Excitatory and inhibitory transmission from the optic tectum to nucleus isthmi and its vicinity in amphibians

Postsynaptic potentials and axonal projections of tegmental neurons responding to electrical stimulation of the toad striatum

The amphibian telencephalic striatum as a major component of the basal ganglia receives multisensory information and projects to the tegmentum and other structures. However, how striatal neurons modulate tegmental activity remains unknown. Here, we show by using intracellular recording and staining in toads that electrical stimulation of the ipsilateral striatum evoked an inhibitory postsynaptic potential (IPSP) in presumably binocular tegmental neurons. Seventy-one neurons were intracellularly stained with Lucifer yellow or horseradish peroxidase. They were located in the anterodorsal tegmental nucleus, anteroventral tegmental nucleus, nucleus profundus mesencephali, and superficial isthmal reticular nucleus, with axons projecting to the tectum, nucleus isthmi, and spinal cord. It appears that the striatum can control visually guided behaviors through the striato-tegmento-spinal pathway and the tegmento-spinal pathway mediated by the tectum and nucleus isthmi.

How the mesencephalic locomotor region recruits hindbrain neurons

This chapter summarizes experiments which were designed to reveal how repetitive electrical stimulation of the mesencephalic locomotor region (MLR) recruits nearby hindbrain neurons into activity, such that locomotion can ensue in the tiger salamander, A. tigrinum. The MLR stimulus strength was subthreshold or near-threshold for locomotor movements to ensue. Such relatively weak stimulation of the MLR produced locomotor movements after a relatively long delay, which featured neuronal interactions in the hindbrain. MLR-evoked spike responses of single hindbrain neurons were recorded before locomotor movements began. This allowed consideration of the build-up of the hindbrain neuronal activity, which was subsequently impressed upon the spinal cord such as to evoke locomotor movements. Each train of MLR stimulus pulses evoked monosynaptic responses in but a small proportion of the hindbrain's neurons. Rather, oligosynaptic responses were routinely evoked, even in the "input" neurons that were activated monosynaptically. Consecutive stimulus volleys recruited a given neuron after a variable number of synaptic translations. It is argued that the hindbrain's input neurons excited a much larger number of other hindbrain neurons. By this means, an MLR-evoked, short-lived propagating wave of excitation (i.e., approximately 2-4 successive synaptic activations) can be spread throughout the hindbrain.

The nucleus isthmi and dual modulation of the receptive field of tectal neurons in non-mammals

The nucleus isthmi in the dorsolateral tegmentum had been one of the most obscure structures in the nonmammalian midbrain for eight decades. Recent studies have shown that this nucleus and its mammalian homologue, the parabigeminal nucleus, are all visual centers, which receive information from the ipsilateral tectum and project back either ipsilaterally or bilaterally depending on species, but not an auditory center as suggested before. On the other hand, the isthmotectal pathways exert dual, both excitatory and inhibitory, actions on tectal cells in amphibians and reptiles. In birds, the magnocellular and parvocellular subdivisions of this nucleus produce excitatory and inhibitory effects on tectal cells, respectively. The excitatory pathway is mediated by glutamatergic synapses with AMPA and NMDA receptors and/or cholinergic synapses with muscarinic receptors, whereas the inhibitory pathway is mediated by GABAergic synapses via GABA(A) receptors. Further studies have shown that the magnocellular and parvocellular subdivisions can differentially modulate the excitatory and inhibitory regions of the receptive field of tectal neurons, respectively. Both the positive and the negative feedback pathways may work together in a winner-take-all manner, so that the animal could attend to only one of several competing visual targets simultaneously present in the visual field. Some behavioral tests seem to be consistent with this hypothesis. The present review indicates that the tecto-isthmic system in birds is an excellent model for further studying tectal modulation and possibly winner-take-all mechanisms.

Behavior of hindbrain neurons during the transition from rest to evoked locomotion in a newt

Trains of electrical stimuli were delivered to the mesencephalic 'locomotor region' in the rough skin newt. The current (3-12 mcA) and the interstimulus interval (100 to 200 ms) were adjusted so that locomotion arose in approximately 10 s, or so that the train remained subthreshold for initiation of locomotion. Impulses of single neurons in the hindbrain were recorded during the transition period from rest to locomotion. Time-locked synaptic responses were bi- or unimodal with typical latencies close to 18, 23 or 28 ms, and weak irregular mode near 13 ms. Impulses that were not locked to the stimuli arose in some silent neurons, and the rate of firing of neurons with background discharge was sometimes enhanced. Composite responses consisted of both time-locked component and impulses distributed throughout the interstimulus interval. The data suggest that short-lived, wave-like propagation of the input volley ceases or is transformed into asynchronous activity after three or four translations. The latter variant could occur if the train reached the threshold for initiation of locomotion. The asynchronous activity persisted throughout interstimulus interval and could coexist with time-locked impulses. Some neurons generated only a few impulses, while others remained active from beginning to end of the train. These active neurons could either spike at a steady rate, or decrement or augment their rate of firing during the train. The time course of their activity was related to the initial rate of firing. The augmenting type of firing in a subset of neurons may arise due to the interaction of neurons with unstable, steady state and decrementing activity.

Localization and source of gamma aminobutyric acid immunoreactivity in the isthmic nucleus of the frog Rana esculenta

The distribution of gamma-aminobutyric acid (GABA)-containing neurons and nerve fibers was studied in the isthmic nucleus of the frog Rana esculenta using light and electron microscopical immunohistochemical techniques. Approximately 0.5% of isthmic cells showed GABA immunopositivity, and the majority of these cells was found in the anterior one-third of the nucleus. A meshwork of GABA-immunostained fine beaded axons filled the entire isthmic nucleus. The GABA-immunoreactive terminals formed pericellular basket-like structures around a few cells both in the medulla and the cortex of the isthmic nucleus. To determine the source of GABA-positive fibers in the isthmic nucleus lesion experiments were carried out. After unilateral tectal ablation no change was observed in GABA immunoreactivity. Hemisectioning the tegmentum close to the anterior border of the isthmic nucleus, transection of the caudal tectal commissure and decussatio veli, or electrical lesioning of the anterodorsal tegmental nucleus all resulted in a moderate decrease in the density of GABA-positive fibers. Our results suggest that the majority of GABA-positive fibers derives from local GABA-positive cells, but some GABAergic afferents seem to arise in the tegmentum.

A new slicing method for the lower vertebrate brain: brain mould and adjustable stage

This paper describes a new method for slicing the lower vertebrate brain which is too small and soft to be sliced using conventional methods. The brain is sliced in a pre-prepared agar mould glued to a special stage placed in a conventional vibratome. The mould is constructed from a plaster model prepared by embedding a paraformaldehyde fixed brain in a paraffin and vaseline mixture. The brain to be sliced is placed within two prepared agar half cylinders which are in turn placed in a pre-prepared larger agar collar glued to a special stage. The study describes in detail the preparation of the interlocking collar and inner cylinders of agar. The plexiglass stage allows the agar block containing the brain, to be rotated and inclined to improve the angle of cut. By using this method, small and soft brain even with thin walls and large ventricles could be sliced coronally, sagittally, horizontally or obliquely. Brain slices obtained by this method have good viability showing spontaneous and evoked activity.