Histamine depolarizes cholinergic septal neurons

Area of Science:

• Neuroscience
• Neuropharmacology

Background:

• Cholinergic neurons in the medial septum and nucleus of the diagonal band of Broca (MS/DBB) are crucial for cognitive functions.
• Histamine (HA) is a neuromodulator in the central nervous system, but its direct effects on MS/DBB neurons are not fully understood.

Purpose of the Study:

• To investigate the direct effects of histamine on cholinergic neurons in the MS/DBB.
• To elucidate the receptor subtypes and ionic mechanisms underlying histamine-induced neuronal responses.

Main Methods:

• Electrophysiological recordings (bridge mode and voltage-clamp) were performed on rat brain slices.
• Pharmacological manipulations using agonists and antagonists for histamine receptor subtypes were employed.
• Ionic substitution experiments and voltage-clamp ramp commands were used to identify ion conductances.

Main Results:

• Histamine (10 microM) induced depolarization and inward currents in 58/59 MS/DBB cholinergic neurons.
• Two distinct phases were observed: a fast depolarization linked to a TTX-insensitive Na+ conductance (H1 receptor-mediated) and a slow depolarization associated with a decrease in inwardly rectifying potassium conductance.
• The fast response was blocked by H1 antagonists (pyrilamine, promethazine) but not H2 antagonists (cimetidine).

Conclusions:

• Histamine directly depolarizes MS/DBB cholinergic neurons primarily through H1 receptor activation, leading to an increase in Na+ conductance.
• A secondary, slower depolarization is mediated by a reduction in inwardly rectifying potassium conductance, independent of classical histamine receptor subtypes.
• These findings reveal a novel mechanism of cholinergic neuron modulation by histamine in the brain.