Evidence for muscarinic acetylcholine receptor subtypes in the pigeon telencephalon

Neurochemistry and circuit organization of the lateral spiriform nucleus of birds: A uniquely nonmammalian direct pathway component of the basal ganglia

We used diverse methods to characterize the role of avian lateral spiriform nucleus (SpL) in basal ganglia motor function. Connectivity analysis showed that SpL receives input from globus pallidus (GP), and the intrapeduncular nucleus (INP) located ventromedial to GP, whose neurons express numerous striatal markers. SpL-projecting GP neurons were large and aspiny, while SpL-projecting INP neurons were medium sized and spiny. Connectivity analysis further showed that SpL receives inputs from subthalamic nucleus (STN) and substantia nigra pars reticulata (SNr), and that the SNr also receives inputs from GP, INP, and STN. Neurochemical analysis showed that SpL neurons express ENK, GAD, and a variety of pallidal neuron markers, and receive GABAergic terminals, some of which also contain DARPP32, consistent with GP pallidal and INP striatal inputs. Connectivity and neurochemical analysis showed that the SpL input to tectum prominently ends on GABAA receptor-enriched tectobulbar neurons. Behavioral studies showed that lesions of SpL impair visuomotor behaviors involving tracking and pecking moving targets. Our results suggest that SpL modulates brainstem-projecting tectobulbar neurons in a manner comparable to the demonstrated influence of GP internus on motor thalamus and of SNr on tectobulbar neurons in mammals. Given published data in amphibians and reptiles, it seems likely the SpL circuit represents a major direct pathway-type circuit by which the basal ganglia exerts its motor influence in nonmammalian tetrapods. The present studies also show that avian striatum is divided into three spatially segregated territories with differing connectivity, a medial striato-nigral territory, a dorsolateral striato-GP territory, and the ventrolateral INP motor territory.

In vitro characterization of gamma oscillations in the hippocampal formation of the domestic chick

Avian and mammalian brains have evolved independently from each other for about 300 million years. During that time, the hippocampal formation (HF) has diverged in morphology and cytoarchitecture, but seems to have conserved much of its function. It is therefore an open question how seemingly different neural organizations can generate the same function. A prominent feature of the mammalian hippocampus is that it generates different neural oscillations, including the gamma rhythm, which plays an important role in memory processing. In this study, we investigate whether the avian hippocampus also generates gamma oscillations, and whether similar pharmacological mechanisms are involved in this function. We investigated the existence of gamma oscillations in avian HF using in vitro electrophysiology in P0–P12 domestic chick (Gallus gallus domesticus) HF brain slices. Persistent gamma frequency oscillations were induced by the bath application of the cholinergic agonist carbachol, but not by kainate, a glutamate receptor agonist. Similar to other species, carbachol‐evoked gamma oscillations were sensitive to GABA_A, AMPA/kainate and muscarinic (M1) receptor antagonism. Therefore, similar to mammalian species, muscarinic receptor‐activated avian HF gamma oscillations may arise via a pyramidal‐interneuron gamma (PING)‐based mechanism. Gamma oscillations are most prominent in the ventromedial area of the hippocampal slices, and gamma power is reduced more laterally and dorsally in the HF. We conclude that similar micro‐circuitry may exist in the avian and mammalian hippocampal formation, and this is likely to relate to the shared function of the two structures.

Transmitter receptors reveal segregation of the arcopallium/amygdala complex in pigeons (Columba livia)

At the beginning of the 20th century it was suggested that a complex group of nuclei in the avian posterior ventral telencephalon is comparable to the mammalian amygdala. Subsequent findings, however, revealed that most of these structures share premotor characteristics, while some indeed constitute the avian amygdala. These developments resulted in 2004 in a change of nomenclature of these nuclei, which from then on were named arcopallial or amygdala nuclei and referred to as the arcopallium/amygdala complex. The structural basis for the similarities between avian and mammalian arcopallial and amygdala subregions is poorly understood. Therefore, we analyzed binding site densities for glutamatergic AMPA, NMDA and kainate, GABAergic GABA_A , muscarinic M₁ , M₂ and nicotinic acetylcholine (nACh; α₄ β₂ subtype), noradrenergic α₁ and α₂ , serotonergic 5-HT_1A and dopaminergic D_1/5 receptors using quantitative in vitro receptor autoradiography combined with a detailed analysis of the cyto- and myelo-architecture. Our approach supports a segregation of the pigeon's arcopallium/amygdala complex into the following subregions: the arcopallium anterius (AA), the arcopallium ventrale (AV), the arcopallium dorsale (AD), the arcopallium intermedium (AI), the arcopallium mediale (AM), the arcopallium posterius (AP), the nucleus posterioris amygdalopallii pars basalis (PoAb) and pars compacta (PoAc), the nucleus taeniae amgygdalae (TnA) and the area subpallialis amygdalae (SpA). Some of these subregions showed further subnuclei and each region of the arcopallium/amygdala complex are characterized by a distinct multi-receptor density expression. Here we provide a new detailed map of the pigeon's arcopallium/amygdala complex and compare the receptor architecture of the subregions to their possible mammalian counterparts.

The avian subpallium: new insights into structural and functional subdivisions occupying the lateral subpallial wall and their embryological origins

The subpallial region of the avian telencephalon contains neural systems whose functions are critical to the survival of individual vertebrates and their species. The subpallial neural structures can be grouped into five major functional systems, namely the dorsal somatomotor basal ganglia; ventral viscerolimbic basal ganglia; subpallial extended amygdala including the central and medial extended amygdala and bed nuclei of the stria terminalis; basal telencephalic cholinergic and non-cholinergic corticopetal systems; and septum. The paper provides an overview of the major developmental, neuroanatomical and functional characteristics of the first four of these neural systems, all of which belong to the lateral telencephalic wall. The review particularly focuses on new findings that have emerged since the identity, extent and terminology for the regions were considered by the Avian Brain Nomenclature Forum. New terminology is introduced as appropriate based on the new findings. The paper also addresses regional similarities and differences between birds and mammals, and notes areas where gaps in knowledge occur for birds.

Muscarinic acetylcholine receptor subtype expression in avian vestibular hair cells, nerve terminals and ganglion cells

Muscarinic acetylcholine receptors (mAChRs) are widely expressed in the CNS and peripheral nervous system and play an important role in modulating the cell activity and function. We have shown that the cholinergic agonist carbachol reduces the pigeon's inwardly rectifying potassium channel (pKir2.1) ionic currents in native vestibular hair cells. We have cloned and sequenced pigeon mAChR subtypes M2-M5 and we have studied the expression of all five mAChR subtypes (M1-M5) in the pigeon vestibular end organs (semicircular canal ampullary cristae and utricular maculae), vestibular nerve fibers and the vestibular (Scarpa's) ganglion using tissue immunohistochemistry (IH), dissociated single cell immunocytochemistry (IC) and Western blotting (WB). We found that vestibular hair cells, nerve fibers and ganglion cells each expressed all five (M1-M5) mAChR subtypes. Two of the three odd-numbered mAChRs (M1, M5) were present on the hair cell cilia, supporting cells and nerve terminals. And all three odd numbered mAChRs (M1, M3 and M5) were expressed on cuticular plates, myelin sheaths and Schwann cells. Even-numbered mAChRs were seen on the nerve terminals. M2 was also shown on the cuticular plates and supporting cells. Vestibular efferent fibers and terminals were not identified in our studies. Results from WB of the dissociated vestibular epithelia, nerve fibers and vestibular ganglia were consistent with the results from IH and IC. Our findings suggest that there is considerable co-expression of the subtypes on the neural elements of the labyrinth. Further electrophysiological and pharmacological studies should delineate the mechanisms of action of muscarinic acetylcholine receptors on structures in the labyrinth.

Distribution of the limbic system-associated membrane protein (LAMP) in pigeon forebrain and midbrain

The limbic system-associated membrane protein (LAMP) is an adhesion molecule involved in specifying regional identity during development, and it is enriched in the neuropil of limbic brain regions in mammals but also found in some somatic structures. Although originally identified in rat, LAMP is present in diverse species, including avians. In this study, we used immunolabeling with a monoclonal antibody against rat LAMP to examine the distribution of LAMP in pigeon forebrain and midbrain. LAMP immunolabeling was prominent in many telencephalic regions previously noted as limbic in birds. These regions include the hippocampal complex, the medial nidopallium, and the ventromedial arcopallium. Subpallial targets of these pallial regions were also enriched in LAMP, such as the medial-most medial striatum. Whereas some telencephalic areas that have not been regarded as limbic were also LAMP-rich (e.g., the hyperpallium intercalatum and densocellulare of the Wulst, the mesopallium, and the intrapeduncular nucleus), most nonlimbic telencephalic areas were LAMP-poor (e.g., field L, the lateral nidopallium, and somatic basal ganglia). Similarly, in the diencephalon and midbrain, prominent LAMP labeling was observed in such limbic areas as the dorsomedial thalamus, the hypothalamus, the ventral tegmental area, and the central midbrain gray, as well as in a few nonlimbic areas such as nucleus rotundus, the shell of the nucleus pretectalis, the superficial tectum, and the parvocellular isthmic nucleus. Thus, as in mammals, LAMP in birds appears to be enriched in most known forebrain and midbrain limbic structures but is present as well in some somatic structures.

Muscarinic acetylcholine receptors in the brain of the zebrafish (Danio rerio) measured by radioligand binding techniques

Muscarinic acetylcholine receptors (mAChRs) play a role in learning, memory and behavior in vertebrate animals. We measured the muscarinic cholinergic receptor levels in extracts from zebrafish (Danio rerio) brain by radioligand binding techniques. Saturation binding experiments with the radioligand [3H]-quinuclidinyl benzilate (QNB) were used to determine receptor number and relative affinity for several agonists and antagonists. Affinity at zebrafish brain receptors was relatively high with a K(d) of 40 +/- 5 pM. The number of receptors, represented by Bmax, was 63 +/- 16 fmol/mg protein. Oxotremorine and carbachol, agonists at muscarinic acetylcholine receptors, bound with displacement curves indicating multiple binding sites. In addition, oxotremorine bound with a higher affinity than did carbachol. The antagonist potency profile at zebrafish receptors in brain was determined to be atropine>>pirenzipine>p-fluoro-hexahydro-sila-difenidol>>otenzepad. The results obtained with zebrafish brain compare favorably to those found in insect, fish and mammalian species. Taken together, the binding results and favorable comparisons to mammalian systems indicate that zebrafish may provide a useful model organism for evaluating the role of cholinergic systems in learning, memory and behavior.

Internal connectivity of the homing pigeon (Columba livia) hippocampal formation: an anterograde and retrograde tracer study

The avian hippocampal formation (HF) is a structure necessary for learning and remembering aspects of environmental space. Therefore, understanding the connections between different HF regions is important for determining how spatial learning processes are organized within the avian brain. The prevailing feed-forward, trisynaptic internal connectivity of the mammalian hippocampus and its importance for cognition have been well described, but the internal connectivity of the avian HF has only recently been investigated. To examine further the connectivity within the avian HF, small amounts of cholera toxin subunit B, primarily a retrograde tracer (n = 15), or biotinylated dextran amine, primarily an anterograde tracer (n = 10), were injected into localized regions of the HF. Examination of the immunohistochemically labeled tissue showed projections from extrinsic sensory processing areas into dorsolateral HF and the dorsal portion of the dorsomedial HF (DMd). DMd in turn projected into the medial (VM) and lateral (VL) ventral cell layers. A projection from VM into VL was found, and together these areas and DM provided input into the contralateral ventral cell layers. Ipsilaterally, a ventral portion of dorsomedial HF (DMv) received input from VL and VM. From DMv, projections exited HF laterally. The highlighted projections formed a discernible feed-forward processing network through the avian HF that resembled the trisynaptic circuit of the mammalian HF.

Localization of dopamine D1A and D1B receptor mRNAs in the forebrain and midbrain of the domestic chick

The distribution and cellular localization of dopamine D1A and D1B receptor mRNAs in the forebrain and midbrain of the domestic chick were examined using in situ hybridization histochemistry with 35[S]-dATP labeled oligonucleotide probes, visualized with film and emulsion autoradiography. Labeling for D1A receptor mRNA was intense in the medial and lateral striatum, and moderately abundant in the pallial regions termed the archistriatum and the neostriatum, in the hypothalamic paraventricular nucleus region, and in the superficial gray layer of optic tectum of the midbrain. D1B receptor mRNA was abundant in the medial and lateral striatum, and in the pallial region termed the hyperstriatum ventrale, and moderately abundant in the intralaminar dorsal and posterior thalamus and in the superficial gray of the optic tectum. At the cellular level, about 75% of neurons in the medial striatum and 59% of neurons in the lateral striatum were labeled for D1A receptor mRNA, whereas about 39% of the neurons in the medial striatum and 21% in the lateral striatum were labeled for D1B receptor mRNA. Large striatal neurons were not labeled for D1A or D1B receptor mRNA. The data suggest that while both D1A and D1B receptors mediate dopaminergic responses in many neurons of the avian striatum, primarily D1A receptors mediate dopaminergic responses in the archistriatum and the neostriatum, while primarily D1B receptors mediate dopaminergic responses in the hyperstriatum ventrale and the thalamus.

Changes in binding to muscarinic and nicotinic cholinergic receptors in the chick telencephalon, following passive avoidance learning

Changes in nicotinic and muscarinic cholinergic receptors 30 min after one-trial passive avoidance training were studied in day-old chicks (Gallus domesticus), by quantitative receptor autoradiography. [3H]-alpha-bungarotoxin (alpha-BgT) and [3H]-quinuclidinyl benzilate (QNB) were used to monitor changes in 15 forebrain regions for nicotinic and muscarinic receptors, respectively. A significant increase occurred bilaterally in the quantity of bound alpha-BgT in the lobus parolfactorius, while the amount of bound QNB decreased significantly, and bilaterally, in the hippocampus, hyperstriatum ventrale, lobus parolfactorius and posterolateral telencephalon, pars dorsalis. The data support an involvement of cholinergic receptor types in the neural mechanisms underlying passive avoidance learning.

The muscarinic acetylcholine antagonist scopolamine impairs short-distance homing pigeon navigation

The present study employed intramuscular (i.m.) injections of the acetylcholine (ACh) receptor antagonist scopolamine hydrobromide (0.10 mg/kg) to investigate the possible involvement of ACh in naturally occurring spatial navigation in homing pigeons (Columba livia). Control pigeons receiving injections of saline or scopolamine methylbromide, an ACh antagonist that does not cross the blood-brain barrier, were oriented in a homeward direction when released from a location 8 km from home. In contrast, pigeons injected with scopolamine hydrobromide (0.10 mg/kg, i.m.) were less well oriented and took more time to return home from the same location. These results suggest that homing pigeon navigation is regulated, in part, by central cholinergic mechanisms.