The Lamprey Forebrain - Evolutionary Implications.

The forebrain plays a critical role in a broad range of neural processes encompassing sensory integration and initiation/selection of behaviour. The forebrain functions through an interaction between different cortical areas, the thalamus, the basal ganglia with the dopamine system, and the habenulae. The ambition here is to compare the mammalian forebrain with that of the lamprey representing the oldest now living group of vertebrates, by a review of earlier studies. We show that the lamprey dorsal pallium has a motor, a somatosensory, and a visual area with retinotopic representation. The lamprey pallium was previously thought to be largely olfactory. There is also a detailed similarity between the lamprey and mammals with regard to other forebrain structures like the basal ganglia in which the general organisation, connectivity, transmitters and their receptors, neuropeptides, and expression of ion channels are virtually identical. These initially unexpected results allow for the possibility that many aspects of the basic design of the vertebrate forebrain had evolved before the lamprey diverged from the evolutionary line leading to mammals. Based on a detailed comparison between the mammalian forebrain and that of the lamprey and with due consideration of data from other vertebrate groups, we propose a compelling account of a pan-vertebrate schema for basic forebrain structures, suggesting a common ancestry of over half a billion years of vertebrate evolution.

The blueprint of the vertebrate forebrain - With special reference to the habenulae.

The medial and lateral habenulae are conserved throughout vertebrate evolution, and form an integrated part in the forebrain control of behavior together with the basal ganglia, the dopamine and serotonin systems and cortex. The lateral habenula plays a role in the control of dopamine activity in the context of aversive behavior and the converse, a reward situation. These circuits are important for a value-based evaluation of the success of prior actions. The medial habenula is involved in mediating escape and freezing behavior. These structures are reviewed with a focus on the lamprey, belonging to the oldest group of now living vertebrate, showing that most aspects of the habenular structure and function have been conserved throughout vertebrate phylogeny.

Cerebrospinal Fluid-Contacting Neurons Sense pH Changes and Motion in the Hypothalamus.

CSF-contacting (CSF-c) cells are present in the walls of the brain ventricles and the central canal of the spinal cord and found throughout the vertebrate phylum. We recently identified ciliated somatostatin-/GABA-expressing CSF-c neurons in the lamprey spinal cord that act as pH sensors as well as mechanoreceptors. In the same neuron, acidic and alkaline responses are mediated through ASIC3-like and PKD2L1 channels, respectively. Here, we investigate the functional properties of the ciliated somatostatin-/GABA-positive CSF-c neurons in the hypothalamus by performing whole-cell recordings in hypothalamic slices. Depolarizing current pulses readily evoked action potentials, but hypothalamic CSF-c neurons had no or a very low level of spontaneous activity at pH 7.4. They responded, however, with membrane potential depolarization and trains of action potentials to small deviations in pH in both the acidic and alkaline direction. Like in spinal CSF-c neurons, the acidic response in hypothalamic cells is mediated via ASIC3-like channels. In contrast, the alkaline response appears to depend on connexin hemichannels, not on PKD2L1 channels. We also show that hypothalamic CSF-c neurons respond to mechanical stimulation induced by fluid movements along the wall of the third ventricle, a response mediated via ASIC3-like channels. The hypothalamic CSF-c neurons extend their processes dorsally, ventrally, and laterally, but as yet, the effects exerted on hypothalamic circuits are unknown. With similar neurons being present in rodents, the pH- and mechanosensing ability of hypothalamic CSF-c neurons is most likely conserved throughout vertebrate phylogeny.SIGNIFICANCE STATEMENT CSF-contacting neurons are present in all vertebrates and are located mainly in the hypothalamic area and the spinal cord. Here, we report that the somatostatin-/GABA-expressing CSF-c neurons in the lamprey hypothalamus sense bidirectional deviations in the extracellular pH and do so via different molecular mechanisms. They also serve as mechanoreceptors. The hypothalamic CSF-c neurons have extensive axonal ramifications and may decrease the level of motor activity via release of somatostatin. In conclusion, hypothalamic somatostatin-/GABA-expressing CSF-c neurons, as well as their spinal counterpart, represent a novel homeostatic mechanism designed to sense any deviation from physiological pH and thus constitute a feedback regulatory system intrinsic to the CNS, possibly serving a protective role from damage caused by changes in pH.

Independent circuits in the basal ganglia for the evaluation and selection of actions.

The basal ganglia are critical for selecting actions and evaluating their outcome. Although the circuitry for selection is well understood, how these nuclei evaluate the outcome of actions is unknown. Here, we show in lamprey that a separate evaluation circuit, which regulates the habenula-projecting globus pallidus (GPh) neurons, exists within the basal ganglia. The GPh neurons are glutamatergic and can drive the activity of the lateral habenula, which, in turn, provides an indirect inhibitory influence on midbrain dopamine neurons. We show that GPh neurons receive inhibitory input from the striosomal compartment of the striatum. The striosomal input can reduce the excitatory drive to the lateral habenula and, consequently, decrease the inhibition onto the dopaminergic system. Dopaminergic neurons, in turn, provide feedback that inhibits the GPh. In addition, GPh neurons receive direct projections from the pallium (cortex in mammals), which can increase the GPh activity to drive the lateral habenula to increase the inhibition of the neuromodulatory systems. This circuitry, thus, differs markedly from the "direct" and "indirect" pathways that regulate the pallidal (e.g., globus pallidus) output nuclei involved in the control of motion. Our results show that a distinct reward-evaluation circuit exists within the basal ganglia, in parallel to the direct and indirect pathways, which select actions. Our results suggest that these circuits are part of the fundamental blueprint that all vertebrates use to select actions and evaluate their outcome.

Evolutionary conservation of the habenular nuclei and their circuitry controlling the dopamine and 5-hydroxytryptophan (5-HT) systems.

The medial (MHb) and lateral (LHb) habenulae are a small group of nuclei that regulate the activity of monoaminergic neurons. Disruptions to these nuclei lead to deficits in a range of cognitive and motor functions from sleep to decision making. Interestingly, the habenular nuclei are present in all vertebrates, suggesting that they provide a common neural mechanism to influence these diverse functions. To unravel conserved habenula circuitry and approach an understanding of their basic function, we investigated the organization of these nuclei in the lamprey, one of the phylogenetically oldest vertebrates. Based on connectivity and molecular expression, we show that the MHb and LHb circuitry is conserved in the lamprey. As in mammals, separate populations of neurons in the LHb homolog project directly or indirectly to dopamine and serotonin neurons through a nucleus homologous to the GABAergic rostromedial mesopontine tegmental nucleus and directly to histamine neurons. The pallidal and hypothalamic inputs to the LHb homolog are also conserved. In contrast to other species, the habenula projecting pallidal nucleus is topographically distinct from the dorsal pallidum, the homolog of the globus pallidus interna. The efferents of the MHb homolog selectively target the interpeduncular nucleus. The MHb afferents arise from sensory (medial olfactory bulb, parapineal, and pretectum) and not limbic areas, as they do in mammals; consequently, the "context" in which this circuitry is recruited may have changed during evolution. Our results indicate that the habenular nuclei provide a common vertebrate circuitry to adapt behavior in response to rewards, stress, and other motivating factors.

Neuronal mechanisms of respiratory pattern generation are evolutionary conserved.

A brainstem region, the paratrigeminal respiratory group (pTRG), has been suggested to play a crucial role in the respiratory rhythm generation in lampreys. However, a detailed characterization of the pTRG region is lacking. The present study performed on isolated brainstem preparations of adult lampreys provides a more precise localization of the pTRG region with regard to both connectivity and neurochemical markers. pTRG neurons projecting to the vagal motoneuronal pool were identified in a restricted area of the rostral rhombencephalon at the level of the isthmic Müller cell I1 close to sulcus limitans of His. Unilateral microinjections of lidocaine, muscimol, or glutamate antagonists into the pTRG inhibited completely the bilateral respiratory activity. In contrast, microinjections of glutamate agonists enhanced the respiratory activity, suggesting that this region is critical for the respiratory pattern generation. The retrogradely labeled pTRG neurons are glutamatergic and surrounded by terminals with intense substance P immunoreactivity. Cholinergic neurons were seen close to, and intermingled with, pTRG neurons. In addition, α-bungarotoxin binding sites (indicating nicotinic receptors) were found throughout the pTRG area and particularly on the soma of these neurons. During apnea, induced by blockade of ionotropic glutamate receptors within the same region, microinjections of 1 µm substance P or 1 mm nicotine into the pTRG restored rhythmic respiratory activity. The results emphasize the close similarities between the pTRG and the mammalian pre-Bötzinger complex as a crucial site for respiratory rhythmogenesis. We conclude that some basic features of the excitatory neurons proposed to generate respiratory rhythms are conserved throughout evolution.

Dopamine differentially modulates the excitability of striatal neurons of the direct and indirect pathways in lamprey.

The functions of the basal ganglia are critically dependent on dopamine. In mammals, dopamine differentially modulates the excitability of the direct and indirect striatal projection neurons, and these populations selectively express dopamine D1 and D2 receptors, respectively. Although the detailed organization of the basal ganglia is conserved throughout the vertebrate phylum, it was unknown whether the differential dopamine modulation of the direct and indirect pathways is present in non-mammalian species. We aim here to determine whether the receptor expression and opposing dopaminergic modulation of the direct and indirect pathways is present in one of the phylogenetically oldest vertebrates, the river lamprey. Using in situ hybridization and patch-clamp recordings, we show that D1 receptors are almost exclusively expressed in the striatal neurons projecting directly to the homolog of the substantia nigra pars reticulata. In addition, the majority of striatal neurons projecting to the homolog of the globus pallidus interna/globus pallidus externa express D1 or D2 receptors. As in mammals, application of dopamine receptor agonists differentially modulates the excitability of these neurons, increasing the excitability of the D1-expressing neurons and decreasing the excitability of D2-expressing neurons. Our results suggest that the segregated expression of the D1 and D2 receptors in the direct and indirect striatal projection neurons has been conserved across the vertebrate phylum. Because dopamine receptor agonists differentially modulate these pathways, increasing the excitability of the direct pathway and decreasing the excitability of the indirect pathway, this organization may be conserved as a mechanism that biases the networks toward action selection.

GABAergic and glycinergic inputs modulate rhythmogenic mechanisms in the lamprey respiratory network.

We have previously shown that GABA and glycine modulate respiratory activity in the in vitro brainstem preparations of the lamprey and that blockade of GABAA and glycine receptors restores the respiratory rhythm during apnoea caused by blockade of ionotropic glutamate receptors. However, the neural substrates involved in these effects are unknown. To address this issue, the role of GABAA, GABAB and glycine receptors within the paratrigeminal respiratory group (pTRG), the proposed respiratory central pattern generator, and the vagal motoneuron region was investigated both during apnoea induced by blockade of glutamatergic transmission and under basal conditions through microinjections of specific antagonists. The removal of GABAergic, but not glycinergic transmission within the pTRG, causes the resumption of rhythmic respiratory activity during apnoea, and reveals the presence of a modulatory control of the pTRG under basal conditions. A blockade of GABAA and glycine receptors within the vagal region strongly increases the respiratory frequency through disinhibition of neurons projecting to the pTRG from the vagal region. These neurons were retrogradely labelled (neurobiotin) from the pTRG. Intense GABA immunoreactivity is observed both within the pTRG and the vagal area, which corroborates present findings. The results confirm the pTRG as a primary site of respiratory rhythm generation, and suggest that inhibition modulates the activity of rhythm-generating neurons, without any direct role in burst formation and termination mechanisms.

The evolutionary origin of the vertebrate basal ganglia and its role in action selection.

The group of nuclei within the basal ganglia of the forebrain is central to the control of movement. We present data showing that the structure and function of the basal ganglia have been conserved throughout vertebrate evolution over some 560 million years. The interaction between the different nuclei within the basal ganglia is conserved as well as the cellular and synaptic properties and transmitters. We consider the role of the conserved basal ganglia circuitry for basic patterns of motor behaviour controlled via brainstem circuits. The output of the basal ganglia consists of tonically active GABAergic neurones, which target brainstem motor centres responsible for different patterns of behaviour, such as eye and locomotor movements, posture, and feeding. A prerequisite for activating or releasing a motor programme is that this GABAergic inhibition is temporarily reduced. This can be achieved through activation of GABAergic projection neurons from striatum, the input level of the basal ganglia, given an appropriate synaptic drive from cortex, thalamus and the dopamine system. The tonic inhibition of the motor centres at rest most likely serves to prevent the different motor programmes from becoming active when not intended. Striatal projection neurones are subdivided into one group with dopamine 1 receptors that provides increased excitability of the direct pathway that can initiate movements, while inhibitory dopamine 2 receptors are expressed on neurones that instead inhibit movements and are part of the 'indirect loop' in mammals as well as lamprey. We review the evidence showing that all basic features of the basal ganglia have been conserved throughout vertebrate phylogeny, and discuss these findings in relation to the role of the basal ganglia in selection of behaviour.

Evolutionarily conserved differences in pallial and thalamic short-term synaptic plasticity in striatum.

The striatum of the basal ganglia is conserved throughout the vertebrate phylum. Tracing studies in lamprey have shown that its afferent inputs are organized in a manner similar to that of mammals. The main inputs arise from the thalamus (Th) and lateral pallium (LPal; the homologue of cortex) that represents the two principal excitatory glutamatergic inputs in mammals. The aim here was to characterize the pharmacology and synaptic dynamics of afferent fibres from the LPal and Th onto identified striatal neurons to understand the processing taking place in the lamprey striatum. We used whole-cell current-clamp recordings in acute slices of striatum with preserved fibres from the Th and LPal, as well as tract tracing and immunohistochemistry. We show that the Th and LPal produce monosynaptic excitatory glutamatergic input through NMDA and AMPA receptors. The synaptic input from the LPal displayed short-term facilitation, unlike the Th input that instead displayed strong short-term synaptic depression. There was also an activity-dependent recruitment of intrastriatal oligosynaptic inhibition from both inputs. These results indicate that the two principal inputs undergo different activity-dependent short-term synaptic plasticity in the lamprey striatum. The difference observed between Th and LPal (cortical) input is also observed in mammals, suggesting a conserved trait throughout vertebrate evolution.

The Basal Ganglia Downstream Control of Action - An Evolutionarily Conserved Strategy.

The motor areas of the cortex and the basal ganglia both contribute to determining which motor actions will be recruited at any moment in time, and their functions are intertwined. Here, we review the basal ganglia mechanisms underlying the selection of behavior of the downstream control of motor centers in the midbrain and brainstem and show that the basic organization of the forebrain motor system is evolutionarily conserved throughout vertebrate phylogeny. The output level of the basal ganglia (e.g. substantia nigra pars reticulata) has GABAergic neurons that are spontaneously active at rest and inhibit a number of specific motor centers, each of which can be relieved from inhibition if the inhibitory output neurons themselves become inhibited. The motor areas of the cortex act partially via the dorsolateral striatum (putamen), which has specific modules for the forelimb, hindlimb, trunk, etc. Each module operates in turn through the two types of striatal projection neurons that control the output modules of the basal ganglia and thereby the downstream motor centers. The mechanisms for lateral inhibition in the striatum are reviewed as well as other striatal mechanisms contributing to action selection. The motor cortex also exerts a direct excitatory action on specific motor centers. An overview is given of the basal ganglia control exerted on the different midbrain/brainstem motor centers, and the efference copy information fed back via the thalamus to the striatum and cortex, which is of importance for the planning of future movements.

The neural bases of vertebrate motor behaviour through the lens of evolution.

The primary driver of the evolution of the vertebrate nervous system has been the necessity to move, along with the requirement of controlling the plethora of motor behavioural repertoires seen among the vast and diverse vertebrate species. Understanding the neural basis of motor control through the perspective of evolution, mandates thorough examinations of the nervous systems of species in critical phylogenetic positions. We present here, a broad review of studies on the neural motor infrastructure of the lamprey, a basal and ancient vertebrate, which enjoys a unique phylogenetic position as being an extant representative of the earliest group of vertebrates. From the central pattern generators in the spinal cord to the microcircuits of the pallial cortex, work on the lamprey brain over the years, has provided detailed insights into the basic organization (a bauplan) of the ancestral vertebrate brain, and narrates a compelling account of common ancestry of fundamental aspects of the neural bases for motion control, maintained through half a billion years of vertebrate evolution. This article is part of the theme issue 'Systems neuroscience through the lens of evolutionary theory'.

ExSTED microscopy reveals contrasting functions of dopamine and somatostatin CSF-c neurons along the lamprey central canal.

Cerebrospinal fluid-contacting (CSF-c) neurons line the central canal of the spinal cord and a subtype of CSF-c neurons expressing somatostatin, forms a homeostatic pH regulating system. Despite their importance, their intricate spatial organization is poorly understood. The function of another subtype of CSF-c neurons expressing dopamine is also investigated. Imaging methods with a high spatial resolution (5-10 nm) are used to resolve the synaptic and ciliary compartments of each individual cell in the spinal cord of the lamprey to elucidate their signalling pathways and to dissect the cellular organization. Here, light-sheet and expansion microscopy resolved the persistent ventral and lateral organization of dopamine- and somatostatin-expressing CSF-c neuronal subtypes. The density of somatostatin-containing dense-core vesicles, resolved by stimulated emission depletion microscopy, was shown to be markedly reduced upon each exposure to either alkaline or acidic pH and being part of a homeostatic response inhibiting movements. Their cilia symmetry was unravelled by stimulated emission depletion microscopy in expanded tissues as sensory with 9 + 0 microtubule duplets. The dopaminergic CSF-c neurons on the other hand have a motile cilium with the characteristic 9 + 2 duplets and are insensitive to pH changes. This novel experimental workflow elucidates the functional role of CSF-c neuron subtypes in situ paving the way for further spatial and functional cell-type classification.

Striatal cellular properties conserved from lampreys to mammals.

The striatum of the lamprey, the first vertebrate group to appear in evolution, shows striking similarities to that of mammals with respect to histochemical markers, afferent and efferent projections and the effect of dopamine depletion, which leads to hypokinetic motor symptoms. The cellular properties of lamprey striatal neurons were studied here using patch-clamp recordings in acute striatal slices. Sixty-five per cent of recorded neurons were characterised by a prominent inward rectification due to a K+ conductance of the Kir type. They had a ramping response with a long delay to the first action potential due to activation of a low-voltage-activated A-type K+ current. Many such inwardly rectifying neurons (IRNs) had a hyperpolarised resting membrane potential and some had spiny dendrites. The remaining 35% of the neurons (non-IRNs) represent a heterogeneous group, including some with characteristics similar to the fast-spiking interneuron of the mammalian striatum. They showed short-lasting, large after hyperpolarisations (AHPs) and discharged action potentials at high frequency. None of the recorded neurons were spontaneously active but they received GABAergic and glutamatergic synaptic input. The fact that most lamprey striatal neurons display inward rectification indicates that this is a conserved characteristic of striatal neurons throughout vertebrate phylogeny. This is a cellular property of critical importance for the operations of the striatum in mammals.

Olfaction in Lamprey Pallium Revisited-Dual Projections of Mitral and Tufted Cells.

The presence of two separate afferent channels from the olfactory glomeruli to different targets in the brain is unravelled in the lamprey. The mitral-like cells send axonal projections directly to the piriform cortex in the ventral part of pallium, whereas the smaller tufted-like cells project separately and exclusively to a relay nucleus called the dorsomedial telencephalic nucleus (dmtn). This nucleus, located at the interface between the olfactory bulb and pallium, in turn projects to a circumscribed area in the anteromedial, ventral part of pallium. The tufted-like cells are activated with short latency from the olfactory nerve and terminate with mossy fibers on the dmtn cells, wherein they elicit large unitary excitatory postsynaptic potentials (EPSPs). In all synapses along this tufted-like cell pathway, there is no concurrent inhibition, in contrast to the mitral-like cell pathway. This is similar to recent findings in rodents establishing two separate exclusive projection patterns, suggesting an evolutionarily conserved organization.

Basal Ganglia-A Motion Perspective.

The basal ganglia represent an ancient part of the nervous system that have remained organized in a similar way over the last 500 million years and are of importance for our ability to determine which actions to choose at any given moment in time. Salient or reward stimuli act via the dopamine system and contribute to motor or procedural learning (reinforcement learning). The input stage of the basal ganglia, the striatum, is shaped by glutamatergic input from the cortex and thalamus and by the dopamine system. All intrinsic neurons of the striatum are GABAergic and inhibitory except for the cholinergic interneurons. Too little dopamine and all vertebrates show symptoms similar to that of a Parkinsonian patient, whereas too much dopamine results in hyperkinesia with involuntary movements. In this article, we discuss the detailed organization of the basal ganglia, with the different cell types, their properties, and contributions to basal ganglia functions. The striatal projection neurons represent 95% of all neurons in the striatum and are subdivided into two types, one that projects directly to the output stage, referred to as the "direct" pathway that promotes action, and the other subtype that targets the output nuclei via intercalated basal ganglia nuclei. This "indirect" pathway has an opposite effect. The striatal projection neurons express a set of ion channels that give them a high threshold for activation, whereas neurons in all other parts of the basal ganglia have a resting discharge that allows for modulation in both an increased and decreased direction. © 2020 American Physiological Society. Compr Physiol 10:1241-1275, 2020.

The evolutionary origin of visual and somatosensory representation in the vertebrate pallium.

Amniotes, such as mammals and reptiles, have vision and other senses represented in the pallium, whereas anamniotes, such as amphibians, fish and cyclostomes (including lampreys), which diverged much earlier, were historically thought to process olfactory information predominantly or even exclusively in the pallium. Here, we show that there is a separate visual area with retinotopic representation, and that somatosensory information from the head and trunk is represented in an adjacent area in the lamprey pallial cortex (lateral pallium). These cortical sensory areas flank a non-primary-sensory motor area. Both vision and somatosensation are relayed via the thalamus. These findings suggest that the basic sensorimotor representation of the mammalian neocortex, as well as the sensory thalamocortical relay, had already evolved in the last common ancestor of cyclostomes and gnathostomes around 560 million years ago.

Neural bases of goal-directed locomotion in vertebrates--an overview.

The different neural control systems involved in goal-directed vertebrate locomotion are reviewed. They include not only the central pattern generator networks in the spinal cord that generate the basic locomotor synergy and the brainstem command systems for locomotion but also the control systems for steering and control of body orientation (posture) and finally the neural structures responsible for determining which motor programs should be turned on in a given instant. The role of the basal ganglia is considered in this context. The review summarizes the available information from a general vertebrate perspective, but specific examples are often derived from the lamprey, which provides the most detailed information when considering cellular and network perspectives.

Individual Dopaminergic Neurons of Lamprey SNc/VTA Project to Both the Striatum and Optic Tectum but Restrict Co-release of Glutamate to Striatum Only.

Dopaminergic neurons in the substantia nigra (SNc) innervate both striatum and the superior colliculus in mammals, as well as its homolog the optic tectum in lampreys, belonging to the oldest group of living vertebrates [1-3]. In the lamprey, we have previously shown that the same neuron sends axonal branches to both striatum and the optic tectum [3]. Here, we show that most neurons in the lamprey SNc and ventral tegmental area (VTA) (also referred to as the nucleus of the posterior tuberculum) express not only tyrosine hydroxylase (TH), in lamprey a marker of dopaminergic neurons [4], but also the vesicular glutamate transporter (vGluT), suggesting that glutamate is a co-transmitter. Remarkably, the axonal branches that project to striatum elicit both dopaminergic and glutamatergic synaptic effects on striatal neurons, whereas the axonal projections to the optic tectum only evoke dopaminergic effects. Thus, axonal branches from the same neuron can use two transmitters in one branch and only one in the other. Previous studies suggest that, along an individual dopaminergic axon, there can be microdomains of either TH or vGluT [5-8]. In addition, the present results demonstrate that entire axonal branches to one target structure can differ from that of branches to another target, both originating from the same dopamine neuron. This implies that a given dopamine neuron can exert different effects on two different target structures. The combined release of dopamine and glutamate may be appropriate in striatum, whereas the effects exerted on the tectal motor center may be better served with a selective dopaminergic modulation.

GABA distribution in lamprey is phylogenetically conserved.

The localization of gamma-aminobutyric acid (GABA) has been well described in most classes of vertebrates but not in adult lampreys. The question if the GABA distribution is similar throughout the vertebrate subphylum is therefore still to be addressed. We here investigate two lamprey species, the sea lamprey, Petromyzon marinus, and the river lamprey, Lampetra fluviatilis, and compare the GABA pattern with that of other vertebrates. The present immunohistochemical study provides an anatomical basis for the general distribution and precise localization of GABAergic neurons in the adult lamprey forebrain and brainstem. GABA-immunoreactive cells were organized in a virtually identical manner in the two species. They were found throughout the brain, with the following regions being of particular interest: the granular cell layer of the olfactory bulb, the nucleus of the anterior commissure, the septum, the lateral and medial pallia, the striatum, the nucleus of the postoptic commissure, the thalamus, the hypothalamus, and pretectal areas, the optic tectum, the torus semicircularis, the mesencephalic tegmentum, restricted regions of the rhombencephalic tegmentum, the octavolateral area, and the dorsal column nucleus. The GABA distribution found in cyclostomes is very similar to that of other classes of vertebrates, including mammals. Since the lamprey diverged from the main vertebrate line around 450 million years ago, this implies that already at that time the basic vertebrate plan for the GABA innervation in different parts of the brain had been developed.