Simultaneous Release of Glutamate and Acetylcholine from Single Magnocellular “Cholinergic” Basal Forebrain Neurons

Abstract

Basal forebrain (BF) neurons provide the principal cholinergic drive to the hippocampus and cortex. Their degeneration is associated with the cognitive defects of Alzheimer's disease. Immunohistochemical studies suggest that some of these neurons contain glutamate, so might also release it. To test this, we made microisland cultures of single BF neurons from 12-to 14-d-old rats. Over 1-8 weeks in culture, neuronal processes made autaptic connections onto the neuron. In 34 of 36 cells tested, a somatically generated action potential was followed by a short-latency EPSC that was blocked by 1 mM kynurenic acid, showing that they released glutamate. To test whether the same neuron also released acetylcholine, we placed a voltage-clamped rat myoball expressing nicotinic receptors in contact with a neurite. In six of six neurons tested, the glutamatergic EPSC was accompanied by a nicotinic (hexamethonium-sensitive) myoball current. Stimulation of the M 2 -muscarinic presynaptic receptors (characterized using tripitramine and pirenzepine) produced a parallel inhibition of autaptic glutamatergic and myoball nicotinic responses; metabotropic glutamate receptor stimulation produced similar but less consistent and weaker effects. Atropine enhanced the glutamatergic EPSCs during repetitive stimulation by 25 Ϯ 6%; the anti-cholinesterase neostigmine reduced the train EPSCs by 37 Ϯ 6%. Hence, synaptically released acetylcholine exerted a negative-feedback inhibition of coreleased glutamate. We conclude that most cholinergic basal forebrain neurons are capable of releasing glutamate as a cotransmitter and that the release of both transmitters is subject to simultaneous feedback inhibition by synaptically released acetylcholine. This has implications for BF neuron function and for the use of cholinesterase inhibitors in Alzheimer's disease.

Chronic Exposure to Nerve Growth Factor Increases Acetylcholine and Glutamate Release from Cholinergic Neurons of the Rat Medial Septum and Diagonal Band of Broca via Mechanisms Mediated by p75NTR

“…Using similar methods, Allen et al (2006) also found that all (6/6) magnocellular basal forebrain neurons that released glutamate coreleased acetylcholine. Despite these in vitro results, whether MS-DBB neurons corelease multiple neurotransmitters in vivo remains unknown.…”

“…It is unclear why many p75 NTRbearing "cholinergic" neurons in our study failed to display observable acetylcholine-mediated autaptic currents. A possible explanation may be that the density of postsynaptic nicotinic receptors was too low in some neurons to allow detection of acetylcholine being released (Allen et al, 2006), or some cholinergic neurons simply did not establish functional cholinergic synapses.…”

“…We now know that the MS-DBB also contains glutamatergic neurons (Colom et al, 2005) and neurons that coexpress markers for multiple neurotransmitter phenotypes (Sotty et al, 2003), indicating that some neurons have the molecular capacity to corelease multiple transmitters. Indeed, cultured basal forebrain neurons have recently been demonstrated to release acetylcholine and glutamate simultaneously (Allen et al, 2006). Nerve growth factor (NGF) is a potent neurotrophin for cholinergic MS-DBB neurons, promoting their survival, neurite outgrowth, and cholinergic phenotype expression (Hartikka and Hefti, 1988).…”

Four Excitatory Postsynaptic Ionotropic Receptors Coactivated at the Motoneuron–Renshaw Cell Synapse

“…For the MN-RC synapse, the only evidence for a corelease from the same MN is the paired recording between a MN and a RC obtained by Nishimaru et al (2005). Finally, individual "cholinergic" neurons of the basal forebrain have been shown, at least in culture, to release both glutamate and ACh (Allen et al, 2006;Huh et al, 2008). In none of the above experiments was a postsynaptic ␣ 7 component identified, but Li et al (2004) recorded in the presence of strychnine, bicuculline, and ␣-bungarotoxin, while Nishimaru et al (2005) and Huh et al (2008) used broad spectrum nicotinic antagonists.…”

Electrophysiological and morphological heterogeneity of slow firing neurons in medial septal/diagonal band complex as revealed by cluster analysis

“…However, recently described septal glutamatergic neurons may also express a slow firing phenotype (Sotty et al, 2003;Manseau et al, 2005). Furthermore, cholinergic basal forebrain neurons are capable of releasing glutamate as a cotransmitter (Allen et al, 2006). Other studies have shown the coexistence of GABA and ChAT in a small number of basal forebrain neurons (Brashear et al, 1986), as well as mRNA for ChAT and GAD 65/67 (Sotty et al, 2003).…”