The midbrain precommand nucleus of the mormyrid electromotor network.

The functional role of the midbrain precommand nucleus (PCN) of the electromotor system was explored in the weakly electric mormyrid fish Gnathonemus petersii, using extracellular recording of field potentials, single unit activity, and microstimulation in vivo. Electromotor-related field potentials in PCN are linked in a one-to-one manner and with a fixed time relationship to the electric organ discharge (EOD) command cycle, but occur later than EOD command activity in the medulla. It is suggested that PCN electromotor-related field potentials arise from two sources: (1) antidromically, by backpropagation across electrotonic synapses between PCN axons and command nucleus neurons, and (2) as corollary discharge-driven feedback arriving from the command nucleus indirectly, via multisynaptic pathways. PCN neurons can be activated by electrosensory input, but this does not necessarily activate the whole motor command chain. Microstimulation of PCN modulates the endogenous pattern of electromotor command in a way that can mimic the structure of certain stereotyped behavioral patterns. PCN activity is regulated, and to a certain extent synchronized, by corollary discharge feedback inhibition. However, PCN does not generally function as a synchronized pacemaker driving the electromotor command chain. We propose that PCN neurons integrate information of various origins and individually relay this to the command nucleus in the medulla. Some may also have intrinsic, although normally nonsynchronized, pacemaker properties. This descending activity, integrated in the electromotor command nucleus, will play an important modulatory role in the central pattern generator decision process.

Regional levels of neurotransmitter-related markers in the brain of the weakly electric fish Gnathonemus petersii.

The regional distribution of neurochemical markers for cholinergic, GABAergic and glutamatergic/aspartatergic transmission has been studied in several brain areas of the weakly electric fish Gnathonemus petersii. In particular, brain regions related to reception, relaying and processing of electrosensory modalities have been examined. The regional distribution of neurotransmitter-related markers is considered in the light of available anatomical data and is discussed with particular emphasis on differences among different parts of the hypertrophic valvula cerebelli and other cerebellar structures.

Ultrastructural features and acetylcholinesterase histochemistry of the rat habenular complex.

The ultrastructural features and the acetylcholinesterase (AChE) localization of the rat habenula have been studied. On the basis of different morphology and AChE content, it is suggested that at least two types of neurons are present in the medial habenula (MHb) and three types of neurons in the lateral habenula (LHb). In particular, actively AChE-synthesizing neurons have been noticed in both LHb and MHb. Some unusual ultrastructural arrangements of the endoplasmic reticulum have been described in habenular neurons. Finally, the most common types of synaptic contacts present in the habenular complex have been surveyed.

Neurotoxic effect of kainic acid on ultrastructure and GABAergic parameters in the goldfish cerebellum.

Kainic acid administration into the cerebellar dorsal lobe of the goldfish causes selective degeneration of some neuronal types. Stellate and Golgi neurons are very sensitive to the neurotoxin and undergo rapid degeneration. On the basis of their differential responses to kainic acid, Purkinje cells can be divided in two distinct sub-populations (i.e. sensitive and insensitive neurons). The degenerative changes of the Purkinje neurons are in addition remarkably slow in comparison with the same cells in mammals or with stellate and Golgi neurons in the goldfish. Granule cells, as well as the cerebellar afferent fiber system, are not significantly affected. Six days after kainic acid administration, the level of glutamate decarboxylase in the cerebellar dorsal lobe drops to about 40% of the control value. This result suggests that the neurons sensitive to kainic acid neurotoxicity are, at least in part, GABAergic. Light- and electron-microscopic autoradiography of cerebellar elements selectively accumulating [3H]GABA, supports this idea. Moderate decreases of acetylcholinesterase and protein content were also noticed in the kainic acid-treated cerebellar dorsal lobe.

Electron microscopic demonstration of neurons and synaptic terminals selectively accumulating (3H)GABA in goldfish optic tectum.

After injection of (3H)GABA in the goldfish tectum, several neural structures selectively accumulating the labeled neurotransmitter were revealed by electron microscopic autoradiography. A number of labeled neurons was observed in the periventricular layer, while some scattered neurons were present in more superficial layers. Different types of axon terminals were also labeled in the stratum marginale, stratum fibrosum and griseum superficiale and stratum griseum centrale. Labeled dendrites, in particular those belonging to the periventricular neurons, were also noted.

Biogenic amine terminals in the goldfish cerebellum and optic tectum: a fluorescence and autoradiographic study.

The intraventricular administration of 3H-noradrenaline was used to demonstrate the presence and the distribution of the monoaminergic terminals in the goldfish cerebellum and optic tectum. Monoaminergic terminal can be selectively labelled by tritiated neurotransmitter and are visualized by light and electron microscopic autoradiography. Further evidence of the presence of catecholamines in these encephalic areas was furnished by the induced-fluorescence technique.