Marco Margnelli

Brain stem-thalamus reciprocal influences in the cat

Abstract Intracellular recordings were made from Mth neurons in acute ‘encephale isole’ preparations during low- and high-frequency BP and Mes stimulation. Low-frequency BP stimulation induced short-latency (1–5 msec) EPSPs on Mth neurons, followed by IPSPs which could also appear in isolation. High-frequency BP activation produced mixed excitatory and inhibitory effects. Low-frequency Mes stimulation mainly induced long-latency and long-lasting IPSPs while high-frequency activation blocked spontaneous firing of Mth neurons with or without increase in membrane potential. Similar effect was seen during high-frequency Mth stimulation. DCN stimulation never affected any of the Mth studied neurons. Control experiments have excluded the co-stimulation of spinothalamic and cerebello-rubro-thalamic pathways. Intracellular injection of hyperpolarizing and depolarizing current showed that BP effects were postsynaptic whereas Mes inhibition was due to removal of excitation (i.e., disfacilitation) on Mth neurons. High-frequency BP stimulation induced an increase of cell firing and reduction of the inhibitory phase in the thalamically induced sequences, while Mes activation reduced the EPSPs without apparent changes of IPSPs. Low- and high-frequency Mth stimulation evoked short-latency (1–4 msec) EPSPs on BP and Mes neurons with an increase of firing which was more evident during high-frequency stimulation. Antidromic invasion in some BP and Mes neurons was also found. The results indicate the existence of a reciprocal brain stem-thalamic linkage which is excitatory in the descending, while it is excitatory and inhibitory in the ascending way from BP regions, and mainly disfacilitatory from the midbrain.

Postsynaptic potentials in the mesencephalic and pontomedullar reticular regions underlying descending limbic influences

Abstract Influences exerted by cortical and subcortical limbic structures upon the brain stem reticular formation were analyzed by intracellular recordings from mesencephalic (MES) and bulbo-pontine (BP) reticular neurons in ence´phale isole´cats. The results indicate that: (1) Limbic effects on reticular neurons are predominantly excitatory. (2) Qualitatively similar responses are revealed in cells tested by stimulation of different limbic sites. (3) The fraction of neurons activates by limbic inputs is higher in the MES as compared to the BP region. (4) Neurons with long ascending or descending axons identified by antidromic invasion are less responsive than unidentified units. (5) Descending limbic influences are transmitted by oligo- and polysynaptic pathways, the former being preferentially linked to BP, the latter to MES regions. The functional significance of the excitatory limbic input to the brain stem reticular formation is discussed.

Variability of peripheral representation in ventrobasal thalamic nuclei of the cat: effects of acute reversible blockade of the dorsal column nuclei.

Abstract The effects of reversible cooling of the dorsal column nuclei on the peripheral responsiveness of 55 specific somatosensory neurons (24 of which were identified as relay cells) belonging to the forepaw focus of the ventralis posterolateralis nucleus of the cat thalamus were studied. The effectiveness of cooling was checked for each unit by observing the disappearance of its responsiveness to dorsal column stimulation. The neurons were isolated from locally anesthetized preparations (17 units), as well as from animals anesthetized with chloralose (21 units) or Nembutal (17 units). Fifteen cells (six from locally anesthetized and nine from chloralose-treated cats) were seen to retain their peripheral responsiveness in spite of the dorsal column nucleus transmission blockade. The area and location of receptive fields showed no changes in 10 such units and exhibited small enlargements and/or displacements in the remaining 5 (3 from locally anesthetized and 2 from chloralose-treated cats). In locally anesthetized preparations, intravenous injections of chloralose administered after removal of the cold block produced the same effects as those seen during cooling on the three units that exhibited enlargements of their receptive fields. The evidence points to a similar mechanism explaining the “unmasking” effects of chloralose and of dorsal column nuclei transmission blockade. Disinhibition of a redundant specific pathway converging on a fraction of thalamocortical relay cells might be suggested. Such a redundant pathway is tentatively identified with the spinocervicothalamic tract.

Variability of peripheral representation in ventrobasal thalamic nuclei of the cat: Effects of chloralose treatment

Abstract The effects of the intravenous administration of α-chloralose on the static and dynamic properties of 20 highly specific somatosensory neurons isolated within the forepaw focus of the thalamic ventralis posterolateralis (VPL) nucleus of locally anesthetized cats were studied. Observations were made (extracellular recording) before, during, and after injection. Nine units of the sample were identified as thalamocortical relay cells. Spontaneous activity, activity during tonic afferent drive maintained by weak peripheral stimuli, responsiveness to stimulation of the dorsal column and dorsolateral funiculus, and location and extent of peripheral receptive fields were examined. In none of the sample units did the drug bring to light aspecific properties such as those previously described in sizable proportions of the cell populations sampled from the VPL nucleus of cats anesthetized with chloralose or subjected to surgical interventions which lower the amount of the ascending or corticofugal influx to the thalamus. After the treatment, eight units (six of which were identified as thalamocortical relay cells) exhibited continuous or (in two cases) discontinuous enlargements of their peripheral receptive fields, which, however, remained rather small compared to the aspecific fields. The effects are tentatively explained by hypothesizing the “unmasking” of a convergent specific input, directed only to a fraction of thalamocortical relay cells and normally blocked by afferent or corticofugal inhibition. It is suggested that the unmasked input could be mediated by the spinocervicothalamic pathway.