The firing patterns of spinal neurons: in situ patch-clamp recordings reveal a key role for potassium currents.

Neuron firing patterns underpin the detection and processing of stimuli, influence synaptic interactions, and contribute to the function of networks. To understand how intrinsic membrane properties determine firing patterns, we investigated the biophysical basis of single and repetitive firing in spinal neurons of hatchling Xenopus laevis tadpoles, a well-understood vertebrate model; experiments were conducted in situ. Primary sensory Rohon-Beard (RB) neurons fire singly in response to depolarising current, and dorsolateral (DL) interneurons fire repetitively. RB neurons exhibited a large tetrodotoxin-sensitive sodium current; in DL neurons, the sodium current density was significantly lower. High-voltage-activated calcium currents were similar in both neuron types. There was no evidence of persistent sodium currents, low-voltage-activated calcium currents, or hyperpolarisation-activated currents. In RB neurons, the potassium current was dominated by a tetraethylammonium-sensitive slow component (I(Ks) ); a fast component (I(Kf) ), sensitive to 4-aminopyridine, predominated in DL neurons. Sequential current-clamp and voltage-clamp recordings in individual neurons suggest that high densities of I(Ks) prevent repetitive firing; where I(Ks) is small, I(Kf) density determines the frequency of repetitive firing. Intermediate densities of I(Ks) and I(Kf) allow neurons to fire a few additional spikes on strong depolarisation; this property typifies a novel subset of RB neurons, and may activate escape responses. We discuss how this ensemble of currents and firing patterns underpins the operation of the Xenopus locomotor network, and suggest how simple mechanisms might underlie the similar firing patterns seen in the neurons of diverse species.

Pharmacology of currents underlying the different firing patterns of spinal sensory neurons and interneurons identified in vivo using multivariate analysis.

The operation of neuronal networks depends on the firing patterns of the network's neurons. When sustained current is injected, some neurons in the central nervous system fire a single action potential and others fire repetitively. For example, in Xenopus laevis tadpoles, primary-sensory Rohon-Beard (RB) neurons fired a single action potential in response to 300-ms rheobase current injections, whereas dorsolateral (DL) interneurons fired repetitively at 10-20 Hz. To investigate the basis for these differences in vivo, we examined drug-induced changes in the firing patterns of Xenopus spinal neurons using whole cell current-clamp recordings. Neuron types were initially separated through cluster analysis, and we compared results produced using different clustering algorithms. We used these results to develop a predictive function to classify subsequently recorded neurons. The potassium channel blocker tetraethylammonium (TEA) converted single-firing RB neurons to low-frequency repetitive firing but reduced the firing frequency of repetitive-firing DL interneurons. Firing frequency in DL interneurons was also reduced by the potassium channel blockers 4-aminopyridine (4-AP), catechol, and margatoxin; 4-AP had the greatest effect. The calcium channel blockers amiloride and nimodipine had few effects on firing in either neuron type but reduced action potential duration in DL interneurons. Muscarine, which blocks M-currents, did not affect RB neurons but reduced firing frequency in DL interneurons. These results suggest that potassium currents may control neuron firing patterns: a TEA-sensitive current prevents repetitive firing in RB neurons, whereas a 4-AP-sensitive current underlies repetitive firing in DL interneurons. The cluster and discriminant analysis described could help to classify neurons in other systems.

Clustered third-base substitutions among wild strains of Escherichia coli.

Nucleotide sequences of translated regions of the trp operon in 12 wild strains of Escherichia coli reveal striking uniformity among eight strains (suggesting recent common ancestry and supporting the importance of periodic selection in natural populations) and clustered substitutions in four strains (implicating events affecting runs of nucleotides).

The neural correlates of visual imagery: A co-ordinate-based meta-analysis.

Visual imagery is a form of sensory imagination, involving subjective experiences typically described as similar to perception, but which occur in the absence of corresponding external stimuli. We used the Activation Likelihood Estimation algorithm (ALE) to identify regions consistently activated by visual imagery across 40 neuroimaging studies, the first such meta-analysis. We also employed a recently developed multi-modal parcellation of the human brain to attribute stereotactic co-ordinates to one of 180 anatomical regions, the first time this approach has been combined with the ALE algorithm. We identified a total 634 foci, based on measurements from 464 participants. Our overall comparison identified activation in the superior parietal lobule, particularly in the left hemisphere, consistent with the proposed 'top-down' role for this brain region in imagery. Inferior premotor areas and the inferior frontal sulcus were reliably activated, a finding consistent with the prominent semantic demands made by many visual imagery tasks. We observed bilateral activation in several areas associated with the integration of eye movements and visual information, including the supplementary and cingulate eye fields (SCEFs) and the frontal eye fields (FEFs), suggesting that enactive processes are important in visual imagery. V1 was typically activated during visual imagery, even when participants have their eyes closed, consistent with influential depictive theories of visual imagery. Temporal lobe activation was restricted to area PH and regions of the fusiform gyrus, adjacent to the fusiform face complex (FFC). These results provide a secure foundation for future work to characterise in greater detail the functional contributions of specific areas to visual imagery.

Structural changes in human serum albumin induced by ingestion of acetylsalicylic acid.

Acetylsalicylic acid (aspirin) acetylates human serum albumin under physiologic conditions in vitro. These investigations were done to determine whether this phenomenon occurs in vivo and to delineate the site(s) of acetylation on the albumin molecule. Albumin was reacted in vitro with aspirin labeled with (14)carbon at the acetyl-1 or the carboxyl carbon. The altered albumin was hydrolyzed with trypsin and peptide mapping performed. Albumin so treated contains a unique peptide, designated "A," and shows diminution of two normal peptides, designated "B" and "C." Peptide "A" is never seen in normal albumin. Amino acid analyses indicate that peptide "A" equals the sum of peptides "B" and "C." Furthermore all three peptides contain lysine but lack arginine. Thus peptide "A" is formed by the acetylation of a lysine residue which is normally susceptible to trypsin and yields peptides "B" and "C." Radioautography of the peptide maps show most of the acetyl-1-(14)C activity in peptide "A." This indicates that one of the lysine residues in this peptide is the preferential site for the transacetylation reaction. Peptide "A," used as a marker for acetylation, is found in albumin from patients who take aspirin but is not demonstrable in albumin from one of these patients while she was taking sodium salicylate. A transacetylation reaction between aspirin and human albumin occurs in vivo and is similar to that observed in vitro.

Laboratory induction and clinical occurrence of combined clindamycin and erythromycin resistance in Corynebacterium acnes.

Corynebacterium acnes strains cross-resistant to clindamycin and erythromycin were observed following long-term selection or mutagenic treatment in the laboratory. Similar strains were found among clinical isolates from patients using clindamycin or erythromycin topically in the treatment of acne vulgaris. Clindamycin resistance was never observed in the absence of resistance to macrolides or other lincosaminides. It is suggested that this resistance may result from an alteration of the 50S ribosomal subunit.

DNA sequence of the tryptophan synthase genes of Pseudomonas putida.

Genes encoding the 2 subunits of tryptophan synthase in Pseudomonas putida have been identified and cloned by their similarity to the corresponding genes in Pseudomonas aeruginosa. The deduced amino acid sequences were confirmed by comparison with regions ascertained earlier by protein sequencing. The Pseudomonas amino acid sequences are 85% identical for the beta subunit and 70% identical for the alpha subunit. These sequences are compared to those of Salmonella typhimurium, where the structure is known from X-ray crystallography. Although amino acid conservation drops to 54% and 36% for the beta and alpha subunits, only 3 single residue gaps are required to maintain alignment throughout and most of the residues identified as important for catalysis or cofactor binding are conserved. The 23 residues surrounding the beta chain lysine that enters into a Schiff base linkage with the pyridoxal phosphate cofactor are compared in 13 species, including representatives from the eukaryotic and both prokaryotic kingdoms; appreciable conservation is apparent. The approximately 100 base pairs separating the trpB gene from its divergently transcribed activator gene are similar in the 2 pseudomonads, but do not resemble those of any other bacterium or fungus studied to date.

Sequence and features of the tryptophan operon of Vibrio parahemolyticus.

The nucleotide sequence of the trp operon of the marine enteric bacterium Vibrio parahemolyticus is presented. The gene order E, G, D, C(F), B, A is identical to that of other enterics. The structural genes of the operon are preceded by a long leader region encoding a 41-residue peptide containing five tryptophan residues. The organization of the leader region suggests that transcription of the operon is subject to attenuation control. The promoter-operator region of the V. parahemolyticus trp operon is almost identical to the corresponding promoter-operator of E. coli. The similarities suggest that promoter strength and operator function are identical in the two species, and that transcription initiation is regulated by repression. The operon appears to lack the internal promoter within trpD that is common in terrestrial enteric species.