cholinergic interneurons (CINs) controls dopamine release through nicotinic acetylcholine receptors (nAChRs) on dopaminergic axons. Anatomical studies suggest that cholinergic terminals signal predominantly through non-synaptic volume transmission. However, the influence of cholinergic transmission on electrical signaling in axons remains unclear. We examined axo-axonal transmission from CINs onto dopaminergic axons using perforated-patch recordings which revealed rapid spontaneous EPSPs with properties characteristic of fast synapses. Pharmacology showed that axonal EPSPs (axEPSPs) were mediated primarily by high-affinity 6-containing receptors. Remarkably, axEPSPs triggered spontaneous action potentials locally in dopaminergic axons, suggesting these axons perform integration to convert synaptic input into spiking, a function associated with somatodendritic compartments. We investigated cross-species validity of cholinergic axo-axonal transmission by recording dopaminergic axons in macaque putamen and found similar axEPSPs. Thus, we reveal that fast synaptic-like neurotransmission underlies cholinergic signaling onto dopaminergic axons, providing insight into how nicotinic receptors shape electrical signaling directly in axon terminals.

neuroscience↗

bioRxiv2020

Axonal mechanisms mediating GABA-A receptor inhibition of striatal dopamine release

Axons of midbrain dopaminergic neurons innervate the striatum where they contribute to movement and reinforcement learning. Past work has shown that striatal GABA tonically inhibits dopamine release, but whether GABA-A receptors directly modulate transmission or act indirectly through circuit elements is unresolved. Here, we use whole-cell and perforated-patch recordings to test for GABA-A receptors on the main dopaminergic neuron axons and branching processes within striatum. Application of GABA depolarized axons, but also decreased the amplitude of axonal spikes, limited propagation and reduced striatal dopamine release. The mechanism of inhibition involved sodium channel inactivation and shunting. Lastly, we show that the positive allosteric modulator diazepam enhanced GABA-A currents on dopaminergic neuron axons and directly inhibited release, but also likely acts by reducing excitatory drive from cholinergic interneurons. Thus, we reveal the mechanisms of GABA-A receptor modulation of dopamine release and provide new insight into the actions of benzodiazepines within the striatum.

neuroscience↗