The influence of descending inputs on breathing pattern formation in the isolated bullfrog brainstem-spinal cord.

This study used in vitro brainstem-spinal cord preparations from the American bullfrog, Rana catesbeiana, to examine the influence of central descending inputs on breathing pattern formation. In preparations with an episodic pattern of fictive breathing, a transection slightly caudal to the optic chiasma produced a continuous breathing pattern and increased the overall frequency of fictive breathing. Following a transection to isolate the medulla, the frequency of fictive breathing decreased and the incidence of other forms of motor output increased. Further transections between the trigeminal and vagus nerve roots resulted in variable and asynchronous discharge from each nerve. These results suggest that a primary respiratory rhythm is produced within the medulla but descending influences stimulate breathing and promote episodic breathing. It would appear that multiple elements of the respiratory control system, including tegmental and medullary sites, play a role in shaping the burst pattern of motor output associated with each breath and that slower rhythms of longer burst duration are generated by more caudal hindbrain sites.

Pulmonary vagal modulation of ventilation in toads (Bufo marinus).

This study examined the role of pulmonary vagal feedback on hypercapnic chemosensitivity and breathing pattern formation in cane toads (Bufo marinus). Decerebrate, paralysed toads were uni-directionally ventilated with air, 2.5% CO(2) or 5.0% CO(2) with the lungs inflated or deflated, before and after pulmonary vagotomy. Motor output from the mandibular branch of the trigeminal nerve served as an index of fictive breathing. As respiratory drive was increased, breathing frequency increased and breaths were clustered into discrete episodes separated by periods of apnea. Lung deflation tended to enhance episodic breathing while inflation biased the system towards apnea at low levels of respiratory drive and a pattern of continuous, small breaths at higher levels of respiratory drive. Following bilateral pulmonary vagotomy there was no increase in ventilation during hypercapnia and lung inflation/deflation had no effect on breathing pattern. In isolated brainstem-spinal cord preparations from the same animals, all variables associated with fictive breathing were unaffected by changes in superfusate pH from 8.0 to 7.6. The breathing pattern from the in vitro preparations was highly variable. This study demonstrates a crucial role for vagal feedback in modulating respiration and the respiratory responses to hypercapnia in B. marinus.

Central respiratory pattern generation in the bullfrog, Rana catesbeiana.

There are two components to breathing pattern generation the production of the pattern of neural discharge associated with individual breaths, and the pattern in which breaths are produced to effect ventilation. Bullfrogs typically breathe with randomly distributed breaths. When respiratory drive is elevated, breathing becomes more regular and often episodic. Studies on in vitro brainstem-spinal cord preparations of the adult bullfrog and in situ preparations of decerebrate, paralyzed, unidirectionally ventilated animals suggest that output from the central rhythm generator in frogs is conditional on receiving some input and that a host of central inputs remain even in the most reduced preparations. There appear to be descending inputs from sites in the dorsal brainstem just caudal to the optic chiasma that cluster breaths into episodes, a strong excitatory input caudal to this site but rostral to the origin of the Vth cranial nerve and, possibly, segmental rhythm generators throughout the medulla that are normally entrained to produce the normal breathing pattern. The data also suggest that the shape of the discharge pattern (augmenting, decrementing) and timing of outputs (alternating vs synchronous) associated with motor outflow during each breath are also dependent on the interconnections between these various sites.

P nucleotides in V(D)J recombination: a fine-structure analysis.

Antigen receptor genes acquire junctional inserts upon assembly from their component, germ line-encoded V, D, and J segments. Inserts are generally of random sequence, but a small number of V-D, D-J, or V-J junctions are exceptional. In such junctions, one or two added base pairs inversely repeat the sequence of the abutting germ line DNA. (For example, a gene segment ending AG might acquire an insert beginning with the residues CT upon joining). It has been proposed that the nonrandom residues, termed "P nucleotides," are a consequence of an obligatory end-modification step in V(D)J recombination. P insertion in normal, unselected V(D)J joining products, however, has not been rigorously established. Here, we use an experimentally manipulable system, isolated from immune selection of any kind, to examine the fine structure of V(D)J junctions formed in wild-type lymphoid cells. Our results, according to statistical tests, show the following, (i) The frequency of P insertion is influenced by the DNA sequence of the joined ends. (ii) P inserts may be longer than two residues in length. (iii) P inserts are associated with coding ends only. Additionally, a systematic survey of published P nucleotide data shows no evidence for variation in P insertion as a function of genetic locus and ontogeny. Together, these analyses establish the generality of the P nucleotide pattern within inserts but do not fully support previous conjectures as to their origin and centrality in the joining reaction.

Antigenic variation is associated with DNA rearrangements in a relapsing fever Borrelia.

Borrelia hermsii, an agent of relapsing fever, undergoes antigenic variation in its host. Surface-exposed proteins with differing primary structures determine the serotype of each organism. Using amino acid sequence data from two of these variable proteins, we synthesized two mixed-sequence oligonucleotides and then used the oligonucleotides to probe mRNA and DNA of three isogenic serotypes of B. hermsii. In Northern blots the probes were specific for the mRNA of the homologous serotype. Southern blots revealed two classes of hybridizing fragments: those common to the three serotypes and those specific for a particular serotype. A serotype-specific DNA fragment, which had hybridized to both oligonucleotide probes, was cloned. Subsequent use of the cloned fragment as a probe provided further evidence that antigenic variation in B. hermsii is associated with DNA rearrangements and with occurrence of expression-linked copies of all, or part, of an antigen-specifying gene.