1 Extracellular single-unit recording and iontophoresis were used to examine the e ects of di erent cholinoceptor agonists and antagonists on the ®ring rate and ®ring pattern of A9 and A10 presumed dopaminergic neurones in the anaesthetized rat. 2 Administration of low currents (1 ± 5 nA) of the selective muscarinic agonists oxotremorine M (Oxo M) and muscarine and of the non-selective muscarinic/nicotinic agonist carbamylcholine (CCh) produced a dose-dependent increase in ®ring rate in most of the A9 and A10 presumed dopaminergic neurones tested. Oxo M-induced activation could be completely blocked by iontophoretic application of the muscarinic antagonist butyl-scopolamine or systemic administration of the muscarinic antagonist scopolamine (300 mg kg 71 , i.v.). 3 Iontophoretic application of the selective nicotinic agonist methylcarbamylcholine (MCCh), but not nicotine, induced a consistent increase in ®ring rate. Surprisingly, the excitatory e ect of MCCh was signi®cantly reduced by the selective muscarinic antagonist scopolamine (300 mg kg 71 , i.v.), but not by the selective nicotinic antagonist mecamylamine (2.2 mg kg 71 , i.v.). Mecamylamine (3 mg kg 71 , i.v.) was also ine ective in reducing the CCh-induced activation of presumed dopamine neurones, suggesting that both CCh and MCCh increased the activity of dopamine neurones via an interaction with muscarinic receptors. 4 Iontophoretic application of the endogenous agonist acetylcholine (ACh) had no or little e ect on the ®ring activity of A10 presumed dopaminergic neurones. However, concomitant application of neostigmine, a potent cholinesterase inhibitor, with acetylcholine induced a substantial activation of these neurones. This activation consisted of two components; one, which was prevalent, was scopolamine (300 mg kg 71 , i.v.)-sensitive, and the other was mecamylamine (2 mg kg 71 , i.v.)-sensitive. 5 In addition to their e ect on ®ring activity, Oxo M, muscarine and concomitant neostigmine/ACh caused a signi®cant increase in burst ®ring of A10 neurones, but not of A9 neurones. 6 These data suggest that dopamine cells, both in the A9 and A10 regions, possess functional muscarinic receptors, the activation of which can increase their ®ring rate and, for A10 neurones, their amount of burst activity. These cholinoceptors would be able to in¯uence the activity of the midbrain dopamine system greatly and may play a role in, and/or be a therapeutic target for, brain disorders in which dopamine is involved (e.g., Parkinson's disease, drug addiction and schizophrenia).

We have previously shown that prebiotics (dietary fibres that augment the growth of indigenous beneficial gut bacteria) such as Bimuno™ galacto-oligosaccharides (B-GOS®), increased N-methyl-D-aspartate (NMDA) receptor levels in the rat brain. The current investigation examined the functional correlates of these changes in B-GOS®-fed rats by measuring cortical neuronal responses to NMDA using in vivo NMDA micro-iontophoresis electrophysiology, and performance in the attentional set-shifting task. Adult male rats were supplemented with B-GOS® in the drinking water 3 weeks prior to in vivo iontophoresis or behavioural testing. Cortical neuronal responses to NMDA iontophoresis, were greater (+30%) in B-GOS® administered rats compared to non-supplemented controls. The intake of B-GOS® also partially hindered the reduction of NMDA responses by the glycine site antagonist, HA-966. In the attentional set-shifting task, B-GOS® -fed rats shifted from an intra-dimensional to an extra-dimensional set in fewer trials than controls, thereby indicating greater cognitive flexibility. An initial exploration into the mechanisms revealed that rats ingesting B-GOS® had increased levels of plasma acetate, and cortical GluN2B subunits and Acetyl Co-A Carboxylase mRNA. These changes were also observed in rats fed daily for 3 weeks with glyceryl triacetate, though unlike B-GOS®, cortical histone deacetylase (HDAC1, HDAC2) mRNAs were also increased which suggested an additional epigenetic action of direct acetate supplementation. Our data demonstrate that a pro-cognitive effect of B-GOS® intake in rats is associated with an increase in cortical NMDA receptor function, but the role of circulating acetate derived from gut bacterial fermentation of this prebiotic requires further investigation.

These data demonstrate that VTA dopamine cells possess functional muscarinic receptors whose activation stimulates the release of dopamine in the VTA, NAC and FC. These results also suggest that muscarinic receptors may modulate the synaptic release of serotonin in the VTA.

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