The intrinsic organization of the ventroposterolateral nucleus and related reticular thalamic nucleus of the rat: a double-labeling ultrastructural investigation with gamma-aminobutyric acid immunogold staining and lectin-conjugated horseradish peroxidase

GABAergic circuitry in the rostral ventral medulla of the rat and its relationship to descending antinociceptive controls

This study used postembedding immunocytochemistry to examine the organization of GABA-immunoreactive synapse in the rostral ventral medulla (RVM) of the rat. To determine whether the outflow neurons of the RVM are under GABAergic control, we examined the distribution of GABA-immunoreactive synapses upon bulbospinal projection neurons that were labelled by retrograde transport of wheatgerm agglutinin-HRP from the cervical spinal cord. To study the possible convergence of GABAergic and periaqueductal gray (PAG) synaptic inputs to RVM neurons, we also made lesions in the PAG and examined the relationship between degenerating PAG axons and GABA-immunoreactive terminals. Approximately 45% of all synapses in the RVM, which includes the midline nucleus raphe magnus and the nucleus reticularis paragigantocellularis lateralis, were GABA-immunoreactive. The vast majority of GABA-immunoreactive terminals contained round, clear, and pleomorphic vesicles and made symmetrical axodendritic synapses; axoaxonic synapses were not found. Almost 50% of the retrogradely labeled dendrites in the NRM were postsynaptic to GABA-immunoreactive terminals. Several examples of convergence of degenerating PAG terminals and GABAergic terminals onto the same unlabelled dendrite were also found. These data indicate that the projection neurons of the RVM are under profound GABAergic inhibitory control. The results are discussed with regard to the hypothesis that the analgesic action of narcotics and electrical stimulation of the midbrain PAG involves the regulation of tonic GABAergic inhibitory controls that are exerted upon spinally-projecting neurons of the nucleus raphe magnus.

Light and electron microscopic evidence for a GABAergic projection from the caudal basal forebrain to the thalamic reticular nucleus in rats

Neurons in the magnocellular nucleus of the caudal basal forebrain extend an axonal projection which arborizes within the reticular nucleus of the thalamus. The present study addresses the ultrastructure and neurochemistry of this projection in rats. Many labeled terminals are apparent within the thalamic reticular nucleus following Phaseolus vulgaris leucoagglutinin injections into the caudal basal nucleus; anterogradely labeled axon terminals most commonly contact both somata and dendrites of reticular nucleus neurons with symmetric membrane specializations. Thus, the majority of the labeled terminals examined contrast with choline acetyltransferase positive terminals which have been previously identified as contacting dendrites and forming asymmetric synapses within this nucleus. Many of the neurons within the caudal basal nucleus which are retrogradely labeled following tracer injections into the thalamic reticular nucleus are gamma-aminobutyric acid (GABA) immunoreactive. In addition, following injections of Phaseolus vulgaris leucoagglutinin or fluoro-ruby into the caudal basal forebrain, some of the labeled axonal swellings and boutons within the thalamic reticular nucleus also contain glutamic acid decarboxylase. These results indicate that a significant component of the projection is GABAergic. These anatomical observations suggest that the projection from the caudal basal nucleus onto the thalamic reticular nucleus could facilitate the relay of information through the dorsal thalamus by inhibiting reticular nucleus neurons, and thus, in turn, disinhibiting thalamic relay neurons.

Fine structure of the magnocellular subdivision of the ventral anterior thalamic nucleus (VAmc) of Macaca mulatta: I. Cell types and synaptology

The nucleus ventralis anterior pars magnocellularis (VAmc) is recognized only in primates and is the major recipient of the nigrothalamic projections. The neuronal and synaptic composition of this nucleus in the rhesus monkey was studied with the use of a variety of neuroanatomical techniques that included quantitative morphometry, anterograde and retrograde labeling with WGA-HRP from the prefrontal cortex, and immunocytochemistry for glutamic acid decarboxylase (GAD). Two major cell types were identified in the nucleus: thalamocortical projection neurons (PN) that were multipolar cells of various sizes, and small GAD-immunoreactive cells, apparently local circuit neurons (LCN). The approximate ratio of the two types of cells was 10:1. The major type of bouton encountered in the neuropil was of medium to large-sized (areas from 1.5 to 12 microns 2) and mostly of en passant type. These terminals formed symmetric contacts, contained moderate amounts of pleomorphic or mostly flat synaptic vesicles and large numbers of mitochondria, and displayed numerous puncta adhaerentia. All of these boutons exhibited positive GAD immunoreactivity. These boutons constituted the only synaptic population on somata and primary dendrites of PN and formed an overwhelming majority on the secondary PN dendrites. There were fewer of these axon terminals on tertiary PN and LCN dendrites. Additionally, boutons with similar features formed synapses on axon hillocks or initial axonal segments of PN, and somata or very proximal parts of primary dendrites of LCN. With the exception of the boutons in the last two locations, all of the other boutons in this group were shown to be terminals of the nigrothalamic afferents in the parallel EM autoradiographic study (Kultas-Ilinsky and Ilinsky: J. Comp Neurol. 294:479-489, '90). The second major bouton population in the VAmc was represented by small to medium-sized terminals (areas range from 0.2 to 2.0 microns 2) that formed distinct asymmetric contacts and contained large numbers of round vesicles and few or no mitochondria. These boutons were labeled anterogradely from the cortex and dominated on distal PN and LCN dendrites. Some of them were found on secondary PN dendrites where they formed synapses either directly or indirectly via LCN dendrites and dendro-dendritic contacts. The latter arrangements, i.e., serial synapses, were also formed between the cortical boutons and PN somata or tertiary dendrites.(ABSTRACT TRUNCATED AT 400 WORDS)

Extensive dual innervation and mutual inhibition by forelimb and hindlimb inputs to ventroposterolateral nucleus projection neurons in the rat

The stimulation of brachial plexus and sciatic nerve resulted in a precisely timed, synchronous volley of inputs to ventroposterolateral (VPL) neurons from either forelimb or hindlimb. Such stimulation activated sensory fibers of all modalities and was therefore modality-nonspecific. Extracellular recordings of modality-nonspecific single-unit evoked responses from VPL showed that 13% of VPL projection neurons responded to both forelimb and hindlimb inputs. We also demonstrated mutually inhibitory interactions between inputs from forelimb and hindlimb in 45% of VPL units. Unlike the somatotopic map produced by others using modality-specific inputs, the modality-nonspecific evoked response map of VPL had a broadly overlapping distribution of evoked responses. This was especially true for the more caudal aspects of VPL. When the delivery of stimuli was appropriately timed, forelimb inputs caused the inhibition of responses to forelimb stimulation; similarly, hindlimb inputs inhibited responses to forelimb stimulation. The inhibition had a variable duration that may reflect a combination of processes, including recurrent inhibitory collateral input from the thalamic reticular nucleus (TRN) or an intrinsic hyperpolarizing inhibitory afterpotential of the VPL neuron. The presence of an extensive converging input on VPL neurons and an inhibitory correlate to this overlapping of inputs may explain the shifting of VPL maps following lesions of peripheral nerve, spinal cord, or dorsal column nuclei (DCN).