Neuron and synapse numbers are important assays in neuroscience. These numbers are estimated by one of four methods: 1) profile counts, 2) assumption-based methods, 3) serial reconstructions, and 4) stereological methods. The criteria for these methods are diverse. This creates a disparity in that some reviewers accept estimates from any of these methods, while others accept only specific methods. An equally important issue is the diversity of sampling strategies, since unbiased estimates of neuronal or synaptic numbers are contingent upon both counting and sampling techniques. The purpose of this commentary is to institute a dialog that will lead to a better understanding of the strengths and weaknesses of the above methods, and to propose guidelines that should lead to more uniform and thus fairer judging of the studies that provide estimates of neuron or synapse numbers. In addition, adoption of more uniform standards for obtaining unbiased numerical estimates should result in the generation of an unbiased database that will be of considerable use in future studies.

Neuron and synapse numbers are important assays in neuroscience. These numbers are estimated by one of four methods: 1) profile counts, 2) assumption-based methods, 3) serial reconstructions, and 4) stereological methods. The criteria for these methods are diverse. This creates a disparity in that some reviewers accept estimates from any of these methods, while others accept only specific methods. An equally important issue is the diversity of sampling strategies, since unbiased estimates of neuronal or synaptic numbers are contingent upon both counting and sampling techniques. The purpose of this commentary is to institute a dialog that will lead to a better understanding of the strengths and weaknesses of the above methods, and to propose guidelines that should lead to more uniform and thus fairer judging of the studies that provide estimates of neuron or synapse numbers. In addition, adoption of more uniform standards for obtaining unbiased numerical estimates should result in the generation of an unbiased database that will be of considerable use in future studies.

No abstract

1. An experimental peripheral neuropathy (EPN) was induced in three monkeys (Macaca fascicularis) by ligation of spinal nerve L7. Behavioral responses to innocuous mechanical stimuli were tested before and after the surgery. Two weeks after the nerve ligation, the activity of spinothalamic tract (STT) neurons was recorded on both sides of the spinal cord with the animal under general anesthesia. Responses of the STT neurons to the following stimuli applied to the skin were recorded: graded mechanical stimuli (brush, press, pinch and squeeze), von Frey filaments of different bending forces (0.077-19.05 g), 5-s heat stimuli ranging from 39 to 53 degrees C, and 15 s cold stimuli (32-8 degrees C). 2. Innocuous mechanical stimulation of the foot did not evoke hindlimb withdrawal in the animals before surgery. Within 24-48 h after nerve ligation, the animals showed hindlimb withdrawal to the same innocuous stimuli. This behavior was more pronounced on the side of the ligation than on the sham-operated side and more frequent during the second week after the surgery. 3. Responses of 51 STT neurons recorded on the side of the ligation (EPN all group) were compared with responses of 33 STT cells recorded on the sham-operated side (control group) and with records from STT neurons in unoperated animals obtained earlier (reference group). Neurons from the EPN all group were divided into two sets according to their rostrocaudal location (EPN R, rostral to L6/7 border, n = 40; EPN C, caudal to L6/7 border, n = 11). 4. Neurons from the EPN all and EPN R groups had significantly higher background frequencies than those from the control and reference groups. Innocuous brush stimuli evoked mean discharge frequencies of approximately 35 Hz in EPN R neurons and only approximately 15 Hz in both control and reference groups. Increased responsiveness of EPN R neurons to innocuous stimuli was also demonstrated by lower thresholds and higher discharge frequencies to von Frey filament stimulation and by discriminative analysis of the responses evoked by graded mechanical stimuli. 5. The responses of the EPN R neurons to heat stimulation of the skin showed decreased thresholds and increased responses to suprathreshold stimuli, resulting in a significant leftward shift of the stimulus-response curve compared with both reference and control groups. The neurons from the control group showed responses comparable to reference group values. 6. Neurons from the reference group tested with the cooling stimuli showed no evoked response above background.(ABSTRACT TRUNCATED AT 400 WORDS)

No abstract

A goal of the present study was to document the behavioral changes observed in a model of painful neuropathy in the primate (Macaca fascicularis). A neuropathic state was induced by tight ligation of the L7 spinal nerve, just distal to the L7 dorsal root ganglion. Sensory testing was done on the ventral surface of the foot, a region that includes the L7 dermatome. Within 1 week following surgery, all monkeys (n = 3) developed a marked sensitivity to mechanical stimulation (with a camel hair brush and von Frey hairs), indicating the presence of mechanical allodynia. In 2 animals, the increased sensitivity to mechanical stimulation was also observed on the contralateral side. The threshold for withdrawal to a heat stimulus decreased, indicating the presence of heat hyperalgesia. Presentation of various cooling stimuli, such as acetone and cold water baths, suggested that cold allodynia had also developed. These behavioral phenomena are similar to those seen in humans diagnosed with peripheral neuropathic pain. The behavioral abnormalities are discussed in relation to the responses of spinothalamic tract cells recorded from primates with the same peripheral nerve injury (Palecek et al. 1992).

No abstract

Excitotoxicity due to excessive synaptic glutamate release is featured in many neurological conditions in which neuronal death occurs. Whether activation of primary sensory pathways can ever produce sufficient over-activity in secondary sensory neurons in the dorsal horn of the spinal cord to induce cell death, however, has not been determined. In this study, we asked whether activity in myelinated afferents (A fibers), which use glutamate as a transmitter, can induce cell death in the dorsal horn. Using stereological estimates of neuron numbers from electron microscopic sections, we found that stimulation of A-fibers in an intact sciatic nerve at 10 Hz, 20 Hz, and 50 Hz in 10-minute intervals at a stimulus strength that activates both Abeta and Adelta fibers resulted in the loss of 25% of neurons in lamina III, the major site of termination of large Abeta fibers, but not in lamina I, where Adelta fibers terminate. Furthermore, sciatic nerve lesions did not result in detectable neuron loss, but activation of A fibers in a previously sectioned sciatic nerve did cause substantial cell death not only in lamina III but also in laminae I and II. The expansion of the territory of A-fiber afferent-evoked cell death is likely to reflect the sprouting of the fibers into these laminae after peripheral nerve injury. The data show, therefore, that primary afferent A-fiber activity can cause neuronal cell death in the dorsal horn with an anatomical distribution that depends on whether intact or injured fibers are activated. Stimulation-induced cell death potentially may contribute to the development of persistent pain.