Expression of Nicotinic Acetylcholine Receptor α4 and β2 Subunits on Direction-Selective Retinal Ganglion Cells in the Rabbit

Target identification and validation of the alpha7 nicotinic acetylcholine receptor as a potential therapeutic target in retinal disease

The role of acetylcholine (ACh) in visual processing in the mammalian retina has been the focus of research for many decades. Pioneering work on the localization of ACh discovered that the neurotransmitter is synthesized and stored in a distinct subpopulation of amacrine (starburst) cells. It has been shown that ACh release is regulated to a low resting "tonic" level, much like what is observed at the neuromuscular junction (NMJ). If there were a dysfunction in the tonic release of ACh, might post-synaptic changes render the targets of ACh [i.e., retinal ganglion cells (RGCs)] vulnerable to disease? During my time at Pharmacia & Upjohn (PNU), selective nicotinic ACh receptor (nAChR) agonists (e.g., PNU-282987) were developed as a possible therapy for central nervous system (CNS) diseases. As RGCs are the main targets of neurodegeneration in glaucoma, could the activation of this target provide neuroprotection? In response to this question, experiments to identify alpha7 nAChRs in the retina (i.e., target ID studies) followed by "proof-of-concept" experiments were conducted. Target ID studies included binding studies with retinal homogenates, [125I]-alpha-bungarotoxin (α-BTX) autoradiography, and fluorescently tagged α-BTX binding in retinal slices. Imaging studies of intracellular calcium dynamics in the retinal slice were conducted. Reverse transcription-polymerase chain reaction (RT-PCR) analysis with alpha7 nAChR knockout mice using the "laser-capture microdissection" technique, in situ hybridization studies, and RT-PCR analysis of the human retina were conducted. Collectively, these experiments confirmed the presence of alpha7 nAChRs on specific cells in the retina. "Proof-of-concept" neuroprotection studies demonstrated that PNU-282987 provided significant protection for RGCs. This protection was dose dependent and was blocked with selective antagonists. More recently, evidence for the generation of new RGCs has been reported with PNU-282987 in rodents. Interestingly, the appearance of new RGCs is more pronounced with eye-drop application than with intravitreal injection. One could postulate that this reflects the neurogenic activation of alpha7 receptors on the retinal pigment epithelium (RPE) (eye drops) vs. a neuroprotective effect on RGCs (injections). In conclusion, there does appear to be a cholinergic retinal "tone" associated with RGCs that could be utilized as a neuroprotective therapy. However, a distinct cholinergic neurogenic mechanism also appears to exist in the outer retina that could possibly be exploited to generate new RGCs lost through various disease processes.

NMDA Receptor Activation and Ca2+/PKC Signaling in Nicotine-Induced GABA Transport Shift in Embryonic Chick Retina

Nicotinic receptors are present in the retina of different vertebrates, and in the chick retina, it is present during early development throughout to post-hatching. These receptors are activated by nicotine, an alkaloid with addictive and neurotransmitter release modulation properties, such as GABA signaling. Here we evaluated the mechanisms of nicotine signaling in the avian retina during the development of neuron-glia cells at a stage where synapses are peaking. Nicotine almost halved [³H]-GABA uptake, reducing it by 45% whilst increasing more than two-fold [³H]-GABA release in E12 embryonic chick retinas. Additionally, nicotine mediated a 33% increase in [³H]-D-aspartate release. MK-801 50 μM blocked 66% of nicotine-induced [³H]-GABA release and Gö 6983 100 nM prevented the nicotine-induced reduction in [³H]-GABA uptake by rescuing 40% of this neurotransmitter uptake, implicating NMDAR and PKC (respectively) in the nicotinic responses. In addition, NO-711 prevented [³H]-GABA uptake and release induced by nicotine. Furthermore, the relevance of calcium influx for PKC activation was evidenced through fura-2 imaging. We conclude that the shift of GABA transport mediated by nicotine promotes GABA release by inducing transporter reversal via nicotine-induced EAA release through EAATs, or by a direct effect of nicotine in activating nicotinic receptors permeable to calcium and promoting PKC pathway activation and shifting GAT-1 activity, both prompting calcium influx, and activation of the PKC pathway and shifting GAT-1 activity.

Acetylcholine regulation of GnRH neuronal activity: A circuit in the medial septum

In vertebrates, gonadotropin-releasing hormone (GnRH)-secreting neurons control fertility by regulating gonadotrophs in the anterior pituitary. While it is known that acetylcholine (ACh) influences GnRH secretion, whether the effect is direct or indirect, and the specific ACh receptor (AChR) subtype(s) involved remain unclear. Here, we determined 1) whether ACh can modulate GnRH cellular activity and 2) a source of ACh afferents contacting GnRH neurons. Calcium imaging was used to assay GnRH neuronal activity. With GABAergic and glutamatergic transmission blocked, subtype-specific AChR agonists and antagonists were applied to identify direct regulation of GnRH neurons. ACh and nicotine caused a rise in calcium that declined gradually back to baseline after 5-6 min. This response was mimicked by an alpha3-specific agonist. In contrast, muscarine inhibited GnRH calcium oscillations, and blocking M2 and M4 together prevented this inhibition. Labeling for choline acetyltransferase (ChAT) and GnRH revealed ChAT fibers contacting GnRH neurons, primarily in the medial septum (MS), and in greater number in females than males. ChAT positive cells in the MS are known to express p75NGFRs. Labeling for p75NGFR, ChAT and GnRH indicated that ChAT fibers contacting GnRH cells originate from cholinergic cells within these same rostral areas. Together, these results indicate that cholinergic cells in septal areas can directly regulate GnRH neurons.