The three main dopamine cell groups of the brain are located in the substantia nigra (A9), ventral tegmental area (A10), and retrorubral field (A8). Several subdivisions of these cell groups have been identified in rats and humans but have not been well described in mice, despite the increasing use of mice in neurodegenerative models designed to selectively damage A9 dopamine neurons. The aim of this study was to determine whether typical subdivisions of these dopamine cell groups are present in mice. The dopamine neuron groups were analysed in 15 adult C57BL/6J mice by anatomically localising tyrosine hydroxylase (TH), dopamine transporter protein (DAT), calbindin, and the G-protein-activated inward rectifier potassium channel 2 (GIRK2) proteins. Measurements of the labeling intensity, neuronal morphology, and the proportion of neurons double-labeled with TH, DAT, calbindin, or GIRK2 were used to differentiate subregions. Coronal maps were prepared and reconstructed in 3D. The A8 cell group had the largest dopamine neurons. Five subregions of A9 were identified: the reticular part with few dopamine neurons, the larger dorsal and smaller ventral dopamine tiers, and the medial and lateral parts of A9. The latter has groups containing some calbindin-immunoreactive dopamine neurons. The greatest diversity of dopamine cell types was identified in the seven subregions of A10. The main dopamine cell groups in the mouse brain are similar in terms of diversity to those observed in rats and humans. These findings are relevant to models using mice to analyse the selective vulnerability of different types of dopamine neurons.

Voltage‐gated K+ channels activating close to resting membrane potentials are widely expressed and differentially located in axons, presynaptic terminals and cell bodies. There is extensive evidence for localisation of Kv1 subunits at many central synaptic terminals but few clues to their presynaptic function. We have used the calyx of Held to investigate the role of presynaptic Kv1 channels in the rat by selectively blocking Kv1.1 and Kv1.2 containing channels with dendrotoxin‐K (DTX‐K) and tityustoxin‐Kα (TsTX‐Kα) respectively. We show that Kv1.2 homomers are responsible for two‐thirds of presynaptic low threshold current, whilst Kv1.1/Kv1.2 heteromers contribute the remaining current. These channels are located in the transition zone between the axon and synaptic terminal, contrasting with the high threshold K+ channel subunit Kv3.1 which is located on the synaptic terminal itself. Kv1 homomers were absent from bushy cell somata (from which the calyx axons arise); instead somatic low threshold channels consisted of heteromers containing Kv1.1, Kv1.2 and Kv1.6 subunits. Current‐clamp recording from the calyx showed that each presynaptic action potential (AP) was followed by a depolarising after‐potential (DAP) lasting around 50 ms. Kv1.1/Kv1.2 heteromers had little influence on terminal excitability, since DTX‐K did not alter AP firing. However TsTX‐Kα increased DAP amplitude, bringing the terminal closer to threshold for generating an additional AP. Paired pre‐ and postsynaptic recordings confirmed that this aberrant AP evoked an excitatory postsynaptic current (EPSC). We conclude that Kv1.2 channels have a general presynaptic function in suppressing terminal hyperexcitability during the depolarising after‐potential.

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A hyperpolarization‐activated non‐specific cation current, Ih, was examined in bushy cell bodies and their giant presynaptic terminals (calyx of Held). Whole‐cell patch clamp recordings were made using an in vitro brain slice preparation of the cochlear nucleus and the superior olivary complex. The aim was to characterise Ih in identified cell bodies and synaptic terminals, to examine modulation by presynaptic cAMP and to test for modulatory effects of Ih activation on synaptic transmission. Presynaptic Ih was activated by hyperpolarizing voltage‐steps, with half‐activation (V1/2) at –94 mV. Activation time constants were voltage dependent, showing an e‐fold acceleration for hyperpolarizations of –32 mV (time constant of 78 ms at –130 mV). The reversal potential of Ih was –29 mV. It was blocked by external perfusion of 1 mm CsCl but was unaffected by BaCl2. Application of internal cAMP shifted the activation curve to more positive potentials, giving a V1/2 of –74 mV; hence around half of the current was activated at resting membrane potentials. This shift in half‐activation was mimicked by external perfusion of a membrane‐permeant analogue, 8‐bromo‐cAMP. The bushy cell body Ih showed similar properties to those of the synaptic terminal; V1/2 was –94 mV and the reversal potential was –33 mV. Somatic Ih was blocked by CsCl (1 mm) and was partially sensitive to BaCl2. Somatic Ih current density increased with postnatal age from 5 to 16 days old, suggesting that Ih is functionally relevant during maturation of the auditory pathway. The function of Ih in regulating presynaptic excitability is subtle. Ih had little influence on EPSC amplitude at the calyx of Held, but may be associated with propagation of the action potential at branch points. Presynaptic Ih shares properties with both HCN1 and HCN2 recombinant channel subunits, in that it gates relatively rapidly and is modulated by internal cAMP.