A new process for the fabrication of regeneration microelectrode arrays for peripheral and cranial nerve applications is presented. This type of array is implanted between the severed ends of nerves, the axons of which regenerate through via holes in the silicon and are thereafter held fixed with respect to the microelectrodes. The process described is designed for compatibility with industry-standard CMOS or BiCMOS processes (it does not involve high-temperature process steps nor heavily-doped etch-stop layers), and provides a thin membrane for the via holes, surrounded by a thick silicon supporting rim. Many basic questions remain regarding the optimum via hole and microelectrode geometries in terms of both biological and electrical performance of the implants, and therefore passive versions were fabricated as tools for addressing these issues in on-going work. Versions of the devices were implanted in the rat peroneal nerve and in the frog auditory nerve. In both cases, regeneration was verified histologically and it was observed that the regenerated nerves had reorganized into microfascicles containing both myelinated and unmyelinated axons and corresponding to the grid pattern of the via holes. These microelectrode arrays were shown to allow the recording of action potential signals in both the peripheral and cranial nerve setting, from several microelectrodes in parallel.

In the anesthetized rat, cocaine (25 mg/kg i.p.), enhanced the frequency potentiation of nociceptively evoked polysynaptic discharges but did not affect the polysynaptic reflex discharge to single nociceptive stimuli or the habituation of this reflex to repetitive pinch stimuli. The non-nociceptive, short-latency reflex discharge was suppressed for 10-15 min after cocaine administration. The neurogenic extravasation response to antidromic cutaneous C-fiber stimulation was unaffected by cocaine. These findings suggest that systemic cocaine, in doses analgesic for the rat, does not suppress spinal nociceptive reflexes.

The effect of antinociceptive doses of cocaine (25 mg/kg, i.p.) on unit responses to noxious somatic stimuli and spontaneous activity of antidromically identified projection neurons in the medial medullary reticular formation (MRF) was studied in the rat. Thirty-three antidromically activated neurons were recorded from the medullary raphe, gigantocellular, or paragigantocellular nuclei in an acute anaesthetized preparation; 25 cells projected to the spinal cord and 8 neurons had rostral projections through the medial forebrain bundle (n = 4) or the medial thalamus (n = 4). After cocaine administration, 24 (73%) of these cells showed immediate (less than 5 min) and prolonged (45-70 min) increases in their level of spontaneous activity. Associated with this increased interstimulus activity, 21 of 29 (72%) neurons responsive to noxious somatic stimulation reduced their responsiveness, relative to prestimulus activity, after cocaine administration. In 5 animals tested, the cocaine-induced changes in spontaneous activity and changes in evoked responsiveness were unaffected by naloxone (1 mg/kg, i.p.) but partially reversed within 5 min of the administration of chlorpromazine (3 mg/kg, i.p.). There were no obvious differences in neuronal response characteristics or the effect of cocaine that correlated with anatomical location or direction of axonal projection. Similar results were obtained while recording from 14 somatically responsive units in chronic, unrestrained, lightly anesthetized or awake rats. These findings provide direct evidence that cocaine, in doses that are antinociceptive for the rat, affects both unit responses to noxious stimuli and the spontaneous activity of caudally and rostrally projecting bulboreticular neurons over a time course that parallels the behavioral antinociception. The observation that unit responses to somatic stimuli were reduced while spontaneous activity was unchanged or increased in most cells suggests that cocaine antinociception may be due to the activation of sensory inhibitory mechanisms mediated by the MRF.