Hippocampal and neostriatal inhibition of extralemniscal thalamic unitary responses in the cat

The role of the basal ganglia in nociception and pain

The involvement of the basal ganglia in motor functions has been well studied. Recent neurophysiological, clinical and behavioral experiments indicate that the basal ganglia also process non-noxious and noxious somatosensory information. However, the functional significance of somatosensory information processing within the basal ganglia is not well understood. This review explores the role of the striatum, globus pallidus and substantia nigra in nociceptive sensorimotor integration and suggests several roles of these basal ganglia structures in nociception and pain. Electrophysiological experiments have detailed the non-nociceptive and nociceptive response properties of basal ganglia neurons. Most studies agree that some neurons within the basal ganglia encode stimulus intensity. However, these neurons do not appear to encode stimulus location since the receptive fields of these cells are large. Many basal ganglia neurons responsive to somatosensory stimulation are activated exclusively or differentially by noxious stimulation. Indirect techniques used to measure neuronal activity (i.e., positron emission tomography and 2-deoxyglucose methods) also indicate that the basal ganglia are activated differentially by noxious stimulation. Neuroanatomical experiments suggest several pathways by which nociceptive information may reach the basal ganglia. Neuroanatomical studies have also indicated that the basal ganglia are rich in many different neuroactive chemicals that may be involved in the modulation of nociceptive information. Microinjection of opiates, dopamine and gamma-aminobutyric acid (GABA) into the basal ganglia have varied effects on pain behavior. Administration of these neurochemicals into the basal ganglia affects supraspinal pain behaviors more consistently than spinal reflexive behaviors. The reduction of pain behavior following electrical stimulation of the substantia nigra and caudate nucleus provides additional evidence for a role of the basal ganglia in pain modulation. Some patients with basal ganglia disease (e.g., Parkinson's disease, Huntington's disease) have alterations in pain sensation in addition to motor abnormalities. Frequently, these patients have intermittent pain that is difficult to localize. Collectively, these data suggest that the basal ganglia may be involved in the (1) sensory-discriminative dimension of pain, (2) affective dimension of pain, (3) cognitive dimension of pain, (4) modulation of nociceptive information and (5) sensory gating of nociceptive information to higher motor areas. Further experiments that correlate neuronal discharge activity with stimulus intensity and escape behavior in operantly conditioned animals are necessary to fully understand how the basal ganglia are involved in nociceptive sensorimotor integration.

Single intralaminar thalamic neurons project to cerebral cortex, striatum and nucleus reticularis thalami. A retrograde anatomical tracing study in the rat

The hypothesis of triple axonal branching to the cortex, striatum and nucleus reticularis thalami (RT) of the forebrain projecting thalamic intralaminar neurons (TIN) was studied by retrograde axonal transport of horseradish peroxidase (HRP), iron-dextran complex and [3H]wheat germ agglutinin [3H]WGA. The best combination of tracers for this purpose was demonstrated to be: HRP-pellet implantation in the rostral cortex, iron-dextran injections into the striatum and [3H]WGA injections into the rostral RT. Prussian blue labeled neurons were observed in the ipsilateral TIN, substantia nigra, mediodorsal nucleus, medial part of the ventral anterior-ventral lateral complex, anterior medial and anterior ventral nuclei. HRP labeled neurons were observed in ipsilateral ventral nuclei, mediodorsal nucleus and the TIN. Radiolabeled neurons were located only in the TIN. HRP-Prussian blue labeled neurons (cortex-striatum branched neurons) were scattered in the TIN. Prussian blue radiolabeled neurons (striatum-RT branched neurons) could be observed in the TIN, as well as a few HRP radiolabeled neurons (cortex-RT branched neurons). Triply labeled neurons were scattered throughout the TIN until the rostral part of the centre median nucleus. These results demonstrate the existence of triple axonal branching on TIN efferent axons directed to the cerebral cortex, striatum and RT. The RT directed branch provides an anatomical basis to describe an intrathalamic regulatory loop well suited to control ascending messages arising from the TIN.

Behavioral alterations and loss of caudate modulation in the centrum medianum--parafascicular complex of the cat following electrolytic lesions of the substantia nigra

Extracellular activity in the centrum medianum--parafascicular complex was examined following electrolytic lesions of the substantia nigra in the cat. Three major changes were observed in the activity of these polysensory medial thalamic neurons: (1) normally quiescent in the intact animal, about 70% of the neurons now displayed spontaneous activity; (2) many of these spontaneously active neurons no longer responded to caudate stimulation and the responses to sensory stimulation were not changed; and (3) while caudate stimulation alone did inhibit some neurons, such stimulation in a conditioning test procedure often failed to inhibit the test response to limb stimulation which 'broke through' the inhibitory period. All of these changes were correlated with the extent of damage to the ipsilateral substantia nigra. Additionally, a noticeable change was observed in the behavior of the lesioned animals; intense circling, contralateral to the side of the lesion, persisted for several days following surgery. Other symptoms were arrest reactions, abnormal rhythmic pawing and sluggishness of movement. The results indicate that the substantia nigra is an important element in funneling caudatofugal activity to the centromedian--parafascicular complex where it interacts with, and modulates incoming sensory activity. The loss of this modulation and the removal of a tonic background inhibition, indicated by the increased proportion of spontaneously active neurons, may be important factors in producing the observed behavioral abnormalities.

Regional properties of dorsal and ventral hippocampus in suppression of intralaminar thalamic unit responses

In chloralose-anaesthetized, Flaxedil-paralysed cats, the suppression of extra-lemniscal thalamic units by dorsal and ventral hippocampus was investigated. Unitary responses to test somatic stimuli, recorded in centrolateral and neighbouring thalamic nuclei, were interacted with conditioning electric stimulation in different regions around the hippocampal arch, including parahippocampal gyrus (entorhinal and retrosplenial areas). Stimulation of dorsal (dhc) and ventral (VHC) hippocampus suppressed roughly equal proportions of responses. However, within each of DHC and VHC, effectiveness depended on the region stimulated. In DHC, fields CA1 and CA3, subiculum (SUB), and retrosplenial area, but not field CA4 or dentate gyrus, usually suppressed extralemniscal units at currents below 1.0 mA. In VHC, the most effective regions were entorhinal cortex, CA3, and CA4 with dentate gryus (FD), while stimulation of CA1 or subiculum was almost ineffective, at currents below 1.0 mA. In VHC, the regions were ranked for effectiveness: Entorhinal cortex=CA3 is greater than FD is greater than SUB is greater than CA1. No topographic relationship was found between hippocampal region and thalamic loci for unit suppression. Lemniscal-type unit responses in ventrobasal thalamus were unaffected by stimulation of the hippocampus or parahippocampal gryus. Interruption of the fornix-fimbria system prevented suppression elicited from CA1 of DHC, or from CA3 but not FD of VHC. It had no effect on suppression elicited from retrosplenial or entorhinal cortex. Hippocampal regional variation of effectiveness in suppressing extralemniscal pathways may contribute to the differential behavioural involvements reported for different hippocampal structures.