A high voltage electrometer for recording and iontophoresis with fine-tipped, high resistance microelectrodes

lontophoretic Devices for Drug Delivery

Transdermal drug delivery of ionized drugs can be enhanced by iontophoresis. Drug in the ionic form, contained in some reservoir, can be "phoresed" through the skin with a small current across two electrodes, one above the reservoir and one at a distal skin location. Positive ions can be introduced from the positive pole, or negative ions from the negative pole. The design and development of iontophoretic devices are rather simple. Some of the principles of operation and the advantages/disadvantages and clinical implications associated with these devices are outlined in this review.

Functional and morphological features of neurons in the midline region of the caudal spinal cord of the cat

Neurons surrounding and dorsal to the central canal in caudal segments of the cat spinal cord were functionally and morphologically characterized. From electrophysiologically obtained responses these neurons were categorized into 3 functional groups based on excitation by somatic afferent stimulation. Eighteen of 36 units were activated by both low threshold and high threshold primary afferent inputs. Of the remaining 18 units, 9 responded only to innocuous intensities of stimulation and the other 9 were excited selectively by noxious peripheral stimulation. Neurons intracellularly marked with horseradish peroxidase formed a heterogeneous population with respect to perikaryal size, dendritic orientation and dendritic extent, and no evident correlations between functional categories and morphological features were observed in light microscopic analyses. Neurons immediately surrounding the central canal were functionally similar to neurons located in the dorsally adjacent gray matter.

The spinal terminations of single, physiologically characterized axons originating in the pontomedullary raphe of the cat

Single myelinated axons were recorded in the dorsolateral funiculus of the cat and physiologically characterized as descending from the midline medulla or midline pons. Following further physiological characterization (e.g., conduction velocity, adequate stimulus, receptive field, activation by stimulation of periaqueductal gray), the axons were labeled with horseradish peroxidase that was iontophoretically ejected from the recording micropipette. Histochemical reaction allowed visualization of the stained axons and their arborizations in the spinal gray matter. The conduction velocities of the sampled axons ranged from 7.3 to 117.2 m/second with a mean of 35.5 m/second. However, unmyelinated axons could not be sampled with the technique employed here. Descending axons could be divided into two groups: (1) those which terminated in laminae I, II, V, and X, and (2) those which terminated in laminae V, VII, and X. Axons from both groups had myelinated parent axons, were activated by periaqueductal gray stimulation, and responded to noxious pinch of their receptive field. Terminal collaterals from both groups of axons were generally transversely oriented. These results suggest heterogeneous functions for these descending axons which may include modulation of nociceptive input to higher centers.

Synaptic complexes formed by functionally defined primary afferent units with fine myelinated fibers

The individual fine myelinated fibers of cutaneous mechanical nociceptors and "D-hair" receptors were identified by electrophysiological recording with micropipette electrodes in cats and monkeys. Their intraspinal terminations were labeled by iontophoresing horseradish peroxidase intracellularly and subsequent diaminobenzidine histochemistry. These terminations were examined with light and electron microscopy to determine the nature and organization of their synaptic contacts. Myelinated fibers of the mechanical nociceptors became unmyelinated before exhibiting many enlargements that made multiple synaptic contacts in the marginal zone (lamina I) and lamina V. Pre- or postsynaptic contacts were found only on enlargements. In the marginal zone of the cat, enlargements made simple axodendritic contacts or were scalloped, central terminals in glomeruli. In glomeruli, myelinated mechanical nociceptor enlargements were presynaptic to several dendritic appendages and postsynaptic to two different types of profiles. One type was interpreted as a presynaptic axon terminal, the other as a presynaptic, vesicle-containing, dendritic appendage. In lamina V of the cat the nociceptor synaptic complexes were similar, but simpler, and only axonal profiles were found to be presynaptic to them. In the monkey marginal zone and deep nucleus proprius, myelinated nociceptor terminations formed the central element of glomeruli, which consisted of postsynaptic dendritic appendages and presynaptic axon terminals. D-hair axons terminated in large numbers of enlargements in the nucleus proprius (laminae III and IV) and inner substantia gelatinosa (lamina IIi). Their large rounded enlargements formed the central terminals in glomeruli and were presynaptic to both ordinary and vesicle-containing dendritic appendages; the presynaptic dendritic profiles also often contacted each other. Profiles interpreted as axonal in origin were the only terminals presynaptic to the primary ending within the D-hair glomeruli. The results suggest that transfer of primary afferent information occurs only at enlargements of the primary fiber and that each primary fiber enters into more than one kind of synaptic arrangement. They also point out that synaptic glomeruli are common to functionally different types of primary afferent fibers and that the internal organization of glomeruli varies with the kind of primary fiber and the locus of the complex.