However, this does not necessarily mean that all of these cells belong to the spinoparabrachial tract, since some of the labelling may result PR-171 manufacturer from uptake of tracer by fibres passing through the injection site. For example, the projection from lamina I to the PAG passes through the rostral part of the parabrachial area (Bernard et al., 1995 and Feil and Herbert, 1995), and although there is a dense terminal arborisation within the LPb it is possible that some axons pass through this region without contributing to this arborisation. If this is the case, then some spino-PAG neurons would not belong to the spinoparabrachial tract, but may be retrogradely labelled

from the LPb. Spinothalamic axons from lamina I ascend near the parabrachial area and are located approximately 500 μm lateral to the external lateral nucleus of the LPb (J.F. Bernard, personal communication). Although these axons are likely to have been included in the LPb injections in several of the

present series of experiments, this should not alter the interpretation, because our previous finding that virtually all spinothalamic lamina I neurons were labelled from LPb was obtained from cases in which the LPb injections did not extend into this region (Al-Khater and Todd, 2009). The uptake of tracer by fibres of passage is unlikely check details to have contributed to the labelling from the dorsal medulla, as these injections PAK5 were located a considerable distance from the main ascending bundle of axons from lamina I, which is in the ventrolateral part of the brainstem at this level (Mehler, 1969, Zemlan

et al., 1978 and Slugg and Light, 1994). However, it causes a significant problem for interpreting the labelling that results from injections of tracer into the CVLM, as we have reported previously (Spike et al., 2003). Although tracer injections into CVLM cannot be used to identify supraspinal targets, they are useful because they can label a very high proportion of lamina I projection neurons in both enlargements. Our previous estimate that there were ∼ 400 lamina I projection neurons on each side in the L4 segment of the rat was based on counts of cells retrogradely labelled from LPb, CVLM and PAG (Spike et al., 2003), and we have since demonstrated that all spinothalamic lamina I cells at this level are included in the population labelled from LPb (see above). Since nearly all lamina I neurons that project to the dorsal medulla are also labelled from LPb, this provides further support for the reliability of our estimate. The present results, together with those of Al-Khater and Todd (2009) suggest that virtually all lamina I projection neurons in C7 can also be labelled from LPb.