Identification of spinothalamic tract cells in fresh, unfixed rat spinal cord

Kainate-receptor-mediated sensory synaptic transmission in mammalian spinal cord

Glutamate, the major excitatory neurotransmitter in the central nervous system, activates three different receptors that directly gate ion channels, namely receptors for AMPA (alpha-amino-3-hydroxy-5-methyl isoxozole propionic acid), NMDA (N-methyl-D-aspartate), and kainate, a structural analogue of glutamate. The contribution of AMPA and NMDA receptors to synaptic transmission and plasticity is well established. Recent work on the physiological function of kainate receptors has focused on the hippocampus, where repetitive activation of the mossy-fibre pathway generates a slow, kainate-receptor-mediated excitatory postsynaptic current (EPSC). Here we show that high-intensity single-shock stimulation (of duration 200 microseconds) of primary afferent sensory fibres produces a fast, kainate-receptor-mediated EPSC in the superficial dorsal horn of the spinal cord. Activation of low-threshold afferent fibres generates typical AMPA-receptor-mediated EPSCs only, indicating that kainate receptors may be restricted to synapses formed by high-threshold nociceptive (pain-sensing) and thermoreceptive primary afferent fibres. Consistent with this possibility, kainate-receptor-mediated EPSCs are blocked by the analgesic mu-opiate-receptor agonist Damgo and spinal blockade of both kainate and AMPA receptors produces antinociception. Thus, spinal kainate receptors contribute to transmission of somatosensory inputs from the periphery to the brain.

The cells of origin of the spinothalamic tract of the rat: a quantitative reexamination

We quantitatively reinvestigated the cells of origin of the spinothalamic tract (STT) of the rat. Injections of Fluoro-Gold that filled the thalamus on one side labeled large numbers of neurons throughout the length of the spinal cord. In 3 cases, we estimated the total number of STT neurons by counting labeled neurons in 18 of the 34 total segments, applying correction factors to these counts, and estimating the numbers of labeled neurons in the 16 remaining unexamined segments. The accuracy of these estimates was tested in two animals in which labeled neurons were counted in all 34 spinal segments. In both cases, the estimated totals of STT neurons differed from the counted totals by less than 5%. In the most effective case, we estimated that more than 9500 STT neurons were labeled. This study indicates that the number of STT neurons in rats is larger than previously reported and suggests that the STT may play an important role in nociception in rats, as it does in primates including humans.

Origin of thalamically projecting somatosensory relay neurons in the immature rat

The locations of spinothalamic (STT) and trigeminothalamic (TTT) neurons in 14-18-day rats using retrograde transport of fluorescent latex microspheres were examined. The aim of this study was to determine whether the connections between the somatosensory relay neurons and the thalamus were established in these immature rats. The majority of the labeled STT and TTT neurons was found in the brainstem and upper cervical cord (C1-C4). These cells were distributed to a number of distinct groups. Among them, the nucleus of trigeminal spinal tract interpolaris (SP51) had the highest number of TTT cells, and the internal basilar nucleus (IB) comprised the largest population of STT cells. Except for the cells located in the dorsal portion of the ventral horn (VHd), most of the labeled STT and TTT cells were contralateral to the thalamic injection sites. This pattern of distribution of the projecting neurons in immature rats is very similar to the pattern observed in adult rats. The development of the STT and TTT projections in 14-18-day rats is therefore largely complete. This result will allow us to extrapolate our understanding of the membrane properties of projection cells obtained in the immature rats to the adult rats.

Calcium channels in isolated rat dorsal horn neurones, including labelled spinothalamic and trigeminothalamic cells

1. Single isolated neurones were prepared from the spinal trigeminal nucleus and the dorsal horn of cervical spinal cord of the rat. Spinothalamic and trigeminothalamic neurones were identified using rhodamine-labelled fluorescent latex microspheres. 2. Calcium currents in these cells were examined by the whole-cell patch-clamp technique. Three types of calcium currents, transient (T) and slow inactivating (N and L) types, were identified by their sensitivities to inorganic blockers and rates of inactivation at two different holding potentials (temperature = 21-25 degrees C). 3. From a holding potential of -100 mV, the ICa,T began to activate at -60 mV. The current reached its maximum amplitude around -30 mV and was inactivated completely when the cell was held more positive than -60 mV. The time constant of the inactivation was between 10 and 50 ms. 4. The slow inactivating component of ICa was dissociated into two components by eliciting ICa from two holding potentials of -100 and of -40 mV. The current (ICa,L) activated from -40 mV was characterized by positive activation potentials and a very slow inactivation (time constant, 700-4000 ms). The current (ICa,N) elicited from a holding potential of -100 mV started to activate at -30 mV and inactivated slowly with time constants ranging between 400 and 1000 ms. 5. Compared with the ICa,T, the inactivation curve for ICa,N was shifted about 30 mV in the depolarizing direction. ICa,N inactivated over a broader range of potentials, and its inactivation and activation curves overlapped. 6. Cadmium blocked ICa,T at a concentration 24 times higher than that which was needed to block slow inactivating currents. The apparent dissociation constant of nickel for ICa,T is twofold lower than that for the slow inactivating currents. 7. Nimodipine (2 microM) decreased the slow inactivating currents, but had no effect on ICa,T. (-)-Bay K 8644 (200 nM) increased both ICa,N and ICa,L and shifted the current activation in the hyperpolarizing direction. This result is different from that obtained in sensory and sympathetic neurones in which ICa,N is insensitive to Bay K 8644.

Electrical properties of acutely isolated, identified rat spinal dorsal horn projection neurons

We used a retrograde marker, rhodamine-labeled fluorescent latex microspheres and the enzyme papain to isolate a group of identified, dorsal horn projection neurons from rat spinal cord. This method has allowed us to study the electrical properties of these acutely dissociated cells with patch-clamp techniques. It will also provide us the opportunity to understand the transmitter responses of these neurons that had not undergone culturing manipulation. The isolated projection cells have a short-lasting action potential and fire repetitively under a sustained depolarization. The ionic conductance mechanisms underlying the action potentials were studied under voltage-clamp conditions. Voltage-dependent inward sodium, inward calcium, outward potassium and calcium-activated potassium currents were observed in these cells.