An intracellular study of the effects of GABA on frog tectal neurones in vitro

Distribution of GABA-Like Immunoreactive Cell Bodies in the Brains of Two Amphibians, Rana catesbeiana and Xenopus laevis

The distribution of the neurotransmitter gamma-aminobutyric acid (GABA) is not well understood for non-mammalian vertebrates. We thus used immunocytochemistry to locate putative GABAergic cells in the brains of male bullfrogs (Rana catesbeiana) and South African clawed frogs (Xenopus laevis). GABA-immunoreactive cell bodies were broadly distributed throughout the brains of both species with similar general patterns. In both, the greatest numbers of GABA-positive cells were found in the olfactory bulb, thalamus, and optic tectum, but virtually no major brain region lacked GABAergic cells. Species differences were also apparent. The density of GABA-immunoreactive cells was substantially higher in many areas of the bullfrog brain, compared to Xenopus. Bullfrogs possessed extensive cell populations in the medial pallium, preoptic area, optic tectum, torus semicircularis and tegmentum but cells were fewer in these locations in Xenopus. In the bullfrog hindbrain, GABA-immunoreactive cell bodies were restricted to very narrow and distinct populations. In Xenopus, however, cells in a similar position were fewer and spread more extensively. The distribution of GABA cells in the brain of these two species supports the hypotheses that GABA is involved in control of olfaction, audition, vision and vocalization. However, differences in the distribution of GABA between the bullfrog and Xenopus suggest that the extent of the GABAergic influence might vary between species.

The long-term resetting of a brainstem pacemaker nucleus by synaptic input: a model for sensorimotor adaptation

The cellular mechanisms behind sensorimotor adaptations, such as the adaptation to a sustained change in visual inputs by prism goggles in humans, are not known. Here we present a novel example of long-term sensorimotor adaptation in a well known neuroethological model, the jamming-avoidance response of a weakly electric fish. The adaptation is relatively long lasting, up to 9 hr in vivo, and is likely to be mediated by NMDA receptors. We demonstrate in a brain slice preparation that the pacemaker nucleus is the locus of adaptation and that it responds to long-lasting synaptic stimulation with an increase in the postsynaptic spike frequency persisting for hours after stimulus termination. The mechanism for the neuronal memory behaves as an integrator, and memory duration and strength are quantitatively related to the estimated amount of synaptic stimulation. This finding is contrary to the idea that neurons respond solely to long-lasting synaptic input by turning down their intrinsic excitability. We show that this positive feedback at the cellular level actually contributes to a negative feedback loop at the organismic level if the entire neural circuit and the behavioral link are considered.

Peptides in frog brain areas processing visual information

Vision is the most important sensory modality to anurans and a great deal of work in terms of hodological, physiological, and behavioral studies has been devoted to the visual system. The aim of this account is to survey data about the distribution of peptides in primary (lateral geniculate complex, pretectum, tectum) and secondary (striatum, anterodorsal and anteroventral tegmental nuclei, isthmic nucleus) visual relay centers. The emphasis is on general traits but interspecies variations are also noted. The smallest amount of peptide-containing neuronal elements was found in the lateral geniculate complex, where primarily nerve fibers showed immunostaining. All peptides found in the lateral geniculate complex, except two, occurred in the pretectum together with four other peptides. A large number of neurons showing intense neuropeptide thyrosine-like immunoreactivity was characteristic here. The mesencephalic tectum was the richest in peptide-like immunoreactive neuronal elements. Almost all peptides investigated were present mainly in fibers, but 9 peptides were found also in cells. The immunoreactive fibers show a complicated overlapping laminar arrangement. Cholecystokinin octapeptide, enkephalins, neuropeptide tyrosine, and substance P (not discussed here) gave the most prominent immunoreactivity. Several peptides also occur in the tectum of fishes, reptiles, birds, and mammals. Peptides in various combinations were found in the striatum, the anterodorsal- and anteroventral tegmental nucleus, and the isthmic nucleus that receive projections from the primary visual centers. The functional significance of peptides in visual information processing is not known. The only exception is neuropeptide tyrosine, which was found to be inhibitory on retinotectal synapses.

Colocalization of GABA, enkephalin and neuropeptide Y in the tectum of the green frog Rana esculenta

The colocalization of GABA, enkephalin and neuropeptide Y immunoreactivity in neurons in the pretectal area and in the mesencephalic tectum of the green frog (Rana esculenta) was studied. Several Met-enkephalin immunoreactive perikarya were found in layer 6 of the tectum and every third of these neurons showed GABA-ir as well. Colocalization of GABA and NPY could also be shown in half of the neuropeptide Y immunopositive cells in the 6th layer of the tectum, but only a few cells were double stained in layers 9 and 4. In the pretectal area no colocalization of the investigated peptides and GABA was found.

Effects of glutamatergic, cholinergic and gabaergic antagonists on tectal cells in toads

The present paper using microiontophoresis analysis describes transmitters and their receptor subtypes used in retinotectal and isthmotectal transmission, and suggests several modes converging retinotectal and isthmotectal afferents on tectal neurons in toads (Bufo bufo gargarizans). Neuronal responses of tectal cells were extracellularly recorded to both visual stimulation and electrical stimulation of the nucleus isthmi, and effects of glutamatergic, cholinergic, GABAergic and glycinergic antagonists on these responses examined. Visual responses in 80% of tectal cells were reversibly blocked by the N-methyl-D-aspartate antagonist 3-Rs-2-carboxypiperazin-4-yl-propyl-1-phosphonic acid, and those of the remaining 20% of cells by the muscarinic antagonist atropine, suggesting that there may be at least two kinds of retinotectal synapse that use glutamate and N-methyl-D-aspartate receptors, and acetylcholine and muscarinic receptors, respectively. Electrical stimulation of the nucleus isthmi elicited excitatory responses in 67% of tectal cells, excitatory-inhibitory responses in 16% of cells, and inhibitory responses in 17% of cells examined. The excitatory responses were reversibly abolished by atropine, but not affected by either 3-Rs-2-carboxypiperazin-4-yl-propyl-1-phosphonic acid or the alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate antagonist 6-cyano-7-nitroquinoxaline-2,3-dione, whereas the inhibitory responses were released by the GABA receptor A antagonist bicuculline, but not influenced by the GABA receptor B antagonist 2-hydroxysaclofen and glycinergic antagonist strychnine. Excitatory and inhibitory components in the excitatory-inhibitory responses were blocked by atropine and bicuculline, respectively. It appears that glutamatergic and cholinergic afferents from the retina, and cholinergic and GABAergic afferents from the nucleus isthmi may converge on tectal neurons in at least five modes of synaptic connections, in agreement with the heterogeneous populations of tectal cells in amphibians.

Neurons in the deep layers of superior colliculus play a critical role in the neuronal network for audiogenic seizures: mechanisms for production of wild running behavior

Recent investigations suggest that the deep layers of superior colliculus (DLSC) play a role in the neuronal network for audiogenic seizures (AGS). The present study examined DLSC neuronal firing and convulsive behavior simultaneously in freely-moving genetically epilepsy-prone rats (GEPR-9s) using chronically implanted microwire electrodes. An abrupt onset of acoustically-evoked firing at approximately 80-90 dB was observed in DLSC neurons of GEPR-9s, which was significantly above the normal threshold. DLSC neurons began to exhibit rapid tonic burst firing 1-2 s prior to the onset of the wild running behavior at the beginning of AGS. As the tonic phase of the seizure began, DLSC firing ceased, and only returned towards normal following post-ictal depression. These neuronal mechanisms may be relevant to other seizure models in which the DLSC is implicated. The temporal pattern of neuronal firing during AGS is specific to DLSC and differs markedly from those observed elsewhere in the AGS neuronal network. The temporal firing pattern suggests that the DLSC plays a primary role in the generation of the wild running phase of AGS. Previous studies indicate that the inferior colliculus is dominant during AGS initiation, and the pontine reticular formation is dominant during the tonic extension phase of AGS. Taken together these data suggest that the neurons in the neuronal network undergo a dominance shift as each specific convulsive behavior of AGS is elaborated.